American Mineralogist, Volume 69, pages ll16-1126' 1984

The hydrous component in garnets: pyralspites

Rocen D. AINesr AND GEoRGE R. RossMAN

Division of Geological and Planetary ScienceszCalifurnia Institute of Technology

Pasadena, California 91 125

Abstract

Natural pyralspite garnets have been found to commonly contain a hydrous component,ranging in concentration from 0.02 to 0.25 wt.% as H2O. Anhydrous pyralspites of crustalorigin are rare. Of forty crustal garnets examined, only two were anhydrous. The most-hydrous garnets were spessartines from igneous pegmatites. Metamorphic garnets hadlower water contents, and frequently also contained hydrous inclusions. The infraredabsorptions of the hydrous component in the garnet end members are characteristic, andconsist of 2 to 4 narrow bands centered at 3640 cm-l in spessartine, 3500 cm-l inalmandine, and 3670 cm-l in pyrope. The IR spectra indicate that the hydrous componentis not in the form of molecular HzO; the most likely form is H+OX- substituting for SiOX-but other substitutions involving multiple OH- groups on one site are consistent with thedata. The concentration of OH (as HzO) in garnets may be determined from the integralabsorptivity (K) in the 3700-3400 cm-r region, although K varies with chemistry from3700-6000 (l mo[! cm-2; in end members to 120-600 in intermediate compositions.

Introduction

The hydrous component found in many garnets isroutinely ascribed to the hydrogarnet substitution, HaOf-e SiOl-. The characteristics of this substitution arepoorly defined in natural garnets, however. This is partic-ularly true of the pyralspite garnets, the pyrope-alman-dine-spessartine series. In this paper we discuss thenature and extent of the hydrous component commonlyfound in pyralspite garnets. In a forthcoming paper, wewill discuss the grandite series. These papers address thequestions of the degree to which all natural garnets maybe expected to contain a hydrous component, and wheth-er that hydrous component is the classic hydrogarnetsubstitution, other substitutional hydroxide, molecularwater or fluid inclusions, or alteration and includedhydrous phases. Previous studies have concentrated onrare garnets containing lVo H2O or more; we have insteadconcentrated on understanding the hydrous component incommon garnets.

Hydrogarnets were first described by Cornu (1906) andFoshag (1920). Winchell (1933), Flint et al. (1941), Pabst(1937, 1942), and Belyankin and Petrov (1941) broughtunderstanding of the natural hydrogarnets to the pointwhere it was realized that the HAO1+- e SiO!- substitu-

I Present address: Lawrence Livermore Natl. Laboratory,Earth Sciences Department, P.O. Box 808, L-202, Livermore,California 94550.

2 Contribution No. 3974.

0003-004x/84/t l l2-l I 16$02.00

tion accounted for the chemistries of hydrous granditesknown at that time. Modern research has focused almostexclusively on synthetic hydrogrossular. A series ofpapers by Cohen-Addad and co-workers proved, usingneutron difraction and other techniques, that in severalsynthetic compounds including hydrogrossular there are,in fact, tetrahedral groups of 4OH- replacing the SiO

-

tetrahedron (Cohen-Addad et al., 1964, 1967; Cohen-Addad 1968. 1969). These results were confirmed byForeman (1968). There are, however, no direct determi-nations of the HaOa tetrahedral grouping in natural gar-

nets. The proton positions in hydrogarnet type substitu-tions have only been verified by neutron ditrraction innatural materials in henritermierite (Aubry et al., 1969)'and by X-ray diffraction in bicchulite (Sahl, 1980) andzunyite (Baumer et al., 1974).

The pyralspite series is not well known for containing ahydrous component. Ackermann et al. (1983) describe thesynthesis ofhydrous pyropes, which they assume containthe hydrogarnet substitution. Wilkins and Sabine (1973)

reported that two spessartines from Sterling Hill' NewJersey, contained up to 2.5 wt.VoH2O. Hsu (1980) synthe-sized hydrospessartine, but the garnets were reported tobe metastable.

In this paper we discuss the extent to which naturalpyralspite garnets contain a hydrous component, and theevidence for the speciation of that component. Thehydrogarnet substitution (H+OX- c SiOi-), or othersubstitutional hydroxides would constitute a structuralhydrous component. They are part of the crystal struc-

l l 1 6

ture and their properties are linked to those of the basicgarnet structure. Other forms for a hydrous componentare basically inclusions of a foreign phase: fluid inclu-sions, alteration, or included hydrous phases. Regardlessofthe actual speciation, we report the content ofhydrouscomponent as H2O because that is the species actuallymeasured by our analytical methods. We have tabulatedthe total amounts of H2O present in pyralspites from avariety of occurrences and localities, and determinedcalibrations for structural H2O content in pyralspites. Themajor tool used in this study was infrared (IR) spectrosco-py, because of its sensitivity to the O-H bond. The use ofIR spectroscopy to determine the speciation of tracehydrous components in minerals is discussed by Ainesand Rossman, (1984a). Water contents were obtainedfrom infrared integral absorptivity, calibrated by thermo-gravimetry, P2O5 cell coulometry, and H2 gas manome-try.

Methods

Infrared spectra were obtained on doubly polished,single crystal slabs of garnet, using techniques describedby Goldman et al. (1977). A perkin-Elmer model 180grating infrared spectrometer was used for both roomtemperature and cryogenic measurements. Near infrared(NIR) measurements were obtained using a Cary l7lgrating spectrometer. IR spectra were recorded digitallyusing an interface with a DEC MINC-I I computer. Thesespectra were corrected for baseline and then used todetermine integrated areas under absorption bands.

