FT-IR Spectroscopic Investigation of Hydrous ... - ScienceDirect

Goiidwctnn Reserirclr, 1! 2. No. I , p p . 89-94.

Gondwana Research

0 I999 Iiiteriicitioiicil Associtrtioii ,fhr Goiidwciiia Rrsccircli, J q i c i i i . ISSN: 1.342-937X

FT-IR Spectroscopic Investigation of Hydrous Components in Sillimanite from Eastern Ghat Granulite Belt, India R. V. Karanth', George Mathew' and T. K. Gundu Rao2

' Department of Geology, M. S, University of Buraclu,

Vadodura - 390 002, India. Regional Sophisticated Instrumentation Centre, IIT, Powai, Mumbai - 400 076, India. (Manuscript raceiiwcl April 21, 1998; uccepted August 5, 1998)

Abstract FT-IR spectra of sillimanite samples from high grade regionally metamorphosed rocks belonging to the granulite terrain (amphibolite to pyroxene granulite facies) deciphers prominent OH features. Heating experiments indicate growth of prominent band at 3161cn~'.Heating above 1000°Call OH features disappear in intensity into broad features with slight shift of bands towards lower energies. Complete dehydration requires temperatures above 1000°C. Coexistence of boron and OH features are also observed in all sillimanite samples. The high temperature behaviour of sillimanite from the granulite terrain discerns that the hydrous species in sillimanitc were incorporated much below 700"C, however, secondary hydration due to pegmatite activity, retrograde metamorphism and migmatisation is not ruled out. Thus a near anhydrous condition were probably not achieved during the granulite facies metamorphism in Eastern ghat granulite terrain. Keywords: FT-IR, Sillimanite, hydrous, boron, granulite, metamorphism.

Introduction The Presence of trace water in silicates influence substantial effect on the properties of minerals (Aines and Rossman 1985).Presence of hydroxyl absorption is known in all three AI,SiO, polymorphs (Wilkins and Sabine, 1973; Beran et al., 1983;Beran and Gotzinger, 1987).Howevkr, the mechanism of OH incorporation into the Al,SiO, structure is not well understood. Sillimanite analyses indicates upto 0.72% of H,O (Aramaki and Roy1963; Deer et a1.,1982). Halenius (1979)proposed Fez++ OH-substitution for A13++ 0" mechanism of OH incorporation in sillimanite,whereas Beran et al. (1983) suggested hydroxide incorporation in sillimanite as charge compensating process for aluminium deficiency. Beran et al. (1989) classified sillimanite OH spectral features mainly into two groups; type4 : along with characteristic 3556cm-' band, a prominent triplet at 3329, 3300 and 3248cm-', with one of the band of this triplet always more intense than the band at 3556cm-' and type-11: the band at 3556cm-' is most intense and components of triplet band are weak or absent. Some spectra do not fall into either of the above group.

In the present paper, the authors have made an attempt to observe whether the OH content really reflects the environment of formation as suggested by Beran et aL(1989) and their possible mode of incorporationin sillimanite.Since the hydrous components in minerals are mobile, the high temperature speciation and their site at higher temperatures would be different from that of room temperature. Therefore understanding the behaviour of hydrous components at higher temperatures provide insight to further apprehend trace water in geological conditions.The above studies were carried out using gem quality samples of sillimanite (fibrolite) obtained from high grade rocks belonging to the Eastern Ghat granulite belt, India. For comparison two samples belonging to hornblende granulite facies (Grew, 1982)from Tamil Nadu was also studied.

Experimental Details All the sillimanite samples shown in Table I are from high grade regionally metamorphosed rocks belonging to the granulite terrain. The rocks of this region have been intensively folded and subjected to deep seated palingenetic

R.V. KARANTH ET AL.

90

Table 1. Details of sillimanite samples obtained from various localities.

No

Colour

Locality

Facies

1. 2. 3.

Greenish yellow Greenish yellow Light brown

Karur (Tr~ichirapalli) Tiruchengodu (Salem) Bhadrachalam (Khammam)

4. 5. 6.

