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Ullrich A.,University of Heidelberg | Miletich R.,University of Heidelberg | Balic-Zunic T.,Copenhagen University | Olsen L.,Copenhagen University | And 3 more authors.
Physics and Chemistry of Minerals | Year: 2010

A compressional study of (Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxenes was carried out at high pressures between 10-4 and 10.2 GPa using in situ single-crystal X-ray diffraction, Raman spectroscopy and optical absorption spectroscopy. Compressional discontinuities accompanied by structural changes, in particular, the appearance of two distinct Ti3+-Ti3+ distances within the octahedral chains at 4.37 GPa, provide evidence for the occurrence of a phase transition in NaTi3+Si2O6. Equation-of-state parameters are K0 = 115.9(7) GPa with K′ = -0.9(3) and K0 = 102.7(8) GPa with K′ = 4.08(5) for the low- and high-pressure range, respectively. The transition involves a C2/c-P 1̄ symmetry change, which can be confirmed by the occurrence of new modes in Raman spectra. Since no significant discontinuity in the evolution of the unit-cell volume with pressure has been observed, the transition appears to be second-order in character. The influence of the coupled substitution Na+Ti3+↔Ca2+Mg2+ on the static compression behavior and the structural stability has been investigated using a sample of the intermediate composition (Na0.54Ca0.46)(Mg0.46Ti0.54)Si2O6. No evidence for a deviation from continuous compression behavior has been found, neither in lattice parameter nor in structural data and the fit of a third-order Birch-Murnaghan equation-of-state to the pressure-volume data yields a bulk modulus of K0 = 109.1(5) GPa and K′ = 5.02(13). Raman and polarized absorption spectra have been compared to NaTiSi2O6 and reveal major similarities. The main driving force for the phase transition in NaTi3+Si2O6 is the localization of the Ti3+d-electron and the accompanying distortion, which is suppressed in the (Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxene. © Springer-Verlag 2009.

Nestola F.,University of Padua | Nestola F.,CNR Institute of Geosciences and Earth Resources | Ballaran T.B.,University of Bayreuth | Angel R.J.,Virginia Polytechnic Institute and State University | And 2 more authors.
American Mineralogist | Year: 2010

Two C2/c pyroxenes, CaMnSi2O6 and CaZnSi 2O6, were investigated at high pressure by single-crystal X-ray diffraction. Their unit-cell parameters were determined at 9 different pressures up to 7.55 and 8.23 GPa, respectively. No evidence of phase transformations was found for either sample throughout the investigated pressure range. The pressure-volume data were fit using a third-order Birch-Murnaghan equation of state and it was possible to refine simultaneously the unit-cell volume V0, the bulk modulus KT0 and its first pressure derivative K′ providing the following coefficients: V0 = 462.01(3) Å3, KT0 = 97.3(4) GPa, K′ = 7.5(2) for CaMnSi2O6 and V0 = 444.79(2) Å3, KT0 = 109.6(5) GPa, K′ = 7.0(2) for CaZnSi2O6. The substitution of Zn for Mn at the M1 crystallographic site causes a strong decrease of the bulk modulus (by about 11%). Intensity data were collected at several pressures for both samples. The main compression mechanism is related to the tetrahedral chain kinking and the consequent shortening of the Ca-O3long bond distances, which show a markedly larger decrease for the CaMnSi2O6 sample. Comparing the results obtained in this work with previous data for Ca/Na-bearing C2/c pyroxenes containing trivalent and divalent 3d-transition elements at M1 site, we identified two distinct compressibility trends: for a similar unit-cell volume at room pressure the pyroxenes with Na on M2 and trivalent cations on M1 are more compressible than those having Ca on M2 and divalent cations at the M1 site. The analysis of the structure evolution for all samples indicates that this is strongly correlated with the evolution of kinking of the tetrahedral chains and the shortening of the M2-O3 bonds.

Periotto B.,Copenhagen University | Nestola F.,University of Padua | Bali-Uni T.,Copenhagen University | Pasqual D.,University of Padua | And 3 more authors.
Solid State Communications | Year: 2012

The equation of state of a single-crystal synthetic silicate NaInSi 2O 6 was determined from the unit cell parameters measured by X-ray diffraction using a diamond-anvil cell at 12 different pressures up to 7.83(1) GPa in order to provide a definitive model capable of predicting the high-pressure behavior of NaMe 3 +Si 2O 6 silicate compounds. The PV data were fitted by a third-order BirchMurnaghan equation of state obtaining the following coefficients: V0=463.42(3) 3, KT0=109.0(6)GPa, K′=3.3(2). Our results show similar results for NaMe 3 +Si 2O 6 compounds and confirm that for this mineral family, the empirical KT0× V0=constant relationship proposed in previous investigations is valid for isostructural compounds only if they share the same valence electron character. In addition, we collected intensity data on the same compound at 16 different pressures up to 9.467 GPa using a diamond-anvil cell with diamond backing plates. The main structural compression mechanism is played by the tetrahedral chain kinking, which is related to the shortening of NaO3 long distances. Comparing our equation of state and high-pressure crystal structural results with previous data on NaMe 3 +Si 2O 6 pyroxenes, we show that their compressibility increases much faster with the size of Me 3 + cation when this one is a 3d transition metal. © 2011 Elsevier Ltd. All rights reserved.