Water contents were calculated by calibrating the inte-gated IR intensities against the water content measuredwith a DuPont moisture evolution analyzer (MEA). Rea-gent grade Mg(OH)2 was used as a standard material; itswater content was calibrated using thermogravimetry.The carrier gas used was N2 or 99Vo N2:lVo 02 forsamples containing ferrous iron. In these samples it ispossible for H2O to oxidize the iron, evolving H2 gaswhich is not detected by the cell. The oxygen gas is addedto react with H2 to regenerate H2O prior to reaching theelectrolytic cell.

100 to 200 mg of coarsely powdered garnet was used forMEA analysis. Samples were held at l l0"C until no moremoisture was measurable, then heated at -50O"/minute to1000'C. The moisture evolved was taken as the watercontent of the garnets. The blank for this technique is30-+ l0 ,rg H2O, as determined from empty boat runs, andruns using identically prepared anhydrous garnet. Theprecision is approximately 5% of the measurement (ex-clusive of the error in the blank) as measured using theMg(OH)2 standard. The primary errors in this measuremenrare due to adsorbed water on furnace walls and tightlybound water on particle surfaces that was added duringgrinding. The water added during grinding ranged up to0.04 wt.%. It was kept below 0.02Vo for the analysesreported here by thoroughly drying the mortar and pestle

nl7

in a vacuum oven at 160'C and by not overgrinding thesamples. The average particle size was 25 p,m,

Water contents were also estimated by thermal gravi-metric analysis using a Mettler TA2000 TGA system. Thelower detection limit is -Ifi) pg with a precision ofapproximately *50 pg. Up to 100 mg of powdered samplewas used per run. The primary error in this measurementis due to bouyancy changes in the sample during thecourse of the run, which is made with constant flow of N2gas. Measurements were made up to 12fi)'C.

Samples 13 and 49 were analyzed by gas volumetricmethods. 5fi) mg of the powdered samples were heated to14fi)" and the evolved gas was converted to H2, thenmeasured manometrically. This analysis was performedin James O'Neil's laboratory at the USGS, Menlo park,California, facility.

Major element analyses were performed using theCaltech MAC5 automated microprobe, with data reduc-tion methods described by Bence and Albee (1968) andthe alpha factors of Albee and Ray (1970). A pyropecollected by T. McGetchin from the Moses Rock dia-treme, Utah, was used as a secondary standard (Champi-on et al., 1975). This garnet is homogeneous and has beenextensively used as a microprobe secondary standard. Itis included in Table l, Sample #110. In no instance wasthere sufficient H2O in these garnets for the microprobeanalyses to be significantly silica deficient.

SamplesA total of 75 pyralspite garnets were surveyed for the

presence of a hydrous component. A summary of thelocalities and occurrences of samples that were analyzedin detail is given in Table l The garnets were museumspecimens, and details of associated phases can not beobtained directly. Table I contains references to thedetailed petrology of the ldcalities, where available. Onlyrepresentative samples from several diatreme and kim-berlite localities are listed in Table l. A more completediscussion of the hydrous mantle garnets encounteredduring the course of this project will be published later.Samples were chosen to represent the common range ofnatural chemistries ofpyralspite garnets, to represent thecommon occurrences, and to be a sufficient size (andlacking inclusions) for accurate analysis.

The necessary size of gemmy material is approximately250 pm in diameter by I mm in thickness for spectroscop-ic analysis. More material is required for the analyticaltechniques used for H2O concentration. Accordingly,analytical results on large samples were used to calibratethe infrared integral absorptivity so that water contentscould be measured on extremely small samples. Theeffects of inclusions on spectroscopic results were deter-mined by studying samples which contained zones of highand low inclusion content. All samples were doublypolished for IR analysis, and the same sample was thenanalyzed by microprobe utilizing the existing polishedsurface.

AINES AND ROSSMAN: HYDROI]S COMPONENT IN PYRALSPITES

AINES AND ROSSMAN: HYDROTJS COMPONENT IN PYRALSPITES

Table 1. Localities and occurrences of garnets used in this study

24J

10l1t213192L22232728293233343549c c

5657616465?88995

Minos Gerais, BrazilRutherforal #2, Amelia, VaRutherford #2, Amelia, vaTanzaniaUnknown, East Africa (?)Minos Gerais, BrazilAsbestos, QuebecFrorklin, N. JerseyUnknownBraziIWrangell, NeskaSruce Pine, N. Car.

Bincon, Callf.Mt. San Jacinto, Calif.Ugelvik, NorwayGore Mt. New YorkGreen Knobs, N. Mex.Anakapelle, IndiaCasey Bay, AntarcticaNort} River, N. YorkGarnet Ridge, Ariz.Garnet Ridg€, Ariz,Garnet Rialge, Ariz.Minas Gerais, Brazilwesselton Mine, S. Afr.Roberts Victor Mine,South AfricaRock Mica Mine,Yancy Co., N. Cor.Broken Hill, AustraliaMoses Rock, UtahChanthabud, TheilandPodsedice, Czech.Garnet Ridge, Ariz.Garnet Ridg€, Ariz.St. Lawrence, N,York

Igneous pegmatiteIgneous pegmatiteIgneous pegmatite

Igneous pegmatite (?)Altered ultramafic boqtl

\Metamor?hle, schistIgneous pegmatite, Zoned.Other zones in Semple 27;27 Eim,28 middle, 29 coreIgneous pegmatiteXenolith in tonaliteUltremafic bodyMetamorphic gneissDiatreme megacrystMetamoryhic, granuliteMetamor?hic, gEnuliteMetamor?hic, granuliteDiabeme megscBrstDiatreme megaclVstDiatreme megacrystIgneouspegmat i te(?)Kimberlite megacrystKyanite bearing eclogitenodulelgneous pegmatite

