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Vibrational characterization of the new gemstone Pezzottaite
No full textPezzottaite 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
No full textRaman 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
No full textThe 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
Characterization of emeralds by micro-Raman spectroscopy
No full textIn recent years the use of Raman spectroscopy as a gemological tool has largely increased. In particular, in the conservation science field, the possibility to have a quick, non-destructive, contactless identification of a gem, maybe mounted on precious archaeological item, made this technique an invaluable procedure for gemologists and conservators.
The results of the Raman analysis are not limited to the simple identification of a gem. In this work we show the large amount of information which is possible to obtain on one of the most important gems, emerald, the green variety of beryl.
We studied by means of a standard micro-Raman spectrometer a large group of emeralds in different forms and of different origin: 15 faceted gems and a series of raw crystals (some of them still embedded in the host rock) coming from Val Vigezzo (Western Alps). Some fakes have been identified between the faceted gems (a garnet, a glass, a “quartz-beryl” sandwich). All the natural gems and crystals have been fully characterized from the vibrational point of view. In particular, the high frequency spectrum, in the OH-rich region, was used to estimate the amount of alkali ions present in the channels of the structure [Łodziński et al., 2005]. To quantify Be and Li, alkali ions in the channels such as Cs, Rb, K, Na, and other elements which better define the structure of beryl such as V, Cr, Mn, Fe analyses with LA-ICP-MS have been performed.
Fig.1: Raman spectra of two simulants found between the faceted gems and the characteristic Cr3+ luminescence of the emerald excited at 473.1 nm.
In addition, solid inclusion were identified and used as a tool to differentiate the provenance of the emeralds. The shape and the position of the characteristic laser-induced luminescence of chromium ions was used to better define the origin of the gems [Moroz et al., 2000].
Fluid inclusions present in the alpine crystals were studied; the identification of the phases and their concentration obtained by micro-Raman spectroscopy was completed by thermal analysis in order to made hypothesis on their genesis.
Fig.2: Raman spectra of an emerald coming from Val Vigezzo and of the liquid and gas phases present in a fluid inclusion; the broad band of liquid water, the sharpest one of the channel water at ~3600 cm-1 and the peak of methane at 2915 cm-1 are clearly visible.
References
Łodziński M., Sitarz M., Stec K., Kozanecki M., Fojud Z., Jurga S. Journal of Molecular Structure 2005; 744–747: 1005–1015
Moroz I., Roth M., Boudeulle M., Panczer G. Journal of Raman Spectroscopy 2000; 31: 485-490
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Effects of cation substitutions on the physical properties of M2M1T2O6 pyroxenes
No full textIn this PhD study, the effects of the cation substitutions on the physical properties of pyroxenes have been discussed. The results of this work extend the knowledge on pyroxenes with different chemical compositions. These properties might be used in the development of ceramic pigments, advanced materials and for the mineralogical phase identification.
First of all, the crystallographic differences between Ge and Si pyroxenes have been examined.
The structure of C2/c Ca rich Ge clinopyroxenes is very close to the low pressure C2/c structural configuration found in Ca-rich Si-pyroxenes. The shear of the unit cell is very similar, and the difference between a Ge end member and the corresponding Si-rich one is less than 1°. Instead, a remarkable difference exists between Ca-poor Si and Ge clinopyroxenes. First, Ca-poor Ge pyroxenes do not display a P21/c symmetry, but retain the C2/c symmetry; second, the observed C2/c structure shows, at room pressure, the configuration with highly kinked tetrahedral chains characteristic of the high pressure C2/c symmetry of Si Ca-poor pyroxenes.
In orthopyroxenes, with Pbca symmetry, Ge-pyroxenes have volume larger than Si-pyroxenes.
