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 (1937), this entry is based upon Le Maitre and others (2002).

TAS Stands for Total Alkali Silica.

Before using the TAS or any other classification, however, the following words of Johannsen (1937) should be kept in mind.

The subtitle of the classification chapter by Johannsen (1937) is "Chacun a son goût" (to each his own taste).

Furthermore, as discussed in considerable detail by Le Maitre and others (2002), 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 (otherwise, use a scheme based on mineralogy, such as the QAPF diagram or one of the other diagrams presented in the entry for igneous rocks). Before classifying rocks using the TAS diagram, the chemical analyses must be recalculated to 100% excluding water and carbon dioxide.

The names provided by Le Maitre et al. (2002) for fields in the TAS diagram are listed below.

B (Basalt) (Use normative mineralogy to subdivide)

O1 (Basaltic andesite)

O2 (Andesite)

O3 (Dacite)

R (Rhyolite)

T (Trachyte or Trachydacite) (Use normative mineralogy to decide)

Ph (Phonolite)

S1 (Trachybasalt) *Sodic and potassic variants are Hawaiite and Potassic Trachybasalt

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