Brownish black Colourless Colourless

Korapu t Koraput (with needles like inclusions) Koraput (transparent)

Hornblende granulite Hornblende granulite Aniphibolite to pyroxene granulite transi tio Pyroxene granulite Pyroxene granulite Pyroxene granulite

migmatisation and hypograde granulite facies metamorphism (Narayanswami, 1975). Fourier Transform Infrared (FT-IR) spectra in the frequency 4000-2000~m-~ for single crystal and 4000400cn1-~for powder samples were recorded using NICOLET Magna IR 550 FT-IR spectrometer. Spectral resolution was set at 4cm-'. Background effects due to atmospheric H,O and CO, were subtracted. Polarised spectra were taken using a grid polarizer deposited on substrate of KRS-5, enclosed in a stainless steel ring. Samples used were inclusion free and were prepared as doubly polished (010) cleavage plates of about 0.1 to 2.0mm thick. However, brown and black samples contain fine needle like inclusions. Similar inclusions in chatoyant sillimanite from Sri Lanka have been identified as ilmenite, spinel and orthopyroxene (Woensdregt, 1990).

Results A variety of OH related features have been observed in the region 4000 - 3100cm-'.Neither detectable overtoneband of H20 at 5200cm-I nor absorbance in the bending mode region (1600 - 1400cm-') of H,O is observed, indicating hydrous species to be OH- group. As shown in Fig.1, the sillimanite samples from the granulite terrain can be classified into three groups: Type-I : those that yield intense 3247cm-' band with associated doublet bands and a moderately intense band around 36OOcm-' (sample 3). Type-II : Intense band around 3600cm-'with doublet/ triplet bands of lower intensity. The t y p e 4 is further divided into type-lln : intense band around 360Ocin-' and doublet/triplet bands around 3240cm-' being absent (sample 1);type-IIb : band around 36OOcm-' and doublet around 3240cm-' of equal intensity or doublet showing moderately intense feature (sample5) or moderately intense band around 3600cm-' with doublet bands at 3307 and 3245cm-' (sample 6). Bands at 2930 and 2854cm-' are due to surface adsorbance of water. The attributes of broad band at 2664cm-' is, however, not understood. Intense peak at 2304cm-' is attributed to asymmetrical vibration of CO,, since CO, is commonly encountered in the region 2400-2300cm-'(Aines

3362

I

3800

1

3000

1

220(

WAVENUMBER km") Fig.1.

FT-IR spectra of four sillimanite showing characteristic OH features along E I In polarisation. All spectra obtained from (010) plates of 1.0mm thickness. Absorbance scale of all four spectra are arbitrary.

and Rossman, 1984). The base of the peak is slightly enlarged and asymmetrical. Small side bands on either side of CO, peak are attributed to combination bands of both stretching and coupled hindered motion of CO, (Aines and Rossman,l984; Mathew et al., 1997). All sillimanite samples were subjected to a stepwise heating from 200°C to 1000°C with samples kept for two hours at each step and spectra were taken after returning to room temperature. Beran et al. (1989)indicated that there is Goridwcinci Rcsecirch, V 2. No. I , 1999

FT-IR SPECTROSCOPY OF SILLIMANITE, EASTERN GHATS

sample 4 2304

I

3?10

A

91

I

3161

w

0

z

3 CT EEm a

I

4000

I

3600

I

3200

I

2800

I

2400

WAVELENGTH (cm") Fig.2.

FT-IR spectra indicating step wise heating experiment of sample 4, showing loss in intensity of OH features at higher temperatures. Unpolarised spectra of l.Omm thick (010) plate.

weight loss at each step with smaller loss in IR intensity. The results of step heating experiment is shown in Fig. 2 (sample 4) and Figs.3a and b (sample 5 and 6). Stepwise heating from room temperature to 500°C does not indicate any significant change in intensity. The broad band centred around 3400cm-' due liquid H,O absorbance is lost only on heating above 500°C. After heating to 800°C for 2hrs, spectra shows significant change characterised by lowering in intensity of triplet bands around 3240cm-'. The band at 3161cm-', which at room temperature (25%) were observed as small hump progressively grows in intensity on heating above 700°C. This observation is in accordance with Beran et al. (1989). However, in addition to the 3161cm-' band, growth of a band is also seen at 3410cm-I. Band at 3562cm-' does not show any change in intensity, whereas 3307 and 3247cm-' bands retain half of their Goridwcinci Resenrcli, V 2, N o . I , 1999

