Description and crystal structure of maghrebite, MgAl₂(AsO ₄)₂(OH)₂·8H₂O, from Aghbar, Anti-Atlas, Morocco: first arsenate in the laueite mineral group

Abstract

Maghrebite, MgAl₂(AsO₄)₂(OH)₂8H₂O, is a new mineral occurring at the Aghbar open pit mine, Bou Azzer district, AntiAtlas, Morocco. Maghrebite is associated with quartz, erythrite, arseniosiderite, wendwilsonite and cabalzarite, and results from the weathering of a Variscan hydrothermal Co-Ni-As-(Cu-U-Mo) vein. Maghrebite forms lozenge-shaped prismatic crystals up to 0.2 mm in length. It is colorless, translucent with vitreous luster. D meas is 2.60 (1) g/cm³, Dcalc (crystal structure) 2.46 g/cm³. The new mineral is biaxial negative without pleochroism. Measured 2V angle is 87 (3) ; the refractive indices measured at 589 nm are: α = 1.562 (2), β = 1.574 (2), γ = 1.586 (2) with the following orientation a // b; β ^ c = 28.8 and γ ^ a = 25.5°. The empirical chemical formula is (Mg0.96Co0.01Ca0.01)Σ=0.98(Al1.94Fe3+ 0.06)Σ=2.00(As2.01Si0.01)Σ=2.02H18.0O18.02. Maghrebite is triclinic, P1, Z = 1, with a = 5.436 (2) Å, b = 10.500 (3) Å, c = 7.075 (2) Å, α = 97.701 (7)°, β = 110.295 (5)°, γ = 102.021 (6) and V = 361.0 (2) Å3. The six strongest lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 9.9 (100) (010), 6.4 (90) (001), 4.90 (80) (–101), 3.198 (60) (002), 2.885 (60) (–131) and 2.622 (60) (13-1). The structure was solved by direct methods and refined to R 1 = 0.045 on the basis of 1064 unique observed reflections. The structure is based upon [Al(AsO4)(OH)(H2O)3] layers parallel to (010) and interlinked by [Mg(H2O)6]+ octahedra. In addition, there is one H2O molecule in the structure that is not bonded to any cation but is held in between the layers by hydrogen bonds only. The topology of the Al arsenate octahedral-tetrahedral layer is that of the laueite type and belongs to the laueite isomer of the laueite-stewartite-pseudolaueite-metavauxite group of structural and geometrical isomers. Maghrebite is an arsenate analogue of gordonite and is the first natural member of the laueite group to contain arsenate as the major anion group.