Unusual Mineral Diversity in a Hydrothermal Vein-type Deposit: the Clara Mine, SW Germany, as a Type Example

The Clara baryte-fluorite-(Ag-Cu) mine exploits a polyphase, mainly Jurassic to Cretaceous, hydrothermal unconformity vein-type deposit in the Schwarzwald, SW Germany. It is the type locality for 13 minerals, and more than 400 different mineral species have been described from this occurrence, making it one of the top five localities for mineral diversity on Earth. The unusual mineral diversity is mainly related to the large number and diversity of secondary, supergene, and low-temperature hydrothermal phases formed from nine different primary ore-gangue associations observed over the last 40 years; these are: chert/quartz-hematite-pyrite-ferberite-scheelite with secondary W-bearing phases; fluorite-arsenide-selenide-uraninite-pyrite with secondary selenides and U-bearing phases (arsenates, oxides, vanadates, sulfates, and others); fluorite-sellaite with secondary Sr- and Mg-bearing phases; baryte-tennantite/tetrahedrite ss-chalcopyrite with secondary Cu arsenates, carbonates, and sulfates; baryte-tennantite/tetrahedrite ss-polybasite/pearceite-chalcopyrite, occasionally accompanied by Ag±Bi±Pb-bearing sulfides with secondary Sb oxides, Cu arsenates, carbonates, and sulfates; baryte-chalcopyrite with secondary Fe- and Cu-phosphates; baryte-pyrite-marcasite-chalcopyrite with secondary Fe- and Cu-sulfates; quartz-galena-gersdorffite-matildite with secondary Pb-, Bi-, Co-, and Ni-bearing phases; and siderite-dolomite-calcite-gypsum/anhydrite-quartz associations. The first eight associations are of Jurassic to Cretaceous age and are related to at least eight different pulses of hydrothermal fluids (plus the meteoric fluids responsible for supergene oxidation); the last association is of Neogene age. Spatial juxtaposition of the various primary associations produces overlaps of the secondary associations. In addition to natural oxidation processes, two anthropogenic additions led to specific mineral associations: (1) lining of the adit walls with concrete resulted in high-pH assemblages of mainly Ca-rich phases, including arsenates and sulfates; and (2) the addition of hydrofluoric acid to counterbalance the high-pH fluids produced by power plant ashes introduced into the exploited parts of the mine resulted in fluoride assemblages of alkali and alkaline earth metals. This contribution describes for the first time all types of assemblages and associations observed and physicochemical considerations and models of formation for some of the supergene associations. The meteoric fluids responsible for element mobilization and redistribution, and for the formation of new, secondary phases, interacted with wall rocks prior to and during percolation through the actual hydrothermal associations. Depending on the amount of reaction with ore, gangue, and host rock phases, the chemical composition of the meteoric fluids and its redox potential may vary over short distances. Hence different mineral assemblages and zoned associations record fluid compositional changes, even on the millimeter to centimeter scale. Unusual mineral diversity at the Clara mine therefore develops from a combination of diverse primary hydrothermal mineralization stages, an unusual number of fluid flow events involving compositionally different fluids, and local equilibrium conditions that change within centimeters during supergene processes involving meteoric fluids and anthropogenic additions.

Nollmotzite, Mg[UV(UVIO2)2O4F3]·4H2O, the first natural uranium oxide containing fluorine

Nollmotzite (IMA2017-100), Mg[UV(UVIO2)2F3O4](H2O)4, is a new uranium oxide fluoride mineral found in the Clara mine, Black Forest Mountains, Germany. Electron microprobe analysis provided the empirical formula (Mg1.06Cu0.02)Σ1.08[UV(UVIO2)2O3.85F3.15][(H2O)3.69(OH)0.31]Σ4.00based on three U and 15 O + F atoms per formula unit. Nollmotzite is monoclinic, space groupCm, witha= 7.1015 (12) Å,b= 11.7489 (17) Å,c= 8.1954 (14) Å, β = 98.087 (14)°,V= 676.98 (19) Å3andZ= 2. The crystal structure [twinned by reticular merohedry; refined toR= 0.0369 with GoF = 1.09 for 1527 unique observed reflections,I> 3σ(I)] is based upon [UV(UVIO2)2F3O4]2–sheets of β-U3O8topology and contains an interlayer with MgF2(H2O)4octahedra. Adjacent sheets are linked through F–Mg–F bonds, as well asviahydrogen bonds. The presence of fluorine and pentavalent uranium in the structure of nollmotzite has potentially important implications for the safe disposal of nuclear waste.

