Mineral - Quartz

Name and first discovery: Formerly known as crystal and named after the Greek word "kruos" for "ice-cold" due to its ice-like appearance. Quartz can be traced back in records 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". One theory also assumes that the word "quartz" originated in the miner's language from a mixture of the words "Querkluft" and "Erz".

Synonyms: α-quartz, deep quartz, quartz, acetulite, conite, lodolite

Polymorphism

In mineralogy, quartz is usually referred to as the most stable and most frequently occurring α-quartz on the surface, also known as deep quartz. However, silicon dioxide (_SiO2) is a polymorphic compound, meaning that it occurs at certain temperatures and pressures in different crystal structures (modifications), with different crystal forms and properties.

The only really stable form is α-quartz, but other modifications can also be metastable on the earth's surface. α-quartz (deep quartz) is trigonal, each silicon ion is surrounded tetrahedrally by four oxygen ions and thus forms a _SiO4 tetrahedron. However, as the smallest repeating formula unit is a silicon ion with 2 oxygen ions, it is referred to as _SiO2. There is a strong covalent bond between silicon and oxygen, which explains the high hardness of quartz. The _SiO4 tetrahedra are linked to each other via the tetrahedron corners, each tetrahedron with four neighbouring tetrahedra. They thus form spiral chains in the direction of the c-axis. This spiral chain structure explains the enantiomorphism of α-quartz. Depending on the direction of rotation of the tetrahedral screws, a left-handed (clockwise tetrahedral screws) or a right-handed (anti-clockwise tetrahedral screws) deep quartz is present.

At a temperature of over 573°C (at a pressure of 1 bar), α quartz (low quartz) transforms into hexagonal β quartz (high quartz). This increases the volume by 0.8%, but when it cools down, deep quartz is immediately formed again, which means that there can be no β-quartz on the earth's surface. Properties such as piezoelectricity or enantiomorphism are lost in β-quartz (high quartz). In high quartz, the SiO4 _tetrahedra are slightly tilted; when cooled to low quartz, the tilting direction determines the orientation of the low quartz. In contrast to the crystal structure, the crystal form of hexagonal β-quartz can be preserved as paramorphosis (low quartz paramorphous to high quartz).

At a temperature of over 870°C (normal pressure), hexagonal β-quartz transforms into hexagonal β-tridymite (HP-tridymite). It consists of equal layers of SiO4 _tetrahedra arranged in hexagonal rings. These layers are stacked on top of each other in an ABAB sequence, leaving continuous tunnels. At temperatures below 130ºC, β-tridymite transforms into a metastable, orthorhombic modification, the α-tridymite. Over time, this transforms into the stable deep quartz. There are also 5 other modifications of tridymite that exist at different temperatures.

At a temperature of over 1,470°C (normal pressure), hexagonal β-tridymite transforms into cubic β-cristobalite (high cristobalite). The structure is very similar to the diamond or ZnS structure. From temperatures of approx. 240-275 °C, β-cristobalite transforms into tetragonal α-cristobalite, which is metastable on the earth's surface. Here, too, cubic paramorphoses are found (α-cristobalite paramorphous to β-cristobalite).

At temperatures of 450 to 800 °C and pressures of over 2.5-3.8 GPa, β-quartz transforms into monoclinic coesite. This corresponds to a depth of approx. 70 km below the earth's surface. Coesite is therefore often an indicator of an ultra-high pressure (UHP) metamorphic event. This can be a meteorite impact or a continent-continent collision, for example. Coesite can also reach the earth's surface from great depths through kimberlite pipes. Coesite is metastable on the surface and transforms relatively quickly into deep quartz. Due to the strong increase in volume, radial cracks in the quartz grain can indicate such a transformation. In rare cases, quartz grains of UHP rocks still contain coesite nuclei.

Tetragonal stishovite is formed at pressures above 8 GPa. In contrast to the low-pressure modifications of quartz, the silicon in stishovite is bound to six oxygen atoms, resulting in a very compact structure (Mohs hardness 9). While low-pressure quartz has a density of approx. 2.65 g/cm3, stishovite has a density of 4.32 g/cm3. Stishovite is only formed by impact metamorphism and is metastable on the earth's surface.

In addition, there is only the orthorhombic seifertite, which is formed at pressures of over 40 GPa. So far, seifertite has only been found in Martian meteorites, where the mineral was formed during the impact on Mars. According to calculations, however, if enough free _SiO2 is available, seifertite could also form in our earth's mantle at depths of 1,700 kilometres.

At particularly low temperatures, there are other modifications of _SiO2. In addition, silicon dioxide is able to absorb crystal water into its structure at low pressures and temperatures. These modifications therefore crystallise from a _SiO2 gel, also known as silica gel. A well-known example is the monoclinic α-mogánite (_SiO2-nH2O). Above approx. 200°C, this can also transform into orthorhombic β-mogánite (_SiO2). α-mogánite transforms into α-quartz at the earth's surface over geological time periods. Organic substances such as methane can also be incorporated into the silica or silica gels, e.g. in chibaite [SiO2 - n(_CH4, _C2H6, _C3H8, _i-C4H10)]. The melting of quartz sand, e.g. by lightning strikes, can also produce amorphous _SiO2 minerals, such as lechatelierite (it is questionable whether this is really a mineral).

Quartz varieties

The most stable and best-known _SiO2 modification discussed here, deep quartz, occurs in different varieties. A distinction is made between macrocrystalline and microcrystalline or cryptocrystalline quartz.

Macrocrystalline quartz varieties

Rock crystal:

Milky quartz:

Amethyst:

Smoky quartz:

Morion:

Rose quartz:

Citrine:

Iron pebble:

Tiger's eye/falcon's eye:

Prasem:

Prasiolite:

Blue quartz:

Aventurine:

Ametrine:

Micro- or cryptocrystalline quartz varieties

Chalcedony:

Amorphous quartz

Habitus

Twins

Growth Forms

Abbreviation: Qz

Famous locations

(⊗) Locations with top levels (TL) Type locality(analysis) Chemical element distribution, see above