Fluorowardite, NaAl₃(PO₄)₂(OH)₂F₂·2H₂O, the fluorine analog of wardite from the Silver Coin mine, Valmy, Nevada

Abstract

Fluorowardite (IMA2012-016), NaAl₃(PO₄)2(OH)₂F₂·2H₂O, the F analog of wardite, is a new mineral from the Silver Coin mine, Valmy, Iron Point district, Humboldt County, Nevada, U.S.A., where it occurs as a low-temperature secondary mineral in complex phosphate assemblages rich in Al, Na, and F. Fluorowardite forms colorless to white or cream-colored, tetragonal-pyramidal crystals up to 0.1 mm in diameter. The streak is white. Crystals are transparent to translucent, with vitreous to pearly luster. The Mohs hardness is about 5, the tenacity is brittle, the fracture is irregular, and crystals exhibit one perfect cleavage on {001}. The calculated density is 2.760 g/cm³. Optically, fluorowardite is uniaxial positive, with ω = 1.576(2) and ε = 1.584(2) (white light) and is non-pleochroic. Electron microprobe analyses (average of 8) provided: Na₂O 6.27, CaO 1.74, MgO 0.42, Al₂O₃ 35.21, Fe₂O₃ 0.72, P₂O₅ 32.49, As₂O₅ 0.64, F 6.76, O=F -2.85, H₂O 13.35 (structure), total 94.74 wt%. The presence of H₂O and OH and the absence of CO₃ were confirmed by FTIR spectroscopy. The empirical formula (based on 14 anions) is: (Na_0.87Ca_0.13Mg_0.04)_Σ1.04(Al_2.96Fe³⁺_0.04)_Σ3.00(P_1.96As_0.03)Σ_1.99O_8.12(OH)_2.35F_1.53·2H₂O. Fluorowardite is tetragonal, P4₁2₁2, a = 7.077(2), c = 19.227(3) Å, V = 962.8(5) ų, and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs in Å(I)(hkl)]: 4.766(100)(004,103); 3.099(75) (211,203); 3.008(62)(115,212); 2.834(28)(204,213); 2.597(56)(205); 1.7628(32)(400,401); 1.6592(29) (multiple); and 1.5228(49)(423, 2·2·10). The structure of fluorowardite (R₁ = 3.15% for 435 F_o > 4σF) contains layers parallel to {001} consisting of Alφ₆ (φ = F, O, OH or H₂O) octahedra, PO₄ tetrahedra, and NaO₆(H₂O)2 polyhedra. The two independent Alφ₆ octahedra link by corner-sharing to form a square array. Each PO₄ tetrahedron shares corners with three adjacent octahedra in the same square array and a fourth corner with an octahedron in the next layer. The Na atoms reside in the “cavities” in the square array, forming bonds only to O atoms in the same layer. Of the two nearly identical OH sites in the wardite structure, only one is occupied by F in the fluorowardite structure. This is an interesting example of a structure in which OH and F are selectively incorporated into two different, but similar, sites as the result of rather subtle hydrogen bonding influences.