Clinopyroxenes (Cpx) are one of the main rock-forming minerals, but stoichiometry of their compositions was called into question. In particular, an idea of hypothetical calcium molecule Eskola (CaEs, Ca_0,5AlSi₂O₆) existence was expressed. This minal has structure vacancy and silica excess. Numerous experimental investigations in CMAS-system (CaO-MgO-Al₂O₃-SiO₂) have showed that the question of non-stoichiometric Cpx existence remains open. This paper presents the results of an experimental study of the diopside Di (CaMgSi₂O₆) – calcium molecule Eskola CaEs (Ca_0,5AlSi₂O₆) cross-section in the CMAS-system. The experiments were carried out in the following pressure and temperature range: P=10^-4 – 3,0 GPa; T=966 – 1525 ⁰C. Experiments at atmospheric pressure (10^-4 GPa) were performed on a vertical shaft electric resistance furnace; high-pressure ones were performed on a «piston-cylinder» type apparatus. Samples obtained were analyzed using electron microprobe (EMP), scanning electron microscope (SEM) and Raman spectrometer. Depending on the P-T conditions, the samples contain the following phases: anorthite An, garnet Grt, diopside Di, clinopyroxene Cpx, quartz Qtz (tridymite Tr – for experiments at atmospheric pressure), and glass L. The data array on the composition of clinopyroxenes crystallized in this cross-section with diopside in various associations is generalized and supplemented. Clinopyroxenes were found to form quaternary solid solutions of diopside Di (CaMgSi₂O₆) – enstatite En (Mg₂Si₂O₆) – calcium molecule Tschermak CaTs (CaAl₂SiO₆) – calcium molecule Eskola CaEs (Ca_0,5AlSi₂O₆ ). The CaTs and CaEs minals contents are positively correlated with the amount of aluminum in clinopyroxene, and this relationship is particularly pronounced for CaTs. It is confirmed that clinopyroxenes in this cross-section can contain an excess of silica at both atmospheric and high pressures. Apparently, the cation vacancy that exists in pyroxene structure can participate in ordering processes. As a result the pyroxenes of another structure (not diopside – C2/c-symmetry) can be crystallized from total compositions in the Di-CaEs cross-section. Additional research is needed to support this hypothesis. Besides, at present investigation it was not possible to establish an unambiguous relationship between the Cpx composition and P-T-parameters, since it is also associated with both the mixture initial composition and the mineral association. Further experiments are required to justify any geothermobarometric dependence.

Banushkina Sofja Victorovna, Junior Researcher, V. S. Sobolev Institute of Geology and Mineralogy SB RAS, 3/1, Akademik Koptyug аv., Novosibirsk, 630090, Russian Federation, e-mail: banushkinasv@igm.nsc.ru 

Turkin Alexandre Ivanovich, Doctor of Sciences (Geology and Mineralogy), Senior Researcher, V. S. Sobolev Institute of Geology and Mineralogy SB RAS, 3/1, Akademik Koptyug аv., Novosibirsk, 630090, Russian Federation, e-mail: turkin@igm.nsc.ru 

Chepurov Anatoliy Il’ich, Doctor of Sciences (Geology and Mineralogy), Senior Researcher, V. S. Sobolev Institute of Geology and Mineralogy SB RAS, 3/1, Akademik Koptyug аv., Novosibirsk, 630090, Russian Federation, e-mail: chepurov@igm.nsc.ru

Banushkina S.V., Turkin A.I., Chepurov A.I. Clinopyroxene Solid Solutions in the CaMgSi₂O₆ – Ca_0,5AlSi₂O₆ Cross-Section at High P-T Parameters. The Bulletin of Irkutsk State University. Series Earth Sciences, 2020, vol. 34, pp. 37-54. https://doi.org/10.26516/2073-3402.2020.34.37 (in Russian)

Banushkina S.V., Surkov N.V., Golitsyna Z.F. Osobennosti plavleniya faz v sechenii diopsid – kal'tsievaya molekula Eskola v intervale davlenii 1 kgs/sm2 – 20 kbar [Features of phase melting in the diopside – Calcium molecule of Eskola cross-section in pressure range 1 kgs/sm2 – 20 kbar]. Vestnik ZabGU [Bulletin of ZabSU], 2019, vol. 25, no. 7, pp. 6-17. (in Russian)

Zharikov V.A., Ishbulatov R.A., Chudinovskikh L.T. Eklogitovyi bar'er i klinopirokseny vysokikh davlenii [Eclogitic barrier and clinopyroxenes of high pressures]. Geologiya i Geofizika [Russian Geology and Geophysics], 1984, no. 12, pp. 54-63. (in Russian)

Khanukhova L.T., Zharikov V.A., Ishbulatov R.A., Litvin Yu.A. Izbytochnyi kremnezem v tverdykh rastvorakh klinopiroksenov vysokogo davleniya po dannym eksperimental'nogo izucheniya sistemy CaMgSi₂O₆-CaAl₂SiO₆-SiO₂ pri 35 kbar i 1200 ⁰S [Excess silica in solid solutions of high pressure clinopyroxenes according to data of experimental investigation of CaMgSi₂O₆-CaAl₂SiO₆-SiO₂ system at 35 kbar and 1200 ⁰C]. Dokladi Akademii nauk SSSR [Reports of the Academy of Sciences USSR], 1976, vol. 229, no. 1, pp. 182-184. (in Russian)

