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Rengeite, Sr₄ZrTi₄Si₄O₂₂, a new mineral, the Sr-Zr analogue of perrierite from the Itoigawa-Ohmi district, Niigata Prefecture, central Japan

¹ Fossa Magna Museum, Itoigawa, Niigata 941-0056, Japan
² Department of Geology, National Science Museum, Tokyo 169-0073, Japan
³ Suihodo Co., Ltd, Ohmi, Niigata 949-0301, Japan

^* E-mail: itoigawa{at}rose.ocn.ne.jp

Rengeite, Sr₄ZrTi₄Si₄O₂₂, is a new member of the perrierite–chevkinite group found in the jades from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P2₁/a, a = 13.97(1), b = 5.675(7), c = 11.98(1) Å, ß = 114.26(8)°, V = 866 ų 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 SiO₂ 22.58, TiO₂ 29.88, ZrO₂ 9.49, Nb₂O₅ 0.24, Ta₂O₅ 0.07, Al₂O₃ 0.20, FeO 0.10, CaO 0.43, SrO 34.32, BaO 0.13, La₂O₃ 0.00, Ce₂O₃ 0.38, Pr₂O₃ 0.10, Nd₂O₃ 0.29, Sm₂O₃ 0.04, total 98.25 wt.%, corresponding to (Sr_3.62Ca_0.08Ce_0.03Nd_0.02 Ba_0.01Pr_0.01)_3.76 (Zr_0.84Ti_0.09Al_0.04Fe_0.02Nb_0.02)_1.01Ti_4.00Si_4.11O₂₂ on the basis of O = 22. The unit-cell parameters and chemical composition imply that rengeite is the Sr and Zr-analogue of perrierite or high-ß 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 VHN₁₀₀ 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.

KEYWORDS: rengeite, new mineral, jadeite, perrierite, Renge metamorphic belt, Japan

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