Description
Quartz is a form of silicon dioxide, SiO2, and is one of the most common minerals in the Earth's crust. It represents more than 12% of the Earth's crust by volume (found in granite and sand, among other places). Quartz has two modifications: α-quartz is trigonal, and β-quartz (a higher-temperature form) is hexagonal. Silicon dioxide also occurs in many other structures, such as α- and β-cristobalite, keatite, tridymite, coesite, stishovite, melangophlogite, and lechatelierite. Although quartz consists of silicon dioxide, it is often classified as a tectosilicate. This is because quartz forms a network of SiO4 tetrahedra, with each oxygen atom shared by two silicon atoms. Individual SiO2 molecules only occur in gaseous silicon dioxide. Quartz can be very pure, as in rock crystal, but often contains impurities. These impurities can consist of ions that are incorporated into the crystal lattice during crystal growth. Quartz crystals can contain between 13 and 15,000 ppm Al3+, between 9 and 1,400 ppm Na+, between 3 and 300 ppm K+, and traces of Fe3+, Ti4+, P5+, H+, and Li+. Aluminum and iron, in combination with ionizing radiation, are responsible for the color of some quartz varieties. Furthermore, quartz crystals can grow over other minerals, as is the case with rutile quartz. Quartz can form both large crystals and microscopic aggregates. The largest quartz crystals are found at Verloren Farm in Namibia. Quartz crystals up to 20 meters, and possibly 50 meters, and dolomite crystals up to 2 meters are found there. When quartz is melted, it often forms a glass upon rapid cooling, as the crystallization process takes a considerable time. Quartz is a piezoelectric material and the vibrations that can be induced in the crystal lattice are used in electronic equipment such as quartz clocks and radios.
The mineral group epidote consists of a number of sorosilicates, the most important of which is the mineral epidote. The mineral epidote is a calcium-iron-aluminum-silicate with the chemical formula Ca2(Fe3+,Al)Al2(SiO4)(Si2O7)O(OH).
The yellow to brownish-green, grey, or black epidote has a vitreous luster, a greyish-white streak colour, and the cleavage is perfect according to crystal level [001]. The crystal system is monoclinic. The average density is 3.45 and the hardness is 7. Epidote is not radioactive. The name epidote is derived from the Greek epidosis, which means “addition”. Epidote is a very common mineral worldwide, particularly in contact metamorphic rocks. It is formed in crystalline limestone and in schists. Beautiful epidote crystals are found particularly in the Knappenwand, near the Großvenediger in the Untersulzbachthal, Salzburg (Austria). Epidote is common in the sand fraction of Dutch Quaternary river sediments. In heavy mineral analysis, as performed in the Netherlands by the Geological Survey during the second half of the twentieth century, the mineral is classified in the so-called unstable group.
Fortune telling with quartz ball and quartz from the Coleman mines.
