(mineralogy) CuSiO2(OH)2 A rare emerald-green mineral that forms hexagonal, hydrous crystals.
Dioptase
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H
Hexagonal -- Rhombohedral
Environment
Oxidized zones of copper ores, particularly in arid climates.
Crystal descriptionUsually crystallized, with the crystals generally quite small; short-prismatic to rhombohedral habit; long-prismatic habit rare.
Physical propertiesBluish emerald green. Luster glassy; hardness 5, specific gravity 3.3-3.4; fracture uneven to conchoidal; cleavage perfect rhombohedral. Transparent to translucent.
CompositionHydrous silicate of copper (50.4% CuO, 38.2% SiO 2 , 11.4% H 2 O).
TestsCrystals dull in hydrochloric acid; decrepitate, blacken, and give water in closed tube; turn brown on charcoal, without fusing.
Distinguishing characteristicsIt is harder than similar green minerals -- the copper sulfates, carbonates, and phosphates. The rhombohedral termination is invariable and very typical. Brochantite is softer and crushes easily to a green powder. Malachite dissolves in hydrochloric acid with effervescence. The bluish look of dioptase's green is very distinctive; experienced collectors can spot this typical shade at some distance. Once seen it becomes unmistakable.
OccurrenceThere is no outstanding U.S. occurrence. Rich crusts of very slender, short, upright and bundled green needles associated with willemite and wulfenite were found at Tiger, Arizona, in the Mammoth Mine. Other Arizona localities include the Christmas Mine and Salome, always as crusts of tiny crystals. Because of the similarity to brochantite and malachite, it is probably more common than generally realized at many of the western copper mines. Copiapó, Chile, has two mines where the thin U.S.-type crystals have been found. Mexico has small crystals too, but Western Hemisphere examples take second place to those of the Eastern Hemisphere.
Tsumeb, Namibia, is remarkable as the source of the largest dioptase crystals, which may be as much as 1 in. (2.5 cm) long. More slender, gemmy prismatic crystals to 2 in. (5 cm) long have been found at Mindouli in the Congo Republic. The classic locality, given as the Kirghiz Steppe, Ural Mountains (now labeled Altyn-Tuba, Karaganda Oblast, Kazakhstan) was of seams in a limestone, on brownish quartz with some quite respectable specimens. Though rare, dioptase has always been one of the most popular and desirable minerals in the eyes of the general collector. Good specimens are usually expensive.
| Wikipedia: Dioptase |
| Dioptase | |
 Dioptase from Altyn Tübe, Kazakhstan, the type locality |
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| 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 Scale hardness | 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 a trigonal mineral, forming 6-sided crystals that are terminated by rhombohedra.
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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 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.
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Use
Dioptase is popular with mineral collectors and it is occasionally cut into small emerald-like gems. 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.
See also
Notes
References and external links
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Wikimedia Commons has media related to: Dioptase |
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
- Dioptase at Mineral Galleries
- Dioptase at Webmineral
- Dioptase at Mindat.org, with an extensive photo gallery
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 | Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Rock & Mineral Guide. Peterson Field Guide to Rocks and Minerals, by Frederick H. Pough. Copyright © 1998 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Dioptase". Read more |
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