Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A "low" series contained the natural ordered material and a "high" series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole "e" structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il̄ structure →- Cl̄ high albite structure: ~0.7-1.9 kcal/mole Il̄ structure equilibrated at low temperature → Il̄ structure equilibrated at high temperature: ~ 1.8-0.8 kcal/mole. ΔHsoln data for the high series overlap with the data of Newton et al. (1980) for synthetic high structural state plagioclases except in the composition range ~An90-An100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl̄ symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il̄ and "e" structures. Values of the enthalpy change due to disordering (ΔHord) also show a number of systematic trends. Firstly, the values for e → Cl̄ are larger than for Il̄ → Cl̄ in the composition range where both e and Il̄ structures are observed (~An65-An72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔHord, which occurs at ~An50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il̄ anorthites and bytownites to a Cl̄ structure have been estimated by assuming that the Cl̄ solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.
ASJC Scopus subject areas
- Geochemistry and Petrology