Solid solutions in micas hydrothermally synthesized from kaolinite at 400 °C: Fid-Bau Portal

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

Bentabol, María / Abad, Isabel

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.

Highlights • Micas with different compositions have been synthesized from Kln in K+Mg-, Na+Mg- and Na+K+ Mg- bearing solutions (400 ºC). • The experiments yielded the assemblage mica + Al-serpentine ± Mg-kaolinite. • Dioctahedral, di,trioctahedral mica (Na+Mg), white mica, biotite (K+Mg-) and phengite,di,trioctahedral mica (Na+K+Mg-). • Increasing concentrations of Mg²⁺ causes an increase of the serpentine/mica ratio, not affecting the mica compositions.