First Occurrence of Titanian Hydroxylclinohumite in Marble-Hosting Gem Spinel Deposits, Luc Yen, Vietnam

Abstract

In this paper, we report the very first occurrence of titanian hydroxylclinohumite in the marble-hosted gem spinel deposits of the Luc Yen district, northern Vietnam. Hydroxylclinohumite is anhedral and associated with forsterite, tremolite, pargasite, diopside, spinel, dolomite and calcite. Hydroxylclinohumite from the Luc Yen deposit was characterized via electron microprobe analysis, single-crystal X-ray diffraction study, and Raman spectrometry. The average composition is (Mg0.69Ti0.29Fe0.02)Σ1.00Mg7.91(SiO4)4.08[(OH)1.10F0.53O0.37]Σ2.00. (ideally (Mg0.7Ti0.3)Σ1Mg8.0(SiO4)4[(OH)1.2F0.5O0.3]2). The compositions of the analyzed hydroxylclinohumites have a narrow range of Mg/(Mg+Fe+Ti) values (0.96–0.97) and a defined hydroxylclinohumite solid-solution series. Compared with other occurrences, the Luc Yen hydroxylclinohumite has an average titanium content, which attains 0.31 atoms per formula unit (3.93 wt.% TiO2) and a low iron content of 0.04 atoms per formula unit (0.42 wt.% FeO). The formation of hydroxylclinohumite is favored by the proportion of Mg, and Si in the precursor rocks and the increased activity of H2O in the fluid phase.