Synthetic Clinopyroxenes Research Articles

Effect of fO2 on Eu partitioning between clinopyroxene, orthopyroxene and basaltic melt: Development of a Eu3+/Eu2+ oxybarometer

We present partition coefficients for Cs, Li, Sr, Ba, Zn, Mn, Co, Ni, Sc, Ga, Y, La, Nd, Sm, Eu, Tb, Yb, Lu, Ti, Zr, Hf, Ta, Nb and P for synthetic clinopyroxene and orthopyroxene crystallized from a Fe-free basalt at 1.5 GPa and four different oxygen fugacities (fO2) ranging from 6 log units above the quartz-fayalite-magnetite oxygen buffer (QFM + 6) to QFM-5 at temperatures of 1275–1300 °C. Partition coefficients for the rare earth elements (REE) vary as a function of their ionic radii, in agreement with lattice strain theory. We use the lattice strain model to evaluate partition coefficients for Eu2+ and Eu3+. Our results suggest that fO2 exerts a primary and important control on Eu, Ni and Co partitioning in clinopyroxene whereas fO2 seemingly has no recognizable effect on the partitioning behavior of these elements in orthopyroxene. However Eu anomalies in orthopyroxene, orthopyroxene Onuma diagrams and calculated proportions of Eu2+ in melts all show evidence that fO2 influence Eu partitioning also in orthopyroxene. We combined our results, available experimental data and predictive models for divalent and trivalent Eu clinopyroxene-melt and orthopyroxene-melt partitioning to parameterize fO2-dependent Eu partitioning models. The reported models reproduce measured Eu partition coefficients within a factor of two. Our model can be applied as clinopyroxene-melt and orthopyroxene-melt oxybarometers if Eu equilibrium partitioning between melt and pyroxene can be demonstrated.

Near-infrared spectroscopy of Ca-rich clinopyroxenes revisited: A new interpretation of anomalous type-B spectra and implications for remote sensing of inner solar system bodies

Four Ca-rich clinopyroxene samples (3 terrestrial clinopyroxenes and a druse clinopyroxene from the D’Orbigny meteorite) have been studied by near-infrared (NIR) spectroscopy and X-ray structure refinement in order to address the issue of anomalous type-B NIR spectra seen in some terrestrial and extraterrestrial Ca-rich clinopyroxenes. The D’Orbigny druse clinopyroxene shows a typical type-B NIR spectrum, with two absorption bands located at ~ 1.1 and 2.3 μm. Based on previous Mössbauer and X-ray structure refinement results on synthetic clinopyroxenes, the 1.1 μm band is assigned to Fe2+ at the M1 site in diopside and high-pigeonite (i.e. pigeonite with C2/c symmetry), and the 2.3 μm band to Fe2+ at the M2 site in high-pigeonite. A comparison of NIR spectral type and X-ray structure refinement of the studied samples indicates that the average structure refinement of all type-B clinopyroxenes show a residual electron density peak in the difference-Fourier synthesis, corresponding to the split M2’ site which is mainly occupied by Fe2+. The results present a new interpretation for type-B NIR spectra of Ca-rich clinopyroxenes.

Co2+-doped diopside: crystal structure and optical properties

Synthetic clinopyroxenes along the CaMgSi2O6–CaCoSi2O6 join were investigated by a combined chemical-structural-spectroscopic approach. Single crystals were synthesized by flux growth methods, both from Ca-saturated and Ca-deficient starting compositions. Single crystal structure refinements show that the incorporation of Co2+ at the octahedrally coordinated cation sites of diopside, increases the unit-cell as well as the M1 and the M2 polyhedral volumes. Spectroscopic investigations (UV–VIS–NIR) of the Ca-rich samples reveal three main optical absorption bands, i.e. 4 T 1g → 4 T 2g(F), 4 T 1g → 4 A 2g(F) and 4 T 1g → 4 T 1g(P) as expected for Co2+ at a six-coordinated site. The bands arising from the 4 T 1g → 4 T 2g(F) and the 4 T 1g → 4 T 1g(P) electronic transitions, are each split into two components, due to the distortions of the M1 polyhedron from ideal Oh-symmetry. In spectra of both types, a band in the NIR range at ca 5000 cm−1 is caused by the 4 A 2g → 4 T 1g(F) electronic transition in Co2+ in a cubic field in the M2 site. Furthermore, an additional component to a band system at 14,000 cm−1, due to electronic transitions in Co2+ at the M2 site, is recorded in absorption spectra of Ca-deficient samples. No variations in Dq and Racah B parameters for Co2+ at the M1 site in response to compositional changes, were demonstrated, suggesting complete relaxation of the M1 polyhedron within the CaMgSi2O6–CaCoSi2O6 solid solution.

