Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold.
There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Europe and Asia.
Quartz is the mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material to abrasion.
Etymology
The word "quartz" is derived from the German word Quarz,[11] which had the same form in the first half of the 14th century in Middle High German and in East Central German[12] and which came from the Polish dialect term twardy, which corresponds to the Czech term tvrdý ("hard").[13] Some sources, however, attribute the word's origin to the Saxon word Querkluftertz, meaning cross-vein ore.[14][15]
The Ancient Greeks referred to quartz as κρύσταλλος (krustallos) derived from the Ancient Greekκρύος (kruos) meaning "icy cold", because some philosophers (including Theophrastus) understood the mineral to be a form of supercooled ice.[16] Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz.[17][18]
Early studies
Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time.[19] He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool the hands. This idea persisted until at least the 17th century. He also knew of the ability of quartz to split light into a spectrum.[20]
In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that regardless of a quartz crystal's size or shape, its long prism faces always joined at a perfect 60° angle.[21]
Crystal habit and structure
Crystal structure of α-quartz (red balls are oxygen, gray are silicon)
Crystal structure of β-quartz
A chiral pair of α-quartz
Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 °C (846 K; 1,063 °F). The ideal crystal shape is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end. In nature, quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive.[22][23]
Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples.[24] The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum.[25]
α-quartz crystallizes in the trigonal crystal system, space groupP3121 or P3221 (space group 152 or 154 resp.) depending on the chirality. Above 573 °C (846 K; 1,063 °F), α-quartz in P3121 becomes the more symmetric hexagonal P6422 (space group 181), and α-quartz in P3221 goes to space group P6222 (no. 180).[26]
These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked.[22][27] However, there is a significant change in volume during this transition,[28] and this can result in significant microfracturing in ceramics during firing,[29] in ornamental stone after a fire[30] and in rocks of the Earth's crust exposed to high temperatures,[31] thereby damaging materials containing quartz and degrading their physical and mechanical properties.
Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties.[32]
Fibrous, variously translucent, cryptocrystalline quartz occurring in many varieties. The term is often used for white, cloudy, or lightly colored material intergrown with moganite. Otherwise more specific names are used.
Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent and has often been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others.[33] These color differentiations arise from the presence of impurities which change the molecular orbitals, causing some electronic transitions to take place in the visible spectrum causing colors.
The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite.[34] Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, onyx, heliotrope, and jasper.[22]
Amethyst
Rock crystal
Amethyst
Blue quartz
Dumortierite quartz
Citrine quartz (natural)
Citrine quartz (heat-altered amethyst)
Milky quartz
Rose quartz
Smoky quartz
Prase
Amethyst is a form of quartz that ranges from a bright vivid violet to a dark or dull lavender shade. The world's largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, and Morocco. Sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine. Amethyst derives its color from traces of iron in its structure.[35]
Inclusions of the mineral dumortierite within quartz pieces often result in silky-appearing splotches with a blue hue. Shades of purple or gray sometimes also are present. "Dumortierite quartz" (sometimes called "blue quartz") will sometimes feature contrasting light and dark color zones across the material.[37][38] "Blue quartz" is a minor gemstone.[37][39]
Citrine
Citrine is a variety of quartz whose color ranges from pale yellow to brown due to a submicroscopic distribution of colloidal ferric hydroxide impurities.[40] Natural citrines are rare; most commercial citrines are heat-treated amethysts or smoky quartzes. However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smoky appearance. It is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness. Brazil is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul. The name is derived from the Latin word citrina which means "yellow" and is also the origin of the word "citron". Sometimes citrine and amethyst can be found together in the same crystal, which is then referred to as ametrine.[41] Citrine has been referred to as the "merchant's stone" or "money stone", due to a superstition that it would bring prosperity.[42]
Citrine was first appreciated as a golden-yellow gemstone in Greece between 300 and 150 BC, during the Hellenistic Age. Yellow quartz was used prior to that to decorate jewelry and tools but it was not highly sought after.[43]
