Quartz
Trigonal
SiO2
Quartz is the most common crystalline form of silica dioxide (SiO2) and the second most abundant mineral in the Earth’s crust after feldspars. Quartz is a common rock-forming minerals that occurs in most acid and intermediate igneous rocks, in many metamorphic rocks, and in terrigenous sedimentary rocks. The name ‘quartz’ likely derives from the Saxon word ‘Querkluftertz’ (cross-vein ore), since it is also a common mineral in veins and metasomatic rocks.
Structure and chemistry
Quartz is a framework silicate and one of the naturally occurring polymorphs of SiO2. There are two forms of quartz: α-quartz, stable below 573 °C at ambient pressure, and β-quartz, stable from 573 to 870 °C. β-quartz has an hexagonal crystal lattice with ‘rings’ of 6 SiO4 tetrahedrons linked to each other. The structure of α-quartz is trigonal and characterized by a slight rotation of the SiO4 tetrahedrons with respect to the β-quartz structure. The α-β transformation, indeed, involves no atom exchange or breaking of bonds and occurs very easily in nature. α-quartz is a chiral substance with crystal class 32 and, therefore, can exist in two forms: right-handed quartz and left-handed quartz. Right-handed quartz has a virtual helix of SiO4 tetrahedrons that rotates clockwise with respect to the long-axis (c-axis). In left-handed quartz this rotation is anticlockwise. Quartz is commonly almost purely SiO2, but it may accommodate some cations in traces like Al3+, Na+, Li+, K+, Ti4+, and Fe3+. In some cases, the impurity of quartz linked to these cations can produce colored varieties of quartz (e.g. smoky, rose, amethyst…). Quartz is commonly twinned. Some common twins are Dauphiné and Brazil twins, occurring between right-handed and left-handed crystals: the two individuals are generally penetrated with each other with their c-axis parallel to one another. Consequently, these twins are optically undetectable in standard thin sections. The Japan law is a twinning between two quartz crystals of the same type (left- or right-handed) where the two crystals are twinned on the prism faces and their c-axes meet at angle of 84°33’.
Properties
Habit: 6-sided double pyramidal prism, commonly anhedral
Hardness: 7
Cleavage: none
Twinning: Dauphiné (twin axis [0001]); Brazil-law {11-20}; Japan-law {11-22}
Color: colorless or white, sometimes black, pink, purple, orange, or green
Luster: vitreous or waxy
Streak: white
Alteration: none
In thin section…
ε: 1.553
ω: 1.544
Color: colorless
Birefringence (δ): 0.009 (first-order greys)
Relief: low
Optic sign: +
[Mindat]
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- Quartz var. Herkimer Diamond (a variety of very transparent quartz) from the Carrara quarries (Apuan Alps, Italy). Size: 7.8 x 5.3 x 3.7 cm. Photo by Robert M. Lavinsky (Irocks).
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- Aggregate of quartz crystals from Tibet. Photo by JJ Harrison.
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- Quartz crystals from Minas Gerais, Brazil. Size: 18×15 cm. Photo by Didier Descouens.
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- Quartz crystals with Japan law twinning. Note the long-axes intersecting at nearly 90°. Size: 4.5 x 3.7 x 1.2 cm. Itamo, Madagascar. Photo by kakov (Flickr).
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- Smoky Quartz from Lincoln County, New Mexico. The ‘smoky’ variety of quartz has grey to black appearance. Photo by Robert M. Lavinsky.
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- Quartz may show different colors due to the presence of impurities. The purple variety is called amethyst. Unknown locality. Photo by James St. John.
Field features
Quartz is colorless, lacks cleavage, and does not alter to secondary minerals. It is rare to find it in its euhedral form (i.e. bipyramidal prisms) and commonly occurs as anhedral or interstitial grains in igneous rocks or as polycrystalline layers in metamorphic rocks. In any case, it is easily identifiable because it appears transparent with grey to dark-grey color, showing a vitreous to greasy luster, and glass-like conchoidal fractures. The luster is vitreous on crystal faces, which are very reflective, and waxy or greasy on the irregular, conchoidal fractures. Massive quartz layers or veins can appear white and opaque, because they consist of inclusion-rich quartz, deformed quartz, or microcrystalline quartz, which are not transparent to light. The hardness of quartz on the Mohs scale is 7 (harder than metal) and, therefore, quartz can engrave a metal plate.
From left to right: two samples of massive quartz and a sample of quartz crystal. The transparency is very different but luster and fracture are similar. Note that crystal faces (euhedral sample on the right) are very reflective (vitreous). Photo by Marli Miller.
Quartz lacks cleavage planes and commonly develops conchoidal fractures with the characteristic waxy or greasy-like luster. Sample width: 11 cm. Photo by Siim Sepp (sandatlas.org).
Quartz in veins tends to appear massive, opaque and white. It is informally called milky quartz. The white color is related to the presence of abundant fluid inclusions. Width of sample 9 cm. Sample from Morocco (Anti Atlas). Photo by Siim Sepp (sandatlas.org).
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- Large quartz (gray) crystals in alkali feldspar (red) pegmatite. Nyelv, Norway. Width of view 50 cm. Photo © Siim Sepp.
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- Granite with abundant quartz (grey, transparent), alkali feldspar (transparent to whitish with cleavage planes and perthites), and plagioclase (white). The mafic minerals are biotite (black, metallic) and muscovite (metallic). Photo © Yannick Vandenberghe.
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- Coarse-grained granite with evident quartz (grey with greasy luster) surrounding reddish alkali feldspar and white plagioclase. The black mineral is amphibole. Photo © James St. John.
