Lexikon

a Streckeinsen or QAPF diagram is a double triangle diagram used to classify igneous rock based on mineralogic composition. The diagram is based on the fundamental work of many petrologists which was fully summarised by Strekeisen in 1973 and 1976. The QAPF diagram was completed and recommended by the International Union of Geological Sciences, IUGS. Geologists use it worldwide as a classification of igneous, especially plutonic igneous rocks. The QAPF by acronym, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". The mineral groups used for classification of igneous rocks are the following: Q = Quartz, A = Alkali feldspar, P = Plagioclase, F = Feldspathoid (Foid), M = Mafic (silicate mineral rich in magnesium and iron). Groups Q, A, P, F comprise the felsic minerals. The sum of Q+A+P+F+M must be 100%. However there can never be more than four non-zero values, as the minerals in groups Q and F are mutually exclusive i.e. if Q is present F must be absent and viceversa. The classification is divided into two parts: If mafic minerals (M) are less than 90% (M<90) the rock is classified according to its felsic minerals in the QAPF double triangle diagram. The basic idea of classification is that minerals belonging to the Q and F mineral groups do not occur simultaneously as primary minerals in the same igneous rock, because the SiO2 surplus in the melt reacts with the Feldspathoids resulting feldspars. For this reason maximum three of the Q, A, P, F mineral groups may occur in an igneous rock type. If mafic minerals make up more than 90% of the rock composition (M≥90), it is an ultramafic rock and it is classified according to its mafic minerals. In this case two triangle diagrams are used (olivine–orthopiroxene–clinopyroxene, olivine–pyroxene–amphibole).