Milky quartz
Milk quartz or milky quartz is the most common variety of crystalline quartz. The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation,[44] making it of little value for optical and quality gemstone applications.[45]
Rose quartz is a type of quartz that exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese in the material. Some rose quartz contains microscopic rutile needles that produce asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the quartz.[46]
Additionally, there is a rare type of pink quartz (also frequently called crystalline rose quartz) with color that is thought to be caused by trace amounts of phosphate or aluminium. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumford, Maine, US, and in Minas Gerais, Brazil.[47] The crystals found are more transparent and euhedral, due to the impurities of phosphate and aluminium that formed crystalline rose quartz, unlike the iron and microscopic dumortierite fibers that formed rose quartz.[48]
Smoky quartz
Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-gray crystal that is almost opaque. Some can also be black. The translucency results from natural irradiation acting on minute traces of aluminum in the crystal structure.[49]
Prase
Prase is a green variety of quartz.[50] The green color is caused by inclusions of amphibole.[51]
Prasiolite, also known as vermarine, is a variety of quartz that is green in color.[52] The green is caused by iron ions.[51] It is a rare mineral in nature and is typically found with amethyst; most "prasiolite" is not natural – it has been artificially produced by heating of amethyst. Since 1950[citation needed], almost all natural prasiolite has come from a small Brazilian mine, but it is also seen in Lower Silesia in Poland. Naturally occurring prasiolite is also found in the Thunder Bay area of Canada.[52]
While the majority of quartz crystallizes from molten magma, quartz also chemically precipitates from hot hydrothermalveins as gangue, sometimes with ore minerals like gold, silver and copper. Large crystals of quartz are found in magmatic pegmatites.[22] Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.[57]
The largest documented single crystal of quartz was found near Itapore, Goiaz, Brazil; it measured approximately 6.1 m × 1.5 m × 1.5 m (20 ft × 5 ft × 5 ft) and weighed over 39,900 kg (88,000 lb).[58]
Mining
Quartz is extracted from open pit mines. Miners occasionally use explosives to expose deep pockets of quartz. More frequently, bulldozers and backhoes are used to remove soil and clay and expose quartz veins, which are then worked using hand tools. Care must be taken to avoid sudden temperature changes that may damage the crystals.[59][60]
Pressure-temperature diagram showing the stability ranges for the two forms of quartz and some other forms of silica[61]
Tridymite and cristobalite are high-temperature polymorphs of SiO2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of SiO2 found in some meteorite impact sites and in metamorphic rocks formed at pressures greater than those typical of the Earth's crust. Stishovite is a yet denser and higher-pressure polymorph of SiO2 found in some meteorite impact sites.[62]Moganite is a monoclinic polymorph. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.[63]
Safety
As quartz is a form of silica, it is a possible cause for concern in various workplaces. Cutting, grinding, chipping, sanding, drilling, and polishing natural and manufactured stone products can release hazardous levels of very small, crystalline silica dust particles into the air that workers breathe.[64] Crystalline silica of respirable size is a recognized human carcinogen and may lead to other diseases of the lungs such as silicosis and pulmonary fibrosis.[65][66]
Synthetic and artificial treatments
A synthetic quartz crystal grown by the hydrothermal method, about 19 centimetres (7.5 in) long and weighing about 127 grams (4.5 oz)
Not all varieties of quartz are naturally occurring. Some clear quartz crystals can be treated using heat or gamma-irradiation to induce color where it would not otherwise have occurred naturally. Susceptibility to such treatments depends on the location from which the quartz was mined.[67]
Prasiolite, an olive colored material, is produced by heat treatment;[68] natural prasiolite has also been observed in Lower Silesia in Poland.[69] Although citrine occurs naturally, the majority is the result of heat-treating amethyst or smoky quartz.[68]Carnelian has been heat-treated to deepen its color since prehistoric times.[70]
Because natural quartz is often twinned, synthetic quartz is produced for use in industry. Large, flawless, single crystals are synthesized in an autoclave via the hydrothermal process.[71][22][72]
While jade has been since earliest times the most prized semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and the Middle East the different varieties of quartz were the most commonly used for the various types of jewelry and hardstone carving, including engraved gems and cameo gems, rock crystal vases, and extravagant vessels. The tradition continued to produce objects that were very highly valued until the mid-19th century, when it largely fell from fashion except in jewelry. Cameo technique exploits the bands of color in onyx and other varieties.
