Roberto Cossio

Ab initio quantum-mechanical calculation of CaCO 3 aragonite at high pressure: thermodynamic properties and comparison with experimental data

Structure and vibrational frequencies (at the Γ point) of CaCO 3 aragonite have been calculated from first principles, by using the hybrid Hartree-Fock/DFT B3LYP Hamiltonian, at different unit-cell volumes in the 185–242 A 3 range. By using the frequencies evaluated at such different volumes, the mode- γ Gruneisen’s parameters were estimated for each vibrational mode, and the zero point and thermal pressure contributions to the total pressure, at each volume and temperature, have then been determined by means of standard thermodynamic formulas, within the limits of the quasi-harmonic approximation. This allowed for the determination of ( i ) the equation of state at different temperatures; ( ii ) the thermal expansion as a function of pressure and temperature, and ( iii ) the evaluation of some thermodynamic properties (entropy and specific heat) together with their temperature dependences. Results were directly compared with relevant experimental data. The agreement of the calculated frequencies with the experimental data, at variable pressure, shows that the ab initio simulation can reproduce, at a relatively low computational cost, the full vibrational spectra of crystalline compounds of mineralogical interest. Moreover, the elastic properties (bulk modulus in particular), thermal expansion and thermodynamic properties, which play an important role in the characterization and in the understanding of the stability relations between carbonate phases, at high-pressure and high-temperature conditions, can be satisfactorily estimated. Precisely, at room temperature and pressure conditions, the calculated bulk modulus was 64.7 GPa, to be compared with an experimental value of 65(1) GPa (mean of three different experimental determinations); the estimated thermal expansion was about 6.1 · 10 −5 K −1 , which is only slightly underestimated with respect to the experimental datum [6.3(2) · 10 −5 K −1 ]; the calculated entropy ( S ) and the constant-pressure specific heat ( C P ) were 87.5 and 83.1 J mol −1 K −1 respectively, which are in close agreement with the experimental data [84(6) and 82.6 J mol −1 K −1 , for S and C P respectively].

Quantitative Modal Determination of Geological Samples Based on X-ray Multielemental Map Acquisition

Multielemental X-ray maps collected by a remote scanning system of the electron beam are processed by a dedicated software program performing accurate modal determination of geological samples. The classification of different mineral phases is based on elemental concentrations. The software program Petromod loads the maps into a database and computes a matrix consisting of numerical values proportional to the elemental concentrations. After an initial calibration, the program can perform the chemical composition calculated on the basis of a fixed number of oxygens for a selected area. In this way, it is possible to identify all the mineral phases occurring in the sample. Up to three elements can be selected to calculate the modal percentage of the identified mineral. An automated routine scans the whole set of maps and assigns each pixel that satisfies the imposed requirements to the selected phase. Repeating this procedure for every mineral phase occurring in the mapped area, a modal distribution of the rock-forming minerals can be performed. The final output consists of a digitized image, which can be further analyzed by common image analysis software, and a table containing the calculated modal percentages. The method is here applied to a volcanic and a metamorphic rock sample.

Petromap: MS-DOS software package for quantitative processing of x-ray maps of zoned minerals

Abstract This paper shows an application of energy dispersive spectrometry (EDS) for digital acquisition of multi-element X-ray compositional maps of minerals in polished thin sections. A square matrix of n EDS spectra with known X , Y coordinates is collected, converted and exported to a personal computer. Each spectrum of the matrix is processed and the apparent concentration of each analyzed element is calculated by means of PETROMAP, a program written in Quick-Basic which applies a quantitative ZAF/FLS correction. The results of processing are comparable to the conventional quantitative microprobe analyses, with similar counting statistics. The output is a numerical matrix, compatible with the most popular graphic and spreadsheet programs from which it is possible to produce two-dimensional wt% oxide, mole fractions and mineral end-members pseudocolored or black/white maps. The procedure has been tested using a metamorphic garnet of the medium-grade Stilo unit (Calabrian Arc, Southern Italy).

µ-XRF Analysis of Trace Elements in Lapis Lazuli-Forming Minerals for a Provenance Study.

This paper presents new developments on the provenance study of lapis lazuli started by our group in 2008: during the years a multi-technique approach has been exploited to obtain minero-petrographic characterization and creation of a database considering only rock samples of known provenance. Since the final aim of the study is to develop a method to analyze archeological findings and artworks made with lapis lazuli in a completely non-invasive way, ion beam analysis techniques were employed to trace the provenance of the raw material used for the production of artifacts. Continuing this goal and focusing the analysis on determination of more significant minero-chemical markers for the provenance study of trace elements in different minerals, the method was extended with the use of micro X-ray fluorescence (µ-XRF), to test the potential of the technique for this application. The analyzes were focused on diopside and pyrite in lapis lazuli samples of known provenance (Afghanistan, Tajikistan, and Siberia). In addition, µ-XRF data were compared with micro proton-induced X-ray emission (µ-PIXE) results to verify the agreement between the two databases and to compare the analytical performance of both techniques for this application.

