Chalcedony (mineral)

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Chalcedony
Quartz-83325.jpg
Chalcedony from Rio Grande do Sul , Brazil (size 23 cm × 12 cm × 3.5 cm)
General and classification
other names

chalcedony

chemical formula SiO 2
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. to Strunz
and to Dana 		see quartz
see quartz
Similar minerals Mogánite , quartzine , quartz
Crystallographic Data
Crystal system trigonal
Crystal class ; symbol trigonal trapezoidal; 32
Twinning polysynthetic according to (11 2 0) (Brazilian law)
Physical Properties
Mohs hardness 6.5
Density (g / cm 3 ) 2.6 to 2.7
Cleavage no
Break ; Tenacity uneven
colour colorless to bluish gray
Line color White
transparency opaque, thinly translucent
shine Glass gloss, fat gloss
Crystal optics
Refractive indices n ω  = 1.54
n ε  = 1.55
Birefringence δ = 0.009
Optical character uniaxial positive
Other properties
Chemical behavior soluble in hydrofluoric acid

The chalcedony ( latinisierte form of Greek χαλκηδών ) or in case Germanised chalcedony is a fibrous, microcrystalline Gefügevarietät of the mineral quartz .

According to older sources, the term chalcedony applies to all fibrous forms of microcrystalline quartz (including quartzine ), to all weakly or not at all colored, massive occurrences of microcrystalline SiO 2 or is used as a generic term for all forms of fine crystalline quartz ( flint , chert , agate , Onyx , jasper  ...). These are, strictly speaking, but rocks, for various Gefügevarietäten of quartz consist of SiO 2 polymorphs moganite and other coloring impurities. In modern mineralogy , the term chalcedony is defined more narrowly (see structure ).

Chalcedony is colorless to bluish gray. Impurities cause a wide variety of colors, mostly brown, reddish or green. Chalcedony is translucent, cloudy, has a waxy sheen and, with a Mohs hardness of 6.5–7, is almost as hard as quartz .

Different names are used for other colors. Red to brown chalcedones are known as carnelian (sarders), the green variety that are colored by nickel oxide is called chrysoprase (artificially colored ones are called green-pickled agate) or plasma, emerald green chalcedones get their dark green color by iron oxide. Plasma is sometimes found with tiny dots of jasper that resemble drops of blood, which is why it is called a heliotrope (blood jasper) or, misleadingly, a blood stone. Widely known as a blood stone, the mineral is hematite , an oxide of iron.

Other names found for chalcedony are jasponix , massik , quartzine , zoesite , blue or Californian moonstone and milk stone .

Etymology and history

The name appears for the first time with Pliny the Elder in his Latin Naturalis Historia (around 77 AD). He names a stone called "Calchedon" from a list of translucent types of jasper. The name is derived from the city of Kalchedon in Bithynia , without it being possible to identify any relationship between the two. The also by Pliny d. Ä. A few sections previously described reddish-sparkling, but slightly blackish stone "Carchedon" (German: the Carthaginian; possibly it is garnet) was also identified with chalcedony in the Middle Ages. This was due to the Greek word χαλκηδών (school Greek pronunciation chalkēdón ), which appeared around 95 AD , a Hapax legomenon that has only been passed on in one ancient source, namely the Revelation of John . There ( Rev 21:19  EU ) it says about the heavenly Jerusalem :

The foundation stones of the city wall are adorned with precious stones of all kinds; the first cornerstone is a jasper, the second a sapphire, the third a chalcedony, the fourth an emerald.

However, it cannot be determined which mineral was exactly meant in the revelation. The Vulgate, however, did not reproduce the word in some manuscripts with “chalcedonius” but with “carcedonius”; the two statements of Pliny were thus confused with each other in the Middle Ages. In the tradition of this time one finds under the name "Chalcedon" according to Pliny '"Carchedon" the description of a reddish carbuncle, which has no similarities with today's mineral. Soon the red recedes into the description; the “blackness” becomes “paleness”, which was always emphasized. The description, which has become indistinct, then leads to a new definition, which Albertus Magnus makes literarily tangible: He is the first to provide a description of chalcedony that no longer fits Pliny, but does match the mineral now named with the term: “A pale, dark-colored one , somewhat cloudy stone ”with an“ almost transparent, muddy and dirty substance, like the lead imitates the silver ”. Up to modern times, the word has become a collective term for microcrystalline, fine-fiber quartz.

Varieties

Education and Locations

Blue-gray chalcedony rose from Knoxville , California , USA. Exhibited in the Mineral Museum Bonn
Chalcedony snail shells

Chalcedony forms together with quartzine and mogánite near the surface in crevices and cavities of acidic and basic magmatites ( agate ), in crevices of metamorphic rocks and in sediments (flint, chert, petrified wood). In the formation of agate going from a crystallization of a SiO 2 - gel or SiO 2 -übersättigten solutions at temperatures between 25 and 200 ° C from. With sedimentary formation, chalcedony and quartzine either crystallize from SiO 2 -containing solutions (e.g. petrified wood) or form during diagenesis from the opal skeletons of diatoms , radiolarians ( radiolarite ) or horn silica sponges ( chert ).

