Okieite, Mg3[V10O28]·28H2O, is a new decavanadate mineral from the Burro mine, Slick Rock district, San Miguel County, Colorado, USA (type locality); the mineral is also found at the Hummer mine, Paradox Valley, Montrose County, also in Colorado. The mineral is rare; it occurs with dickthomssenite on montroseite- and corvusite-bearing sandstone. Crystals of okieite from the Burro mine are equant to prismatic, commonly appearing like curving columns (up to about 3 mm in length) and often exhibiting rounded faces. The streak of okieite is light orange yellow, and the luster is vitreous. The Mohs hardness is ca. 1½, the tenacity is brittle, the fracture is curved or conchoidal, there is no cleavage, and the measured density is 2.20(2) g/cm3. Okieite is biaxial (–), with α = 1.720(3), β = 1.745(3), γ = 1.765(3) (white light); 2V = 84(2)° with strong r < v dispersion. The optical orientation is X ^ a = 37°, Y ^ c = 28°, Z ^ b = 31°. No pleochroism is observed in okieite. The empirical formula from electron-probe microanalysis (calculated on the basis of V = 10 and O = 56 apfu as indicated by the structure) is Mg2.86[H0.28V5+10O28]·28H2O. Okieite is triclinic, forumla, with a 10.55660(19), b 10.7566(2), c 21.3555(15)Å, α 90.015(6), β 97.795(7), γ 104.337(7)°, and V 2326.30(19)Å3, as determined by single-crystal X-ray diffractometry. The strongest four diffraction lines in the powder diffractograms are [d in Å(I)(hkl)]: 9.71(100)forumla; 8.32(19)forumla; 11.04(17)(002); and 6.42(12)(110, forumla. The atomic arrangement of okieite [R1 = 0.0352 for 11,327 I > 2σI reflections] consists of a {V10O28}6– (decavanadate) structural unit and a {[Mg(H2O)6]3·10H2O}6+ interstitial complex. Only hydrogen bonding links the structural unit with the components of the interstitial complex. Okieite is isostructural with synthetic Mg3[V10O28]·28H2O. The name okieite is for Craig (“Okie”) Howell of Naturita, Colorado.

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