Lecture 5: Chemical & Mineralogical Classification of Magmas

Simple classification in Lecture 1 of Mafic to Felsic magmas & Si, Mg, contents. Now, attach real values.

Chemical makeup of magmas - 11 major oxides:

Classification of igneous rocks can both be dependent on the mineralogy (modal classification) or the oxide chemistry (includes normative calculation)

mode = mineralogical composition of rock - volume %

norm = calculated mineralogical composition based on conversion of chemical analysis into the formulas of minerals. Transforms weight percents of major oxides into minerals that might have formed with complete equilibrium under plutonic conditions

Done by computer.

  1. Allot major oxides sequentially to different minerals (follows Bowen's Reaction Series)
  2. No hydrous minerals (use anhydrous equivalents)
  3. No solid solution, just endmember minerals

Ideal situation: norm = mode (but hardly ever happens)

 

Degree of silica saturation in norm

See basalt tetrahedron (handouts) for division of fields

 

Degree of alumina saturation in norm

Corundum in norm

Anorthite and diopside in norm

Acmite in norm (NaFeSi2O6)

 

Comparison of normative and modal mineralogy in peraluminous, metaluminous, and peralkaline rocks

  Peraluminous Rocks Metaluminous Rocks Peralkaline Rocks
Normative minerals:
Corundum
Anorthite & diopside
Acmite
Modal minerals:
Muscovite
Andalusite
Garnet
Topaz
Tourmaline
Biotite
Hornblende
Augite
Orthopyroxene
Na-rich pyroxenes
(aegerine-augite)
Na-rich amphibolte
(riebeckite)

Comparing silica and alumina saturations: