Experimental delineation of the “e” ⇌ Ibar 1 and “e” ⇌ Cbar 1 transformations in intermediate plagioclase feldspars

Approximately 125 hydrothermal annealing experiments have been carried out in an attempt to bracket the stability fields of different ordered structures within the plagioclase feldspar solid solution. Natural crystals were used for the experiments and were subjected to temperatures of ～650°C to ～1,000°C for times of up to 370 days at (P_{{text{H}}_{text{2}} {text{O}}} ) =600 bars, or (P_{{text{H}}_{text{2}} {text{O}}} ) =1,200 bars. The structural states of both parent and product materials were characterised by electron diffraction, with special attention being paid to the nature of type e and type b reflections (at h+k=(2n+1), l=(2n+1) positions). Structural changes of the type C (bar 1) → I (bar 1) , C (bar 1) → “e” structure, I (bar 1) → “e” and “e” structure → I (bar 1) have been followed. There are marked differences between the ordering behaviour of crystals with compositions on either side of the C (bar 1) ? I (bar 1) transition line. In the composition range ～ An₅₀ to ～ An₇₀ the e structure appears to have a true field of stability relative to I (bar 1) ordering, and a transformation of the type I (bar 1) ? e has been reversed. It is suggested that the e structure is the more stable ordered state at temperatures of ～ 800°C and below. For compositions more albite-rich than ～ An₅₀ the upper temperature limit for long range e ordering is lower than ～ 750°C, and there is no evidence for any I (bar 1) ordering. The evidence for a true stability field for “e” plagioclase, which is also consistent with calorimetric data, necessitates reanalysis both of the ordering behaviour of plagioclase crystals in nature and of the equilibrium phase diagram for the albite-anorthite system. Igneous crystals with compositions of ～ An₆₅, for example, probably follow a sequence of structural states C (bar 1) → I (bar 1) → e during cooling. The peristerite, Bøggild and Huttenlocher miscibility gaps are clearly associated with breaks in the albite, e and I (bar 1) ordering behaviour but their exact topologies will depend on the thermodynamic character of the order/disorder transformations.