Low-temperature compatibility relations of the assemblage quartz-paragonite and the thermodynamic status of the phase rectorite

Abstract

The reaction 3 Na-montmorillonite + 2 albite ⇌ 3 paragonite + 8 quartz has been studied experimentally using starting materials composed of natural low albite, kaolinite and quartz. Rate studies at 2, 4 and 7 kb demonstrate that the reaction takes place at ∼335−315° C from lower to higher pressures. Attempts to reverse this reaction with runs lasting several months were without success. Comparison with pertinent data from natural mineral assemblages indicate that despite non-reversal, the data presented here may be very near to the true lower thermal compatibility limit of the assemblage quartz-paragonite.

The above reaction becomes metastable beyond the upper pressure stability limit of the phase Na-montmorillonite; it is replaced here by another reaction 1 albite + 1 kaolinite ⇌ 1 paragonite + 2 quartz + 1 H₂O, as suggested originally by Zen (1960). A P-T-grid showing possible compatibility relations of the assemblage quartz-paragonite is provided (Fig. 4). Perusal of natural assemblages belonging to the subsystem Na₂O-Al₂O₃-SiO₂-H₂O lends credence to this grid.

In course of the rate studies reported here, various regular paragonite-sodium montmorillonite mixed-layer phases were encountered (Fig. 2); the 1∶1 regular mixed-layer phase represents the synthetic analogue of the mineral rectorite (sometimes called allevardite), widely recorded from deep diagenetic and anchimetamorphic environments. Results of rate-studies (Fig. 3) suggest that the mixed-layer phases are all transient, metastable products obtained during the transformation of the albite-Na-montmorillonite assemblage to paragonite-quartz. As such, rectorite and related mixed-layer phases on the join montmorillonite-paragonite, are always less stable relative to the assemblage Na-montmorillonite-paragonite.