Diatreme megacrystNodu}G in alkali basalt

DiatremeDiatremeMetamoryhic, granulite

Sinkankosr 1968Sinkenkos, 1968

Grice & Williams, l9?9Frondel, 19?2

Sinkenkos, 19?6Parker, 1952

Jahns & Wright, 1951Hill, 1984Ca6well, 1968Levin, 1950Smitl & Levy, 19?6Grew, 1982Grew, 1981Levin, 1950Hunter & Smith, 1981; Swizter' 19??Hunter dc Smith' 1981; Switzer, 19??I{unter & Smith, 1981; Switzer, 19??

Parker, 1952

Mccetchin & Silver, 19?2

Hintze, 189?, p. 63Hunter & Smith, 1981; Switzer, 19??Hunter & Smith' 1981; Switzer' 1977Levin, 1950

SessSpe.ssAlm-SpessSess-NmPy-AlmSpess-AlmGro6sGrGs$pessPy-AlmAlml$essAlmSpess-Alm

Spess-AlmAlmryAIm-PyrPyPy-AlmPy-AfmPy-Almry-ilmPVry-AlmSpessPVGr6s-ry

Alm-Spess

Alm-SpessryAlm-Py

ryryPyAlm

7?6567256725

1500115002150031500,1N.M.N.H.f 11318r15005150061500?1?38

150089968150091501015011Ed Grew #3080LEd Grew #24348t024415013150131501315014Harvard *12528N.M.N,H.#8?3?5

1731

11826150169964

1501?150r31501315012

10{

105110l l l112113114118

Results

Forty pyralspite garnets of crustal origin were studied;all but two were hydrous, as indicated by the presence ofabsorption bands in the 3400 to 3700 cm-r region. Peaksin this region are typical of O-H stretching absorptions(e.g., Aines and Rossman, 1984a). Typical "water" con-tents are 0.lwt.Vo, ranging up to 0.25 wt.Vo.This hydrouscomponent is structural, not due to inclusions or alter-ation which may considerably increase the apparentwater content. Pyralspites of mantle origin are also fre-quently hydrous.

Figures 1 and 2 show the major element chemistry ofthe garnets studied and the integrated IR absorptivity dueto structural hydrous component. Water contents may becalculated from integrated IR absorption as shown in thescale bar in Figure I which was calibrated using MEA andTG results. Only the concentration of structural hydrouscomponent is shown in Figures I and 2. Some samplesalso contained hydrous inclusions or were altered and therelative amount of water held in structural versus non-structural sites could not be determined. These are shownas squares containing a cross in Figures I and2. Samplesthat were badly altered, largely metamorphic garnets, arenot included in these figures. The pyrope-almandine andspessartine-almandine series are plotted against grossular

because most pyralspites contain some calcium. In manyrespects it is appropriate to consider the octahedral Al3+garnets (pyrope, almandine, spessartine, grossular) as asingle group. One representative end-member grossular isincluded in Table l. A discussion of the grossular-

andradite series is in preparation.

"Water" contents

Figures I and 2 show the content of hydrous compo-nent in the garnets studied in terms ofintegrated infraredabsorption measured from 3750 cm-r to 3400 cm-'' Thismethod of measuring the hydrous component is verysensitive, limited only by the sample thickness available,but must be calibrated by another method in order toobtain absolute "HzO" concentrations' The primary cali-bration method used was P2O5 cell coulometry using themoisture evolution analyzer. Table 2 shows the results ofMEA calibration of the IR integrated absorbance. Sam-ples listed here were analyzed first by IR, then ground forMEA. The strong zoning present in some samples (for

example up to a factor of l0 for points separated by 4 mmin sample numbers 27-29) reqired that the same areaused for IR be powdered for MEA.

The result of primary interest in Table 2 is the largevariation in integral molar absorptivity seen between end-member and intermediate-chemistry garnets' Spessartine

*Mi, F,,u,,_u,i lno E qE35@57 ffi. . 21 . .4x 2324

A^TNES AND ROSSMAN: HYDROT]S COMP)NENT IN PYRALSPITES l l 1 9

Fig. l. Chemistries and ..water" contents of pyrope_almandine garnets studied. Cation contents are plotted as mole7a. None of the garnets deviate significantly from M3*Al2Si3Or2,M : Ca,Mg,Fe. The vertical axis is integral infrared absorbancein the region 3750 to 3400 cm-t, which is related to water contentas shown in the scale and in Table 2. Squares with no verticalextension and which are filled with a cross represent samplesthat were too badly altered to determine the content of structuralhydrous component, even though the IR pattern indicated thatone was present. Sample numbers refer to Table l.

Fig. 2. Chemistries and "water" contents of spessartrne_almandine garnets studied. Same scale as Fig. L None of thesesamples dif fers signif icantly f iom M32*Al2Si3Or2, M =Mn,Fe,Ca. Vertical extension has not been shown for samples 4and 13, which have very large integral absorbances due to thelarge molar absorptivity of these end member garnets; theirwater contents are similar to those of the other samples (seeTable 2).