Efforts to synthesize quartz began in the mid-nineteenth century as scientists attempted to create minerals under laboratory conditions that mimicked the conditions in which the minerals formed in nature: German geologist Karl Emil von Schafhäutl (1803–1890) was the first person to synthesize quartz when in 1845 he created microscopic quartz crystals in a pressure cooker.[76] However, the quality and size of the crystals that were produced by these early efforts were poor.[77]
Elemental impurity incorporation strongly influences the ability to process and utilize quartz. Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing siliconwafers in the semiconductor industry, are expensive and rare. These high-purity quartz are defined as containing less than 50 ppm of impurity elements.[78] A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.[79] Quartz may also be found in Caldoveiro Peak, in Asturias, Spain.[80]
By the 1930s, the electronics industry had become dependent on quartz crystals. The only source of suitable crystals was Brazil; however, World War II disrupted the supplies from Brazil, so nations attempted to synthesize quartz on a commercial scale. German mineralogist Richard Nacken (1884–1971) achieved some success during the 1930s and 1940s.[81] After the war, many laboratories attempted to grow large quartz crystals. In the United States, the U.S. Army Signal Corps contracted with Bell Laboratories and with the Brush Development Company of Cleveland, Ohio to synthesize crystals following Nacken's lead.[82][83] (Prior to World War II, Brush Development produced piezoelectric crystals for record players.) By 1948, Brush Development had grown crystals that were 1.5 inches (3.8 cm) in diameter, the largest at that time.[84][85] By the 1950s, hydrothermal synthesis techniques were producing synthetic quartz crystals on an industrial scale, and today virtually all the quartz crystal used in the modern electronics industry is synthetic.[72]
Rock crystal jug with cut festoon decoration by Milan workshop from the second half of the 16th century, National Museum in Warsaw. The city of Milan, apart from Prague and Florence, was the main Renaissance centre for crystal cutting.[93]
Synthetic quartz crystals produced in the autoclave shown in Western Electric's pilot hydrothermal quartz plant in 1959
Fatimid ewer in carved rock crystal (clear quartz) with gold lid, c. 1000
Almost all the industrial demand for quartz crystal (used primarily in electronics) is met with synthetic quartz produced by the hydrothermal process. However, synthetic crystals are less prized for use as gemstones.[94] The popularity of crystal healing has increased the demand for natural quartz crystals, which are now often mined in developing countries using primitive mining methods, sometimes involving child labor.[95]
^ abcDeer, W. A.; Howie, R.A.; Zussman, J. (1966). An introduction to the rock-forming minerals. New York: Wiley. pp. 340–355. ISBN0-582-44210-9.
^Antao, S. M.; Hassan, I.; Wang, J.; Lee, P. L.; Toby, B. H. (1 December 2008). "State-Of-The-Art High-Resolution Powder X-Ray Diffraction (HRPXRD) Illustrated with Rietveld Structure Refinement of Quartz, Sodalite, Tremolite, and Meionite". The Canadian Mineralogist. 46 (6): 1501–1509. doi:10.3749/canmin.46.5.1501.
^Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (29 January 1990). "Quartz"(PDF). Handbook of Mineralogy. Vol. III (Halides, Hydroxides, Oxides). Chantilly, VA: Mineralogical Society of America. ISBN0962209724. Archived(PDF) from the original on 1 April 2010. Retrieved 21 October 2009.
^Morgado, Antonio; Lozano, José Antonio; García Sanjuán, Leonardo; Triviño, Miriam Luciañez; Odriozola, Carlos P.; Irisarri, Daniel Lamarca; Flores, Álvaro Fernández (December 2016). "The allure of rock crystal in Copper Age southern Iberia: Technical skill and distinguished objects from Valencina de la Concepción (Seville, Spain)". Quaternary International. 424: 232–249. Bibcode:2016QuInt.424..232M. doi:10.1016/j.quaint.2015.08.004.
^Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. p. 205. ISBN9780195106916.
^Tutton, A.E. (1910). "Rock crystal: its structure and uses". RSA Journal. 59: 1091. JSTOR41339844.
^Nicolaus Steno (Latinized name of Niels Steensen) with John Garrett Winter, trans., The Prodromus of Nicolaus Steno's Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid (New York, New York: Macmillan Co., 1916). On page 272Archived 4 September 2015 at the Wayback Machine, Steno states his law of constancy of interfacial angles: "Figures 5 and 6 belong to the class of those which I could present in countless numbers to prove that in the plane of the axis both the number and the length of the sides are changed in various ways without changing the angles; … "
^Knapek, Michal; Húlan, Tomáš; Minárik, Peter; Dobroň, Patrik; Štubňa, Igor; Stráská, Jitka; Chmelík, František (January 2016). "Study of microcracking in illite-based ceramics during firing". Journal of the European Ceramic Society. 36 (1): 221–226. doi:10.1016/j.jeurceramsoc.2015.09.004.
