Jingsuiite, TiB2, a new mineral from the Cr-11 podiform chromitite orebody, Luobusa ophiolite, Tibet, China: Implications for recycling of boron

The new mineral jingsuiite (TiB2, IMA-2018-117b), together with osbornite-khamrabaevite solid solution (TiN-TiC), deltalumite, and a potential new mineral, hexagonal Ti-10(Si,P,)(7), constitute four inclusions up to 50 mu m across in corundum recovered from the Cr-11 podiform chromitite orebody near Kangjinla, Luobusa ophiolite, Tibet, China. EELS, EDS, and 3D electron diffraction were applied to study the phases. In one inclusion, jingsuiite forms a rounded grain 40 mu m across. Associated osbornite-khamrabaevite solid solution forms an irregular mass up to 10 mu m across having the composition Ti(N0.5C0.5) and the Ti-10(Si,P,)(7) phase forms an incomplete overgrowth up to 20 mu m thick around the grain of jingsuiite. In a second inclusion, jingsuiite, osbornite-khamrabaevite solid solution, Ti-10(Si,P,)(7) and deltalumite form a lamellar intergrowth 100 mu m long composed of tablets of the four phases up to 50 mu m long x 4 mu m in thickness. Jingsuiite has a primitive hexagonal cell with a = 3.04(6), b = 3.04(6), c = 3.22(6) angstrom, alpha = 90 degrees, beta = 90 degrees, gamma = 120 degrees, V = 25.8 (9) angstrom(3), space group P6/mmm, Z = 1. Its structure was determined ab initio and dynamically refined on the basis of three-dimensional electron diffraction data; it is equivalent to that of synthetic TiB2. Results of EELS analyses of jingsuiite in foil no. 5357 (N = 20) gave B 61.87(1.22), C 1.53 (1.26), Ti 36.62 (1.45) at% from which an empirical formula of Ti-1.10(B1.86C0.05)(sigma 1.91) was calculated on the basis of 3 atoms. The ideal formula is TiB2. Our preferred scenario is that corundum with entrapped Ti-Si-P-Fe intermetallic melts was precipitated from basaltic magmas during exhumation following deep subduction. Enrichment of B in the melt pockets is attributed to the highly reducing conditions that led to the segregation of siderophile elements into intermetallic melts and to the siderophile behavior of B, thereby concentrating it in the intermetallic melts in preference to silicate melt. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum grains such as Ti, FeTiSi2, and TiSi2 could have crystallized from alloy melts at the lowest T accessible on the liquidus, i.e., <1300 degrees C. The presence of TiB2 in four inclusions in the Cr-11 orebody suggests incorporation of crustal sediments in the ophiolite followed by deep subduction to the Transition Zone where qingsongite (cubic BN) is inferred to have crystallized and subsequently exhumed to shallower levels where hexagonal BN and jingsuiite presumably crystallized.