Synthesis and upper stability of paragonite

Abstract

The mineral paragonite, NaAl₂[AlSi₃O₁₀ (OH)]₂, has been synthesized on its own composition starting from a variety of different materials. Indexed powder data and refined cell parameters are given for both the 1M and 2M₁ polymorphs obtained. The upper stability limit of paragonite is marked by its breakdown to albite + corundum + vapour. The univariant equilibria pertaining to this reaction have been established by reversing the reaction at six different pressures, the equilibrium curve running through the following $P_{{\text{H}}_{\text{2}} {\text{O}}} - T -$ intervals: 1 kb: 530°–550° C 2 kb: 555°–575° C 3 kb: 580°–600° C 5kb: 625°–640° C 6 kb: 620°–650° C 7 kb: 650°–670° C.

Comparison with the upper stability limit of muscovite (Velde, 1966) shows that paragonite has a notably lower thermal stability thus explaining the field observation that paragonite is absent in many higher grade metamorphic rocks in which muscovite is still stable.

The enthalpy and entropy of the paragonite breakdown reaction have been estimated. Since intermediate albites of varying structural states are in equilibrium with paragonite, corundum and H₂O along the univariant equilibrium curve, two sets of data pertaining to the entropy of paragonite (S ⁰₂₉₈ ) as well as the enthalpy (Δ H ⁰_f,298 ) and Gibbs free energy (Δ G ⁰_f,298 ) of its formation were computed, assuming (1) high albite and (2) low albite as the equilibrium phase. The values are: (1) (2) S ⁰₂₉₈ 67.8±3.9 cal deg⁻¹ gfw⁻¹ 63.7±3.9 cal deg⁻¹ gfw⁻¹ ΔH ⁰_f,298 −1417.9±2.7 kcal gfw⁻¹ −1420.2±2.6 kcal gfw⁻¹ ΔG ⁰_f,298 −1327.4±4.0 kcal gfw⁻¹ −1328.5±4.0 kcal gfw⁻¹.