Hartkoppeite, a new mineral with a novel polyoxometalate sandwich cluster

Hartkoppeite (IMA2025-056), Ca4Mn2+Mo6+6As5+4O32(OH)2(H2O)14·5H2O, is a newly approved mineral species from the Fuchs quarry, Hartkoppe, Sailauf, Bavaria, Germany. Hartkoppeite occurs as elongated and striated tabular crystals up to about 0.2 mm long. Tablets are flattened on {110}, elongated and striated parallel to [001], and exhibit the forms {110}, {1-10}, and {001}. The mineral is colorless and transparent with vitreous luster and white streak. The Mohs hardness is ∼2½. The mineral has brittle tenacity, stepped fracture, and three cleavages: {110} very good, {001} and {1-10} good. The measured density is 2.88(2) g·cm-3. The mineral is optically biaxial (+), α = 1.692(5), β = 1.693(5), γ = 1.718(5) (white light); 2Vmeas = 22(1)°; moderate r < v dispersion; nonpleochroic. Electron probe microanalysis provided the empirical formula (Ca3.88Mn2+0.12)Σ4.00Mn2+1.00Mo6+6.01As5+3.99O32.38(OH)1.62(H2O)14·K0.38(H2O)4.62. Hartkoppeite is triclinic, P-1, a = 13.34610(15), b = 13.53840(12), c 14.3087(2) Å, α = 71.080(5), β = 82.936(6), γ = 68.792(5)°, V = 2280.00(10) Å3, and Z = 2. The crystal structure (R1 = 0.0478 for 6216 I > 2σI reflections) contains a novel polyoxometalate sandwich cluster of formula {Ca4Mn2+2(H2O)10[Mo6+6As5+4O32(OH)2]2}8-.