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.

  1. GRADATION IN SILICA CONTENT

    - referred to as acid or basic, implying a range of silica content
  2. .

  3. COLOUR GRADATION

  4. Ultramafic vs. ultrabasic

  5. SATURATION CONCEPT

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

SiO2 Saturation


HANDOUT 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

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.

  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)