Magnesiobermanite, MgMn3+2(PO4)2(OH)2⋅4H2O, the Mg analogue of bermanite: Description and crystal structure

Abstract

ABSTRACTMagnesiobermanite, MgMn3+2(PO4)2(OH)2⋅4H2O, is a new secondary phosphate mineral and the Mg analogue of bermanite found in a granitic pegmatite at the White Rock No.2 quarry situated in the Bimbowrie Conservation Park, South Australia, Australia. Magnesiobermanite occurs as aggregates of twinned, bladed to tabular crystals, up to 1.2 mm across. Individual crystals are up to 0.3 mm in length. The crystals are orange–red to brownish red, with a vitreous lustre and a salmon-pink streak. The mineral is brittle with a conchoidal fracture and a good cleavage on {100}. The mineral is biaxial (–), with α = 1.690(2), β = 1.719(2) and γ = 1.734(2) (white light). The calculated 2V is 70.4°. Electron microprobe analyses provided: MgO 9.59, Mn2O3 27.41, Fe2O3 8.84, Al2O3 0.18, P2O5 33.27, H2O 20.94, total 100.23 wt.%. The empirical formula (based on 14 O atoms) is: Mg1.02(Mn3+1.49Fe3+0.47Al0.02)1.98(PO4)2.01(OH)1.95⋅4.01H2O. Magnesiobermanite is monoclinic, space group P21, with the unit-cell parameters: a = 5.4215(11), b = 19.072(4), c = 5.3889(11) Å, β = 110.21(3)°, V = 522.89(18) Å3 and Z = 2. The crystal structure was refined to an R1 index of 2.43% based on for 3222 observed reflections with Fo > 4σ(Fo). Magnesiobermanite is isostructural with bermanite, Mn2+Mn3+2(PO4)2(OH)2⋅4H2O, from which it derives its name. The structure is based upon a sheet of the form [(M(OH)2(PO4)2] in the (010) plane. Sheets are linked in the b direction by [M3(H2O)4O2] octahedra and by hydrogen bonds.