Davidbrownite-(NH4), (NH4,K)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, a new phosphate-oxalate mineral from the Rowley mine, Arizona, USA

Kampf AR1, Cooper MA2, Rossman GR3, Nash BP4, Hawthorne FC2 and Marty J5
 

1Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, USA

2Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

3Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA  91125-2500, USA

 4Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA



ABSTRACT

Davidbrownite-(NH4), (NH4,K)5(V4+O)2(C2O4)[PO2.75(OH)1.25]4·3H2O, is a new mineral species from the Rowley mine, Maricopa County, Arizona, U.S.A. It occurs in an unusual bat-guano-related, post-mining assemblage of phases that include a variety of vanadates, phosphates, oxalates and chlorides, some containing NH4+. Other secondary minerals found in association with davidbrownite-(NH4) are antipinite, fluorite, mimetite, mottramite, quartz, rowleyite, salammoniac, struvite, vanadinite, willemite and wulfenite. Crystals of davidbrownite-(NH4) are light green-blue needles or narrow blades up to about 0.2 mm in length. The streak is white, luster is vitreous, Mohs hardness is ca. 2, tenacity is brittle and fracture is splintery. There are two good cleavages in the [010] zone, probably {100} and {001}. The measured density is 2.12(2) g·cm-3. Davidbrownite-(NH4) is optically biaxial (+) with α = 1.540(2), β = 1.550(5), γ = 1.582(2) (white light); 2V = 58.5(5)°; moderate r > v dispersion; orientation Z = b, Y ≈ a. Pleochroism: X pale blue, Y = nearly colorless, Z = light blue; Y < X < Z. Electron microprobe analysis gave the empirical formula [(NH4)3.11K1.73Na0.09]Σ4.93[(V4+1.92Mg0.01Al0.02)Σ1.95O2](C2O4) [(P3.94As0.12)Σ4.06O10.94(OH)5.06]·3H2O, with the C and H content provided by the crystal structure. Raman and infrared spectroscopy confirmed the presence of NH4 and C2O4. Davidbrownite-(NH4) is monoclinic, P21/c, with a = 10.356(6), b = 8.923(5), c = 13.486(7) Å, β = 92.618(9)°, V = 1244.9(12) Å3, and Z = 2. The crystal structure of davidbrownite-(NH4) (R1 = 0.0524 for 2062 Io > 2sI reflections) consists of a chain structural unit with the formula {(V4+O)2(C2O4)[PO2.75(OH)1.25]4}5–, and a disordered interstitial complex containing five large monovalent cations (NH4+ and K+) and three H2O groups pfu. Strong hydrogen bonds form links within and between the chains.