Table 2. Integral molar extinction coefficients

Semple HcO. K# - - ComDositionNumbor (w1%) (liter mofrcm-z) (m6te r)

0.084 r.04 3?000.25 1.01 59000.30 t.02 58200.059 t.03 1160.17 t.02 6200.18 1.01 .t 80000.06 r.0l 1800.08 t.02 .. 2800.27 t.02 ll030.15 t.03 3,100.22 1.03 320

. Determincd by MBA enalysis. Blrotl besed ondr.PUeate meaaurements end known error in MEAmearurement which is lsrgely e function ol theamount of sample available.

.. Determined by H2 gss volume. por Stample lg,MEA gf,ve identlcdl results. No MEA avallable forSemple 19.

# Determinfd from integral absor?tion from 3?50 to3400 cm-r.

f* Theee samples wgre cheeked for completedetrydration et l000oc by heeting a slab in air foFfour hours. All lost at leest g5% of thelr original IRintensity.

and grossular have a molar absorptivity (K) of around5000 to 6000 (/ mole-r cm-2; while the spessartine-almandines and pyrope-almandines have absorptivities of200 to 500. Some of the variability seen in Table 2 may beascribed to analytical error. Major sources of error in-clude alteration in the sample and surface hydroxyl whichforms during grinding even in a low humidity atmosphere.This averaged 0.02 to 0.04 wt.%. These problems mayaccount for the variability among each group, but theextreme difference between the end-members and inter-mediate chemistries is considerably larger than any possi-ble error.

Two of the samplesin Table2 were analyzed by the H2gas volume technique. Sample 49 was not analyzed byMEA because of small sample size. Sample 13 wasanalyzed by both techniques and both gave identicalresults within error: MEA (two analyses) 0.18% (t0.01);H2gas0.l7Vo (*0.02). Sample 13 is from the same localityas the material used by Westrum, et al. (1979) in a studyof the thermophysical properties of grossular. They ob-tained a water content of 0.19 wt.Vo. Several sampleswere also measured using thermogravimetry. The errorsdue to buoyancy for small samples were considerablylarger than the corrected weight losses, and all samplesstudied in this way gave considerably greater weightlosses than indicated by MEA. Sample 13, for instance,lost 0.53 wt.Vo from 100 to 1100'C. The sample weighedll2 mg. Considerably larger samples are required foraccurate TG work, and no single sample was largeenough. An average sample of500 purple and red garnetsfrom the Garnet Ridge, Arizona, diatreme was groundand sieved to pass 2fi) mesh but not pass 325 mesh. In this

24 * *o

1 0L2 **13 **3549

104r13 * *114

ll20

way a l-gram sample was available for TG, and it yieldeda weight loss of 0.14%. The same sample gave an MEAresult of 0.12Vo, confirming that the values in Table 2 atenot artifacts of small sample size. Individual garnets fromthe same locality yielded from 0 to 0.22 wt.%o H2O byMEA, in accord with their integrated IR absorbances(Table 2).

The complete dehydration of the garnets measured byMEA was verified in four cases (Table 2) by heating adoubly polished slab at 1000'C for four hours. All fourshowed greater than a 957o loss of integrated intensity inthe infrared spectrum of the O-H region after this treat-ment, indicating that the MEA analysis at 10fi)"C is valid.TG curves for single samples and the 500-stone averageGarnet Ridge sample show continuous weight loss from200' to 1000' when heated at 5'C/min. The structuralhydrous component in pyralspites is not stable to veryhigh temperatures, if at all, at P,o, : I bar.

Infrared analysis

IR spectroscopy was used to determine the nature ofthe hydrous component, if present, and to calculate thetotal water content from the integral absorption intensity.

End-members. The IR absorption patterns for typicalend-members of the octahedral Al3* garnets are shown inFigure 3. Only the grossular and spessartine are true end-members. The pyrope and almandine both contain -20

molTo of almandine and pyrope component, respectively.The examples shown in Figure 3 are all gemmy crystalswith no hydrous inclusions or alteration.

Two important results of the infrared analysis of pyral-spites are exhibited in Figure 3. (1) End member patternsare composed of multiple absorption peaks. (2) Thelocation of the center of the multiple peaks varies with thechemistry. The grossular and spessartine end-memberpatterns are best known since multiple examples of themwere available. All end-member grossulars and spessar-tines studied have IR spectra similar to those in Figure 3'The multiple peaks in almandine are not as well-resolvedas those in grossular and spessartine. All pyropic garnetsstudied have patterns similar to that seen in Figure 3, butas discussed below this is probably not an end-memberspectrum. The end-member pyrope pattern is not welldetermined because of the absence of stoichiometricpyropes in nature and the low water content of commonMg2*-rich garnets.

The spectra shown in Figure 3 were obtained at - 196'Cto enhance the separation between peaks. At room tem-perature the spectra differ only in appearing less wellresolved; all the peaks may be identified but may occur asshoulders rather than distinct peaks. The spectra inFigure 3, as well as the other spectra in this paper, havebeen corrected to remove the effects of a broad Fe2+absorption centered at 4240 cm-l. The Fe2+ absorptionproduces a strongly sloping background which was re-

A/NES AND ROSSMAN: HYDROUS COMPONENT IN PYRALSPITES

l 1

r 9

s 2

l o

g 2

3800 3600 31@

u A V E N U M B E R , c m - l

Fig. 3. Typical single crystal infrared spectra of end-memberpyralspite garnets obtained at -196'C. From bottom to top:grossular, sample 13, 0.14 mm thick, spessartine, sample 14' 0.06

mm thick; almandine, sample 23, plotted as ll'0 mm thick;pyrope, sample 49, plotted as 10.0 mm thick. End-memberhydrous garnets of these chemistries always give IR patterns like

these with the exception of the peaks shown here of absorbanceless than 0.2, which are not reproducible and are due to

inclusions or alteration. These spectra have been corrected for a

sloping baseline due to an Fe2* electronic absorption at 4200

cm- I , with the exception of the pyrope which is uncorrected to

show the baseline. In the absence of a hydrous component, allgarnets are featureless in the 3700-3400 cm I region.

