Platy (Te Puke, New Zealand), cylindrical (Spruce Pine, North Carolina), and spherical (North Gardiner mine, Huron, Lawrence County, Indiana) halloysite samples were analyzed by high-pressure differential thermal analysis (HP-DTA) to determine the effect of morphological and chemical differences on their respective thermal stability. In halloysite, these morphological differences imply structural features. The metastable phase relations of each are analogous to those of kaolinite. At 1 bar, the platy, cylindrical, and spherical samples showed peak temperatures (maximum deflection in the dehydroxylation endotherm) of 560°, 578°, and 575°C, respectively, whereas at about 600 bars the peak temperatures were 622°, 655°, and 647°C. At low pressures the observed reaction is related to dehydroxylation: halloysite (H) metahalloysite (MH) + vapor (V), whereas higher pressures produce melting reactions, either H + V = metaliquid (ML) for conditions of P(H2O) = P(total), or H + MH = ML for P(H2O) < P(total). The PT conditions of the invariant point, H + MH + ML + V, for each system are: Te Puke, 612° ± 4°C, 25 ± 7 bars; Spruce Pine, 657 ± 2°C, 30 ± 7 bars; North Gardiner, 652° ± 2°C, 34 ± 7 bars. The lower thermal stability of the Te Puke sample may be related to its higher iron content, although additional data are necessary to confirm that it is not related also to the platy structure. Furthermore, morphological differences between the cylindrical and spherical varieties appear to have had little effect on the energy required to dehydroxylate these halloysite structures. Exceptionally high values obtained for the dehydroxylation enthalpies using the van't Hoff equation, compared with values derived using other methods, may be explained by a 10–15-bar excess in the intracrystalline H2O fugacity during dehydroxylation. Intracrystalline fugacity is defined here as the H2O fugacity within crystallites and is not related to the partial pressure of H2O around individual particles.

Clays are involved in a variety of natural and managed processes, and calculation of their stability conditions is important. Such calculations are still fraught with large uncertainties owing to the lack of experimental constraints on the thermodynamic properties of clays, and bracketing of equilibrium reactions at low temperature is barely possible. Experiments aimed at studying the thermal stabilities of, and composition-temperature relations among, smectite, illite, kaolinite, pyrophyllite, mica, and chlorite at different levels of SiO2, K2O, MgO, and Al2O3 species in solution were conducted under a strong thermal gradient in the simple K2O-Al2O3-SiO2-H2O (KASH), MgO-Al2O3-SiO2-H2O (MASH), and KMASH systems. The crystallization series observed in the different experiments match to some degree those observed in active geothermal systems where clay minerals precipitate from oversaturated solutions. Smectite and/or ordered mixed-layer materials, smectite-donbassite, or possibly pyrophyllite-donbassite were observed to crystallize in both KASH and MASH experiments. Similar crystallization sequences and clay composition variations with temperature were observed in most cases when the relative positions of the starting solids were switched. The experimental results were used to refine the thermodynamic properties of K- and Mg-smectite. Stability diagrams calculated by energy minimization and activityactivity diagrams are consistent with the experimental mineral variations, suggesting that smectite is thermodynamically stable at temperatures as high as 300°C in the presence of diluted water and quartz and K-feldspar-free systems.

Phase equilibria in the system Pd–Ag–S were studied using the silica-glass tube method at 400°C and 550°C. In the system we synthesised three ternary phases: coldwellite (Pd3Ag2S), kravtsovite (PdAg2S) and a new phase Pd13Ag3S4. At 400°С, coldwellite forms a stable association with vysotskite (PdS) and vasilite (Pd16S7); vysotskite and kravtsovite; phase Pd4S and a Ag–Pd alloy; it also coexists with a new phase Pd13Ag3S4. Kravtsovite is stable up to 507°C; the presence of kravtsovite in the mineral assemblage reflects its formation below this temperature. The occurrence of coldwellite, vysotskite and Ag2S together in equilibrium reflects the formation of this mineral assemblage above this temperature (507°C). Coldwellite is stable up at 940°С. Mineral assemblages defined in this study can be expected in Cu–Ni–PGE mineral deposits, associated with mafic and ultramafic igneous rocks, in particular in mineralisations with known silver–palladium sulfides.