Comparison between major and trace element concentrations in garnet performed by EPMA and micro-PIXE techniques

Abstract This paper deals with the application of the electron and proton microprobe (EPMA and micro-PIXE) in order to determine major and trace elements in metamorphic garnet samples. The selected garnet samples come from a metapelitic rock belonging to the tectonic unit of Monte Rosa Nappe (Italy). Quantitative spot analysis profiles and compositional X-ray maps of both major and trace (yttrium) elements are reported. Major elements show a smoothed and continuous compositional zoning characterized by concentric variations from core to rim. Yttrium displays a strong enrichment in the core and a flat pattern at the rim. No correlation is shown between major and yttrium distribution. Only a rough correlation may be supposed with manganese. A good agreement was found between EPMA and micro-PIXE yttrium data, for values higher than 80–100 ppm. For lower values micro-PIXE technique is strongly recommended. The combination of X-ray two-dimensional maps and quantitative microanalyses allowed to evaluate the distribution of major and trace elements in a petrologically significative rock-forming mineral, to define the type of the chemical zoning and finally to attest difference in diffusivity between major and trace elements. Because of the slow diffusivity of yttrium, its quantitative determination in garnets is fundamental to reconstruct the temperature path suffered by polymetamorphic garnet-bearing rocks.

Textural and Mineralogical Analysis of Volcanic Rocks by µ -XRF Mapping

In this study, µ-XRF was applied as a novel surface technique for quick acquisition of elemental X-ray maps of rocks, image analysis of which provides quantitative information on texture and rock-forming minerals. Bench-top µ-XRF is cost-effective, fast, and non-destructive, can be applied to both large (up to a few tens of cm) and fragile samples, and yields major and trace element analysis with good sensitivity. Here, X-ray mapping was performed with a resolution of 103.5 µm and spot size of 30 µm over sample areas of about 5×4 cm of Euganean trachyte, a volcanic porphyritic rock from the Euganean Hills (NE Italy) traditionally used in cultural heritage. The relative abundance of phenocrysts and groundmass, as well as the size and shape of the various mineral phases, were obtained from image analysis of the elemental maps. The quantified petrographic features allowed identification of various extraction sites, revealing an objective method for archaeometric provenance studies exploiting µ-XRF imaging.

A New Approach for Provenance Studies of Archaeological Finds: Inferences from Trace Elements in Carbonate Minerals of Alpine White Marbles by a Bench-to-Top μ-XRF Spectrometer

The metamorphic rocks outcropping in the Western Alps are characterised by a great variety of white marbles which have been used since the antiquity. This variety mostly includes nine historical Piedmont white marbles (Ornavasso, Candoglia, Crevola, Pont Canavese, Foresto, Chianocco, Prali, Brossasco, and Garessio marbles) coming from well-known quarry sites and belonging to different metamorphic geological units of the Western Alps. The petrographical, minerochemical, and C–O isotopic data of these white marbles have been integrated with CaO and trace element (Fe, Mn, and Sr) concentration determined on single crystals of carbonate minerals (i.e., calcite and/or dolomite) by means of a bench-to-top µ-XRF spectrometer. Principal component analysis and hierarchical cluster analysis were performed on a data set of 178 observations containing CaO, Fe, Mn, and Sr concentration as well as the maximum grain size (MGS), δ18O and δ13C. The use of only five selected variables (CaO, Fe, Mn, Sr, and δ18O) has provided the correct allocation of each individual observation to its relevant class. Therefore, this approach based mostly on a noninvasive µ-XRF determination will be useful to define the provenance of unknown marbles of alpine origin used in antiquity for cultural heritage.

A mineralogical application of micro-PIXE technique: Yttrium zoning in garnet from metamorphic rocks and its petrologic meaning

Micro-PIXE has been applied to a specific petrologic problem particularly affected by the limitation of other techniques. Selected garnet crystals from metapelitic rock samples equilibrated under high pressure low temperature metamorphic conditions coming from Western Alps were analysed. The yttrium concentration reveals a strongly (two order of magnitude) zoned distribution decreasing from core to rim in garnet crystals. Quantitative spot analyses range from about 1500 ppm at the core to 50 ppm at the rim. The extremely precise (LOD < 5 ppm) Y content determination by micro-PIXE technique allowed to better define the thermal rock history, applying a recently calibrated geothermometer.

Compositional X-Ray Maps of Metamorphic and Magmatic Minerals

This paper deals with the applications of qualitative and quantitative x-ray compositional maps collected by either an energy-dispersive spectrometer (EDS) or a wavelength-dispersive spectrometer (WDS) on magmatic and metamorphic minerals.

Alpine corundum-bearing veins from the Etirol-Levaz Austroalpine continental slice (Valtournenche, Aosta, Italy)

This short manuscript presents a detailed investigation into the metamorphic evolution of a unique corundum-bearing chloritite from the layered metagabbro body within the Etirol-Levaz continental slice in Valtournenche, Aosta, Italy. This sample represents the first occurrence of corundum and hogbomite-bearing mafic rocks in the Western Italian Alps. The data presented here mostly summarizes the content of the article Regis et al. (submitted to European Journal of Mineralogy).