Phys Chem Minerals (1997) 25:39±47 Springer-Verlag 1997 ORIGINAL PAPER H. Yang ´ R.M. Hazen ´ L.W. Finger ´ C.T. Prewitt R.T. Downs Compressibility and crystal structure of sillimanite, Al2SiO5, at high pressure Received December 16, 1996 / Revised, accepted February 26, 1997 Abstract The unit-cell dimensions and crystal structure for metamorphic rocks, because of their abundance in of sillimanite at various pressures up to 5.29 GPa have metamorphosed pelitic sediments and simple pressure- been refined from single-crystal X-ray diffraction data. temperature phase equilibrium relations. From the crys- As pressure increases, a and b decrease linearly, whereas tal-chemical point of view, the Al2SiO5 polymorphs are c decreases nonlinearly with a slightly positive curvature. of particular interest because Al occurs in three different The axial compression ratios at room pressure are coordinations; in addition to one-half of total Al that is ba:bb:bc=1.22:1.63:1.00. Sillimanite exhibits the least octahedrally coordinated in each structure, the remaining compressibility along c, but the least thermal expansivity Al is four-, five-, and six-coordinated in sillimanite, and- along a (Skinner et al. 1961; Winter and Ghose 1979). alusite, and kyanite, respectively. Therefore, a detailed The bulk modulus of sillimanite is 171(1) GPa with knowledge of the responses of three polymorphic struc- K©=4 (3), larger than that of andalusite (151 GPa), but tures, especially the different types of Al-O bonds, to high smaller than that of kyanite (193 GPa). The bulk moduli temperature and pressure is essential to the understanding of the [Al1O6], [Al2O4], and [SiO4] polyhedra are of the crystal chemistry, physical properties, and phase re- 162(8), 269(33), and 367(89) GPa, respectively. Compar- lations of the Al2SiO5 polymorphs. The high-temperature ison of high-pressure data for Al2SiO5 polymorphs reveals crystal-chemistry of sillimanite, andalusite, and kyanite that the [SiO4] tetrahedra are the most rigid units in all was investigated by Winter and Ghose (1979). Ralph et these polymorphic structures, whereas the [AlO6] octah- al. (1984) and Yang et al. (1997a) studied the pressure ef- edra are most compressible. Furthermore, [AlO6] octa- fects on the crystal structures of andalusite and kyanite, hedral compressibilities decrease from kyanite to sillima- respectively. In this paper, we report a high-pressure X- nite, to andalusite, the same order as their bulk moduli, ray structure study of sillimanite up to 5.29 GPa and com- suggesting that [AlO6] octahedra control the compression pare our results with those for andalusite and kyanite to of the Al2SiO5 polymorphs. The compression of the provide a better understanding of the high-pressure crys- [Al1O6] octahedron in sillimanite is anisotropic with the tal-chemistry of the Al2SiO5 system. longest Al1-OD bond shortening by 1.9% between The crystal structure of sillimanite was first determined room pressure and 5.29 GPa and the shortest Al1-OB by Taylor (1928) using X-ray diffraction intensities esti- bond by only 0.3%. The compression anisotropy of silli- mated from rotation photographs. Further structure refine- manite is primarily a consequence of its topological an- ments were carried out by Burnham (1963) and Winter and isotropy, coupled with the compression anisotropy of Ghose (1979) from single-crystal X-ray diffraction data. the Al-O bonds within the [Al1O6] octahedron. Neutron diffraction studies of the sillimanite structure were performed by Finger and Prince (1972) and Peterson and McMullan (1986). Summaries of crystal structural studies Introduction on three Al2SiO5 polymorphs have been given by Papike and Cameron (1976), Ribbe (1980), and Kerrick (1990). The Al2SiO5 polymorphs, sillimanite, andalusite, and kya- nite, have been considered as primary thermobarometry H. Yang ()) ´ R.M. Hazen ´ L.W. Finger ´ C.T. Prewitt Experimental procedures Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015-1305 Room-pressure X-ray diffraction data measurements R.T. Downs Department of Geosciences, University of Arizona, Tucson, The sample used in this study is