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 Al 3+ , Na + , Li + , K + , Ti 4+ , and Fe 3+ . 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 ; Japan-law
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]
[HoM]
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.
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.
Quartz from deformed mylonitic pegmatite
This pegmatite was deformed in a shear zone after its crystallization, likely at temperatures between 500 and 700 °C. Quartz shows several microstructures indicative of high temperature grain boundary migration recrystallization [GBM-recrystallization page].
Sample: mylonitic pegmatite
Assemblage: muscovite, quartz, alkali feldspar, tourmaline, andalusite
Locality: Fosso del Pontimento, Calamita, Island of Elba, Italy
Sample courtesy Giovanni Musumeci
