Significance of Riebeckite and Ferrohastingsite in Microperthite Granites

Riebeckite and ferrohastingsite occur separately in closely related hypersolvus granites of eastern Massachusetts. Mineralogical, chemical, and phase equilibria data suggest that the Quincy riebeckite granite and the Peabody ferrohastingsite granite crystallized at similar temperatures, most likely 650-750°C. The chemical composition of the parent magma and oxygen fugacity played important rolrs in determining the composition of the amphibole. Parent composition favored a calcium-iron-aluminum amphibole, ferrohastingsite, NaCa₂Fe²⁺₄Fe³⁺Al₂Si₆O₂₂(0H)₂, in the Peabody, and a soda-iron amphibole, riebeckite, Na₂Fe²⁺₃Fe³⁺₂Si₃O₂₂(OH)₂, in the Quincy. Higher oxygen fugacities favored the ferric-rich amphibole, in the Quincy; lower oxygen fugacities favored the ferric-poor amphibole, ferrohastingsite, in the Peabody. Mineral relations in the Quincy suggest an increase in oxygen fugacity with decrease in temperature. Two new chemical analyses, one of the Quincy granite riebeckite and one of the Peabody granite ferrohastingsite, are reported.