Quartz (SiO2) is the most abundant mineral in terrigenous sedimentary rocks and is exceedingly durable (surviving multiple generations of weathering and deposition). Quartz and silica occur in many varieties—true quartz in the form of megaquartz, chert, microquartz, or chalcedony and various other forms of silica, mainly opal (opal-A and opal-CT [cristobalite]).
Major characteristics:
Color: Typically clear, but cloudy and colored varieties occur where crystals are especially rich in water or mineral inclusions. In thin section, these colors are rarely seen, although red and pale brown varieties related to included hematite and limonite are sometimes found.
Common crystal habit: Hexagonal, bipyramidal.
Pleochroism: None.
Cleavage/fracture: No cleavage, but fractures and healed fractures can be common.
Relief and optic sign: Low; uniaxial (+).
Birefringence: First-order (in a 30-µm section) typically first-order white and gray to pale straw yellow.
Other: No twinning; may contain trains of water, gas or mineral inclusions including ones, such as rutile and tourmaline needles, that may provide clues to
grain provenance. Optical behavior varies based on deformation history; grains can have undulatory (undulose or sweeping) or nonundulatory
(nonundulose or straight) extinction. Quartz grains commonly have syntaxial quartz overgrowths that form diagenetically. Minerals that may have similar appearance and distinguishing differences:
Feldspars: Commonly show some cleavage, more alteration (vacuolization, sericitization, partial to complete dissolution), presence of characteristic
twinning patterns and are biaxial. Can be differentiated by staining.
Apatite: Normally lower birefringence, has a higher relief and is uniaxial (-).
Gypsum and barite: Both have birefringence similar to quartz, but barite has slightly higher relief and gypsum has lower relief; both gypsum and barite
have strong cleavage.
Zeolites: Numerous zeolite minerals can be confused with fibrous varieties of quartz. Most of the common zeolite minerals are biaxial (+/) and most have somewhat lower birefringence than quartz.
Provenance indicators:
Quartz can occur as single crystals or polycrystalline aggregates that may provide clues to the provenance of the grains, but quartz is common to most rock types other than basalts.
Semicomposite and polycrystalline quartz can be found in metamorphic and plutonic rocks as well as hydrothermal vein deposits and fractures. For metamorphic quartz, the size of the crystals may represent increasing metamorphic grade; larger crystals form under higher temperatures and pressures.
Grain size can make provenance determination more difficult. With decreasing grain size, the ability to see undulatory quartz or polycrystalline/composite
grains becomes more difficult. Since crystal sizes within polycrystalline grains may be large, grains formed from their breakdown may not exhibit polycrystallinity or undulatory extinction.
Chert and opaline silica can be found in either sedimentary or volcanic rocks — in the former as direct precipitates and in the latter as recrystallized glass.
Good photomicrographs as well.
See additional constituents, textures, cements, and porosities of sandstones and associated rocks (here) in "A Color Illustrated Guide To Constituents, Textures, Cements, and Porosities of Sandstones and Associated Rocks,"PeterA. Scholle, AAPG Memoir 28, U.S. Geological Survey, 1979.
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