Phase relations in the binary system anorthite-K-feldspar at 10 kbar were studied with a solid-media piston cylinder apparatus. The eutectic character of this system is confirmed, with the eutectic point located at An30Kf70 (± 3 mol%), and 1215 ± 15°C, 10 kbar. The mutual solubilities between anorthite and K-feldspar at the solidus are greatly increased at 10kbar compared with those at 1 atm. The maximum solubility of K-feldspar in anorthite is c. 5% at 1 atm, c. 18% at 10 kbar; and that of anorthite in K-feldspar is c. 3% at 1 atm, c. 7% at 10 kbar. The increased solubilities are attributed to the increase in the eutectic temperature at high isostatic pressures. The results imply increased ternary feldspar solid solutions at high pressures and support the existence of homogeneous feldspars with extensive ternary composition under high-P, T and anhydrous conditions.

Phase relations in the system PbS-PbSe-PbTe were investigated in the temperature range between 1150 and 300°C. There is a complete solid solution series along the join PbS-PbSe, whereas miscibility gaps exist below ∼ 350°C and 805°C along joins PbSe-PbTe and PbS-PbTe, respectively. Both gaps extend into the ternary system, and the range of the gap between PbS and PbTe reduces in size with temperature as well as Se-content.

By using the subregular model of mixing (Ganguly and Saxena, 1987), mixing energetics and activity composition relations of PbX were derived. The temperature-dependence of the interaction parameters was calculated to be 5057.6 − 1.7T, 9605.2 − 4.4T, 3933.2 − 3.1T (T < 500°C), 6410.3 − 6.35T (T < 500°C) 1649 − 0.26T, 1594.6 − 0.2T, and 12906.3 − 17.3T for WWSte, WTeS, WSeTe, WTeSe, WSSe, WSeS, and WSSeTe, respectively. A critical temperature along the join PbS-PbSe at ∼ 100°C was obtained from interaction parameter calculation, which suggests the separation of the PbS-PbSe series.

Correlation of solid-solution ranges established in this study with natural occurrences was made.

Using previously published data and a few new data the compositions of protopyroxene, orthopyroxene, and pigeonite, coexisting with liquid in the system CaO-MgO-SiO2, are reviewed and shown to depend subtly on the number and nature of the coexisting phases such as forsterite, silica, other pyroxene, and liquid. Broadly, protopyroxene contains 0 to 1.0 wt. % CaO, orthopyroxene 1.4 to 2.2%, and pigeonite 3.0 to 6.5%, but pigeonite coexisting with olivine contains up to 1.5 wt. % more CaO than pigeonite coexisting with SiO2 at the same temperature.

In the system CaO-MgO-Al2O3-SiO2 some new analyses and all previously published microprobe analyses of pyroxenes fall into three compositional groups, which are hereafter used to name the pyroxene irrespective of whatever description the original authors gave to their products. For these groups, the compositional subtleties depend on the number and nature of coexisting phases and these are illustrated using sketches of the solid solution polyhedron for each phase, to illustrate, in particular, the decomposition of pigeonite as temperature drops, at 1276°C when forsterite, diopside and liquid coexist, but at other temperatures when other phases coexist. The assemblage orthopyroxene, anorthite, diopside, forsterite is established as the solidus at 1244°C by X-ray diffraction and by new microprobe analyses, and not protopyroxene with anorthite, diopside, and forsterite as was previously accepted. Liquidus surfaces for forsterite and protopyroxene, forsterite and orthopyroxene, and forsterite and pigeonite, are drawn, with temperature data, and several crystallization paths demonstrate the double resorption boundaries, forsterite-protopyroxene-orthopyroxene-liquid and forsterite-orthopyroxene-pigeonite-liquid, and demonstrate crystallization of four pyroxenes, resorption of three of them, and reprecipitation of a second generation of one of them.

No new data are presented for the solidus of the system CaO-MgO-SiO2, but a reinterpretation of the existing data suggests that the solvi of these alumina-free pyroxenes do not show an increase in CaO as temperature decreases. This is contrary to previous interpretations which have mingled data for alumina-bearing and alumina-free pyroxenes. The reinterpretation also favours the reaction protoenstatite + pigeonite ⇌ orthopyroxene at 1375°C and a revision of the previously accepted P-T (Schreinemaker's) net for the pyroxenes is proposed which does not accept orthopyroxene ⇌ protopyroxene + diopside at 1100°C at one bar, which previously has always been accepted by extrapolation from the experimental data at 1 kbar.

