Pyrophyllite Solid Solutions in the System Al₂O₃-SiO₂-H₂O

Pyrophyllite solid solutions have been synthesized in the system AL₂O₃-SiO₂-H₂O between 400° and 565°C at 2 kbar from a variety of starting materials using sealed gold tubes and conventional hydrothermal techniques. The duration of experiments was 3–9 weeks. Products were characterized by X-ray measurements and by infrared spectroscopy.

Appreciably larger basal spacings were observed for pyrophyllite synthesized from gels than for natural pyrophyllite; small amounts of quartz were present in these samples. Basal spacings of pyrophyllite synthesized from mixtures of kaolinite and quartz approach but do not equal those of natural pyrophyHite.

Rehydroxylation of dehydroxylated pyrophyllite under hydrothermal conditions suggests that enlargement of basal spacings cannot be due to dehydroxylation. Substitution of the type Al³⁺ + H⁺ = Si⁴⁺ is proposed to explain the observed variations. The coupled substitution of Al³⁺ and (OH)^- for Si⁴⁺ and O^2- results in expanded basal spacings due largely to the formation of OH on the basal surface and in increased thermal stability due to the formation of H-bonds between oxygens in adjacent silica sheets. Analyses of natural pyrophyllite also suggest limited substitution of this type.

Failure to recognize these substitutions in synthetic samples accounts, in part, for discrepancies in the reported thermal stability of pyrophyllite. Phase characterization, often lacking, is essential in experimental studies of mineral stabilities.