Niigataite, CaSrAl₃(Si₂O₇)(SiO₄)O(OH): Sr-analogue of clinozoisite, a new member of the epidote group from the Itoigawa-Ohmi district, Niigata Prefecture, central Japan

Abstract

Niigataite, CaSrAl₃(Si₂O₇)(SiO₄)O(OH), is a new member of the epidote group. It is monoclinic, P2₁/m, a=8.890(4), b = 5, 5878(18), c = 10.211(4) Å, b = 115.12(3)°, V = 459.3(3)ų and Z = 2. The 8 strongest X-ray powder diffractions are d_obs(Å)(I/I₀)(hkl): 2.90(100)(113), 2.79(48)(020), 2.70(26)(013), 3.22(25)(201), 2.11(24)(221), 2.60(24)(311), 5.05(23)(102) and 1.397(22)(040). Electron microprobe analysis gave the composition SiO₂ 35.49, TiO₂ 0.75, Al₂O₃ 24.86, Fe₂O₃ 7.08, MnO 0.22, MgO 0.07, CaO 14.09, SrO 14.75, H₂O(calc.) 1.77 total 99.08 wt%, corresponding to a formula Ca_1.00(Sr_0.72Ca_0.28)_Σ1.00(Al_2.48Fe_0.45Ti_0.05Mn_0.02Mg_0.01)_Σ3.01Si_3.00O₁₃. H calculated on the basis of H = 1 and O = 13 per unit formula. It is transparent, pale gray with a yellowish green tint. Cleavage is perfect on one direction. Streak is white. The Vickers microhardness is 642-907 kg/mm² (100g load) corresponding to Mohs’ 5-5.5. The calculated density is 3.63 g/cm³. It occurs as anhedral grains in close association with chlorite and diaspore in druse of prehnite rock in the seashore of Miyabana, Ohmi Town, Niigata Prefecture, central Japan. Niigataite is considered to be crystallized under the presence of Sr-rich metamorphic solution in the late stage of the formation of prehnite rock. Sr enrichment is caused by crystallization of prehnite, which is the most abundant phase having no acceptability of Sr.