Andalusite-kanonaite series; lattice and optical parameters

Va. Polytech. Inst. and State Univ., Blacksburg, VA, United States

20 crystals ranging compositionally between pure andalusite (Al ₂ SiO ₅ ) and 62 mole percent kanonaite (MnAlSiO ₅ ), some containing Fe (super 3+) , 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 (symbols: n _a , n _b , and n _c ) were determined (to + or -0.0005) by spindle stage methods, and the values for x and y in the general formula Al (sub l-x-y) Mn (super 3+) _x Fe (super 3+) _y AlSiO ₅ were determined from electron microprobe analyses. Substitution of Fe (super 3+) increases n _a , n _b and n _c significantly more than does Mn (super 3+) . Regressions performed on the data for 11 specimens (= andalusites) 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) Aa, D(calc) = 3.149 g/cm ³ , molar volume = 51.45(4) cm ³ and (for 589.3 nm) the indices 1.6328(3), 1.6386(2) and 1.6436(4) with 2V(calc) = -85.5 degrees .--Modified Journal abstract.

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