Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: A case study from palaeo-lake Fucino (central Italy).

RATIONALE: The suitability of organic matter (OM) removal pre-treatments in isotopic studies of lacustrine carbonates is currently under debate. Naturally occurring OM seems to have a negligible effect on the bulk isotopic composition of carbonates compared with changes induced by pre-treatments. This study provides further insights into the possible effects induced by commonly used pre-treatments on natural lacustrine carbonates. METHODS: Sixteen samples from the Fucino F1-F3 lacustrine succession (Abruzzo, central Italy) were characterised for their mineralogical and geochemical composition and each was split into three identical aliquots. One aliquot was left untreated while the remaining two were treated with NaOCl and H2 O2 dilutions. The same treatment was applied to an internal standard consisting of pure Carrara marble. The treated and untreated samples were analysed for their carbon (δ13 C values) and oxygen (δ18 O values) isotope compositions using an Analytical Precision AP2003 isotope ratio mass spectrometer. RESULTS: The samples had variable proportions of endogenic and detrital components, the detrital portion being more (less) abundant during colder (warmer) climate phases. We observed that neither the NaOCl nor the H2 O2 treatment was able to completely remove OM and therefore there was selective removal of compounds within the OM pool. A possible effect of pre-treatment is the loss of carbonates intimately interspersed within the OM, as suggested by the evolution of isotopic ratios towards the local detrital array. CONCLUSIONS: Our study highlights sample-specific changes in geochemistry associated with sample pre-treatments; however, such changes do not seem to lead to either systematic and/or predictable isotopic shifts. We suggest that the suitability of NaOCl or H2 O2 pre-treatments for OM removal should be evaluated on a case-by-case basis. In the specific case of lacustrine marls from palaeo-lake Fucino containing relatively low amounts of OM and in which both detrital and endogenic carbonates occur, both pre-treatments should be avoided.

Hexavalent chromium release over time from a pyrolyzed Cr-bearing tannery sludge.

Pyrolysis in an inert atmosphere is a widely applied route to convert tannery wastes into reusable materials. In the present study, the Cr(III) conversion into the toxic hexavalent form in the pyrolyzed tannery waste referred to as KEU was investigated. Ageing experiments and leaching tests demonstrated that the Cr(III)-Cr(VI) inter-conversion occurs in the presence of air at ambient temperature, enhanced by wet environmental conditions. Microstructural analysis revealed that the Cr-primary mineral assemblage formed during pyrolysis (Cr-bearing srebrodolskite and Cr-magnetite spinel) destabilized upon spray water cooling in the last stage of the process. In the evolution from the higher to the lower temperature mineralogy, Cr is incorporated into newly formed CrOOH flakes which likely react in air forming extractable Cr(VI) species. This property transforms KEU from an inert waste to a hazardous material when exposed to ordinary ambient conditions.

Phase transition at high pressure in Cu2CO3(OH)2 related to the reduction of the Jahn-Teller effect.

Hydroxycarbonates with the general formula Me(2)(CO(3))(OH)(2) are widely used materials in industrial processes and are widespread in nature. The Cu term, malachite, Cu(2)CO(3)(OH)(2), is monoclinic, P2(1)/a. Substitution of Cu(2+) with other bivalent cations such as Mg, Zn, Fe, Cu or Ni is possible and leads to a different structure type, rosasite, P2(1)/a or P2(1)/b11 in the same cell setting as malachite. Rosasite structure is topologically similar to malachite, but the symmetry elements are oriented differently with respect to structural units. The stability of the malachite-like structure (MS) compared with the rosasite-like structure (RS) has been suggested to be related to the Jahn-Teller effect in CuO(6) coordination polyhedra. For this reason the hypothesis of the phase transition of malachite, Cu(2)CO(3)(OH)(2), to a rosasite structure at high pressure, as a result of the reduced Jahn-Teller effect, has been tested and confirmed by powder and single-crystal diffraction structural studies: above 6 GPa the malachite structure is no longer stable and transforms to a RS structure. RS Cu(2)CO(3)(OH)(2) is 3% more dense than malachite and the bulk modulus is remarkably higher, 80 (2) GPa compared with 48 (4) GPa. The longer apical Cu-O bonds in the distorted Me1 octahedral site are progressively shortened with increasing pressure, revealing a decrease in the Jahn-Teller effect at high pressure. The transition has a first-order character, is reversible with a significant hysteresis, and there is no evidence of any intermediate phase between the two structures. We then have further evidence that in the Me(2)(CO(3))(OH)(2) compounds, the two main structural types, MS and RS, are closely related. The former structure is stabilized only when Cu is the prevalent cation in the octahedral sites, and it can transform directly to the RS as a function of thermodynamic changes.

Occurrence and characterization of tremolite asbestos from the Mid Atlantic Ridge.

Tremolite is one of the most common amphibole species and, in the fibrous form (i.e., characterized by crystals/particles consisting of fibres with length > 5 µm, width < 3 µm and aspect ratio > 3), one of the six asbestos minerals. Until now the attention of crystallographers has focused only on samples from continental environment. Here we report the first chemical and structural data of a tremolite asbestos found along the Mid Atlantic Ridge (MAR) at the eastern intersection of the Romanche Transform Fault (Equatorial MAR). Tremolite is associated with chlorite and lizardite and was formed through the green shale facies lower than zeolite in a predominantly fluid system. MAR tremolite asbestos shows very slight deviations from the ideal crystal structure of tremolite. Differences in cation site partitioning were found with respect to tremolite asbestos from ophiolitic complexes, attributed to the different chemical-physical conditions during the mineral formation. In particular, oceanic tremolite asbestos is enriched in Al and Na, forming a trend clearly distinct from the continental tremolites.

Crystal structure study of a cobaltoan dolomite from Kolwezi, Democratic Republic of Congo.

A structural study has been undertaken on a cobaltoan dolomite, with chemical formula CaMg0.83Co0.17(CO3)2 (cal-cium magnesium cobalt dicarbonate), from Kolwezi, Democratic Republic of Congo. Pale-pink euhedral cobaltoan dolomite was associated with kolwezite [(Cu1.33Co0.67)(CO3)(OH)2] and cobaltoan malachite [(Cu,Co)2(CO3)(OH)2]. A crystal with a Co:Mg ratio of 1:5.6 (SEM/EDAX measurement), twinned on (11 -2 0) was used for crystal structural refinement. The refinement of the structural model of Reeder & Wenk [Am. Mineral. (1983 ▶), 68, 769-776; Ca at site 3a with site symmetry -3; Mg site at site 3b with site symmetry -3; C at site 6c with site symmetry 3; O at site 18f with site symmetry 1] showed that Co is totally incorporated in the Mg site, with refined occupancy Mg0.83Co0.17, which compares with Mg0.85Co0.15 from chemical data. The Co substitution reflects in the expansion of the cell volume, with a pronounced increasing of the c cell parameter.