A New Cation Ordering Pattern in Amesite-2H2
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American Mineralogist, Volume 66, pages 185-195, 1981 A new cation ordering pattern in amesite-2H2 CYNTHIA S. ANDERSON AND S. W. BAILEY Department of Geology and Geophysics, University of Wisconsin-Madison Madison, Wisconsin 53706 Abstract The crystal structure of amesite-2H2 from the Saranovskoye chromite deposit, North Urals Mountains, USSR, was refined in space group C1 to a residual of 5.7% with 1719 independ- ent reflections. The study was undertaken to resolve conflicting interpretations of the optics, twinning, and infrared spectra of amesite from the Urals relative to data reported previously for amesite from Antarctica. There is nearly complete ordering of tetrahedral and octahedral cations in the Urals specimen, but the pattern of ordering differs from that found for the same polytype of amesite from Antarctica. Tetrahedral ordering of Si,AI in the Urals specimen preserves the identity of the ideal space group P63, but octahedral ordering of Mg,Allowers the symmetry to PI. Tetrahedra lying on the pseudo-63 axis are all Si-rich. Octahedral Al is in the B site of each layer, but the degree of octahedral order is slightly different in the two layers. Local charge balance between adjacent layers is achieved by localization of all tet- rahedral and octahedral Al in spirals around lines that parallel the Z axis and are spaced at intervals of (a + b)/2. We postulate that the presence, pattern, and degree of ordering can account for the observed sector and polysynthetic twinning, for differences in the bonding and geometry at the two interlayer junctions, and for the slightly tric1inic shape of the unit cell. The possibility of other ordering patterns is examined. The normal assumption that all crystals of the same mineral or the same polytype have the same ordering pattern, crystalliza- tion conditions being similar, is not necessarily valid. Introduction suggest that the difference in twinning may result from a difference in ordering patterns in the two sam- This study was undertaken because of conflicting pIes, rather than from the presence or absence of or- evidence as to the likelihood of cation ordering in dering. amesite-2H2 from the Saranovskoye chromite deposit Preliminary examination of 30 crystals from the in the North Urals Mountains, USSR. Hall and Bai- Urals sample (NMNH #103312) indicates that sev- ley (1979) cite the biaxial optical nature and sector eral polytypes are present. The 2H2 polytype is most twinning of the Urals amesite as evidence of lower abundant, but 6R2, 6R1, and 2Hl polytypes also are symmetry than the ideal space group of P63. They at- present (terminology of Bailey, 1969, and of Hall et tribute this reduction in symmetry to cation ordering al., 1976). X-ray photographs reveal that some crys- by analogy with amesite-2H2 from Antarctica, which tals have a disordered stacking of layers, in that the they determined by structural refinement to have k :F3n reflections appear as coalesced streaks and not nearly complete ordering of tetrahedral and octahe- as individual spots. All crystals examined with dral cations. Serna et ale (1977), however, concluded crossed nicols under the petrographic microscope are that the Urals amesite was disordered on the basis of noticeably biaxial and twinned. their study of the infrared patterns of a series of syn- The 2H2 polytype has alternating interlayer shifts thetic and natural amesites. Hall and Bailey point of -b/3 and +b/3 and alternating occupancy of the I out one difference in the two samples that may be and II sets of octahedral positions in successive layers significant. The Urals crystals are twinned in more (equivalent to 1800 rotations). Steinfink and Brunton complex patterns than the Antarctic crystals, in that (1956) studied amesite-2H2 from Saranovskoye. They polysynthetic twin lamellae parallel to the (010) attributed the biaxial nature to strain, and assumed prism edges are present in some crystals in addition the ideal hexagonal symmetry of P63 in their refine- to the ubiquitous 6-fold sector twins on (001). They ment. With this assumption, they determined the cat- 0003-004Xj81jOl02-0185$02.00 185 186 ANDERSON AND BAILEY: CATION ORDERING IN AMESITE ion distribution to be random. The present refine- tering factors for the cation sites were adjusted ap- ment assumes triclinic symmetry. propriately. Repeated cycles of refinement, using sigma weights and varying only the positional atomic Experimental coordinates, reduced the residual to R. = 11.8%. An untwinned sector was excised from a crystal of At this point, four of the eight hydrogen atoms amesite-2H2 that showed sector twinning without were located successfully by use of difference elec- polysynthetic lamellae. The shape of this sector ap- tron density maps. The hydrogen atoms were