Apexite, NaMg(PO₄)·9H₂O, a new struvite-type phase with a heteropolyhedral cluster

Abstract

Apexite (IMA2015-002), NaMg(PO₄)·9H₂O, is a new mineral from the Apex mine, Lander County, Nevada, U.S.A., where it occurs as a low-temperature secondary mineral on massive quartz matrix in association with andersonite, calcite, čejkaite, gaylussite, and goethite. Apexite forms colorless needles up to 0.5 mm in length. The streak is white. Crystals are transparent and have vitreous to satiny luster. The Mohs hardness is about 2, the tenacity is brittle, the fracture is curved, and crystals exhibit one perfect cleavage on {100}. The measured density is 1.74(1) g/cm³ and the calculated density is 1.741 g/cm³. Electron microprobe analyses provided: Na₂O 9.26, MgO 14.42, P₂O₅ 23.31, H₂O 53.01 (structure), total 100.00 wt% (normalized). The empirical formula (based on 13 O apfu) is: Na_0.91Mg_1.09P_1.00O_13.00H_17.91. Apexite is triclinic, P1̄, a = 6.9296(7), b = 11.9767(13), c = 14.9436(19) Å, α = 92.109(6), β = 102.884(7), γ = 105.171(7)°, V = 1160.9(2) ų, and Z = 4. The eight strongest lines in the X‑ray powder diffraction pattern are [d_obs in Å(I)(hkl)]: 14.63(35)(001); 5.11(61)(021,1̅1̅1,110,1̅20); 4.68(75)(02̅2,1̅1̅2,12̅1,01̅3); 4.301(96)(102,013,022,1̅13); 4.008(44) (1̅1̅3,12̅2); 2.876(46)(040); 2.762(100)(2̅1̅3,23̅1,03̅4,2̅04,015); and 2.507(30)(212,025,2̅2̅3). Apexite is a new struvite-type phase with a unique structure (R₁ = 4.44% for 1401 F_o > 4σF) consisting of four components: (1) a [Na₂Mg₄(H₂O)₁₄]¹⁰⁺ heteropolyhedral cation cluster; (2) a trans edge-sharing chain of Na(H₂O)₆ octahedra; (3) an isolated PO₄ group; and (4) an isolated H₂O group. The structural components are linked to one another only via hydrogen bonds. Its structure is related to that of hazenite.