Malinovskaya E.K., Doroshev A.M., Bulatov V.K., Brai G. Klinopirokseny serii CaMgSi₂O₆-CaAl₂SiO₆-Ca_0.5AlSi₂O₆ v assotsiatsii s anortitom, kvartsem, koesitom i granatom [Climopyroxenes of series CaMgSi₂O₆-CaAl₂SiO₆-Ca_0.5AlSi₂O₆ in the association with anorthite, quartz, coesite and garnet]. Geokhimiya [Geochemistry], 1991, no. 2, pp. 216-226. (in Russian)

Lavrent'ev, Yu.G., Karmanov N.S., Usova L.V. Elektronno-zondovoe opredelenie sostava mineralov: mikroanalizator ili skaniruyushchii elektronnyi mikroskop? [Electron probe determination of mineral composition: microanalyzer or scanning electron microscope?]. Geologiya i Geofizika [Russian Geology and Geophysics], 2015, vol. 56, no. 8, pp. 1473-1482. (in Russian)

Surkov N.V., Gartvich Yu.G, Babich Yu.V. Eksperimental'noe issledovanie fazovoi diagrammy secheniya CaMgSi₂O₆ – Ca_0,5AlSi₂O₆ pri davlenii 3,0 GPa [Experimental investigation of phase diagram of CaMgSi₂O₆ – Ca_0,5AlSi₂O₆ section at pressure 3.0 GPa]. Dokladi Akademii nauk [Reports of the Academy of Sciences], 2004, vol. 398, no. 4, pp. 533-537. (in Russian)

Surkov N.V., Gartvich Yu.G., Izokh O.P. Ustoichivost' i fazovye vzaimootnosheniya nestekhiometrichnykh klinopiroksenov v sechenii diopsid-»Ca-molekula Eskola» pri vysokikh davleniyakh [Stability and phase relationships of non-stoichiometric clinopyroxenes in the diopside – “Calcium molecule of Eskola” section at high pressures]. Geokhimiya [Geochemistry], 2007, no. 6, pp. 632-642. (in Russian)

Surkov N.V., Banushkina S.V., Gartvich Yu.G. Osobennosti plavleniya assotsiatsii s α – diopsidom v sechenii CaMgSi₂O₆ – Ca_0,5AlSi₂O₆ pri atmosfernom davlenii [Melting features of association with α – diopside in the section CaMgSi₂O₆ – Ca_0,5AlSi₂O₆ at atmospheric pressure]. Vestnik ZabGU [Bulletin of ZabSU], 2018, vol. 24, no. 7, pp. 51-59. (in Russian)

Khanukhova L.T., Zharikov V.A., Ishbulatov R.A., Litvin Yu.A. Tverdye rastvory piroksenov v sisteme NaAlSi₂O₆-CaAl₂SiO₆-SiO₂ pri 35 kbar i 1200 ⁰S [Solid solutions of pyroxenes in the system NaAlSi₂O₆-CaAl₂SiO₆-SiO₂ at 35 kbar and 1200 ⁰C]. Dokladi Akademii nauk SSSR [Reports of the Academy of Sciences USSR], 1976, vol. 231, no. 1, pp. 185-187. (in Russian)

Angel R.J., Chupelas A., Ross N.L. Stability of high-density clinoenstatite at upper-mantle pressures. Nature, 1992, vol. 358, pp. 322-324.

Arlt T., Angel R.J. Displacive phase transition in C-centred clinopyroxenes: spodumene, LiScSi₂O₆ and ZnSiO₃. Physics and Chemistry of Minerals, 2000, vol. 27, no. 10, pp. 719-731.

Zhao S., Nee P., Green H.W., Dobrzhinetskaya L.F. Ca-Eskola component in clinopyroxene: Experimental studies at high pressures and high temperatures in multianvil apparatus. Earth and Planetary Science. Letters, 2011, vol. 307, pp. 517-524.

Cameron M., Papike J.J. Crystal chemistry of silicate pyroxenes. Prewitt C.T. (ed.). Mineralogical Society of America. Reviews in Mineralogy, 1980, vol. 7: Pyroxenes, pp. 5-92.

Cameron M., Papike J.J. Structural and chemical variations in pyroxenes. American Mineralogist, 1981, vol. 66, pp. 1-50.

Christy A., Angel R.J. A model for the origin of the cell-doubling phase transition in clinopyroxene and body centered anorthite. Physics and Chemistry of Minerals, 1995, vol. 22, pp. 129-135.

Eskola P. On the eclogites of Norway. Videnskaps Skr. J. Kristiania. 1. Mat. Nat. kl, 1921, vol. 8, pp. 163–170. 

Gasparik T. Experimental study of subsolidus phase relations and mixing properties of clinopyroxene in the silica-saturated system CaO-MgO-Al₂O₃-SiO₂. American Mineralogist, 1986, vol. 71, pp. 686-693.

