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Revision 1 1 2 Fluorowardite, Naal3(PO4)2(OH)2F2·2H2O, The 1 Revision 1 2 3 Fluorowardite, NaAl3(PO4)2(OH)2F2·2H2O, the fluorine analogue of wardite from the Silver Coin 4 mine, Valmy, Nevada. 5 1 2 3 3 6 ANTHONY R. KAMPF *, PAUL M. ADAMS , ROBERT M. HOUSLEY , AND GEORGE R. ROSSMAN 7 8 1Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition 9 Boulevard, Los Angeles, CA 90007, USA 10 2126 South Helberta Avenue, #2, Redondo Beach, California 90277, USA 11 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, 12 CA 91125, USA 13 *Email: [email protected] 14 15 Abstract 16 Fluorowardite (IMA2012-016), NaAl3(PO4)2(OH)2F2·2H2O, the F analogue of wardite, is 17 a new mineral from the Silver Coin mine, Valmy, Iron Point district, Humboldt County, Nevada, 18 USA, where it occurs as a low-temperature secondary mineral in complex phosphate assemblages 19 rich in Al, Na, and F. Fluorowardite forms colorless to white or cream-colored, tetragonal- 20 pyramidal crystals up to 0.1 mm in diameter. The streak is white. Crystals are transparent to 21 translucent, with vitreous to pearly luster. The Mohs hardness is about 5, the tenacity is brittle, 22 the fracture is irregular, and crystals exhibit one perfect cleavage on {001}. The calculated 23 density is 2.760 g/cm3. Optically, fluorowardite is uniaxial positive, with ω = 1.576(2) and ε = 24 1.584(2) (white light) and is non-pleochroic. Electron microprobe analyses (average of 8) 25 provided: Na2O 6.27, CaO 1.74, MgO 0.42, Al2O3 35.21, Fe2O3 0.72, P2O5 32.49, As2O5 0.64, F 26 6.76, O=F -2.85, H2O 13.35 (structure), total 94.74 wt%. The presence of H2O and OH and the 27 absence of CO3 were confirmed by FTIR spectroscopy. The empirical formula (based on 14 3+ 28 anions) is: (Na0.87Ca0.13Mg0.04)Σ1.04(Al2.96Fe 0.04)Σ3.00(P1.96As0.03)Σ1.99O8.12(OH)2.35F1.53·2H2O. 3 29 Fluorowardite is tetragonal, P41212, a = 7.077(2), c = 19.227(3) Å, V = 962.8(5) Å , and Z = 4. 30 The eight strongest lines in the X-ray powder diffraction pattern are [dobs in Å(I)(hkl)]: 31 4.766(100)(004,103); 3.099(75)(211,203); 3.008(62)(115,212); 2.834(28)(204,213); 32 2.597(56)(205); 1.7628(32)(400,401); 1.6592(29)(multiple); and 1.5228(49)(423,2·2·10). The 33 structure of fluorowardite (R1 = 3.15% for 435 Fo > 4σF) contains layers parallel to {001} 34 consisting of Alφ6 (φ = F, O, OH or H2O) octahedra, PO4 tetrahedra, and NaO6(H2O)2 polyhedra. 35 The two independent Alφ6 octahedra link by corner-sharing to form a square array. Each PO4 36 tetrahedron shares corners with three adjacent octahedra in the same square array and a fourth 37 corner with an octahedron in the next layer. The Na atoms reside in the “cavities” in the square 38 array, forming bonds only to O atoms in the same layer. Of the two nearly identical OH sites in 39 the wardite structure, only one is occupied by F in the fluorowardite structure. This is an 40 interesting example of a structure in which OH and F are selectively incorporated into two 41 different, but similar, sites as the result of rather subtle hydrogen bonding influences. 42 43 Keywords: fluorowardite; new mineral; crystal structure; hydrogen bonding; FTIR spectroscopy; 44 Raman spectroscopy; electron microprobe analysis; Silver Coin mine, Valmy, Nevada. 45 46 Introduction 47 Wardite, NaAl3(PO4)2(OH)4·2H2O, was first described by Davison (1896) from cavities in 48 variscite nodules from Utah. Although not specifically mentioned in that paper, the type locality 49 is the well-known Clay Canyon deposit near Fairfield in Utah County, which is also the type 50 locality for englishite, gordonite, millisite, montgomeryite, and overite. Since that time, wardite 51 has been reported from many other localities world-wide, but has previously not been reported to 52 contain significant amounts of F. The structure of wardite was solved by Fanfani et al. (1970) 53 using a crystal from the type locality. They reported the structure to include two distinct OH sites. 54 The recognition of wardite crystals in an F-rich secondary phosphate assemblage at the 55 Silver Coin mine near Valmy, Nevada, led us to extensively survey wardite crystals for high F 56 contents that could correspond to the F analogue. We found F to be present in most of the wardite 57 crystals in this assemblage, with contents reaching levels sufficient to take the place of nearly 58 half of the OH in the structure. The refinement of the structure of one of these crystals (see 59 below) showed the F to selectively occupy one of the OH sites, where it is strongly dominant 60 over OH. While it is not entirely clear whether it is possible for both OH sites to be dominated by 61 F, its dominance at one of the sites is sufficient to qualify the phase as a new mineral and the F 62 analogue of wardite. 