An Automatic Deconvolution Method for Modified Gaussian Model using the Exchange Monte Carlo Method: Application to Reflectance Spectra of Synthetic Clinopyroxene

Deconvolution analysis of reflectance spectra has been a useful method to infer mineral composition and crystal structure. Many of the recent deconvolution analyses of reflectance spectra of major rock-forming minerals, such as olivine and pyroxene, have been based on a modified Gaussian Model (MGM). The numerical algorithm of the widely used MGM, however, utilizes the steepest descent method, which has a local minima problem. With inaccurate initial parameters, the steepest descent method converges into a local minimum, thus the analyzer must manually adjust initial parameters and calculate the model repeatedly to obtain the desired solution. In order to avoid the local minimum problem, we utilized Bayesian spectral deconvolution with the exchange Monte Carlo method, which is an improved algorithm of the Markov chain Monte Carlo method, aimed to both avoid local minima traps and remove the arbitrariness originated from initial parameters. We applied the model to visible to near infrared reflectance spectra of 31 synthetic clinopyroxene samples with wide ranging Mg, Fe and Ca compositions (solid solution). We obtained results consistent with the previous studies based on conventional MGM analyses, suggesting that the exchange Monte Carlo method can yield results consistent with the conventional MGM analyses purely based on the observed data. We also find that the center wavelengths of 1 μm absorption bands of high-Ca pyroxene samples have a linear dependence on Fe/Mg component. Both 1 μm and 2 μm absorption bands seem to follow approximation lines in the three-dimensional spaces of center wavelengths, Ca and Fe components. The successful application of the exchange Monte Carlo method to a wide range of clinopyroxenes would have a potential to expand the applicability of MGM to a variety of space/ground-based observations, especially when we cannot rely on prior information of the mineralogy.

Structural and magnetic phase transitions in the synthetic clinopyroxene LiCrGe2O6: a neutron diffraction study between 0.5 and 1473 K

The pyroxene-type compound LiCrGe2O6, the Li- and Ge-analogue to the silicate mineral kosmochlor, has been synthesized at 1373 K and investigated by neutron diffraction between 0.5 and 1473 K in order to investigate the variation in magnetic and crystal structure with temperature. A structural phase transition from a low-temperature P21/c to a high-temperature C2/c structure was found around 1140 K. The two different structures exhibit different thermal expansion behavior with temperature with a reversal of the largest thermal expansion from the c-axis to the b-axis in the P21/c and C2/c phase, respectively. The structural phase transition is accompanied by a large volume increase of 1.9 % and sharp discontinuities in bond lengths, especially for the Li–O and—to a lesser extent—for the Cr–O bonds. At low temperature, some additional nonlinear changes in lattice parameters occur, which are associated with a magnetoelastic couplings of the lattice. Magnetic ordering is observed below 6 K in the neutron diffraction data. Data could be indexed with k = (0 0 0), giving rise to magnetic space group P21′/c. This model of the magnetic structure has a pure antiferromagnetic arrangement of spins, both within and between the M1 chains. The spins are oriented within the a–c plane with an almost nil component along [0 1 0]. A shift of the Cr atom out of the center in the equatorial plane of the octahedron is observed below 6 K and is associated with the magnetic phase transition.

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.

