Wenjiite, Ti10(Si,P,☐)7, and kangjinlaite, Ti11(Si,P)10, new minerals in the ternary Ti-P-Si system from the Luobusa ophiolite, Tibet, China

Abstract

Abstract The new minerals wenjiite, Ti10(Si,P,☐)7 (IMA2019-107c) and kangjinlaite, Ti11(Si,P)10 (IMA2019-112b) occur with badengzhuite, zhiqinite, and a K-bearing dmisteinbergite-like mineral in a spheroid 20 μm across enclosed in corundum from the Cr-11 podiform chromitite orebody near the Kangjinla, Luobusa ophiolite, Tibet, China. In addition, wenjiite occurs with deltalumite, jingsuiite, osbornitekhambaraevite, and the K-bearing dmisteinbergite-like mineral in a lamellar intergrowth 100 μm long, also enclosed in corundum from the same locality. The new minerals were characterized by energy-dispersive spectroscopy and three-dimensional electron diffraction, which enabled us to obtain an ab initio structure solution and dynamical refinement from grains a few micrometers across hosted in a FIB lamella. Four analyses of wenjiite from the spheroid gave in wt% Si 21.67, P 6.24, Ti 66.39, V 1.37, Cr 2.20, Mn 0.97, and Fe 1.17 (normalized to 100), which corresponds to (Ti0.93Cr0.03Mn0.01Fe0.01V0.02)10 (Si0.79P0.21)6.51 on the basis of 10 cations excluding Si and P. The simplified formula is Ti10(Si,P)6.5, or more generally Ti10SixPy, where x > y and 6 ≤ (x + y) ≤ 7, i.e., Ti10(Si,P,☐)7. Wenjiite has hexagonal symmetry, space group: P63/mcm (no. 193), with a = 7.30(10) Å, c = 5.09(10) Å, V = 235(6) Å3, Z = 1, and is isostructural with xifengite, mavlyanovite, synthetic Ti5Si3, and synthetic Ti5P3.15. Four analyses of kangjinlaite gave in wt% Si 25.56, P 9.68, Ti 62.35, V 0.21, Cr 0.83, Mn 0.42, and Fe 0.95 (normalized to 100), which corresponds to (Ti10.65V0.03Cr0.13Mn0.06Fe0.14)Σ11.01(Si7.43P2.55)Σ9.99. The simplified formula is Ti11(Si,P)10. Kangjinlaite is tetragonal, with space group: I4/mmm (no. 139), a = 9.4(2) Å, c = 13.5(3) Å, V = 1210(50) Å3, Z = 4, and is isostructural with synthetic compounds of the Ho11Ge10 type, being the most compact of these phases. Despite there now being over 70 compounds containing 38 elements isostructural with Ho11Ge10, synthesis of an analog of kangjinlaite has not been previously reported in either the Ti-P or Ti-Si binary systems or in a multicomponent system. The previously deduced crystallization sequence with decreasing temperature of the four minerals in the spheroid is wenjiite → kangjinlaite → zhiqinite + badengzhuite. This sequence is consistent with relationships reported in 9 binary systems containing intermetallic compounds of Ge and Sn isostructural with Mn5Si3 and Ho11Ge10. In eight of these systems the Mn5Si3 analog melts congruently, whereas the Ho11Ge10 analog never does. Instead, the Ho11Ge10 analog melts peritectically, generally to an Mn5Si3 analog and less commonly to compounds with 5:4 stoichiometry. Final crystallization of the spheroid to zhiqinite + badengzhuite is expected to be well below the temperature of 1500 °C for the congruent melting of zhiqinite in the Ti-Si system, i.e., in the range of ~1100–1300 °C.