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- Sample of granite with gray, glassy crystals of quartz, pinkish crystals of alkali feldspar, whitish crystals of plagioclase, and black crystals of hornblende. Photo © James St. John.
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- Alkali feldspar granite, with red alkali feldspar, transparent grey quartz, and black amphibole and biotite. Åland archipelago, Finland. Photo © Hildegard Wilske.
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- Quartzolite dyke (consisting entirely of quartz) crosscutting biotite schist in the aureole rocks of the Calamita, Island of Elba, Italy. Photo: Samuele Papeschi/Geology is the Way.
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- Eolian sand from the Sahara (Erg Murzuk, Libya), consisting entirely of well rounded pure quartz grains. The orange hue is due to the presence of very fine-grained hematitic pigment. Width of view 15 mm. Photo by Siim Sepp (sandatlas.org).
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- Quartzite: a metamorphic rock consisting almost entirely of quartz derived from quartz-rich sandstone. Sample from Ireland. Photo by Siim Sepp (sandatlas.org)
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- This gneiss shows a well-defined foliation with crenulations. The dark bands are rich in biotite mica. The light-colored bands are dominated by quartz and feldspar. The small red crystals are garnet. Joshimath, Himalaya, northern India. Photo by James St. John.
Quartz in thin section
Quartz is transparent and colorless at PPL and shows first-order grey interference colors at CPL. It shows very low relief and lacks perfect cleavage (some rare, feeble cleavage planes may be present). All these features allow to easily distinguish quartz from feldspars and cordierite. Moreover, quartz is resistant to alteration and appears cleaner than feldspars, which, on the other hand, commonly alter to phyllosilicates. Quartz is length-slow and uniaxial positive.
In igneous rocks, it commonly forms anhedral grains i.e. interstitial quartz in plutonic rocks or resorbed grains in volcanic ones. In metamorphic rocks, it commonly occurs as more or less deformed polycrystalline layers.
Embayed quartz phenocrystals in a rhyolite. Note the clearness of quartz. San Vincenzo, Tuscany (Italy). PPL Image (field of view: 2 mm). Photo by Alessandro da Mommio (alexstrekeisen.it).
Embayed quartz phenocrystals in a rhyolite. Note the clearness of quartz. San Vincenzo, Tuscany (Italy). CPL Image (field of view: 2 mm). Photo by Alessandro da Mommio (alexstrekeisen.it).
Interstitial quartz associated with microcline (cross-hatched) in an alkali feldspar granite. CPL Image (field of view: 7 mm). Photo by Alessandro da Mommio (alexstrekeisen.it).
Quartz with foam structure in a metamorphic rock. CPL Image (field of view: 7 mm). Photo by Alessandro da Mommio (alexstrekeisen.it).
Metamorphic quartz grains with undulose extinction (light/dark bands) and lobate grain boundaries from a micaschist. CPL image. Field of view: 2.4 mm. Calamita Schists. Calamita, Isle of Elba, Italy. Photo: Samuele Papeschi/Geology is the Way.
Intensely recrystallized amoeboid quartz grains with lobate grain boundaries and abundant subgrains (i.e. patchy areas with different extinction). CPL image. Field of view: 1.4 mm. Calamita Schists. Calamita, Isle of Elba, Italy. Photo: Samuele Papeschi/Geology is the Way.
Occurrence
Quartz is the second most abundant mineral on the Earth’s crust after feldspars. In Igneous rocks it forms from the crystallized ‘excess’ silica that remains after the formation of feldspars in the latest stages of crystallization. Quartz is a rock-forming minerals of most acid and intermediate igneous rocks and may occur as accessory also in silica-oversaturated mafic rocks. It never occurs together with feldspathoids, which form in silica-undersaturated conditions.
Quartz is chemically and physically resistant to erosion and concentrates in terrigenous sedimentary rocks. Quartz-rich sandstones and conglomerates may indicate protracted transport and erosion, with consequent removal of feldspars and enrichment in quartz, or a quartz-rich source. In sedimentary environments, quartz may also form during diagenesis of siliceous rocks produced by the accumulation of shells of siliceous organisms.
Quartz is an important phase during metamorphism of quartz-bearing metasediments and igneous rocks: in the low-grade quartz may resist deformation and metamorphism but at higher metamorphic grade it tends to recrystallize. In metamorphic rocks, there are both quartz-forming and quartz-consuming reactions. Therefore, quartz may appear during the metamorphism of quartz-absent rocks (e.g. mafic rock) or be consumed entirely (e.g. in quartz-poor metapelites) by the growth of metamorphic minerals. Quartz is also a very common mineral in veins and mineral ores, occurring in many metasomatic rocks. In such settings, quartz frequently hosts fluid inclusions.
References and Further Reading
Frondel, C. (1945). Secondary Dauphiné twinning in quartz. American Mineralogist: Journal of Earth and Planetary Materials, 30(5-6), 447-460.
Gotze, J. (2009). Chemistry, textures and physical properties of quartz-geological interpretation and technical application. Mineralogical Magazine, 73(4), 645-671.
Krinsley, D. H., & Doornkamp, J. C. (2011). Atlas of quartz sand surface textures. Cambridge University Press.
Murray, R. C. (1957). Hydrocarbon fluid inclusions in quartz. AAPG Bulletin, 41(5), 950-952.
Sterner, S. M., Hall, D. L., & Keppler, H. (1995). Compositional re-equilibration of fluid inclusions in quartz. Contributions to Mineralogy and Petrology, 119(1), 1-15.
See also
Alexstrekeisen.it – Quartz
Sandatlas.org – Quartz
The Quartz Page
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