intensity. As shown in Figs. 3a and b, on further heating at 800°C for 14hrs,the samples 5 and 6 shows further decrease in intensity of bands at 3307 and 3247cm-' and progressive growth of 3161cm-' band accompanied by slight decrease in intense band around 3600cm-'. Beran et al. (1989) have also observed that the growth of 3161cm-' band begins at 700°C and it progressively intensifies on further heating. They correlated such growth as reminiscent of the OH band in topaz, that grows at high temperature owing to site reequilibration. In topaz, however, hydroxyl sites interconvert on heating rather than a separate growth of band. It was observed in present samples that the band at 2664cm-' increased in intensity and 2304cm-' band shows a progressive decrease in intensity on heating to 800°C. On further heating to 9OO0C, OH absorption features around 3248cm-' band is transformed into a broad band.

R.V.KARANTH ET AL.

92

I 1040 "C(Zhrs)

sample 5

I!I

800 "C (14 hrs)

-1 W

0

z = l m oc 0

3 a

I

3800

I 3000

I

2200

WAVENUMBER (cm")

Fig.3.

3800

3000

2200

WAVENUMBER (cm")

Unpolarised FT-IR spectra depicting changes in OH features on heating of typical type-IIb sillimanite. Spectra shows growth of 3161 and 3410cm' on heating with progressive decrease in intensity of OH features. Shift and broadening on heating above 1000°C. Increase in intensity of 2664 and decrease of 2304cm' band in a) sample 5 and b) sample 6.

Further heating to 1040"C, all samples show decrease in intensity of band at 3557-3562cm-' into a broad feature accompanied by shifting towards lower wavenumber at 3450 and 3465cin-'.The broad feature at 3248cm-I also shifts towards 3117cm-' The band at 2664cm-' does not show significant change, however, 2304cm-' band show a decrease in intensity indicating some decarbonation. Heating above 1040°C was not carried out due to inaccessibility of high temperature furnace. The presence of CO, even at 1000°C elucidates tightly locked characteristics in the lattice.

Discussion In accordance with Beran et al. (1989) observation, the OH features are unrelated to various coloured varieties of sillimanite described by Rossman et al. (1982), in particular the dark brown and colourless varieties; since both uncoloured and light to dark coloured samples show OH spectral features. According to Lamp and valley, (1988) water activities during crystallisation were greatest during amphibolite facies metamorphism than in pyroxene granulite facies. Beran et a1.(1989)observed that the average intensities of OH content decrease from amphibolite facies to pyroxene granulite facies. However, in the present samples, a comparative intensities of 3557 - 3562cm-' and 3247cm-' band as listed below are: sample 1 (0.14, 0.00);

sample3 (0.155,0.225);sample 4 (0.15,0.12);sample 5 (0.295, 0.310) and sample 6 (0.039,0.22).It indicates that, intensity of OH spectral features is also greatest for pyroxene granulite facies. However, in all the four samples belonging to pyroxene granulite facies, the 3557-3562cm-'band is either more intense or nearly equal in intensity to 3247cm-' band rather than the typical type-I absorbance features. Sample 1 from Tamil Nadu was found to be an exception. Although this sample belongs to hornblende granulite facies (Grew, 1982) it shows intense 3557cm-' band with no OH spectral features around 3247cm-'(Fig. 1).The Tamil Nadu samples were almost opaque and hence the sample 1was thinned to l.Omm and sample 2 thinned considerably to 100pm so as to allow transmittance of IR. The sample 2 did not reveal any OH features except broad band of H,O centred around 3400cn~'.Various intense OH absorbance features observed in sillimanite also could be due to secondary hydration as water could be incorporated as submicroscopic lamellae of hydrated aluminosilicate along discrete planes in the sillimanite structure (Beran et al., 1989) Grew and Hinthorne (1983) indicated that rocks containing boron bearing sillimanite form at higher temperatures under low water activities. They also noticed that sillimanite associated with kornerupine can incorporate from 0.035 to 0.43wt% B203.Although the presence of B203 in the present study is not analysed chemically, its presence Goiidrvtrrici Reseirrclr,

