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Raman spectroscopy for uid inclusion analysis

Maria Luce Frezzotti a,b,⁎, Francesca Tecce b, Alessio Casagli a

a Dipartimento Scienze della Terra, Università di Siena, Via Laterina 8, 53100 Siena, Italyb Istituto Geologia Ambientale e Geoingegneria - CNR, c/o Dipartimento Scienze della Terra, Università “ La Sapienza” , P.le Aldo Moro 5, 00185 Roma, Italy

a b s t r a c ta r t i c l e i n f o

Article history:Received 7 June 2011Accepted 18 September 2011Available online 25 September 2011

Keywords:

Raman spectroscopyFluid inclusionsGeological uidsRaman spectra database

Raman spectroscopy is a versatile non-destructive technique for uid inclusion analysis, with a wide eld of applications ranging from qualitative detection of solid, liquid and gaseous components to identication of polyatomic ions in solution. Raman technique is commonly used to calculate the density of CO 2 uids, thechemistry of aqueous uids, and the molar proportions of gaseous mixtures present as inclusions. Ramanspectroscopy has been applied to measure the pH range and oxidation state of uids. The main advantagesof this technique are the minimal sample preparation and the high versatility. Present review summarizesthe recent developments of Raman spectroscopy in uid inclusions research to provide support for laboratoryanalyses.

© 2011 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33. Methods of analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44. Gaseous uids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1. CO2 uids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2. Gaseous mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5. Aqueous uids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.1. Analyses of solutes: monoatomic ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.2. Analyses of solutes: polyatomic ions and molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6. Identication of mineral phases: a catalog of reference Raman spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.1. Native elements, halides, oxides and suldes (Table 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2. Carbonates (Table 3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.3. Sulfates, phosphates, and borates (Tables 4 and 5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.4. Silicates (Tables 6 and 7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

Fluid inclusions (Fig. 1) represent the only rst-hand informationon uids in the Earth's interior (e.g., Roedder, 1984; Wilkinson,2001). They are acknowledged in an enormous range of lithologies(e.g., hydrothermal ore deposits, metamorphic rocks, igneous rocks,and geothermal systems), and pressure and temperature conditions.

Fluid inclusions are generally small closed volumes (i.e., b50 μ m in di-ameter; Fig. 1), in which pressure and temperature are interdepen-dent variables. Both are related by the equation of state of theenclosed uid, resulting in a nearly linear relation in the P –T space(isochore). Therefore, a key requirement for research and applica-tions is the ability to characterize uid composition and density.These two properties are usually obtained by petrographic and micro-thermometric methods (Poty et al., 1976).

Raman spectroscopy is the non-destructive technique which bet-ter characterizes liquid and gaseous compounds, solid phases, and

solute species in

uid inclusions. One of the main advantages is that

Journal of Geochemical Exploration 112 (2012) 1–20

⁎ Corresponding author. Tel.: +39 0577 233929; fax: +39 0577 233938.E-mail addresses: [email protected] (M.L. Frezzotti),

[email protected] (F. Tecce), [email protected] (A. Casagli).

0375-6742/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.gexplo.2011.09.009

Contents lists available at SciVerse ScienceDirect

Journal of Geochemical Exploration

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j g e o e x p


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it allows the chemical and structural characterization of samples assmall as 1 μ m in diameter, a resolution not possible by conventionalpetrography, microthermometry, and other spectroscopic methods(e.g., infrared spectroscopy). Raman spectroscopy has become a con-ventional method in uid inclusion research starting from the 70's(Burke and Lustenhouwer, 1987; Dhamelincourt et al., 1979; Dubessy

et al., 1982, 1989; Guilhaumou, 1982; Pasteris et al. 1986, 1988;Rosasco et al., 1975; Seitz et al., 1987). The continuing interest and

Fig. 1. Photomicrographs of uid inclusions: a) primary H2O uid inclusions aligned following chevron halite bands, evaporite from Vitravo diapir, Crotone, Italy. b) Primary H2Ouid inclusions in anhydrite from a geothermal well (2410 m depth), Sabatini Volcanic District, Italy. c) Plane of liquid-rich and vapor-rich H 2O uid inclusions in sanidine fromsyenite, Sabatini Volcanic District, Italy. d) H2O uid inclusion containing calcite and anhydrite daughter minerals (same provenance as in c). e) Tri-phase H2O–CO2 (L 1+L 2+G)uid inclusions from an Alpine quartz vein, Binn, Switzerland. f) CO2 uid inclusions in orthopyroxene, peridotite from Italy.

Fig. 2. Energy level scheme for elastic (Rayleigh) and inelastic (Raman) scattering atthe frequency of the light source (νl), and Raman and Rayleigh spectra. The molecularvibration of the analyzed sample is of frequency νm. Fig. 3. Schematic diagram of a Raman spectrometer.

2 M.L. Frezzotti et al. / Journal of Geochemical Exploration 112 (2012) 1– 20


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the importance of this technique is demonstrated by the number of publications and of review papers in this research eld (e.g., Burke,1994, 2001; Burruss, 2003; McMillan et al., 1996; Nasdala et al.,2004).

Present review gives an introduction to Raman spectroscopy forthe analysis of geological uids trapped as inclusions. Our approach

is instructional and we focus on selected examples from the literatureand from our laboratory experience, but only as far as concerning theroutine analysis. The theoretical and experimental treatment of thisspectroscopy is on a basic level, and more advanced approaches,such as high-pressure and/or temperature and cryoscopic Ramanmeasurements of uid inclusions are not discussed in detail. As arst step toward the use of Raman spectroscopy for the study of geo-logical uids, we provide a catalog of reference spectra for mainphases that can be present in uid inclusions.

2. Fundamentals

Raman spectroscopy is based on inelastic scattering of light by

matter in its solid, liquid, or gas state. Monochromatic light scatteredby matter contains radiations with frequencies different from theexciting light. This effect, predicted by Smekal (1923), was demon-strated by Raman (1928), and named after him. The discovery of anew optical scattering phenomenon won him the Nobel prize inphysics in 1930. In several liquids Raman observed scattered light,which had energy greater than the incoming light (Raman anti-Stokes, see below). The observation of an increase in energy con-vinced him that he was in presence of a new light-scattering effect,since energy decreasing light-scattering, such as uorescence, wasalready known at that time (Raman and Krishnan, 1928). Landshergand Mandelstam (1928a,b) also found this effect independently andalmost simultaneously in Moscow.

A straightforward way to explain the Raman scattering of light isby quantum mechanical model, which considers the interaction of photons with molecules in terms of energy-transfer mechanisms(cf., Colthup et al., 1975; Karr, 1975, and references therein). A mol-ecule has different vibrational energy levels, the ground state n = 0,and the excited states n =1, n =2, n =3 etc., which are separated

by a quantum of energy ΔE = hν m, where h is the Plank's constantand ν m is the frequency of the molecular vibration. The incident vis-ible light (λ=400–750 nm) with energy ν l induces transitions tovirtual vibrational energy levels in molecules. A virtual level is notan actual energy level of the molecule and it is generated whenlight photons interact with the molecule, raising its energy. This vir-tual level is unstable, and light is instantaneously released as scat-tered radiation.

Returning to theinitialstateoccurs by emitting light of frequencyν l, ν l−ν m, and ν l+ν m. The concept is illustrated in Fig. 2. TheRayleigh or elastic scattering occurs when the transition starts andnishes at the same vibrational energy level without loss of energy(i.e., no frequency change;ν l). Inelastic scattering (Raman effect) in-duces a change to lower (ν l−ν m) and higher (ν l+ν m) frequencies

in scattered light, which are known as Stokes and anti-Stokes lines,with ν m representing a fundamental rotational, vibrational or latticefrequency of the molecule. Rayleigh scattering can account for thewide majority of light scattered by molecules, being the Raman ef-fect extremely weak – in the order of some 10−6–10−8 of incidentphotons – and variable, as the intensity of the Raman scattering isproportional to the fourth power of the frequency of the incidentlight.

Raman spectroscopy is the measurement of the photons arisingfrom inelastic (Raman) scattering of light. A Raman spectrum is theplot of light intensity expressed as arbitrary units, or counts, versusthe frequency of scattered light (i.e., Raman vibrational modes) infrequency units (wavenumbers ˜ ν= νc =

1λ

in cm−1, where c is the

d Rutile

4000

180

Rt

Rt

I

-1

C

C

a b

c e CalciteDiamond

DmdCc

300 600 900 1200 1500 cm

Fig. 4. Raman spectral images of daughter mineral distribution in an aqueous uid inclusion. a) Optical microphotograph of analyzed uid inclusion in garnet from ultra-high pres-

sure metamorphic rocks, western Italian Alps, reporting the grid of single point measurements. b) Single point Raman spectrum showing the selected wavenumber intervals fordaughter mineral mapping [diamond (red), rutile (blue), and calcite (green)]. c, d, and e) Spectral images of diamond, rutile, and calcite distribution in the uid inclusion. Thecolor intensity of the mineral phases (from black to white) reects the increase in the intensity of the Raman band. The aqueous uid in the inclusion has no signicant Ramansignal in the investigated region, and thus does not interfere with the measurement; modied from Frezzotti et al. (2011).

