chalcedony

Chalcedony
A cut and polished Chalcedony geode
General
Category	Oxide mineral
Chemical formula	Silica (silicon dioxide, SiO2)
Identification
Molar mass	60 g / mol
Color	Various
Crystal system	Trigonal
Cleavage	Absent
Fracture	Uneven, splintery, conchoidal
Mohs Scale hardness	6 - 7
Luster	Waxy, vitreous, dull, greasy, silky
Streak	White
Diaphaneity	Translucent
Specific gravity	2.59 - 2.61
References	[1]

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite^[2]. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, whilst moganite is monoclinic.

Chalcedony has a waxy luster, 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 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

Agate is a variety of chalcedony with multi-colored concentric banding.

Carnelian

Carnelian

Carnelian (also spelled cornelian) is a clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from a pale orange, to an intense almost-black coloration. Similar to carnelian is sard, which is brown rather than red.

Chrysoprase

Chrysoprase

Chrysoprase (also spelled chrysophrase) is a 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, or bloodstone

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

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.

Mtorolite

Mtorolite

Mtorolite is a green variety of chalcedony, which has been colored by chromium. It is principally found in Zimbabwe.

Onyx

Several onyx forms

Onyx is a variant of agate with black and white banding. Similarly, agate with brown and white banding is known as sardonyx.

History

Chalcedony cameo of Titus head, 2nd Century AD

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.^[3] 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. ^[4] 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.

Chalcedony knife, AD 1000-1200

Geochemistry

Structure

Chalcedony was once regarded to be a fibrous variety of cryptocrystalline quartz ^[5]. More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite^[2]. The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%^[6]. The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association^[7][8].

Solubility

Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio.^{[citation needed]} It has also been suggested that the higher solubility is due to the moganite component ^[6].

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.

References

1. ^ Rudolf Duda and Lubos Rejl: Minerals of the World (Arch Cape Press, 1990)
2. ^ ^{a b} Heaney, Peter J., 1994. Structure and Chemistry of the low-pressure silica polymorphs. In: Reviews in Mineralogy v. 29; Silica: Physical Behavior, geochemistry and materials applications. Ed. Heaney, P.J., Prewitt, C.T., Gibbs, G.V., 1-40.
3. ^ C. Michael Hogan, Knossos fieldnotes, Modern Antiquarian (2007)
4. ^ Section 12 of the translation of Weilue - a 3rd century Chinese text by John Hill under "carnelian" and note 12.12 (17)
5. ^ Chalcedony mineral information and data. http://www.mindat.org/min-960.html
6. ^ ^{a b} Heaney, Peter J., and Jeffrey E. Post. "The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties." Science ns 255 (1992): 441-443. JSTOR. Aug. 2007. Keyword: moganite.
7. ^ Origlieri, Marcus. "Moganite: a New Mineral -- Not!" Lithosphere (1994). Aug. 2007 <http://fgms.home.att.net/moganite.htm>.
8. ^ Nickel, Ernest H., and Monte C. Nichols. "IMA/CNMNC List of Mineral Names." Materials Data. June 2007. Aug. 2007 <http://www.geo.vu.nl/users/ima-cnmmn/MINERALlist.pdf>.

See also

• List of minerals

External links

• http://www.mindat.org/min-960.html
• http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/chalcedony.html