Silica Saturation

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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

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.