View source for Dioptase

{{Short description|Cyclosilicate mineral}}
{{Infobox mineral
| boxbgcolor= #115544
| boxtextcolor = #fff
| name        = Dioptase
| category    = [[Silicate mineral#Cyclosilicates|Cyclosilicates]]
| image       = Dioptase, quartz 1100.2612.jpg
| imagesize   = 260px
| caption     = 
| formula     = {{chem2|Cu6Si6O18*6H2O}}
| IMAsymbol   = Dpt<ref>{{cite journal |last=Warr|first=L. N. |date=2021 |title=IMA–CNMNC approved mineral symbols |journal=[[Mineralogical Society of Great Britain and Ireland|Mineralogical Magazine]] |volume=85 |issue=3 |pages=291–320 |doi=10.1180/mgm.2021.43 |bibcode=2021MinM...85..291W |s2cid=235729616 |doi-access=free}}</ref>  
| molweight   = 
| strunz      = 9.CJ.30
| system      = [[hexagonal crystal family|Trigonal]]
| class       = Rhombohedral ({{overline|3}}) <br/>[[Hermann–Mauguin notation|H–M symbol]]: ({{overline|3}})
| symmetry    = ''R''{{overline|3}} (No. 148)
| unit cell   = a&nbsp;=&nbsp;14.566, c&nbsp;=&nbsp;7.778&nbsp;[Å]; Z&nbsp;=&nbsp;18
| color       = Dark [[teal]], emerald green
| habit       = Six-sided prisms terminated by [[rhombohedron]]s, to massive
| twinning    = 
| cleavage    = Perfect in three directions
| fracture    = [[conchoidal fracture|Conchoidal]]
| tenacity    = [[brittleness|Brittle]]
| mohs        = 5
| luster      = Vitreous
| refractive  = nω = 1.652 – 1.658 nε = 1.704 – 1.710
| opticalprop = Uniaxial (+)
| birefringence = δ = 0.052
| pleochroism = 
| streak      = Green
| gravity     = 3.28–3.35
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Transparent to translucent
| other       = 
| references  = <ref name=Webmin>{{WebMineral |url=https://webmineral.com/data/Dioptase.shtml |title=Dioptase Mineral Data}}</ref><ref name=Mindat>{{Mindat |id=1295 |name=Dioptase}}</ref>
}}

'''Dioptase''' is an intense [[Shades of green#Emerald|emerald]]-green to [[blue-green|bluish-green]] [[mineral]] that is [[Silicate mineral#Cyclosilicates|cyclosilicate]] of [[copper]]. It is transparent to [[transparency and translucency|translucent]]. Its [[lustre (mineralogy)|luster]] is vitreous to sub-[[Lustre (mineralogy)#Adamantine lustre|adamantine]]. Its formula is {{chem2|Cu6Si6O18*6H2O}}, also reported as {{chem2|CuSiO2(OH)2}}. It has a [[Mohs scale|Mohs hardness]] of 5, the same as [[tooth enamel]]. Its [[specific gravity]] is 3.28–3.35, and it has two perfect and one very good [[cleavage (crystal)|cleavage]] directions. Additionally, dioptase is very fragile, and specimens must be handled with great care. It is a [[hexagonal crystal family|trigonal]] mineral, forming six-sided [[crystal]]s that are terminated by [[rhombohedron|rhombohedra]].

It is popular with [[mineral collecting|mineral collectors]] and is sometimes cut into small [[gemstone|gems]]. It can also be pulverized and used as a [[pigment]] for painting.

