Itoigawaite, a new mineral, the Sr analogue of lawsonite, in jadeitite from the Itoigawa-Ohmi district, central Japan

Itoigawaite, SrAl2Si2O7(OH)(2). H2O, is a new member of the lawsonite group found in the Itoigawa-Ohmi district, central Japan. It is orthorhombic, Cm cm, a = 6.031(2), b = 8.945(2), c = 13.219(4) Angstrom, Z = 4. The stronges t lines in the X-ray powder diffraction pattern are 4.68 (s) (111), 4.26 (v s) (021), 3.31 (Vs) (113), 2.75 (vs) (114, 202), 2.68 (vvs) (130), 2.63 (s) (131), 2.50 (s) (220, 203, 132), 2.45 (s) (221). Electron microprobe analy sis gave SiO2 32.98, TiO2 0.87, Al2O3 27.67, Fe2O3 0.39, Mg 0.27, CaO 0.45, SrO 27.71, H2O (diff.) 9.66, total 100.00 wt.%, corresponding to (Sr0.97Ca 0.03Mg0.02)(Sigma 1.02) (Al1.97Ti0.04Fe0.02)(Sigma 2.03)Si1.99H3.89O10 on t he basis of O = 10. It is transparent, blue in colour with vitreous lustre. The streak is white and the cleavage good in one direction. The hardness i s 5-5.5. The calculated density is 3.3 g cm(-3). It is colourless in thin s ection and optically biaxial positive with alpha = 1.664 (2), beta = 1.674 (2), gamma = 1.688 (2) and 2V(calc.) = 81 degrees. It occurs in a thin vein let cutting a lavender-coloured Ti-bearing jadeitite. The veinlet includes irregularly-shaped aggregates composed of minute tabular crystals of itoiga waite up to 50 mu m across and minute jadeite crystals with interstitial na trolite aggregates. Itoigawaite is considered to be crystallized from fluid during later stage activity of high-P/T metamorphism. The name is for the locality.