TAS classification

Dacite is also defined by silica and alkali contents in the TAS classification.

The chemical name of a felsic rock is given according to the TAS classification of Le Maitre (1975).

These chemical differences are consistent with the position of trachyte in the TAS classification, and they account for the feldspar-rich mineralogy of the rock type.

Phonolites are silica under-saturated, as illustrated by the position of phonolite in the TAS classification and QAPF diagrams.

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.

Hekla's basaltic andesite lava generally has a SiO content of over 54%, compared to the 45-50% of other nearby transitional alkaline basalt eruptions (see TAS classification).

QAPF diagrams are not used to classify pyroclastic rocks or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification (Total-Alkali-Silica) is used.

Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO and the alkali oxides, i.e., NaO + KO, consistent with the TAS classification.

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).