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Bulk modulus variation along the diopside–kosmochlor solid solution

¹ Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany
² Dipartimento di Geoscienze, Università di Padova, Corso Garibaldi 37, I-35137 Padova, Italy
³ Dipartimento di Scienze della Terra, Università di Parma, Viale G.P. Usberti 157/A, I-43100, Parma, Italy
⁴ Hashi Institute for Silicate Science, Nishinakanobu 1-9-25, Shinagawa, Tokyo 142-0054, Japan

^* Corresponding author, e-mail: tiziana.boffa-ballaran{at}uni-bayreuth.de

Three pyroxene single-crystals with compositions NaCrSi₂O₆ (Ko₁₀₀Di₀), (Na_0.75Ca_0.25)(Cr_0.75Mg_0.25)Si₂O₆ (Ko₇₅Di₂₅) and (Na_0.25Ca_0.75)(Cr_0.25Mg_0.75)Si₂O₆ (Ko₂₅Di₇₅) have been studied by means of in situ high-pressure X-ray single-crystal diffraction using a diamond anvil cell up to ~7.6 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) was used to fit the P–V data measured for the three samples. The bulk modulus, K_T0, increases from 113.1(5) GPa for Ko₂₅Di₇₅ to 130.2(7) GPa for Ko₁₀₀Di₀, whereas the first pressure derivative, K', decreases from 5.8 to 4.1 with increasing Ko content. The bulk modulus varies linearly with composition according to the expression: K_T0 (GPa) = 107.4(1) + 0.228(1) x % molar Ko and the extrapolated value for pure diopside is in good agreement with the datum obtained from Brillouin scattering measurements and lies between data obtained from other X-ray diffraction studies. An inverse behaviour of volume compressibility and thermal expansion with composition is observed, i.e. Na-richer compositions are stiffer and less expandable. Softening of the clinopyroxene structure with increasing diopside component is mainly due to the softening of the b direction and can be ascribed to the narrowing of the M1 octahedral chains which is greater in diopside than in kosmochlor.

Key-words: high-pressure X-ray diffraction, single-crystal diffraction, clinopyroxene, solid solution, diopside, kosmochlor.