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

Matsubaraite, Sr 4 Ti 5 (Si 2 O 7 ) 2 O 8 , is a new member of the perrierite-chevkinite group found in the jadeitite from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P 2 1 / a (pseudo C 2/ 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 C 2/ m , and that matsubaraite is Sr and Ti-analogue of perrierite-(Ce), Ce 4 Fe 2 Ti 3 (Si 2 O 7 ) 2 O 8 , or Zr-free analogue of rengeite, Sr 4 ZrTi 4 (Si 2 O 7 ) 2 O 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 (Mohs9 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.