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. (28th February 2018)

• Record Type: Journal Article
• Authors: Tribaudino, Mario; Mantovani, Luciana; Mezzadri, Francesco; Calestani, Gianluca; Bromiley, Geoffrey
• Is Part Of: Mineralogical magazine. Volume 82:Number 1(2018)
• Page Start: 211
• Page End: 228
• Publication Date: 2018-02-28
• Subjects: pyroxene, -- phase transition, -- cobalt, -- crystal structure, -- high temperature; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/minmag.2017.081.036
• Languages: English
• ISSNs: 0026-461X

Abstract

ABSTRACT: A P 21 / c synthetic (Ca0.2 Co0.8 )CoSi2 O6 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 C 2/ c pyroxenes along the series CaCoSi2 O6 –Co2 Si2 O6 . Strong similarities in the crystal chemical mechanism of the transition with the synthetic CaFeSi2 O6 –Fe2 Si2 O6 and CaMgSi2 O6 –Mg2 Si2 O6 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 with a set of natural and synthetic C 2/ c and P 21 / c pyroxenes, show that the average cation radius in the M 2 site is the driving force for the phase transition from C 2/ c to P 21 / c . The longest M 2–O3 distances and the O3–O3–O3 angles follow the same trend, dictated only by the ionic radius in M 2, in either synthetic or natural pyroxenes, regardless of the ionic radius of the M 1 cations. The transition also affects the difference between bridging and non-bridging oxygen atoms and the extent of tetrahedral deformation, whereas the M 1–O, M 2–O1 and M 2–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 C 2/ c and P 21 / c transitions are similar, and different to the high-pressure transition. 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 P 21 / c structure to a higher temperature and larger cation radius. Finally, our results show that the monoclinic P 21 / c Ca-poor clinopyroxene, i.e the mineral pigeonite, crystallizes only at conditions where the structure is HT- C 2/ c, and changes to the P 21 / c symmetry during cooling.