Kenngottite, Mn32+Fe43+(PO4)4(OH)6(H2O)2, is a new phosphate mineral from the Krásno ore district near Horní Slavkov, Czech Republic. It occurs in association with phosphosiderite, fluorapatite, Mn-rich dufrénite, frondelite, rockbridgeite, morinite, beraunite, strengite, natrodufrénite, fluorite and a K–Mn oxide. It forms brown aggregates up to 3 mm across in tiny cavities of hydrothermally altered black rockbridgeite-group minerals. These aggregates are composed of imperfect laths to fibrous crystals up to 0.5 mm in size. Kenngottite has a light brown streak and a vitreous to pearly lustre. It does not fluoresce under either short- or long-wave ultraviolet light. Its aggregates are opaque; individual crystals or tiny fragments are translucent to transparent. Cleavage is not directly observed, probably parallel to {100}, the Mohs hardness is ~4–5 (by analogy with souzalite and gormanite), and the mineral is brittle with an uneven, stepped fracture. The calculated density is 3.40 g/cm3. Kenngottite is optically biaxial positive, the indices of refraction are α = 1.785(1), β = 1.790(5), γ = 1.810(2), and 2Vmeas. is 50(10)°. Kenngottite is monoclinic, space group P2/a, a = 13.909(10), b = 5.186(4), c = 12.159(9) Å, β = 98.88(1)°, V = 866.5(11) Å3, Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are as follows: d (Å)/I(hkl): 4.87/47(202¯⁠,110); 3.458/89(401¯⁠,310); 3.209/100(203,311,113¯⁠,013,312¯⁠); 3.023/31(113,004); 2.623/46(114¯⁠,204,020); 2.429/49(510,220,314¯⁠); 1.9506/28(024,224¯⁠) and 1.5772/34(624¯⁠). The chemical analyses by electron microprobe yielded (in wt%) Na2O 0.03, MnO 17.82, CaO 0.31, ZnO 0.32, Fe2O3 35.30, Al2O3 2.71, P2O5 30.80, As2O5 0.15, SiO2 0.08, H2Ocalc. 9.60, total 97.12. The resulting empirical formula on the basis of 24 O atoms per formula unit (apfu) is (Mn2.29Fe0.53Ca0.05Zn0.04Na0.01)Σ2.92 (Fe3.51Al0.49)Σ4.00 [(PO4)3.96(AsO4)0.01(SiO4)0.01]Σ3.98(OH)6.42(H2O)1.66. The crystal structure of kenngottite was solved using single-crystal X-ray diffraction data and refined to wRobs = 0.088 for 350 reflections with I > 2σ(I). The structure contains trimeric clusters of face-shared octahedra (Mn–Fe–Mn) that are connected into chains along [001] by sharing edges with Mn-centred octahedra. The chains are corner-connected along [100] to linear corner-shared trimers of Fe-centred octahedra and along [010] via corner-connected PO4 tetrahedra. Monoclinic kenngottite is structurally related to triclinic souzalite and gormanite. It is named in honor of the prominent mineralogist Prof. Gustav Adolf Kenngott (1818–1897) from the University of Zürich (Switzerland) for his contributions to systematic mineralogy.

GeoRef Subject
Central Europe Czech Republic crystal structure Europe phosphates hydrates metasomatism
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