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 standard-less 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 standard-less 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) A, c = 8.015(3) A, V = 451.6(2) A3, Z = 2. The 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 (#227) with a = 11.388(4) A, V = 1477.0(8) A3, 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.