Tamaite, the Ca-analogue of ganophyllite, from the Shiromaru mine, Okutama, Tokyo, Japan

Abstract

Tamaite, (Ca, K, Ba, Na)₃₋₄Mn₂₄(Si, Al)₄₀(O, OH)₁₁₂·21H₂O, the Ca-analogue of ganophyllite, is found from the Shiromaru mine, Okutama, Tokyo, Japan. It is monoclinic, P2₁/a, a=16.64(1), b=27.11(2), c=25.35(2)A, β=98.74(7)°, Z=4. The strongest lines in the X-ray powder diffraction pattern are 12.6 (vvs) (002), 3.46 (m) (027), 3.13 (s) (008), 2.84 (s) (38 -2), 2.69 (vs) (38 -4), 2.60 (s) (602), 2.46 (s) (60 -6), 2.38 (m) (604), 1.623 (m) (98 -2), 1.609 (m) (3 10 11). Electron microprobe and Karl-Fischer analyses gave SiO₂ 41.23, Al₂O₃ 7.79, FeO 0.16, MnO 35.17, MgO 0.23, CaO 1.94, BaO 2.03, Na₂O 0.34, K₂O 0.82, H₂O 11.07, total 100.78 wt%, and lead to the empirical formula, (Ca_1.65K_0.83Ba_0.63Na_0.53)_Σ3.64(Mn_23.71Mg_0.27Fe_0.11Al_0.12)_Σ24.21(Si_32.81Al_7.19)_Σ40.00 O_95.27(OH)_16.73·21H₂O on the basis of O=133. It is transparent and colorless to pale yellowish brown in color with vitreous to pearly luster. The streak is white and cleavage perfect on {001}. The Mohs' hardness is about 4. The measured and calculated densities are 2.85(5) g/cm³ and 2.83 g/cm³, respectively. It is colorless in thin section and optically biaxial negative with β=1.612(2) and 2 V<15°. It occurs as veinlets up to 1.5 mm thick composed of micaceous platy crystals under 0.5 mm in diameter and as spotted crystals in celsian-barian orthoclase veinlets cutting host rocks of metamorphosed manganese ore deposit. The mineral is considered to be formed during later stage activity of low grade metamorphism.