Abstract

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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