Raman modes in Pbca enstatite (Mg2Si2O6): an assignment by quantum mechanical calculation to interpret experimental results

Raman spectra of orthoenstatite have been computed from first principles, employing the hybrid Hamiltonian WC1LYP.[1] The calculated data show excellent agreement with the experimental data from literature with an absolute average difference of ~5 cm1. The quantum mechanical simulation allowed the assignment of Raman features to specific vibrational modes. This enabled to assess quantitatively the contributions of internal (tetrahedral stretching) and external (tetrahedral chains and M1 and M2 cations) vibrations. Moreover, the mass substitution of 56Fe for 24Mg in the M1 and M2 sites and of 30Si and 18O for the 28Si and 16O sites, pointed out the relative contributions of the cations to each mode within different sites. The description of the Raman modes enabled to relate the major experimental peaks to specific structural vibrations, and to link the changes in crystal structure to those modes with pressure, temperature and composition. The results provide new clues to identify most suitable peaks for the investigation of the intracrystalline ordering of Fe and Mg in the M1 and M2 sites, and of Al in the tetrahedral and octahedral sites. Moreover we have been able to identify those peaks which are related to structural features, like tetrahedral bond distances

Caratterizzazione in microscopia SEM-EDS del cemento endodontico MTA: un confronto tra diversi prodotti

RiassuntoObiettiviValutare le caratteristiche microscopiche dei cementi endodontici MTA disponibili in commercio e di un cemento di Portland.Materiali e metodiAnalisi su quattro varietà di MTA: Ogna Aureoseal®, Proroot white MTA®, Angelus white e gray MTA® e un cemento di Portland. I campioni, previo esame in diffrazione X, sono stati consolidati; i provini a cemento solidificato sono stati inglobati in una resina epossidica e lucidati in superficie: questo ha permesso un'osservazione SEM-EDS per evidenziarne la struttura e la natura chimica e mineralogica.RisultatiLe analisi mostrano significative differenze tra i vari tipi di MTA presi in esame e tra questi e il cemento di Portland usato come riferimento.I cementi utilizzati a scopo odontoiatrico hanno granuli di minori dimensioni, una maggiore idratazione e un minor contenuto di ferro e zolfo. Tra i costituenti mineralogici caratteristici del MTA si riscontrano agenti radio-opacizzanti (Bi2O3 e CaWO4). Una sostanziale differenza tra i cementi MTA è la quantità variabile nel contenuto di magnesio e alluminio che, dopo il consolidamento, presentano una granulometria inferiore nei campioni Ogna Aureoseal® e Proroot white MTA®; la presenza di apatite e tungstato di calcio (CaWO4) è invece caratteristica del cemento MTA Ogna Aureoseal®.ConclusioniLe differenze tra i campioni MTA e tra questi e il cemento di Portland sono significative sia a livello di tessitura microscopica sia di composizione chimica.AbstractObjectivesTo determine the mineralogical, chemical, and microtextural differences between commercially available MTA products, clarifying common differences with Portland and between each other.Materials and methodsThe study was done on four commercially available MTA cements, that is Ogna Aureoseal®, Proroot white MTA®, Angelus white and gray MTA® and on a commercial Portland cement. After preliminary X-ray diffraction characterization of the powder, the samples were consolidated following the same procedure used in clinical practice. The consolidated material was embedded in epoxy and polished, and subsequently investigated, by means of SEM-EDS analysis of back-scattered electron images on all samples and of quantitative compositional mapping on MTA.ResultsThe MTA and Portland cements differ for the grain size, the hydration rate, and homogeneity of the set products, and for lower Fe and S content in MTA. Previously reported presence of opacizing agents (Bi2O3 and CaWO4) was confirmed in all samples, whereas low Al content was not observed in all MTA. Although the MTA products are all mainly composed by di-calcium and tri-calcium silicates, the post-setting texture and the chemical and mineralogical constitution differ. Finer texture was observed in Ogna Aureoseal® and Proroot white MTA®; in Ogna Aureoseal® the presence of apatite and Ca-tungstate was reported; Mg and Al content changed in samples, following the clinker composition.ConclusionsSignificant differences exist between Portland cement and MTA, but also among different MTA formulations. MTA products can be classified as a form of Portland cement, specifically modified for dentistry purposes