Optical absorption spectroscopy study of three synthetic V3+-bearing clinopyroxenes

Optical absorption spectra of synthetic clinopyroxenes of NaVSi 2 O 6 , LiVSi 2 O 6 and CaVAlSiO 6 composition consist of a high-energy absorption edge and two broad absorption bands ν 1 and ν 2 at around 15,000 cm −1 and 22,000 cm −1 caused by spin-allowed electronic transitions 3 T 1 g ( 3 F ) → 3 T 2 g ( 3 F ) and 3 T 1 g ( 3 F ) → 3 T 1 g ( 3 P ) of V 3+ in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi 2 O 6 and LiVSi 2 O 6 pyroxenes the relative intensities of the polarized components of ν 1 - and ν 2 -bands are rather different, revealing differences in the distortion of VO 6 -octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr 3+ in Cr-bearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V 3+ in NaVSi 2 O 6 -pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr 3+ , the integral intensity of the ν 1 -band remains nearly constant within uncertainty whereas that of the ν 2 -band somewhat decreases, in contrast to Cr 3+ for which the spin-allowed bands significantly decrease. Comparing with k oct loc -value of CrO 6 -octahedron in NaCrSi 2 O 6 kosmochlor, ~196 GPa, the VO 6 -octahedron in NaVSi 2 O 6 clinopyroxene structure is considerably softer, with k oct loc ≈ 163 GPa. The distances derived from the optical absorption spectra can fairly well be approximated by a linear dependence. The Racah parameter B of V 3+ evidently decreases with increasing pressure from ~554 cm −1 at 10 −4 GPa to 508 cm −1 at 10.7 GPa, thus indicating that V – O bonds in the VO 6 -octahedra become more covalent.

Thermal expansion and high-temperature P21/c–C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6

A synthetic clinopyroxene with composition LiFe3+Ge2O6, monoclinic s.g. P21/c, a = 9.8792(7), b = 8.8095(5), c = 5.3754(3) A, β = 108.844(6)°, V = 442.75(16) A3, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P21/c to C2/c at a transition temperature Ttr = 789 K, first order in character, with a sudden volume increase of 1.6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below Ttr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > Ttr the thermal volume expansion is due to dilatation of the structure in the \((\bar{1}\,0\,1) \) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P21/c expands more than the C2/c phase. The evolution of Q2 (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q02 = 0.538(2) at Ttr, with Tc of 481(7) K, b/a = −2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe3+Si2O6) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P21/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13.3%, compared to ca. 1.3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > Ttr ( Ttr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at Ttr of the kinking angle (O3–O3–O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T < Ttr [206.8(5)° at 703 K] and extends by 12° at the transition.

Experimental Investigation of the Upper Thermal Stability of Mg-rich Actinolite; Implications for Kiruna-Type Iron Deposits

The occurrence of actinolite in magnetite deposits of possible mag-matic origin has prompted an experimental investigation of theupper thermal stability of Mg-rich actinolite to determine how thestability of actinolite changes with increasing Fe content.Experiments were carried out primarily on the compositionalre-equilibration of natural tremolite [molar Fe/(FeþMg)¼Fe-number ¼0 014] in the presence of synthetic clinopyroxene(Ca

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)-diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W-CaTs-Di(V) = -0.29 +/- 0.11 cm(3) mol(-1) and W-Di-CaTs(V) = -1.14 +/- 0.14 cm(3) mol(-1). Heat capacities were determined between 5 and 923 K by heat-pulse at 5-302 K and differential-scanning calorimetry at 143-923 K. The precision of the low and high temperature C (p) data is better than +/- 1%. Polynomials of the form C (p) = a + bT(-1/2) + cT(-2) + dT(-3) were fitted to the C (p) data in the temperature range between 250 and 925 K. Thermal entropy values [S (298) - S (0)] and [S (900) -S (0)] as well as enthalpies [H (298) - H (0)] and [H (900) - H (0)] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.

