四国三波川帯上須戒鉱山産Baに富むtweddillite

抄録

Ba-rich tweddillite were found from the Kamisugai mine in Sambagawa metamorphic belt, which is embedded in piemontite schist, showing green-schist facies metamorphism. These minerals occur as veinlets or aggregates of fine needle crystals in albite veins cutting braunite-hollandite-piemontite ore, associated with okhotskite and calcite. This mineral is purplish red in color, prismatic crystal up to 1mm in length, reddish brown to yellowish brown in pleochroism and show anomalous interference in thin section. This mineral is strongly zoned with cores richer in Ba than the rims. EPMA analysis for tweddillite with highest Ba content gave the empirical formula : Ca_1.02(Sr_0.63Ba_0.41)_Σ1.04(Mn³⁺_1.67Fe³⁺_0.75Al_0.58)_Σ3.00Si_2.98O_12.5 on the basis of 12.5 oxygens (O₁₂(OH)). Ba cation occupy up to 40% of A2 site and Mn³⁺ + Fe³⁺ / Mn³⁺ + Fe³⁺ + Al value attains up to 81% of M sites. Cell dimensions of Ba-rich tweddillite are a = 8.96(1), b = 5.714(2), c = 10.34(2)Å, β = 114.9(1)°, V = 480(1) ų. These minerals were formed at the low temperature condition during the retrograde Sambagawa metamorphism.