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FT-IR SPECTROSCOPY OF SILLIMANLTE, EASTERN GHATS

is deciphered in the FT-IR spectrum in the range 1600 1000cm-' (Figs. 4a and b). Absorptions due to BO, groups show complex spectrum in the range 1180 - 900cm-' (Ross, 1974;Grew and Rossman, 1985),whereas BO, groups show absorbance features between 1450 - 1200cm-I.However, the region between 1100 - 800cm-' is largely inaccessible under our experimental conditions due to the opacity of the samples above 1050cm-'.The presence of strong absorption band at 1225cm-I(Figs. 4a and b) and associated features indicate the presence of 80, group in all samples rather than B0,as argued by Grew and Rossman (1985).They, however, suggested that the spectral features of boron are due to its presence in three fold co-ordination. The B - 0 polyhedron are either in the form of triangle with boron coplanar with the oxygen or highly distorted tetrahedron, where boron occupies slightly out of plane of the three oxygens and is weakly bonded to a fourth oxygen (B0,-0). A tetrahedrally co-ordinated boron does not absorb in the range 12001500cm-',whereas trigonally co-ordinated does (Ross,1974). Grew and Rossman (1985)inferred that, boron incorporation in sillimanite appears not to involve hydroxyl as no OH spectral features were detected on a slice of lOOpm thick sillimanite sample from Paderu, India. In the present study, the authors have observed that on thinning of the sample, intensity of OH also decreases especially for observing the range below 1400cm-I.Therefore, we presume that it was due to extreme thinning of sample that, Grew and Rossman (1985) noticed zero OH absorbance in Paderu sample from Vishakapatnam granulite terrain. Samples 4 to 6 from Koraput belonging to the pyroxene granulite facies are associated with Khondalite suite of rocks and yet contain intense OH features together with boron.

93

Thus invoking boron bearing sillimanite forming at high temperatures have low water activities does not seem true. Heating to 1040°C (Figs. 2, 3a and b) results in OH absorbance features to be in form of broad bands. The 3476cm-'band increases in intensity and shifts from 3476 to 3465cm-'. The growth band at 3161cm-' disappears and forms broad band centred around 3117cm-'. Broadening of band with progressive disappearance indicate that above 1OOO"C, the OH molecules are probably transformed into liquid H20 species. According to Freund (1974) just prior to dehydration, slight broadening of 0-H peaks is interpreted as evidence of proton tunnelling between OH sites. This is one way to form H20 molecules which are observed as dehydrated species. H,O molecules now would be free to diffuse rapidly out of the sample on further heating. High temperature behaviour of sillimanite hydrous species discerns that the hydrous species were probably trapped much below 700C, as above this temperature OH features should have shown an additional band at 3161cm-'.As observed by Beran et a1.(1989)sillimanite that form under lower temperature conditions does have type-I spectra, however, under high temperature conditions spectra can be either type-IIa or type-1%. Thus the above inference deduce that a near anhydrous conditionswere not achieved during the pyroxene granulite facies metamorphism in Eastern Ghat Granulite terrain. However, a secondary hydration is not ruled out, as incorporation of OH in these samples may also be due to, 1. late stage pegmatite activity; 2. high water activities along retrograde P-T path and 3. at some stage polymetamorphism and migmatisation. Thus deciphering the mechanism by which water is incorporated in sillimanite and further

1225

sample 3 (E II c)

1300

2.8

3557 3247

1.2-

r ; 1

I

I

I

I

1600

WAVENUMBER (em")

1500

1400 13w 1200 WAVENUMBER (crn")

1100

Fig. 4. Polarised spectra along E I I n and E I I c showing presence of intense band of BO, group at 1225cm' with associated small absorbance bands at 1282,1372 and 1390cm-'in a)sample 3 and b) sample 4. Gontlwnnn Resecirch, V 2, No. I , I999

94

R.V. KARANTH ET AL.

elucidating whether they a r e primary or secondary hydration would throw important insight into metamorphic petrology.