3M.L. Frezzotti et al. / Journal of Geochemical Exploration 112 (2012) 1– 20


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velocity of light; Fig. 2). Typically, only Stokes Raman scattered fre-quencies are presented since they have the same energy but areabout 10 times more frequent than their anti-Stokes counterparts.The Rayleigh scattered frequency (i.e., light-source wavenumber)lies at 0 cm−1 and Raman frequencies are expressed as relative wave-numbers, or Raman shifts. On this scale, frequencies correspond to

the energy levels of different molecular vibrations and are indepen-dent from the wavelength of the light source: a mode at 464 cm−1

will occur whether the light source wavelength is 514.5 or 632.8 nm.

A spectrum comprises one or more bands which reect the vibra-tional energies of the molecules within the analyzed sample; thesein turn are related to the nature of the bonding. Main molecular vibra-tions include stretching and bending modes, stretching frequenciesbeing generally higher than bending frequencies. In order fora normalmode of vibration to be Raman active, it should produce a change in

the polarizability of the molecule. The“selectionrules” for Raman scat-tering depend on: 1) the creation of an induced dipole in the molecule(polarization); 2) the modication of the dipole by a molecular vibra-tion; 3) the successive scattering of a photon from the modied dipole(McMillan and Hess, 1988, and references therein). As a thumbnailrule, those molecules which are not easily polarized are poor Ramanscatterers. One example is H2O which has a strong dipole momentbut electrons are not easily polarized and Raman scattering is weak.

3. Methods of analysis

The basic instrumental set up requires a monochromatic lightsource, generally a laser, focused on a sample (solid, liquid, or gas-eous); the light is scattered, collected at a 90° or 180° angle, and ana-

lyzed by a detector (Fig. 3). The

rst dispersive Raman spectrometershad the sun or a mercury lamp as the exciting source, a prism mono-chromator as the light disperser, and a photographic lm as detector(Colthup et al., 1975; Kohlrausch, 1943). In modern commercial in-struments, polarized laser light sources in the UV, visible, and IR areused to excite molecular samples, because of the high intensity andnarrow bandwidth of wavelengths that are emitted (monochromatic-ity), and multi-channel charge-coupled devices (CCD) are generallyused as detectors. Their combination, together with notchholographiclters to eliminate the Rayleigh line, results in more intense Ramanscatter, with considerably reduced measuring time in obtaining high

Table 1

Main Raman vibrations (cm−1) of major gaseous species and of solutes in aqueousuids.

Gasses Main vibrations Ref.

COS 857 1SO2 s 1151 2

w 524CO2 Fermi doublet s 1285 3vs 1388

13CO2 w 1370O2 1555 1CO 2143 1N2 2331 4H2S 2611 1C3H8 2890 1CH4 vs 2917 5

w 3020C2H6 2954 1NH3 3336 1H2 vs 4156 6

w 4126w 4143w 4161w 1032w 586w 354

H2O vapor vs 3657–3756 7w 1595

Solutes Main vibrations Ref.

Si(OH)40 750–800 8, 9

Si2O(OH)60 590–680 8, 9

ClO4− vs 928 10

w 645w 460

SO42− vs 980 10

w 620w 450

NO3− vs 1049 10

w 690

w 1355HSO4

− vs 1050 11w 890

HCO3− vs 1017 12

m 1360CO3

2− vs 1064 12w 684m 1380

CO2 in solution vs 1384 12m 1276

HS− and H2S 2570–2590 11NH4

+ vs 3040 13sh 2870

B(OH)30 vs 877 14

w 495H2O liquid vs 2750–3900* 15 a,b

w 1630

vs = very strong; m = medium; w = weak; sh = shoulder; * Broad bands of severalhundreds of cm−1; 1 Burke, 2001; 2 Herzberg, 1945; 3 Rosso and Bodnar, 1995; 4Herzberg, 1950; 5 Brunsgaard-Hansen et al., 2002; 6 Dubessy et al., 1988; 7 Fraleyet al., 1969; 8 Zotov and Keppler, 2000; 9 Hunt et al., 2011; 10 Ross, 1972; 11Dubessy et al., 1992; 12 Davis and Oliver, 1972; 13 Schmidt and Watenphul, 2010;14 Schmidt et al., 2005; 15 a,b Walrafen, 1964, 1967. Ref. = References. Underlinedvibrations indicate most intense Raman modes.

Fig. 5. Raman spectra and relative wavenumbers of most common gaseousuid speciesin uid inclusions. Note that the hypothetical CO2 Raman band at 1340 cm

−1 is reallytwo bands at 1285 and 1388 cm−1, see text.



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signal to noise spectra (i.e., tens of seconds), and low detection limits.A detailed description of the different instrumental set up can befound in scientic Journals (e.g., Vibrational Spectroscopy, Elsevier;and Journal of Raman Spectroscopy, Wiley) and in the web at the

pages of single manufacturing companies.Fluid inclusion analysis is based on the samefundamental principle:

the laser excites the molecules to generate scattering. Raman

microspectrometers are the common analytical set up, where the exci-tation of the sample and collection of the scattered light at a 180° angle(backscattering) are achieved using a ordinary optical microscope fo-cused within singleuid inclusions by means of high-magnication ob-

jectives (50× or 100×). Instruments offer perfect visualization of thesubsurface of samples and of the laser spot, which makes easy the

choice of the appropriate inclusion to be analyzed. The volume of theanalyzed sample (spot size) depends mostly on the numerical aperture(N.A.) of the objective, and on the excitation wavelength. As an exam-ple, fora 514.5 nm excitationsourceand a 100× magnicationobjectivewith N.A.=0.9, the spot size is 1 ×1 ×5 μ m3.

Thick double-polished sections are easily studied and require nospecial preparation. Fluid inclusions can be studied down to 1 μ m di-ameter in situ, where microstructures are preserved and the differentpopulations of uid inclusions can be discriminated. This is possiblebecause of the confocal arrangement of the optical pathway which al-lows a good spatial resolution perpendicular to the optical axis, aswell as along the optical axis of the microscope (depth) (see, Nasdalaet al., 1996, 2004). However, the depth resolution degrades with in-creasing optical penetration depth, therefore it is better to analyze

uid inclusions not deeper than 30 μ m within a sample.The choice of laser wavelength inuences the performance of thespectrometer. The characteristics of each laser are different, so thatno laser may be ideal for every uid inclusion analysis. In general,the optical power of the laser line and the ef ciency of Raman CCDdetectors tend to increase with decreasing wavelength. However,the cost of the laser, the likelihood of uorescence (see below), andthe risk of sample heating increase as well. The most popular choicesare: (1) the green light Ar ion (λ=514.5 nm) water- or air-cooled;(2) the blue light Ar ion (λ=488 nm) air-cooled; and (3) the redlight He\Ne (λ=632.8 nm).

Raman microspectrometers can be equipped with a programma-ble x– y microscope stage which allows sample areas to be mappedin the same way as with EDS and WDS microprobes. Single spot spec-tra are collected by multiple steps within a grid pattern, as illustratedin Fig. 4a. Each analyzed point contains the information of a wholespectrum (Fig. 4b). Generated Raman maps are chemical or structuralimages where integrated areas of single bands or band ratios, charac-teristics for the presence of a certain chemical species in a compositesample, are illustrated (Figs. 4c, d, e). The x– y resolution in a map de-pends on the distance between the single measuring points, while thedepth resolution along z is determined by the confocal instrumentsettings (see above). The best resolution is achieved by setting thedistance between two measuring points smaller than the laser spotsize (“oversampling”). By increasing the distance between twospots, the spatial resolution decreases, but larger areas can be ana-lyzed in a shorter time. Spatially resolved Raman spectra can beused to identify the distribution of uid or mineral species within sin-gle uid inclusions (Frezzotti et al., 2011; Korsakov et al., 2011).

Fluorescence and the presence of overlapping bands from hostmineral are possible competing effects during analysis, since theyoften overpower and conceal the weak Raman features from theuid inclusions. Fluorescence generally appears as a very broad back-ground, often much more intense than the Raman scattering. This ef-fect may commonly arise from epoxy used to embed or polish therock sections and can be easily eliminated using non-uorescent ep-oxies and/or cleaning the sample. However, uorescence can also beemitted by uids contained in inclusions (e.g., hydrocarbons) or bythe surrounding host mineral (e.g., Fe-bearing minerals). These lastcases are much more dif cult to cope with. Increasing the wavelengthof the light source is a way of overcoming uorescence: red or near-infrared lower lasers (λ=630–1060 nm) should not, in principle,give rise to uorescence (Carey, 1999). Another practical method to

mitigate a uorescent background consists in repeating spectral accu-mulations for several times in order to bleach out this effect by pro-tracted exposure to laser light (photo-bleaching).

