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Definition of tas classification

A classification scheme for volcanic rocks based on their bulk chemical composition.
TAS stands for Total Alkali versus Silica.

Volcanic rocks often cannot be classified by their mineralogy because the rock is finely crystallized or even made of volcanic glass, and so get classified by their chemistry.

Within the IUGS classification of igneous rocks this is done by determining the chemistry of the rock as oxides and plotting the total of the alkali oxides (Na2O + K2O) against the total silica (SiO2) content as weight percent. The corresponding diagram is called a TAS diagram.

The official field names and rock definitions can be found in the figures below. Some, but not all, of the rock names are closely related to those in the QAPF diagram, but the mineralogy and chemistry may result in slightly different names for the same rock (sorry but thats petrology for you!).





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