Abstract

Postite, Mg(H2O)6Al2(OH)2(H2O)8(V10O28)·13H2O, is a new mineral species from the Vanadium Queen mine, La Sal Creek Canyon, and the Blue Cap mine, Lyon Canyon Creek, San Juan County, Utah, U.S.A. Postite occurs as very thin, needle-like prisms with pyramidal terminations; crystals commonly occur in parallel bundles and grow in divergent and “jackstraw” masses. Individual crystals are up to 1 mm long and 50 μm in diameter, and are golden-yellow with a yellow streak. The mineral is transparent, with a subadamantine luster; it does not fluoresce in short- or long-wave ultraviolet radiation. Postite has a Mohs hardness of approximately 2 and brittle tenacity. The mineral has one good cleavage on {001} and at least two perfect cleavages parallel to [001], possibly {100} and {010}. The fracture is splintery. The density calculated from the empirical formula using the single-crystal cell data is 2.226 g/cm3. Postite is biaxial (+) with a 2V angle of 71º. Indices of refraction for postite are α 1.727(3), β 1.733(3), and γ1.745(3). The optical orientation is X = c, Y = b, Z = a. Dispersion was not observed and pleochroism was not perceptible. Electron probe microanalysis and the crystal structure solution gave the empirical formula (Mg0.97Na0.06Ca0.04Sr0.01K0.01)∑1.09Al1.94 [(OH)1.92(H2O)0.08]‡”2.00 (V10O28)·27H2O. The simplified structural formula of postite is Mg(H2O)6Al2(OH)2(H2O)8(V10O28) ·13H2O. Postite is orthorhombic, Pccn, with a 16.3357(6), b 24.2434(17), c 11.7343(4) Å, V 4647.2(4) Å3, and Z = 4. The strongest four lines in the diffraction pattern are [d in Å(I)(hkl)]: 8.937(100)(111), 12.190(90)(020), 3.771(24)(113), and 8.248(22)(200). The atomic arrangement of postite was solved and refined to R1 = 0.0358. The structural unit is a decavanadate polyanion, [V10O28]6−; charge balance in the structure is maintained by the [Mg(H2O)6Al2(OH)2(H2O)8 ·13 H2O]6+ interstitial unit. The interstitial unit consists of a [Mg(H2O)6]2+ monomer, an [Al2(OH)2(H2O)8]4+ edge-sharing dimer, and thirteen additional H2O molecules. The linkage between the structural unit and the interstitial unit results from hydrogen bonding between oxygen atoms of the structural unit with hydrogen atoms of the [Al2(OH)2(H2O)8]4+ edge-sharing dimer and those of isolated H2O molecules. The new mineral is named in honor of Dr. Jeffrey E. Post (b. 1954), Curator-in-Charge of the National Gem and Mineral Collection, U. S. National Museum of Natural History (Smithsonian Institution).

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