Abstract: The effects of (OH)/F substitutions on thermally induced reactions in the apophyllite group have been investigated using TGA and DTA methods, supplemented by X-ray diffraction, infra-red and other techniques to identify the products of reaction. The samples studied cover the range between fluorapophyllite and hydroxyapophyllite, and include some unusual ammonium varieties. The presence or absence of fluorine exerts a major control on thermally induced reactions, not only during low-temperature dehydration, but also on high temperature recrystallization reactions up to 900°C+. All apophyllites dehydrate in two stages, the first being loss of a proportion of the water with only minor distortion of the crystal lattice, whereas the second results in total collapse into an amorphous material. Higher fluorine concentrations stabilize the structure, shifting dehydration reactions to higher temperatures, and permitting more water molecules to be lost during the first stage without destruction of the structure. Fluorine is not lost during the second-stage dehydration, as previously believed, but most is retained and influences subsequent recrystallization reactions.

Dehydrated hydroxyapophyllites remain amorphous until ca. 900°C, when high-temperature wollastonite crystallizes. Fluorapophyllites initially form a metastable CaF₂.SiO₂ phase by 700°C, low-temperature wollastonite by 800°C, both disappearing above 900°C when fluorine loss occurs and high-temperature wollastonite is produced. TGA and DTA provide simple methods of distinguishing between hydroxy- and fluorapophyllites.

Ammonian apophyllites, with up to 25% of the stoichiometric K⁺ replaced by NH₄⁺ (in the samples studied), can be recognized by their DTA patterns, the endothermic peak for the second stage reaction being significantly reduced, and that for the first, dehydration, stage being correspondingly enhanced. Such samples have weight losses well above the calculated water content, the excess being due to evolution of ammonia. Ammonian varieties also have an extra infra-red absorption band at 1460 cm⁻¹.