Macraeite, [(H₂O)K]Mn₂(Fe₂Ti)(PO₄)₄[O(OH)](H₂O)₁₀  ⋅  4H₂O, a new monoclinic paulkerrite-group mineral, from the Cubos–Mesquitela–Mangualde pegmatite, Portugal

Abstract

Macraeite, [(H2O)K]Mn2(Fe2Ti)(PO4)4[O(OH)](H2O)10 ⋅ 4H2O, is a new monoclinic member of the paulkerrite group, from the Cubos–Mesquitela–Mangualde pegmatite, Mangualde, Portugal. It was found in phosphate nodules of weathered triplite, heterosite, and lithiophilite. Associated minerals are strengite, triplite, bermanite, phosphosiderite, and switzerite. Macraeite forms colourless to light-greenish-yellow pseudo-rhombic dodecahedral-shaped crystals up to 0.15 mm. The crystals are equant with forms {010}, {001}, {111}, and {1‾11}. The calculated density is 2.39 g cm−3. Optically, macraeite crystals are biaxial (+), with α=1.605(3), β=1.611(3), γ=1.646(3) (measured in white light), and 2V(meas) = 45(3)°. The empirical formula from electron microprobe analyses and structure refinement is A1[(H2O)0.83K0.17]Σ1.00 A2[K0.65(H2O)0.35]Σ1.00 M1(Mn1.98□0.022+)Σ2.00 M2(Fe1.093+Al0.31Ti0.524+Mg0.08)Σ2.00 M3(Ti0.664+Fe0.343+)Σ1.00 (PO4)4 X[O0.87F0.53(OH)0.60]Σ2.00(H2O)10 ⋅ 4H2O. Macraeite has monoclinic symmetry with space group P21/c and unit-cell parameters a=10.562(2) Å, b=20.725(4) Å, c=12.416(2) Å, β=90.09(3)°, V=2717.8(9) Å3, and Z=4. The crystal structure was refined using synchrotron single-crystal data to wRobs=0.065 for 4990 reflections with I>3σ(I). Macraeite is isostructural with the paulkerrite-group minerals rewitzerite and paulkerrite, with ordering of K and H2O at different A sites (A1 and A2) of the general formula A1A2M12M22M3(PO4)4X2(H2O)10 ⋅ 4H2O, whereas in the orthorhombic member, benyacarite, K and H2O are disordered at a single A site.