Chalcedony is also one of the cement phases in the pore spaces of sandstones.

Microstructure

Chalcedony almond

A massive, granular structure is characterized by the lack of a preferred orientation of the chalcedony fibers. The individual chalcedony fibers are rarely longer than 1–3 µm. Such granular chalcedony makes up the main mass of flint tubers.

Of nodular structure is when the Chalcedonfasern grow starting radialstrahlig of a seed crystal and form spherical aggregates. The diameter of these spherulites is usually 0.1-0.2 mm. Spherolithic chalcedony forms the horizontally layered areas in agate glands and occurs occasionally in flint and chert. When the polarizers are crossed, the spherulites show a characteristic extinction image, the Bertrand's cross, under the polarization microscope .

A parabolic structure is formed when the chalcedony fibers grow radially on the surface of a rock cavity. Neighboring tufts of fibers hinder each other in their growth. Even at a short distance from the growth nucleus, only almost parallel fiber bundles grow into the cavity. Chalcedony grown parabolically forms the concentric bands in agates. Under the polarization microscope, the characteristic wrinkled banding can be seen in the parabolic chalcedony bands . It is characteristic of chalcedony and is absent from quartzine .

structure

The term chalcedony is used for all deep quartz crystals that have grown perpendicular to the crystallographic c-axis along the prism surfaces (110) or (1 1 0) fibrously grown (Flörke et al. 1991). The fibers have a thickness of less than one μm and are typically twisted in the longitudinal direction. The optical character of the fiber direction is length-fast . This means that the axis with the higher refractive index in the crystal is oriented perpendicular to the fiber direction. This distinguishes chalcedony from another fibrous structure - a variety of deep quartz, quartzine . Quartzine is length-slow , i.e. H. the higher refractive index of the quartz lies parallel to the fiber direction.

Structurally, chalcedony hardly differs from deep quartz . However, electron microscopic examinations revealed a very high density of lattice defects for all microcrystalline quartz varieties. It is characterized by a dense sequence of twinning according to Brazilian law. This can be thought of as a stacking of (101) layers made of right and left quartz. In microcrystalline quartz, these layers have variable thicknesses of some unit cells. Structurally thus take chalcedony and Quarzin also an intermediate position between unverzwillingtem low quartz (right or left quartz) and moganite one. This phase, which was only recognized by the IMA in 1994 as a separate SiO 2 modification, can be imagined as a deep quartz with the maximum possible number of Brazilian twins. Layers of right and left quartz of the smallest possible thickness of just one unit cell form a regular sequence in mogánite. This close structural relationship is the reason that chalcedony always contains significant amounts of mogánite. Another characteristic structural property of chalcedony that quartzine lacks is the twisting of the fibers about their longitudinal direction. This twist takes place within a continuous crystal and is due to spiral crystal growth along lattice defects (screw dislocations).

use

Cylinder seal from the Achaemenid Empire (6th to 4th century BC)

Chalcedony is a semi-precious stone and is used as a material for cameos , gems , ring stones, seal stones and many other jewelry and utensils. It has been processed for a long time and is therefore often found in antiques. Large pieces are also material for columns, architectural ornaments, table tops and vases, which can be polished up with a nice polish. In Stone Age cultures, chalcedony was used to make blades; see flint .

Precautions

In the case of strong UV light, color loss can affect the beauty of the stone, as well as in the case of excessive direct sunlight or light.

See also

literature

Web links

Commons : Chalcedony  - collection of images, videos and audio files

Individual evidence

  1. ↑ Pliny the Elder: Naturalis Historiae, Book 37, chap. 7, section 115, books.google.de
  2. ↑ Erika Zwierlein-Diehl: Antique gems and their afterlife . Verlag Walter de Gruyter, Berlin 2007, p. 307, books.google.de
  3. ↑ a b c Hans Lüschen: The names of the stones. The mineral kingdom in the mirror of language . 2nd Edition. Ott Verlag, Thun 1979, ISBN 3-7225-6265-1 , p. 195 ff .
  4. ↑ Pliny the Elder: Naturalis Historiae, Book 37, chap. 7, section 92, books.google.de ; see. Walter Bauer: Greek-German dictionary on the writings of the New Testament and early Christian literature . 6th, completely revised edition, ed. v. Kurt Aland and Barbara Aland. Verlag Walter de Gruyter, Berlin 1988, Sp. 1745. For identification with garnet cf. Hans Lüschen: The names of the stones. The mineral kingdom in the mirror of language . 2nd Edition. Thun 1979, p. 195.
  5. ^ Jörg Traeger : Renaissance and Religion. The Art of Faith in the Age of Raphael . Munich 1977, p. 110, books.google.de
  6. ↑ See Liddell-Scott-Jones sv
  7. ↑ Albertus Magnus : De mineralibus II, 2,3 and I, 2,1, quoted in. after: Hans Lüschen: The names of the stones. The mineral kingdom in the mirror of language . 2nd Edition. Thun 1979, 196.
  8. ↑ a b c d Name search at EPI, the Institute for Gemstone Testing (you need to enter chalcedony )
  9. ↑ Mückenstein description on steine-und-mineralien.de. Retrieved September 11, 2019.