Critical Review of Synthesis and Occurrence of Plagioclase Feldspars and a Possible Phase Diagram

Attainment of equilibrium in plagioclase feldspars is severely hampered by the charge linkage between Ca and Al on the one hand and Na and Si on the other hand (Goldsmith): exsolution is governed therefore by the activation energy for migration of tetrahedrally bonded atoms. Fine-scale intergrowths with structural continuity of the aluminosilicate framework are the initial products of exsolution rather than separate phases. Plagioclase with type e diffractions (denoted e-plagioclase) is interpreted as an intergrowth on a unit-cell scale of domains approaching pure albite and anorthite in composition, separated by disordered boundaries (Smith and Ribbe). It represents an attempt to achieve a mechanical mixture of low albite and P-anorthite, believed to be the stable feldspar assemblage at low temperature. Three types of intergrowths develop on a micrometer scale: peristerite (An₂₋₁₆), Bøggild intergrowth (An₄₃₋₅₈), and Huttenlocher intergrowth (An66-85). The Bøggild intergrowth of plagioclases near An₃₅₋₄₀ and An₆₀₋₆₅ (Nissen) is believed to result from a solvus, perhaps cresting near 500°-600° C. The peristerite and Huttenlocher intergrowths may result from hanging binary loops, the former between intermediate plagioclase and low albite solid solutions, and the latter between intermediate plagioclase and I-anorthite solid solutions meeting the solidus near An₉₀₋₉₅. A strong tendency toward metastability near phase boundaries inhibits formation of intergrowths: for example, from An₁₆ to An₂₅, the peristerite intergrowth does not develop on a micrometer scale, although some specimens form a complex fine-scale intergrowth (Korekawa et al.). In the one-phase region of plagioclase solid solution, considerable ordering (Eberhard, MacKenzie) occurs before the two-phase regions are encountered. Inversions to monalbite solid solution and P-anorthite are shown. The phase diagram is presented in terms of first-order inversions. If transitions are not first order, modifications will be required. For example, if the transition from high to low albite is continuous with a sigmoid variation of Al-Si order with temperature, the binary loop might be replaced by an asymmetric solvus. The cell dimension β^ of pure anorthite varies with temperature of equilibration, and the relaxation time is consistent with Si, Al disorder.