RRUFFITE, Ca2Cu(AsO4)2{middle dot}2H2O, A NEW MEMBER OF THE ROSELITE GROUP, FROM TIERRA AMARILLA, CHILE

Abstract

A new mineral species, rruffite, ideally Ca 2 Cu(AsO 4 ) 2 ·2H 2 O, has been found in the oxidation zone of the Cu–As orebody in the Maria Catalina mine, Tierra Amarilla, Chile. It is a secondary mineral and occurs in granular or blocky aggregates and druses. Associated minerals include quartz, barite, mansfieldite, alumopharmacosiderite, conichalcite, metazeunerite, and barahonite-(Al). Rruffite is pale or light blue, transparent with white streak and vitreous luster. It is brittle, has a Mohs hardness of ~3 and a perfect cleavage on {010}, and is twinned on (100). The measured and calculated densities are 3.79(3) and 3.77(2) g/cm 3 , respectively. Rruffite is insoluble in water, acetone, and hydrochloric acid. Optically, it is biaxial (−), with α 1.725(1), β 1.734(1), γ 1.740(1), 2 V meas 80(2)°, Y = b , X ∧ c 49°, and it does not fluoresce under long- or short-wave ultraviolet light. The dispersion is weak, with r v . An electron-microprobe analysis yielded the empirical formula Ca 2.01 Cu 1.01 (AsO 4 ) 2.02 ·1.9H 2 O. Rruffite, isostructural with roselite, is monoclinic, with space group P 2 1 / c and unit-cell parameters a 5.8618 (2), b 12.7854 (5), c 5.7025 (2) A, β 109.425(2)°, and V 403.05(3) A 3 . Its structure is characterized by isolated CuO 4 (H 2 O) 2 octahedra that are linked by corner-sharing with AsO 4 tetrahedra to form the krohnkite-type chains parallel to the c axis. These chains are linked together by large Ca cations and hydrogen bonding. Owing to the strong Jahn–Teller effect, the M O 4 (H 2 O) 2 octahedron in rruffite is the most distorted of all known roselite-group minerals ( M = Cu, Co, Mn, Mg, and Zn) in terms of the quadratic elongation of the octahedra. The Raman spectra of rruffite resemble those of arsenate minerals of the roselite group.