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

摘要

Davidbrownite-(NH4), (NH4,K)(5)(V4+O)(2)(C2O4)[PO2.75(OH)(1.25)]4 center dot 3H(2)O, is a new mineral species from the Rowley mine, Maricopa County, Arizona, USA. 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 similar to 0.2 mm tong. The streak is white, the lustre is vitreous, Mohs hardness is ca. 2, tenacity is brittle and fracture is splintery. There are two good cleavages in the [0101 zone, probably 11001 and {001}. The measured density is 2.12(2) g cm(-3). Davidbrownite-(NH4) is optically biaxial (+) with alpha = 1.540(2), beta = 1.550(5) and gamma = 1.582(2) (white light); 2V = 58.5(5)degrees; moderate r > v dispersion; and orientation Z = b and Y approximate to a. Pleochroism: X = pale blue, Y = nearly colourless, Z = light blue; and Y < X < Z. Electron microprobe analysis gave the empirical formula [(NH4)(3.11)K1.73Na0.09[Sigma(4.93)f[(V1.924+Mg0.01Al0.02)(Sigma 1.95)O-2)(C2O4) (P3.94As0.12)Sigma O-4.06(10.94)(OH)(5.06)]center dot 3H(2)O, 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, P2(1)/c, with a = 10.356(6), b = 8.923(5), c = 13.486(7) angstrom,beta = 92.618(9)degrees, V= 1244.9(12) angstrom(3) and Z = 2. The crystal structure of davidbrownite-(NH4) (R-1 = 0.0524 for 2062 I-o > 2 sigma I 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.