Co-Ordination of Boron in Sillimanite

132 SHORT COMMUNICATIONS Kirkpatrick, R. J. (1981) In Kinetics of Geochemical Scarfe, C. M., Takahashi, E., and Yoder, H. S. (1980) Processes (A. C. Lasaga and R. J. Kirkpatrick, eds.). Carnegie Inst. Washington Yearb. 79, 290-6. Mineral. Soc. Am. Washington, DC, 321-98. Schnorr, W. (1915) Z. Kristallogr. 54, 289-331. Kohli, C. S., and Ives, M. B. (1972) J. Crystal Growth, Siesmayer, B., Heimann, R., Franke, W., and Lacmann, R. 16, 123-30. (1975). J. Crystal Growth, 28, 157-61. Kuo, L.-C., and Kirkpatrick, R. J. (1983) EOS, Trans. Smith, F. J. (1963) Physical Geochemistry. Addison Amer. Geophys. Union, 64, 349. Wesley, Mass., 624 pp. Lacmann, R., Franke, W., and Heimann, R. (1974) J. Tsukamoto, K., Abe, T., and Sunagawa, I. (1983) J. Crystal Growth, 26, 106-16, 117-21. Crystal Growth, 63, 215-18. Lofgren, G. (1980) In Physics of Magmatic Processes Watson, E. B. (1982) Contrib. Mineral. Petrol. 80, 73-87. (R. B. Hargraves, ed.). Princeton Univ. Press, 487- Wells, A. F. (1946) Phil. Mag. 37, 605-30. 551. --(1983) J. Petrol. 24, 229-55. [Manuscript received 3 May 1984] Nicolas, A., and Prinzhofer, A. (1983) Ibid. 24, 288- 306. © Copyright the Mineralogical Society KEYWORDS: crystal growth, igneous rocks, magmas. Department of Geology, University of St Andrews, Fife KY 16 9ST, Scotland C. H. DONALDSON MINERALOGICAL MAGAZINE, MARCH 1985, VOL. 49, PP. 132-135 Co-ordination of boron in sillimanite ION-MICROPROBE analyses of six sillimanites em- 1 in the E // c polarization and 1317 em- 1 in associated with kornerupine show that the silli E //a. These bands, which are less conspicuous in manite can incorporate from 0.035 to 0.43 wt. % the boron-poor sillimanite 3083D (0.035% B20 3) B2 0 3 (Grew and Hinthorne, 1983). Boron appears and are weak or absent in the Antarctic sillimanite, 1 to substitute for silicon concomitantly with Mg lie in the range 1100 to 1450 em - in which B03 substitution for AI such that the atomic Mg/B ratio groups absorb (Moenke, 1974; Weir and Schroeder, is close to 0.5. This substitution results in a 1964; Weir, 1966). Absorption due to B04 groups deficiency of cationic charge, which Grew and lies in the 1180 to 800 em - 1 region. Unfortunately, Hinthorne (1983) attributed to a submicroscopic this region is largely inaccessible under our experi rearrangement of the sillimanite structure involv mental conditions due to the opacity of the samples 1 ing loss of oxygen. A possible substitution scheme is at wave numbers below 1050 em- . We suggest 2(B + xMg) --+ 2(Si + xAl) + (1 + x)O, where x ~ 0.5. that the observed spectral features between 1372 In the present study, we have addressed the ques and 1128 em - 1 in the two boron-bearing silli tion of co-ordination of boron in sillimanite. As manites are due to B in threefold co-ordination. boron can occur in trigonal or tetrahedral co The B-0 polyhedron could be either a triangle in ordination with oxygen, there is no compelling which the B is coplanar with the three oxygens, or a reason that B substitution for Si implies tetrahedral highly distorted tetrahedron, in which the B lies co-ordination for B. slightly out ofthe plane of the three closest oxygens Two of the boron-bearing sillimanites analysed and is weakly bonded to a fourth oxygen extended by Grew and Hinthorne (1983), samples E 2724 and from the boron: B03 -0. 3083D from South India (these authors' samples The absorption features appear not to be caused no. 2 and 4), and a sillimanite from a pegmatite by submicroscopic inclusions in sillimanite ofboro lacking borosilicate minerals from Antarctica, silicates with B03 groups such as grandidierite sample 556 (no. 8, see also Grew, 1980; Rossman et (which is present as a constituent in E 2724), al., 1982), are in fragments suitable for single crystal tourmaline (present in 3083D), or dumortierite. infra-red studies. Spectra were obtained on (010) M. A. Carpenter (written communication, 1983) cleavage fragments// c and// a with a Perkin Elmer has examined a small sample of sillimanite E 2724 180 spectrometer (fig. 1). Prominent absorption with the transmitting electron microscope (TEM) bands in the B-rich sillimanite E 2724 (0.42% B2 0 3) and did not observe any submicroscopic features are centred at 1372, 1327, 1281, 1244, and 1128 that could represent incipient exsolution of a B-rich SHORT COMMUNICATIONS 133 WAVELENGTH, JJm TABLE I. Intensity ratios of infra-red absorption 7 8 9 features attributed to boron in sillimanite -+/ ~---------T- SILLIMANITE V I 0f~ ANT ARCTIC A I 556 3083D E 2724 w u I Sample no. locality Antarctica India India z 0 8 <! I co 1 ~ 0 6 I Absorption (em - ) 0 (j) J 1244 0.239 1.0 off scale co 0 4 I <! / 1281 0.376 1.56 10 0 2 ~ 1128 too weak 1.49 10 1327 1.10 10 1600 1400 1200 1000 8 9 wt.