Portable Raman Spectrometer for in situ analysis of asbestos and fibrous minerals

Asbestos inhalation is associated with fatal respiratory diseases and raises concerns from the perspective of workplace safety and environmental impacts. Asbestos and asbestos-like minerals naturally occur in rocks and may become airborne when outcrops or soils are disturbed by anthropic activities. In situ detection of these minerals is a crucial step for the risk evaluation of natural sites. We assess here whether a portable Raman spectrometer (pRS) may be used in the identification of asbestos and asbestos-like minerals at the mining front during exploitation. pRS performance was tested at three geologically different mining sites in Italy and New Caledonia and compared with a high-resolution micro-Raman spectrometer (HRS). About 80% of the overall in situ analyses at the mining front were successfully identified by pRS, even when intermixed phases or strongly disaggregated and altered samples were analyzed. Chrysotile and tremolite asbestos, asbestos-like antigorite, and balangeroite were correctly detected during surveys. The major difficulties faced during in situ pRS measurements were fluorescence emission and focussing the laser beam on non-cohesive bundles of fibers. pRS is adequate for discriminating asbestos and asbestos-like minerals in situ. pRS may support risk assessment of mining sites to better protect workers and environmen

The structure of P21/c (Ca0.2Co0.8)CoSi2O6 pyroxene and the C2/c - P21/c phase transition in natural and synthetic Ca, Mg, Fe2+ pyroxenes

A P21/c synthetic (Ca0.2Co0.8)CoSi2O6pyroxene was synthesized by slow cooling from melt at high pressure. Single crystals suitable for X-ray diffraction were obtained and refined. The results were compared to those of C2/c pyroxenes along the series CaCoSi2O6-Co2Si2O6. Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2O6-Fe2Si2O6and CaMgSi2O6-Mg2Si2O6pyroxenes, both at an average and local level are apparent. The results, examined together with two new refinements of pigeonite in the ureilites ALHA77257 and RKPA80239 and with a set of natural and synthetic C2/c and P21/c pyroxenes, show that the average cation radius in the M2 site is the driving force for the phase transition from C2/c to P21/c. The longest M2-O3 distances and the O3-O3-O3 angles follow the same trend, dictated only by the ionic radius in M2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M1-O, M2-O1 and M2-O2 distances are unaffected by the transition and are determined only by the ionic radius of the bonding cation. The structural changes between the ionic radius and the high temperature C2/c and P21/c transitions are similar, and different to the high-pressure transition. Analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 Å. Increasing strain stabilizes the P21/c structure to a higher temperature and larger cation radius. Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the mineral pigeonite, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling

Micro-Raman mapping of the polymorphs of serpentine

Serpentinites are rocks, often used in buildings, formed in large extent by minerals of the serpentine group: chrysotile, antigorite, lizardite, and polygonal serpentine. The fibrous type (e.g. chrysotile) of serpentine group minerals, along with several amphibole varieties (e.g. actinolite and tremolite), are the major components of asbestos family. The exposure to fine fibrous asbestos powder is linked to diseases such as pleural mesothelioma and asbestosis. The identification of the main varieties of the serpentine group, laminated or fibrous, becomes an issue of great interest for public health. This work introduces an analytical strategy able to distinguish the different serpentine polymorphs directly on the sample, allowing the analysiswithin their textural environment, evidencing at themicrometer scale the mineral reactions of the phases. Samples coming from the Koniambo massif (Grande Terre Island, New Caledonia) were studied by means of optical microscopy, scanning electron microscopy–energy dispersive X-ray spectroscopy, and Raman spectroscopy. Raman peaks observed in the high wavenumber spectral range of 3550–3850 cm-1, associated with OH stretching vibrations, allow the iscrimination of the all four serpentine varieties. The relationship between the different varieties of serpentine, at a micrometric scale, in complex samples, has been investigated by two-dimensional Raman mapping