CHAMOSITE

(Fe,Mg,Fe³⁺)₅Al(Si₃Al)O₁₀(OH,O)₈
Monoclinic

Frondel and Ito (1975), in the description of baumite, noted a brunsvigite with appreciable amounts of Mn and Zn. This material is described here as chamosite and was further discussed by Guggenheim and Bailey (1989). Chamosite is not known from Sterling Hill.

Description

Franklin chamosite occurs as crude, radial, lustrous, 2-5 mm, greenish-black crystals in veinlets in a lizardite (baumite) assemblage. The luster is vitreous, and the density is 3.13 g/cm³. Optically, it is biaxial, negative, with 2V = 0^o, a = 1.657, and b = g = 1.67.

Composition

Chamosite is an iron aluminum silicate hydroxide mineral of the chlorite group. Chemical analysis by Jun Ito yielded SiO₂ 25.3, Al₂O₃ 14.0, Fe₂O₃ 2.02, FeO 21.0, MnO 9.96, MgO 7.22, ZnO 9.60, CuO 0.08, CaO 0.04, Na₂O 0.02, K₂O 0.08, H₂O^- 0.2, H₂O⁺ 10.6, total = 100.12 wt. %. Microprobe analysis by the writer yielded SiO₂ 25.0, Al₂O₃ 16.9, total Fe as FeO 25.3, MgO 5.7, ZnO 10.2, MnO 7.1, with H₂O 9.8 wt. % by difference, total = 100 wt. %.

Occurrence and paragenesis

Chamosite was found associated with crude, broken, hexagonal, barrel-shaped, prismatic crystals of willemite, abundant lizardite (baumite in part), and green crystals of aegirine. The assemblage is anomalous for Franklin, but is assuredly from here. The material was found on the Buckwheat Dump, and the known material is all from one find (J. L. Baum, personal communication).

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