Dioptase

Dioptase

Dioptase
Dioptase from Altyn Tübe, Kazakhstan, the type locality
General
Category	Mineral
Chemical formula	Copper silicate hydrate: CuSiO3·H2O
Identification
Color	Dark blue green, emerald green
Crystal habit	Six sided prisms terminated by rhombohedrons to massive
Crystal system	Trigonal; bar 3
Cleavage	Perfect in three directions
Fracture	Conchoidal and brittle
Mohs Scalehardness	5
Luster	Vitreous
Streak	Green
Specific gravity	3.28–3.35
Refractive index	1.65–1.71

Dioptase is an intense emerald-green to bluish-green copper cyclosilicate mineral. It is transparent to translucent. Its luster is vitreous to sub-adamantine. Its formula is CuSiO₃·H₂O (also reported as CuSiO₂(OH)₂). It has a hardness of 5, the same as tooth enamel. It specific gravity is 3..28–3.35, and it has two perfect and one very good cleavage directions. Additionally, dioptase is very fragile and specimens must be handled with great care. It is atrigonal mineral, forming 6-sided crystals that are terminated by rhombohedra.

History

Late in the 18th century, copper miners at the Altyn-Tyube (Altyn-Tube) mine, Karagandy Province,Kazakhstan ^[1] thought they found an emerald deposit of their dreams. They found fantastic cavities in quartz veins in a limestone, filled with thousands of lustrous emerald-green transparent 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. Later Fr. René Just Haüy (the famed French mineralogist) in 1797 determined that the enigmatic Altyn-Tyube mineral was new to science and named it dioptase (Greek,dia, "through" and optima, "vision"), alluding to the mineral's two cleavage directions that are visible inside unbroken crystals.

Occurrence

Dioptase from the Tsumeb Mine, Tsumeb, Namibia

Dioptase is a very rare mineral found mostly in desert regions where it forms as a secondary mineral in the 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 solubility. However, in dry climates and with enough time, especially in areas of a mineral deposit where acids are buffered 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 wonderfully lustrous and transparent, with its crystal often perched on an attractive snow-white carbonate matrix. Dioptase is also found in the deserts of the southwestern USA. 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 Argentina. Here tiny bluish-green dioptase is found on and in quartz. It appears at this occurrence, dioptase is primary and has crystallized with quartz, native copper, and malachite.

Dioptase crystals fromDemocratic Republic of the Congo, displaying the dioptasecrystal habit of six sidedprisms terminated byrhombohedrons

Use

Dioptase is popular with mineral collectors and it is occasionally cut into small emerald-likegems. 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.