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Hydrous components of grossular-andradite garnets from Thailand:

thermal stability and exchange kinetics

BONGKOT PHICHAIKAMJORNWUT1,*, HENRIK SKOGBY2, PRAYOTE OUNCHANUM1, PHISIT LIMTRAKUN1

and APICHET BOONSOONG1

1 Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand*Corresponding author, e-mail: [email protected]

2 Department of Mineralogy, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden

Abstract: Grossular-andradite garnet from three localities of skarn deposits in Thailand were studied by FTIR spectroscopy toinvestigate the occurrence of hydrous components. Characteristic OH absorption bands were present in the spectra of all samples,with intensities corresponding to OH concentrations in the range 0.01–0.34 wt% H2O. The distribution of absorption bands varyrelatively strongly among samples, partly correlated with sample composition. Fourteen samples were chosen for step-wise heatingexperiments at 700–900 �C in air or H2 atmosphere, with subsequent acquisition of IR spectra. Diffusion coefficients for thedehydration and rehydration reactions were obtained by fitting the changing OH concentrations calculated from the absorbance ofthe individual OH bands. The results indicate that the rehydration reaction is normally faster than the dehydration reaction. Aspreviously reported, individual bands show a somewhat different behaviour during the experimental heat treatments, which indicatesthat several different types of OH defects occur in these grossular-andradite solid-solutions. Mossbauer spectra acquired before andafter heat treatments show that hydrogen exchange occurs without charge compensation by the Fe redox state.

Key-words: hydrous component, OH bands, grossular-andradite, FTIR spectroscopy, kinetics.

1. Introduction

The occurrence of hydrogen defects in nominally anhy-drous minerals in upper-mantle minerals, such as pyrox-ene, olivine, and garnet, is a well studied phenomenon(Rossman, 1990; Beran, 1999). These minerals may pro-vide important storage sites for water in specific regions ofthe upper mantle (Schreyer, 1988; Pawley & Wood, 1995;Schmidt, 1995; Ingrin & Skogby, 2000). Quantitative ana-lysis of trace hydrogen is therefore desirable for a properunderstanding of its role in geological processes. Granditegarnets, Ca3(Al,Fe)2Si3O12, are known to incorporategreater concentrations of hydrogen than other membersof the garnet group (e.g., Johnson, 2006), whereas sampleswithin the pyralspite series also normally contain ahydrous component, but at much lower concentration.

The hydrogarnet substitution, (OH)4 , SiO4, has beenestablished in both synthetic and natural garnets, and repre-sents the best characterized type of OH incorporation ingarnets in the tetrahedral site. However, the observed com-plexity of typical spectra of low OH content grossular inthe OH region is consistent with multiple site occupancy(Rossman & Aines, 1991; Rossman, 1996). In general,defects associated with hydrogen incorporation in anhy-drous minerals may be quite numerous and complex, andthis appears to be the case also for garnets. From studies of

non-cubic garnets (anisotropic) with compositions withinthe uvarovite-grossular solid solution (Andrut et al., 2002),it has been suggested that OH defects can be incorporatedalso in the dodecahedral and octahedral sites (Beran &Libowitzky, 2006).

Kinetic data of hydrogen diffusion rates are of consider-able interest for the genetic interpretation of hydrogentraces in garnets. The exchange of hydrogen with theenvironment may take place by two different types ofmechanisms: by moving H interstitials without movingthe associated ‘‘non-hydrogen point defect’’ and by mov-ing also the associated ‘‘non-hydrogen point defect’’(Ingrin & Skogby, 2000). The hydrogen exchange in dehy-dration-rehydration reactions of nominally anhydrousiron-containing minerals often occurs by the redox reac-tion: Fe3þ þ O2� þ ½ H2 ¼ Fe2þ þ OH� (e.g. Ingrin &Skogby, 2000). The kinetics of this reaction is compara-tively fast and may occur during ascent processes of theupper-mantle minerals, which would then lead to anincrease in Fe3þ. Pyrope garnet seems to be the onlyupper-mantle silicate mineral with low water content buthigh ferric iron concentration suggesting that major dehy-drogenation by the dehydrogenation-oxidation reactioncannot be ruled out (Ingrin & Skogby, 2000). However,other dehydration reactions may also be considered and, ina study of grossular garnet, Forneris & Skogby (2004)

0935-1221/11/0023-2146 $ 6.75DOI: 10.1127/0935-1221/2011/0023-2146 # 2011 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

Eur. J. Mineral.

2012, 24, 107–121

Published online October 2011

suggested that dehydration may also occur without corre-sponding change in Fe2þ/Fe3þ ratio.

