Study of the variation of the optical properties of calcite with applied stress, useful for specific rock and material mechanics

Calcite (CaCO3, trigonal crystal system, space group R3 ¯ c) is a ubiquitous carbonate phase commonly found on the Earth’s crust that finds many useful applications in both scientific (mineralogy, petrology, geology) and technological fields (optics, sensors, materials technology) because of its peculiar anisotropic physical properties. Among them, photoelasticity, i.e., the variation of the optical properties of the mineral (including birefringence) with the applied stress, could find usefulness in determining the stress state of a rock sample containing calcite by employing simple optical measurements. However, the photoelastic tensor is not easily available from experiments, and affected by high uncertainties. Here we present a theoretical Density Functional Theory approach to obtain both elastic and photoelastic properties of calcite, considering realistic experimental conditions (298 K, 1 atm). The results were compared with those available in literature, further extending the knowledge of the photoelasticity of calcite, and clarifying an experimental discrepancy in the sign of the p41 photoelastic tensor component measured in past investigations. The methods here described and applied to a well-known crystalline material can be used to obtain the photoelastic properties of other minerals and/or materials at desired pressure and temperature conditions

Fibre-reinforced geopolymer composites micro-nanochemistry by SEM-EDS simulations

The focus of the present study is on fibre-reinforced geopolymer composites, whose optimization and application necessarily need a detailed chemical characterization at the micro-nanoscale. In this regard, many geopolymer composites presenting micro and nanometric architectures pose a challenge for scanning electron microscopy with energy dispersive X-ray microanalysis (SEM-EDS) quantification, because of several potential sources of errors. For this reason, the present work reports a SEM-EDS Monte Carlo approach to carefully investigate the complex physical phenomena related to the cited quantification errors. The model used for this theoretical analysis is a simplified fibre-reinforced geopolymer with basalt-derived glass fibres immersed in a potassium-poly(sialatesiloxo) matrix. The simulated SEM-EDS spectra showed a strong influence on the measured X-ray intensity of (i) the sample nano-to-micro architecture, (ii) the electron beam probing energy and (iii) the electron probe-sample-EDS detector relative position. The results showed that, compared to a bulk material, the X-ray intensity for a nano-micrometric sized specimen may give rise to potential underestimation and/or overestimation of the elemental composition of the sample. The proposed Monte Carlo approach indicated the optimal instrumental setup depending on the sample and on the specific SEM-EDS equipment here considered

Simulated infrared and Raman spectroscopy, complex dielectric function and refractive index dataset of monoclinic C2/m stoichiometric clinochlore Mg6Si4O10(OH)8 as obtained from Density Functional Theory

This article reports a simulated dataset of the vibrational (infrared and Raman) and optical properties (complex dielectric function and refractive index) of clinochlore, an important mineral belonging to the phyllosilicate family [1]. The data here reported were calculated from ab initio Density Functional Theory (DFT) simulations at B3LYP level, including a correction for the dispersive forces (B3LYP-D* approach) and all-electron Gaussian-type orbitals basis sets. This dataset was calculated between 0 cm–1 and 4000 cm–1 and comprises infrared, reflectance and Raman spectra, frequency-dependent complex dielectric function and complex refractive index of clinochlore. The data was validated against available experimental spectroscopic results reported in literature and can be of help in several application fields, for instance fundamental georesource exploration and exploitation, in applied mineralogy, geology, material science, and as a reference to assess the quality of other theoretical approaches

Monte Carlo strategy for SEM-EDS micro-nanoanalysis of geopolymer composites

The development, optimization and application of new geopolymer composite materials must necessarily go through a precise and accurate physico-chemical and mineralogical characterization down to the micro and nanoscale. In this regard, SEM-EDS X-ray microanalysis is widely and successfully employed by the scientific community and industry. However, the nano-to-micrometre sized architecture of many geopolymer composites introduces many difficulties and issues in SEM-EDS quantification, with potential large sources of error that should carefully be taken into account and investigated. In this work, a SEM-EDS Monte Carlo approach is proposed to study the complex physical phenomena at the basis of the quantification issues and errors, through the investigation of: (i) a not completely reacted sodium-poly(sialate-siloxo) geopolymer, and (ii) a geopolymer composite with a potassium-poly(sialate-siloxo) matrix and basalt-derived glass fibres reinforcement. The Monte Carlo simulation evinced a strong influence of the nano-microsized specimen architecture (e.g., basalt fibre size and shape, different elemental composition between fibre and matrix) on the measured X-ray intensity, with contributions also depending on the SEM electron beam energy. The proposed approach provided fundamental indications for selecting optimal operative conditions depending on the type of geopolymer sample, shape, size with the specific SEM-EDS setup and silicon drift EDS detector here used