Samples along the system CaCoGe2O6 - CoCoGe2O6 have been synthesized at three different temperatures: 1050 °C, 1200 °C and 1250 °C. The aim of these solid state syntheses was to obtain a solid solution at ambient pressure, since the analogues Si-system needs high pressure. Unfortunately, very limited solution occurs because the structure forms of the two end member (high temperature for CaCoGe2O6 and high pressure CoCoGe2O6) are incompatible. The phase diagram of this system has been sketched and compared to that of Si. The cobalt end member (CoCoGe2O6) is stable at ambient pressure in two symmetries: at 1050 °C C2/c and 1200 °C Pbca. The impurity phase formed during these experiments is cobalt spinel.
Raman spectroscopy has been used to investigate the vibrational properties of Ca-pyroxenes CaCoGe2O6, CaMgGe2O6, CaMgSi2O6 and CaCoSi2O6.
A comparison between silicate and germanate pyroxenes shows significant changes in peak positions of the corresponding modes caused mainly by the difference of the Ge-Si atomic weight along with the distortion and compression of the coordination polyhedra. Red shift in Raman spectra of germanates has been calculated by a rough scale factor calculated by a simple harmonic oscillator model, considering the different bond lengths for 4-coordinated Si ~ 1.60- 1.65 Å vs Ge–O distance ~1.70 - 1.80 Å.
The Raman spectra of CaMgGe2O6 and CaCoGe2O6 have been classified, in analogy with silicate (Wang et al., 2001) counterparts, in different ranges:
- R1 (880-640 cm-1): strong T-O stretching modes of Ge and non-bridging O1 and O2 atoms within the GeO4 tetrahedron;
- R2 (640-480 cm-1): stretching/bending modes of Ge-Obr-Ge bonds (chain stretching and chain bending);
- R4 (480-360 cm-1): O-Ge-O vibrations;
- R3 (360-240 cm-1): motions of the cations in M2 and M1 sites correlated with tetrahedral chain motion and tilting tetrahedra;
- R5 (below 240 cm-1): lattice modes.
The largest shift with respect to CaMgSi2O6 - CaCoSi2O6 is shown by the T-O stretching and chain modes.
High-pressure Raman spectroscopy (up to about 8 GPa) on the same samples of Ca-pyroxenes using an ETH-type diamond anvil cell shows no phase transition within the P-ranges investigated, as all the peak positions vary linearly as a function of pressure. Our data confirm previous experimental findings on Si-diopside (Chopelas and Serghiou, 2000). In the investigated samples, all the Raman peaks shift upon compression, but the major changes in wavenumber with pressure are attributed to the chain bending (Ge-Obr-Ge bonds) and tetrahedra stretching modes (Ge-Onbr). Upon compression, the kinking angle, the bond lengths and T-T distances between tetrahedra decrease and consequently the wavenumber of the bending chain mode and tetrahedra stretching mode increases. Ge-pyroxenes show the higher P-induced peak-position shifts, being more compressible than corresponding silicates.
The vibrational properties of CaM2+Ge2O6 (M2+ =Mg, Mn, Fe, Co, Ni, Zn) are reported for the first time. The wavenumber of Ge-Obr-Ge bending modes decreases linearly with increasing ionic radius of the M1 cation. No simple correlation has been found with M1 atomic mass or size or crystallographic parameters for the peak at ~850 cm-1 and in the low wavenumber regions.
The magnetic properties of the system CaCoSi2O6 - CoCoSi2O6 have been investigated by magnetometry.
The join is always characterized by 1 a.p.f.u. of cobalt in M1 site and this causes a pure collinear antiferromagnetic behaviour of the intra-chain superexchange interaction involving Co ions detected in all the measurements, while the magnetic order developed by the cobalt ions in M2 site (intra-chain) is affected by weak ferromagnetism, due to the non-collinearity of their antiferromagnetic interaction. In magnetically ordered systems, this non-collinearity effect promotes a spin canting of anti-parallel aligned magnetic moments and thus is a source of weak ferromagnetic behaviour in an antiferromagnetic. The weak ferromagnetism can be observed only for the samples with Co content higher than 0.5 a.p.f.u. in M2, when the concentration is sufficiently high to create a long range order along the M2 chain which is magnetically independent of M1 chain. The ferromagnetism was detected both in the M(T) at 10 Oe and M(H)
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