Quartz

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A Color Guide to the Petrography of Sandstones, Siltstones, Shales, and Associated Rocks
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Series Memoir
Chapter Grains: Quartz and Silica
Author Dana S. Ulmer-Scholle, Peter A. Scholle, Juergen Schieber, Robert J. Raine
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Quartz crystals with adularia (orthoclase) from Dona Ana Co., New Mexico. Photograph courtesy of the New Mexico Bureau of Geology and Mineral Resources.

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

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.

Figure 1 Plot of modern beach sands and their source terranes shows how the amount of nonundulatory, undulatory, and polycrystalline quartz grains and the number of crystals in polycrystalline grains can help determine quartz provenance (from Basu et al., 1975[1]).

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 (see the table below). 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 (Figure 1).

Overview of quartz types, characteristics, and provenance (adapted from Krynine, 1946[2], and Folk, 1980[3])
Quartz types/source Extinction characteristics Other features
Common or plutonic (granitic or many others) Unit (straight) to slightly undulatory Some vacuoles and/or microlites; subsequant to xenomorphic grains
Vein or pegmatitic Unit or semi-composite to undulatory, possible shearing Abundant vacuoles, possible vermicular chlorite; comb structure common; typically found in large clasts
Volcanic Unit (straight) Euhedral bipyramidal shapes, round corners, and straight sides with embayments; virtually no inclusions; posssible negative crystals
Schistose metamorphic Unit to slightly undulatory Elongate composite grains with straight borders; common mica inclusions
Stretched metamorphic Strongly undulatory; subunit borders crenulate, granulate, or smooth Elongate, lenticular-shaped crystal subunits; some microlites and vacuoles
Recrystallized metamorphic Unit to slightly undulatory Straight boundaries between equant interlocking grains in a mosaic; some microlites or vacuoles

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.

Common inclusions and provenance

Amphiboles (actinolite and others): metamorphic, intermediate to mafic igneous rocks and skarn deposits.

Apatite: felsic to intermediate volcanic rocks, skarns, carbonatites, calcium-rich metamorphic rocks, pegmatites and hydrothermal vein deposits.

Chlorite: fracture fills in low to medium grade metamorphic rocks, skarns and pegmatites.

Iron oxides (hematite, goethite and others): very common, and they form in a variety of environments ranging from anoxic (magnetite) to highly oxidizing (hematite, goethite) and low (goethite), medium (hematite) to high temperature (magnetite) conditions.

Sillimanite: metamorphic rocks.

Titanium minerals (such as rutile, titanite, anatase and others): metamorphic rocks, pegmatites and hydrothermal vein deposits.

Tourmaline: pegmatites.

Zircon: common in igneous and metamorphic rocks.

Hydrocarbons, bitumen and methane: trapped during growth of the quartz crystals from fluids containing organics. These can sometimes be used to reconstruct basin temperature and fluid histories.

Petrographic examples

Triassic Sherwood Sandstone Group, County Antrim, Northern Ireland, U.K., polycrystalline quartz grain of metamorphic origin. The grain has sutured internal boundaries between composite crystals and a preferential alignment of the constituent crystals. In addition, crystal boundaries are diffuse. XPL, AFeS, KFS, BDI, Scale bar = 0.07 mm.
Lower Cretaceous Travis Peak Formation, Eastland County, Texas, monocrystalline quartz grain with exceptionally abundant vacuoles (probably liquid-filled inclusions) and semicomposite extinction (Krynine’s “hydrothermal” quartz). Such vacuole-rich quartz grains are most commonly derived from hydrothermal-vein sources. The quartz overgrowth (cement) around the upper part of the grain has far fewer inclusions. XPL, Scale bar = 0.26 mm.

See also

References

  1. Basu, A., S. W. Young, L. J. Suttner, W. C. James, and G. H. Mack, 1975, Re-evaluation of the use of undulatory extinction and polycrystallinity in detrital quartz for provenance interpretation: Journal of Sedimentary Research, v. 45, p. 873–882, doi: 10.1306/212F6E6F-2B24-11D7-8648000102C1865D.
  2. Krynine, P. D., 1946, Microscopic morphology of quartz types: Annals of 2nd Pan-American Congress of Mining and Geological Engineers, p. 35–49.
  3. Folk, R. L., 1980, Petrology of sedimentary rocks: Texas Hemphill’s Book Store, Austin, 182 p.

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