HUGHESITE, Na3Al(V10O28)*22H2O, A NEW MEMBER OF THE PASCOITE FAMILY OF MINERALS FROM THE SUNDAY MINE, SAN MIGUEL COUNTY, COLORADO

Abstract

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 (V 5+ ,V 4+ ,Fe 2+ ) 8 O 28 •4H 2 O, and montrosite, (V 3+ ,Fe 2+ ,V 4+ )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 2 V 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 2.99 Al 1.05 (V 10 O 28 )•22 H 2 O (based upon V = 10 apfu ). The ideal formula of hughesite is Na 3 AlV 10 O 28 •22H 2 O. The density calculated from the empirical formula using the single-crystal cell data is 2.29 g/cm 3 . Hughesite is triclinic, space group P 1, 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) A 3 , and Z = 1. The strongest four lines in the powder-diffraction pattern [ d in A(I) hkl ]: are 12.24(100)001, 8.25(38)100, 9.50(30)010, and 8.99(28)011. The crystal structure, refined to R 1 = 0.0496, consists of two components, the structural unit, which contains the decavanadate (V 10 O 28 ) 6− polyanion, and the fully hydrated interstitial complex, composed of two separate components, a Na 3 (OH 2 ) 12 O 2 trimer with two distinct cation sites, and a Al(OH 2 ) 6 monomer.