Natural sodium phlogopite coexisting with potassium phlogopite and sodian aluminian talc in a metamorphic evaporite sequence from Derrag,Tell Atlas,Algeria

In its only natural occurrence known thus far sodium phlogopite is found in a dolomite containing large porphyroblasts of albite, three other magnesium phyllosilicates, dravite-uvite tourmaline, quartz, rutile, and pyrite. Sodium phlogopites are close to the ideal formula NaMg₃AlSi₃O₁₀](OH)₂, although they may possibly contain additional Li. They are invariably coated by thin rims of potassium phlogopite with octahedral and tetrahedral occupancies different from those of sodium phlogopite. These rims may have prevented the retrograde hydration of sodium phlogopite which seems to be the main reason for its general absence in natural rocks. For the low-grade metamorphic conditions undergone by the dolomite a solvus relationship is indicated between sodium and potassium phlogopite.Sodium phlogopite also coexists, at least prior to the appearance of K phlogopite, with a talc phase containing Na and Al^4] substituting for Si. This type of substitution leading from pure talc to sodium phlogopite was found to extend as far as 36 mole percent. However, the nature of this phase as a genuine solid solution or as a disordered mixed-layer between talc and sodium phlogopite could not be identified as yet. The final phyllosilicate appearing in millimeter-size porphyroblasts is an ordered 11 mixed layer between clinochlore and sodian aluminian talc representing a new mineral.Metamorphic temperatures at the supposedly low water and CO₂ fugacities are estimated to have been below 400 °C.