The hydrogen contents of pyralspite and grandite gar-nets from various localities have been studied by Aines &Rossman (1984); Bell et al. (1995); Wang et al. (1996) andBlanchard & Ingrin (2004a, 2004b) studied the hydrogendiffusion in pyrope-rich garnets. Kurka et al. (2005) stu-died the kinetics of H-D exchanges in grossular and sug-gested that at least two different types of OH defects arepresent in grossular. In this paper, we studied the kineticsof hydrogen diffusion in grossular-andradite garnets withthe aim to increase the knowledge on OH defects anddehydration-rehydration reactions.

2. Methodology

2.1. Sample description and preparation

Single crystals of natural grossular-andradite from threelocalities (Phu Kha Hill – KPK; Phra Ngam Hill – KPN;and Thap Kwai Hill – KTK) in the middle of Thailand wereused in this study. The sample locations are situated in theMueang District, Lop Buri Province, central part of thecountry. This area is located at a contact zone between aPermo-Triassic diorite intrusion and Permian carbonaterocks. The intrusion has thermally metamorphosed thehost rock into marble and skarn rocks. Calcic garnetbelonging to the grossular-andradite series occurs in theskarn rock, which consists of wollastonite, pyroxene,quartz, calcite, epidote, plagioclase, feldspar, and hornble-nde. The geological setting and more details regarding theoccurrences were described by Intayot (2007). The garnetcrystals are 0.5–3 mm in diameter and yellowish green tobrownish green in colour. Fluid inclusions and tiny crys-tals, presumably pyroxene, were observed in the grossular-andradite samples.

The samples were cut and polished on both sides to athickness in the range of 130–340 mm. The samples wereexamined under the optical microscope to select inclu-sions-free areas for FTIR measurements.

2.2. Thermal annealing procedure

Fourteen grossular-andradite samples were divided intothree sets, and each set was stepwise annealed at 700,800, and 900 �C at ambient pressure. A horizontal glass-tube furnace was used for these experiments. The sampleswere placed in a gold sample holder and pushed into thepreheated furnace. The temperature was controlled by a Pt/Pt-Rh 10 % thermocouple located above the samples. Thetemperature is estimated to be correct within �2 �C. Forextraction experiments the samples were heated in air andthe glass-tube was left open; for re-hydration experimentsthe glass-tube was flushed with hydrogen at a flow rate of100 cm3/min. Before and after each heat-treatment in H2

gas, a CO2 gas-flow was used to flush out all oxygen. TheH2 gas-flow was started at least 3 minutes before theexperiment begun.

2.3. FTIR analysis

Unpolarized spectra were recorded with a Bruker Equinox55 spectrometer equipped with a NIR light source, CaF2

beamsplitter, and a liquid nitrogen cooled InSb-detector.After each step of the sequential annealing experiments anFTIR spectrum was recorded by 400 scans in the area5000–2000 cm�1 with a resolution of 4 cm�1. In order toavoid interference by atmospheric water vapour, the sam-ple chamber was continuously purged with dried air. Thedoubly polished crystal sections were positioned directlyon stainless-steel apertures under a binocular microscopeusing transmitted light, and were fixed by aid of a thermo-plastic resin. To avoid inclusions and zoned areas, smallapertures in the range 100–200 mm had to be used. Asufficient signal strength could still be obtained by meansof the high-sensitive InSb detector. Background spectrawere acquired on identical free apertures. For every set ofexperiments all FTIR measurements were subsequentlytaken with the crystals carefully re-positioned on the aper-tures so that the same spot was analysed. Raw spectra werebackground corrected using the baseline correction func-tion of the Peakfit Software (Jandel Scientific). The OHabsorption envelope was then fitted with a number ofbands, normally 6–15, which were kept as few as possiblewhile still obtaining a satisfying fit.

The total water content was calculated from the summedintegrated absorbance using the wavenumber-dependentcalibration of Libowitzky & Rossman (1997). The choiceof a calibration method for garnets is not unambiguoussince considerable deviation exists in available calibra-tions (e.g., Maldener et al., 2003). However, the absoluteconcentrations are not crucial for studies of hydrogenexchange kinetics which rely on changes in relative values.

The diffusion coefficients of the relative decrease of theintegrated absorbance values in the OH region of the IRspectra were calculated using the Fick’s second law solu-tion for one-dimensional diffusion (Carslaw & Jaeger,1959; Ingrin et al., 1995; Kurka et al., 2005; Ingrin &Blanchard, 2006). For hydrogen extraction, the OH con-centration is described by the following equation:

CavðtÞC0¼ 8

p2X1

n¼0

1

2nþ 1ð Þ2�Dð2nþ 1Þ2p2t

4L2

!(1)

Where C0¼ the initial hydrogen concentration, Cav(t)¼ theaverage concentration across the whole thickness of thesample at a certain time of annealing, L ¼ the half-thick-ness of the sample, t ¼ the time of annealing, and D ¼ thediffusion coefficient. For the following rehydration experi-ments, the equation:

CavðtÞCs¼ 1�

CavðtÞC0¼ 8

p2X1

n¼0

1

2nþ 1ð Þ2�Dð2nþ 1Þ2p2t

4L2

! !

