TAS classification

TAS classification – Total Alkali Silica – is employed to classify common volcanic rocks based upon alkali and silica content (recalculated to exclude CO2 and H2O). The TAS classification employs the fact that relative proportions of alkalis and silica play an important role in determining mineral assemblages. The typical silica content of rocks: ◙ Acid rocks (felsic), such as dacite and rhyolite: greater than 63% silica (mostly light colored minerals such as feldspars and quartz). ◙ Intermediate rocks, such as andesite or latite, have compositions between those of the felsic and mafic rocks. ◙ Basic rocks (mafic), such as basalt: 45 to 55% silica (mostly dark minerals plus plagioclase feldspar and/or feldspathoid minerals). ◙ Ultrabasic rocks (ultramafic), such as komatiite or plutonic peridotite: less than 45% silica (mostly dark minerals olivine and pyroxenes). The TAS classification is used only for volcanics rocks for which the mineral mode cannot be determined. Where mineralogy is known, systems such as the QAPF diagram are applied. The TAS classification is not applicable to all volcanic rocks. Some volcanic rocks cannot be named using the TAS diagram, while additional chemical, mineralogic, or textural criteria are necessary for the classification of rocks with unusual mineral assemblages, such as the lamprophyres.

B (Basalt) (normative mineralogy, geochemical calculation of the whole rock geochemistry, is used to subdivide) O1 (Basaltic andesite) O2 (Andesite) O3 (Dacite) R (Rhyolite) T (Trachyte or Trachydacite) (using normative mineralogy to differentiate) Ph (Phonolite) S1 (Trachybasalt) *sodic variant is Hawaiite, and potassic variant is Potassic Trachybasalt S2 (Basaltic trachyandesite) *sodic variant is Mugearite, and potassic variant is Shoshonite S3 (Trachyandesite *sodic variant is Benmoreite, and potassic variants is Latite Pc (Picrobasalt) U1 (Basanite or Tephrite) (using normative mineralogy to differentiate) U2 (Phonotephrite) U3 (Tephriphonolite) F (Foidite) (named according to dominant feldspathoid where possible) Sodic (as used in the TAS classification) above means that Na2O - 2 is greater than K2O, and potassic that Na2O - 2 is less than K2O (ultrapotassic igneous rocks) Rocks with low alkali content are typically ferromagnesian, mafic volcanics. more : Flow Charts for Classification of Igneous Rocks carbonatite classification charnockite classification Flow chart for melilitic, kalsilitic, leucitic rocks... and lamprophyres, kalsilitic rock classification, lamproite classification, melilitic rock classification high-Mg classification norm ne v. norm ab classification plutonic QAPF pyroclastic rock classification TAS ultramafic classification volcanic QAPF