Fig. 6. Raman spectroscopy applied to CO2 density measurement. a) Main spectral fea-tures of CO2 uids, which consist of the two bands of the Fermi doublet, bounded bythe hot bands. The distance between the Fermi doublet (Δ) depends on uid density.

b) Superdense CO2 uid inclusions (d N1.178 g/cm3

) spectral features, including: i) in-creased Δ (≥106 cm−1), ii) shifting of bands to lower wavenumbers, iii) increasedband intensity ratio, iv) broadened band bases, and v) attened hot bands (van denKerkhof and Olsen, 1990); analyzed uid inclusions are in pyroxenes from peridotitexenoliths, Hawaii; modied from Frezzotti and Peccerillo (2007). c) CO2 density as afunction of Δ (cm−1), as derived from the equations of: 1) Rosso and Bodnar (1995),2) Kawakami et al. (2003), 3) Yamamoto and Kagi (2006), 4) Song et al. (2009), 5)Fall et al. (2011), and 6) Wang et al. (2011). The inset shows that the maximum differ-ence in CO2 densities derived from the different equations is about 0.1 g/cm

3; redrawnand modied from Wang et al. (2011).



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Interpretation of spectra of crystalline phases is often complicated,due to the fact that Raman scattering intensity depends upon latticeorientation. Consequently, variations of band intensity ratios shouldbe taken into account in the analysis of most minerals. Knowledgeof the orientation of main crystallographic axes, and/or repetition of analysis after 90° rotation to get random orientations is helpful inmineral identication (Nasdala et al., 2004). In addition, due to latticegeometries, some minerals are very weak Raman scatterers. Unfortu-nately, among these there are major chloride species (e.g., NaCl, KCl,and CaCl2), which represent relevant constituents of aqueous uidinclusions.

The intensity of the Raman scattering can vary by many order of magnitudes depending on the nature of the molecules. Detectionlimits for single components within a single uid inclusion dependon several contributing factors, including uid inclusions size andgeometry (i.e., number of molecules of the analyzed constituent),nature of the other constituents in uid inclusions, and analytical

conditions (e.g., intensity of the laser light, depth of the inclusionin the analyzed sample, etc.). Several approaches can be used, andthey will be discussed in the following sections.

4. Gaseous uids

A custom application of Raman spectroscopy to uid inclusionanalysis is the qualitative identication of major gaseous uid compo-nents. The characterizing Raman bands for most important geologicaluids are reported in Table 1 and Fig. 5. Most gasses show a singlesymmetric stretching strong band, whose wavenumber is traditional-ly reported at ambient P –T conditions, since a progressive slightwavenumber downshift is known to occur with increasing uid den-sity (Burke, 2001; van den Kerkhof, 1988b).

Early work on uid inclusions allowed to recognize CO2, CH4, andN2 as relevant geological uids (e.g., Dubessy et al., 1989; Frezzottiet al., 1992; Touret, 2001; van den Kerkhof, 1988a,b, 1990). H2S,

COS, SO2, CO, H2, NH3 and O2 have also been detected in appre-ciable amounts in some uids (Bény et al., 1982; Ferrando et al.,2010; Frezzotti et al., 2002; Giuliani et al., 2003; Grishina et al.,1992; Peretti et al., 1992; Siemann and Ellendorff, 2001; Tsunogaeand Dubessy, 2009). Identication of hydrocarbons heavier thanCH4 is also possible (e.g., Guilhaumou, 1982; Hrstka et al. 2011;Makhoukhi et al., 2003; Munz, 2001; Orange et al., 1996; Pironon,1993; Pironon and Barrès, 1990; Potter et al., 2004; Rossetti andTecce, 2008; Schubert et al., 2007; Weseł ucha-Birczyńska et al.,2010), although uorescence often does not allow conventional anal-ysis (see e.g., Pironon et al., 1998).

4.1. CO 2 uids

The Raman spectrum of molecular CO2 shows two strong bands at1285 and 1388 cm−1, and two symmetrical weak bands below 1285

and above 1388 cm−1, the so-called hot bands (Colthup et al., 1975;Dhamelincourt et al., 1979; Dubessy et al., 1999; Rosasco et al.,1975; Rosso and Bodnar, 1995; van den Kerkhof and Olsen, 1990).The two sharp bands appear because of a resonance effect, proposedby Fermi (1931) in order to explain the doublet structure in theregion of CO2 symmetric stretching vibration. A small peak at1370 cm−1 is the 13CO2.

Fig. 6a and b shows examples of spectra of CO2 uid inclusionshaving different densities. The distance between the Fermi doublet(Δ, in cm−1) is proportional to uid density (Garrabos et al., 1980;van den Kerkhof, 1988b; Wang and Wright, 1973). Several equations(e.g., Fall et al., 2011; Kawakami et al., 2003; Rosso and Bodnar, 1995;Song et al., 2009; Wang et al., 2011; Yamamoto and Kagi, 2006 ) havebeen proposed to calculate the density (d) of pure CO2 uid inclusions

based on the distance between the Fermi doublet Δ (Fig. 6c). CO2density can be determined in the range from 0.1 to 1.24 g/cm 3 withan accuracy better than 5% (Wang et al., 2011).

Fig. 7. Quantitative Raman analysis of H2 and CH4 contained in the gas bubble of an aqueous uid inclusion in vesuvianite from rodingites, western Italian Alps (Ferrando et al.,2010). Relative mole% of H2 and CH4 in the gas bubble is calculated with Eq. (1) based on band area integration, and considering the relative Raman cross sections (σ ) and the in-strumental ef ciency (ζ) at the wavenumbers of H2 and CH4. (σ of CH4 is 3.5 times higher than of H2; Burke, 2001).



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We observe a very good agreement between density data derived

from Raman spectroscopy and from microthermometry, also for CO2uids containing minor amounts of other gaseous species (i.e.,b5 mol% CH4 or N2; Frezzotti and Peccerillo, 2007). These twomethods are complementary for the characterization of uid inclu-sioncomposition and densities. Although the precision of microther-mometricmeasurements is higher, the Raman densimeter permitstoanalyze very small uid inclusions (b5 μ m in diameter), and/or lowdensity uids.

The relative intensities of the 13CO2 and the associated 12CO2

band (Fig. 6a) have been used to calculate the carbon isotope ratiosin single uid inclusions. The development of Raman as a mass-spectroscopy, however, is still at a very early stage of development;reported δ13C determinations have uncertainties ≥20‰ (Arakawaet al., 2007; Dhamelincourt et al., 1979), andconsent only to discrim-

inate between inorganic and organic CO2 at best. This is due to thedif culty in controlling all parameters inuencing intensity of scattering, probably including a dependence of 13CO2 and

12CO2

band intensities on uid density. Nevertheless, Raman mass-spec-troscopy remains a particularly attractive prospective since it couldpermit to analyze samples several order of magnitude smaller thangenerally used by mass-spectrometry.

4.2. Gaseous mixtures

When uid inclusions consist of mixtures of two or more gas spe-cies, the relative molar fractions of the end-members can be calculat-ed. The prerequisite to quantitative Raman analysis is the knowledgeof two essential parameters (cf., Burke, 2001): (1) the Raman scatter-ing cross-section, which indicates the activity of a certain gas compo-nent in a mixture (Schrötter and Klöckner, 1979); and (2) thevariation of the instrumental ef ciency at the different wavenumbersfor a specic excitation wavelength. The rst parameter is dependenton the laser excitation wavelength. A list of major gas species cross-sections for the 632.8 nm red light (e.g., He\Ne laser source), the514.5 nm green light (Ar-ion laser source), and the 488 nm bluelight (Ar-ion laser source) is reported in Burke (2001). The second pa-rameter requires an empirical calibration for each Raman microspect-

rometer, by measuring synthetic or natural gas-mixture standards of known composition and density (Beeskow et al., 2005; Chou et al.,1990; van den Kerkhof, 1988b).

The molar fraction ( X ) of end-member components in a gas mix-ture can be obtained using the following equation (Beeskow et al.,2005; Burke, 2001; Dubessy et al., 1989; Morizet et al., 2009; Nasdalaet al., 2004; Wopenka and Pasteris, 1986, 1987):

X a ¼

Aaσ a ζa

∑ Aiσ i ζi

ð1Þ

where X a, Aa,σ a and ζ a, are the molar fraction, the band area, theRaman cross-section and the instrumental ef ciency for gas a, respec-tively, whileΣ Ai,σ i, and ζ i represents the sum of values for all gas spe-cies in the uid inclusion. In order to get reliable quantitativeanalyses, no change in the analytical conditions should be made dur-ing measurements (i.e., laser intensity, focus, number of accumula-tions, and accumulation time). Accuracy of analyses is reportedbetter than 5% (Pasteris et al., 1988; van den Kerkhof, 1988b). Notethat when CO2 uids are involved, the sum of the two bands formingthe Fermi doublet should be used (Dubessy et al., 1989).

