TAS classification


The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportions of alkalis and silica play an important role in determining actual mineralogy and normative mineralogy. The classification appears to be and can be simple to use for rocks that have been chemically analyzed. Except for the following quotation from Johannsen, this entry is based upon Le Maitre and others.

Use of the TAS classification

TAS Stands for Total Alkali Silica.
Before using the TAS or any other classification, however, the following words of Johannsen should be kept in mind.
The subtitle of the classification chapter by Johannsen is "Chacun a son goût".
Furthermore, as discussed in considerable detail by Le Maitre and others, the classification cannot be applied to all volcanic rocks. Certain rocks cannot be named using the diagram. For others, additional chemical, mineralogic, or textural criteria must be used, as for lamprophyres.
The TAS classification should be applied only to rocks for which the mineral mode cannot be determined. Before classifying rocks using the TAS diagram, the chemical analyses must be recalculated to 100% excluding water and carbon dioxide.

The TAS diagram

The names provided by Le Maitre et al. for fields in the TAS diagram are listed below.
B
O1
O2
O3
R
T
Ph
S1 *Sodic and potassic variants are Hawaiite and Potassic Trachybasalt
S2 *Sodic and potassic variants are Mugearite and Shoshonite
S3
U1
U2
U3
F