Abstract: The anhydrous structures of three-layer phyllosilicates are destroyed by heating, as in differential thermal analyses, over a range of temperatures starting from about 775°C, mean temperature for pyrophyllite, and rising to about 1075°C, a mean temperature for biotite. Parallel to the trend of increasing temperature of stability (and destruction) is an increased ratio of substituted Al for Si in the tetrahedral layers and, in some occurrences, of Mg for Al in the octahedral layers. These substitutions increase the degree of ionic bonding (and decrease the covalent bonding) within the oxygen framework. The electronegativity difference between the cation and oxygen increases as Al substitutes for Si, and Mg for Al. Increase in ionic character of a compound is compatible, from chemical considerations, with increase in thermal stability and susceptibility of the compound to alteration by water.
The principle illustrated by the phyllositicates applies likewise to mineral groups of other silicate structure, such as the anorthite-albite series, the forsterite-fayalite series, and others.