moved using a scaled baseline obtained from a rigorously

anhydrous garnet, samPle #111.The consistent nature of the IR spectra of the end-

member pyralspites is striking. No exceptions to this

behavior were observed in this study; if end-member

garnets were hydrous, they gave the IR spectra seen in

Figure 3. This is strong evidence that the absorption

pattern is intrinsic to garnet rather than from an alteration

product or included phase. In other words, there is a

structural hydrous component in natural pyralspites

Alteration. Many pyralspites contain a hydrous compo-

nent that is clearly not structural and, in fact, is not part

s 6

o 2

1 S

0 6

0 6

HAVELENGTH. Ph

of the garnet but rather part of an included phase. Waterof this type may be recognized by two methods. 0) Ingarnets with varying concentrations of inclusions or al-tered regions, any component of the IR spectrum thatchanges from region to region in the same manner as theinclusion concentration is assumed to be due to thealteration or included phases. (2) Any comp.onenr of theIR spectrum of a garnet that is inconsistent with thespectra expected for a garnet of that chemistry is assumedto be due to alteration. Criterion (2) could only be appliedafter many garnets were studied and the patterns ofspectral behavior were established.

Figure 4 shows how these criteria are applied to twoactual cases. The top two spectra are of an almandinefrom Wrangell, Alaska. The upper trace is in a region ofmany inclusions, some of which appear to be chlorite,based on microprobe analysis. Below that is a spectrumof an inclusion-free, gemmy region of the crystal. Identi-cal spectra are obtained in all inclusion-free portions ofthis crystal. The intrinsic, structural hydrous component

IIAVTLENGIH, pn2.7 2.8 2.5

t . 8 ATIERD AI.MIOIilES

t .

t . {t .t . 0

38m s0 3M

IIAVENIHBER, cn-lFig. 4. Example of two altered almandine garnets showing

the spectral features which allow alteration and inclusions to bedistinguished from a structural hydrous component. The uppertwo traces are both from a single crystal of sample 23. Theuppermost trace shows a region of inclusions, the lower trace agemmy area which gives a typical pattern for almandinestructural hydrous component. The peaks in the upper trace inthe 3700-3600 cm- I region are due to the inclusions or alteration.The sample is plotted as 3.8 mm thick. The lowermost traceshows a 0.7 mm thick almandine from a granulite, sample llg.The extremely broad band underlying the sharp peaks ischaracteristic of incipient alteration. It extends to 3000 cm-t. Allthree of these spectra have a sharp peak at ?1625 cm t which isthe characteristic O-H stretching frequency of muscovite, andmay be attributed to muscovite inclusions.

ll2l

in this garnet gives rise to the peak at 3550 cm-r, a peakseen in other almandines, while the higher wavenumberpeaks are due to included hydrous phases. The lowestspectrum in Figure 4 is of a spessartine-almandine garnetin which there is incipient alteration. The extremelybroad band (-400 cm-t in width) seen underlying thesharper peaks is characteristic of incipient alteration inwhich crystallites have not yet formed. It is similar to thespectra seen for fluid inclusions (Aines and Rossman,1984a). This garnet has obvious cloudy zones ofalterationextending several hundred micrometers inward fromlarge cracks.

Intermediate chemistries. The spectral patterns of theintermediate composition pyralspites are more complexthan those of the end-members. Once alteration andincluded phases are removed from consideration, twocommon factors are seen in the behavior of the intermedi-ate garnets. (1) Multiple peaks occur, and (2) sharp bandsof the sort seen in Figure 3 do not occur. The sharpestpeaks seen in end-member spessartine are -10 cm-l inwidth (at half height) while the sharpest bands seen inintermediate compositions are 40-50 cm-r wide, as broadas the entire set of sharp bands seen in the end-members.

Figure 5 shows the infrared spectra of typical membersof the spessartine-almandine series. The chemistries ofthese samples are shown in Figure 2. Although thespectra of the intermediate compositions are complex,they appear to be simple combinations of the end-memberpatterns. The broad bands of the intermediate composi-tions appear to be averaged versions of the multiple sharpbands seen in the end-members as, for instance, the twomost spessartine-rich samples at the top of Figure 5.

Despite a consistency of spectral behavior in theMn2+-Fe2+ series, the transition between spectral pat-terns is not entirely a smooth function of chemistry.Mn2+-rich samples are always dominated by the bands at-3640 cm-r and -3600 cm-1, and Fe2+ garnets aredominated by the bands at -3470 cm-r and -3540 cm-r.However, as seen in the Mn2+-rich samples of Figure 5(the top two), the relative intensities of the 3640 cm-r and3600 cm-r bands may vary widely. This behavior is alsoseen for the Fe2+-related peaks at -3470 cm-r and 3540cm-r. Several spessartines of near end-member chemis-try contain minor peaks in the region observed for Fe2*-related peaks. (These samples contain lamellar bands,-200 pm across, that a-re slightly more brown than thebulk sample.) The consistent peak locations and slightlyvariable peak intensities suggest that there is a structuralhydrous substitution present, but that factors other thanthe bulk chemistry may control the extent or structure ofthe substitution.

