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Quartz is a silicate mineral—composed of silicon and oxygen—that is in abundance within the Earth’s crust. It can be found naturally as lumps of the just the mineral itself, or it can be present in a number of rocks—with the most common and well-known example being in granite (alongside feldspar and mica). While it is one of the most well-known minerals, this article is dedicated to looking at the different characteristics and properties which quartz has.
Quartz can be found in almost every mineral environment and can also be found within all three rock classes (sedimentary, igneous and metamorphic). Chemically, quartz is only composed of silicon and oxygen atoms and has a chemical formula of SiO2. At the atomic level, the silicon and oxygen atoms are arranged in a continuous framework of tetrahedra, where four oxygen atoms surround a single silicon atom, but the oxygen atoms are located between two tetrahedron (i.e. the oxygen are utilized in two tetrahedron, hence the molecular formula). The tetrahedron arrange themselves into a 3D network by forming helical structures. These helical structures take two common forms, which are threefold and sixfold helices, and six helices (regardless of the type) join together to form ‘channels’ through the mineral.
There are two different structural forms of quartz, which are known as α-quartz and β-quartz. While both forms are chiral (even though the individual tetrahedrons are achiral), α-quartz adopts a trigonal crystal structure, whereas β-quartz adopts a hexagonal crystal structure. Β-quartz is known as the high temperature form of quartz, and the atomic structure rearranges easily—via a rotation of the tetrahedra—once α-quartz is heated up to 573 °C. Quartz can also form left-handed and right-handed crystals, depending on which way the helices turn during formation, and the morphology and crystal structures of each handed form are mirror opposites of the other form.
There are many different aspects and properties to a mineral, from the color, to its hardness, to how its layers cleave under stress, and more. Quartz can be many different colors. In its purest form, it is a clear or white colour, but different impurities within the atomic lattice can cause the color to change to purple, pink, brown, black, gray, green, orange, yellow, blue, or red. In some cases, the color can be banded or multiple colors depending on the impurity levels. For example, one common form of quartz that has impurities is amethyst, where the presence of iron or other transition metal ions causes the color to turn purple/lilac. Therefore, depending on whether a specimen is pure or not, it’s transparency can range from transparent to opaque.
Quartz also has a vitreous lustre, which means that it interacts with light in the same way that glass does. In terms of whether you can see visible crystals, it depends on how transparent the quartz sample is. Transparent quartz samples tend to be macrocrystalline and the different crystals can be seen with the human eye. However, translucent and opaque samples are cyptocrystalline, and the crystals are only just about visible with a microscope, never mind the human eye. Quartz also has a white streak (i.e. the colour of the mineral when it is in a powdered form).
In terms of hardness, quartz is relatively hard for a mineral, and has a Moh’s scale hardness of 7 (out 10 – with diamond being a 10). Quartz is also very stable to high temperatures and harsh chemicals, with it requiring very strong acids, such as hydrofluoric acid, to dissolve it. Many other minerals will dissolve in weaker acids. However, quartz is very brittle, but because it does not exhibit cleavage, it is relatively strong to mechanical stress. It is, however, susceptible to conchoidal (rounded and shell-like) fractures and some of the crystal faces can exhibit striations—which are a series of parallel marks/grooves in the crystal face.
Quartz also exhibits a number of other properties. Quartz has a specific gravity between 2.6 and 2.7 depending on the type of quartz, and it can polarize any light which passes through it (how it polarizes the light depends on the handedness of the helices), with a birefringence of up to 0.009. Quartz is also piezoelectric, so it can generate an electrical current under an applied stress, as well as being triboluminescent (the generation of light when it is crushed or damaged), and some forms can also exhibit fluorescence.
Sources and Further Reading
Quartz - Geology.com
The Mineral Quartz - Minerals.net
Quartz - Geology Science
Properties of Quartz and Glass - Lumen
The Mineral Quartz - Galleries.com
Quartz - University of Minnesota
Quartz - Mindat