QAPF diagram

In a Streckeisen diagram, also called QAPF diagram is a schematic diagram for the classification of igneous rocks. It is named after the Bernese petrographers Albert Streckeisen ( 1901-1998 ).

Representation

The representative point on the graph is determined by the existence of the modal felsic minerals and its percentage proportion in the rock mass. Be considered in the Streckeisen diagram thus only the mineral phases quartz (Q ), alkali feldspar (A), plagioclase (P) and Foide (F). Albite while the alkali feldspar is attributed. The diagram is made up of two triangles concentration. The two triangles touching each other on the feldspar baseline as free quartz and Foide in a magma can not crystallize at the same time, but would respond to feldspar. The cornerstones of the Streckeisen diagram are formed by rocks in which the respective minerals are the only felsic share, but this more mafic minerals does not exclude. Divided is generally in plutonic and volcanic rocks, which for both rock types each have their own classification double triangle is applied.

The composition of a rock resulting from the distance of each point in the Streckeisen diagram to the corners, so the rocks with the respective pure basic groups of minerals, and is usually determined by the counting of crystals in thin sections. For example, a rock with 30 % quartz, 50 % alkali feldspar and plagioclase 20% in Streckeisen diagram would be classified for plutonic rocks as granite, while a vulcanite the same mineral composition would be called rhyolite.

Rock types

The rocks of the Streckeisen diagram for plutonic rocks include granite, Quarzolit, diorite, gabbro, tonalite, monzonite, syenite, anorthosite, Foidolit and their many hybrids. The volcanics are shown rhyolite, dacite, trachyte, latite, basalt, andesite, phonolite, tephrite, Foidit and transitional forms. As a rough guide can serve the rule that the proportion of mafic minerals of the vertices A ( alkali ) to P ( plagioclase ) and the Q ( quartz) to F ( Foid ) increases towards the rule.

Applicability and Limitations

The Streckeisen diagram may only be used for the classification of igneous rocks, if:

  • The rock no melilite leads
  • The proportion of dark ( mafic ) minerals is less than 90% ( then the Ultramafitdreieck olivine - orthopyroxene - clinopyroxene used )
  • The rock does not exceed 10% Glass
  • The rock is neither a Pyroklastit nor a carbonatite (> 50 % carbonate minerals ), a Charnockite, a kimberlite, lamproite or lamprophyre one is.

Since volcanic rocks are often extremely finely crystalline and contain rock glasses, the proportions of individual felsic mineral phases generally can not be determined with microscopic methods. Therefore, other classification systems are often used for volcanic rocks, which are based on the chemical analysis of rock samples, such as the distinction between the TAS diagram. Alternatively, from a chemical rock analysis, a normative mineral composition, for example, be based on the CIPW norm calculated, which is used for classification in the Streckeisen diagram.

Swell

  • John D. Winter: An introduction to igneous and metamorphic petrology. Prentice Hall, Upper Saddle River NJ 2002, ISBN 0-1324-0342-0.
Feldspathoid TAS classification
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