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Andalusite–kanonaite series: lattice and optical parameters

1982; Mineralogical Society of America; Volume: 67; Linguagem: Inglês

1945-3027

Mickey E. Gunter, F. Donald Bloss,

Mineralogy and Gemology Studies

For 20 crystals ranging compositionally between pure andalusite (Al2SiO5) and 62 mole percent kanonaite (MnAlSiO5), some containing Fe3*, cell edges a, b, and c were determined by back-reflection Weissenberg techniques, refractive indices for sodium light vibrating parallel to a, b, and c (symbolsi nu, n6, and n) were determined (to -+0.0005) by spindle stage methods, and the values for .r and y in the general formula Alr-^-v Mnl+Fe3y+AlSiO5 were determined from electron microprobe analyses. Multiple linear regressions disclosed that cell edges a and b increased more with .r than with y but that c increased with y but hardly at all with -x. These changes are inferred to result primarily from dimensional changes in the M(l) octahedra if occupied by Mn3* (Jahn-Teller distortion and elongation of the M(lFO(4) distance) or by Fe3* (enlargement without such distortion). Substitution of Fe3* increases fls, n6 tnd n. are non-linearly related to [,r + y]. Surprisingly, the resultant curves all intersect at or near a common point and near-isotropic andalusites may exist near [x + y] equal 0.066. The curve intersections explain the differences in optic orientation between andalusites (c : X) and kanonaites and viridines (c = Z). Relative to the {110} cleavage, therefore, andalusite is characterized by (-) elongation whereas kanonaites and viridines possess (+) elongation. Regressions performed on the data for I I specimens (= nn6ulusites) for which [x + y] < 0.05 predict the following properties for pure andalusite: a = 7.7923(3), b : 7.8965(2), c : 5.5534(4)A, D(calc) = 3.149 g/cm3, molar volume : 51.45(4) cm3 and (for 589.3 nm) the indices 1.6328(3), 1.6386(2) and 1.6436(4) with 2V(calc) = -3t.to. The indices most commonly cited by modern textbooks-l .629, 1.633 and 1.638-are for a (+) crystal whereas andalusite is (-). Ironically, they overlook those reported by Taubert (1905), namely, 1.6326, 1.6390 and l.&40.