In Fig. 7 is reported for example an aqueous uid inclusion con-tained in vesuvianite from vein in rodingite from Bellecombe, ItalianWestern Alps (Ferrando et al., 2010). In the gas bubble, bands of CH4and H2 have been obtained using an Ar-ion laser (λ=514.5 nm) asthe excitation source. The integrated measurements of the single

gas Raman band area ( A) are reported along with the relativecross-sections (σ ) of H2 and CH4 and the instrumental ef ciency(ζ ) of the Raman spectrometer at 2917 and at 4156 cm−1. UsingEq. (1), the resulting composition of the gas phase in the Alpine in-clusion is equal to 82 mol% H2 and 18 mol% CH4.

In more complex gaseous–aqueous uid mixtures, the quantita-tive analysis of the different components is much more dif cultand often requires measurements at high temperatures. Empiricalequations for (semi)quantitative analyses of H2O-CH4±NaCl andH2O-CO2±NaCl systems have been proposed based on relative bandareas in spectra (e.g., Azbej et al., 2007; Guillaume et al., 2003; Luet al., 2007). In these complex uid mixtures, analysis should includedetection of gasses dissolved in water (e.g., CO2 or CH4), and thecharacterization of clathrate hydrates (ice-like compounds formed

from CO2, CH4, or N2 and water under low-T and high-P conditions;Azbej et al., 2007; Dubessy et al., 2001; Fall et al., 2011; Orangeet al., 1996; Pironon et al., 1991) .

Fig. 8. Raman spectra of water contained in uid inclusions, presenting examples for:a) low-salinity (b1 NaCl wt.%) liquid water, b) high-salinity (20 NaCl wt.%) liquidwater, and c) optically-hidden water in a CO2-rich uid inclusion, peridotite fromEthiopia.



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5. Aqueous uids

The Raman modes of water consist of two main O\H stretchingmodes at 3657 and 3756 cm−1 and one very weak H\O\H bendingmode at 1595 cm−1 (Carey and Korenowski, 1998; Fraley et al.,1969). However, the Raman spectrum of liquid H2O consists of sev-eral large overlapping bands in the OH stretching region from 2750to 3900 cm−1 (Fig. 8a and b), and of a weak bending mode at~1630 cm−1 (Walrafen, 1964, 1967). Reduced to minimum terms,such spectral complexity results from the strong interactions of asingle water molecule with the neighboring molecules, formingintermolecular O\H\O bridging networks (Hare and Sorensen

1992; Sun, 2009).The characteristics of the Raman spectrum of water have been

used to prove the presence of H2O in small CO2 uid inclusions(b5–10 μ m in size; Frezzotti and Peccerillo, 2007; Frezzotti et al.,2010; Hidas et al., 2010). Here, a water lm of a thickness of 0.2 μ mwrapping the CO2 uid cannot be identied with optical techniques,although it may correspond to as much as 10–20 mol% of H2O. A de-tailed description of the method can be found in Dubessy et al.(1992) and in McMillan et al. (1996). One example is illustrated inFig. 8c from CO2 uid inclusions in peridotites from Ethiopia. Thedominant spectral features of optically unnoticed water are the vibra-tional bands at 3658 and 3750 cm−1 characteristic of OH− stretchingvibrations for isolated molecules of H2O (i.e., lack of signicant Hintermolecular bonding).

Raman spectroscopy allows determination of the appropriatewater content of melt inclusion glass in minerals of granites and peg-matites (e.g., Behrens et al., 2006; Chabiron et al., 2004; Di Muro et al.,

2006; Severs et al. 2006; Thomas, 2000; Thomas and Davidson, 2006;Thomas et al., 2008b; Zajacz et al., 2005). Note that, during cooling of a natural water-rich melt inclusions, often SiO2 is deposited on the in-clusion wall and makes an apparent aqueous uid inclusion fromwhat was primary a melt inclusion (Thomas et al. 2011a).

5.1. Analyses of solutes: monoatomic ions

Qualitative and (semi)quantitative Raman analysis of water-richuid inclusions typically focuses on determination of solutes.Monoatomic charged cations, such as Na+, K+, Ca2+, and Mg2+

have too weak Raman spectra to be analyzed in uid inclusions. A

way to obtain spectra is by nucleation of salt-hydrates at low tem-peratures, but this requires the combination of the Raman micro-spectrometer with a uid inclusion cooling stage. Spectra arereported for all major salt-hydrates, such as NaCl·2H2O, FeCl3·6H2O,CaCl2·6H2O, MgCl2·12H2O, KCl·MgCl2·6H2O, FeCl2·6H2O, LiCl·5H2O(Bakker, 2004; Baumgartner and Bakker, 2009, 2010; Derome et al.,2007; Dubessy et al., 1982, 1992; Samson and Walker, 2000; Schiffries,1990).

Chlorine ions have the power of breaking certain hydrogen bondsin aqueous solutions. The variation of OH stretching bands induced bydifferent Cl concentrations in aqueous uid inclusions (Fig. 8a and b)has been intensively investigated with different approaches. Semi-quantitative estimation of the salt content in aqueous uid inclusionsrequires development of a specic calibration for each spectrometer

and it is complementary to measurements of phase transitions atlow temperatures by microthermometry (e.g., eutectic and nal melt-ing temperatures).

Fig. 9. Raman spectroscopy applied to solute analysis in aqueous uids. a) Band of SO42− ions in a uid inclusion in feldspar from syenite, Sabatini volcanic district, Italy. b) Bands of

native sulfur in a uid inclusion in orthopyroxene from peridotite, Italy. c) Bands of CO32− and HCO3

− ions in a uid inclusion from ultra-high pressure metamorphic rocks, westernItalian Alps. d) Bands of Si(OH)4

0, and deprotonated H4-nSiO4n− monomers in a uid inclusion from metamorphic rocks from western Italian Alps. The Raman modes of anhydrite

(Anh),quartz (Qtz), and Mg-calcite (MgCc)daughter minerals are also shown. Raman bands ofhost minerals are marked with asterisks. c and d: modied from Frezzotti et al., 2011.



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Mernagh and Wilde (1989) proposed a formula to calculate NaClwt.%, with a relative error of 15%:

NaClwt:% ¼ α 2 Y − X ð Þ X þ Y

2− X =Y I 3400cm−1ð ÞI 3200cm−1ð Þ

0B@1CA−β ð2Þ

where X is equal to the integral of the OH− band from 2800 to3300 cm−1, Y is equal to the integral from 3300 to 3800 cm−1, I is theintensity at the specied wavenumbers, and α and β are regressionparameter specic for each spectrometer (cf., McMillan et al., 1996).

The idea behind Eq. (2) was to link theshape of the two halves formingtheOH stretching band to theamount of Cl− in solution. More recently,calibration curves were expanded also to LiCl, KCl, MgCl2, CaCl2, and to

Table 2

Main Raman vibrations (cm−1) of selected native elements, halides, suldes, oxides and hydroxides.

Native elements, halides and suldes Main vibrations Ref.

DiamondC

1332 [1]

Graphite

C

1355 1580 2

SulfurS 8

mw 157 m 220 s 462 3w 187 w 246 w 437

Arsenic As

mw 220 4w 225vs 253

HaliteNaCl

358 [1]

SylviteKCl

vw 291 [1]vw 213

FluoriteCaF 2

m 322 vw 641 [1]

CryoliteNa 3(AlF 6 )

vw 485 m 555 mw 620 5

ElpasoliteK 2NaAlF 6

135 326 559 1009 6387

PyriteFeS 2

w 342 vs 428 7s 377

MarcasiteFeS 2

vs 324 7s 387

ChalcopyriteCuFeS 2

vs 293 w 322 7w 352w 378

CovelliteCuS

vw 263 vs 471 7

Blende ZnS

w 218 w 300 w 419 w 639 [1]w 274 w 310 w 669

vs 349Galena

PbS

vs 136 m 270 [1]

Oxides and hydroxides Main vibrations Ref.

RutileTiO 2

w 139 m 238 vs 444 vs 609 w 920 [1]m 696

AnataseTiO 2

vs 143 w 395 w 514 mw 638 8vw 195

BrookiteTiO 2

s 127 s 247 s 318 w 412 mw 645 9vs 150 w 366

SpinelMgAl 2O4

w 313 vs 408 mw 666 w 76810

MagnetiteFe 2+Fe 2

3+O4

w 193 w 306 s 538 vs 668 11

Hematite s 223 vs 409 m 609 vs 1313 12Fe 2O 3 vs 290 w 498Ilmenite

FeTiO 3

w 232 mw 373 vs 685 13

Gibbsite Al(OH) 3

mw 242 m 322 vs 538 w 979 14m 255 vs 380 vs 569

Diaspore AlO(OH)

w 331 vs 448 3

Corundum

Al 2O 3

mw 378 vs 417 m 644 w 750 15

Goethiteα-FeO(OH)

w 242 vs 389 m 547 mw 681 12mw 299

vs = very strong; s = strong; m = medium;mw = medium weak; w = weak;vw = very weak; [1] = Raman SpectraDatabase,Siena Geouids Lab (http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm); 2 Wopenkaand Pasteris, 1993; 3 Giulianiet al.,2003; 4 Thomas and Davidson, 2010; 5 Nazmutdinov et al.,2010; 6 R.Thomas, pers.comm.;7 MernaghandTrudu,1993; 8 Clark et al.,2007; 9 Yanqing et al.,2000; 10 Slotznick andShim, 2008; 11 Shebanova andLazor,2003; 12 Kuebleret al.,2006; 13 Rullet al., 2007; 14 Ruanet al.,2001; 15 Xuet al., 1995; Ref. = References.