The phase equilibria in the system Ag–Pd–Te were studied by the technique of using an evacuated silica glass tube at 350° and 450°C. Five ternary phases were synthesized: sopcheite (Pd3Ag4Te4), lukkulaisvaaraite (Pd14Ag2Te9),telargpalite (Pd2–xAg1+xTe) and the previously unknown phases Pd7.5–xAg0.5+ xTe3 and Pd2+xAg2–xTe.The synthetic telargpalite has a compositional range from 26 to 29 wt.% Ag, with the formula Pd2–xAg1+xTe, where x varies from 0.09 to 0.22. The phase Pd2+xAg2–xTe has a compositional range from 34 to 35 wt.% Ag, where x varies from 0.18 to 0.24. The phase Pd7.5–xAg0.5+xTe3 forms a solid solution from 4 to 11 wt.% Ag, where x varies from 0.02 to 0.83. Phases Pd20Te7and Pd13Te3 dissolve up to 3.5 and 2 wt.% Ag, respectively. Other binary palladium tellurides do not dissolve Ag. The phase Pd3Ag4Te4, an analogue of the mineral sopcheite, forms a stable association with hessite and kotulskite it also coexists with lukkulaisvaaraite. Sopcheite is stable up to 383°C. Natural occurrences of hessite, kotulskite and lukkulaisvaaraite together in equilibrium indicate formation above this temperature. Phase relations defined the mineral assemblages that can be expected to occur in nature.The phase Pd7.5–xAg0.5+xTe3 potentially represents a new mineral; it will probably be found in association with lukkulaisvaaraite and telargpalite or telluropalladinite, among other platinum-group minerals. The phasePd2+xAg2–x Te can be found in association with telargpalite. Mineral assemblages defined in this study can be expected in Cu-Ni-PGE mineral deposits, associated with mafic and ultramafic igneous rocks, particularly in mineralized zones with known silver-palladium tellurides.

Understanding the controls on the evolution of natural feldspars is greatly assisted by coupling experimental determinations of feldspar/melt equilibria with thermodynamic modelling and the calculation of crystallisation paths. Such a combined approach permits the evaluation of the influence of intensive and extensive variables on feldspar compositions. Feldspar compositional paths are influenced only to a minor degree by pressure. The presence of H2O or other melt component incompatible in feldspar has a more major effect, not only in increasing the temperature interval over which feldspar crystallises, but also in decreasing the amount of Ab enrichment of the feldspar which occurs during crystallisation. The amount and behaviour of H2O in the magma has a pronounced influence on feldspar compositions when two feldspars are stable. Under conditions of high bulk H2O content or constant activity of H2O, plagioclase and alkali feldspar compositions evolve by increasing Ab content similar to the behaviour manifested in the simple binary systems. If the bulk H2O content is low, however, and the H2O content increases during crystallisation, plagioclase evolves by increasing Ab content until alkali feldspar is stabilised. From that point on, plagioclase compositions change mainly by decreasing Or content, while alkali feldspar evolves mainly by increasing Or content.

Liquidus phase relationships at H2O-saturated and -undersaturated conditions and 2 kbar in the systems Qz-Or-Ab (SiO2-KAlSi3O8-NaAlSi3O8), Qz-Or-Ab-Al2O3, and subsystems are compared and discussed. In the peraluminous systems (i.e. when melts are saturated with respect to mullite) the liquidus temperatures are lowered by 40-55°C for compositions in the quartz primary field and by 15-25°C for cotectic compositions. The composition of the Qz-Ab eutectic and of the minimum are slightly shifted towards more Qz-rich compositions (minimum composition at P(H2O) = 2 kbar in the system Qz-Or-Ab-A12O3, saturated with respect to mullite: Qz40Or23Ab37). In melts saturated with sillimanite or mullite, the effect of high Al content may be lower for the Qz-Or than for the Qz-Ab eutectic.

The depression of the liquidus temperatures may be partly related to the higher H2O solubility in melts saturated with respect to mullite. The solubility of H2O in a melt with a composition of Qz28Or34Ab38 at 2 kbar and 800°C is 5·77 ± 0·15 wt% H2O and 6·36 ± 0·30 wt% H2O in a melt with the same Qz/Or/Ab proportions but saturated with respect to mullite.

The effect of high Al contents on the Mg and Fe contents of Ca-free granite melts was investigated at 775°C-3 kbar (melts coexisting with phlogopite), and at 820°C-2 kbar (melts coexisting with biotite and spinel), under NNO buffer conditions. Less than 0·15 wt% MgO is incorporated in subaluminous melts coexisting with phlogopite, whereas peraluminous melts (2·9 wt% normative corundum) contain 0·6–0·7 wt% MgO. A similar behaviour of the MgO content is observed for melts coexisting with biotite. In contrast, no significant effect of high Al contents on the FeO content of melt coexisting with biotite was observed. This suggests that the Fe/Mg ratio may be significantly lower in peraluminous than in subaluminous granitic melts.