Halilsarpite, a new arsenate analogue of walentaite, from the Oumlil mine, Bou Azzer district, Morocco

Abstract

Abstract. Halilsarpite, [Mg(H2O)6][CaAs23+(Fe2.673+Mo0.336+)(AsO4)2O7], is a new secondary mixed arsenate/arsenite mineral from the Oumlil mine, Bou Azzer district, Morocco. It forms lemon-yellow spherules, about 0.1 mm in diameter, composed of platelets, some tens of micrometres in diameter but only 1–2 µm thick. The platelets are flattened on {100} and exhibit perfect cleavage on {100}. The calculated density is 2.89 g cm−3. Optically, halilsarpite is biaxial (−), with α=1.646(calc), β=1.765(5), γ=1.815(5) (white light), and 2V (meas.) = 62(1)∘. The partial orientation is X=a. Dispersion is weak with r<v, and pleochroism is in shades of yellow, X<Y≈Z. Electron microprobe analyses coupled with crystal-structure refinement results give the empirical formula [(Mg0.42Ca0.19Fe0.113+ □0.28)Σ1.00 (H2O)6][((Ca0.5Mg0.2Na0.11□0.14)Σ0.95 (As3+)2.05)∑3.00(Fe2.443+ Mo0.566+)Σ3.00 ((As0.9855+P0.0155+)Σ1.00O4)2O6.9(OH)0.1]. Halilsarpite is orthorhombic, Imma, with a=26.4890(10), b=7.4205(3), c=10.4378(4) Å, V=2051.67(14) Å3, and Z=4. The structure was solved and refined using single-crystal X-ray data to wRobs=0.046 for 1402 unique reflections to a resolution of 0.75 Å. Halilsarpite is isostructural with walentaite and natrowalentaite, which have structures based on heteropolyhedral layers of configuration [M1(M2)2(TO4)2(O,OH)6] with surface-attached As3+O3 groups and Ca and Na cations, and with interlayer hydrated cations and H2O molecules. The octahedrally coordinated sites M1 and M2 are occupied by Fe3+ in walentaite, while M2 = Fe0.673+W0.336+ in natrowalentaite. Walentaite and natrowalentaite are phosphates, with T=P5+. Halilsarpite differs from these minerals in having T=As5+ and in having partial replacement of Fe3+ by Mo6+ in the M2-centred octahedra.