Hagstromite, Pb8Cu2+(Te6+O6)2(CO3)Cl4, a new lead–tellurium oxysalt mineral from Otto Mountain, California, USA

Abstract

AbstractHagstromite, Pb8Cu2+(Te6+O6)2(CO3)Cl4, (IMA2019-093) is a new tellurate mineral from Otto Mountain near Baker, California, USA. It occurs on quartz in association with cerussite, fuettererite and thorneite. It is a secondary oxidation zone mineral and is presumed to have formed by oxidation of earlier formed tellurides, chalcopyrite and galena. Hagstromite occurs as light yellow–green blades, up to ~100 μm long. Crystals are transparent with adamantine to silky lustre. The mineral is brittle with two cleavages providing splintery fracture; the Mohs hardness is probably between 2 and 3. The calculated density is 7.062 g cm–3. Hagstromite is optically biaxial (+), with calculated indices of refraction for α = 2.045, β = 2.066 and γ = 2.102; 2Vmeas = 76(1)°; and optical orientation X = b, Y = a and Z = c. The Raman spectrum of hagstromite exhibits similarities with those of agaite and thorneite and confirms the presence of CO32–. The electron microprobe analyses provided the empirical formula Pb8.07Cu2+0.98Te6+1.96C1.17Cl3.83O15.34. Hagstromite is orthorhombic, space group Ibam, with a = 23.688(17), b = 9.026(8), c = 10.461(8) Å, V = 2237(3) Å3 and Z = 4. The crystal structure of hagstromite (R1 = 0.0659 for 284 I > 2σI reflections) contains a novel Cu2+Te6+2O12 chain assembled of corner-sharing Cu2+O4 squares and Te6+O6 octahedra. The O atoms in the chains form bonds with Pb2+ cations, which in turn bond to Cl– and CO32– anions, thereby creating a framework structure.