Residual electron density at theM2 site inC2/c clinopyroxenes: Relationships with bulk chemistry and sub-solidus evolution

The systematic study of both natural and synthetic clinopyroxenes often indicates the presence in the difference Fourier map of a maximum of residual density of up to 0.8 electrons, here labelledM2′, close to theM2 site along the diad axis, defining a square pyramid co-ordination polyhedron. To investigate the nature and the crystalchemical implications of this feature, a limited but representative set of clinopyroxenes of volcanic, metamorphic and synthetic origin has been investigated by X-ray structure refinement (at 0.7 and 0.4 Å resolution), by microprobe analysis and by transmission electron microscopy. The most important results are: a) at increasing resolution, the height of theM2′ peak increases while its co-ordinates move towardM2; b) as (Ca + Na) content approaches 1.0 atom per formula unit,M2′ vanishes; c)M2′ has been found in clinopyroxenes which show differing incipient exsolution microstructures, from spinodal decomposition to non-periodic fluctuations, as well as in homogeneous specimens. The presence ofM2′ is interpreted in terms of the simultaneous coexistence in the crystals of two different structural models, approximately diopside and clinoenstatite. An accurate evaluation of the totalM2 + M2′ site occupancy is strongly suggested in XREF work, particularly when thermodynamic and kinetic considerations have to be obtained with accurate determinations of site occupancy factors as a starting point.