The Effect of Local Bond-Valence Requirements on the Stability and Chemical Composition of Orthorhombic and Monoclinic Amphiboles

The compositions of rock-forming minerals are a function of temperature, pressure and bulk composition of the rocks in which they occur. Examining these changes is a major part of petrology, and our geothermobarometers are, for the most part, based on thermodynamic analysis of these changes as a function of temperature and pressure. However, this approach does not address the mechanistic causes of these variations in composition; I will attempt to do so here for orthorhombic and monoclinic amphiboles. The amphibole structures consist of units of polymerized tetrahedra and polymerized octahedra. For these structures to be stable, these units must be dimensionally commensurate, and they must remain dimensionally commensurate with changing temperature and pressure. However, aluminosilicate tetrahedra show much lower thermal expansion and much higher compressibility than (Mg,Fe,Al)-containing octahedra. Thus with changing temperature and pressure, the absolute and relative sizes of the tetrahedra and octahedra change as a function of thermal expansion and elastic compression. Thus the relative sizes of the tetrahedron chain and the octahedron strip change, and the linkage of these two modules is affected. As a result, these two units change their conformations, and contents of the tetrahedra and octahedra change to maintain (1) articulation, (2) local bond-valence requirements, and (3) electroneutrality. Here, I examine how the chemical compositions and site ordering of amphibole structures are expected to change as a function of temperature and pressure in order to conform to these requirements for structure stability.