Taran M.N.,Ukrainian Academy of Sciences | Ohashi H.,HASHI Institute for Silicate Science | Langer K.,TU Berlin | Vishnevskyy A.A.,Ukrainian Academy of Sciences
Physics and Chemistry of Minerals | Year: 2011

Comparison of polarized optical absorption spectra of natural Ca-rich diopsides and synthetic NaCrSi2O6 and LiCrSi2O6 clinopyroxenes, evidences as vivid similarities, as noticeable differences. The similarities reflect the fact that in all cases Cr3+ enters the small octahedral M1-site of the clinopyroxene structure. The differences are due to some iron content in the natural samples causing broad intense near infrared bands of electronic spin-allowed dd transitions of Fe2+(M1, M2) and intervalence Fe2+/Fe3+ charge-transfer transition, and by different symmetry and different local crystal fields strength of Cr3+ in the crystal structures. The positions of the spin-allowed bands of Cr3+, especially of the low energy one caused by the electronic 4A2g → 2T1g transition, are found to be in accordance with mean M1-O distances. The local relaxation parameter e{open} calculated for limCr 3+ → 0 from the spectra and interatomic 〈Cr-O〉 and 〈Mg-O〉 distances yields a very high value, 0.96, indicating that in the clinopyroxene structure the local lattice relaxation around the "guest" ion, Cr3+, deviates greatly from the "diffraction" value, e{open} = 0, than in any other known Cr3+-bearing systems studied so far. Under pressure the spin-allowed bands of Cr3+ shift to higher energies and decrease in intensity quite in accordance with the crystal field theoretical expectations, while the spin-forbidden absorption lines remain practically unshifted, but also undergo a strong weakening. There is no evident dependence of the Racah parameter B of Cr3+ reflecting the covalence of the oxygen-chromium bond under pressure: within the uncertainty of determination it may be regarded as practically constant. The values of CrO6 octahedral modulus, kloc poly, derived from high-pressure spectra of natural chromium diopside and synthetic NaCrSi2O6 kosmochlor are very close,̃203 and̃196 GPa, respectively, being, however, nearly twice higher than that of MgO6 octahedron in diopside, 105(4) GPa, obtained by Thompson and Downs (2008). Such a strong stiffening of the structural octahedron, i. e. twice higher value of kloc Cr 3+ comparing with that of kloc Mg 2+, may be caused by simultaneous substitution of Ca2+ by larger Na+ in the neighboring M2 sites at so-called jadeite-coupled substitution Mg2+ + Ca2+ → Cr3+ + Na+. It is also remarkable that the values of CrO6 octahedral modulus of NaCrSi2O6 kosmochlor obtained here are nearly twice larger than that of 90(16) GPa, evaluated by high-pressure X-ray structural refinement by Origlieri et al.(2003). Taking into account that the overall compressibility of the clinopyroxene structure should mainly be due to the compressibility of M1- and M2-sites, our kloc Cr 3+-value,̃196 GPa, looks much more consistent with the bulk modulus value, 134(1) GPa. © 2010 Springer-Verlag.

Taran M.N.,Ukrainian Academy of Sciences | Ohashi H.,HASHI Institute for Silicate Science
European Journal of Mineralogy | Year: 2012

Optical absorption spectra of synthetic clinopyroxenes of NaVSi 2O6, LiVSi2O6 and CaVAlSiO 6 composition consist of a highenergy absorption edge and two broad absorption bands ν1 and ν2 at around 15,000cm -1 and 22,000cm-1 caused by spin-allowed electronic transitions 3T1g (3F)→3T 2g (3F) and 3T1g (3F) →3T1g (3P) of V3+ in octahedral M1-site of the clinopyroxene structure. In polarized spectra of NaVSi 2O6 and LiVSi2O6 pyroxenes the relative intensities of the polarized components of ν1- and ν2-bands are rather different, revealing differences in the distortion of VO6-octahedra in the two structures. In spite of a formal similarity to the absorption spectra of Cr3+ in Crbearing clinopyroxenes, the pressure-induced behaviour of the spin-allowed bands of V3+ in NaVSi2O6-pyroxene is noticeably different: although they also shift to higher energies as do the bands of Cr3+, the integral intensity of the ν1-band remains nearly constant within uncertainty whereas that of the ν2-band somewhat decreases, in contrast to Cr3+ for which the spin-allowed bands significantly decrease. Comparing with kloc oct -value of CrO 6-octahedron in NaCrSi2O6 kosmochlor, ∼196 GPa, the VO6-octahedron in NaVSi2O6 clinopyroxene structure is considerably softer,with koct loc ≈ 163 GPa. The 〈V-O〉 distances derived from the optical absorption spectra can fairly well be approximated by a linear dependence. TheRacah parameter B ofV3+ 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. © 2012 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart.

Tribaudino M.,University of Parma | Ohashi H.,Hashi Institute for Silicate Science
Periodico di Mineralogia | Year: 2011

Synchrotron radiation powder diffraction patterns were taken on synthetic Co3Al2Si3O12 garnet at BM8-GILDA (ESRF), at T = 298, 423, 573, 723 and 873 K. The cell parameters and atomic positions were refined by Rietveld analysis; the volume thermal expansion coefficient is αV = 28.5(9) x10-6 K-1. The results on synthetic Co3Al2Si3O 12 garnet were compared with the high temperature structure data of X3Al2Si3O12 garnets with X = Mg, Ca, Mn and Fe taken from literature. The thermal expansion of the unit cell is very similar in X3Al2Si3O12 garnets, whereas the thermal expansion of the longest bonds in the distorted cubic cage surrounding the X cation decreases with increasing cation size. Contrarily to the compression behaviour, the thermal expansion in X3Al 2Si3O12 garnets does not change significantly as a function of composition.

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