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Ca6Si6O17(OH)2
Monoclinic, C2/m, a = 8.55, b = 7.34, c
= 7.03 Å, b = 90o, Z = 2
Xonotlite was described from Franklin by Bauer and Berman (1935) and Palache (1935), from which this description is largely taken; it has not been studied since, and it has not been found at Sterling Hill. Xonotlite has been mislabeled calciothomsonite.
Xonotlite occurs as colorless acicular crystals, up to 2 cm, which appear white in the aggregate. The crystals form matted white aggregates and radial clusters, some randomly arranged. Crystals forming on clinohedrite may have an epitactic relation, as yet unstudied. The luster is vitreous; the cleavage is parallel to the elongation; and the density is 2.71 g/cm3. Optically, xonotlite is biaxial, positive, with small 2V, a = 1.586, b = 1.586, and g = 1.594. The fluorescence in ultraviolet is moderate violet, stronger in shortwave than longwave. Although xonotlite resembles many species, the restricted and diagnostic paragenesis at Franklin, together with its fluorescence, serve to identify it unambiguously. Frondel (1972), on the basis of an unpublished study, has noted that many similar minerals are not present in this assemblage and that crystals fitting the description of xonotlite are indeed that species.
Xonotlite is a calcium silicate hydroxide mineral, related in structure to wollastonite. It has not been re-analyzed since the study of Palache (1935) which gave: SiO2 48.60, CaO 44.64, MnO 0.49, MgO 0.25, H2O 3.97, total = 97.95 wt. %.
Xonotlite occurs in vugs in a porous recrystallized assemblage consisting predominantly of hancockite, manganaxinite, prehnite, willemite, and garnet, and lesser amounts of roeblingite, clinohedrite, barite, ganophyllite, and numerous other species. This first occurrence was found in 1933 in the north end of the 500 level of the Franklin Mine. This assemblage was recovered in abundance by collectors, and thus is not rare, although fine xonotlite specimens are prized.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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