HISTORICAL CLASSIFICATION SYSTEMS
Several aspects which historically have played and continue to play a role in the classification of igneous rocks should also be considered.
GRADATION IN SILICA CONTENT - referred to as acid or basic, implying a range of silica content
- Acidic > 66 wt% SiO2
Granites ~ 72 wt% SiO2, granodiorites ~ 68 wt% SiO2
- Intermediate - 52 to 66 wt% SiO2
Andesite 57 wt% SiO2
- Basic - 45 to 52 Wt% SiO2
Basalts range from 48 to 50 wt%
- Ultrabasic - < 45 wt% SiO2
peridotites 41 to 42 wt% SiO2
- Felsic rocks are light coloured, contain felsic minerals (e.g. qtz, feldspar, feldspathoids) which are themselves light in colour and have a low density which contribute to the pale colour of the rock.
- Mafic Rocks are denser and dark coloured, the result of containing mafic minerals (pyroxene, amphibole, olivine, biotite). These minerals contribute to the green, brown and black colour of these rocks.
Ultramafic vs. ultrabasic
Used in reference to the SiO2 and Al2O3 which are the two most abundant components of igneous rocks.
HANDOUT SiO2 Saturation
Minerals present in igneous rocks can be divided into two groups:
- Those which are compatible with quartz or primary SiO2 mineral (tridymite, cristobalite) these minerals are saturated with respect to Si, e.g feldspars, pyroxenes.
- Those which never occur with a primary silica mineral. These are undersaturated minerals, e.g. Mg-rich olivine, nepheline.
The occurrence of quartz with an undersaturated mineral causes a reaction between the two minerals to form a saturated mineral.
2SiO2 + NaAlSiO4 ===> NaAlSi3O8
Qtz + Ne ===> Albite
SiO2 + Mg2SiO4 ===> 2MgSiO3
Qtz + Ol ===> En
Rock Classification (Silica saturation)
- Oversaturated - contains primary silica mineral
- Saturated - contains neither quartz nor an unsaturated mineral
- Unsaturated - contains unsaturated minerals
HANDOUT Al2O3 Saturation
Four subdivisions of rocks independant of silica saturation, based on the molecular proportions of Al2O3, Na2O, K2O and CaO applied mainly to granitic lithologies.
- Peraluminous - Al2O2 > (Na2O + K2O + CaO)
- Metaluminous - Al2O3 < (Na2O + K2O + CaO) but Al2O3 > (Na2O + K2O)
- Subaluminous - Al2O3 = (Na2O + K2O)
- Peralkaline - Al2O3 < (Na2O + K2O)