Similar behavior is seen in the Mg2+-Fe2+ series (Fig.6), but the smooth progression of spectral pattern withchemistry seen in Figure 5 is not present. Again, there arefour fundamental bands, and the high wavenumber pair at-3660 cm-r and 3560 cm-r are associated with Mg2+-richgarnets, and the low wavenumber pair at 3470 cm-r and

AINES AND ROSSMAN: HYDROUS COMPONENT IN PYRALSPITES

l4Jo=,<6e.o</,cE!<

0.E

0.0

9 .4

0.2

n22

IIAVILENGTH, pn2.7 2 .8 2 .9

38m 3600 3{00

}IAVENUI'IBER, cn-lFig. 5. Infrared spectra of typical members of the spessartine-

almandine series showing the progression of spectral patterns atintermediate chemistries. From bottom to top: sample 23,plotted as 5.5 mm; sample 104, plotted as 1.8 mm; sample 27,plotted as 13 mm; sample 29, plotted as 0.7 mm; sample 5, 0.06mm thick, and sample 4, 0.06 mm thick. All spectra recorded at- 196'C.

3540 cm-r are associated with Fe2*-rich garnets. At theintermediate chemistries (middle trace of Fig. 6), howev-er, the highest and lowest wavenumber bands are greatlydiminished in intensity. The IR pattern is dominated by aband that appears to be a combination of the 3560 cm-land 3540 cm-r bands.

Near infrared spectroscopy. The 5200 cm-r overtoneband of molecular water allows one to distinguish be-tween molecular H2O and OH- ions. This band arisesfrom combined bending and stretching, and cannot occurfor OH- ion (e.g., Aines and Rossman, 1984a). Figure 7shows the near infrared (NIR) spectrum of a hydrousspessartine. Superimposed upon three broad bands(-1000 cm-r wide) are a number of sharp bands (-20cm-l wide). The broad bands are electronic transitions ofFe2*. The sharp bands are overtone and combination

AINES AND ROSSMAN: HYDROUS COMPONENT IN PYRALSPITES

TA'IEII}IGTI|,2.1 2.t

ALMANDIIIE

llAVEt{tt{BER, cn-lFig. 6. Infrared spectra of typical members of the pyrope-

almandine series showing the progression of spectral pattemwith chemistry. From bottom to top: sample 23, plotted as 15mm; sample 21, plotted as 20 mm; sample 49, plotted as 9.5 mmthick. Spectra recorded at -196"C.

modes related to the fundamental O-H modes seen at-3600 cm-r (Fig. 5). None occur at -5200 cm-r, thediagnostic H2O frequency. All the O-H is in the form ofOH- ions. The NIR region is difficult to study in detailbecause of very low intensities in the hydrous pyralspites,but no band at -52C{J cm-l has been seen.

Discussion

Structure of the hydrous defect

The pyralspite garnets contain two classes of hydrouscomponent: a structural component, and alteration-relat-ed water. The alteration-related hydrous component alsoincludes hydrous inclusions and will not be discussedfurther. The structural hydrous component is of interestbecause: (l) the "water" contained in the garnet is arecord of the environment of formation or later events,and (2) the garnet structure must be changed to accommo-date a hydrous component and this may also change thephysical properties of the garnet.

The criteria used to distinguish the structural hydrouscomponent from alteration and inclusions, and the spec-troscopic features that we have observed for the structur-al component, provide considerable information about itscharacter. (l) The O-H defect responsible for the hydrouscomponent is OH-, not HzO; the diagnostic 5200 cm-rH2O band is not seen. (2) The IR spectra depend only onthe chemistry of the garnet, not its origin' There is onlyone type of structural hydrous component. (3) Multiple

Prl2.9

8.8

0 .6

a=,

E 0.{at)ct<

9.2UOz,<6cz,c><tct< 310s0s0

SPESSARTTNENEAR INFRARED

!{7H'o'eio.'

aooo ao60 4?/eoWAVENUMBER, . '- l

Fig. 7. Near-infrared spectrum of sample 78 showing thesharp overtone absorptions of O-H vibrations superimposedupon the extremely broad absorptions of trace Fe2+ in thesample, which occur at7900,5900 and 4200 cm-t. The ovenoneO-H absorptions occur at 7000, 6000, and 4600-4000 cm r. NoO-H absorption peaks occur in the 5200 cm-r region, indicatingthat there is no molecular water present.

OH* groups occur in close enough proximity to forcethem all to vibrate simultaneously, resulting in multiplepeak patterns in which the relative intensities of the peaksare constant. These facts provide the basis for a structuralinterpretation of the hydrous component. It would be amistake to automatically assume that the hydrogarnetsubstitution is present in natural pyralspites simply be-cause they are hydrous. However, the above facts all fitan interpretation based on hydrogarnet.

In the hydrogarnet substitution, four OH- groupsreplace SiOf- tetrahedra (Cohen-Addad et al., 1967;Foreman, 1968). The four protons are slightly outside thevolume of the tetrahedron as defined by the oxygencenters. These four O-H groups can be expected to yielda total of 3N - 6 = 18 vibrations, four of which arestretching vibrations and would occur near 3400 to 3700cm-I. Not all four must occur, depending on the exactsymmetry of the group. The presence of hydrogarnetsubstitution in an end-member garnet should be evi-denced by an infrared spectrum in the 3500 cm-l regionwith up to four peaks. If complete garnet symmetry ismaintained there will be two peaks.

