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(Na,Ca)(Ti3+,Mg)Si2O6-clinopyroxenes at high pressure: influence of cation substitution on elastic behavior and phase transition |
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| Authors: | Angela Ullrich Ronald Miletich Tonci Balic-Zunic Lars Olsen Fabrizio Nestola Manfred Wildner Haruo Ohashi |
| Institution: | 1. Institut für Geowissenschaften, Universit?t Heidelberg, Im Neuenheimer Feld 236, 69120, Heidelberg, Germany 2. Department of Geography and Geology, University of Copenhagen, ?ster Voldgade 10, 1350, Copenhagen, Denmark 3. Dipartimento di Geoscienze, Universita di Padova, Via Giotto 1, 10137, Padua, Italy 4. Institut für Mineralogie und Kristallographie, Universit?t Wien, Geozentrum, Althanstr. 14, 1090, Vienna, Austria 5. HASHI Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo, 142-0054, Japan |
| Abstract: | 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 K 0 = 115.9(7) GPa with K′ = −0.9(3) and K 0 = 102.7(8) GPa with K′ = 4.08(5) for the low- and high-pressure range, respectively. The transition involves a C2/c–P `1] \overline{1} 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 K 0 = 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. |
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