(2)

108 B. Phichaikamjornwut, H. Skogby, P. Ounchanum, P. Limtrakun, A. Boonsoong

was used for the increase in OH concentration, where Cs

represents the saturation concentration achieved during thefinal rehydration experiment. Most of the samples were notcompletely dehydrated at the end of the extraction experi-ments. These initial hydrogen concentrations, present atthe start of rehydration experiments, were subtracted fromthe Cav and Cs values. Only samples with a small (,15 %)initial hydrogen concentration were used for fitting ofdiffusion coefficients, since Equations (1) and (2) arevalid only for a homogeneous starting concentration, andnot applicable to the diffusion profiles inherent to thedehydration experiments.

2.4. Mossbauer spectroscopy

Mossbauer spectra were recorded at room temperature onthe same single crystals as used for the FTIR measurement,using a 57Co point source of 10 mCi nominal activity. Thesamples were positioned over circular holes in a lead foilunder a binocular microscope in order to avoid inclusionsand zoned areas. However, in order to obtain a sufficientsignal strength, an aperture of 400 mm had to be used,

larger than what was used for the IR measurements, andoccasionally some minor inclusions could not be avoided.

The software MDA (Jernberg & Sundqvist, 1983) wasused for spectral fitting after calibration against a-Fe foil,folding and reduction from initially 1024 to 256 channels.The spectra were fitted with one ferric Fe-doublet, and onedoublet assigned to ferrous iron, when appropriate. Duringthe fitting procedure, the position of the Fe2þ doublet wasfixed in cases where it was very weak.

2.5. Chemical analysis

Finally, all samples were analysed using the CAMECA SX-50 electron microprobe at Uppsala University, Sweden,10–25 spots were analysed on each crystal using a beamcurrent of 15 nA and an acceleration voltage of 20 kV. Mg,Al, Fe, and Cr were analysed using the standards: MgO,Al2O3, Fe2O3 and Cr2O3; Si and Ca were analysed usingwollastonite; and Ti and Mn were analysed using MnTiO3.The crystal-chemical formulas were calculated from thegarnet composition. Details of samples and their chemicalcomposition are presented in Table 1. Several samples

Table 1. Garnet samples and their chemical compositions obtained by EPMA.

Wt% oxide % end-members

MgO Al2O3 SiO2 CaO TiO2 Cr2O3 MnO Fe2O3 H2O Pyr Alm Sps Uva Gro And

KPK 39-1-1 0.00 9.70 36.91 35.02 0.22 0.03 0.40 17.81 0.10 0.00 0.00 0.83 0.00 45.12 54.05KPK 54-9 0.06 0.04 34.69 33.55 0.00 0.00 0.19 30.96 0.15 0.16 0.00 0.49 0.00 0.25 99.09KPK 54-10 0.00 12.56 36.87 35.63 1.31 0.00 0.48 12.74 0.31 0.00 0.00 1.00 0.00 57.64 41.35KPK 54-11 0.00 11.34 36.73 35.26 1.44 0.60 0.39 13.83 0.34 0.00 0.00 0.84 0.00 53.14 46.02KPK 56-12-2 0.00 11.88 37.25 35.44 0.03 0.01 0.43 15.05 0.13 0.00 0.00 1.00 0.00 54.45 44.55KPK 56-12-3 0.02 0.03 35.06 33.19 0.00 0.00 0.11 31.09 0.13 0.00 0.00 0.33 0.00 0.25 99.42KPK 56-12-9 0.00 5.60 36.14 34.67 0.01 0.01 0.26 24.12 0.09 0.00 0.00 0.66 0.00 26.38 72.96KPN 09 0.01 17.85 38.84 36.10 0.04 0.01 0.56 6.83 0.02 0.00 0.00 1.16 0.00 79.17 19.67KPN 10 0.01 17.33 38.26 36.44 0.03 0.01 0.50 7.18 0.03 0.00 0.00 0.99 0.00 78.17 20.84KPN 11 0.03 17.50 38.11 36.29 0.22 0.01 0.55 6.66 0.13 0.00 0.00 1.16 0.00 79.32 19.52KTK 05 0.26 14.46 37.69 36.07 0.47 0.01 0.32 10.65 0.01 0.98 0.00 0.65 0.00 65.91 32.46KTK 07 0.25 15.25 37.95 35.68 0.49 0.01 0.27 9.42 0.02 0.99 0.00 0.66 0.00 69.27 29.09KTK 09 0.29 15.41 38.06 35.88 0.74 0.01 0.46 9.27 0.02 1.15 0.00 0.99 0.00 68.92 28.94KTK 10 0.24 14.59 37.65 36.03 0.47 0.00 0.31 10.51 0.02 0.98 0.00 0.65 0.00 66.24 32.13

