Matsubaraite, Sr_4Ti_5 (Si_2O_7)O_8 a new mineral, the Sr-Ti analogue of perrierite in jadeitite from the Itoigawa-Ohmi district, Niigata Prefecture, Japan

摘要

Matsubaraite, Sr_4Ti_5(Si_2O_7)_2O_8, is a new member of the perrierite-chevkinite group found in the jadeitite from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P2_1/a (pseudo C2/m), a = 13.848(7), b = 5.626 (2), c = 11.878 (6) A, β = 114.19(4) °, V = 844 A~3, and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.16 (70) (400), 3.09 (95) (-403), 3.01 (90) (-313), 2.96 (95)(311), 2.71 (100) (004), 2.17 (90) (-421). Electron microprobe analysis gave SiO_2 22.60, TiO_2 39.06, SrO 38.84 total 100.50 wt.%, corresponding to Sr_(3.92)Ti_(5.11)Si_(3.93)O_(22) on the basis of O = 22. The crystal structure analysis (R = 0.0372) revealed that the space group is pseudo C2/m, and that matsubaraite is Sr and Ti-analogue of perrierite-(Ce), Ce_4Fe_2Ti_3(Si_2O_7)_2O_8 or Zr-free analogue of rengeite, Sr_4ZrTi_4(Si_2O_7)_2O_8. It is transparent, grey with blue tint with adamantine luster. Streak is white, cleavage not observed. The hardness is VHN_(100) 681-743 kg mm~(-2) (Mohs'5.5). The calculated density is 4.13 g cm~(-3). It occurs as long prismatic euhedral to subhedral crystal with interstitial natrolite in a boulder of lavender-colored Tibearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, Niigata Prefecture, central Japan. Matsubaraite is considered to have crystallized during later stage activity of high-P/T metamorphism. The mineral is named after Dr Satoshi Matsubara of department of Geology, National Science Museum in recognition of his works on strontium dominant minerals from Japan.