Davidbrownite-(NH₄), (NH₄,K)₅(V⁴⁺O)₂(C₂O₄)[PO_(2.75)(OH)_(1.25)]₄·3H₂O, a new phosphate–oxalate mineral from the Rowley mine, Arizona, USA

Abstract

Davidbrownite-(NH₄), (NH₄,K)₅ (V⁴⁺O)₂(C₂O₄)[PO_(2.75)(OH)_(1.25)]₄·3H₂O, 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 ~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-(NH₄) 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 [(NH₄)_(3.11)K_(1.73)Na_(0.09)]_(Σ4.93)[(V⁴⁺_(1.92)Mg_(0.01)Al_(0.02))_(Σ1.95)O₂](C₂O₄) [(P_(3.94)As_(0.12))_(Σ4.06)O_(10.94)(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) ų and Z = 2. The crystal structure of davidbrownite-(NH₄) (R₁ = 0.0524 for 2062 I_o > 2σI reflections) consists of a chain structural unit with the formula {(V⁴⁺O)₂(C₂O₄)[PO_(2.75)(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.