Monte Carlo SEM-EDS Nano-Microanalysis Strategy of Historical Mineral Pigments: The Simulation of the Egyptian Blue from Pompeii (Italy) as an Example

A correct determination of the mineral and chemical composition of specimens is of the utmost importance to answer questions regarding the Cultural Heritage field. Because of the preciousness and often very low quantity of sample available, with textures and sizes in the nano-to-micrometric range, scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) is one of the most suited and exploited nano-microanalytical techniques. In these cases, to avoid severe mistakes and quantification errors in SEM-EDS, it is mandatory to consider several effects related to the transport of electrons and X-rays in the material, which in turn are dependent on the SEM-EDS setup. In the present work, a Monte Carlo SEM-EDS nano-microanalytical simulation strategy is proposed and applied to a practical selected case. The Egyptian blue mineral pigment, which is found in Pompeian murals, is used here as an example and model system to show the effects of real size variations (0.1–10 µm), basic geometrical shapes of the pigment (prismatic and spherical) and typical SEM setups, sample holders and substrates. The simulations showed a great—sometimes not intuitive—dependence of the X-ray intensity on the thickness and shape of the samples and SEM-EDS parameters, thereby influencing the analysis and quantification. The critical overview of the results allowed the determination of the correct procedure and technical SEM-EDS parameters and indicated how to apply the Monte Carlo simulation strategy to other Cultural Heritage cases

Thermally Treated Waste Silt as Filler in Geopolymer Cement

This study aims to investigate the feasibility of including silt, a by-product of limestone aggregate production, as a filler in geopolymer cement. Two separate phases were planned: The first phase aimed to determine the optimum calcination conditions of the waste silt obtained from Società Azionaria Prodotti Asfaltico Bituminosi Affini (S.A.P.A.B.A. s.r.l.). A Design of Experiment (DOE) was produced, and raw silt was calcined accordingly. Geopolymer cement mixtures were made with sodium or potassium alkali solutions and were tested for compressive strength and leaching. Higher calcination temperatures showed better compressive strength, regardless of liquid type. By considering the compressive strength, leaching, and X-ray diffraction (XRD) analysis, the optimum calcination temperature and time was selected as 750 °C for 2 h. The second phase focused on determining the optimum amount of silt (%) that could be used in a geopolymer cement mixture. The results suggested that the addition of about 55% of silt (total solid weight) as filler can improve the compressive strength of geopolymers made with Na or K liquid activators. Based on the leaching test, the cumulative concentrations of the released trace elements from the geopolymer specimens into the leachant were lower than the thresholds for European standards

Resolution Limits in the Study of Cardiolipin Crystals by TEM, SAED and AFM

Aggregates of cardiolipin molecules have been studied by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and atomic force microscopy (AFM). It is found that cardiolipin molecules are very susceptible to electron damage; the lethal dose being of 80 $\rm el\cdot nm^{-2}$. The finest organized structures revealed by TEM after image recording with the minimum dose technique are rather coarse, about 1 nm, whereas SAED reveals the presence of periodicities down to 0.2 nm, in accordance to the images obtained by AFM, whose application has enabled us to reveal substructures of a size of about 0.2 nm

Scanning electron microscopy and X-ray energy dispersive spectroscopy studies of defects in lead glasses

Scanning electron microscopy (SEM) observations, quantitative X-ray energy dispersive spectroscopy (EDS) and windowless EDS (WEDS) of typical defects in lead glasses are presented. The micrographs, obtained mainly by means of a high efficiency backscattered electron (BSE) detector, showed the presence of cords of varying shape, diameter (10-200 μm) and composition, spherical inclusions and cracks; a high density of low Z deposits were found along and in the proximity of the edges of the cracks. Quantitative microanalyses performed on the cords showed the presence of about 3 wt% of Al2O3 and 2 wt% of ZrO2 originating at the interface between the glass and the refractory materials used to hold the melt, and a Pb content lower than the matrix. A high Pb content was instead found in the spherical inclusions, whereas WEDS showed the presence of C in the deposits. These morphological and microchemical results provide to clarify the mechanisms of defect formation

Dataset on the piezo-spectroscopic behaviour of hydroxylapatite: Effect of mechanical stress on the Raman and Infrared vibrational bands from ab initio quantum mechanical simulations

This article reports data on the vibrational (Raman and Infrared) behavior of hydroxylapatite [OHAp, Ca10(PO4)6(OH)2, space group P63] under mechanical stress, which were discussed in details in the work of Ulian and Valdrè (2017) [1]. The dataset has been obtained by ab initio quantum mechanical means, by employing Density Functional Theory methods, in particular the B3LYP hybrid functional, all-electron Gaussian-type orbitals basis sets and a correction to take into account the effects of dispersive forces