The holotype specimen, composed of numerous crystals on matrix, is deposited in the collection of the National Museum of Natural History of the Smithsonian Institution, Washington D.C., U.S.A. 1952), Professor of Mineralogy at the University of Vermont. The name of the mineral honors John Michael Hughes (b. It consists of two components: (1) the structural unit, which contains the decavanadate (V 10O 28) 6− polyanion, and (2) the fully hydrated interstitial complex, composed of two separate components, a Na 3(OH 2) 12O 2 trimer with two distinct cation sites, and a Al(OH 2) 6 monomer. It exhibits a strong r > v optical axes dispersion, and is pleochroic with X = Y = light golden yellow 2σ( I). Crystals are platy, with one good cleavage on (001), spear-shaped or blocky, averaging 2 mm thick. Hughesite crystals are orange to golden orange, transparent to translucent with a subadamantine luster (upon partial dehydration, they become opaque).
Crystals of hughesite occur in efflorescent crusts on the sandstone walls of mine workings and in rock fractures, through the oxidation of the primary vanadium oxides corvusite and montroseite. Within the Slick Rock district, uranium and vanadium minerals occur in ore deposits, commonly referred to as rolls, or roll-front deposits.
Uranium–vanadium ore production within the Sunday mine complex is confined to the Upper Jurassic Wash Member, permeable, carbonaceous fluvial sandstone of the Morrison Formation. The Slick Rock mining district was the location of the first uranium production in the Colorado Plateau geological province. Hughesite (IMA 2009-035a), ideally Na 3AlV 10O 28♲2H 2O, is a new mineral species of the pascoite family from the Sunday mine, Gypsum Valley, San Miguel County, Slick Rock District, Colorado, U.S.A. The holotype is deposited in the Reference Collection of Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, Italy. The basic structural units observed in clinometaborite are six-membered B 3O 3 rings containing two three-coordinated boron atoms and an additional boron atom in a distorted tetrahedral coordination, because of interaction with a H 2O molecule. The structure was refined to a final R 1 of 0.043 for 977 observed reflections with I > 2σ( I). Single-crystal diffraction data were collected (using a BRUKER Apex II diffractometer, graphite-monochromated Mo Kα radiation) and the diffraction pattern confirmed the monoclinic lattice, space group P2 1/ a, with a = 7.127(2), b = 8.842(3), c = 6.773(2) Å, β = 93.21(1)°, and V = 426.1(2) Å 3, values that virtually coincide with those of the synthetic compound. X-ray powder-diffraction data (obtained with Philips PW1830 diffractometer, Cu Kα radiation) provided the following unit-cell parameters: monoclinic unit-cell, space group P2 1 /a, with a = 7.1243(6), b = 8.8468(10), c = 6.7699(7) Å, β = 93.23(1)°, V = 426.00(6) Å 3, and Z = 12. No other data on chemical composition were provided. The infrared spectrum (recorded on a Jasco IRT-3000 spectrometer), shows absorption bands at 11 cm −1 typical of the E′ vibration modes of the BO 2 − ion. Analyses of clinometaborite with an electron microprobe in energy-dispersive mode indicate only the presence of oxygen, a trace amount of Na due to contaminant material, and no elements with atomic number greater than 11. The optical properties of the synthetic counterpart were reported: β-HBO 2 is biaxial (−) with α = 1.434, β = 1.570, and γ = 1.588 (λ = 589 nm). The mineral is not fluorescent either under short-wave or long-wave ultraviolet radiation. The crystal habit is prismatic twinning was not observed. The unaltered mineral is colorless and translucent, with a vitreous luster it becomes chalky white after some months of exposure to open air, and it transforms into orthoboric acid. Clinometaborite forms aggregates of stout crystals up to 2 mm long in an altered pyroclastic breccia, associated with metaborite, sassolite, and adranosite. This new species was found as a sublimate in an active medium temperature (~250 ☌) intracrater fumarole at La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral is named after its composition and symmetry. Clinometaborite (IMA 2010-022), ideally β-HBO 2, is the monoclinic modification of metaboric acid.
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