High-pressure Raman spectroscopy on low albite

The pressure dependence of the Raman spectrum of low albite, NaAlSi3O8, has been investigated from 0.0001 to 10.4 GPa, at room temperature, on a single crystal compressed hydrostatically in a diamond anvil cell. The Raman vibrational features move to higher wavenumbers \u3c5i with increasing pressure, due to the decrease in the unit-cell volume corresponding to a drastic shrinkage of the framework. The slopes \u394\u3c5i/\u394P of the four investigated bending modes (i.e. at 478, 507, 578 and 815 cm 121, at 0.0001 GPa) show evident changes at ~6.5 and ~8.5 GPa. This behaviour may be ascribed, in the absence of phase transitions, to the evolution of the compressional mechanisms at the atomic scale found in previous high-pressure studies on albite (mainly by X-ray diffraction), through a model based on tilts of rigid tetrahedra. The Raman data of this study allowed also to bracket the pressure range in which the occurrence of the first change in the compressional behaviour was found by X-ray diffraction

High-pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes

In situ high-pressure Raman spectra were collected on four pyroxenes, with composition CaCoSi2O6, CaMgSi2O6, CaCoGe2O6 and CaMgGe2O6, up to P = 7.6 and 8.3 GPa for silicates and germanates, respectively. The peak wavenumbers \ucf\u85i increase almost linearly with pressure; the slope d\ucf\u85i/dP is more pronounced for the modes at higher wavenumbers, and higher in germanates than in silicates. No phase transition or change in the compressional behaviour was observed within the investigated P-range. The strong dependence of the peak position with pressure of the high energy stretching modes is due to the high sensitivity of the vibrational frequencies probed by Raman spectroscopy to subtle changes in the tetrahedral deformation, which are overlooked by single-crystal X-ray diffraction

Vibrational characterization of the new gemstone Pezzottaite

Pezzottaite is a rare Cs-bearing mineral with ideal composition Cs(Be2Li)Al2Si6O18, discovered in November 2002. Pezzottaite is probably the only new mineral species with some relevance in gemology, thanks to its optical properties, rarity and beauty. It is considered as a member of the “beryl group”, along with beryl sensu-scricto (Be3Al2Si6O18;), bazzite (Be3Sc2Si6O18), stoppaniite (Be3Fe2Si6O18) and indialite (Mg2Al3(AlSi5O18)). The chemical composition and the spectroscopic features of pezzottaite from Ambatovita (central Madagascar) and a Cs-rich beryl from Monte Capanne (Isola d’Elba, Italy) were investigated by standard gemmological analysis, electron microprobe analysis in wavelength dispersive mode (EMPA-WDS), X-ray diffraction and micro-Raman spectroscopy. The density and the refractive index of pezzottaite were found to be higher than those of beryl due to the entrance of a large amount of alkali. However, an unambiguous distinction between pezzottaite and Cs-rich beryl cannot be done only on the basis of density and optical properties. Pezzottaite and Cs-rich beryl are usually distinguished on the basis of chemical analysis, considering a conventional upper-limit of caesium in Cs-rich beryl of Cs2O ~ 9 wt%, or by X-ray diffraction, as pezzottaite has different symmetry. In any case, the discrimination is not easy and requires advanced and expensive techniques. Chemical analysis of our samples showed an high amount of cesium (Cs2O 12.91 wt%) for pezzottaite, while the Cs-beryl has 1.27 wt%. The crystal structure of the samples has been investigated through X-ray diffraction. The pezzottaite has a trigonal symmetry (space group R-3c, with a~15.9 and c~27.8 Å), while beryl is hexagonal (space group P6/mcc, with a~9.2 and c~9.2 Å). The increase of cell parameters is due to the entrance of lithium, that replaces beryllium in the tethaedra. The replacement causes a positive charge deficit neutralized by cesium in the channels. The samples of pezzottaite and Cs-rich beryl were investigated by micro-Raman spectroscopy, a non-destructive and rapid tool of investigation. The un-polarized Raman spectrum of pezzottaite over the extended region 100-3650 cm-1 was collected for the first time, and compared with the spectrum of a Cs-beryl (Figure 1 and 2). In particular, Cs-beryl has showed only a intense peak at 3604 cm-1, ascribable to H2O stretching vibrations. On the other hand, two weak Raman bands at 3,591 and 3,545 cm-1, ascribable to the fundamental H2O or OH stretching vibrations respectively, were observed, despite the mineral should be nominally anhydrous. The Raman spectroscopy was useful to understand the type of water (type “I” or type “II”) and then to evaluate presence of alkali in the channels. In addition, the Raman spectrum of pezzottaite shows two intense and characteristic bands at 110-112 cm-1 and 1100 cm-1, which are not present in the beryl spectrum (Figure 1). Even if the true nature of the two bands is not completely understood, Raman spectroscopy appears to be a promising and inexpensive tool for a quicker identification of pezzottaite. Figure 1: Raman spectra of pezzottaite (above) and Cs-beryl (below) in the region 100-1,200 cm-1. Figure 2: Raman spectra of pezzottaite (above) and Cs-beryl (below) in the region 3,500-3,650 cm-1

