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Name and First Discovery : Formerly referred to as crystal, quartz was named after the Greek word "kruos" meaning "ice-cold" due to its ice-like appearance. Records of quartz date back to 300-325 BC. One of the first scientists to use the word "quartz" was the German physician Georgius Agricola (1494-1555), also known as the "father of mineralogy." Another theory suggests that the word "quartz" originated in mining terminology from a blend of the words "cross-valley" and "ore."
Synonyms : α-quartz, low quartz, quartz, acetulite, conite, lodolite
In mineralogy, quartz usually refers to α-quartz, also called low quartz, which is the most stable and abundant form found at the surface. Silicon dioxide ( SiO₂ ), however, is a polymorphic compound, meaning it exists in different crystal structures (modifications) with varying crystal forms and properties at specific temperatures and pressures.
The only truly stable form is α-quartz , although other modifications can also exist metastably at the Earth's surface. α-quartz (low quartz) is trigonal; each silicon ion is tetrahedrally surrounded by four oxygen ions, thus forming a SiO₄ tetrahedron. Since the smallest repeating formula unit is a silicon ion with two oxygen ions, it is called SiO₂ . A strong covalent bond exists between silicon and oxygen, which explains the high hardness of quartz. The SiO₄ tetrahedra are linked to each other via the tetrahedral vertices, each tetrahedron to four neighboring tetrahedra. Along the c-axis, they thus form helical chains. This helical chain structure explains the enantiomorphism of α-quartz. Depending on the direction of rotation of the tetrahedral screws, the quartz crystal is either left-handed (tetrahedral screws clockwise) or right-handed (tetrahedral screws counterclockwise).
At temperatures above 573°C (at a pressure of 1 bar), α-quartz (low quartz) transforms into hexagonal β-quartz (high quartz). During this transformation, the volume increases by 0.8%, but upon cooling, low quartz is immediately regenerated, which is why β-quartz cannot exist at the Earth's surface. Properties such as piezoelectricity and enantiomorphism are lost in β-quartz (high quartz). In high quartz, the SiO₄ tetrahedra are slightly tilted; upon cooling to low quartz, the direction of this tilt determines the orientation of the low quartz. In contrast to the crystal structure, the crystal form of hexagonal β-quartz can be preserved as a paramorph (low quartz paramorph after high quartz).
At temperatures above 870°C (standard pressure), hexagonal β-quartz transforms into hexagonal β-tridymite (HP-tridymite). It consists of identical layers of SiO₄ tetrahedra arranged in hexagonal rings. These layers are stacked in an ABAB sequence, leaving continuous tunnels. β-tridymite transforms into a metastable, orthorhombic modification, α-tridymite , at temperatures below 130°C. This eventually transforms into stable low-temperature quartz. In addition, there are five other modifications of tridymite that exist at different temperatures.
At temperatures above 1,470°C (standard pressure), hexagonal β-tridymite transforms into cubic β-cristobalite (high cristobalite). Its structure is very similar to that of diamond or ZnS. At temperatures of approximately 240–275°C, β-cristobalite transforms into the metastable tetragonal α-cristobalite found at the Earth's surface. Cubic paramorphs are also observed in this form (α-cristobalite paramorph after β-cristobalite).
At temperatures of 450 to 800 °C and pressures exceeding 2.5–3.8 GPa, β-quartz transforms into monoclinic coesite . This corresponds to a depth of approximately 70 km below the Earth's surface. Coesite is therefore often an indicator of an ultra-high-pressure metamorphic (UHP) event. This could be, for example, a meteorite impact or a continent-continent collision. Coesite can also be brought to the Earth's surface from great depths by kimberlite pipes. At the surface, coesite is metastable and transforms relatively quickly into low-level quartz. Due to the significant increase in volume, radial cracks in the quartz grain can indicate such a transformation. In rare cases, quartz grains from UHP rocks still contain coesite cores.
At pressures above 8 GPa, tetragonal stishovite forms. Unlike low-pressure quartz, in stishovite the silicon is bonded to six oxygen atoms, resulting in a very compact structure (Mohs hardness 9). While low-pressure quartz has a density of approximately 2.65 g/cm³, stishovite has a density of 4.32 g/cm³. Stishovite forms only through impact metamorphism and is metastable at the Earth's surface.
Beyond this, only orthorhombic seifertite exists, which forms at pressures above 40 GPa. So far, seifertite has only been found in Martian meteorites, where the mineral formed during the impact on Mars. However, calculations suggest that seifertite could also form in Earth's mantle at depths of 1,700 km if enough free SiO₂ is available.
At particularly low temperatures, further modifications of SiO₂ exist. Furthermore, silicon dioxide is able to incorporate water of crystallization into its structure at low pressures and temperatures. Therefore, these modifications crystallize from a SiO₂ gel, also known as silica gel. A well-known example is monoclinic α-moganite ( SiO₂ · nH₂O ). Above approximately 200°C, this can also transform into orthorhombic β-moganite ( SiO₂ ). α-Moganite transforms into α-quartz at the Earth's surface over geological timescales. In addition , organic substances such as methane can be incorporated into the silica or silica gels, for example in chibaite [ SiO₂ · n( CH₄ , C₂H₆ , C₃H₈ , iC₄H₁₀ )]. The melting of quartz sand, e.g. by lightning strikes, can also create amorphous SiO2 minerals, such as lechatelierite (it is questionable whether it is actually a mineral).
The most stable and best-known SiO₂ modification discussed here, low quartz, occurs in various varieties. A distinction is made between macrocrystalline and micro- or cryptocrystalline quartz.
Macrocrystalline quartz varieties
Rock crystal: Rock crystal is the completely colorless and transparent variety of macrocrystalline quartz without discoloring impurities or lattice defects.
