Pseudodickthomssenite (IMA2021-027), Mg(VO3)2·8H2O, is a new mineral discovered at the Pickett Corral mine, Bull Canyon, Montrose County, Colorado, USA. The mineral formed from the oxidation of montroseite-corvusite assemblages in a moist environment and occurs on asphaltite and on montroseite-corvusite-bearing sandstone in association with dickthomssenite, gypsum, huemulite, lasalite, and trebiskyite. Pseudodickthomssenite is known only from a diverging cluster of striated needles up to about 500 μm long and 20 μm in diameter; the mineral is light tan in color, with a white streak and a silky luster. The crystals are brittle but somewhat flexible in thin fibers. There are two excellent cleavages, {011} and forumla, and the fracture is splintery. The measured density is 1.97(2) g/cm3. Electron probe microanalysis provided the empirical formula [Mg0.99Ca0.01]Σ1.00(V5+O3)2·8H2O. Pseudodickthomssenite is triclinic, Pforumla, a = 7.3566(6) Å, b = 9.4672(9) Å, c = 9.5529(9) Å, α = 104.205(7)°, β = 100.786(7)°, γ = 100.157(7)°, V = 616.08(10) Å3, and Z = 2. The structure of pseudodickthomssenite (R1 = 0.0307 for 1124 I > 2σI reflections) contains V5+O5 polyhedra that link by edge-sharing to form a zig-zag [VO3] chain. MgO2(H2O)4 octahedra link the [VO3] chains into a Mg(H2O)4[VO3]4 sheet and the sheets are linked to one another via a complex network of hydrogen bonding involving Mg(H2O)6 octahedra and isolated H2O groups. The structure is very similar to that of dickthomssenite.

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