Rruffite, Ca2Cu(AsO4)2·2H 2O, a new member of the roselite group, from Tierra Amarilla, Chile

Hexiong Yang, Robert A. Jenkins, Robert T. Downs, Stanley H. Evans, Kimberly T. Tait

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Abstract

A new mineral species, rruffite, ideally Ca2Cu(AsO 4)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 α 1.725(1), β 1.734(1), γ 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 Ca 2.01Cu1.01(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 CuO 4(H2O)2 octahedra that are linked by corner-sharing with ASO4 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 MO 4(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.

Original languageEnglish (US)
Pages (from-to)877-884
Number of pages8
JournalCanadian Mineralogist
Volume49
Issue number3
DOIs
StatePublished - Jun 1 2011

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Keywords

  • Arsenate
  • Chile
  • Crystal structure
  • Raman spectra
  • Roselite
  • Rruffite
  • Tierra Amarilla
  • X-ray diffraction

ASJC Scopus subject areas

  • Geochemistry and Petrology

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