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Rruffite (IMA 2009-077) type-description

Posted by Marco E. Ciriotti  
Marco E. Ciriotti July 29, 2011 09:14AM
▪ Yang, H., Jenkins, R.A., Downs, R.T., Evans, S.H., Tait, K.T. (2011): Rruffite, Ca2Cu(AsO4)2·2H2O,a new member of the roselite group, from Tierra Amarilla, Chile. Canadian Mineralogist, 49, 877-884.

A new mineral species, rruffite, ideally Ca2Cu(AsO4)2·2H2O, 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/cm3, respectively. Rruffite is insoluble in water, acetone, and hydrochloric acid. Optically, it is biaxial (–), with alpha 1.725(1), β 1.734(1), gamma 1.740(1), 2Vmeas 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 Ca2.01Cu1.01(AsO4)2.02·1.9H2O. Rruffite, isostructural with roselite, is monoclinic, with space group P21/c and unit-cell parameters a 5.8618 (2), b 12.7854 (5), c 5.7025 (2) Å, β 109.425(2)°, and V 403.05(3) Å3. Its structure is characterized by isolated CuO4(H2O)2 octahedra that are linked by corner-sharing with AsO4 tetrahedra to form the kröhnkite-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 MO4(H2O)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.
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