Sample Colour Formula proportions wt% H2O

KPK 39-1-1 Brownish green (Ca 5.97 Mn 0.05) 6.02 (Al 1.82 Fe3þ2.15 Ti 0.03) 4.00 Si 5.87 O 24 0.01

KPK 54-9 Brownish green (Ca 6.03 Mn 0.03 Mg 0.01) 6.07 (Al 0.01 Fe3þ3.95) 3.96 Si 5.81 O 24 0.15

KPK 54-10 Brownish green (Ca 5.93 Mn 0.06) 5.99 (Al 2.30 Fe3þ1.50 Ti 0.15) 3.95 Si 5.73 O 24 0.31

KPK 54-11 Brownish green (Ca 5.89 Mn 0.05) 5.94 (Al 2.09 Fe3þ1.64 Ti 0.17) 3.90 Si 5.73 O 24 0.34

KPK 56-12-2 Brownish green (Ca 5.96 Mn 0.06) 6.02 (Al 2.20 Fe3þ1.80) 4.00 Si 5.85 O 24 0.13



KPN 09 Pale yellowish green (Ca 5.95 Mn 0.07) 6.02 (Al 3.22 Fe3þ0.80) 4.02 Si 5.95 O 24 0.02

KPN 10 Pale brownish green (Ca 6.01 Mn 0.06) 6.07 (Al 3.15 Fe3þ0.84) 3.99 Si 5.89 O 24 0.03

KPN 11 Pale yellowish green (Ca 5.97 Mn 0.07) 6.04 (Al 3.17 Fe3þ0.78) 3.95 Si 5.85 O 24 0.13

KTK 05 Pale brownish green (Ca 6.03 Mn 0.04 Mg 0.06) 6.13 (Al 2.66 Fe3þ1.26 Ti 0.05) 3.97 Si 5.88 O 24 0.01




Hydrous components of grossular-andradite from Thailand 109

displayed chemical zonations and minor solid inclusions,e.g. diopside.

3. Results

3.1. Infrared analysis

All of the studied samples revealed significant absorptionbands in the OH-region of their infrared spectra. Theabsorption bands vary considerably among samples, bothin terms of band distribution and integrated intensity, andvariation within the same crystal was sometimes alsoobserved.

Rossman & Aines (1991) studied natural grossular-hydrogrossular garnets and divided them into seven classesbased on their prominent OH absorption bands. However,the spectral patterns for the present grossular-andraditesamples normally do not fit into the classification schemeof Rossman & Aines (1991). Instead, we have distin-guished five groups of absorption spectra for the presentsamples. Typical spectra of these groups are shown inFig. 1 and described in more detailed in Table 2, wherethe relation to the classification of Rossman & Aines(1991) is also indicated.

3.2. Thermal annealing

The spectral evolution during thermal treatment in air isillustrated in Fig. 2a–e for the five groups of spectra. Forthe re-hydration experiments, the corresponding spectraare shown in Fig. 3a–e. The calculated OH concentrationsbased on the integral IR absorbance for both hydrogenextraction and rehydration experiments are summarizedin Table 3.

Figure 2a shows an example of the evolution of theGroup I infrared spectra during stepwise annealing at 900�C in air for a total of 36 hours. This sample (KPK 54-10:Gr58An41) shows a stepwise decrease of absorption bandsduring annealing with the high-wavenumber bandsdecreasing faster than the low-wavenumber bands. Theband at 3615 cm�1 is the one that disappear first, andthen the bands at 3588 and 3578 cm�1. The broad band at3518 cm�1 is only slightly decreasing in intensity duringheating, but becomes narrower.

Sample KTK 09 (Gr69An29) (Fig. 2b) was heated at 800�C up to 279 hours. The bands decrease stepwise, withhigh-wavenumber bands decreasing faster than low-wave-number bands. Finally, the intensity relation of the bands at3551 cm�1 and 3600 cm�1 becomes reversed.

Sample KTK 05 (Gr66An32) (Fig. 2c) was heated at 900�C for a total of 36 hours. The spectrum of this sample isquite complicated with comparatively weak absorptionbands. The bands at higher wavenumber disappear fasterthan those at low wavenumbers.

Sample KPN 10 (Gr78An21, Fig. 2d) was heated at 900�C for 36 hours. Again, high-wavenumber bands decrease

faster than low-wavenumber bands. The absorption band at3686 cm�1 is relatively stable.

