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Mcbirneyite, Cu3(VO4)2, a new copper vanadate mineral, has been discovered in the summit crater fumaroles of Izalco volcano, El Salvador. The mineral formed as a sublimate from the volcanic gases and is associated with stoiberite, fingerite, ziesite, euchlorine and thenardite. Mcbirneyite is the naturally occurring analog of synthetic Cu3V2O8. Mcbirneyite is triclinic, P 1 , a=5.3418(9) A, b=6.5100(8) A, c=5.1798(7) A, α=88.61(1)°, β=68.11(1)°, γ=69.22(1)°. The five most intense lines in the diffraction pattern are 4.01 (80; 01 1 ), 3.12 (100; 120), 2.82 (100; 10 1 ), 2.641 (80; 211) and 2.428 A (80; 021). The crystal structure of the mineral has been determined; the independent structure solution demonstrates that mcbirneyite is isostructural with stranskiite, CuZn2 (AsO4)2, and a synthetic Cu phosphate, Cu3(PO4)2. The structure is constructed of (1 1 0) layers of Cu atoms in square-planar and square-based pyramidal coordination with oxygen; adjacent layers are linked by isolated VO4 tetrahedra. Mcbirneyite is black with a metallic luster. The calculated density (Z=1) is 4.50 g cm−3. The mineral is opaque to transmitted light; in reflected light in air the mineral is medium-dark gray and nonpleochroic, with reflectance values as follows: 470 nm, 18.5%; 546 nm, 17.5%; 589 nm, 18.7%; 650 nm, 20.6%. The mineral is named in honor of Alexander R. McBirney, Editor-in-Chief, Journal of Volcanology and Geothermal Research. |
