Читать книгу Earth Materials - John O'Brien - Страница 64

The behavior of materials in Earth systems can be modelled using thermodynamic calculations and/or empirical results from laboratory investigations. The results of such calculations and/or investigations are commonly summarized on phase stability diagrams. A phase is a mechanically separable part of the system. Phase stability (equilibrium) diagrams display the stability fields (conditions under which a phase is stable) for various phases in a system of specified composition. These fields are separated by phase stability boundary lines that represent the conditions under which phase changes from one phase to another occur. Phase stability diagrams related to igneous systems and processes summarize relationships between liquids (melts) and solids (crystals) in a system. Such diagrams usually have temperature increasing upward on the vertical axis and composition shown on the horizontal axis. At high temperatures the system is completely melted. The stability field for 100% liquid is separated from the remainder of the phase diagram by a phase boundary line called the liquidus that represents the temperature above which the system exists as 100% melt and below which it contains some crystals. The low temperature stability field for 100% solid crystals is separated from higher temperature conditions by a phase boundary line called the solidus. At intermediate temperatures between the solidus and liquidus, the system consists of two types of stable phases in equilibrium, both liquid and solid crystals. Phase equilibrium diagrams, based on both theoretical and laboratory analyses, exist for a variety of multicomponent systems. A one‐component and five representative two‐component systems related to the discussion of igneous rocks and processes (Chapters 7–10) are discussed below. Metamorphic phase diagrams are discussed in Chapter 18. For discussions of systems that are beyond the scope of this text, including three‐ and four‐component systems, the reader is referred to mineralogy books by Wenk and Bulakh (2016), Nesse (2016), Dyer et al. (2008) and Klein and Dutrow (2007) and to petrology books by Frost and Frost (2019), Philpotts and Ague (2009), Winter (2009) and Best (2000). Some of the more important terms you will encounter in this discussion are defined in Table 3.1.

Table 3.1 A list of some common terms used in discussing phase diagrams.