Davidbrownite-(NH 4 ), (NH 4 ,K) 5 (V 4+ O) 2 (C 2 O 4 )[PO 2.75 (OH) 1.25 ] 4 ·3H 2 O, a new phosphate–oxalate mineral from the Rowley mine, Arizona, USA

Davidbrownite-(NH 4 ), (NH 4 ,K) 5 (V 4+ O) 2 (C 2 O 4 )[PO 2.75 (OH) 1.25 ] 4 ·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 NH 4 + . Other secondary minerals found in association with davidbrownite-(NH 4 ) are antipinite, fluorite, mimetite, mottramite, quartz, rowleyite, salammoniac, struvite, vanadinite, willemite and wulfenite. Crystals of davidbrownite-(NH 4 ) are light green–blue needles or narrow blades up to ~0.2 mm long. 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 [010] zone, probably {100} and {001}. The measured density is 2.12(2) g cm –3 . Davidbrownite-(NH 4 ) is optically biaxial (+) with α = 1.540(2), β = 1.550(5) and γ = 1.582(2) (white light); 2V = 58.5(5)°; moderate r > v dispersion; and orientation Z = b and Y ≈ a . Pleochroism: X = pale blue, Y = nearly colourless, Z = light blue; and Y < X < Z . Electron microprobe analysis gave the empirical formula [(NH 4 ) 3.11 K 1.73 Na 0.09 ] Σ4.93 [(V 4+ 1.92 Mg 0.01 Al 0.02 ) Σ1.95 O 2 ](C 2 O 4 ) [(P 3.94 As 0.12 ) Σ4.06 O 10.94 (OH) 5.06 ]·3H 2 O, with the C and H content provided by the crystal structure. Raman and infrared spectroscopy confirmed the presence of NH 4 and C 2 O 4 . Davidbrownite-(NH 4 ) is monoclinic, P 2 1 / c , with a = 10.356(6), b = 8.923(5), c = 13.486(7) Å, β = 92.618(9)°, V = 1244.9(12) Å 3 and Z = 2. The crystal structure of davidbrownite-(NH 4 ) ( R 1 = 0.0524 for 2062 I o > 2σ I reflections) consists of a chain structural unit with the formula {(V 4+ O) 2 (C 2 O 4 )[PO 2.75 (OH) 1.25 ] 4 } 5– , and a disordered interstitial complex containing five large monovalent cations (NH 4 + and K + ) and three H 2 O groups pfu. Strong hydrogen bonds form links within and between the chains.