recog- proximates a parallelogram with sides 0.25 X 0.5mm nized as spherical volumes of excess electron density, and thickness 0.08 mm. An electron microprobe anal- representing Y3to ~ electrons, situated near predicted ysis gives a structural formula (Mg1.936Alo.943Fe~~2S locations. Cro.o74Do.o22)(Sil.o27Alo.973)Os(OH)4 on the basis of Continued refinement, varying first isotropic B val- seven formula oxygens. ues and later anisotropic B values for all atoms ex- Although the ideal space group of P63 is hexago- cept hydrogen, reduced the residual to R. = 6.1 %. nal, the intensity distribution on X-ray photographs The remaining four hydrogen atoms then were lo- shows that the true symmetry is triclinic. The struc- cated successfully by means of difference electron ture was refined using an orthohexagonal cell of density maps. Weissenberg photographs were taken space group CI. This is the same space group used by of levels that contained reflections whose observed Hall and Bailey (1979) in their study of amesite from intensities were significantly different from the calcu- Antarctica. The true X and Y directions were picked lated intensities. Data for six reflections were dis- as being parallel to the optical extinction directions carded because the photographs indicated that the on (001). Unit-cell dimensions were obtained by observed intensities might be incorrect due to overlap least-squares refinement of 15 low- to medium-angle by significant white radiation streaks. reflections on an automated single crystal diffrac- Further least-squares refinement, initially varying tometer. The refined values are a = 5.307(1), b the hydrogen atom locations and then varying the 9.195(2), c = 14.068(3)A, a = 90.09(2)°, fJ = positional atomic coordinates for all the atoms along 90.25(2) 0, and y = 89.96(2) °. with the anisotropic B values for all but the hydrogen Data were collected on a Syntex P2. automated atoms, produced a final weighted residual of 5.7% single-crystal diffractometer using monochromatic (4.9% unweighted). The central hydrogen atom H(I) MoKa radiation. The 28: 8 variable scan mode was moved unreasonably close (o.4A) to its OH(I) host, used to collect 1719 independent reflections, aver- and was held constant during the final least-squares aged over all octants out to 28 = 60°. Crystal and cycles at the position indicated by the difference elec- electronic stability were checked after every 50 re- tron density map. The other central hydrogen atom flections by monitoring a standard reflection. Reflec- H(II) moved to within 0.7A of its OH(II) host dur- tions were considered as observed if I > 2(1(1)where I ing refinement. This position also is suspect, but was was calculated from I = [S-(B.+B2)/Br]Tn S being retained for bond length calculations. the scan count, B. and B2 the background, Br the ra- Final difference electron density maps were flat at tio of background time to scan time, and Tr the 28 the sites of all atomic coordinates, but had small scan rate in degrees per minute. Values of (1(1)were peaks nearby that might be due to slight anisotropic calculated from standard counting statistics. In- fJ misfit. Scattering factors from the International Ta- tegrated intensities were corrected for Lp and absorp- bles for X-ray Crysta'/Zography appropriate for 50% tion factors. ionization were used throughout the refinement proc- ess. Fortran programs ORFLS, SHELLS, and ORFFE Refinement were used for the refinement and the determination The structure ofamesite from Antarctica (Hall and of structural parameters and errors. The experimen- Bailey, 1979) was used as the starting point for a tal data,. final atomic coordinates, bond lengths, least-squares refinement of the data. The tetrahedral thermal ellipsoid orientations,. and other important cation T(I) was kept stationary in order to fix an ori- gin in this non-centro symmetric structure. The sense of the Z axis was determined by comparison of ob- ·Tables 1 and 4 of observed and calculated structure amplitudes served and calculated structure amplitudes. A few and of thermal ellipsoid orientations may be obtained by ordering document AM -81-146 from the Business Office, Mineralogical So- cycles of refinement indicated that the ordering pat- ciety of America, 2000 Florida Avenue, N. W., Washington, D. C. terns for the two amesites are different, and the scat- 20009. Please remit $1.00 in advance for the microfiche. ANDERSON AND BAILEY: CATION ORDERING IN AMESITE 187 Table 2. Final atomic coordinates for amesite-2H 2 from Saranovskoye a 8 Atom Bcqui v 811 822 33 ~12 813 '}~ ( ) ] ( ) T(l) 0 0.0136(7) 0.0035(2) o . 00 14 1 o . 00 3 3 0.0003(2) 0 . 000 1(1 ) 0 0.0410 I. 27 ) - T(2) 0.0023(5) 0.3339(3) 0.0425(2) 1.24 0.0122(7) 0.0031(2) o. 0016 (1) 0.0008(3 0.0005(2) -0.0002(1) (1) ( ) T(ll) 0.0136(4) 0.0058(2) 0.5410(1) 1.29 0.0137(6) 0.0031(2) 0.0016 0.0012(3) 0.0003(2) - 0 .