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Table 3

Main Raman vibrations (cm−1) of selected carbonates.

Carbonates CO32− vibrations Ref.

T ν4 ν2 ν1 ν3

CalciteCaCO 3

s 284 mw 711 vs 1085 vw 1435 [1]mw 156

AragoniteCaCO 3

s 154 w 704 vw 854 vs 1085 vw 1463 [1]mw 206

VateriteCaCO 3

m 301 w 740 vs 1090 vw 1465 2sh 118 w 750 s 1074

Mg-Calcite(Ca,Mg)CO 3

s 281 mw 714 vs 1087 vw 1438 [1]mw 155

MagnesiteMgCO 3

s 329 w 738 vs 1094 w 1444 3mw 212

DolomiteCaMg(CO 3) 2

s 299 w 725 vs 1097 vw 1443 [1]ms 176

NatriteNa 2CO 3

w 698 vs 1078 w 1428 4

K-CarbonateK 2CO 3

s 141 m 697 vs 1064 m 1405 5m 192 sh 1043

ZabuyeliteLi 2CO 3

mw 96 w 712 vs 1091 w 1459 4

Siderite

(Fe,Mg)CO 3

m 301 sh 738 vs 1090 vw 1442 [1]

w 194Rhodochrosite

MnCO 3

s 289 mw 718 vs 1087 vw 1416 [1]mw 185

StrontianiteSrCO 3

mw 149 w 700 vs 1073 vw 1450 [1]mw 183 vw 1057sh 250

WitheriteBaCO 3

s 136 w 692 vs 1059 w 1420 [1]m 152w 227

CerussitePbCO 3

s 150 m 682 m-w 839 vs 1056 s 1378 [1]mw 180sh 215

Smithsonite ZnCO 3

m 303 w 731 vs 1093 mw 1408 [1]mw 196

NahcoliteNaHCO 3

mw 688 vs 1048 w 1432 [1]m 1271

KaliciniteKHCO 3

w 635 vs 1028 4w 673 mw 1277

Hydrated carbonates CO32− vibrations OH− Ref.

T ν4 ν1 ν3

MalachiteCu 2(OH) 2(CO 3)

vs 154 w 721 sh 1098 vs 1492 vs 3468 [1];vs 178 mw 3386 6vs 434ms 272ms 537

AzuriteCu 3(OH) 2(CO 3) 2

s 397 vs 1095 vw 1457 vs 3453 [1];m 246 sh 937 6mw 170mw 279

ArtiniteMg 2(OH) 2(CO 3)· 3H 2O

s 147 w 704 vs 1094 vs 3593 7s 173 s 3229 8w 472 s 3030

Hydromagnesite

Mg 5(CO 3)4(OH) 2·4H 2O

m 184 vs 1119 sh 1487 n.a. 7

m 202m 232

DypingiteMg 5[(OH)(CO 3) 2] 2·5H 2O

mw 203 w 727 vs 1122 mw 1447 vs 3648 8mw 249 mw 1092 m 3421w 311 mw 3515w 434

DawsoniteNaAl(CO 3)(OH) 2

ms 189 vs 1091 mw 1505 vs 3282 9m 260 w 1065 m 3250mw 587

ThermonatriteNa 2(CO 3)·H 2O

s 156 vs 1062 sh 1432 n.a. [1]m 185w 230

TronaNa 3H(CO 3) 2· 2H 2O

mw 140 vs 1060 w 1430 n.a. [1]mw 185w 225

GaylussiteNa 2Ca(CO 3) 2·5H 2O

s 164 w 723 vs 1071 vs 2944 [1];sh 265 s 3334 10

ν1=Symmetric stretching vibration; ν2 = Out-of-plane bending vibration; ν3 = Antisymmetric stretching vibration; ν4 = In-plane bending vibration; T = Translational latticemodes; OH−= OH stretching vibrations; vs = very strong; s = strong; ms = medium strong; m = medium; mw = medium weak; w = weak; vw = very weak; sh = shoulder;[1] = Raman Spectra Database, Siena Geouids Lab (http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm). 2 Carteret et al., 2009; 3 Gillet, 1993; 4 Thomas et al., 2011a,b;5 Koura et al., 1996; 6 Frost et al., 2002; 7 Edwards et al., 2005; 8 Frost et al., 2008; 9 Frost and Bouzaid, 2007; 10 Frost and Dickfos, 2007; n.a. = not analyzed; Ref. = References.



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Table 4

Main Raman vibrations (cm−1) of selected sulfates.

Sulfates SO42− vibrations Ref.

ν2 ν4 ν1 ν3

AnhydriteCaSO4

mw 430 w 611 vs 1018 mw 1131 2mw 500 w 629

w 676Mg-sulfate

MgSO4

ms 451 s 608 vs 1023 ms 1136 3ms 475 vw 681 s 1053 w 1220ms 499 vw 697 vw 1256

ThenarditeNa 2SO4

w 452 w 621 vs 994 w 1103 [1]w 469 w 632 w 1153

GlauberiteNa 2Ca(SO4) 2

w 485 m 618 vs 1002 m 1106 [1]m 644 m 1140

BurkeiteNa6 (CO 3)(SO4) 2

mw 451 mw 620 vs 994 4w 474 mw 633 m 1065*

mw 644Sulfohalite

Na6 (F,Cl)(SO4) 2

m 471 m 634 vs 1002 m 1125 [1]

ArcaniteK 2SO4

mw 457 mw 622 vs 983 w 1093 5w 1109w 1145

Aphthitalite

(K, Na) 3Na(SO4) 2

m 457 s 619 vs 984 m 1104 [1]

mw 447 mw 1093Celestine

SrSO4

m 452 ms 656 vs 1000 ms 1156 [1]vw 627 mw 1190

BariteBaSO4

s 461 w 617 vs 988 w 1143 [1]

AnglesitePbSO4

mw 438 w 608 vs 978 w 1160 [1]mw 450 vw 641 vw 1068

Hydrated sulfates SO42− vibrations OH− Ref.

ν2 ν4 ν1 ν3

GypsumCaSO4· 2H 2O

s 494 w 621 vs 1008 w 1142 vs 3405 [1];m 414 mw 3491 6

EpsomiteMgSO4·7H 2O

mw 447 vw 612 vs 984 vw 1061 vs 3303 3vw 1095 s 3425vw 1134

ExahydriteMgSO4·6H 2O

w 445 vw 610 vs 984 w 1146 vs 3428 3w 466 vw 1085 m 3258

PentahydriteMgSO4·5H 2O

m 447 vw 602 vs 1005 vw 1106 vs 3391 3vw 371 vw 1159 vs 3343

m 3553m 3494m 3289

StarkeyiteMgSO4·4H 2O

vw 401 vw 565 vs 1000 w 1156 vs 3427 3vw 462 vw 616 vw 1086 s 3481

vw 664 vw 1116 m 3558vw 1186 m 3331

SanderiteMgSO4· 2H 2O

m 447 w 597 vs 1034 m 1164 vs 3446 3w 492 w 630 m 3539

KieseriteMgSO4·H 2O

m 436 m 629 vs 1046 mw 1117 vs 3297 3w 481 w 1215

K-AlumKAl(SO4) 2·12H 2O

mw 455 mw 614 vs 989 mw 1130 vs 3396 7w 442 s 974 w 1104 m 3072

Alunite

KAl 3[(OH) 3(SO4)] 2

mw 509 mw 654 vs 1026 mw 1190 vs 3509 [1];

w 485 w 1079 vs 3482 8Syngenite

K 2Ca(SO4) 2·H 2Omw 474 w 642 vs 983 w 1142 vs 3301 [1];w 494 w 662 s 1007 w 1168 s 3378 9

GörgeyiteK 2Ca5(SO4)6 ·H 2O

m 480 m 631 vs 1013 w 1108 vs 3525 10w 433 w 595 vs 1005 w 1115 m 3580w 440 w 602 w 1085 w 1162w 457 w 654

MirabiliteNa 2SO4·10H 20

m 458 mw 627 vs 989 w 1129 vs 3506 11m 3340

CesaniteNa 3Ca 2(OH)(SO4) 3

vs 448 vs 626 vs 1004 sh 1104 n.a. [1]mw 474 mw 647

ν1 = Symmetric stretching vibration; ν2 = Out-of-plane bending vibration; ν3 = Anti-symmetric stretching vibration; ν4 = In-plane bending vibration; * = Symmetric stretchingvibration of CO3 group. Peak intensities as in Table 3. [1] = Raman Spectra Database, Siena Geouids Lab (http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm);2 Thompson et al., 2005; 3 Wang et al., 2006; 4 Korsakov et al., 2009; 5 Montero and Schmolz, 1974; 6 Kloprogge and Frost, 2000; 7 Barashkov et al., 2004; 8 Frost et al., 2006;9 Kloprogge et al., 2002; 10 Kloprogge et al., 2004; 11 Hamilton and Menzies, 2010; n.a. = not analyzed; Ref. = References.



