The structure of P21/c (Ca0.2Co0.8)CoSi2O6 pyroxene and the C2/c - P21/c phase transition in natural and synthetic Ca, Mg, Fe2+ pyroxenes

Mario Tribaudino, Luciana Mantovani, Francesco Mezzadri, Gianluca Calestani, Geoffrey Bromiley

Research output: Contribution to journalArticlepeer-review

Abstract

The P21/c synthetic (Ca0.2Co0.8)CoSi2O6 pyroxene was synthesized by slow cooling from melt at high pressure. Single crystals suitable for X-ray diffraction were obtained and refined. The results were compared to those of C2/c pyroxenes along the series CaCoSi2O6-Co2Si2O6. Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2O6-Fe2Si2O6 and CaMgSi2O6-Mg2Si2O6 pyroxenes, both at an average and local level are apparent.
The results, examined together with two new refinements of pigeonite in the ureilites ALHA77257 and RKPA80239 and of a set of natural and synthetic C2/c and P21/c pyroxenes, show that the average cation radius in the M2 site is the driving force for the phase transition from C2/c to P21/c . The longest M2-O3 distances and the O3-O3-O3 angles follow the same trend, dictated
2
only by the ionic radius in M2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M1-O, M2-O1 and M2-O2 distances are unaffected by the transition and are determined only by the ionic radius of the bonding cation. The structural changes between the ionic radius and the high temperature C2/c and P21/c transitions are similar, and different from the high-pressure transition.
The analysis of natural and synthetic pyroxenes shows that the transition with composition occurs in strain free pyroxenes for a critical radius of 0.85 Å. Increasing strain stabilizes the P21/c structure to higher temperature and larger cation radius.
Finally, our results show that the monoclinic P21/c Ca-poor clinopyroxene, i.e the pigeonite mineral, crystallizes only at conditions where the structure is HT-C2/c, and changes to the P21/c symmetry during cooling.
Original languageEnglish
JournalAmerican Mineralogist
Early online date28 Feb 2018
DOIs
Publication statusPublished - 2018

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