Thermal expansion inC2/cpyroxenes: a review and new high-temperature structural data for a pyroxene of composition (Na0.53Ca0.47)(Al0.53Fe0.47)Si2O6(Jd53Hd47)

Abstract

AbstractSingle-crystal X-ray data collection was performedin situatT= 300 and 700°C on a sample synthesized along the jadeite–hedenbergite series. The structural data, together with those of a previous investigation of a crystal of the same composition, were compared to those of the endmembers. The evolution of the displacement parameters with temperature shows significant residuals for the O1, O2 and O3 oxygen atT= 0 K, most significantly in the O1, which were interpreted as an indication of positional disorder.Volume thermal expansion and axial deformation ellipsoids were calculated for the above sample together with those of a series ofC2/cpyroxenes. Pyroxenes with a divalent M2 cation, Ca, Fe and Mg have a greater expansion than those with a monovalent M2, like Na and Li; the Na pyroxene endmembers with Al, Cr and Fe were observed to show greater expansion than corresponding Li ones.The greater axial expansion is found along thebaxis, except in LiCrSi2O6; the changes along thebaxis are related to the volume thermal expansion. The axial orientation and anisotropy of the two axes onto the (010) plane is different in Na, Li and Ca-Mg-Fe pyroxenes, but the overall expansion onto the (010) plane, given by the sum of the scalar expansion along the two axes on (010), is very similar in pyroxenes.The deformation along thebaxis with temperature and composition is driven by the deformation along b of the octahedral M1 chain; most important is the contribution from the O1−O1 shared edge between M1 octahedra in the same octahedral chain.