Matsubaraite, Sr4Ti5(Si2O7)2O8, a new mineral, the Sr-Ti analogue of perrierite in jadeitite from the Itoigawa-Ohmi district, Niigata Prefecture, Japan

Abstract

Matsubaraite, Sr4 Ti5(Si2 O7)2 O8, is a new member of the perrierite-chevkinite group found in the jadeitite from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P21/a (pseudo C2/m). a = 13.848(7), b = 5.626(2), c = 11.878(6) Å, β = 114.19(4)°, V = 844 Å3 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 SiO2 22.60, TiO2 39.06, SrO 38.84 total 100.50 wt.%, corresponding to Sr3.92Ti5.11 Si3.93O22 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), Ce4Fe2Ti3 (Si2 O7)2O8, or Zr-free analogue of rengeite, Sr4ZrTi4 (Si2 O7)2 O8. It is transparent, grey with blue tint with adamantine luster. Streak is white, cleavage not observed. The hardness is VHN100 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 Ti-bearing 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.