from Reinbolt Hills, Antarctica, Arizona 85721, USA and was kindly supplied by Pete J. Dunn of the Smithsonian Insti- 40 Table 1 Crystal data and other relevant information on sillimanite at various pressures P (GPa) a () b () c () V (3) Total Refls. R (int.) Rw R refls. >3 s (I) 0.00* 7.4857 (8) 7.6750 (9) 5.7751 (7) 331.80 (7) 532 409 0.036 0.033 0.65 7.4803 (13) 7.6600 (9) 5.7687 (11) 330.54 (6) 1.23* 7.4732 (14) 7.6520 (9) 5.7631 (12) 329.56 (7) 1134 198 0.027 0.034 0.034 1.96 7.4595 (13) 7.6372 (9) 5.7580 (11) 328.03 (7) 2.54* 7.4537 (11) 7.6238 (8) 5.7560 (10) 327.09 (6) 1112 195 0.030 0.027 0.029 3.22 7.4446 (12) 7.6094 (8) 5.7515 (11) 325.82 (7) 3.72* 7.4345 (10) 7.5989 (7) 5.7507 (10) 324.88 (5) 1100 192 0.029 0.032 0.034 4.61 7.4224 (12) 7.5836 (8) 5.7462 (10) 323.44 (6) 5.29* 7.4146 (12) 7.5739 (7) 5.7450 (10) 322.61 (6) 1086 185 0.028 0.029 0.030 * X-ray intensity data were collected at these pressures Table 2 Atomic positional and isotropic displacement parame- P (GPa) 0.00 1.23 2.54 3.72 5.29 ters of sillimanite at various pressures Al1 x 0 0 0 0 0 y 0 0 0 0 0 z 0 0 0 0 0 Biso 0.37 (2) 0.34 (3) 0.31 (3) 0.31 (3) 0.32 (3) Al2 x 0.1417 (1) 0.1414 (2) 0.1411 (2) 0.1409 (2) 0.1405 (2) y 0.3452 (1) 0.3444 (2) 0.3437 (2) 0.3434 (2) 0.3427 (2) z 0.25 0.25 0.25 0.25 0.25 Biso 0.44 (2) 0.47 (4) 0.38 (4) 0.39 (4) 0.41 (4) Si x 0.1532 (1) 0.1536 (2) 0.1541 (2) 0.1543 (2) 0.1548 (2) y 0.3404 (1) 0.3394 (2) 0.3395 (2) 0.3389 (2) 0.3386 (2) z 0.75 0.75 0.75 0.75 0.75 Biso 0.42 (2) 0.43 (4) 0.44 (3) 0.43 (4) 0.47 (3) OA x 0.3602 (2) 0.3611 (5) 0.3618 (5) 0.3620 (5) 0.3633 (5) y 0.4089 (2) 0.4069 (5) 0.4062 (5) 0.4060 (5) 0.4055 (5) z 0.75 0.75 0.75 0.75 0.75 Biso 0.52 (3) 0.60 (8) 0.59 (8) 0.54 (8) 0.53 (8) OB x 0.3566 (2) 0.3554 (5) 0.3557 (4) 0.3559 (5) 0.3551 (5) y 0.4339 (2) 0.4348 (5) 0.4336 (5) 0.4334 (5) 0.4330 (5) z 0.25 0.25 0.25 0.25 0.25 Biso 0.55 (3) 0.48 (8) 0.39 (7) 0.51 (8) 0.60 (8) OC x 0.4767 (2) 0.4758 (5) 0.4748 (5) 0.4745 (5) 0.4728 (5) y 0.0009 (2) 0.0010 (7) 0.0021 (7) 0.0028 (7) 0.0031 (7) z 0.75 0.75 0.75 0.75 0.75 Biso 0.91 (4) 0.87 (8) 0.81 (7) 0.78 (8) 0.76 (8) OD x 0.1255 (2) 0.1255 (3) 0.1262 (3) 0.1266 (3) 0.1269 (3) y 0.2232 (2) 0.2228 (3) 0.2219 (3) 0.2211 (3) 0.2204 (3) z 0.5145 (2) 0.5150 (7) 0.5141 (7) 0.5152 (7) 0.5146 (7) Biso 0.63 (3) 0.47 (5) 0.56 (4) 0.51 (5) 0.53 (5) tution [specimen no: NMNH 139740; also sample no. 566 de- powder X-ray diffraction: a=7.4885(7), b=7.6756(3), and scribed by Grew (1980)]. Because of its good quality, this sample c=5.7734(5) . has been used for optical absorption studies (Grew and Rossman X-ray diffraction intensity data from one octant of reciprocal 1976), determination of elastic constants (Vaughan and Weidner space with 0£2q£60 were collected using w scans of 1 width in 1978), infrared absorption studies (Kieffer 1979), and low- step increments of 0.025 and 3-s per step counting time. Two stan- temperature heat-capacity measurements (Robie and Hemingway dard reflections were checked every 5 hours; no significant or sys- 1984; Hemingway et al. 1991). The chemical composition of the tematic variations in intensities of the standard reflections were ob- sample reported by Grew (1980) is SiO2, 36.73; Al2O3, 61.98; served. Digitized step data were integrated by the method of Leh- Fe2O3, 1.10 (wt.%) or (Al1.98Fe0.2)SiO5. A single-crystal fragment mann and Larsen (1974) with background manually reset when nec- (0.110.080.04 mm) showing sharp diffraction profiles was select- essary. Corrections were made for Lorentz and polarization effects, ed from a crushed sample for the study. A Picker four-circle dif- but not for X-ray absorption by the crystal (m=10.95 cm±1). fractometer equipped with a Mo X-ray tube (b-filtered) was used The initial structural model of sillimanite was taken from Winter for all X-ray measurements. Unit-cell parameters were determined and Ghose (1979). Least-squares refinements were performed using by fitting the positions of 18 reflections with 20<2q<35 following an updated version of RFINE4 (Finger and Prince 1975) in the space the procedure of King and Finger (1979), yielding a=7.4857(8), group Pbnm.