Hydrothermal Synthesis Studies of Potash-Bearing Sesquioxide-Silica Systems

Abstract

The structural properties of synthetic silicates, primarily sheet silicates, crystallized within potash-bearing sesquioxide-silica-water systems, were studied by means of X-ray powder diffraction data. The prototype system of penological importance is the one with alumina as sesquioxide component. As regards this system the main attention was directed at the join pyrophyllite-muscovite. The fairly continuous increase of the b dimension with increasing potassium content recorded for phases along this join is indicative of the formation of solid solutions, the scheme of which is tentatively outlined. The polymorphism of muscovite is also touched upon.

Al-free systems were also studied in which a series of transition-metal ions from Cr³⁺ to Ni³⁺ was used as sesquioxide component (the valency state of the ions may change during the run). Under these conditions several micas of extraordinary composition were formed, such as ferri-ferri-muscovite, nickel-nickel-muscovite (?), chromium mica, nickel-nickel-biotite, cobalt-cobalt-biotite, nickel-ferri-biotite, manganese-ferri-biotite. In systems containing both aluminium and transition-metals various “phlogopites”, including Mn²⁼-phlogopite, were also synthesized. The unit cell dimensions of the trioctahedral micas were discussed in the light of recent conceptions of the mica structure.

An attempt was made to define the upper limit of ferric ion substitution in the octahedral layer of muscovite. Substitution of ferric ion for tetrahedral aluminium in potash felspar was also studied and the occurrence of extensive solid solutions between Al-sanidine and Fe-sanidine as well as between less disordered modifications was established.