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

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 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