Igneous Rock Classification

Igneous rocks are composed dominantly of silicate minerals, reflecting the fact that oxygen and silicon are the most abundant elements in the Earth’s crust. The silicate minerals found in abundance in igneous rocks form a relatively small set: quartz, K-feldspar, plagioclase, muscovite, biotite, Ca-clinoamphibole (such as hornblende), Ca-clinopyroxene (such as augite), orthopyroxene, and olivine. The relative proportions of these minerals, together with the feldspathoids, provide the basis for classifying igneous rocks. ^[1]

The QAPF Classification

The conventional classification scheme for igneous rocks is based on the relative amounts of four components: quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F). One fundamental constraint governs the entire diagram: quartz and feldspathoid minerals are mutually exclusive and are not normally found in the same rock. This is a chemical consequence of their formation conditions - the presence of excess silica drives the system toward quartz, while silica deficiency drives it toward feldspathoids; both cannot coexist at equilibrium. The classification distinguishes between plutonic rocks, which are usually coarse grained, and volcanic rocks, which are usually fine grained. Which category a rock belongs to must first be determined from field relationships or other information before the diagram can be applied. ^[1]

Modal Mineralogy and Point Counting

The QAPF classification uses modal mineralogy - the volume percent of each mineral in the rock - as input. Modal mineralogy is measured by point counting on a petrographic thin section. A mechanical stage moves the slide in systematic increments beneath the microscope crosshairs, and the geologist identifies and tallies the mineral at each crosshair position. After a statistically sufficient number of counts, the fraction of counts for each mineral provides its volume percent in the rock. ^[1]

Classification of Volcanic Rocks

The fine grain size of volcanic rocks typically prevents accurate determination of modal mineralogy - individual grains are too small to identify reliably under the microscope. The QAPF diagram therefore cannot easily be applied to most volcanic rocks in practice. Two alternatives are widely used. The first is chemical classification based on bulk SiO₂ content and the combined amount of alkalis (Na₂O + K₂O); several standard diagrams of this type are in common use. The second is normative mineralogy - a calculated set of minerals that could potentially form from the bulk chemical analysis under standard assumptions about how elements are allocated. Normative calculations convert a bulk chemical analysis into hypothetical mineral proportions, which can then be plotted on a classification scheme. ^[1]

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