• journal_title：The Canadian Mineralogist
• Contributor：John Rakovan ; Gregory R. Schmidt ; Mickey E. Gunter ; Barbara Nash ; Joe Marty ; Anthony R. Kampf ; William S. Wise
• Publisher：Mineralogical Association of Canada
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.49.5.1253
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：49
• issue：5
• firstpage：1253
• section：Articles

We report on the discovery, description and solution of the structure of a new member of the pascoite family of minerals, hughesite, from the Sunday mine, Gypsum Valley, San Miguel County, Slick Rock District, Colorado, USA (38°4’19” N, 108°48’15” W). Orange to golden orange crystals of hughesite occur in efflorescent crusts, averaging 2 mm thick, on the sandstone walls of mine workings and in rock fractures. Hughesite forms through the oxidation of corvusite, (Na,Ca,K)_1–x (V⁵⁺,V⁴⁺,Fe²⁺)₈O₂₈•4H₂O, and montrosite, (V³⁺,Fe²⁺,V⁴⁺)O(OH), the primary vanadium oxide phases present, as they react with acidic, oxidizing groundwater. Crystals vary in habit, including blocky, spear-shaped, and platy, with one good cleavage on (001). Crystals are transparent to translucent with a subadamantine luster, and upon partial dehydration, they become opaque. Hughesite is biaxial (–), with α 1.698(5), β 1.740(5), γ 1.770(5), and the measured 2V_589nm is 84(2)°. It exhibits a strong r > v dispersion, and is pleochroic with X = Y light golden yellow, and Z dark golden yellow. Its absorption is characterized by Z > Y = X. Electron-probe microanalysis and the crystal-structure solution provided the empirical formula Na_2.99Al_1.05(V₁₀O₂₈)•22 H₂O (based upon V = 10 apfu). The ideal formula of hughesite is Na₃AlV₁₀O₂₈•22H₂O. The density calculated from the empirical formula using the single-crystal cell data is 2.29 g/cm³. Hughesite is triclinic, space group P1̄, with a 8.668(4), b 10.295(4), c 12.908(5), α 105.826(9), β 97.899(9), γ 103.385(9)°, V 1053.0(8) ų, and Z = 1. The strongest four lines in the powder-diffraction pattern [d in Å(I)hkl]: are 12.24(100)001̄, 8.25(38)100, 9.50(30)01̄0, and 8.99(28)01̄1. The crystal structure, refined to R₁ = 0.0496, consists of two components, the structural unit, which contains the decavanadate (V₁₀O₂₈)⁶⁻ polyanion, and the fully hydrated interstitial complex, composed of two separate components, a Na₃(OH₂)₁₂O₂ trimer with two distinct cation sites, and a Al(OH₂)₆ monomer.