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other more complex salt systems (Dubessy et al., 2002; Sun et al.,2010). The methods described above are all similar, they only differin the selected bands of water in the OH-stretching region taken asstandards.

5.2. Analyses of solutes: polyatomic ions and molecules

Polyatomic charged anions have Raman spectra characterizedby the presence of one or more bands (Table 1). Band area and

intensity, although proportional to the solute concentration, cannotbe linearly transformed into absolute concentrations, since these areconsiderably inuenced also by measurement conditions (e.g., laserpower, optical arrangement, etc.; McMillan et al., 1996; Nasdalaet al., 2004). Semi-quantitative analysis of polyatomic solutes inuid inclusions has been in some cases possible based on relative

band-intensity ratios, using selected bands of water as standard.The application of intensity ratios eliminates the inuence of mea-surement conditions. Note that during analyses high laser power

Table 5

Main Raman vibrations (cm−1) of selected phosphates and borates.

Phosphates PO43− vibrations Ref.

ν2 ν4 ν1 ν3

ApatiteCa5(PO4) 3(OH,F,Cl)

w 428 w 578 vs 960 w 1026 [1]w 446 w 588 w 1040

FluorapatiteCa5(PO4) 3F

mw 432 m 592 vs 965 m 1053 2w 449 w 608 mw 1081

mw 581 w 1042Chlorapatite

Ca5(PO4) 3Cl

w 430 w 581 vs 963 mw 1039 3w 1127

HerderiteCaBePO4 (F,OH)

584 983 1005 4595

Triplite(Mn,Fe,Mg,Ca) 2(PO4)(F,OH)

425 610 980 1034 4;5

Berlinite AlPO4

437 1111 1229 4;461 5

AmblygoniteLiAl(PO4)F

601 1011 4644

LacroixiteNaAl(PO4)F

609 1001 4623

Na-phosphateNa 3PO4

391 524 910 5482 544 942

993Lazulite

(Mg,Fe)Al 2(PO4) 2(OH) 2

344 611 1059 1100 5630 1136741

Xenotime(Y,Yb)PO4

485 642 998 1394 51056

Monazite(La,Ce,Nd,Th)PO4

m 466 m 620 vs 987 mw 1054 6

Borates Main vibrations Ref.

Metaboric acidHBO 2 (monoclinic)

428 518 782 5475 533

Metaboric acidHBO 2 (orthorhombic)

401 595 809 5415475

Li-metaborate

LiBO 2

713 1419 4

SassoliteH 3BO 3

w 500 vs 880 7

HambergiteBe 2BO 3(OH,F)

vs 153 w 992 7

Na-tetraborateNa 2B4O7 ·10H 2O

385 461 576 756 852 948 1036 5

Li-tetraborateLi 2B4O7 ·5H 2O

391 446 543 772 845 1028 5493 896 1097

1352Borax

Na 2B4O5(OH)4·8H 2O344 405 571 776 943 5

463 997Ca–Mg-hexaborates

CaB6 O10, MgB6 O10 with 4 to 7.5 H 2O

634 852 953 4638 855 964641 861

HydroboraciteCaMgB6 O11·6H 2O

181 322 524 606 753 837 5257 383 548 876

398 564Cs-RamaniteCsB5O8·4H 2O

m 98 vs 548 m 768 m 907 8mw 293

Rb-RamaniteRbB5O8·4H 2O

mw 101 w 508 w 765 w 914 8vs 554

ν1 = Symmetric stretching vibration; ν2 = In-plane bending vibration; ν3 = Antisymmetric stretching vibration; ν4 = Out-of-plane bending vibration. Peak intensities as inTable 3. [1] = Raman Spectra Database, Siena Geouids Lab (http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm); 2 Penel et al., 1997; 3 Kuebler et al., 2006; 4 Rickerset al., 2006; 5 R.Thomas, pers. comm.; 6 Silva et al., 2006; 7 Thomas and Davidson, 2010; 8 Thomas et al., 2008a,b; Ref. = References.



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could result in heating the inclusion uid with consequent possiblechanges in the speciation of ions.

The study of the speciation of sulfur in aqueous solution to deter-mine the redox potential (H2S/SO4

2−) and pH range (SO42−/HSO4

−;HS−/H2S) of geological uids represents one of the rst applicationsof Raman microspectroscopy to uid inclusion research (Boiron et al.1999; Dubessy et al., 1983, 1992, 2002; Rosasco and Roedder, 1979).Sulfate ions give rise to a main S\O stretching band at ~980 cm−1

(Fig. 9a) and to two additional weak bands around 620, and450 cm−1 (Table 1; Ross, 1972; Schmidt, 2009). Only the 980 cm−1

band is generally strong enough to be observed in uid inclusions,and has very low detection limits (0.01–0.05 mol/kg; Dubessy et al.1982, 1983; Rosasco and Roedder, 1979). Bisulfate ions (HSO4

−) canbe identied by their main S\O and S\OH stretching modes at

~1050 and 890 cm−1, respectively (Table 1). Hydrogen sulde (H2S0and HS−) is characterized by S\H stretching modes in the 2570–2590 cm−1 range.

The carbonate ion CO32− fundamental stretching mode is expectedat 1064 cm−1. Other less intense bands at ~1380, and 684 cm−1 maybe observed in concentrated solutions. HCO3

− has a very strong C\OHstretching mode at ~1017 cm−1, and a less intense C\O stretchingmode at ~1360 cm−1 (Table 1). Raman studies of carbonates andbicarbonates in solution were initiated by Davis and Oliver (1972)and Dubessy et al. (1982), although these ions were not detected inuid inclusions at that time. Absence was attributed mainly totheir low Raman scattering compared, for example, to that of sulfateions, and to their relatively low solubility in geological uids(cf., Burke,2001; Dubessy et al., 1982; McMillan et al., 1996). More recently,there has been increasing Raman evidence for signicant HCO3

−

(aq)

and CO32−

(aq) in uid inclusions (Fig. 9c) mainly from pegmatites,ore deposits, and high pressure metamorphic rocks (Frezzotti et al.,

2011; Hrstka et al., 2011; Thomas et al., 2006, 2009a,b, 2011a; Xieet al., 2009).CO3

2−(aq) concentrationsas low as 0.36 wt.% can be mea-

sured using a modied technique by Sun and Qin (2011) (R. Thomas,

Table 6

Main Raman vibrations (cm−1) of selected orthosilicates and tectosilicates.

Orthosilicates Main vibrations Ref.

Forsterite(Mg 0.9,Fe0.1) 2SiO4

227 303 423 548 608 824 921 [1]856 964882

PyropeMg 3 Al 2Si 3O12211 364 563 650 871 902 1066 3928

AlmandineFe 3

2+ Al 2Si 3O12

170 216 323 500 863 916 1038 3342 556 897370

SpessartineMn 3 Al 2Si 3O12

175 221 321 500 630 849 905 1029 3350 552 879

GrossularCa 3 Al 2Si 3O12

181 247 373 420 550 827 1007 3280 848

880Uvarovite

Ca 3Cr 2Si 3O12

176 242 370 509 828 3272 526 894

590Andradite

Ca 3(Fe 3+, Ti) 2Si 3O12

174 236 325 452 516 816 995 3370 494 574 842

874Kyanite

Al 2SiO5

302 405 562 669 952 4325 419360 437386 486

Sillimanite Al 2SiO5

142 235 310 456 597 708 874 907 1127 [2]964

Andalusite Al 2SiO5

293 323 453 553 719 834 920 1065 [2]361 992 1111

Zircon ZrSiO4

202 356 438 974 1008 [2]225

Tectosilicates Main vibrations Ref.

OrthoclaseKAlSi 3O8

157 284 458 514 751 814 967 1035 1137 [1]177 477 583 1062197

MicroclineKAlSi 3O8

159 263 455 514 651 749 813 1007 1128 [1]178 267 475 1142199 286

SanidineKAlSi 3O8

163 284 462 514 767 813 1123 [1]475

AlbiteNaAlSi 3O8

183 210 457 508 764 816 977 1032 [2];5292 480 1098

QuartzSiO 2

128 206 356 402 520 608 807 1066 1161 6265 464 698

485Coesite

SiO 2

116 204 326 427 521 785 815 1036 1144 6151 269 355 466 837 1065 1164176

CristobaliteSiO 2

114 230 420 792 1075 [2]273286

[1] = Raman Spectra Database, Siena Geouids Lab (http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm); [2] = Raman Spectra Database Lyon (http://www.ens-lyon.fr/LST/Raman). 3 Kolesov and Geiger, 1998; 4 Mernagh and Liu, 1991; 5 Sendova et al., 2005; 6 Palmeri et al., 2009. Ref. = References.



