GUNTERITE, Na_4(H_2O)_(16)(H_2V_(10)O_(28))·6H_2O, A NEW MINERAL SPECIES WITH A DOUBLY-PROTONATED DECAVANADATE POLYANION: CRYSTAL STRUCTURE AND DESCRIPTIVE MINERALOGY

摘要

Gunterite, Na_4(H_2O)_(16)(H_2V_(10)O_(28))·6H_2O, 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 α 1.735(5), β 1.770(5) and γ 1.825(5); 2V is equal to 78° (white light). The dispersion v < r is strong and parallel. Optical orientation; X = b, Y ≈ 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): (Na_(3.20)K_(0.02)Ca_(0.87))_(Σ4.09)[H_(1.06)(V_(9.99)Al_(0.01))_(Σ10)O_(28)]·22H_2O. The simplified formula of gunterite is Na_4(H_2O)_(16)(H_2V_(10)O_(28))·6H_2O. There is extensive substitution of Ca for Na in gunterite, yielding a structural formula of (Na_(4–x)C_ax)_(Σ4.00)(H_2O)_(16)(H_(2–x)V_(10)O_(28))·6H_2O; 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) A, β 130.187(9)°, V 2026.6(4) A~3, and Z = 2. The strongest four lines in the diffraction pattern [d in A(I)hkl] are: 10.01(100)201,001, 8.44(72)110, 8.09(46)111, and 2.997(29)331,401. The atomic arrangement of gunterite was refined to R_1 = 0.0632. The structural unit is a doubly-protonated decavanadate polyanion, {H_2[V_(10)O_(28)]}. 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 A apart, and significant Ca occupies that site. Several of the H_2O 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.