Maurogemmiite, $Ti_{10} Fe_{3} O_{3}$ and paulrobinsonite, $Ti_{8} Fe_{4} O_{2}$ : new minerals in a coesite-bearing fragment from the Luobusa ophiolite, Tibet, China

Maurogemmiite, Ti10Fe3O3, and paulrobinsonite, Ti8Fe4O2, are two new mineral species in a fragment 0.45 mm × 0.8 mm in size extracted from chromitite orebody #31 in the Luobusa ophiolite near Luobusa Village, Tibet, China (29°13.86’N, 92°11.41’E). The fragment comprises (1) an alloy core consisting of a wangxibinite + ‘osbornite’ intergrowth, Ti and the new minerals; (2) an inner rim of Ti and (3) an outer rim of coesite, kyanite and amorphous Ti-aluminosilicate. Maurogemmiite forms irregular grains up to 30 μm across enclosed in paulrobinsonite, which isolated it from the wangxibinite (TiFe) + ‘osbornite’ intergrowth. Two standardless EDS analyses and O taken from the structurally refined model gave O 6.40, Al 0.26, Si 1.96, Ti 65.73, Fe 24.79, Ni 0.85, sum 100 wt.%. The empirical formula normalised to Ti = 10 is Al0.07Si0.51Ti10Fe3.23Ni0.11O2.91. Two standardless EDS analyses of paulrobinsonite gave O 3.73, Al 0.28, Si 1.82, Ti 56.50, Fe 35.65, Ni 2.03, sum 100 wt.%. The empirical formula normalised to Ti = 8 is Al0.07Si0.44Ti8Fe4.33Ni0.23O1.58. Three-dimensional electron diffraction (3DED) data on maurogemmiite delivered a primitive hexagonal cell, space group P63/mmc (#194) with a = 8.065(1) Å, c = 8.015(3) Å, V = 451.6(2) Å3 and Z = 2. ... mehrThe structure is a compact framework with Ti1, Ti3 and Fe1 in coordination 12 and Ti2 in coordination 13. Both Ti2 and Ti3 show a wide range of interatomic distances, which result in interstitial positions occupied predominantly by O and partially by non-stoichiometric Fe. The 3DED data on paulrobinsonite delivered an F-centred cubic cell, space group Fdm (#227) with a = 11.388(4) Å, V = 1477.0(8) Å3 and Z = 8. The structure is also a compact framework with Ti1 and Fe1 in coordination 12 and Ti2 in coordination 14, whereas the remaining Fe and all O atoms occupy interstitial positions in nearly regular octahedral coordination with Ti2.