The TAS classification can be used to assign names to many common types of
volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic r ...
s 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
Normative mineralogy is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative chemical analysis according to the principles of geochemistry.
Normative mineral calculations ...
. 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).
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 (1937) should be kept in mind.
:''Many and peculiar are the classifications that have been proposed for igneous rocks. Their variability depends in part upon the purpose for which each was intended, and in part upon the difficulties arising from the characters of the rocks themselves. The trouble is not with the classifications but with nature which did not make things right. … Rocks must be classified in order to compare them with others, previously described, of similar composition and appearance. If this cannot be done on a genetic basis, then an artificial system must answer in order to serve as a card index to rock descriptions. Although this may be an evil thing, it is, at least, the least of several evils.''
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
lamprophyre
Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxid ...
s.
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
A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for ...
or one of the other diagrams presented in the entry for
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
s). 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. (2002) for fields in the TAS diagram are listed below.
B (
Basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
) (Use
normative mineralogy
Normative mineralogy is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative chemical analysis according to the principles of geochemistry.
Normative mineral calculations ...
to subdivide)
O1 (
Basaltic andesite
Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Amer ...
)
O2 (
Andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...
)
O3 (
Dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
)
R (
Rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
)
T (
Trachyte
Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and al ...
or Trachydacite) (Use normative mineralogy to decide)
Ph (
Phonolite
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...
)
S1 (
Trachybasalt
Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and li ...
) *Sodic and potassic variants are
Hawaiite
Hawaiite is an olivine basalt with a composition between alkali basalt and mugearite. It was first used as a name for some lavas found on the island of Hawaii.
It occurs during the later stages of volcanic activity on oceanic islands such as Haw ...
and Potassic Trachybasalt
S2 (Basaltic trachyandesite) *Sodic and potassic variants are
Mugearite
Mugearite () is a type of oligoclase-bearing basalt, comprising olivine, apatite, and opaque oxides. The main feldspar in mugearite is oligoclase.
Mugearite is a sodium-rich member of the alkaline magma series. In the TAS classification of volc ...
and
Shoshonite
Shoshonite is a type of igneous rock. More specifically, it is a potassium-rich variety of basaltic trachyandesite, composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored v ...
S3 (
Trachyandesite
Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alka ...
*Sodic and potassic variants are
Benmoreite
Benmoreite is a silica-undersaturated volcanic rock of intermediate composition. It is a sodium-rich variety of trachyandesite and belongs to the alkalic suite of igneous rocks.
Nepheline benmoreite
An origin by fractionation from basanite t ...
and
Latite
Latite is an igneous, volcanic rock, with aphanitic- aphyric to aphyric-porphyritic texture. Its mineral assemblage is usually alkali feldspar and plagioclase in approximately equal amounts. Quartz is less than five percent and is absent in a f ...
Pc (
Picrobasalt
Picrite basalt or picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts (20-50%) and black to dark brown pyroxene, mostly augite.
The olivine-rich p ...
)
U1 (
Basanite
Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...
or
Tephrite
Tephrite is an igneous, volcanic (extrusive) rock, with aphanitic to porphyritic texture. Mineral content is usually abundant feldspathoids (leucite or nepheline), plagioclase, and lesser alkali feldspar. Pyroxenes (clinopyroxenes) are common ...
) (Use normative mineralogy to decide)
U2 (
Phonotephrite
Phonotephrite or phono-tephrite is a strongly alkaline volcanic rock with a composition between phonolite and tephrite. This unusual igneous rock contains 7 to 12% alkali content and 45 to 53% silica content (see TAS diagram). It can be describ ...
)
U3 (
Tephriphonolite Tephriphonolite or tephri-phonolite is a mafic to intermediate extrusive igneous rock in composition between phonotephrite and phonolite. It contains 9 to 14% alkali content and 48 to 57% silica content (see TAS diagram). Tephriphonolite is rou ...
)
F (
Foidite
Foidolite () is a rare phaneritic (coarse-grained) intrusive igneous rock in which more than 60% (by volume) of light-coloured minerals are feldspathoids. Crystals of alkali feldspar, plagioclase, biotite, amphibole, pyroxene, and/or olivin ...
) (Name according to dominant
feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...
when possible.
Melilitite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is ( Ca, Na)2( Al, Mg, ...
s also plot in this area and can be distinguished by additional chemical criteria.)
*Sodic as used above means that Na
2O - 2 is greater than K
2O, and potassic that Na
2O - 2 is less than K
2O. Yet other names have been applied to rocks particularly rich in either sodium or potassium (as are
ultrapotassic igneous rocks Ultrapotassic igneous rocks are a class of rare, volumetrically minor and generally ultramafic or mafic silica-depleted igneous rocks.
Ultrapotassic rocks are defined by molar K2 O/ Na2O >3 in much of the scientific literature. In other papers wr ...
).
References
*Albert Johannsen, A Descriptive Petrography of the Igneous Rocks. Volume 1, Introduction, Textures, Classifications, and Glossary. The University of Chicago Press, Chicago, Illinois, 1937.
*R. W. Le Maitre (editor), A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efremova, J. Keller, J. Lamere, P. A. Sabine, R. Schmid, H. Sorensen, and A. R. Woolley, Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press, 2002.
{{DEFAULTSORT:Tas Classification
Igneous petrology
Igneous rocks