ALBERT

Description: An exceptional new occurrence of the mineral tamarugite, NaAl(SO 4 ) 2 · 6H 2 O, from a short karst cavity (Diana Cave, Baile Herculane; SW Romania) is described. It was formed by corrosion of the bedrock (limestone and marls) by a SO 4 2– -rich steam condensate resulting from oxidized S 2– ions escaping from the thermo-mineral water emerging from depth in the cave. Tamarugite forms dull white earthy aggregates. Scanning-electron microscope (SEM) observations reveal tabular subhedral crystals never exceeding 15 μm across. The cell parameters refined from the powder data for the monoclinic space group P 2 1 / a are: a = 7.358(6), b = 25.23(2), c = 6.093(5) Å, β = 95.16(5) °, V = 1126.98(1) Å 3 . The 34 S values of the cave sulphates and the thermal water confirm marine evaporites as the source of sulphur. The sulphate-acid alteration of limestone with contribution of Al 3+ and Na + from the marls and the thermal water is responsible for the formation of tamarugite. The steam-condensate alteration paragenesis includes native sulphur, bassanite, anhydrite, epsomite, pickeringite, halotrichite, apjohnite and alunite, as well as quartz and halite, all primary and secondary speleogenetic by-products.

Description: 〈span〉Rudabányaite was found in cavities of siliceous sphaerosiderite and limonite rocks at the Adolf mine area of the Rudabánya ore deposit (northern-East Hungary). The new mineral forms small crystals up to 0.6 mm and aggregates of a few mm across. Usually they have a xenomorphic shape, only occasionally cubic symmetry is morphologically discernible; the crystal forms {110} and {100} were recognized. The crystals are transparent, have yellowish-orange to brownish-yellow colour and a lemon-yellow streak, the lustre is adamantine. The Mohs’ hardness is 3–4. No cleavage was observed. Rudabányaite is optically isotropic. The density could not be measured due to lack of material; ρ(calc.) = 8.04 g/cm〈sup〉3〈/sup〉. Electron-microprobe analyses gave the average composition (in wt%) Ag〈sub〉2〈/sub〉O 29.39, Hg〈sub〉2〈/sub〉O 52.62, As〈sub〉2〈/sub〉O〈sub〉5〈/sub〉 13.69, Cl 4.62, SO〈sub〉3〈/sub〉 0.19, O=Cl −1.04, sum 99.47. The empirical formula based on four oxygen atoms is (Ag〈sub〉2.06〈/sub〉Hg〈sub〉2.05〈/sub〉)〈sub〉Σ=4.11〈/sub〉(As〈sub〉0.97〈/sub〉S〈sub〉0.02〈/sub〉)〈sub〉Σ=0.99〈/sub〉O〈sub〉4〈/sub〉Cl〈sub〉1.06〈/sub〉; the idealized formula as derived from chemical analyses and crystal-structure investigation is [Ag〈sub〉2〈/sub〉Hg〈sub〉2〈/sub〉][AsO〈sub〉4〈/sub〉]Cl. The crystal-structure investigation was performed on single-crystal X-ray data; the refinements on 〈span〉F〈/span〉〈sup〉2〈/sup〉 converged at 〈span〉wR〈/span〉2(〈span〉F〈/span〉〈sup〉2〈/sup〉) = 0.068 and 〈span〉R〈/span〉1(〈span〉F〈/span〉) = 0.031 for all 972 unique data and 53 variable parameters. Rudabányaite crystallizes in space group F4¯3c, 〈span〉a〈/span〉 = 17.360(3) Å, 〈span〉V〈/span〉 = 5231.8 Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 32. The crystal structure is characterised by two crystallographically different [〈span〉M〈/span〉〈sub〉4〈/sub〉]〈sup〉4+〈/sup〉 cluster cations forming tetrahedra; 〈span〉M〈/span〉 = (Ag,Hg) with a ratio Ag:Hg ~ 1:1. There is not any evidence for an order between the Ag and Hg atoms. Small amounts of the 〈span〉M〈/span〉 atoms are displaced by ~0.5 Å. Topologically, the barycentres of the [〈span〉M〈/span〉〈sub〉4〈/sub〉]〈sup〉4+〈/sup〉 clusters and the As atom positions of the crystal structure of rudabányaite form a cubic primitive lattice with 〈span〉a〈/span〉′ = ½〈span〉a〈/span〉 = 8.68 Å; half of the voids are occupied by Cl atoms.〈/span〉

Description: The crystal structure of synthetic BaMg(CO 3 ) 2 whose mineral name is norsethite was re-investigated by single-crystal X-ray diffraction. Complementary in situ high- and low-temperature studies by means of vibrational spectroscopy (Raman, IR), powder X-ray diffraction techniques and thermal analyses were performed. Diffraction images (298 K) revealed weak superstructure reflections caused by the displacement of the O atoms in the earlier considered R 3I m structure model ( a = 5.0212(9), c new = 2 c old = 33.581(6) Å, R 3I c , Z = 6, R 1 = 0.011, sin/ 〈 0.99 Å –1 ). Thermal analyses reveal decarbonatization in two decomposition steps above 750 K, and the heat-flow curves (difference scanning calorimetry) give clear evidence of a weak and reversible endothermal change at 343±1 K. This agrees with a discontinuity in the IR and single-crystal Raman spectra. The changing trend of the c/a ratio supports this discontinuity indicating a temperature-induced structural transition in the range between 343 and 373 K. As the change of the unit-cell volume is almost linear, the character of the transition is apparently second order and matches the mechanism of a subtle displacement of the oxygen atom position. The apparent instability of the R 3I c structure is also evidenced by the remarkably larger anisotropic displacement of the oxygen atom.

