Several aspects which historically have played and continue to play a role in the classification of igneous rocks should also be considered.

1. 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

2. COLOUR GRADATION

   • 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

3. SATURATION CONCEPT

Used in reference to the SiO₂ and Al₂O₃ which are the two most abundant components of igneous rocks.

SiO₂ Saturation

Minerals present in igneous rocks can be divided into two groups:

1. Those which are compatible with quartz or primary SiO₂ mineral (tridymite, cristobalite) these minerals are saturated with respect to Si, e.g feldspars, pyroxenes.
2. 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)

1. Oversaturated - contains primary silica mineral
2. Saturated - contains neither quartz nor an unsaturated mineral
3. Unsaturated - contains unsaturated minerals

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.

1. Peraluminous - Al₂O₂ > (Na₂O + K₂O + CaO)
2. Metaluminous - Al₂O₃ < (Na₂O + K₂O + CaO) but Al₂O₃ > (Na₂O + K₂O)
3. Subaluminous - Al₂O₃ = (Na₂O + K₂O)
4. Peralkaline - Al₂O₃ < (Na₂O + K₂O)