Milky quartz: The characteristic turbidity of milky quartz is caused by dense hydrothermal fluid and gas inclusions in the submicroscopic range.
Amethyst: The violet color of amethyst is caused by the incorporation of trivalent iron ions (Fe³⁺) onto silicon positions that have been oxidized by ionizing radiation.
Smoky quartz: Smoky quartz owes its brown to smoky grey coloration to artificial or natural lattice defects at aluminium centers that have been activated by radioactive irradiation.
Morion: Morion refers to a deep black, almost opaque variety of smoky quartz with an extremely high density of radiation-induced color centers.
Rose quartz: The pink color of rose quartz is primarily based on nanometer-fine exsolutions of a fibrous silicate mineral of the dumortierite group in the crystal structure.
Citrine: The yellow to orange color of true citrine results from submicroscopically fine inclusions of iron oxides or specific defect centers caused by Fe³⁺ ions.
Iron silica: Iron silica is a variety of quartz that is intensely red, yellow or brown colored by microcrystalline inclusions of hematite or limonite.
Hyacinth: Historically and mineralogically, hyacinth refers to a reddish-brown variety of quartz, the color of which is mostly due to iron oxide infiltration.
Jacinto de Compostela: This variety refers to idiomorphic, intensely brick-red colored authigenic quartz from Spain, due to hematite inclusions.
Tiger's eye: Tiger's eye is a pseudomorphic product of quartz after crocidolite, whose golden-brown color and chatoyancy (cat's eye effect) are based on the oxidation of embedded iron fibers.
Hawk's eye: Hawk's eye represents the bluish-grey precursor of tiger's eye, in which the fibrous crocidolite inclusions are present unoxidized in the quartz matrix.
Ametrine: Ametrine is a rare crystal zone quartz in which amethyst and citrine regions coexist due to different oxidation states of the iron in the same crystal.
Prasem: Prasem gets its characteristic leek-green color from dense, felt-like inclusions of minerals from the actinolite or chlorite group.
Prasiolite: Prasiolite is a leek-green quartz that is produced almost exclusively by burning (heating) certain amethysts or yellowish quartzes.
Blue quartz: The blue color of this quartz is based on the Rayleigh scattering of light by the finest, needle-like inclusions of minerals such as crocidolite or rutile.
Aventurine: Aventurine quartz exhibits a glittering shimmer (aventurescence) caused by light reflection off embedded platelets of fuchsite (green) or hematite (reddish-brown).
Rutilated quartz: Rutilated quartz is a rock crystal or smoky quartz that is permeated by macroscopically visible, needle-like inclusions of the titanium suboxide rutile.
Macrocrystalline quartz varieties
Chalcedony: Chalcedony is the fibrous, cryptocrystalline modification of silicon dioxide, which structurally consists of an intergrowth of quartz and moganite.
Agate: Agate is a rhythmically or concentrically banded chalcedony, whose different color zones are created by periodic deposition and trace elements.
Moss agate: Moss agate is a colorless to whitish chalcedony characterized by green, moss-like fine structures made up of chlorite or hornblende inclusions.
Snakeskin agate: This variety exhibits a wrinkled, scaly surface resulting from surface weathering, dehydration, or silica rearrangement.
Fire agate: Fire agate is a botryoidal chalcedony whose opalescent play of colors is due to thin-film interferences on microscopic hematite layers.
Grape agate: Grape agate (botryoidal chalcedony) consists of grape-like aggregated spherulites of cryptocrystalline quartz, often of violet or green color.
Rainbow agate: Under strong backlighting, rainbow agate displays an iridescent play of colors, caused by light diffraction at extremely fine, regular chalcedony layers.
Cloud agate: Cloud agate refers to a chalcedony with irregular, diffuse and nebulous white or grey distributions within a more transparent matrix.
Carnelian: Carnelian is a flesh-red to orange-brown chalcedony, whose color is caused by finely distributed inclusions of iron oxide (hematite).
Heliotrope: Heliotrope (bloodstone) is a dark green chalcedony or jasper that has punctate, intensely red inclusions of hematite.
Chrysoprase: Chrysoprase is the most valuable variety of chalcedony, whose apple-green color is due to embedded, nickel-containing layered silicates such as kerolite.
Sard: Sard is a brown to chestnut-brown variant of chalcedony, which owes its color to a higher hydration level of embedded iron oxides (limonite).
Onyx: Onyx is a variety of agate with parallel, perfectly flat and strictly two-colored – mostly black and white – chalcedony layers.
Pietersite: Pietersite is a breccia rock consisting of silicified, mechanically fractured and re-cemented fibers of crocidolite and toasite.
Jasper: Jasper is a fine-grained to cryptocrystalline quartz that is completely opaque due to high impurities of foreign minerals (such as clay or iron oxides).
Crystal growth forms and structural varieties
Skeletal quartz: Skeletal quartz is formed by preferential edge growth under high supersaturation, which leaves the crystal faces incomplete and creates cavities.
Phantom quartz: Phantom quartz documents growth interruptions in which thin mineral layers were deposited on older crystal faces before the quartz continued to grow purely.
Window quartz: Window quartz is a subspecies of skeletal quartz in which the crystal faces have window-like depressions framed by raised edges.
Scepter quartz: A scepter quartz is formed when a younger generation of quartz grows epitaxially and usually wider on the tip of an older, stem-like quartz crystal.
Thread quartz: Thread quartz forms in tectonic fractures, with a white, fluid-filled "thread" marking the continuous rupture and synantetic healing of the crystal.
Gwindel: A gwindel is a parallel or subparallel grown quartz whose crystals exhibit a regular, helical torsion along a vertical axis.
Herkimer: Herkimer “diamonds” are short prismatic, biterminated (double-ended) rock crystals of high transparency from hydrothermal geodes.
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