Experimental multipole-refined and theoretical charge density study of LiGaSi2O6 clinopyroxene at ambient conditions

The synthetic LiGaSi2O6 clinopyroxene is monoclinic C2/c at room-T. Its experimental electron density, ρ(r), has been derived starting from accurate room-T single-crystal diffraction data. Topological analysis confirms an intermediate ionic-covalent character for Si–O bonding, as found by previous electron-density studies on other silicates such as diopside, coesite and stishovite. The non-bridging Si–O bonds have more covalent character than the bridging ones. The Ga–O bonds have different bonding characters, the Ga–O2 bond being more covalent than the two Ga–O1 bonds. Li–O bonds are classified as pure closed-shell ionic interactions. Similar to spodumene (LiAlSi2O6), Li has sixfold coordination, but the bond critical points associated to the two longest bonds are characterized by very low electron density values. Similar to what previously found in spodumene and diopside, O···O interactions were detected from the topological analysis of ρ(r), and indicate a cooperative interaction among the lone pairs of neighbouring oxygen atoms. In particular, this kind of interaction has been obtained for the O1···O1 edge shared between two Ga octahedra. Integration over the atomic basins gives net charges of −1.39(10), 2.82(10), 1.91(10) and 0.82(8) e for O (averaged), Si, Ga and Li atoms, respectively. Periodic Hartree–Fock and DFT calculations confirm the results obtained by multipole refinement of the experimental data. Moreover, the theoretical topological properties of the electron density distribution on the Si2O6 group are very similar to those calculated for spodumene.

A recipe for laboratory-grown crystals

&#x0D; &#x0D; &#x0D; &#x0D; Clinopyroxenes are among the first minerals to crystallize out of a ferromagnesian silicate magma. They commonly exhibit “sectorzoning”, a phenomenon whereby the crystal incorporates elements in different proportions on non-equivalent crystal faces. By growing clinopyroxene crystals in the laboratory, it is possible to investigate controls on compositional variation, which provides insight on magmatic processes. The goal of this research was to develop an experimental method for growing synthetic clinopyroxene (a silicate mineral) in a carbonate melt rather than in a silicate one. This is advantageous since silicate residue on the clinopyroxene crystal may damage crystal faces, which contain important information on growth features, unlike carbonate residue which is easily dissolved leaving crystal faces intact. The carbonate melt was modeled after the alkali-rich carbonatite lavas erupting at Oldoinyo Lengai, Tanzania by using powdered clinopyroxene, magnetite and alkali carbonates containing up to 5% wt. water as starting materials, and running the experiment at conditions of 800∞C and 10 kbars. Clinopyroxene crystals in a carbonate crystalline matrix were retrieved from the experiment capsules, and cleaned for imaging and analysis with the atomic force microscope (AFM), scanning electron microscope (SEM) and electron microprobe (EMP). This experimental approach provides well-preserved crystal faces whose surfaces can be examined at nanoscale resolution. This technique could be applied to a wide range of synthetic silicate minerals, and the resulting observations help to better understand the relationship between crystal surface structure and trace element uptake during crystal growth.&#x0D; &#x0D; &#x0D; &#x0D;

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin dia- mond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) A 3 , K0 = 129(1) GPa, K' = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compress- ibilities βb > βc ~ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show signiÞ cant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to Þ t into the pyroxene structure.

High temperature single crystal investigation in a clinopyroxene of composition (Na0.5Ca0.5)(Cr0.5Mg0.5)Si2O6

The structure of a synthetic clinopyroxene of composition (Na 0.5 Ca 0.5 )(Cr 0.5 Mg 0.5 )Si 2 O 6 was investigated at room and high temperature. At room temperature a high resolution data collection (2𝛉 ≤ 110°) was done; at high temperature the data were collected up to 2𝛉 ≤ 50° and at T = 25, 200, 400 and 620°C. The observed symmetry was always C 2/ c , and no evidence of a reduction to the long range ordered P 2 1 / n symmetry was found. The analysis of the evolution of the B eq vs T suggests instead the presence of local clustering of NaCr and CaMg pairs. A comparison with the high temperature behaviour of diopside, kosmochlor and jadeite shows a significant decrease in volume expansion with the coupled NaM 3+ substitution for CaMg. This is mainly due to a decrease in the expansion along the b axis, and is related to a smaller expansion in the M1 polyhedron for the substitution of a trivalent cation in the M1 site and to the consequent higher stiffness of the O1A1 - O1B1 shared edge between the M1 polyhedra.