The spessartine end-member pattern contains six peaks(Fig. 3) in violation of the four peak limit. However, thetwo lower-wavenumber peaks vary independently of thefour high-wavenumber peaks (Fig. 5). They apparentlyarise from a separate defect. The grossular and almandineend-member patterns do not violate the four peak limit,and are consistent with the possibility of hydrogarnetsubstitution. Intermediate chemistry pyralspites are morecomplex, however. This complexity could be caused bymultiple, non-hydrogarnet O-H defects. The simple be-havior in binary systems suggests, however, that themultiple peaks are a reflection of site occupancy in thedodecahedral sites which contain the divalent ions. Fig-ure 8 shows a portion of the garnet structure, a tetrahe-

tl23

dral site and its two dodecahedral neighbors. Not shownare four octahedral neighbors that share corners with thetetrahedral site. The presence ofdifferent cations sharingedges with a tetrahedral site "filled" by H4OX- may beexpected to significantly alter the spectra of the HaOagroup by reducing the symmetry of the site and becauseof the large diferences in cation size and cation oxygenbond lengths in the pyralspite garnets (Novak and Gibbs,l97l). The consistent change in the spectra in accord withthe dodecahedral cation chemistries strongly suggeststhat the hydrous defect is located in the tetrahedral site.

Since the H2O contents of the garnets studied areextremely low, considerably fewer than one HaOa substi-tution would occur per unit cell. Accordingly, thereshould be no coupling of vibrations between possibleHaOa tetrahedra, and each may be considered indepen-dently. In a garnet of intermediate chemistry in a binarysystem, three combinations of dodecahedral neighbors(for instance, Mn-Mn, Mn-Fe, Fe-Fe) are possible andshould yield three distinct infrared spectra. The interme-diate-chemistry garnets' spectra can be described ascomposed of three components: one for each end-mem-ber, and a new component only seen in the intermediatechemistry samples. This new component appears to bedue to mixed-neighbor pairs around tetrahedral sites. Thespectral complexity in Figures 5 and 6 may be explainedby this. The sites with neighboring Mg-Fe may give riseto absorption at -3580 cm-r in pyrope-almandines, andthe 3585 cm-r peak in spessartine-almandine may beassigned to sites with Mn-Fe neighbors.

The absence of resolvable fine structure in the interme-diate garnets makes the assignment of actual O-H defectstructures difficult. This apparent blurring of the finestructure arises as a result of intermediate chemistries,apparently due to the large number of local environmentsthat are slightly different giving rise to a similarly large

Dodecohedro l Co t ions

A/NES AND ROSSMAN: HYDROTJS COMPONENT IN PYRALSPITES

0 . 1

UJC)z6u.oU'o

PyrolspitesMg Fe Mn

Ugrond itesCo

Fig. 8. Portion of the garnet structure showing the twododecahedral sites that share edges with the tetrahedral site thatmay contain HaOa.

lt24

number of O-H defects that are slightly di-fferent andwhose O-H absorption frequencies are offset by a fewwavenumbers. The juxtaposition of many of these in anIR spectrum would result in a blurring of the sharp end-member pattern. This may be most clearly seen in the topthree spectra of Figure 5.

The presence of three sets of peaks in intermediate-chemistry garnets is consistent with the hydrogarnetsubstitution, but is not compelling evidence. An exampleof one problem with this interpretation is seen in thespessartine spectra in Figures 3 and 5. The postulatedMn,Fe band at 3585 cm-t is seen in the supposedly end-member spectra. This sample is approximately Mne5Fe5,so some Mn-Fe pairs are expected, but the intensity ofthe 3585 cm-r peaks are far too large to be the result ofrandom mixing of Mn2* and Fe2* dodecahedra with HaOatetrahedra. If the postulated HnOa defects exist, theymust heavily favor association with Mn-Fe neighbors. Asimilar effect is seen for Mg-Fe (Fig. 6) where the Mg2+end-member pattern is never seen alone, and the postulat-ed Mg-Fe pattern is dominant in all pyropes studied.

Occurrence and "water" content

The most hydrous pyralspite garnets encountered werespessartines and spessartine-almandines from igneouspegmatites. Garnets from metamorphic occurrences weremuch lower in structural H2O, but frequently containedhydrous inclusions. Garnets of mantle origin were fre-quently hydrous, and is discussed in detail by Aines andRossman (1984b). The majority of the garnets studiedwhich were anhydrous were from the mantle, but onlytwo samples of "crustal" origin were rigorously anhy-drous. Sample 112 is from a region known for garnetsoccurring in metamorphosed mafic rocks, and sample I I Iis from an alkali-olivine basalt flow. Both samples mayhave dehydrated as the host rock was cooling. Anhydrouscrustal pyralspites are rare.

The garnets used in this study were chosen to representthe range of occurrences of pyralspites, but the range ofwater contents that we observed is quite limited, from 0.0to 0.25 wt.7o clustering around 0.1 to 0.15 wt.Vo. Thissuggests that the solubility of water in pyralspites islimited, and that the garnets we observed with greaterthan 0.2 wt.Vo may be saturated with water. The rangethat we observe, however, suggests that the water con-tent of garnets may be a useful indicator of water fugacitypending experimental calibration of water solubilities.

