Glaucochroite (olivine, CaMnSiO 4 ) from Franklin, New Jersey; its composition, occurrence, and formation

Glaucochroite, CaMnSiOo, is a member of the olivine group, known primarily from the Zn-Mn-Fe deposit at Franklin, New Jersey. Euhedral crystals occur in a number of assemblages with nasonite, willemite, clinohedrite, hardystonite, diopside, and cuspidine. Massive, coarse-grained, blue glaucochroite occurs with willemite, franklinite, calcite, hardystonite, leucophoenicite, and andradite. Massive, fine-grained brown glaucochroite occurs with esperite, hodgkinsonite, calcite, willemite, zincite, and franklinite, and as "calctephroite," an inhomogeneous, sheared or altered material. Glaucochroite was not observed in association with tephroite, rhodonite, or wollastonite, all of which occur at Franklin. Fourteen glaucochroite analyses conform closely to the ideal formula, with little solid solution toward tephroite. Glaucochroite can form either by the heterogeneous reaction calcite + bustamite + tephroite : glaucochroite + CO, or by gradual extension of olivine composition from tephroite to glaucochroite above the olivine solvus. At Franklin, glaucochroite is interpreted as forming instead of the commoner assemblage rhodonite + calcite in local areas with high activity of water and low activity of carbon dioxide, but under the same temperature and pressrue conditions as the deposit as a whole. Cuspidine, CauSirO?(F,OH)r, is here confirmed as occurring at Franklin. It occurs with glaucochoite and hardystonite, and in solution vugs in willemite. Two microprobe analyses averaged SiO, 32.6, CaO 62.3, F 9.7, less O : F 4.1, total 100.5 wt0/0, conforming closely to the ideal composition.