Fluorowardite, NaAl3(PO4)2F2(OH)2(H2O)2,

the fluorine analogue of wardite from the Silver Coin mine, Valmy, Nevada.



Kampf AR
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA

Adams PM
126 South Helberta Avenue, #2, Redondo Beach, California 90277, USA

Housley RM and Rossman GR
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA


Abstract

Fluorowardite, NaAl3(PO4)2F2(OH)2(H2O)2, is a new mineral from the Silver Coin mine, Valmy, Iron Point district, Humboldt County, Nevada, USA, 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 6.06, MgO 0.41, CaO 1.87, Al2O3 36.87, P2O5 32.10, F 7.81, O=F -3.29, H2O 14.64 (structure), total 96.47 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: (Na0.81Ca0.14Mg0.04)Σ0.99Al3.00P1.88O7.55(OH)2.74(H2O)2.00F1.71. Fluorowardite is tetragonal, P41212, a = 7.077(2), c = 19.227(3) , V = 963.0(5) 3  and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [dobs in (I)(hkl)]: 4.766(100)(004,103); 3.0993(75)(211,203); 3.0078(62)(115,212); 2.8344(28)(204,213); 2.5965(56)(205); 1.7628(32)(400,401); 1.6592(29)(multiple); and 1.5228(49)(423,2210). The structure of fluorowardite (R1 = 3.15% for 435 Fo > 4sF) 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.



color fluorowardite

Colorless fluorowardite on blue turquoise. Stacked image. Field of view, approximately 200 micrometers. Individual tetragonal crystals show near-cubic faces.Silver Coin mine, Nevada.



SEM fluorowardite
Back scattered electron image of fluorowardite from the Silver Coin mine, showing compositional zonation (Na,Ca where Ca is the brighter.
The small circles near the center of the image are from beam damage from the SEM.
The white crystals in the lower half of the image are turquoise and the black areas are void space.








revised 29-May-2015