^Rickwood, P. C. (1981). "The largest crystals"(PDF). American Mineralogist. 66: 885–907 (903). Archived(PDF) from the original on 25 August 2013. Retrieved 7 March 2013.
^McMillen, Allen. "Quartz Mining". Encyclopedia of Arkansas. Central Arkansas Library System. Retrieved 28 November 2020.
^Groman-Yaroslavski, Iris; Bar-Yosef Mayer, Daniella E. (June 2015). "Lapidary technology revealed by functional analysis of carnelian beads from the early Neolithic site of Nahal Hemar Cave, southern Levant". Journal of Archaeological Science. 58: 77–88. Bibcode:2015JArSc..58...77G. doi:10.1016/j.jas.2015.03.030.
^Walker, A. C. (August 1953). "Hydrothermal Synthesis of Quartz Crystals". Journal of the American Ceramic Society. 36 (8): 250–256. doi:10.1111/j.1151-2916.1953.tb12877.x.
^von Schafhäutl, Karl Emil (10 April 1845). "Die neuesten geologischen Hypothesen und ihr Verhältniß zur Naturwissenschaft überhaupt (Fortsetzung)" [The latest geological hypotheses and their relation to science in general (continuation)]. Gelehrte Anzeigen. 20 (72). München: im Verlage der königlichen Akademie der Wissenschaften, in Commission der Franz'schen Buchhandlung: 577–584. OCLC1478717. From page 578: 5) Bildeten sich aus Wasser, in welchen ich im Papinianischen Topfe frisch gefällte Kieselsäure aufgelöst hatte, beym Verdampfen schon nach 8 Tagen Krystalle, die zwar mikroscopisch, aber sehr wohl erkenntlich aus sechseitigen Prismen mit derselben gewöhnlichen Pyramide bestanden. ( 5) There formed from water in which I had dissolved freshly precipitated silicic acid in a Papin pot [i.e., pressure cooker], after just 8 days of evaporating, crystals, which albeit were microscopic but consisted of very easily recognizable six-sided prisms with their usual pyramids.)
^Nacken, R. (1950) "Hydrothermal Synthese als Grundlage für Züchtung von Quarz-Kristallen" (Hydrothermal synthesis as a basis for the production of quartz crystals), Chemiker Zeitung, 74 : 745–749.
^Brush Development's team of scientists included: Danforth R. Hale, Andrew R. Sobek, and Charles Baldwin Sawyer (1895–1964). The company's U.S. patents included:
Sobek, Andrew R. "Apparatus for growing single crystals of quartz", U.S. patent 2,674,520; filed: 11 April 1950; issued: 6 April 1954.
Sobek, Andrew R. and Hale, Danforth R. "Method and apparatus for growing single crystals of quartz", U.S. patent 2,675,303; filed: 11 April 1950; issued: 13 April 1954.
Sawyer, Charles B. "Production of artificial crystals", U.S. patent 3,013,867; filed: 27 March 1959; issued: 19 December 1961. (This patent was assigned to Sawyer Research Products of Eastlake, Ohio.)
^Pierce, G. W. (1923). "Piezoelectric crystal resonators and crystal oscillators applied to the precision calibration of wavemeters". Proceedings of the American Academy of Arts and Sciences. 59 (4): 81–106. doi:10.2307/20026061. hdl:2027/inu.30000089308260. JSTOR20026061.
^Pierce, George W. "Electrical system", U.S. patent 2,133,642, filed: 25 February 1924; issued: 18 October 1938.
^The International Antiques Yearbook. Studio Vista Limited. 1972. p. 78. Apart from Prague and Florence, the main Renaissance centre for crystal cutting was Milan.
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![Rock crystal jug with cut festoon decoration by Milan workshop from the second half of the 16th century, National Museum in Warsaw. The city of Milan, apart from Prague and Florence, was the main Renaissance centre for crystal cutting.[93]](File:Milan_Jug_with_cut_festoon_decoration.jpg)
