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Rengeite, Sr 4 ZrTi 4 Si 4 O 22 , a new mineral, the Sr-Zr analogue of perrierite from the Itoigawa-Ohmi district, Niigata Prefecture, central Japan

2001; Cambridge University Press; Volume: 65; Issue: 1 Linguagem: Inglês

10.1180/002646101550163

1471-8022

Hiroshi Miyajima, S. Matsubara, Ritsuro Miyawaki, Kenichi Yokoyama, Kazuo Hirokawa,

Nuclear materials and radiation effects

Abstract Rengeite, Sr 4 ZrTi 4 Si 4 O 22 , is a new member of the perrierite–chevkinite group found in the jades from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P2 1 / a , a = 13.97(1), b = 5.675(7), c = 11.98(1)Å, β = 114.26(8)°, V = 866 Å 3 and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.12 (s) (40-3), 3.05 (vvs) (31-3), 2.99 (vs) (311), 2.84 (s) (020), 2.74 (s) (004), 2.20 (s) (31-5). Electron microprobe analysis gave SiO 2 22.58, TiO 2 29.88, ZrO 2 9.49, Nb 2 O 5 0.24, Ta 2 O 5 0.07, Al 2 O 3 0.20, FeO 0.10, CaO 0.43, SrO 34.32, BaO 0.13, La 2 O 3 0.00, Ce 2 O 3 0.38, Pr 2 O 3 0.10, Nd 2 O 3 0.29, Sm 2 O 3 0.04, total 98.25 wt.%, corresponding to (Sr 3.62 Ca 0.08 Ce 0.03 Nd 0.02 Ba 0.01 Pr 0.01 ) ∑3.76 (Zr 0.84 Ti 0.09 Al 0.04 Fe 0.02 Nb 0.02 ) ∑1.01 Ti 4.00 Si 4.11 O 22 on the basis of O = 22. The unitcell parameters and chemical composition imply that rengeite is the Sr and Zr-analogue of perrierite or high-β analogue of strontiochevkinite. It is transparent, dark brown with adamantine lustre. Its streak is pale greenish brown, and no cleavage was observed. The hardness is VHN 100 606–698 kg mm −2 (Mohs 5–5.5). The calculated density is 4.12 g cm −3 . It is strongly pleochroic from pale green to pale greenish brown where the REE contents are <1 wt.% and pale violet to greenish brown where the REE contents are between 3 and 10 wt.%. It occurs as anhedral grains in close association with titanite, zircon and tausonite in a pebble of blue titanian omphacite-jadeite rock from the seashore of Oyashirazu, Ohmi Town, in a boulder of lavender-coloured Ti-bearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, and in a boulder of green jade from the bed of the Hime-kawa river, Itoigawa City, Niigata Prefecture, central Japan. Rengeite is considered to have crystallized by interaction between pre-existing minerals (rutile, anatase, titanite and zircon) and Sr-rich metamorphic fluid during later stage activity of high- P/T metamorphism. The name is for Mt. Renge near the locality and the Renge metamorphic belt where jadeitite deposits are found.