New data on ferri–ghoseite in Sanbagawa quartz schist from the Iimori region, Wakayama Prefecture, Japan: solid solution between magnesio–riebeckite and clino–suenoite

抄録

Chemically heterogeneous amphibole, ranging in composition from magnesio–riebeckite through ferri–ghoseite to clino–suenoite, was found in a specimen of Sanbagawa quartz schist from the Iimori region of the western Kii Peninsula, central Japan. The amphibole exhibits a continuous solid solution between ^BNa and ^BMn²⁺ (^BMn²⁺ = 0–1.82 atoms per formula unit). Most of the amphibole crystals comprise a Mn–poor core and a Mn–rich rim, and ferri–ghoseite often occurs near the boundary between core and rim. The crystal structure of a single crystal fragment of ferri–ghoseite, which has an averaged composition of ^A(Na_0.16K_0.02)_Σ0.18^B(Na_0.83Ca_0.09Mn²⁺_1.08)_Σ2.00^C(Mg_3.78Mn²⁺_0.52Fe³⁺_0.66
Al_0.04)_Σ5.00^T(Si_7.95Al_0.05)_Σ8.00O₂₂^W[(OH)_1.90F_0.10]_Σ2.00 based on electron–microprobe analyses, was refined to a R1 of 6.7%, has unit cell parameters of a = 9.6389(7), b = 18.0534(10), c = 5.3138(3) Å, and β = 102.896(2)°, and is in space group C2/m with Z = 2. The site populations for B cations of the ferri–ghoseite are ^M4(Na_0.83Ca_0.09)^M4’Mn²⁺_1.08, which also confirms the ^B(Na,Mn²⁺) solid solution. Sector–zoned aegirine occurs in the amphibole–bearing quartz schist from Iimori, and it is assumed that most of the metamorphic minerals in the quartz schist formed under non–equilibrium conditions. Therefore, taking into account the miscibility gap between sodium amphibole and clino–suenoite, the solid solution between ^BNa and ^BMn²⁺ in the amphibole can be inferred to have resulted from rapid, non–equilibrium crystallization rather than high–T equilibrium crystallization.