Whiteite-(CaMnMn), CaMnMn₂Al₂[PO₄]₄(OH)₂·8H₂O, a new mineral from the Hagendorf-Süd granitic pegmatite, Germany

Abstract

Abstract Whiteite-(CaMnMn), CaMnMn 2 Al 2 [PO 4 ] 4 (OH) 2 ·8H 2 O, is a new hydrous phosphate of Ca, Mn and Al, which is closely related to both jahnsite-(CaMnMn) and the minerals of the whiteite group. It is monoclinic, P 2/ a , with a = 15.02(2), b = 6.95(1), c =10.13(3) Å, β = 111.6(1)°, V = 983.3(6) Å 3 , Z = 2 (from powder diffraction data) or a = 15.020(5), b = 6.959(2), c = 10.237(3) Å, β = 111.740(4)°, V = 984.3(5) Å 3 , Z = 2 (from single-crystal diffraction data). The mineral was found in the Hagendorf Süd granitic pegmatite (Germany) as small (up to 0.5 mm in size) crystals elongated on a and tabular on {010}. The crystals are either simply or polysynthetically twinned on {001}. They crystallize on the walls of voids within altered zwieselite crystals or form coronas (up to 1 mm in diameter) around cubic crystals of uraninite. The mineral is transparent, colourless to pale yellow (depending on Al–Fe 3+ substitution), with a vitreous lustre and a white streak. The cleavage is perfect on {001}, the fracture is stepped and the Mohs hardness is 3½. In transmitted light, the mineral is colourless; dispersion was not observed. Whiteite-(CaMnMn) is biaxial (+), α = 1.589(2), β = 1.592(2), γ = 1.601(2) (589 nm), 2V meas = 60(10)°, 2V calc = 60.3°. The optical orientation is X = b , Z^ a = 5°. The calculated and measured densities are D calc = 2.768 and D meas = 2.70(3) g cm –3 , respectively. The mean chemical composition determined by electron microprobe is Na 2 O 0.53, MgO 0.88, Al 2 O 3 11.66, P 2 O 5 34.58, CaO 4.29, MnO 17.32, FeO 8.32, ZnO 2.60 wt.%, with H 2 O 19.50 wt.% (determined by the Penfield method), giving a total of 99.68 wt.%. The empirical formula calculated on the basis of four phosphorus atoms per formula unit, with ferric iron calculated to maintain charge balance, is (Ca 0.63 Zn 0.26 Na 0.14 ) Σ1.03 (Mn 0.60 Fe 0.40 2+ ) Σ1.00 (Mn 1.40 Fe 0.37 2+ Mg 0.18 Fe 0.06 3+ ) Σ2.01 (Al 1.88 Fe 0.12 3+ ) Σ2.00 [PO 4 ] 4 (OH) 2 ·7.89H 2 O. The simplified formula is CaMnMn 2 Al 2 [PO 4 ] 4 (OH) 2 ·8H 2 O. The mineral is easily soluble in 10% HCl at room temperature. The strongest X-ray powder-diffraction lines [listed as d in Å (I) (hkl)] are as follows: 9.443(65)(001), 5.596(25)(011), 4.929(80)(210), 4.719(47)(002), 3.494(46)(400), 2.7958(100)(022). The crystal structure of whiteite-(CaMnMn) was refined for a single crystal twinned on (001) to R 1 = 0.068 on the basis of 5702 unique observed reflections. It is similar to the structures of other members of the whiteite group. The mineral is named for the chemical composition, in accordance with whiteite-group nomenclature.