Solid solutions in micas hydrothermally synthesized from kaolinite at 400 °C

Abstract A study of mineral assemblages obtained experimentally from kaolinite was carried out by X-ray diffraction and transmission/analytical electron microscopy in the systems K2O-MgO-Al2O3-SiO2-H2O-HCl, Na2O-MgO-Al2O3-SiO2-H2O-HCl, and Na2O-K2O-MgO-Al2O3-SiO2-H2O-HCl at 400 °C and 260 bars. Three sets of reactions with different Mg2+ concentrations and variable Na/K ratios were studied, at increasing run times. All the experiments yielded the assemblage mica + Al-serpentine ± Mg-kaolinite. Transitional kaolinite appears as large platy particles with homogeneous compositions (mean Mg content = 0.42 atoms per formula unit). Serpentine crystals, frequently curved, show an Al-rich composition. Micas form thin platy crystals with variable compositions in the three chemical systems. In the Na-free system, the reactions yielded white mica with high Fe + Mg contents and biotite. In the K-free system, Na-dioctahedral and Na-di,trioctahedral mica formed. The Na + K-bearing system yielded a two-mica assemblages consisting of phengite + di,trioctahedral mica, both with variable Na/K ratios, although with K as the main cation in interlayer positions in di,trioctahedral micas.