15/21


pers. comm.). Higher carbonate concentrations can be determinedeasily. These results are of particular interest since they suggest that al-kaline aqueous solutions may represent relevant geological uids.

Raman spectroscopy is a powerful technique to study the speciationof silica in aqueous uids at different P –T and pH conditions (e.g., Huntet al., 2011; Newton and Manning, 2003, 2008; Zotov and Keppler,2000, 2002). In neutral solutions, SiO2 dissolves predominantly as neu-tral monomers (Si(OH)4

0) and dimers (Si2O(OH)60) under most crustal

and upper mantle P –T conditions. Si(OH)40(aq) can be identied by a

Raman band in the 750–800 cm−1 region (Table 1). In alkaline uids,increasing dissociation of monomers and dimers in deprotonated spe-cies (e.g., SiO(OH)3

−, Si2O2(OH)5−) yields additional Raman bands in

the 950–1100 cm−1 region, as shown in Fig. 9d.B(OH)3

0 is the predominant boron species in aqueous uids over awide range of P –T – pH conditions. The Raman spectrum of B(OH)3

0(aq)

shows a strong band at 877 cm−1 and an additional weaker bandat 495 cm−1 (Table 1; Janda and Heller, 1979; Schmidt et al., 2005).A method of determining the B(OH)3

0(aq) concentration in uid

Fig. 10. Raman spectra of carbon phases in

uid inclusions; a) Diamond in a CO2

uid inclusion from peridotites, Hawaii; modi

ed from Frezzotti and Peccerillo (2007). b) Graphitein a CO2 uid inclusion from peridotites, Italy. Excitation light source: Ar ion laser (λ=514.5 nm). G_G-band, or order band; D_D-band, or disorder band. Note that the Ramanwavenumber of the D-band decreases with increasing wavelength of the excitation light source: for example using a He –Ne laser light (λ=632.8 nm), the graphite D-band isexpected at about 1330 cm−1.

Table 7

Main Raman vibrations (cm−1) of selected phyllosilicates and inosilicates, both single and double chains.

Phyllosilicates Main vibrations OH− Ref.

BiotiteK 2(Mg,Fe

2+)6-4(Fe 3+,Al,Ti)0-2(Si6-5 Al 2-3O 20)(OH,F)4

178 549 679 717 3658 2767 3680

Muscovite

KAl4(Si6 Al 2O 20)(OH,F)4

178 216 385 407 639 702 914 1117 3627 [1]

197 261 754 957PhlogopiteK 2(Mg,Fe

2+)6 (Si6 Al 2O 20)(OH,F)4

192 279 331 680 792 1038 3673 3715 [1]372 1096

ParagoniteNa 2 Al4(Si6 Al 2O 20)(OH)4

203 413 647 708 1062 3631 [1]218 465 756272

TalcMg 6 (Si8O 20)(OH)4

113 295 335 434 678 786 1018 3677 3196 366 793 1055

Clinochlore(Mg, Fe 2+)5 Al(OH)8(AlSi 3O10)

104 358 548 679 3477 3605 4198 3647

3679Chrysotile

Mg 3Si 2O5(OH)4

231 345 620 1105 3657 3703 5;389 692 3718 6

3745Antigorite

(Mg,Fe 2+) 3(OH)4Si 2O5

230 375 520 683 1044 3658 3709 5;3687 3729 6

3774Lizardite

Mg 3(OH)4Si 2O5

233 350 510 630 1096 3708 5;388 690 3723 6

Inosilicates Main vibrations Ref.

EnstatiteMgSiO 3

237 343 414 664 1011 [1]382 684

DiopsideCaMgSi 2O6

320 662 1009 7389

Hornblende(Na,K)0-1Ca 2(Mg, Fe

2+,Fe 3+,Al)5(Si6-7 Al 2-1O 22)(OH,F) 2

224 667 1040 [1]

PargasiteNaCa 2Mg 4 Al 3Si6 O 22(OH) 2

229 665 1017 8

OH− = OH stretching vibrations; [1] Raman Spectra Database, Siena (http://www.dst.unisi.it/geouids/raman/ spectrum_frame.htm); 2 Kuebler et al., 2006; 3 Fumagalli et al.,2001; 4 Kleppe et al., 2003; 5 Rinaudo et al., 2003; 6 Auzende et al., 2004; 7 Thompson et al., 2005; 8 Downs, 2006. Ref. = References.



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inclusions has been presented by Thomas (2002), with a minimumdetection limit of 0.050 wt.%. Nitrate and phosphate ions have notyet been reported in uid inclusions, while NaOH(aq) and LiOH(aq)can be present in some ore-forming uids (Thomas et al., 2011b)

6. Identication of mineral phases: a catalog of reference

Raman spectra

Fluid inclusions may contain mineral phases, which form by dif-ferent processes, including direct uid precipitation (daughter min-erals) and reaction of uid contained within inclusions with thehost mineral (step-daughter minerals) (Fig. 1; Roedder, 1984). Min-

erals including or included within uid inclusions can be readily iden-tied by comparison of their spectral ngerprints with referencespectra.

A catalog of about 140 spectra of minerals which are of interest inuid inclusion research is presented in Tables 2–7, as a supplement tothe web Raman mineral library available at: http://www.dst.unisi.it/geouids/raman/spectrum_frame.htm. Each table reports mineralname andformula, a list of themain Raman modes observed, and refer-ences. Main vibrations are reported using the ν notation in scattering

geometries, where the symmetric stretching vibration (ν1) representsthe strongest Raman mode. Reference spectra catalog also includesselected gas and solute species that were discussed above and listedin Table 1. All measured spectra correspond well to spectra reportedin literature. Relatively pure phases and/or phases contained withinuid inclusions were measured on a Horiba (Jobin Yvon) Labram spec-trometer at the University of Siena, using a water-cooled Ar ion laser(λ=514.5 nm) as the excitation source. Present catalog intends to pro-vide a rst library dedicated to uid inclusion research.

6.1. Native elements, halides, oxides and sul des (Table 2)

Carbon is by far the strongest Raman scatterer and the most stud-ied phase by Raman spectroscopy. In C\O±H uid mixtures, precip-

itation of C (graphite, or diamond at higher pressures) re

ects adecrease in f O2 buffer conditions in the uid–rock system (e.g.,redox reactions), often induced by a change in P and/or T . The processhas been studied and modeled in natural and synthetic uid inclu-sions by various authors (e.g., Frezzotti et al., 1994; Huizenga, 2001;Luque et al., 1998, 2009; Sterner and Bodnar 1984; van den Kerkhof et al., 1991). Fig. 10 reports the spectra of diamond and graphitedetected within uid inclusions. Diamond is characterized by a verystrong mode at 1332 cm−1 (sp3 bonds; Table 2). Well-crystallizedgraphite shows one intense bands at 1580 cm−1 (sp2 bonds; so-called G-band or order band). In microcrystalline graphite and disor-dered carbon, presence of defects gives rise to an additional band at1350 cm−1 (D-band or disorder band; excitation light source at514 nm), which increases in intensity with increasing disorder,and to an upshift to 1600 cm−1 of the G-band (e.g., Wopenka andPasteris, 1993 and references therein).

The area ratio of the order–disorder bands has been proved to rep-resent a reliable geothermometer in natural graphite (i.e., increasingdisorder at decreasing temperature; Beyssac et al., 2002; Wopenkaand Pasteris, 1993). However, caution should be used in applyingthe order–disorder geothermometer to graphite contained withinuid inclusions. The crystallinity of graphite precipitated from uidsdoes not show large variations and it is generally rather high – evenat moderate temperatures – unlike what observed in natural graphite(Cesare and Maineri, 1999; Luque et al. 1998, 2009).

The solubility of uncharged molecules of S in water is appreciable,and S 8

0 in uid inclusions (Fig. 9b) has been recognized by the domi-nant broad bands at 462 (S\S stretching) and 220 cm−1 (S\S\Sbending). Additional minor bands may occur at 153, 187, 246, and

437 cm−1 (Giuliani et al., 2003). Spectra of chlorides (e.g., haliteand sylvite) have not been reported from uid inclusions. The prob-lem with halides is that they are extremely weak Raman scatterers:one exception is represented by uorides (Table 2; Burruss et al.,1992; Rickers et al., 2006). Raman bands of most common oxideand hydroxide minerals are listed in Table 2. The three polymorphsof TiO2 are also reported, although only rutile has been observed inuid inclusions (Frezzotti et al., 2007).

