Q

◊ quartz ◙ quartzite

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QAPF diagram

The QAPF diagram is employed for classification of igneous rocks based upon proportions of the minerals: Q quartz : A alkali feldspar : P plagioclase feldspar : and F feldspathoid (foid). These mineralogical classification diagrams are employed most frequently for the classification of phaneritic, plutonic igneous rocks (▼), but can also be applied to fine-textured, non-mafic volcanic rocks (▼). Mafic rocks contain little of the felsic QAPF minerals, so this classication scheme cannot be applied to rocks such as peridotites and pyroxenites. For volcanic rocks that contain volcanic glass (obsidian), the TAS classification (Total-Alkali-Silica) is used. The feldspars are family of common tectosilicate, monoclinic, rock forming minerals that crystallized from melts in a continuous series. The alkali feldspars (A) form solid solutions between K-feldspar endmember polymorphs (KAlSi3O8) and albite (NaAlSi3O8). The plagioclase feldpars (P) form solid solutions between albite and anorthite (CaAl2Si2O8) as the ratio of sodium to calcium substituted in the mineral's crystal lattice structure decreases. The feldspathoids are low-silica 'versions' of feldspars. In Bowen's Reaction Series, the calcium-rich, alkali feldspars such as anorthite are the most refractory members (freeze at highest temperatures), plagioclases are intermediate, and sodium-rich feldspars such as the polymorph orthoclase are the most fusible members (freeze at lowest temperatures). Quartz, which crystallizes last out of melts is the most fusible and surface-stable mineral in the reaction series.

click to enlarge image for phaneritic, plutonic rocks: Q= quartz, A= alkali feldspar, P=plagioclase feldspar, F=feldspathoid. Rock types: quartzolite, quartz rich granitoids, alkali feldspar granite, granite and syeno-granite and monzogranite, granodiorite, tonalites, quartz alkali feldspar syenite, quartz syenite, quartz monzonite, quartz monzodiorite and quartz monzogabbro, quartz diorite and quartz gabbro and quartz anorthosite, alkali feldspar syenite, syenite, monzonite, monzodiorite and monzogabbro, diorite and gabbro and anorthosite, foid-bearing alkali feldspar syenite, foid-bearing syenite, foid-bearing monzonite, foid-bearing monzodiorite and foid-bearing monzogabbro, foid-bearing diorite and foid-bearing gabbro and foid-bearing anorthosite, foid syenite, foid monzonite, foid monzodiorite and foid monzogabbro, foid diorite and foid gabbro, foidolite.

click to enlarge image for field classification for phaneritic igneous rocks: Rock types: granitoid, syenitoid, dioritoid gabbroid anorthosite, foid syenitoid, foid dioritoid and foid gabbroid, foidolite

click to enlarge image for fine-grained texture (volcanic rocks) for which modal mineralogical composition can be determined: Rock types: alkali feldspar rhyolite, rhyolite, dacite, quartz alkali feldspar trachyte, quartz trachyte, quartz latite, basalt andesite, alkali feldspar trachyte, quartz trachyte, quartz latite, basalt andesite, foid-bearing alkali feldspar trachyte, trachyte, latite, phonolite, tephritic phonolite, phonolitic basanite and phonolitic tephrite, basanite tephrite, phonolitic foidite, tephritic foidite, foidite.

click to enlarge image for field classification for fine-grained igneous rocks: Basic rock types: rhyolitoid, dacitoid, trachytoid, andesitic basaltoid, phonolitoid, tephritoid, foiditoid

quartz

mineral / chemical formula		properties / significance	occurrence
quartz (silica) SiO2 3D frameworks (lattices) – tectosilicates [AlxSiyO2(x+y)]x− [gallery, 2, 3]		tectosilicate, silica tetrahedra, hexagonal rhombohedral crystals in numerous varieties named for color and for microstructure, numerous growth forms	commonest rock forming mineral, found in igneous, sedimentary, and metamorphic rocks, hydrothermal veins and pegmatites, granite, sandstone, limestone

quartzite

Quartzite is a tough, weathering-resistant metamorphic rock derived from sandstone, usually through the agency of tectonic compression at the root of orogenic belts. In metamorphic quartzite ( meta-quartzite) material from the parent sandstone's quartz grains and cement have been recrystallized into an interlocking mosaic of quartz crystals, so that any original texture and structure is often obliterated. Minor amounts of the former cementing materials, such as iron oxide, carbonate and clay often form streaks and lenses within the quartzite. Whereas pure quartzite is white to grey, shades of pink and red are due to iron oxide, and other colors arise through minor impurities of other minerals. Because quartzite is particularly resistant to chemical weathering, it often forms ridges and resistant hilltops that are covered only by very thin soil and minimal vegetation. Orthoquartzite is a very pure quartz sandstone composed of well rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite.

links: quartzite: hand-specimens: pink, gray, colored, wp, pointe de projectile en quartzite; close-up: 1, green Masi Quartzite from the north of Norway, Sioux quartzite; formations: Sioux Quartzite glacial erratic, transported quartzite block, hematite stain quartzite ridge in Killarney Park, Ontario, Canada, 2, quartzite vein, Ef with interbedded quartzite and phyllite, overhanging western wall of quartzite rock of Simpsons Gap, Shinumo Quartzite, quartzite outcrop on summit ridge of Sabattus Mountain, ME, bedding ripples in quartzite quarry, 2, Hurwitz Horst quartzite, Dripping Springs Quartzite, layered rock face; thin-section: sutured grain boundaries, 2, quartz arenite —> quartzite (characteristic low birefringence): orthoquartzite: Mt Battie orthoquartzite, Mt Battie quartzite cobble conglomerate