Kokinosite, Na2Ca2(V 10O28)•24H2O, is a new mineral species from the St. Jude mine, Slick Rock district, San Miguel County, Colorado, USA. Kokinosite typically occurs as tablets or blades up to 0.05 mm thick, with stepped faces, and 1 mm in maximum dimension. Crystals are yellow orange, although sometimes they are tinted orange brown; the streak of the phase is yellow. The luster of kokinosite is subadamantine, and the mineral is transparent; it does not fluoresce in short- or long-wave radiation. The Mohs hardness is ca. 1½. Kokinosite possesses a brittle tenacity, and displays one good cleavage on {01 1¯}. No parting was observed, and fracture is irregular. Density (calc.) = 2.353 g cm−3 on the basis of the empirical formula and the single-crystal cell data. Kokinosite is biaxial (−), with α 1.725(3), β 1.770(5), and γ 1.785(3). The 2V was measured as 56.6(3)° from extinction data, and 2Vcalc = 58.7°. Dispersion is r < v, moderate. The optic orientation was incompletely determined, with X ^ a = 15°, Yb. The mineral is pleochroic: X, Z = orange yellow, Y = orange, and X = Z < Y. Electron probe microanalysis and the crystal structure solution gave the empirical formula (based on V = 10 and O = 52 apfu) (Na1.88K0.01)∑1.89(Ca1.81Sr0.12)∑1.93(V10O28)•24H2O. The simplified structural formula is Na2Ca2(V10O28)•24H2O. Kokinosite is triclinic, P1¯, with a 8.74899(19), b 10.9746(3), c 12.8216(9)Å, α 114.492(8), β 105.093(7), γ 91.111(6)°, V 1070.25(11) Å3, and Z = 1. The strongest four lines in the diffraction pattern are [d in Å(I)(hkl)]: 9.88(100)(010,0 1¯1), 7.92(36)( 1¯01), 8.42(33)(100), and 6.01(31)(1 1¯1,110). The atomic arrangement of kokinosite was solved and refined to R1 = 0.0379 for 3806 Fo > 4σF reflections. The structural unit is a decavanadate polyanion, (V10O28)6−; charge balance in the structure is maintained by the [Na2Ca2(H2O)24]6+ interstitial unit that exists as Na(H2O)6 octahedra and Ca(H2O)8 polyhedra that link into chains by edge- and corner-sharing. The linkage between the structural unit and the interstitial unit occurs solely by hydrogen bonding. The new mineral is named in honor of Michael Kokinos (b. 1927) of Shingle Springs, California, a well-known mineral collector and member of the Micromounters’ Hall of Fame.

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