Arlt T., Angel R.J., Miletich R., Armbruster T., Peters T. High-pressure P21/c –C2/c phase transitions in clinopyroxenes: Influence of cation size and electronic structure. American Mineralogist, 1998, vol. 83, pp. 1176-1181.

Cameron M., Sueno S., Prewitt C.T., Papike J.J. High-temperature crystal chemistry of acmite, diopside, hedenbergite, jadeite, spodumeme, and ureyite. American Mineralogist, 1973, vol. 58, pp. 594-618.

Knapp N., Woodland A.B., Klimm K.. Experimental constraints in the CMAS system on the Ca-Eskola content of eclogitic clinopyroxene. European Journal of Minerals, 2013, no. 25, pp. 579–596.

Levien L., Prewitt C.T. High-pressure structural study of diopside. American Mineralogist, 1981, vol. 66, pp. 315-323.

Mao H.K. The system jadeite (NaAlSi₂O₆)-anorthite (CaAl₂Si₂O₈) at high pressures. Carnegie Institution of Washington, Year Book, 69. Baltimore: Port City Press, 1970, pp. 163-168.

McCormick T.C. Crystal-chemical aspects of nonstoichiometric pyroxenes. American Mineralogist, 1986, vol. 71, pp. 1434-1440.

Millholland C.S., Presnall D.C. Liquidus phase relations in the CaO-MgO-Al₂O₃-SiO₂ system at 3.0 GPa; the aluminous pyroxene thermal divide and high-pressure fractionation of picritic and komatiitic magmas. Journal of Petrology, 1998, vol. 39, pp. 3–27.

Oberti R, Caporuscio F.A. Crystal chemistry of clinopyroxenes from mantle eclogites: a study of the key role of the M2 site population by means of crystal-structure refinement. American Mineralogist, 1991, vol. 76, pp. 1141-1152.

Okamura F.P., Chose S., Ohashi H. Structure and crystal chemistry of calcium Tschermak's pyroxene, CaAlAlSiO₆. American Mineralogist, 1974, vol. 59, pp. 549-557.

Okui M., Sawada H., Marumo F. Structure refinement of a nonstoichiometric pyroxene synthesized under ambient pressure. Physics and Chemistry of Minerals, 1998, vol. 25, no. 5, pp. 318-322.

Page F.Z., Essene E.J., Mukasa S.B. Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A. American Mineralogist, 2005, vol. 90, pp. 1092–1099.

Zhang L., Song S., Liou J.G., Ai Y., Li X. Relict coesite exsolution in omphacite from Western Tianshan eclogites, China. American Mineralogist, 2005, vol. 90, pp. 181-186.

Smyth J.R. Peraluminous omphacite: cation vacancies in mantle derived pyroxene (abstr.). Transactions American Geophysical Union, 1977, vol. 58, p. 523.

Smyth J.R. Cation vacancies and the crystal chemistry of breakdown reactions in kimberlitic omphacites. American Mineralogist, 1980, vol. 65, pp. 1185–1191.

Sobolev N.V., Kuznetsova I.K., Zyuzin N.I. The petrology of grospydite xenoliths from the Zagadochnaya kimberlite pipe in Yakutia. Journal of Petrology, 1968, no. 9, part 2, pp. 253-280.

Surkov N.V., Darmenko O.L. Experimental investigation of clinopyroxene stability in the section Di-CaTs-CaEs at pressure 30 kbar. Experiment in Geosciences, 2002, vol. 10, no 1, pp. 32-34.

Katayama I., Parkinson C.D., Okamoto K., Nakajima Y., Maruyama S. Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan. American Mineralogist, 2000, vol. 85, pp. 1368-1374.

Konzett J., Frost D.J., Proyer A., Ulmer P. The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15 GPa with possible implications for the formation of oriented SiO₂-inclusions in omphacite. Contributions to Mineralogy and Petrology, 2008, no. 155, pp. 215-228.

Sang L., Vanpeteghem C.B., Sinogeikin S.V., Bass J.D. The elastic properties of diopside, CaMgSi₂O₆. American Mineralogist, 2011, vol. 96, pp. 224-227.

Tribaudino M., Nestola F., Cámara F., Domeneghetti M.C. The high-temperature P21/c-C2/c phase transition in Fe-free pyroxene (Ca_0.15Mg_1.85Si₂O₆): Structural and thermodynamic behavior. American Mineralogist, 2002, vol. 87, no. 5-6, pp. 648-657.

Wenk H.R., Weiss L.E. Al-rich calcic pyroxene in pseudotachylite: an indicator of high pressure and temperature? Tectonophysics, 1982, vol. 84, no. 2-4, pp. 329-341.

Wood B.J., Henderson C.M.B. Composition and unit-cell parameters of synthetic non-stoichiometric tschermakitic clinopyroxenes. American Mineralogist, 1978, vol. 63, pp. 66–72.

Zhu Y., Ogasawara Y. Phlogopite and coesite exsolution from super-silicic clinopyroxene. International Geology Review, 2002, vol. 44, pp. 831-836. https://doi.org/10.2747/0020-6814.44.9.831