Under magmatic conditions some minerals react with free silica to form other (more silica-rich) minerals. These reactant minerals are said to be undersaturated (with respect to SiO2). Other minerals are stable (can coexist) with free silica (generally in the form of quartz) and are said to be saturated (with respect to SiO2).
Typical reactions are:
quartz + nepheline =======> albite
quartz + kalsilite =======> orthoclase
quartz + kalsilite =======> leucite
quartz + Mg-rich olivine =======> enstatite
Shand (1927) proposed the following list of minerals, subdivided on the basis of silica saturation and/or undersaturation, i.e. those that coexist with quartz (+Q) and those that do not coexist with quartz (-Q).
Saturated (+Q) Undersaturated (-Q)Undersaturated and saturated minerals can coexist stably under magmatic conditions, but quartz, tridymite and christobalite can only coexist stably with saturated minerals. For example Q + ne is an impossible igneous assemblage, as is Q + ol (Mg - rich) (see reactions above), but Q + ol (Fe- rich) is stable.
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