Hartkoppeite, a new mineral with a novel polyoxometalate sandwich cluster

Abstract 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) 10Mo6+6As5+4O32 (OH) 228-.