Occurrence and crystal chemical features of protoferro-anthophyllite and protomangano-ferro-anthophyllite from Cheyenne Canyon and Cheyenne Mountain, U.S.A. and Hirukawa-mura, Suisho-yama, and Yokone-yama, Japan

抄録

The crystal chemical features and occurrence of the new orthoamphiboles (space group Pnmn), protoferro-anthophyllite and protomangano-ferro-anthophyllite, are described. Protoferro-anthophyllites (PFA) are found in fayalite masses from granitic pegmatites at Cheyenne Canyon and Cheyenne Mountain, Colorado, U.S.A. and at Hirukawa-mura, Gifu Prefecture, Japan. Protomangano-ferro-anthophyllites (PMFA) are found in Mn-rich fayalite masses from granitic pegmatites at Suisho-yama, Fukushima Prefecture, and in bedded manganese ore deposits at Yokone-yama, Tochigi Prefecture, Japan. The empirical formulas of PFA from Hirukawa-mura and PMFA from Yokone-yama are (Fe_1.54Mn_0.42Mg_0.14)Fe₅(Si_7.9Al_0.1)O₂₂(OH)₂ and (Mn_1.38Fe_0.59)(Fe_4.1Mg_0.9Ca_0.03)Si₈O₂₂(OH)₂, respectively. The unit cell parameters of PFA are a = 9.388(2), b = 18.387(4), and c = 5.347(1) Å, and those of PMFA are a = 9.425(2), b = 18.303(4), and c = 5.345(1) Å. These minerals are isostructural with synthetic protofluorian-lithian-anthophyllite (Pnmn, Z=2), and have a straight double-tetrahedral chain (O(5)-O(6)-O(5) angle of 177° and 179°, respectively) and M(1), M(2), and M(3) octahedra of regular shape. PFA and PMFA are optically biaxial negative with refractive indices α = 1.690, β = 1.710, and γ = 1.726, and α= 1.695, β = 1.714, and γ = 1.731, respectively. The density (calc.) of PFA is 3.54 and that of PMFA is 3.44 g/cm³. Some PFA and PMFA include fine clinoamphibole lamellae (space group C2/m). PFA may be produced by hydrothermal alteration of fayalite, and PMFA may be similarly produced by alteration of Mn-rich fayalite and also by metasomatism in bedded manganese ore deposits.