Acknowledgements The authors are grateful to Mr. U. R. Shenoy and Mr. A. Basha, Bangalore and Mr. Kuldeep Singh, Vishakhapatnam and Mr. V. K. Singh, Bhubaneshwar for providing gem quality samples for experiments. This work is part of research project No. ESS/23/009/94 provided by DST, New Delhi to RVK. Financial assistance to GM by CSIR (New Delhi) - Senior Research Fellowship (No.9/114/(100)/98/ EMR-I-KK) is also gratefully acknowledged.

References Aines, R. D. and Rossman, G. R. (1984) The high temperature behaviour water and CO, in beryl and cordierite. Amer. Mineral. v. 69, pp. 319-327. Aines, R. D. and Rossman, G. R. (1985) The high temperature behaviour of trace hydrogen components in silicate minerals. Amer. Mineral. v. 70, pp. 1169-1179. Aramaki, S. and Roy, R. (1963) A new polymorph of Al,SiOS and further studies in the system of Al,O,-Si0,-H,O., Amer. Mineral. v. 48, pp. 1322-1347. Beran, A., Hafner, St. and Zeeman, J. (1983) Utersuchungen uberden Einbau von Hydroxil gruppen im Edelstein sillimanit. Neus Jahrbuch fur Mineralogie Monatshefte, pp. 219-226. Beran, A. and Gotzinger, M. A. (1987) The quantitative IR spectroscopic determination of structural OH groups in kyanite. Mineral. and Petr. v. 36, pp. 41-49. Beran, A., Rossman, G. R. and Grew, E. S. (1989)The hydrous component of sillimanite. Amer. Mineral. v. 74, pp. 812-817.

Deer, W. A., Howie, R. A. and Zussman, J. (3 982) Rock forming minerals, Vol lA, Orthosilicates, Longman, London. Freund, F. (1974) Ceramics and thermal transformation of minerals. (In V. C. Farmer, Ed) The infrared spectra of minerals, Min. Soc. America, pp. 205-206. Grew, E. S. (1982) Sapphirine, kornerupine and sillimanite orthopyroxenes in the charnockiticregion of S. India. J. Geol. Soc. India. v. 23, pp. 469-505. Grew, E. S. and Hinthorne, J. R. (1983) Boron in sillimanite. Science v. 221, pp. 547-549. Grew, E. S. and Rossman, G. R. (1985)Coordination of boron in sillimanite, Mineral. Mag. v. 49, pp. 132-135. Halenius, Uf. (1979). Static and location of iron in sillimanite. Neus Jahrbuch fur Mineralogia Monatshefte, pp. 165-174. Lamp, W. M. and Valley, J. W. (1988) Granulite fxies amphibole and biotite equilibria and calculated peak metamorphic water activities. Contrib. Mineral and Petrol. v. 100, pp. 349360. Mathew, G., Karanth, R. V., Gundu Rao, T. K. and Deshpande, R. S. (1997) Channel constituents of alkali poor Orissan beryls : An FT-IR spectroscopic investigation. Curr. Sci. V. 73, pp. 1004-1111. Narayanswami, S. (1975)Proposal for chranockite-khondalite system in the Archean shield of Peninsular India. In “The Precambrian geology of the peninsular shield”. Geol. Surv. India. Misc. Pub. No. 23. Ross, S. D. (1974) Borates. (In V. C. Farmer, Ed) The infrared spectra of minerals, Min. Soc. America, pp. 205-226. Rossman, G. R., Grew, E. S. and Dollase, W. A. (1982)The colour of sillimanite. Amer. Mineral. v. 67, pp. 749-761. Wilkins, R.W.T. and Sabine, W. (1973) Water content in some nominally anhydrous silicates. Amer. Mineral. v. 58, pp. 508-516. Woensdregt, C. S. (1990)Electron microscopical investigations of oriented inclusions causing asterism in star diopside and star quartz, and chatoyance in cat’s eye sillimanite., Publ. Ph.D. thesis, pp. 105-126.

Gorrc~~viirzir Reseiirclr. c! 2, No. I , 1999