GUNTERITE, Na4(H2O)16(H2V10O28)•6H2O, A NEW MINERAL SPECIES WITH A DOUBLY-PROTONATED DECAVANADATE POLYANION: CRYSTAL STRUCTURE AND DESCRIPTIVE MINERALOGY

Gunterite Na-4(H2O)(16)(H2V10O28)center dot 6H(2)O, is a new mineral species from the West Sunday mine, Slick Rock district, San Miguel County, Colorado, U. S. A. Crystals of gunterite are tabular on {001} and generally stacked into elongate curved multiple crystals up to 0.5 mm in maximum dimension; the crystals are orange-yellow, with a yellow streak. The mineral displays a subadamantine luster, and is transparent; it does not fluoresce in short-or long-range ultraviolet radiation. Gunterite has a hardness of about 1, a brittle tenacity, and an irregular fracture; no cleavage or parting was observed. The density calculated from the empirical formula using the single-crystal cell data is 2.398 g cm(-3). Gunterite is biaxial (+), with alpha 1.735(5), beta 1.770(5) and gamma 1.825(5); 2V is equal to 78 degrees (white light). The dispersion v < r is strong and parallel. Optical orientation; X = b, Y approximate to c; pleochroism: X yellow, Y orange, Z yellow; Y > X > Z. Gunterite is soluble in water at room temperature. Electron-probe microanalysis and the crystal-structure solution provided the empirical formula (V + Al = 10 apfu): (Na3.20K0.02Ca0.87)(Sigma 4.09)[H-1.06(V9.99Al0.01)(Sigma 10)O-28]center dot 22H(2)O. The simplified formula of gunterite is Na-4(H2O)(16)(H2V10O28)center dot 6H(2)O. There is extensive substitution of Ca for Na in gunterite, yielding a structural formula of (Na4-xCax)(Sigma 4.00)(H2O)(16)(H2-xV10O28)center dot 6H(2)O; the average value of x from the chemical analyses is 0.85. Gunterite is monoclinic, C2/m, with a 19.848(2), b 10.1889(11), c 13.1184(15) angstrom, beta 130.187(9)degrees, V 2026.6(4) angstrom(3), and Z = 2. The strongest four lines in the diffraction pattern [d in angstrom(I) hkl] are: 10.01(100)(2) over bar 01,001, 8.44(72) 110, 8.09(46)(1) over bar 11, and 2.997(29)(3) over bar 31,401. The atomic arrangement of gunterite was refined to R-1 = 0.0632. The structural unit is a doubly-protonated decavanadate polyanion, {H-2[V10O28]}. The interstitial units linking the structural units contain two Na polyhedra; the Na2 polyhedron is split into two partially occupied sites that are 0.76 angstrom apart, and significant Ca occupies that site. Several of the H2O molecules in the interstitial complex are disordered. The mineral is named in honor of Mickey E. Gunter, Professor of Mineralogy at the University of Idaho.