6.2. Carbonates (Table 3)

Carbonates are common phases in uid inclusions, and a recentexample of Raman identication of multiple carbonates in uid in-clusions in pegmatites is reported in Thomas et al. (2011a). Raman

vibrational modes are dependent on the main carbonate groups,modied by the interactions with the bonded mineral lattice. CO3

2−

exhibits three main distinct internal vibrational modes over the

Fig. 11. Comparison of the Raman spectra of calcite, dolomite, and magnesite in the in-terval 0–1600 cm−1. Main CO3

2− group vibrations are illustrated. ν1 = Symmetricstretching vibration; ν3 = Antisymmetric stretching vibration; ν4 = In-plane bendingvibration; T = Translational lattice modes. Calcite, skarn from Vulsini volcanic district,Italy. Dolomite, eclogite from Sulu, China. Magnesite, peridotite from Baldissero, south-ern Italian Alps.



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range 400–1400 cm−1. Generally, strong Raman modes appeararound 1050–1100 cm−1 due to the symmetric stretching vibration(ν1) of the carbonate group, while weaker (around 20 time less in-tense) Raman bands near 700 cm−1 and 1400 cm−1 are due to thein-plane bending mode (ν4) and the antisymmetric stretch (ν3) of CO3, respectively. Lattice modes show Raman shifts below400 cm−1. As shown in Fig. 11, close similarities exist in the Ramanmodes of the CO3 group between different carbonate minerals. Howev-er, signicantdifferences are evident in the positions of their respectivelattice modes over the range 100–350 cm−1 (T in Fig. 11 and Table 3):for example bands of CaCO3 (156 and 284 cm

−1), CaMg(CO3)2 (176,299 cm−1), and MgCO3 (212 and 329 cm

−1) are distinct and identi-able without dif culty.

Raman spectroscopy is well suited to distinguish among the poly-morphs calcite, aragonite and vaterite (Table 3). Calcite has mainRaman modes at 1085 (ν1), 1450 (ν3), and 712 cm

−1 (ν4). Aragonitehas the main vibrational mode at 1085 cm (ν1), and weak vibrationsat 1463 (ν3) and 704 cm

−1 (ν4), and an additional very weak bandat 854 cm−1 (ν2). In vaterite, the main vibration mode (ν1) forms adoublet at 1074 and 1090 cm−1. A doublet is also present at 740 and750 cm−1 (ν4). The most intense lattice Raman modes are at 284,206, and 301 cm−1 for calcite, aragonite and vaterite, respectively.Mg-calcite shows a slight upshift of the main stretching band to1087 cm−1 and has a broader band base than pure calcite (Burke,2001). In hydrated (i.e., hydrous and OH-bearing) carbonates, the OHstretching vibrations give rise to additional broad Raman bands located

between 3000 and 3700 cm−1 (Table 3).

6.3. Sulfates, phosphates, and borates (Tables 4 and 5)

Sulfates and phosphates are strong Raman scatterers. The Ramanbands of these minerals are due to the vibrations within SO 4 and PO4tetrahedra. Differences among spectra listed in Tables 4 and 5 resultfrom the nature of metals within the main molecular unit, from thebond strength between the main molecular units and neighboringatoms, and from the different degrees of distortion of the main mo-lecular unit in the mineral lattice (cf., Nasdala et al., 2004, and refer-ences therein).

In sulfates, the strongest Raman band due to the symmetricstretching vibration (ν1), of SO4 tetrahedra is at about 1000 cm

−1, at

lower wavenumbers than that of CO3 groups: 1018 cm−1 for anhy-drite, 994 cm−1 for thenardite, and 1008 cm−1 for gypsum(Table 4). Additional weaker bands over the ranges 400–500 cm−1,

600–700 cm−1, and 1100–1200 cm−1 are due to the in-plane (ν2)and the out-of-plane (ν4) bending modes, and to the asymmetricstretching of SO4 tetrahedra. The Raman bands of hydrated sulfatesare closely related to those of the sulfate ion in aqueous solution(i.e., 980, 620, and 450 cm−1; Table 1), and show a progressive shifttoward higher wavenumbers with decreasing of the hydration state(cf., hydrous magnesium sulfate list in Table 4; Wang et al., 2006).

In borates, the distribution of the main Raman bands is mainly de-pendent on the mineral structure and on the type of borate ion (i.e.,boron–oxygen ratio, charge, and hydroxyl groups present); vibration-al modes are observed in the regions: 490–670, 690–800, 820–910,and 950–1040 cm−1 (Table 5). Borates in uid inclusions have beeninvestigated by Williams and Taylor (1996), Peretyazhko et al.(2000), Thomas (2002), and Rickers et al. (2006).

6.4. Silicates (Tables 6 and 7 )

Silicate minerals are critically important to uid inclusion re-search: quartz, olivine, pyroxenes and garnet represent very commonhost phases for uid inclusions, and their Raman bands should befully characterized before analyzing uid inclusions. In addition,they can be found as daughter mineral phases in uid inclusions, be-cause of high silica solubility in aqueous uids at most crustal andupper mantle P –T conditions (e.g., Manning, 2004).

Compared to carbonates and sulfates, silicate minerals areweaker Raman scatterers, due to the low polarizability of the Si\O

bonds. Silicates having different chemical composition or/and struc-ture are discriminated from their spectral features. Fig. 12 comparesmain vibration regions for the different classes of silicates. In ortho-silicates, Raman bands are determined by the vibration modes of theisolated SiO4 tetrahedra, similarly to what observed in sulfates andphosphates. Olivine and garnet show the main stretching modes of SiO4 group in the 800–1050 cm

−1 region (Table 6). A very good cor-relation has been shown between the wavenumbers of the SiO 4main bands and cation substitution (e.g., Mg/Fe+Mg in olivine)which permits the determination of the chemical composition of these minerals (e.g., Guyot et al. 1986).

In inosilicates and phyllosilicates, where tetrahedra are to someextent connected, bands generated by the vibration modes of theSi\Ob\Si bonds (Ob = bridging oxygen) dominate the spectra.

Spectra of pyroxenes show main Si\O bending and stretchingmodes over the 600–700 and the 900–1050 cm−1 regions, respec-tively (Fig. 12; Wang et al., 2000). Clinopyroxene (diopside-

Fig. 12. Raman vibrational mode regions for major silicate classes. Main vibrational regions of borates, phosphates, sulfates, and carbonates are reported for comparison.



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hedembergite series) and orthopyroxene (enstatite–ferrosilite se-ries) can be easily distinguished by the number of bands in the600–700 cm−1 region: orthopyroxene has two bands, while clino-pyroxene has only a single band (cf., Table 7). Most amphibolesand phyllosilicates are very weak Raman scatterers and reconnais-sance within uid inclusion is often dif cult. As shown in Fig. 13,

however, the position and the shape of the more intense OH stretch-ing band(s) can be used to distinguish among minerals containinghydroxyl groups.

Tectosilicate spectra are dominated by vibrations of Si and Oatoms within the framework structures of fully linked tetrahedra(McMillan and Hess, 1990). Main bands, which occur over the range380–530 cm−1 (Fig. 12), are due Si\O\Si symmetric stretchingand bending modes. The Raman frequencies of main modes show arelationship with the size of rings made by tetrahedra (Sharmaet al., 1983): four-membered ring structures, such as feldspars andcoesite, have main modes above 500 cm−1, whereas structures with

six-membered rings, such as quartz, tridymite, cristobalite, and neph-eline have main modes in the 380–480 cm−1 region (Table 6).

7. Concluding remarks

Raman analysis of uid inclusions permits to qualitatively detect

or identify gaseous and liquid phases, as well as enclosed (or enclos-ing) minerals. In some cases, quantitative analyses are possible (e.g.,relative mole% in gas mixtures, and solute concentration in aqueousuids). Major advantages of Raman spectroscopy are the minimalsample preparation, and the excellent volume resolution:uid inclu-sionsas smallas thelaser spot size (1–2 μ m) can be precisely locatedand analyzed within double polished thick sections. In addition,Raman is a non-destructive technique, meaning that there is noneed to decrepitate uid inclusions.

Fluorescence, that can cover the Raman spectrum, represents themost signicant disadvantage during analysis, and the risk of uores-cence must be always considered when selecting uid samples to an-alyze (e.g., hydrocarbons). Another signicant disadvantage is theabsence of adequate libraries of reference spectra. This last inconve-

nience is in part remedied by the compilation of a small spectral li-brary dedicated to uid inclusion research, presented in our reviewpaper.

Raman spectroscopy has been used to successfully analyze uidinclusions with an increasing number of publications through theyears. No other technique can analyze liquid, gas and solid constitu-ents in uid inclusions. Incorporating this exclusive method withevolving new technologies (e.g., spectral imaging) provides a brightfuture for this “old” technique in the analysis of geological uids.

Acknowledgments

Present research was in part supported by the PRIN 2008-BYTF98.We acknowledge helpful reviews by R. Thomas and an anonymous

reviewer of an earlier version of the manuscript. We are grateful tothe Museo di Mineralogia of the University of Rome “La Sapienza”and to the Museo di Mineralogia of the University of Siena for provid-ing several mineral samples for Raman analysis. Raman facilities inSiena were provided by PNRA, the Italian research program forAntarctica.

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