FORSTERITE Chapter15. Nesosilicates

FORSTERITE

Mg₂SiO₄
Orthorhombic, Pbnm, a = 4.777, b = 10.349, c = 6.050 Å, Z = 4.

Forsterite, a magnesium silicate mineral of the olivine group, is rare at Sterling Hill and unknown from Franklin. Francis (1980) described Sterling Hill forsterite (HU-#113721) occurring as < 0.5 mm light-brown blebs with franklinite crystals and massive zincite in a calcite matrix; willemite is present as traces. A chemical analysis was given by Francis (1985) and is presented in Table 1. Forsterite also occurs as 1-2 mm pinkish-brown crystals with phlogopite and calcite and was reported by Squiller (1976) from the outer zincite zone at Sterling Hill. A manganoan forsterite associated with franklinite was reported from the Sterling Hill east-limb surface exposure, at approximately 500 N; the composition of this material is approximately (Mg_0.93Mn_0.80Zn_0.24Fe_0.03) SiO₄ (Johnson, 1990). A Sterling Hill replacement assemblage which included forsterite, clinohumite, chondrodite, humite, dolomite, and calcite was reported by Johnson (1990) from marble above the east limb, about 5 cm from the ore-contact. The composition of this material, very close to the end-member, is approximately (Mg_1.74Zn_0.17Fe_0.10Mn_0.02)SiO₄.

FOOTER LBI




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