Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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(alpha and beta), coesite, and stishovite. Each polymorph of silica is stable under a different set or range of temperature and pressure conditions. A phase stability diagram (Figure 3.6), where pressure increases upward and temperature increases to the right, shows the stability fields for the silica minerals. The stability fields represent the temperature and pressure conditions under which each mineral phase is stable. Each stability field is bounded by phase boundaries, lines that define the limits of the stability field as well as the conditions under which phases in adjoining fields can coexist in equilibrium. Where three phase boundaries intersect, a unique set of conditions is defined under which three stable phases can coexist simultaneously.

Schematic illustration of phase diagram for silica depicting the temperature–pressure stability fields for the major polymorphs and the liquid phase.

      Source: Adapted from Wenk and Bulakh (2004). © John Wiley & Sons.

      In other situations depicted in Figure 3.6, two phases coexist under the conditions marked by phase stability boundary lines rather than points. As a result, the phase rule (P = C + 2 − F) yields 2 = 3 − F, so that F must be 1. For example, under the conditions at point Y (900 °C, 9.2 GPa), both coesite and stishovite can coexist. If the temperature increases the pressure must also increase, and vice versa, in order for the system to remain on the phase stability boundary line where these two phases coexist. There is only one independent variable or 1 degree of freedom. The temperature and pressure cannot be changed independently. In a similar vein, two phases, one solid and one liquid, can coexist anywhere on the melting curve that separates the liquid and a single solid stability fields.

      However, for any point within a phase stability field (e.g., point Z) only one phase is stable (e.g., low quartz). The phase rule (P = C + 2 − F) yields 1 = 1 + 2 − F, so that F must be 2. This means that the temperature and the pressure can change independently without changing the phase composition of the system. For point Z, the temperature and pressure can increase or decrease in many different ways without changing the phase that is stable, as long as they remain within the stability field. There are two independent variables and 2 degrees of freedom. All points to the right of the melting curve in the liquid field represent the stability conditions for a single phase, liquid silica.

      One can also use this diagram to understand the sequence of mineral transformations that might occur as Earth materials rich in silica experience different environmental conditions. From a liquid silica system cooling at a pressure of 0.3 GPa cristobalite will begin to crystallize at ~1650°. As the system continues to cool, it will reach the cristobalite/tridymite phase boundary (~1460 °C), where cristobalite will be transformed into tridymite. Ideally, the system will continue to cool until it reaches the tridymite/high quartz phase boundary. Here it will be transformed into high quartz, then cool through the high quartz field until it reaches the low quartz/high quartz phase boundary, where it will be converted to low quartz and continue to cool. Two phases will coexist only at phase boundaries during phase transformations that take finite amounts of time to complete (Chapter 4).

      Similarly, a system undergoing decompression and cooling as it slowly rises toward the surface might follow line W–W′ on the phase diagram. It will start as coesite and be converted into alpha quartz (low quartz) as it crosses the phase boundary that separates them. Note that low quartz is the common form of quartz in low temperature, low pressure Earth materials.

      3.2.3 Two component phase diagram: plagioclase

Schematic illustration of plagioclase phase stability diagram at atmospheric pressure, with a complete solid solution between the two end member minerals albite (Ab) and anorthite (An).

      To examine the information that can be garnered from the plagioclase phase stability diagram, let us examine the behavior of a system, with equal amounts of the two end member components albite and anorthite. whose composition can be expressed as An50 (Figure 3.7). On the phase diagram, the system is located on the vertical An50 composition line. This line is above the liquidus (100% liquid) at high temperatures, between the liquidus and solidus (liquid + solid) at intermediate temperatures and below the solidus (100% solid) at low temperatures. If this system is heated sufficiently, it will be well above the liquidus temperature for An50 and will be 100% melt, much like an ideal magma. Now let us begin to cool the An50 system