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INTRODUCTION TO NATURAL IMMISCIBLE SYSTEMS
High temperature melts, fluids and gases progressively organize themselves structurally during cooling, usually causing separation of solids, liquids or gases. In many different chemical systems this phase separation results in distinct chemical separation (immiscibility), with associated contrasting physical properties in the separating phases. Because of the variety in chemistries, and relative changes in entropy and volumes of the natural mixtures compared to the separated phases, immiscibility can occur in different chemistries during heating and compression, as well as during cooling and decompression.
Although the main emphasis in this volume is on fluid–fluid equilibria, there are good examples and much literature on liquid–liquid equilibria in synthetic silicate melts and natural magmas. In fact much of our understanding of phase separation (for geochemists generally taken simply as liquid–vapor equilibria) actually originates from liquid–liquid immiscibility studies of silicate melts. For a one-component system there are three distinct regions in PT space where either solid or liquid or vapor exist as the stable phase. These are separated by three univariant curves (i) the solid–liquid curve, the solidus, (ii) the liquid–vapor curve, which can lead to the most commonest form of critical point (see Fig. 1a⇓), and (iii) the solid–vapor curve, which reflects direct vaporization (sublimation) or condensation of solids from gas. The latter is relevant in some industrial processes and to condensation of stars at vacuum pressures. For each added component with solid, liquid and vapor phases, there is the possibility of mixing of each of the phases, or not. In silicates we are used to recognize immiscibility gaps (solvus) among chemically related minerals (e.g., alkali feldspars), which are miscible at high temperatures in the subsolidus region but are immiscible with cooling and undergo phase separation. We are less used to consider immiscibility between two anhydrous melts (silicate–silicate, silicate–carbonate, silicate–oxide, silicate–sulfide, etc.).