Ferrobustamite, Wo82Fs18, from the Ofuku mine, Yamaguchi Prefecture, Japan, is close to the Fe-poor end of the ferrobustamite solid solution. Its crystal data are: space group A1, a = 7.862, b = 7.253, с = 13.967 A, α = 89°44' β = 95°28' γ = 103°29', and 12[Ca0.82Fe0.15Mn0.03SiO3] in the unit cell. Our refinement of its structure reveals that it possesses pseudo-mirrors which are more nearly true mirror planes than those in the Wo50Fs50 structure, and the small size of one cation site, М3, in the structure induces the concentration of the Fe(Mn) content into it, thus giving the solid solution the Fe-poor limit of Wo83.3Fs16.7. The Mössbauer spectra of the specimen showed that, after a heat treatment at 1,140°C, the degree of concentration of Fe at М3 decreases, suggesting that the site occupancies of Fe atoms vary with the temperature of formation and the cooling process of the crystal.

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