The distribution of bonds associated with the M2 sites in various well-ordered pyroxene minerals is determined using a topological analysis of electron density in the manner proposed by Bader (1998). Each M2 atom is bonded to 2 O1 and to 2 O2 atoms, and to zero, one, two, or four bridging O3 atoms. Each of the symmetries displayed by pyroxenes have their own bonding systematics, and each pyroxene-to-pyroxene phase transition involves a change in bonding to M2. As a function of temperature or pressure, the bonding changes appear as a well-defined sequence of steps that can be related to the degree of distortion from the ideal closest packing of anions. It is proposed that the condition at which an individual phase transition occurs is related to M2-Si repulsion through a shared edge. The bonding analysis should provide a qualitative means to interpret the behavior of all pyroxene structures over T, P, and x, and may guide the interpretation of the changes in properties observed by techniques other than X-ray diffraction, such as Raman spectroscopy.
The electron density distribution in a mineral is measureable, and in some ways, provides all the information required to understand the properties of minerals. Through the analysis of the electron density distributions of a large variety of mineral species, Gibbs et al. (2014) examine and challenge the fundamental tenet of Pauling’s Rules, which is that atoms are spheres of a single fixed size. Their analysis provides an updated model of crystal chemistry that is both self-consistent and does what new models should do, explains the older ones.