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'{{Infobox mineral | name = Chalcedony | category = Oxide mineral | boxwidth = | boxbgcolor = | image = Chalcedony geode.JPG | imagesize = 250 | caption = A cut and polished Chalcedony [[geode]] | formula = [[Silica]] (silicon dioxide, SiO₂) | molweight = 60 g / mol | color = Various | habit = | system = Trigonal | twinning = | cleavage = Absent | fracture = Uneven, splintery, conchoidal | mohs = 6 - 7 | luster = Waxy, vitreous, dull, greasy, silky | refractive = | opticalprop = | birefringence = | pleochroism = | streak = White | gravity = 2.59 - 2.61 | density = | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Translucent | other = | references = <ref>Rudolf Duda and Lubos Rejl: ''Minerals of the World'' (Arch Cape Press, 1990)</ref> }} '''Chalcedony''' is a [[cryptocrystalline]] form of [[silica]], composed of very fine intergrowths of the minerals [[quartz]] and [[moganite]]<ref name="heany_1994">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.</ref>. 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. {{TOClimit|limit=3}} == Varieties == Chalcedony occurs in a wide range of varieties. Many semi-precious [[gemstone]]s are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows: ====Agate==== [[Image:Quartz - Agateplate, redbrown-white.jpg|thumb|left|180px|Agate]] '''[[Agate]]''' is a variety of chalcedony with multi-colored concentric banding. <br clear="all" /> ====Carnelian==== [[Image:carnelian sard (mineral specimen).jpg|thumb|left|180px|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. <br clear="all" /> ====Chrysoprase==== [[Image:Chryzopraz 2.jpg|thumb|left|180px|Chrysoprase]] '''[[Chrysoprase]]''' (also spelled '''chrysophrase''') is a green variety of chalcedony, which has been colored by [[Nickel(II) oxide|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.) <br clear="all" /> ====Heliotrope==== [[Image:Quarz - Heliotrop (Blutjaspis).JPG|thumb|left|180px|Heliotrope, or bloodstone]] '''[[Heliotrope (mineral)|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'''. <br clear="all" /> ====Moss agate==== [[Image:mossagate.pebble.750pix.jpg|thumb|left|180px|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. <br clear="all" /> ====Mtorolite==== [[Image:Mtorolite zimbabwe trimmed.jpg|thumb|left|180px|Mtorolite]] '''[[Mtorolite]]''' is a green variety of chalcedony, which has been colored by [[chromium]]. It is principally found in [[Zimbabwe]]. <br clear="all" /> ====Onyx==== [[Image:Onyks grudki , Brazylia.jpg|thumb|left|180px|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]]. <br clear="all" /> == History == [[Image:Titus cameo.jpg|right|thumb|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 civilization|Minoan]] [[Crete]] at the Palace of [[Knossos]], chalcedony seals have been recovered dating to circa 1800 [[Before Christ|BC]].<ref>C. Michael Hogan, [http://www.themodernantiquarian.com/site/10854/knossos.html#fieldnotes ''Knossos fieldnotes'', Modern Antiquarian (2007)]</ref> People living along the [[Central Asia]]n trade routes used various forms of chalcedony, including [[carnelian]], to carve [[intaglio]]s, 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]]. <ref> Section 12 of the translation of ''Weilue'' - a 3rd century Chinese text by John Hill under "carnelian" and note 12.12 (17)</ref> Hot wax would not stick to it so it was often used to make [[Seal (device)|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]]. [[Image:ChalcedonyKnifeNPS.jpg|left|thumb|300px|Chalcedony knife, AD 1000-1200]] <br clear="all" /> == Geochemistry== ===Structure=== Chalcedony was once regarded to be a fibrous variety of cryptocrystalline [[quartz]] <ref>Chalcedony mineral information and data. http://www.mindat.org/min-960.html</ref>. More recently however, it has been shown to also contain a [[monoclinic]] polymorph of [[quartz]], known as [[moganite]]<ref name="heany_1994" />. The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%<ref name="heany_1992"> 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.</ref>. The existence of moganite was once regarded as dubious, but it is now officially recognised by the [[International Mineralogical Association]]<ref>Origlieri, Marcus. "Moganite: a New Mineral -- Not!" Lithosphere (1994). Aug. 2007 <http://fgms.home.att.net/moganite.htm>.</ref><ref> 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>.</ref>. ===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. {{Fact|date=February 2007}} It has also been suggested that the higher solubility is due to the moganite component <ref name="heany_1992" />. =====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. <center><table class="wikitable"> <tr><th>Temperature</th><th>Quartz Solubility (mg/L)</th><th>Chalcedony Solubility (mg/L)</th></tr> <tr><th rowspan="1">0.01°C</th><td>0.68</td><td>1.34</tr> <tr><th rowspan="1">25.0°C</th><td>2.64</td><td>4.92</tr> <tr><th rowspan="1">50.0°C</th><td>6.95</td><td>12.35</tr> <tr><th rowspan="1">75.0°C</th><td>14.21</td><td>24.23</tr> <tr><th rowspan="1">100.0°C</th><td>24.59</td><td>40.44</tr> </table></center> ==References== <references /> == 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 {{Silica minerals}} [[Category:Minerals]] [[Category:Quartz varieties]] [[Category:Lithics]] [[br:Kalkedon (Maenad)]] [[ca:Calcedònia (mineral)]] [[cs:Chalcedon]] [[de:Chalcedon (Mineral)]] [[et:Kaltsedon]] [[es:Calcedonia]] [[eo:Kalcedono]] [[eu:Kaltzedonia]] [[fr:Calcédoine]] [[ko:옥수]] [[is:Kalsedón]] [[it:Calcedonio]] [[he:כלקדון (מינרל)]] [[la:Calcedonius]] [[lv:Halcedons]] [[lt:Chalcedonas]] [[nl:Chalcedoon]] [[ja:玉髄]] [[no:Kalsedon]] [[nn:Kalsedon]] [[pl:Chalcedon (minerał)]] [[pt:Calcedônia]] [[ro:Calcedonie]] [[ru:Халцедон]] [[scn:Calcidoniu]] [[sk:Chalcedón]] [[sr:Калцедон]] [[fi:Kalsedoni]] [[sv:Kalcedon]] [[tr:Kalsedon]] [[uk:Халцедон]] [[vi:Canxedon]] [[zh:玉髓]]'