% B2 0 3 SILLIMANITE Infra-red* 0.01 0.063 0.42 I 0 INDIA Ion Microprobet 0.035 0.42 w u z 0 8 <!co * Estimated from most reliable features and scaled to ~ 0 6 0 the 0.42 % B 0 analysed in E 2724. (j) 2 3 co 0 4 t Estimated precision is ±0.04 forE 2724 (Grew and <! Hinthorne, 1983). 0 2 Dash: not measured. 1600 1400 1200 1000 -I WA'/ENUMBER, em FIG. l. Single crystal infra-red spectra of sillimanite However, a B-bearing sillimanite from the korne (sample No. 556) from Reinbolt Hills, Antarctica (top) and rupine rock of Waldheim, Saxony, has an atomic of sillimanite (sample no. E 2724) from Paderu, India Mg/B ratio of 0. 7 and contains only 0.33 wt. % (bottom). Crystals about 34t-tm thick. Polarizations: solid Fe2 0 3 (Table II). Thus oxidizing environments, line E // c; dashed line E //a. which result in sillimanite Fe2 0 3 contents above 1 wt. % (e.g. Grew, 1980), are not essential for B incorporation in sillimanite. Moreover, B incor poration in sillimanite appears not to involve phase from sillimanite. However, Carpenter hydroxyl, for none was detected in an infra-red cautions that isolated, widely dispersed inclusions trace from 3000 to 4000 em - 1 obtained on a slice of of a B-rich phase would probably escape detec E 2724 100 J.lm thick (sensitivity, 0.01% H 0). tion by the TEM. A further indication that the 2 spectral features are caused by B in the silli manite is the correlation of their intensities with B2 0 3 contents analysed with the ion microprobe TABLE II. Composition of sillimanite from Wald (Table 1). heim kornerupine rock (sample no. 5105c) The pattern for the Antarctic sample indicates that traces of boron are present even in sillimanite from a rock lacking borosilicates. As Pearson and Recalculated Weight% to three cations Shaw (1960) and Evers and Wevers (1984) report 20 to 180 ppm B (equivalent to 0.006 to 0.06 wt. % Si02 35.96 0.978 B 0 ) in sillimanite from environments not en 2 3 Al 2 0 3 62.02 1.988 riched in boron, it appears that sillimanite may in Cr2 0 3 0.09 0.002 general contain traces of boron at the O.OX% Fez03 0.33 0.007 (B 2 0 3 ) level. MgO 0.27 0.011 The crystal-chemical mechanism by which B is Bz03 0.32 0.015 incorporated in sillimanite remains an open ques Total 98.99 3.0 tion. The analytical data of Grew and Hinthorne (1983) suggest that B substitution is coupled with Electron microprobe analysis, except boron, which was Mg, the atomic Mg/B ratio is close to 0.5, and that analysed with ion microprobe by method of Grew and sillimanites richer in boron (0.30-0.43 wt. % B20 3) Hinthorne (1983). Ti02, MnO, Na2 0, K 2 0, and CaO ~ also contain significant Fe20 3 (1.29-1.39 wt. %). 0.03%. 134 SHORT COMMUNICATIONS Fluorine was not detected with the ion micro oriented such that the c axis of grandidierite is probe in any of the sillimanites analysed by Grew parallel to the c axis of sillimanite, and a or b of and Hinthorne (1983) or in the Waldheim silli grandidierite parallel to [110] of sillimanite. Thus manite. One mechanism for B incorporation as the B03 groups, which are oriented nearly parallel B03 or B03 -0 groups is rearrangement of the to (010) in grandidierite (Stephenson and Moore, sillimanite structure in the immediate vicinity of the 1968), would lie roughly parallel to (110) in silli boron-oxygen group to resemble the grandidierite manite. In grandidierite two B-0 bonds make a 1 1 11 stucture, (Mg,Fez+)v Alv Ali Si vW 0 9 , which was small angle to c, while the third is perpendicular. refined by Stephenson and Moore (1968). McKie This orientation could result in the abundant (1965) first noted that sillimanite and grandidierite absorption features in E // c and fewer features in have similar cell dimensions. However, Stephenson E //a present in our patterns. However, of the and Moore (1968) suggested that the grandidierite four sharp bands von Knorring et a/. (1969) and structure was more closely related to andalusite, a Povarennykh (1970) attributed to trigonal boron in suggestion consistent with infra-red data (Pova spectrometer tracings of powdered grandidierite, rennykh, 1970). namely 1465-1458 em -t (medium intensity), 1415- The structures of grandidierite and sillimanite 1407 cm- 1 (strong), 1375-1370 cm- 1 (medium), contain chains of Al06 octahedra parallel to c and and 1315-1308 em -t (strong), only two are close to have a nearly identical c axis repeat (5.760 A for bands in the sillimanite pattern, namely 1372 em -t grandidierite, McKie, 1965, vs.

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