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Geology is the Way

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’.

Double terminated quartz (var. Herkimer Diamond) from Ace of Diamonds Mine, Town of Newport, New York, USA. Size: 4.3 x 3.6 x 3.0 cm. Photo by Robert M. Lavinsky.

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]

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).

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 Materials30(5-6), 447-460.
Gotze, J. (2009). Chemistry, textures and physical properties of quartz-geological interpretation and technical application. Mineralogical Magazine73(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 Bulletin41(5), 950-952.
Sterner, S. M., Hall, D. L., & Keppler, H. (1995). Compositional re-equilibration of fluid inclusions in quartz. Contributions to Mineralogy and Petrology119(1), 1-15.

        

See also
Alexstrekeisen.it – Quartz
Sandatlas.org – Quartz
The Quartz Page

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Mineral Properties
Minerals

 

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