| Dioptase | |
|---|---|
 | |
| General | |
| Category | Cyclosilicates |
| Formula | Cu6Si6O18·6H2O |
| IMA symbol | Dpt[1] |
| Strunz classification | 9.CJ.30 |
| Crystal system | Trigonal |
| Crystal class | Rhombohedral (3) H–M symbol: (3) |
| Space group | R3 (No. 148) |
| Unit cell | a = 14.566, c = 7.778 [Å]; Z = 18 |
| Identification | |
| Color | Darkteal, emerald green |
| Crystal habit | Six-sided prisms terminated byrhombohedrons, to massive |
| Cleavage | Perfect in three directions |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 5 |
| Luster | Vitreous |
| Streak | Green |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.28–3.35 |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.652 – 1.658 nε = 1.704 – 1.710 |
| Birefringence | δ = 0.052 |
| References | [2][3] |
Dioptase is an intenseemerald-green tobluish-greenmineral that iscyclosilicate ofcopper. It is transparent totranslucent. Itsluster is vitreous to sub-adamantine. Its formula isCu6Si6O18·6H2O, also reported asCuSiO2(OH)2. It has aMohs hardness of 5, the same astooth enamel. Itsspecific gravity is 3.28–3.35, and it has two perfect and one very goodcleavage directions. Additionally, dioptase is very fragile, and specimens must be handled with great care. It is atrigonal mineral, forming six-sidedcrystals that are terminated byrhombohedra.
It is popular withmineral collectors and is sometimes cut into smallgems. It can also be pulverized and used as apigment for painting.
Dioptase was used to highlight the edges of the eyes on the threePre-Pottery Neolithic Blime plaster statues discovered at 'Ain Ghazal, known asMicah, Heifa and Noah.[4] These sculptures date back to about 7200 BC.[4]
Late in the 18th century, copper miners at the Altyn-Tyube (Altyn-Tube) mine,Karagandy Province,Kazakhstan[3] thought they had found theemerald deposit of their dreams. They found fantastic cavities inquartz veins in alimestone rock, filled with thousands of lustrous transparent emerald-green crystals. The crystals were dispatched toMoscow,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, themineralogist Fr.René Just Haüy determined that the enigmatic Altyn-Tyube mineral was new to science and named it dioptase (Greek,dia, "through" andoptos, "visible"), alluding to the internal cleavage planes that can be seen inside unbroken crystals.[5]
Dioptase is an uncommon mineral found mostly indesert regions where it forms as a secondary mineral in theoxidized zone of coppersulfide mineral deposits. However, the process of its formation is not simple. The oxidation of copper sulfides should be insufficient to crystallize dioptase, assilica is normally minutely soluble in water except at highlyalkalinepH. The oxidation of sulfides will generate highlyacidic fluids rich insulfuric acid that should suppress silica's solubility. However, in dry climates and with enough time, especially in areas of a mineral deposit where acids arebuffered bycarbonate, minute quantities of silica may react with dissolved copper forming dioptase andchrysocolla.
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 inTsumeb,Namibia. Tsumeb dioptase is transparent and often highly sought after by collectors. Dioptase is also found in the deserts of the southwesternUS. A notable occurrence is the old Mammoth-Saint Anthony Mine nearMammoth, Arizona where small crystals that make finemicromount specimens are found. In addition, many small, pale-green colored crystals of dioptase have come from the Christmas Mine nearHayden, Arizona. Another classic locality for fine specimens is Renéville,Congo-Brazzaville. Finally, an interesting occurrence is the Malpaso Quarry in and near Agua de OroArgentina. Here tiny bluish-green dioptase is found on and inquartz. It appears in this case that dioptase is primary and has crystallized with quartz, native copper, andmalachite.
Dioptase is popular withmineral collectors, and it is occasionally cut into small emerald-likegems. Dioptase andchrysocolla are the only relatively common copper silicate minerals. A dioptase gemstone should never be exposed toultrasonic cleaning or the fragile gem will shatter. As a ground pigment, dioptase can be used in painting.[6] Dioptase dust is toxic due to its copper content and accidental ingestion can lead to serious health problems.[7]
The most famous (and expensive) dioptase mineral locality is atTsumeb, Namibia.[8]
Dioptase is a cyclosilicate mineral consisting of Si6O18 rings which are linked together byJahn–Teller distorted octahedral d9 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 C4Vsymmetry. 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 nm is observed. Dioptase is anti-ferromagnetic at low temperatures (Néel temperature of 70 K). Above 70 K, it obeys theCurie–Weiss law.[9]