High pressure behavior, transformation and crystal structure of synthetic iron-free pigeonite

A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di15En85, unit-cell parameters at room pressure: a = 9.6525(6)Å, b = 8.8461(2)Å, c = 5.2036(5)Å, β = 108.370(5)°, V = 421.68(4)Å3]. A first order P21/c-C2/c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections (h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Murnaghan equation of state with V0 = 421.68(8) Å3, KT0 = 102(2) GPa, and K′ = 8(1) for the low-symmetry phase (P21/c) and with V0 = 411.06(3) Å3 and KT0 = 108(2) GPa for the high-symmetry phase (C2/c), with K′ fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: βb > βa ≅ βc > βasinβ for both phases. In comparison with pure clinoenstatite, Di15En85 shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus.

X-ray Rietveld analyses of synthetic clinopyroxenes in the join CaMgSi2O6-CaMgTi2O6

X-ray Rietveld analyses of synthetic clinopyroxenes in the join CaMgSi2O6-CaMgTi2O6

A transmission electron microscope investigation of theC2/c→P21/cphase transition in clinopyroxenes along the diopside-enstatite (CaMgSi2O6-Mg2Si2O6) join

A transmission electron microscope investigation was performed on a series of synthetic clinopyroxenes with compositions between Ca 0.7 Mg 1.3 Si 2 O 6 and Ca 0.5 Mg 1.5 Si 2 O 6 . For samples with Ca content lower than 0.6 atoms per formula unit (apfu) selected-area electron diffraction (SAED) patterns showed the presence of reflections violating the C 2/ c space group ( b type, h+k odd), indicating a transition to the P 2 1 / c symmetry at room temperature. Antiphase domains induced by the transition could be imaged, with irregular boundaries and size decreasing with increasing Ca content. The antiphase domain size and shape appears unrelated to the non-periodic mottled texture that could be imaged in samples with Ca lower than about 0.65 apfu. A comparison with data reported in the literature suggests a different behavior in the cell parameters of the P 2 1 / c clinopyroxenes along the diopside-enstatite (Di-En) join. For compositions between Di 60 En 40 to about Di 40 En 60 , cell parameters deviate little with respect to the trend shown by Ca-rich C 2/ c clinopyroxenes, whereas for compositions richer in Mg than about Di 40 En 60 , a significant deviation is present, which is marked by discontinuities in the c and β cell parameters.

