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

Abstract Maurogemmiite, Ti 10 Fe 3 O 3 , and paulrobinsonite, Ti 8 Fe 4 O 2 , 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 Al 0.07 Si 0.51 Ti 10 Fe 3.23 Ni 0.11 O 2.91 . 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 Al 0.07 Si 0.44 Ti 8 Fe 4.33 Ni 0.23 O 1.58 . Three-dimensional electron diffraction (3DED) data on maurogemmiite delivered a primitive hexagonal cell, space group P 6 3 / mmc (#194) with a = 8.065(1) Å, c = 8.015(3) Å, V = 451.6(2) Å 3 and 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 Fd $\bar 3$ m (#227) with a = 11.388(4) Å, V = 1477.0(8) Å 3 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.