A QAPF diagram is a doubled-triangle plot diagram used to classify intrusive igneous rocks based on their mineralogy. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)", which are the four mineral groups used for classification in a QAPF diagram. The percentages (ratios) of the Q, A, P and F groups are normalized, i.e., recalculated so that their sum is 100%.
QAPF diagrams are created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks [1] as fostered by Albert Streckeisen (whence their alternative name: Streckeisen diagrams). Geologists worldwide use the diagrams in classifying igneous, especially plutonic rocks. [2] [3] [4]
QAPF diagrams are mostly used to classify plutonic rocks (phaneritic rocks), and can be used to classify volcanic rocks (aphanitic rocks) if modal mineralogical compositions have been determined. But QAPF diagrams are not used to classify pyroclastic rocks or volcanic rocks if modal mineralogical compositions are not determined. There the TAS classification (Total-Alkali-Silica) is used. TAS is also used if volcanic rock contains volcanic glass (such as obsidian). QAPF diagrams are not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites). Instead, an alternate triangle plot diagram is used; (see Streckeisen diagram, lower right.)
An exact name can be given only if the mineralogical composition is established, which cannot be determined in the field.
The QAPF diagram presents for use the proportions (ratios) of four plutonic mineral(s) or mineral groups, which are: quartz (Q), the Alkali feldspars (A), the plagioclase feldspars (P), and the feldspathoids (F).
Because F and Q groups cannot simultaneously form in plutonic rocks—due to the difference in their respective silica contents—the QAPF diagram is drawn as two mutually exclusive triangle plots, i.e., QAP and FAP. These are joined along one side such that, between them, each of the two triangle plots exclude either the Q group or F group minerals. (Other mineral groups may occur in samples, but they are disregarded in this classification method.)
To use this classification method, the concentrations (the modes) of the four mineral groups must be determined or estimated, and then normalized to 100%. Thus, for a rock identified as having, say, 20% mica, 30% quartz (Q), 30% alkali feldspar (A), and 20% plagioclase (P), the mica is disregarded, and the normalized ratios (proportions) of the Q, A, and P groups are calculated as 37.5%, 37.5% and 25% = 100%.
Of these, the (again) normalised relative proportions of A and P are 37.5/62.5 = 60% and 25/62.5 = 40%. The rock can now be plotted on the diagram by finding a horizontal line representing 37.5% quartz and then plotting a point on it 60% of the way across from the A side to the P side. For this example the rock can be classified as a Monzogranite.
And, a plutonic rock that contains no feldspathoids (F group), no alkali feldspar (A group), but contains plagioclase-feldspar (P group), many pyroxenes (not labeled in a QAPF diagram), and few quartz grains (Q group)—is probably gabbro; (see right edge of the Streckeisen diagram, at side P).
This diagram makes no distinction between rock types at the same QAPF plot position and classification, but of different bulk chemical compositions with respect to other minerals such as olivine, pyroxenes, amphiboles or micas. For example, because non-Q, -A, -P and -F minerals are disregarded the system does not distinguish between gabbro, diorite, and anorthosite.
The QAPF diagram is not used for all plutonic rocks; the ultramafic plutonic rocks are the most important of groups that have separate classification diagrams; (see Streckeisen diagram).
Gabbro is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include basalt, diabase and gabbro. Mafic rocks often also contain calcium-rich varieties of plagioclase feldspar. Mafic materials can also be described as ferromagnesian.
Plagioclase ( PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz) is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or "record-groove" effect.
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 alkali metals. It is the volcanic equivalent of syenite.
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 solidifies rapidly close to the Earth's surface.
Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic (coarse-grained) texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with alkali feldspar making up less than 10% of the total feldspar content. Quartz (SiO2) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid (QAPF) content of the rock. Amphiboles and biotite are common accessory minerals.
Nepheline syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.
Diorite is an intrusive igneous rock formed by the slow cooling underground of magma that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silica (mafic) gabbro and high-silica (felsic) granite.
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites. As only two of the three defining mineral groups need to be present for the rock to be called a granitoid, foid-bearing rocks, which predominantly contain feldspars but no quartz, are also granitoids. The terms granite and granitic rock are often used interchangeably for granitoids; however, granite is just one particular type of granitoid.
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, such as batholiths, dikes, sills, laccoliths, and volcanic necks.
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 oxide, >3% potassium oxide, high sodium oxide, and high nickel and chromium.
Essexite, also called nepheline monzogabbro, is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.
Rhyodacite is a volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of those plutonic rocks that are intermediate in composition between monzogranite and granodiorite. Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.
A quartz latite is a volcanic rock or fine grained extrusive rock composed mostly of alkali feldspar and plagioclase with some quartz. It forms from the rapid cooling of magma of intermediate composition but moderately enriched in alkali metal oxides.
TAS stands for Total Alkali Silica. The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali and silica contents. These chemical parameters are useful because the relative proportions of alkalis and silica are important in determining both normative mineralogy and actual mineralogy. The classification can be simple to use for rocks that have been chemically analyzed. Except for the following quotation from Johannsen (1937), this discussion is based upon Le Maitre et al (2002).
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.
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.
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 likely very small amounts of leucite or analcime.
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 America and the Andes of South America.