Sample KPK 54-9 (andradite 99 %, Fig. 2e) was heatedat 800 �C for 279 hours. Already after the first heating step,the water content decreased to half of original water con-tent. The absorption band at 3563 cm�1 disappears first.The prominent band at 3611 cm�1 decreases faster than theband at 3582 cm�1 which finally shifts to 3592 cm�1. It isinteresting to note that also Kurka (2005) observed a verystable band at 3592 cm�1 in andradite. These spectra aredifferent from andradite from serpentinite studied byAmthauer & Rossman (1998) that has a very simple spec-trum, dominated by an absorption band at 3555 cm�1 andvery weak absorption band at 3610 cm�1.

Fig. 1. Typical examples of the five spectral groups and their end-member components. From the bottom to top: Group I to GroupV. Spectra are normalized to 1 mm thickness.


Figure 3a–e shows the spectral evolution of the samplesused in the rehydration experiments. Dot-lines (top spec-tra) represent the initial spectra whereas the dash-lines(lowermost spectra) represent the last spectra of the dehy-dration experiments.

Sample KPK 54-10 (Fig. 3a) was heated stepwise at800 �C in H2 gas for a total of 108 hours. The OH bandsincreased stepwise during experiments, with a faster rate forlow-wavenumber bands compared to high-wavenumberbands. The initial hydrogen content was restored to�56 %.

Table 2. Summary of the five spectral groups of Thai grossular-andradite garnets.

Group Composition Major bands (cm�1) Minor bands (cm�1) Class (Rossman & Aines, 1991)

I Gr53–58An41–46 3641, 3613, 3602 3680, 3658, 3587, 3559 2þ7þ6II Gr66–60An29–32 3600, 3612 3658, 3640, 3585, 3552 2þ7III Gr66An32 3613 3686, 3660, 3641, 3586, 3566 5IV Gr78–79An20–21 3662, 3631 3686, 3614, 3565 1þ3V An53 – An99.4 3611, 3581, 3563 3633 5þ3

‘‘Class’’ refers to the division of type of spectra of Rossman & Aines (1991).

Fig. 2. The evolution of FTIR absorption spectra of grossular-rich (a–d) and andradite-rich (e) garnets with increasing time of annealing inair (summed time for consecutive steps).


Sample KTK 09 (Fig. 3b) was heated at 800 �C in H2 gasup to 81 hours. Already after the first heating step of 1 hourthe prominent spectra increase and the hydrogen content isfinally restored close to 100 %.

Sample KTK 05 (Fig. 3c) was heated at 800 �C in H2 gasfor 108 hours. The spectral evolution is complex, but aftera stepwise increase the initial hydrogen concentration isrestored to 85 %.

Sample KPN 10 (Fig. 3d) was heated at 800 �C in H2 gasfor 108 hours. The absorption bands increase stepwise,with a faster rate for the low-wavenumber bands comparedto high-wavenumber bands. The band at 3686 cm�1

increases only slightly and seems to be stable during

long-time heating. The original concentration is restoredto �52 %.

Sample KPK 54-9 (Fig. 3e) was heated at 800 �C in H2

gas for 81 hours. Already after 1 hour annealing, the Hcontent is restored to 94 % of the original content, with allbands similar to those of the initial spectrum. In the secondstep, the hydrogen content increases to more than 100 %,then shows a decrease after prolonged heating treatment(down to 87 % of original content after 81 hours).

A general observation from the heating experiments isthat the low-wavenumber bands appear more stable thanthe high-wavenumber bands; in most cases the high-wave-number bands decrease faster during dehydration

Fig. 3. The evolution of FTIR absorption spectra of grossular-rich (a–d) and andradite-rich (e) garnets with increasing time of annealing in H2

gas (summed time for consecutive steps). The bottom spectra represent the starting point, obtained after the hydrogen extraction experiments.The top spectra are the original spectra obtained on the untreated crystals.


Table 3. Experimental data for hydrogen extraction and rehydration.

Sample KPK 54-11 KPK 56-12-3 KPN 11 KTK 07

End-member Gr53An46 An73Gr26 Gr79An20 Gr69An29

Thickness (mm) 145 133 217 220T (�C) Time (h) H2Otot (ppm)700 (air) 0 3380 1254 1335 184

1 3497 881 1466 2133 3149 732 1348 1849 2940 573 1328 188

31 2424 414 1288 191103 2228 349 1115 164271 2078 306 996 157

-log D 15.0 13.6 15.0 15.7700 (H2) 0.33 2052 1112 987 180

1.33 2499 1114 1056 1714.33 2778 1132 1082 184

12.33 2805 1132 1153 19836.33 2882 1132 1229 187

128.33 3057 1107 1283 193

Sample KPK 39-1-1 KPK 54-9 KPK 56-12-9 KPN 09 KTK 09 KTK 10

End-member An54Gr45 An99 An73Gr26 Gr79An20 Gr69An29 Gr66An32

Thickness (mm) 166 248 168 338 290 294T (�C) Time (h) H2Otot (ppm)800 (air) 0 1010 1473 934 249 219 204

