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Incompatible Phases

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:

  • 2SiO2 + NaAlSiO4 =======> NaAlSi3O
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    quartz + nepheline =======> albite

  • 2SiO2 + KAlSiO4 =======> KAlSi3O8
  • quartz + kalsilite =======> orthoclase

  • SiO2 + KAlSiO4 =======> KAlSi2O6
  • quartz + kalsilite =======> leucite

  • SiO2 + Mg2SiO4 =======> 2MgSiO3
  • 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)
all feldspars leucite all pyroxenes nepheline all amphiboles sodalite micas cancrinite fayalite (Fe-rich olivine) analcite spessartine Mn3Al2(SiO4)3 forsterite (Mg-rich olivine) almandine Fe3Al2(SiO4)3 melanite (Ti garnet) sphene andradite zircon Ca3(Fe,Ti)2(SiO4)3 topaz pyrope Mg3Al2(SiO4)3 magnetite perovskite ilmenite melilite apatite corundum calcite

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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