Gunterite, Na4(H2O)16(H2V10O28)·6H2O, 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, Yc; pleochroism: X yellow, Y orange, Z yellow; Y > X > Z. Gunterite is soluble in water at room temperature. Electronprobe microanalysis and the crystal-structure solution provided the empirical formula (V + Al = 10 apfu): (Na3.20K0.02Ca0.87)∑4.09[H1.06(V9.99Al0.01)∑10O28]·22H2O. The simplified formula of gunterite is Na4(H2O)16(H2V10O28)·6H2O. There is extensive substitution of Ca for Na in gunterite, yielding a structural formula of (Na4−xCax)∑4.00(H2O)16(H2−xV10O28)·6H2O; 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) Å, β 130.187(9)°, V 2026.6(4) Å3, and Z = 2. The strongest four lines in the diffraction pattern [d in Å(I)hkl] are: 10.01(100)2̄01,001, 8.44(72)110, 8.09(46)1̄11, and 2.997(29)3̄31,401. The atomic arrangement of gunterite was refined to R1 = 0.0632. The structural unit is a doubly-protonated decavanadate polyanion, {H2[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 Å 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.

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