Abstract

The crystal structure of an Fe-free vesuvianite from Asbestos, Quebec, was determined. The sample has the tetragonal space group P4/n, unlike most reported vesuvianites, which have P4/nnc. This vesuvianite has cell parameters of a = 15.504(2) and c = 11.808(2) with a final R value of 5.48% for 3623 unique reflections with  
|F|0>3σ|F|0
. The crystal shows a weak acentricity and a number of violations of glide-plane extinction criteria. The formula is (Ca18.68Mn0.06Mg0.26)Al4AI(Al6.51Mg1.49)(Si17.49A10.51)O69(OH)9.0.

The arrangement of atoms along each fourfold axis is ordered and periodic, but it is also polar. The space group in vesuvianite is determined by the pattern of the two fourfold axes per unit cell. Gradation exists in vesuvianite from an ordered P4/n to a completely disordered P4/nnc vesuvianite. Domains with an ordered P4/nc pattern can account for the acentricity found in the Asbestos vesuvianite.

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