Rosenhahnite, a new hydrous calcium silicate, forms narrow veins in brecciated, finegrained diopside-garnet metasedimentary rock in Mendocino County, California. It is colorless to buff and in cavities forms etched, lath-like crystals up to 5 mm in maximum dimension. The crystals are triclinic, and {010} is invariably the principal form. Cleavages are {001} perfect, {100} and {010} good. Hardness is 4.5-5, Refractive indices are α = 1.625, β = 1.640, γ — 1.646; 2V measured by universal stage is −64 ± 4°.

The formula is (CaSiO3)3·H2O with minor substitution of Ba for Ca.

The cell dimensions, are a=6.946, b = 9.474, c = 6.809, all ±0.006 Å, α=108°39′, β = 94°49′, γ = 95°43′; Z = 2, density (calc.) 2.905, (meas.) 2,89 ± 0.02. Morphological evidence does not permit a choice between space groups P1 and P1.

Rosenhahnite loses water very slowly between 400-500°C. Single crystals of rosenhahnite are coverted to single crystals of triclinic wollastonite in perfect topotactic relation. There is no indication of formation of intermediate phases, even though the composition of xonotlite lies between those of rosenhahnite and wollastonite.

Rosenhahnite is the natural counterpart of a phase reported by Pistorius (1963) to be stable at temperatures below' about 450°C and at H2O pressures above about 20 kilobars.

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