REE-Sr-Ti-dominant silicate in jadeitite from Itoigawa, Niigata, Central Japan

Abstract

A REE-Sr-Ti-dominant silicate mineral occurs as reddish brown aggregate (5 mm) in jadeitite at Itoigawa, Niigata Prefecture, Japan. The aggregate are composed of tiny rhombohedral crystals (max 0.1 mm) closely coexisted with REE-bearing matsubaraite, REE-Sr-bearing clinozoisite and natrolite. The mean of 11 electron microprobe analyses (WDS) gave SiO2 24.96, Al2O3 0.47, P2O5 0.10, TiO2 2.96, FeO 0.18, MgO 0.19, SrO 21.26, CaO 0.43, La2O3 13.94, Ce2O3 22.79, Pr2O3 2.23, Nd2O3 6.56, Sm2O3 0.30, ThO2 1.49, F 0.54, -O = F2 -0.23, total 98.16 wt.%. This leads an empirical formula (O = 27.5); (Sr3.59Ca0.13Ce2.43La1.50Nd0.68Pr0.24Th0.10Sm0.03) Σ 8.70(Ti0.65Al0.16Mg0.08Fe0.05) Σ 0.94 [SiO3 F0.50(OH)0.50](SiO3.83)6.27. Single-crystal XRD investigation showed it is hexagonal, s.p., a = 10.793(8), c = 19.31(3) Å;, V = 1958(4) ų. This mineral is isostructural with synthetic RE9Fe(AlO3OH)(SiO4)6, and is independent from any known mineral species.