Subsolidus phase relations of the plagioclase feldspar solid solution

Carpenter, M. A. (1994) Subsolidus phase relations of the plagioclase feldspar solid solution. In: Feldspars and their Reactions Feldspars and their Reactions. NATO ASI Series, 421 (421). Kluwer Academic, Netherlands, pp. 221-269. ISBN 978-94-010-4483-7

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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 I1¯ ordering. A spinodal mechanism for exsolution can be explained if the I1¯⇌e1 ⇌ 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 C1¯ and e1 structures or of a break between e1 and e2 structures at ~An50. Again, a thermodynamically continuous C1¯⇌e1 ⇌ 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.

Item Type: Book Section
Subjects: 03 - Mineral Sciences
Divisions: 03 - Mineral Sciences
Volume: 421
Page Range: pp. 221-269
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 19 Jan 2016 14:03
Last Modified: 19 Jan 2016 14:05

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