63 The name is based upon the mineral being the F analogue of wardite. Note that “fluoro-“ 64 rather than “fluor-“ is used as the prefix to make pronunciation more straightforward. The new 65 mineral and name have been approved by the Commission on New Minerals, Nomenclature, and 66 Classification of the International Mineralogical Association (IMA2012-016). Two cotype 67 specimens are housed in the collections of the Mineral Sciences Department, Natural History 68 Museum of Los Angeles County, catalogue numbers 57659 and 63810. Specimen 57659 is also a 69 cotype for meurigite-Na (Kampf et al. 2009). 70 71 Occurrence and paragenesis 72 Fluorowardite occurs in the phosphate stope at the Silver Coin mine, Valmy, Iron Point 73 district, Humboldt County, Nevada, USA (40°55'44"N 117°19'26"W). It occurs in association 74 with alunite, barite, cacoxenite, chlorargyrite, fluorapatite, goethite, gorceixite (F-rich), 75 iangreyite, iodargyrite, jarosite, kidwellite, kintoreite/plumbogummite, krásnoite, leucophosphite, 76 lipscombite/zinclipscombite, meurigite-Na, metavariscite, millisite (F-rich), morinite, quartz, 77 rockbridgeite, strengite/variscite, and turquoise/chalcosiderite (minerals separated by slashes 78 exhibit variations in chemistry between the two species). A partial list of mineral species 79 occurring at the Silver Coin mine is given by Thomssen and Wise (2004). The Silver Coin mine 80 is the type locality for zinclipscombite (Chukanov et al. 2006), meurigite-Na (Kampf et al. 2009), 81 iangreyite (Mills et al. 2011), and krásnoite (Mills et al. 2012). Fluorowardite is a low- 82 temperature secondary mineral in complex phosphate assemblages rich in aluminium, sodium, 83 and fluorine. 84 85 Physical and optical properties 86 Fluorowardite occurs as colourless to white or cream-coloured, tetragonal-pyramidal 87 crystals truncated by the basal pinacoid. The forms observed are {001} (prominent and lustrous), 88 {011} and/or {012} (prominent, irregular and striated parallel to [100]), and {100} (common, 89 irregular and striated parallel to [100]); A variety of other minor forms, e.g. {114}, are observed 90 on SEM images, but are uncommon (Figs. 1 to 3). No twinning was observed. Crystals occur as 91 isolated individuals up to 0.1 mm in diameter and as drusy aggregates. 92 The streak is white. Crystals are transparent to translucent with vitreous to pearly luster. 93 Fluorowardite does not fluoresce in long or short wave ultraviolet light. The Mohs hardness is 94 about 5, the tenacity is brittle, the fracture is irregular, and crystals exhibit one perfect cleavage 95 on {001}. Attempts to measure the density by sink-float failed because of the small size of the 96 crystals and their near invisibility in available liquids. The calculated density based on the 97 empirical formula and the unit cell refined from the single-crystal data is 2.760 g/cm3. 98 Fluorowardite is unreactive and insoluble in concentrated HCl, concentrated H2SO4, and 70% 99 HNO3, observed over the course of several hours. Optically, fluorowardite is uniaxial positive, 100 with ω = 1.576(2) and ε = 1.584(2), measured in white light. The mineral is non-pleochroic. 101 102 Infrared spectroscopy 103 An FTIR spectrum (Figure 4) was obtained with a Thermo-Nicolet Model 6700 104 spectrometer equipped with a Continuum microscope. A small amount of material was crushed in 105 a diamond compression cell and analyzed in transmission through one diamond window. Band 106 assignments are according to Breitinger et al. (2004). The main observed bands (in 107 wavenumbers) are: 3615 and 3544 (OH stretching), 3274 and 3153 (H2O stretching), 1659 (H2O 108 bending), 1162 and 1131 [δAl2(OH)] and 1080 (PO4 antisymmetric stretching), and 1008 (PO4 109 symmetric stretching). 110 111 Raman spectroscopy 112 Raman spectroscopic micro-analyses were carried out using a Renishaw M1000 micro- 113 Raman spectrometer system. Light from a 514.5 nm argon laser was focused onto the sample 114 with a 100× objective lens, and at 100% power could provide approximately 5 mw of power at 115 the sample, in a spot size of about 1 μm. Spectral peak positions were periodically calibrated 116 against a silicon standard and rarely varied more than 1 cm-1. All spectra were obtained with a 117 dual-wedge polarization scrambler inserted directly above the objective lens to minimize the 118 effects of polarization. 119 As a reference for the fluorowardite, we first obtained Raman spectra along the a- and c- 120 axes of a Rapid Creek, Yukon Territory, Canada, wardite crystal from the Caltech collection 121 (CIT-15080).
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