Subsolidus Phase Relations of the Plagioclase Feldspar Solid Solution

Abstract

Experimental, microstructural and calorimetric evidence for the stability relations among plagioclase feldspars is reviewed, and a new phase diagram proposed. The incommensurate “e” structure is now believed to have a field of equilibrium stability at intermediate compositions in the solid solution. Two phase fields are interpreted as arising from breaks in the ordering behaviour as a function of composition. The Huttenlocher gap may extend up to ~800°C and is presumed to occur between the stability fields of e₁ and $I\bar 1$ ordering. A spinodal mechanism for exsolution can be explained if the $I\bar 1 \rightleftharpoons {\text{e}}_{\text{1}}$ ⇌ e₁ transition is taken to be thermodynamically continuous. The Bøggild gap may also extend up to ~800°C as a consequence either of a break between $C\bar 1$ and e₁ structures or of a break between e₁ and e₂ structures at ~An₅₀. Again, a thermodynamically continuous $C\bar 1 \rightleftharpoons {\text{e}}_{\text{1}}$ ⇌ e₁ transition would allow a spinodal exsolution mechanism. As in previous reviews, the peristerite gap is ascribed to the influence of an increase in the equilibrium degree of Al/Si order in pure albite below ~650–700°C. Incommensurate ordering is now known to occur also in anorthite under metastable conditions and this has implications for the nature of the e structure in crystals with intermediate compositions. Finally, schematic free energy curves and selected Landau free energy expansions are shown to provide a basis for future thermodynamic models of the overall mixing and ordering behaviour.