The Calcium-Aluminum-Silicate System Phase Diagram
The calcium-aluminium-silicate (CAOS) system phase diagram is one of the most important tools for understanding and predicting the phases of magmatic rocks. This diagram, which is also known as an AFM diagram (after the elements it contains: Al-Fe-Mg) shows the relationship between the major and minor components of silicate systems at different levels of temperature, pressure and composition. The diagram can be used by various professionals in the petrology disciplines as well as in geochemistry.
The calcium-aluminium-silicate (CAOS) system is a multi-component system of three main components: Calcium (Ca), Aluminum (Al) and Silica (SiO2). Each component has its own unique properties, which can affect the physical and chemical properties of the other components in the system. It is possible to plot two-component variables on the diagram, such as the ratio of Ca to Al and the fraction of silica (relative abundances). The three components can form various solid-state minerals and liquids at various temperatures and pressures. The most common minerals found in magmatic rocks are feldspars, micas, and quartz. Solutions of Ca-Al-silicate system components can also form glass-like melts called hyaloclastites which are often used to study glassy rocks and their structure.
The diagram is divided into two main areas: plagioclase stability (plot in the upper right corner of the diagram) and alkali feldspar stability (plot in the upper left corner). Plagioclase stability refers to the range of temperatures and pressures under which the plagioclase component of the CAOS system is stable. Plagioclase is a calcium-bearing mineral which forms when the Ca-Al-silicate system becomes saturated with silica. Alkali feldspar stability refers to the range of temperatures and pressures under which the alkali feldspar component of the CAOS system is stable. Alkali feldspar is an aluminum-bearing mineral which forms when the Ca-Al-silicate system becomes saturated with alumina.
The main features of the diagram are the plagioclase corner, the alkali feldspar corner, the two-field area and the three-field area. In the plagioclase corner, plagioclase is the dominant solid phase, while in the alkali feldspar corner, alkali feldspar is the dominant solid phase. In the two-field area, two solid phases can be present, while in the three-field area, three solid phases can be present.
The CAOS system phase diagram is an important tool for understanding magmatic rocks and their composition. It can be used to estimate the relative abundances of the major and minor components of magmatic rocks, and predict the type of minerals and glassy melts that may be present. It can also be used to determine the range of temperatures and pressures under which different minerals are stable and can thus be used to identify what type of magma or rock may form from a given set of starting materials. Through study of the CAOS system, it is possible to gain a better understanding of the petrology and geochemistry of rocks, as well as gain insights into the formation of magmas.