Description: 〈span〉Rudabányaite was found in cavities of siliceous sphaerosiderite and limonite rocks at the Adolf mine area of the Rudabánya ore deposit (North-East Hungary). The new mineral forms small crystals up to 0.6 mm and aggregates of a few mm across. Usually they have a xenomorphic shape, only occasionally cubic symmetry is morphologically discernible; the crystal forms {110} and {100} were recognized. The crystals are transparent, have yellowish-orange to brownish-yellow colour and a lemon-yellow streak, the lustre is adamantine. The Mohs’ hardness is 3–4. No cleavage was observed. Rudabányaite is optically isotropic. The density could not be measured due to lack of material; ρ(calc.) = 8.04 g/cm〈sup〉3〈/sup〉. Electron-microprobe analyses gave the average composition (in wt%) Ag〈sub〉2〈/sub〉O 29.39, Hg〈sub〉2〈/sub〉O 52.62, As〈sub〉2〈/sub〉O〈sub〉5〈/sub〉 13.69, Cl 4.62, SO〈sub〉3〈/sub〉 0.19, O=Cl −1.04, sum 99.47. The empirical formula based on four oxygen atoms is (Ag〈sub〉2.06〈/sub〉Hg〈sub〉2.05〈/sub〉)〈sub〉Σ=4.11〈/sub〉(As〈sub〉0.97〈/sub〉S〈sub〉0.02〈/sub〉)〈sub〉Σ=0.99〈/sub〉O〈sub〉4〈/sub〉Cl〈sub〉1.06〈/sub〉; the idealized formula as derived from chemical analyses and crystal-structure investigation is [Ag〈sub〉2〈/sub〉Hg〈sub〉2〈/sub〉][AsO〈sub〉4〈/sub〉]Cl. The crystal-structure investigation was performed on single-crystal X-ray data; the refinements on 〈span〉F〈/span〉〈sup〉2〈/sup〉 converged at 〈span〉wR〈/span〉2(〈span〉F〈/span〉〈sup〉2〈/sup〉) = 0.068 and 〈span〉R〈/span〉1(〈span〉F〈/span〉) = 0.031 for all 972 unique data and 53 variable parameters. Rudabányaite crystallizes in space group F4¯3c, 〈span〉a〈/span〉 = 17.360(3) Å, 〈span〉V〈/span〉 = 5231.8 Å〈sup〉3〈/sup〉, 〈span〉Z〈/span〉 = 32. The crystal structure is characterised by two crystallographically different [〈span〉M〈/span〉〈sub〉4〈/sub〉]〈sup〉4+〈/sup〉 cluster cations forming tetrahedra; 〈span〉M〈/span〉 = (Ag,Hg) with a ratio Ag:Hg ~ 1:1. There is not any evidence for an order between the Ag and Hg atoms. Small amounts of the 〈span〉M〈/span〉 atoms are displaced by ~0.5 Å. Topologically, the barycentres of the [〈span〉M〈/span〉〈sub〉4〈/sub〉]〈sup〉4+〈/sup〉 clusters and the As atom positions of the crystal structure of rudabányaite form a cubic primitive lattice with 〈span〉a〈/span〉′ = ½〈span〉a〈/span〉 = 8.68 Å; half of the voids are occupied by Cl atoms.〈/span〉

Description: The Diana Cave in SW Romania develops along a fault line and hosts a spring of hot ( T avg = 51 °C), sulfate-rich, sodium-calcium-chloride bearing water of near-neutral pH. Abundant steam and H 2 S rises from the thermal water to condensate on the walls and ceiling of the cave. The sulfuric acid produced by H 2 S oxidation/hydrolysis causes a strong acid-sulfate weathering of the cave bedrock generating a sulfate-dominated mineral assemblage that includes rapidcreekite, Ca 2 (SO 4 )(CO 3 )·4H 2 O closely associated with gypsum and halotrichite group minerals. Rapidcreekite forms bundles of colorless tabular orthorhombic crystals elongated along [001] and reaching up to 1.5 mm in length. For verifying the hydrogen bond scheme and obtaining crystal-chemical details of the carbonate group a single-crystal structure refinement of rapidcreekite was performed. Its unit-cell parameters are: a = 15.524(2), b = 19.218(3), c = 6.161(1) Å; V = 1838.1(5) Å 3 , Z = 8, space group Pcnb . Chemical composition (wt%): CaO 35.65, SO 3 24.97, CO 2 13.7, H 2 O 23.9, Na 2 O 0.291, MgO 0.173, Al 2 O 3 0.07, total 98.75%. The empirical formula, based on 7 non-water O atoms pfu, is: Ca 1.98 Na 0.029 Mg 0.013 Al 0.004 (S 0.971 O 4 )(C 0.97 O 3 )·4.13H 2 O. The 34 S and 18 O values of rapidcreekite and other cave sulfates range from 18 to 19.5 CDT and from –9.7 to 7.8 SMOW, respectively, indicating that the source of sulfur is a marine evaporite and that during hydration of the minerals it has been an abundant 18 O exchange with percolating water but almost no oxygen is derived from O 2(aq) . This is the first description of rapidcreekite from a cave environment and one of the very few natural occurrences worldwide. We also report on the mineral stability and solubility, parameters considered critical to understand the co-precipitation of carbonates and sulfates, a process that has wide applications in cement industry and scaling prevention.