Two new super-reduced phases from Luobusa chromitite: jianmuite, ZrTi4+Ti3+5Al3O16, and a Ti3+-rich calcium amphibole, □Ca2(Mg2Ti3+3)(Si5Al3)O22(OH)2

Corundum from the Cr-11 chromitite orebody in the Luobusa ophiolite near Kangjinla (Tibet, China) (Xiong et al., 2020) contain a great variety of ultra-reduced inclusions (URIs, i.e. phases without oxygen, e.g. phosphides, silicides, nitride) and super-reduced inclusions (SRIs, i.e. phases with oxygen, but also with metals in an unusual, reduced state, e.g. Ti3+). Given the micrometric to submicrometric size of these inclusions, their chemical and structural characterization is only possible by using advanced analytical protocols that involve a focused ion beam (FIB) and transmission electron microscope (TEM). In particular, three-dimensional electron diffraction (3DED) (Gemmi et al., 2019) combined with programs for the dynamical refinement (i.e., structural refinement that takes into account dynamical scattering) (Brázda et al., 2019) constitute powerful tools to solve the structure of crystal phases that are too small for conventional single-crystal X-ray diffraction methods. In this work, we present two new phases found as SRIs in a FIB lamella from a corundum grain extracted from the Cr-11 orebody. Jianmuite, ideally ZrTi4+Ti3+5Al3O16 (Bosi et al., 2023), is a Zr-Ti-Al-oxide similar in chemistry and paragenesis to carmeltazite, ZrAl2Ti3+4O11 (Griffin et al., 2018), but with a space group I-4, a = 10.3675(10) Å, c = 9.1825(10) Å. Remarkably, this mineral was also found among refractory phases in the matrix of the Allende meteorite, a CV3 carbonaceous chondrite. In the same lamella, we also found another potential new mineral, with a structure consistent with the amphibole supergroup (Hawthorne et al., 2012), but a novel composition with the ideal formula □Ca2(Mg2Ti3+3)(Si5Al3)O22(OH)2. The charge arrangement is unique as is Ti3+ being the dominant trivalent C cation (through the hypothetical homovalent substitution CAl3+ = CTi3+), so a new rootname and the prefix “titano-“ (never used so far in the amphibole supergroup) are needed. The occurrence of Ti3+ in terrestrial minerals is petrologically significant as it constrains the redox conditions under which the Ti3+-bearing minerals crystallized.