Detection limits of selected rare-earth elements in electron-probe microanalysis

Based on statistical analysis of repetitive measurements, definitions of concentration at the detection limit from the literature were specified as four types: (a) peak separation (PKSEP), (b) qualitative detection limit (QUAL), (c) quantitative detection limit (QUANT), and (d) the minimum amount of an element guaranteed detectable (MAGD). As a new approach, for a given mean value of accumulated counts on the background, a reverse calculation yields the mean value of the accumulated counts on the peak fulfilling criteria (a) to (d). Based on the reverse calculation, a diagram was constructed that allows to use the averages of accumulated raw counts of peak and background to decide whether the analysed concentration can be used as quantitative result. The method is valid for any analytical condition and does not need a matrix correction. The maximum concentrations of La, Nd, Sm, Gd, Ho and Yb in olivine, orthopyroxene, clinopyroxene and garnet (sub-solidus model garnet lherzolite assemblage) were investigated experimentally in the system CaO - MgO - Al 2 O 3 - SiO 2 . REE-Lα lines were measured at 20 kV / 60 nA on PET (La, Nd, Sm. Gd) and on LIF (Ho, Yb) monochromator crystals with counting times of 80 s (La) to 190 s (Yb) on the peak and total analysis times of ca. 5 min. The maximum concentrations of REE in synthetic garnet and clinopyroxene, partially also in orthopyroxene, make these phases suitable for starting materials of quasi-reversal experiments. The calculated concentrations at the quantitative detection limit for olivine, orthopyroxene, clinopyroxene and garnet range from 175 ppm [μg/g] for La to 275 ppm for Gd on PET, and from 180 ppm for Ho to 200 ppm for Yb on LIF. In case of La and Nd, compared to the used linear relationship, a full PAP correction yields values higher by about 15%. The background count rates on the REE-Lα peak positions for orthopyroxene, clinopyroxene and garnet in the system CaO - MgO - Al 2 O 3 - SiO 2 , compared to those measured on pure enstatite, diopside and pyrope standards, are mainly higher due to solid solution effects. REE concentrations less than ca. 4000 ppm did not result in a higher count rate of the background for olivine and orthopyroxene. For clinopyroxene and garnet the effect is larger due to higher solubility of REE. Concentrations at the quantitative detection limit were calculated for samples with similar count rates as the synthetic standards. On REEAIO 3 (La, Nd, Sm, Gd, Ho) and REE 3 Al 5 O 12 (Yb, Lu) at 20 kV / 20 nA they range from 450 ppm (La) to 900 ppm (Yb), and on REEPO 4 at 20 kV / 50 nA and 20 kV / 60 nA they range from 350 ppm (La) to 500 ppm (Gd), and from 350 ppm (Ho) to 400 ppm (Yb). The calculated concentrations are influenced by the sensitivity and the stability of the individual spectrometers. Pre-requisite is a careful selection of the background positions.

Crystal structures of (Na,Ca)(Sc,Zn)Si2O6 clinopyroxenes formed at 6GPa pressure.

Crystal structures have been refined from single-crystal X-ray data for eight synthetic clinopyroxenes in the system NaScSi2O6-CaZnSi2O6, crystallized at 1470-1570 K and 6GPa pressure. The structures are isomorphous with other sodium and/or calcium pyroxenes. The space group is C2/c, Z=4. Sc and Zn occupy a distorted octahedral(M1) site. The O1-Si-O2 angles correlate with the differences, dbr-nbr=mean -mean (br=bridging, nbr=nonbridging) in such a way as to follow two different trends, suggesting that there are two different electronic states for the octahedral Zn2+ ions.

Synthesis of the clinopyroxenes join CaMgSi2O6-NaAlSi2O6 for jewellery

Clinopyroxenes of the join CaMgSi2O6(diopside)-NaAlSi2O6 (jadeite) were synthesized in the temperature range 800–1900‡C and under varying pressure, 10–55 kbar. The stability regions of various compositions of diopside-jadeite have been established experimentally using different compositions of glass materials: stoichiometric composition NaAlSi2O6, 0.1CaMgSi2O6-0.9NaAlSi2O6, 0.2CaMgSi2O6-0.8NaAlSi2O6, 0.3CaMgSi2O6-0.7NaAlSi2O6, and 0.4CaMgSi2O6-0.6NaAlSi2O6. Unit cell parameters of synthetic clinopyroxenes with the above compositions were determined. The physical properties, such as hardness, toughness, density, and refractive index, etc., were also measured. The results show that synthetic clinopyroxenes have the same properties as the natural one. The gem quality of diopside-jadeite clinopyroxenes was achieved by synthesised on the basis of the above experiments. Various colouring agents, such as Cr2O3, Co2O3, NiO2, Fe2O3, TiO2, MnO, CuO, and their combinations, FeO-Cr2O3, etc., were added to obtain the different colours of gem. In addition, small amounts of the rare-earth oxides, such as CeO2, Nd2O3, Sm2O3, Dy2O3, Eu2O3, Er2O3, Pr6O11, Lu2O3 and CuO-Eu2O3, Co2O3-Nd2O3, etc., were also added to produce fluorescent clinopyroxenes for jewellery.