A yellow to yellow-orange or yellow-green variety of quartz.
The cause of the color is still under debate. At least some citrine is colored by aluminum-based and irradiation-induced color centers related to those found in smoky quartz (Lehmann, 1972; Maschmeyer et al., 1980). Accordingly, transitions to smoky quartz ("smoky citrine") exist, many citrines show smoky phantoms. Like smoky quartz, these types of citrines pale when heated above 200-500°C and turn yellow again when irradiated (Lehmann, 1970). There appear to be at least two types of yellow Al-based color centers with different thermal stability (Schmetzer, 1988). Since the yellow color centers are often more stable than the smoky color centers, some smoky quartz can be turned into citrine by careful heating (Nassau and Prescott, 1977). Natural citrine as well as citrine produced by heating smoky quartz is dichroic in polarized light.
It has also been suggested that iron is the cause of color, as artificial crystals grown in a iron-bearing solution turn out yellow. However, the dichroic behavior of the lab-grown crystals differs from that in natural citrine (Rossmann, 1994).
Note: Natural citrine is very rare. Large quantities of amethyst, usually of lesser quality, are heated to turn it yellow or orange and sold as "citrine." Because the color is now caused by finely distributed iron minerals (mostly hematite and goethite), heated amethyst is not citrine in the strict sense, and also shows no dichroism in polarized light.
Thin coatings of iron oxides on colourless quartz, as well as inclusions of yellow iron oxides ("limonite"), may simulate citrine.
Quartz colored by inclusions, or coatings, of any kind is not called citrine.
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Classification of Citrine
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Physical Properties of Citrine
Crystallography of Citrine
Other Names for Citrine
References for Citrine
Dana, 1892, 187 - "Syst. Min. 6th Ed"
Chudoba, K. F. (1962) Some relations between the causes of amethyst, smoky quartz, and citrine colors as given by modern science. Mineralogicheskii Sbornik (Lvov): 16: 91-105.
Samoilovich, M.I., Tsinober, L.I., Kreishop, V.N. (1969) The nature of radiation-produced citrine coloration in quartz. Soviet Physics-Crystallography: 13: 626-628.
Lehmann, G. (1972) Yellow color centers in natural and synthetic quartz. Physik der Kondensierten Materie: 13: 297-306.
Nassau, K., Prescott, B.E. (1977) Smoky, blue, greenish yellow, and other irradiation-related colors in quartz. Mineralogical Magazine: 41: 301-312.
Maschmeyer, D., Niemann, K., Hake, K., Lehmann, G., Räuber, A. (1980) Two modified smoky quartz centres in natural citrine. Physics and Chemistry of Minerals: 6: 145-156
Schmetzer, K. (1988) Thermal stability of yellow color centers in natural citrine. Neues Jahrbuch für Mineralogie, Monatshefte: 2: 71-80.
Rossman, G.R. (1994) Colored varieties of the silica minerals. In: Reviews in Mineralogy, Vol.29, Silica - Physical behavior, geochemistry and materials applications, Mineralogical Society of America.
Pan, Y., Nilges, M.J., Mashkovtsev, R.I. (2008) Radiation-induced defects in quartz. II. Single-crystal W-band EPR study of a natural citrine quartz. Physics and Chemistry of Minerals: 35: 387-397.