Nizamoffite, Mn²⁺Zn₂(PO₄)₂(H₂O)₄, the Mn analogue of hopeite from the Palermo No. 1 pegmatite, North Groton, New Hampshire

Nizamoffite, ideally Mn²⁺Zn₂(PO₄)₂(H₂O)₄, is a new mineral from the Palermo No.1 pegmatite in North Groton, Grafton County, New Hampshire, U.S.A. It formed as the result of secondary alteration of primary triphylite and associated sphalerite. The crystals occur as colorless prisms up to 1 mm in length and 0.5 mm in diameter. The prisms are elongated and lightly striated parallel to [001] and exhibit the forms {100}, {010}, {230}, {011}, {031}, and {111}. The mineral is transparent and has a white streak, vitreous luster, Mohs hardness of about 3½, brittle tenacity, irregular fracture, and three cleavages: perfect on {010}, good on {100}, and fair on {001}. The measured and calculated densities are 3.00(1) and 2.961 g/cm³, respectively. It is optically biaxial (−), α = 1.580(1), β = 1.590(1), γ = 1.591(1) (white light), 2V_meas = 28(1)°, and 2V_calc = 35°. Nizamoffite exhibits strong dispersion, r < v. The optical orientation is X = a, Y = c, Z = b, and the mineral is nonpleochroic. Electron-microprobe analyses (average of 10), with H₂O calculated on structural grounds, provided: CaO 0.20, MgO 0.61, MnO 15.80, ZnO 33.34, Fe₂O₃ 2.81, Al₂O₃ 0.10, P₂O₅ 32.05, H₂O 15.95, total 100.23 wt%. The empirical formula (based on 12 O atoms) is: (Mn²⁺_0.99Ca_0.02)_∑1.01(Zn_1.82Fe³⁺_0.12Mg_0.07)_∑2.01(P_1.00O₄)₂(H_1.96O)₄. The mineral dissolves readily in cold, dilute HCl. Nizamoffite is orthorhombic, Pnma, with the unit-cell parameters: a = 10.6530(4), b = 18.4781(13), c = 5.05845(15) Å, V = 995.74(8) ų, and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs in Å(I)(hkl)]: 9.27(71)(020); 4.62(37)(040,220); 4.43(24)(111); 3.424(52)(240,221); 2.873(100)(241); 2.644(36)(400,331); 2.540(33)(420,161,002); and 1.953(36)(281). Nizamoffite is isostructural with hopeite. The structure (R₁ = 1.7% for 1014 F_o > 4σF) contains corner-sharing zigzag chains of ZnO₄ tetrahedra along [001]. The chains are connected by corner sharing with PO₄ tetrahedra to form sheets parallel to {010}. Three of the four PO₄ vertices link to ZnO₄ tetrahedra in the sheet, while the fourth links to an octahedron between the sheets. Each octahedron links to one tetrahedron from each of two adjacent sheets, thereby linking the sheets in the [010] direction. The octahedron contains Zn in hopeite and Mn in nizamoffite.