1 608 750 629 203 228 1143 430 508 589 176 209 1097 331 375 369 141 185 106

15 335 307 268 107 157 9331 266 253 250 78 129 7863 224 233 228 59 83 61

135 216 185 205 49 66 47279 135 132 139 30 32 24

-log D 13.1 12.4 12.3 13.0 13.6 13.1800 (H2) 1 481 1385 730 175 188 132

3 553 1534 722 186 201 1249 416 1368 749 183 197 106

81 519 1280 891 199 223 120-log D 11.7 11.0 12.0 11.2 11.4 10.4

Sample KPK 54-10 KPK 54-12-2 KPN 10 KTK 05

End-member Gr58An41 Gr54An45 Gr78An21 Gr66An32

Thickness (mm) 138 155 248 223900 (air) 0 3080 1317 333 145

0.05 2322 743 299 1400.15 2215 647 255 1160.48 1737 497 215 931.48 1377 363 150 644.48 980 209 99 35

12.48 883 171 61 2336.48 320 141 47 13

-log D 12.7 11.9 12.3 12.3800 (H2) 0.05 502 518 46 37

0.15 895 554 55 740.48 1322 607 88 1071.48 1527 672 158 1094.48 1612 767 167 115

12.48 1597 710 159 10836.48 1678 767 170 112

108.48 1722 819 174 123-log D 11.9 11.3 11.7 11.2

t, cumulative time; –log D, negative logarithm of the diffusion coefficient; Gr, grossular; An, andradite.


experiments, but are restored more slowly during rehydra-tion experiments compared to the low-wavenumber bands.

Examples of fitting of diffusion coefficients for thedehydration experiments on samples from the five typesof spectral groups are shown in Fig. 4. Most samples showa smoothly decreasing OH concentration that can be ade-quately fitted by Equation (1) down to 20–30 % of the

initial OH concentration. However, during prolonged heat-ing, the kinetics appear to slow down considerably. Asimilar behaviour was observed on grossular by Kurka(2005) and Kurka et al. (2005).

For the rehydration experiments, the diffusion coeffi-cients (Fig. 5; Table 3) typically showed faster kineticsthan for hydrogen extractions. To account for this

Fig. 4. General fit of the dehydration behaviour by Equation (1) for the five different spectral groups and values of D at temperatures of 800and 900 �C.


unexpectedly fast kinetics we adjusted the annealing tem-perature and time for some of the experiments. It waspossible to restore more than 50 % of the original hydrogencontents for all studied samples by the thermal treatmentsin H2 atmosphere, and sometimes even up to 100 %.

The calculated diffusion coefficients based on the Hextraction experiments (Table 3) were fitted to anArrhenius law (Fig. 6).

In order to account for the observed difference indehydration behaviour for OH coupled to low- and

Fig. 5. General fit of rehydration by Equation (2) for the five different spectral groups and values of D at temperature of 800 �C.


high-wavenumber bands mentioned above, additionalkinetic fitting strategies were tried. However, kinetic fitsapplied to the integrated area of individual absorptionbands in the IR spectra were not considered meaningful,since the fitted band widths vary considerably as a conse-quence of the pronounced band overlap. Instead, the bandswere grouped in high- and low-wavenumber regions that

largely reflect the observed dehydration behaviour, and thesummed integrated absorption area of these groups ofbands were fitted separately (Fig. 7). The obtained diffu-sion coefficients are generally lower for the low-wavenum-ber bands compared to high-wavenumber bands, with theexception of the andradite sample (KPK 54-9), whichshowed faster kinetics for low-wavenumber bands.

Fig. 6. Arrhenius plot showing diffusion laws for dehydration of four groups of grossular-andradite samples.

Fig. 7. Fits of hydrogen extraction data by Equation (1) for high- and low-wavenumber absorption bands.


Arrhenius laws for the high-wavenumber and low-wave-number bands are shown in Fig. 8.

3.3. Mossbauer analysis

Mossbauer spectroscopy was used to analyse ferric andferrous iron in the garnet crystals before and after theheat-treatment experiments (Table 4). Most of theMossbauer spectra showed only ferric iron doublets,assigned to the octahedral site (Fig. 9a). Weak absorptiondoublets caused by ferrous iron were observed for threesamples (Fig. 9b). However, the hyperfine parameters ofthe Fe2þ doublets (Table 4) are strongly different fromthose expected for grossular-andradite garnets(McCammon, 1995) which are characterized by a muchhigher quadrupole splitting. The centroid shifts and quad-rupole splittings of the Fe2þ doublets are in good agree-ment with those reported for diopside (McCammon, 1995)which was also observed as an inclusion during the

microprobe analyses. It is interesting to note that theFe2þ doublets do respond to the heat treatments, by adecrease in intensity after heating in air and an increasein intensity after heat treatment in hydrogen (Table 4;Fig. 10). This is in agreement with previous studies onhydrogen exchange coupled to Fe redox reactions in pyr-oxenes at similar experimental conditions (cf. Skogby,2006). No evidence for Fe2þ in the garnet structure couldbe observed, not even after the rehydrogenation experi-ments under hydrogen atmosphere.