Raman spectroscopy of CaCoSi2O6-Co2Si2O6 clinopyroxenes

Raman spectra were collected on a set of synthetic clinopyroxenes along the series CaCoSi2O6–Co2Si2O6. Changes in peak position and peak width show: (1) evidence of a phase transition from C2/c to P21/c, at Ca0.4Co1.6Si2O6, in agreement with previous X-ray observations; (2) peak broadening for intermediate compositions, with sharper peaks close to the end members. The phase transition is revealed by a decrease or inversion in the slope of the peak position versus composition and by peak splitting of the peaks at 660 and 1,000 cm−1, related to Si–O bending and stretching modes within the tetrahedral chains, respectively. The observed changes with composition depend more on variation in bond lengths due to structural rearrangement with cation substitution, rather than by changes of the M2 cation mass. A comparison with the structurally analogous CaMgSi2O6–Mg2Si2O6 (Diopside-Estatite, Ca-Mg) series shows that one of the two splitted peaks is fainter than the Ca–Co pyroxenes. Therefore the frequency of the peak at about 1,000 cm−1 does not change for Ca–Mg substitution, whereas it shifts by as much as 20 cm−1 between CaCoSi2O6 and Co2Si2O6. Despite the mechanism of cation substitution is qualitatively similar in the two series, the effect of structural changes and polyhedral deformation on the Raman spectra appeared different. Peak broadening in samples with intermediate compositions could be interpreted as arising by compositional disorder, due to coexistence of local Ca-rich and Co-rich configurations which affect the short range interactions and therefore the Raman frequencies

Raman spectroscopy of CaM2+Ge2O6(M2+= Mg, Mn, Fe, Co, Ni, Zn) clinopyroxenes

The Raman spectra of Ge-clinopyroxenes CaM2+Ge2O6 (M2+12 = Mg, Mn, Fe, Co, Ni, Zn), general formula M2M1T2O6, are reported for the first time. Their spectral features are discussed by comparison with corresponding Si-pyroxenes. The vibrational frequencies of germanates may be roughly obtained by a scale factor of about ~ 0.8 by those of the corresponding silicates, due to the Ge-Si mass difference. The main peaks in the germanate Raman spectra at ~ 850 and ~540 cm-1 may be related to Ge-O tetrahedral stretching and chain bending, respectively; minor peaks between 200 and 400 cm-1 are ascribed to bending and stretching of the non-tetrahedral cations. Within Gepyroxenes, possible correlations between crystallographic parameters and the vibrational frequencies are investigated. The main stretching mode at ~ 850 cm-1 shows wavenumber changes with M2+ substitutions, but no simple correlation can be found with M2+ cation mass or size. On the other hand, the chain bending wavenumber linearly decreases with increasing ionic radius of the M2+ cation: the expansion of the M1 polyhedron reduces the chain kinking angle and the Ge-Ge distances correspondingly increase