==History==
Dioptase was used to highlight the edges of the eyes on the three [[Pre-Pottery Neolithic B]] [[lime plaster]] statues discovered at '[[Ayn Ghazal (archaeological site)|Ain Ghazal]], known as [[Micah, Heifa and Noah]].<ref name=bmMHN>{{cite web |url=https://www.britishmuseum.org/explore/highlights/highlight_objects/me/l/lime_plaster_statues.aspx|title=Lime plaster statues  |website=britishmuseum.org|publisher=British Museum|archive-date=May 11, 2013|archive-url=https://web.archive.org/web/20130511103233/https://www.britishmuseum.org/explore/highlights/highlight_objects/me/l/lime_plaster_statues.aspx|url-status=dead|accessdate=November 19, 2020}}</ref> These sculptures date back to about 7200 BC.<ref name=bmMHN />

Late in the 18th century, copper miners at the Altyn-Tyube (Altyn-Tube) mine, [[Karagandy Province]], [[Kazakhstan]]<ref name=Mindat/> thought they had found the [[emerald]] deposit of their dreams. They found fantastic cavities in [[quartz]] veins in a [[limestone]] rock, filled with thousands of lustrous transparent emerald-green  crystals. The crystals were dispatched to [[Moscow]], [[Russia]], for analysis. However, the mineral's inferior hardness of 5 compared with emerald's greater hardness of 8 easily distinguished it. Eventually, in 1797, the [[mineralogist]] Fr. [[René Just Haüy]] determined that the enigmatic Altyn-Tyube mineral was new to science and named it dioptase ([[Greek language|Greek]], ''dia'', "through" and ''optos'', "visible"), alluding to the internal cleavage planes that can be seen inside unbroken crystals.<ref>{{Cite web |title=Dioptase |url=https://www.mindat.org/min-1295.html |website=mindat.org}}</ref>

==Occurrence==
Dioptase is an uncommon mineral found mostly in [[desert]] regions where it forms as a secondary mineral in the [[oxidation|oxidized]] zone of copper [[sulfide]] mineral deposits. However, the process of its formation is not simple. The oxidation of copper sulfides should be insufficient to crystallize dioptase, as [[silica]] is normally minutely soluble in water except at highly [[alkaline]] [[pH]]. The oxidation of sulfides will generate highly [[acidic]] fluids rich in [[sulfuric acid]] that should suppress silica's solubility. However, in dry climates and with enough time, especially in areas of a mineral deposit where acids are [[buffering agent|buffer]]ed by [[carbonate]], minute quantities of silica may react with dissolved copper forming dioptase and [[chrysocolla]].

The Altyn Tube mine in Kazakhstan still provides handsome specimens; a brownish quartzite host distinguishes its specimens from other localities. The finest specimens of all were found at the Tsumeb Mine in [[Tsumeb]], [[Namibia]]. Tsumeb dioptase is transparent and often highly sought after by collectors. Dioptase is also found in the deserts of the southwestern [[US]]. A notable occurrence is the old Mammoth-Saint Anthony Mine near  [[Mammoth, Arizona]] where small crystals that make fine [[micromount]] specimens are found. In addition, many small, pale-green colored crystals of dioptase have come from the Christmas Mine near [[Hayden, Arizona]]. Another classic locality for fine specimens is Renéville, [[Congo-Brazzaville]]. Finally, an interesting occurrence is the Malpaso Quarry in and near Agua de Oro [[Argentina]]. Here tiny bluish-green dioptase is found on and in [[quartz]]. It appears in this case that dioptase is primary and has crystallized with quartz, native copper, and [[malachite]].

==Use==
Dioptase is popular with [[mineral collector]]s, and it is occasionally cut into small emerald-like [[gemstone|gem]]s. Dioptase and [[chrysocolla]] are the only relatively common copper silicate minerals. A dioptase gemstone should never be exposed to [[ultrasonic cleaning]] or the fragile gem will shatter. As a ground pigment, dioptase can be used in painting.<ref>{{cite web|url=https://www.connect.ecuad.ca/~vsager/FNDT%20150%20Spring%2012/making%20paint.html |title=Making Your Own Water-Based Paint |website=connect.ecuad.ca |date= |accessdate=2019-03-12}}</ref>  Dioptase dust is toxic due to its copper content and accidental ingestion can lead to serious health problems.<ref>{{Cite web|date=2021-05-22|title=Beautiful Green Dioptase / Schöne grüne Dioptase / 美しい緑の翠銅鉱 {{!}} GeoRarities|url=https://georarities.com/2021/05/22/beautiful-green-dioptase-schone-grune-dioptase-%e7%be%8e%e3%81%97%e3%81%84%e7%b7%91%e3%81%ae%e7%bf%a0%e9%8a%85%e9%89%b1/|access-date=2021-05-23|language=en}}</ref>