4. Discussion and conclusions

In their previous study of hydrogen exchange kinetics ingrossular with the composition Gr82An14Py2, Kurka et al.(2005) observed significant variations in the dehydrogena-tion kinetics of different OH defects causing the individualabsorption bands and suggested that at least two types of

Fig. 8. Arrhenius plot showing dehydration diffusion laws for high-wavenumber (a) and low-wavenumber bands (b) of four groups ofgrossular-andradite samples.


OH defects were present in the grossular structure. Thepresent study largely confirms these observations for a sam-ple set expanded towards the andradite end-member (An20 –An99). In line with Kurka et al. (2005), we can identify oneset of absorption bands characterized by comparatively fastkinetics, including the bands at 3641, 3615 and 3604 cm�1

(Group I), 3658, 3639 and 3613 cm�1 (Group II), 3686, 3660and 3613 cm�1 (Group III), 3686, 3662, 3632 and 3614cm�1 (Group IV), and 3611 and 3563 cm�1 (Group V).

These bands correspond more or less to the ‘‘classic hydro-garnet’’ defect identified by Rossman & Aines (1991) bythe bands at 3662 and 3598 cm�1 and Kurka et al. (2005)by the bands 3687, 3657 and 3600 cm�1. The other set ofbands is characterized by slower kinetics, normally occur-ring at lower wavenumbers including the bands at 3560and 3518 cm�1 (Group I), 3551 and 3585 cm�1 (Group II),3641, 3586 and 3566 cm�1 (Group III), 3642 and 3565cm�1 (Group IV), and 3633 and 3582 cm�1 (Group V).

Table 4. Hyperfine parameters of Mossbauer spectra of grossular-andradite garnets.

Fe2þ Fe3þ

Temp. IS DQ FWHM Int. IS DQ FWHM Int.Sample (�C) (mm/s) (mm/s) (mm/s) (%) (mm/s) (mm/s) (mm/s) (%)

KPK 54-11 700 1.091 2.186 0.116 4.5 0.397 0.575 0.116 95.6KPK54-11H 700 1.079 2.226 0.111 3.4 0.396 0.591 0.111 96.6KPK 54-11R 700 1.108 2.214 0.111 5.1 0.394 0.575 0.111 94.9KPK 56-12-3 700 – – – – 0.400 0.551 0.126 100.0KPN 11 700 – – – – 0.389 0.584 0.194 100.0KTK 07 700 – – – – 0.392 0.584 0.194 100.0KPK 39-1-1 800 1.121 2.275 0.113 5.2 0.397 0.565 0.113 94.8KPK 39-1-1H 800 – – – – 0.401 0.550 0.127 100.0KPK 39-1-1R 800 – – – – 0.397 0.557 0.121 100.0KPK 54-9 800 – – – – 0.398 0.550 0.147 100.0KPK 54-9H 800 – – – – 0.400 0.546 0.140 100.0KPK 54-9R 800 – – – – 0.403 0.553 0.159 100.0KPK 56-12-9 800 – – – – 0.397 0.565 0.113 100.0KPK 56-12-9H 800 – – – – 0.399 0.557 0.120 100.0KPK 56-12-9R 800 – – – – 0.399 0.558 0.127 100.0KPN 09 800 – – – – 0.392 0.572 0.136 100.0KTK 09 800 – – – – 0.388 0.577 0.135 100.0KPK 54-10 900 1.118 2.220 0.120 2.4 0.396 0.572 0.120 97.6KPK 54-10R 800 – – – – 0.391 0.582 0.124 100.0KPK 56-12-2 900 – – – – 0.396 0.396 0.115 100.0KPN 10 900 – – – – 0.401 0.550 0.127 100.0KTK 05 900 – – – – 0.391 0.575 0.123 100.0

IS, isomer shift; DQ, quadrupole splitting; FWHM, full width at half maximum; Int, intensity in % of total absorption area; H, afterDehydration; R, after Rehydration.FWHM values reported after subtraction of source line width.Fe2þ doublets are assigned to diopside inclusions.

Fig. 9. Mossbauer spectra showing only ferric iron in sample KPN 09 (a); and both ferric and ferrous iron doublets (thin solid line) in sampleKPK 54-11 (b). The ferrous iron doublet is assigned to diopside inclusions.


