Chalcedony

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite^[1]. 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. Its color is usually white to gray, grayish-blue or a shade of brown ranging from pale to nearly black. Other shades have been given specific names: A clear red chalcedony is known as carnelian or sard. A green variety colored by nickel oxide is called chrysoprase. Prase is a dull green. Onyx has flat black and white bands. Plasma is a bright to emerald-green chalcedony that is sometimes found with small yellow spots of jasper. Heliotrope is similar to plasma, but with red spots of jasper, hence its alternative name of bloodstone. Chalcedony with concentric banding is known as agate. Flint is also a variety of chalcedony.   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.

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Geochemistry: Chalcedony

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.^{[citation needed]}

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

• See Section 12 of the translation of Weilue - a 3rd century Chinese text by John Hill under "carnelian" and note 12.12 (17)

1. ^ 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.

See also

• List of minerals