The most famous (and expensive) dioptase mineral locality is at [[Tsumeb]], Namibia.<ref>Mineralogical Record "TSUMEB!" (special issue), Vol. 8, No. 3,
May - June 1977. [https://mineralogicalrecord.com/detail.asp?id=67 Table of contents] {{Webarchive|url=https://web.archive.org/web/20200318082321/https://mineralogicalrecord.com/detail.asp?id=67 |date=2020-03-18 }}</ref>

==Crystal structure and properties==
Dioptase is a cyclosilicate mineral consisting of Si<sub>6</sub>O<sub>18</sub> rings which are linked together by [[Jahn–Teller effect|Jahn–Teller distorted]] octahedral d<sub>9</sub> Cu(II) ions. Each copper ion is coordinated by four cyclosilicate oxygens and two water molecules. Although the copper ions are six-coordinate, they can be viewed as square planar. The copper centers have approximately C<sub>4V</sub> [[Molecular symmetry|symmetry]]. Each Cu(II) shares a square planar edge with another Cu(II) and corners with two more. The copper ions are responsible for the mineral's color and magnetic properties. A broad visible absorption band at 752&nbsp;nm is observed. Dioptase is anti-ferromagnetic at low temperatures ([[Néel temperature]] of 70 K). Above 70 K, it obeys the [[Curie–Weiss law]].<ref>{{Cite journal|last1=Newnham|first1=R. E.|last2=Santoro|first2=R. P.|date=1967|title=Magnetic and Optical Properties of Dioptase|journal=Physica Status Solidi B|language=de|volume=19|issue=2|pages=K87–K90|doi=10.1002/pssb.19670190247|bibcode=1967PSSBR..19...87N}}</ref>

==Gallery==
<gallery  widths="200px" heights="200px"> 
File:Dioptasetsumeb5.jpg|Dioptase from the Tsumeb Mine, [[Tsumeb]], Namibia 
File:Cerussite-Dioptase-di02a.jpg|Dioptase with [[Cerussite]], Christoff Mine, [[Kunene Region]], Namibia. 6.9 × 5.7 × 4.8&nbsp;cm
File:Chrysocolla-Dioptase-k-115a.jpg|Dioptase on [[Chrysocolla]],  Otjikotu,  [[Kunene Region]], Namibia. 6.8 × 5.5 × 4.5&nbsp;cm
File:Dioptase-146729.jpg|Dioptase crystal, Kimbedi, [[Pool Department]], Republic of Congo (Brazzaville). Size: 2.0 × 1.7 × 1.4&nbsp;cm
File:Dioptase-18577.jpg|Pristine dioptase crystals,  Tsumeb Mine, [[Tsumeb]], Namibia. 4 × 4 × 1&nbsp;cm
File:Dioptase-Shattuckite-163280.jpg| Dioptase on [[Shattuckite]], Kaokoveld Plateau, Kunene Region, Namibia. Size: 2.5 × 2.1 × 2.0&nbsp;cm
</gallery>

==See also==
*[[Aventurine]]
*[[Malachite]]

==References==
{{reflist}}
* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., {{ISBN|0-471-80580-7}}
{{commons category|Dioptase}}

[[Category:Gemstones]]
[[Category:Copper(II) minerals]]
[[Category:Cyclosilicates]]
[[Category:Trigonal minerals]]
[[Category:Minerals in space group 148]]