Impact related changes in mineral properties:ti-magnetite from basaltic rocks at Lonar crater, India

Samples from vesicular Lonar impactite have been investigated by Mossbauer spectroscopy and SEM in combination with elemental analysis. Observation reveal the new-formation of oxide minerals have occurred with nucleation in particular at the walls of the vesicles interpreted as having formed from condensation of the gas phase having caused the vesicles to form in the first place. Elemental analysis of Ti-magnetites gives very similar compositions irrespective of the oxides grains being vesicle or matrix associated (Fe/Ti ratios approximately 3). The room temperature spectra of the oxides show magnetically ordered components due to A, B, and C sites in the substituted spinel. The occurrence of these ordered components are not in accordance with expectations from the bulk chemical compositions, suggesting the presence of fine scaled intergrowth of Ti-oxide and Ti-magnetite

Thermal behaviour of pyrope at 1000 and 1100 °C: mechanism of Fe2+ oxidation and decomposition model

The mechanism of thermally induced oxidation of Fe2+ from natural pyrope has been studied at 1000 and 1100 degreesC using Fe-57 Mossbauer spectroscopy in conjunction with XRD, XRF, AFM, QELS, TG, DTA and electron microprobe analyses. At 1000 degreesC, the non-destructive oxidation of Fe2+ in air includes the partial stabilization of Fe3+ in the dodecahedral 24c position of the garnet structure and the simultaneous formation of hematite particles (15-20 nm). The incorporation of the magnesium ions to the hematite structure results in the suppression of the Morin transition temperature to below 20 K. The general garnet structure is preserved during the redox process at 1000 degreesC, in accordance with XRD and DTA data. At 1100 degreesC, however, oxidative conversion of pyrope to the mixed magnesium aluminium iron oxide, Fe-orthoenstatite and cristoballite was observed. During this destructive decomposition, Fe2+ is predominantly oxidized and incorporated into the spinel structure of Mg(Al,Fe)(2)O-4 and partially stabilized in the structure of orthoenstatite, (Mg,Fe)SiO3. The combination of XRD and Mossbauer data suggest the definite reaction mechanism prevailing, including the refinement of the chemical composition and quantification of the reaction products. The reaction mechanism indicates that the respective distribution of Fe(2+)and Fe3+ to the enstatite and spinel structures is determined by the total content of Fe2+ in pyrope

Mineralogical and geochemical characterization of the concotto artefacts from firing places of Longola protohistoric settlement (Naples)

A set of concotto samples, a type of fired mixture of various raw materials, coming from firing place of the perifluvial protohistoric site of Longola (Campania region, southern Italy) was characterized by means of a multi-analytical approach, i.e. polarized light optical microscopy (OM), X-ray powder diffraction (XRD), scanning electron microscopy and energy dispersive spectrometry (SEM-EDS), X-ray fluorescence (XRF), inductively-coupled plasmamass spectrometry (ICP-MS) and Mössbauer spectroscopy. The specimens generally show a heterogeneous texture, a red-coloured groundmass composed of a mixture of tiny quartz, feldspar, poorly-crystallized goethite, hematite and clay minerals. The crystal fractions show significant amounts of quartz and alkali feldspar and variable percentages of clinopyroxene, leucite, biotite, hematite, magnetite and traces of muscovite. Random secondary phases of calcite, vivianite and gypsum have been identified. The coarser fraction is formed by several types of inclusions, i.e. tephra fragments (pumices, scoriae), volcanic and sedimentary rocks. Illite and very subordinate smectite (montmorillonite) were detected by X-ray diffraction. Trace elements also show a contribution of volcanic raw materials. The Mössbauer analyses at 296 K show the occurrence of paramagnetic Fe3+ iron (clay minerals) and magnetic Fe3+ iron (hematite). For the 80 K spectrum, however, two doublets (ferrous and ferric iron in clay minerals) and one sextet (hematite). The concotto samples do not suffer high temperatures, perhaps much lower than 500-600 °C in a mainly oxidizing atmosphere. Only sporadically, if any, higher temperatures (> 800 °C) might have been reached