Limitations to "water" concentrations inpyralspites

The hydrogarnet substitution has not been previouslyrecognized as an important component of natural pyral-spite garnets. The garnets studied here indicate that thislack of identification is due to the extremely low level ofthe substitution, and not to its frequency of occurrence.Two hypotheses have been previously advanced for the

A/NES AND ROSSMAN: HYDROUS COMPONENT IN PYRALSPITES

apparent lack of hydrogarnet substitution in pyralspite,and may be applied instead to explain the consistentlylow levels. The first is that the tfoO|- e SiO!- substitu-tion results in a large volume increase (Cohen-Addad,1967; Meagher, 1980; Martin and Donnay, 1972) and assuch is not favored at the high pressures typical ofpyralspite formation. The second hypothesis (Zabinski,1966) is that Mg-O and Fe-O bond lengths in pyralspitesare slightly longer than the average lengths for thesebonds in silicates. and that the additional increase inlength that would occur during unit cell expansion associ-ated with hydrogarnet substitution destabilizes the hy-drous pyralspites. Ca-O bond lengths are comparativelyshort relative to such bond lengths in other silicates,favoring hydrogarnet formation in grossular-andraditegarnets.

These theories may serve to explain the lack of pyral-spites containing large amounts of water, such as the 2 to6% H2O hydrogrossulars described by Zabinski (1966).The relatively constant H2O contents of the garnets inthis study suggest that low solubility limits are indeed thecontrolling factor in H+O+ substitution in pyralspites'Two studies of synthetic, hydrothermal pyralspites havebeen made in which the solubility was tested. Hsu (1980)

synthesized grossular-spessartines at P11re = 2 kbar, T :

42W50"C. The hydrogrossular Hsu generated gave IRspectra identical to those in this study, with peaks at 3660and 3620 cm-l. The hydrospessartines were identified onthe basis of cell volume expansion (Hsu, 1968) analogousto the grossular-hydrogrossular expansion (e.g', Shoji,1974; lto and Frondell, 1967) and are reported to bemetastable. However, the sharp bands reported by Hsuas related to HaOa in hydrospessartine appear to be due tooil contamination (Ackermann et al., 1983), a conclusionwith which we agree. Thus, there is no spectroscopicevidence of the hydrogarnet substitution given by thesehydrothermally produced spessartines and their exactnature remains undetermined. This, combined with themetastability of the synthetic hydrospessartines, makes itdifficult to determine the expected extent of H$1-substitution in natural spessartines.

Ackermann et d. (1983) studied hydrothermally pro-duced pyropes. They found a single, sharp IR absorptionband at 3600 cm-I. and a broad band centered near 3470cm-r due to fluid inclusions. They report that the 3600cm- I band is due to hydrogarnet substitution based on thepeak location in synthetic hydrogrossular. Based on theevidence presented here, this seems unlikely because ofthe dependence of peak location on chemistry. While thepyrope end-member pattern is not well defined by thisstudy, it occurs in the 3670 cm-l region based on thespectra of the Mg-rich garnets we studied. The presenceof fluid inclusions leaves open the possibility of hydrousinclusions or alteration products in their pyropes. Fluidinclusion IR absorption dominates the spectra of theirpyropes. Ackermann et al. report that the intensity oftheir 3600 cm-r band corresponds to a water content of

0.05% based on the muscovite peak-height molar absorp-tivity (s) of 170 ( mole-rH2O cm-l. That value is basedon peak height, not integral absorbance as reported here,but on the same basis the e from this srudy would be -5 {mole-rH2O cm-l for intermediate chemistries, to -150fmole-lH2O cm-l for end members, measured using themost intense peak in the spectrum. Peak height molarabsorptivities were not used in this study because of theambiguity introduced by multiple peaks due to a singlehydrous species.

Further work is required to determine the extent andnature of the hydrous component in synthetic pyropes,but the hydrous component seen by Ackermann et al. ispresent at similar concentrations to those in naturalgarnets. If they were in fact the same defects, this wouldbe strong evidence of solubility being the limiting factor innatural hydrogarnet substitution. However, the infraredspectra observed by Ackermann et al. do not seem to beconsistent with those we observed, and it is unlikely thatboth represent the same hydrous defect.

Conclusions

Natural pyralspite garnets commonly contain a hy-drous component at levels of 0.01 to 0.25 wt.%o. Anhy-drous pyralspites are rare. The hydrous component hasthe following characteristics :

l. The hydrogen speciation is OH-, not H2O.2. Multiple hydroxides (2 to 4) vibrate together, indi-

cating close proximity in the structure.3. IR absorption spectra of different chemistry pyral-

spites are consistent with dodecahedral cation occupancyaffecting a hydrogen defect in the retrahedral site.

These characteristics are suggestive ofthe hydrogarnetsubstitution, H+OX- = SiOX-, but other structural formsfor the hydrogen defect(s) which have the above charac-teristics cannot be ruled out. Natural garnets also fre-quently contain a hydrous component in the form ofalteration or included hydrous phases. The presence of astructural hydrous component may be best differentiatedfrom these by infrared spectroscopy. The presence ofhydrogarnet substitution in natural garnets may prove tobe a useful indicator of water fugacity. However, theeffects of alteration and hydrous inclusions must becarefully considered, particularly in metamorphic garnetswhere these effects are common.

Acknowledgments

We would like to thank Douglas Smith (U. Texas), Edward S.Grew (UCLA), Georg Amthauer (Marburg), Stein B. Jacobsen(Caltech), R. Larry Edwards (Caltech), L. T. Silver (Caltech),John S. White (Washington, DC) and Mary L. Johnson (Har-vard) for generously providing samples used in this study, andAnne M. Hofmeister (Caltech) and Robert E. Criss (USGS) forconducting the hydrogen manometry. This study was in partfunded by the National Science Foundation (Grant EAR-7919987).

n25

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Manuscript received, October 14, 1983;

acceptedfor publication, June 20, 1984.