Lead-bearing phyllotungstite from the Clara mine, Germany with an ordered pyrochlore–hexagonal tungsten bronze intergrowth structure

Lead-bearing phyllotungstite from the Clara mine in the central Black Forest, Germany has a formula (Cs0.41)Na0.14K0.05Pb2+2.01Ca0.26[W6+10.87Fe3+3.13O35.75(OH)6.25](O(H2O)3). X-ray diffraction patterns exhibit pseudohexagonal symmetry, but refinement of single-crystal synchrotron data has shown that the true symmetry is orthorhombic, Cmcm, with a = 7.298(1), b = 12.640(2), c = 19.582(4) Å, and that the pseudohexagonal character is due to submicrometre-scale cyclical twinning by rotation about the pseudohexagonal c axis. The structure can be described in terms of an ordered intergrowth, parallel to (001), of (111)py blocks with pyrochlore-type structures, which are ~6 Å in width, and two-layer wide regions with a hexagonal tungsten bronze (HTB) type structure. Caesium atoms occupy 18-coordinate cavities in the HTB regions, and H2O molecules occupy Φ sites in the A2B2O6Φ pyrochlore blocks. The lowering of symmetry from hexagonal to orthorhombic is due to partial ordering of W and Fe in the octahedral B sites and of Pb and vacancies in the A sites of the pyrochlore blocks. The ideal formula for the intergrowth structure (with no vacancies) is C 2A10[B14(O,OH)42]Φ4, where C is the cavity site in the HTB slabs. The mineral has only 21% occupancy of the C site and 25% occupancy of the A site, but full occupancy of the Φ site. There may be some mixing of Cs and H2O between the C and Φ sites.

The crystal structure and compositional range of mckinstryite

The previously unknown crystal structure of mckinstryite, originally described as Ag1.18Cu0.82S or (Ag,Cu)2S, was solved and refined using single-crystal X-ray diffractometer data collected from a sample from the Clara mine, Black Forest (Mo-Kα radiation, CCD area detector, R1(F) = 3.85%). Mckinstryite has the refined formula Ag4.92Cu3.08S4 or Ag1.23Cu0.77S (idealized Ag5Cu3S4 or Ag1.25Cu0.75S) and crystallizes in space group Pnma (no. 62), with a = 14.047(3) Å, b = 7.805(2) Å , c = 15.691(3) Å, V = 1720.3(7) Å3, Z = 8. The structure contains five Ag, six Cu and eight S sites in the asymmetric unit. One of the Ag sites shows minor Cu-for-Ag substitution. The topology is based on flat, interrupted (010) layers of Cu and S atoms (all atoms on y = 0.25), in which the Cu atoms show triangular or two-coordination to S (interrupted {6,3} tiling). These layers alternate with uneven layers consisting of Ag atoms showing irregular three- to two-coordination to S. Some fairly short Ag–Cu contact distances (2.781–2.884 Å) strongly indicate that metal-metal interaction plays an important role in mckinstryite. The topology is related to that of stromeyerite (∼AgCuS) which contains complete flat layers of Cu atoms triangularly coordinated to S atoms, alternating with layers of loosely packed Ag atoms.A critical evaluation of literature data on the chemical composition and unit-cell parameters of mckinstryite confirms the presence of a small compositional range of mckinstryite which extends approximately from Ag1.18Cu0.82S to Ag1.25Cu0.75S, with the presently studied sample being fairly Agrich. The accurate limits of this range at ambient temperature are still to be determined.