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.

The structures of bustamite and the poly-types of wollastonite are shown to be derived from variations in the stacking sequence of a single type of structural unit. This structural unit is a column of unit cells of wollastonite along the b-axis and is bounded by (100) and (001) planes (P1̄ cell); these units may be stacked together along [100] and [001] with either zero displacement between successive units, or a relative displacement of 1/2[010]. Regular stacking sequences give rise to the ordered structures of wollastonite polytypes and of bustamite, while the transformations between these structures proceed by the propagation of line defects with Burgers vectors of 1/2[010] which thus change the stacking sequence of the structural units.

Metasediments of the Tayvallich Subgroup of the Dalradian at Kinnairds Head, Fraserburgh are metamorphosed to sillimanite + K-feldspar grade and form part of the classic high-T low-P Buchan metamorphic terrain. Pelitic samples constrain peak-metamorphic conditions to 615±13°C and 2.2±0.2 kbar. At or close to the metamorphic peak, irregular garnetiferous aplites and autopegmatite bodies intruded the metasediments. Thin marble bands within the sequence are dominated by calcite with diopside, and equilibrated with relatively CO2-rich, internally buffered fluids. Where these are in close proximity to granitoid pegmatites, wollastonite dominates the matrix, and fractures and veins running through the rock contain concentrations of grossular and vesuvianite. With increasing distance from the pegmatite, vesuvianite and then grossular disappear, and wollastonite is only patchily developed. Such occurrences require a flushing of the marble by metasomatic (siliceous and aluminous) aqueous fluids which were derived from the de-watering of the adjacent pegmatite as it crystallized. The large quantities of dissolved silica led to pervasive wollastonite formation for several metres. The smaller quantities of Al reacted to form Ca-Al-silicates which were confined to the fractures.

The Fountain Hill skarn, which was produced by thermal metamorphism of impure limestone in the Omey Granite aureole, consists of wollastonite, calcite, grossular-andradite garnet, diopside, B-free vesuvianite and small amounts of albite, K-feldspar and quartz. Zoning in vesuvianite is, overall, independent of birefringence but oscillatory concentric zoning in the mineral is controlled mainly by Ti. In addition to extensive within-site substitutions (e.g. F for OH in the OH-sites, Fe2+ for Mg, Fe3+ for Al, Ti for Al etc. in the Y-sites), there are significant cross-site combined substitutions involving Y-X and Y-Z but not X-Z sites so that ideal solution models for this mineral are not applicable. The thermodynamic mole fraction of Hoisch (1985) is modified to account for the excess of the ΣY cations in the Y sites and can be applied to both B-bearing and B-free vesuvianites. Using the thermodynamic dataset of Holland and Powell (1990) and taking into account the existence of andalusite, sillimanite and corundum in associated pelites, leads to the conclusion that the metamorphic conditions were about 640±20°C and 3.3±0.3 kb at 0.15±0.05 XCO2.

We have performed an experimental study of the influence of varying size and charge on cation partitioning between wollastonite and silicate-carbonate melt in the system CaCO3-SiO2. The experimental conditions (3 GPa, 1420°C) lie close to the wollastonite II tc/I tc phase boundary. Results for 1+, 2+, 3+ and 4+ partitioning show parabolic dependence of partition coefficients on ionic radius, which can be fitted to the elastic strain model of Blundy and Wood (1994), wherein partitioning is described using three parameters: site radius (r0), site elasticity (apparent Young's Modulus) and the ‘strain-free’ partition coefficient (D0) for an element with radius r0. The apparent Young's Modulus of the Ca site in wollastonite, obtained from modelling the 2+ partitioning data, is 99±3 GPa, similar to the bulk-crystal value for the polymorph wollastonite I tc. r0 decreases with increasing charge on the substituent cation, while D0 also shows an approximately parabolic dependence on charge, with a maximum for 2+ cations. Partition coefficients for divalent cations Zn, Co, Fe, Cd, Mn and Pb are lower than would be predicted from their ionic radii alone, indicating a preference for the melt. This may be a consequence either of cation-carbonate complexation in the melt, or of the more distorted nature of cation co-ordination environments in melts.

Wollastonite (CaSiO3)–diopside (CaMgSi2O6) glass-ceramic scaffolds have been successfully fabricated using two different additive manufacturing techniques: powder-based 3D printing (3DP) and digital light processing (DLP), coupled with the sinter-crystallization of glass powders with two different compositions. The adopted manufacturing process depended on the balance between viscous flow sintering and crystallization of the glass particles, in turn influenced by the powder size and the sensitivity of CaO–MgO–SiO2 glasses to surface nucleation. 3DP used coarser glass powders and was more appropriate for low temperature firing (800–900 °C), leading to samples with limited crystallization. On the contrary, DLP used finer glass powders, leading to highly crystallized glass-ceramic samples. Despite the differences in manufacturing technology and crystallization, all samples featured very good strength-to-density ratios, which benefit their use for bone tissue engineering applications. The bioactivity of 3D-printed glass-ceramics after immersion in simulated body fluid and the similarities, in terms of ionic releases and hydroxyapatite formation with already validated bioactive glass-ceramics, were preliminarily assessed.