AskDefine | Define chalcedony

Dictionary Definition

chalcedony n : a milky or grayish translucent to transparent quartz [syn: calcedony]

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English

Noun

  1. A form of fine-grained quartz that is transparent or has a milky translucence and exhibits a conchoidal fracture.

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Extensive Definition

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, whilst moganite is monoclinic.
Chalcedony has a waxy lustre, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black.

Varieties

Chalcedony (kal-SED-uh-nee) occurs in a wide range of varieties. Many semi-precious gemstones are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows:

Agate

Agate is a variety of chalcedony with concentric banding. Agate with black and white banding is known as onyx.

Carnelian

Carnelian (also spelled cornelian) is a clear-to-translucent reddish-brown variety of chalcedony. Its shade may vary from a pale orange, to an intense almost-black coloration. Similar to carnelian is sard, which is generally harder and darker.

Chrysoprase

Chrysoprase (also spelled chrysophrase) is green variety of chalcedony, which has been colored by nickel oxide. (The darker varieties of chrysoprase are also referred to as prase. However, the term prase is also used to describe green quartz, and to a certain extent is a color-descriptor, rather than a rigorously defined mineral variety.)

Heliotrope

Heliotrope is a green variety of chalcedony, containing red inclusions of iron oxide. These inclusions resemble drops of blood, giving heliotrope its alternative name of bloodstone. A similar variety, in which the spots are yellow instead of red is known as plasma.

Moss agate

Moss agate (also known as tree agate or mocha stone) contains green filament-like inclusions, giving it the superficial appearance of moss or blue cheese. It is not a true form of agate, as it lacks agate's defining feature of concentric banding.

History

As early as the Bronze Age chalcedony was in use in the Mediterranean region; for example, on Minoan Crete at the Palace of Knossos chalcedony seals have been recovered dating to circa 1800 BC. People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted portion of a gem projecting from the ring setting), and beads that show strong Graeco-Roman influence. Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan. Hot wax would not stick to it so it was often used to make seal impressions. The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadıköy district of Istanbul.

Geochemistry

Structure

Chalcedony was once regarded to be a fibrous variety of cryptocrystalline quartz . More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite. The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.

Solubility

Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be due to the fact that chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio. It has also been suggested that the higher solubility is due to the moganite component .
Solubility of quartz and chalcedony in pure water
This table gives equilibrium concentrations of total dissolved silicon as calculated by PHREEQC using the llnl.dat database.
TemperatureQuartz Solubility (mg/L)Chalcedony Solubility (mg/L) 0.01ºC0.681.34 25.0ºC2.644.92 50.0ºC6.9512.35 75.0ºC14.2124.23 100.0ºC24.5940.44

References

chalcedony in Breton: Kalkedon (Maenad)
chalcedony in Catalan: Calcedònia (mineral)
chalcedony in Czech: Chalcedon
chalcedony in German: Chalcedon (Mineral)
chalcedony in Estonian: Kaltsedon
chalcedony in Spanish: Calcedonia
chalcedony in Esperanto: Kalcedono
chalcedony in Basque: Kaltzedonia
chalcedony in French: Calcédoine
chalcedony in Korean: 옥수
chalcedony in Italian: Calcedonio
chalcedony in Hebrew: כלקדון (מינרל)
chalcedony in Latin: Calcedonius
chalcedony in Latvian: Halcedons
chalcedony in Lithuanian: Chalcedonas
chalcedony in Dutch: Chalcedoon
chalcedony in Japanese: 玉髄
chalcedony in Norwegian: Kalsedon
chalcedony in Polish: Chalcedon (minerał)
chalcedony in Portuguese: Calcedônia
chalcedony in Romanian: Calcedon (mineral)
chalcedony in Russian: Халцедон
chalcedony in Slovak: Chalcedón
chalcedony in Serbian: Калцедон
chalcedony in Finnish: Kalsedoni
chalcedony in Swedish: Kalcedon
chalcedony in Turkish: Kalsedon
chalcedony in Ukrainian: Халцедон
chalcedony in Chinese: 玉髓
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