The differences in dehydration kinetics indicate that twogroups of absorption bands are associated with OH defectswith different bonding character. However, a further pos-sibility that might cause the apparently different kineticbehaviour is thermal re-ordering of hydrogen. Cationordering is well-known to occur among octahedral sitesin several ferromagnesian silicates at temperatures similarto the present experiments (e.g., Skogby, 1992). The moremobile H ion can also be expected to order between dif-ferent associated defects in the crystal structure as a func-tion of temperature, and recently Yang & Keppler (2011)showed this to be the case for OH defects in olivine by insitu measurements of infrared spectra at elevated tempera-tures. For the present samples it is possible that theobserved difference in kinetic behaviour is partially causedby redistribution of hydrogen from local environmentsassociated with high-wavenumber bands to those asso-ciated with low-wavenumber bands.

The diffusion coefficients derived from the H extractionexperiments of the grossular-andradite in this studybroadly fall in the same range as those for grossular, pyropeand other silicate minerals, as can be seen in the Arrheniusplot in Fig. 11. However, the dehydration kinetics for thepresent samples are significantly higher than for grossularfrom Madagascar (Kurka et al., 2005), and more similar tothat obtained for mantle pyrope (Blanchard et al., 2004b).

In their study of grossular, Kurka et al. (2005) noted thatthe evolution of hydrogen concentration versus time couldnot be properly modelled for experiments at lower tem-peratures. In particular, the dehydrogenation rate appearsto decrease at the lower concentrations obtained after longexperimental times. We observe a similar evolution forseveral of our samples. The explanation for this behaviourcould be that the diffusion coefficients are dependent onhydrogen concentration as suggested by Wang et al. (1996)for pyrope, or that the different OH defects causing thecomplex grossular-andradite IR spectra have differentkinetics, as favoured by Kurka et al. (2005). It is difficultto determine which of the two effects is significant for thepresent samples.

A key aspect regarding hydrogen exchange reactions insilicates is how charge balance is maintained. The redoxreaction Fe2þ þ OH� ¼ Fe3þ þ O2� þ ½H2 has beenshown, or assumed, to be central for hydrogen extraction/incorporation in several studies on natural samples, includ-ing diopside (Skogby & Rossman, 1989; Ingrin et al., 1995),olivine (Mackwell & Kohlstedt, 1990) and grossular (Kurkaet al., 2005). Analytical Fe redox data for garnets are veryscarce, but in a study on grossular Forneris & Skogby (2004)showed that the Fe redox reaction is indeed active, but is notthe only mechanism that provides charge balance for hydro-gen exchange. For the present samples, the Mossbauer

Fig. 10. Mossbauer spectra of sample KPK 54-10 (a) and sample KPK 54-11 (b) showing ferrous iron doublets (thin solid line) before andafter heat-treatment (H ¼ hydrogen extraction and R ¼ rehydration experiments). The ferrous iron doublet is assigned to diopsideinclusions.


spectra of the studied grossular-andradite samples show thatthey contain Fe only in the trivalent form. In well-resolvedMossbauer spectra the detection limit for different Fe oxida-tion states can be considered to be around 1 % of total Fe,which means that only a small amount of Fe2þ that mighttake part in a redox reaction could remain undetected. Forsample KPK 54-10, 1 % of total Fe corresponds to 0.017apfu (atom per formula unit, based on 24 oxygens). Thismeans that only 0.017 hydrogen apfu could be charge com-pensated by undetected Fe2þ following the redox reaction,which constitutes only 5 % of the total amount of hydrogenof 0.32 apfu. Likewise, the increase in Fe2þ that would beexpected if the redox reaction was active during the rehy-drogenation experiments should easily be detected. SampleKPK 54-10 took up more than 50 % of its original OHcontent during the heat treatments in hydrogen, which cor-responds to 0.16 H apfu. The same number of Fe2þ atomswould be formed following the redox reaction, which wouldresult in a Fe2þ doublet in the Mossbauer spectrum corre-sponding to 10 %. For the few samples with considerablylower hydrogen or higher Fe contents the Mossbauer data isless conclusive, but we may conclude that hydrogenexchange does not occur via the redox reaction for themajority of our samples. Other mechanisms that can beconsidered to maintain charge balance is diffusion of pointdefects, e.g. cation vacancies, as has been suggested forolivine (Kohlstedt & Mackwell, 1998). However, the rela-tively fast kinetics observed for the dehydrogenation experi-ment in the present study is not in agreement with cation or

vacancy diffusion, which normally occurs with considerablyslower kinetics (e.g. Brady, 1995).

Acknowledgements: We thank H. Harryson at UppsalaUniversity for help with EPMA analyses and also J. Ingrinfor his comments. We are grateful for valuable reviews fromJ. Dubessy and an anonymous reviewer. This project wassupported by the Ministry of Education, Thailand, and theSwedish Research Council.

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Received 30 November 2010

Modified version received 8 April 2011

Accepted 23 August 2011


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