Dioptase

Object/Artifact

-

Crater Rock Museum

Name/Title

Dioptase

Entry/Object ID

2004.1.6

Description

Assemblage Zone: copper ore deposits Chemical Composition: Cu^2+SiO2(OH)2 Crystal System: Triclinic System Description: Common Name: Dioptase Chemistry: H2CuSiO4, Hydrated copper silicate Class: Silicates Subclass: Cyclosilicates Location: Found in the Tsumed Mine, Tsumed, Otjikoto Region, Namibia. Description: This specimen has beautiful emerald green and deep green-blue perfect crystals on a snow white carbonate matrix Estimated Value: $20,000 PHYSICAL CHARACTERISTICS: Color is deep green. Luster is vitreous. Transparency crystals are transparent to translucent. Crystal System is trigonal; bar 3 (Dioptase is one of the few rhombic silicates). Crystal Habits include stubby crystals that are usually well formed prisms topped with the three faces of a rhombohedon. Prisms are six sided and usually not very long. Simple rhombohedrons are also common, at times a second rhombohedron will modify the primary rhombohedron faces producing a second set of three smaller faces. Crusts and massive forms are also seen. Cleavage is perfect in three directions forming rhombohedrons. Fracture is conchoidal and brittle. Hardness is 5 (harder than other green copper minerals). Specific Gravity is approximately 3.3+ (slightly above average) Streak is green. Associated Minerals are dolomite, calcite, cerussite, limonite, chrysocolla and other copper minerals. Other Characteristics: index of refraction is 1.65 - 1.71. Notable Occurrences include Tsumeb, Nambia; Zaire; Russia; California and Arizona, USA and Chile. Best Field Indicators are crystal habit, deep color and hardness. Fracture: uneven to conchoidal Hardness: 5 Apatite Luster: Glassy Occurrence: Occurs in the oxidation zone of copper ores particularly in arid climates. 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 finest specimens of all were found at the now-shut Tsumeb Mine in Tsumeb, Namibia. Tsumeb dioptase is wonderfully lustrous and transparent, with its crystal often perched on an attractive snow-white carbonate matrix. Rock Color: Medium Rock Type: Sedimentary Specific Gravity: 3.3 - 3.4 Streak: Green Texture: Crystalline Variety: Dioptase is one of the few silicates to crystallize in the same symmetry class as dolomite and forms crystals that can have a typical carbonates' rhombohedral shape. Dioptase shares this symmetry with the typically ice clear silicate phenakite and the fluorescent but rarely well crystallized willemite. Specimens of dioptase are often deeply colored and show well developed crystals. The faces of the rhombohedrons, and even the prism faces, are very reflective due to a fairly high luster. Crystals can be quite clear but at times they seem to be cloudy due to the deep color. The special attributes of a high quality specimen of dioptase are hard to describe adequately but must be observed in person in order to appreciate their value. Fine crystals of dioptase are still available in large supplies but their sources are few and the continued demand may put a squeeze on their abundance in the future.

Collection

Suomynona Mineral Collection, Suomynona Collection

Acquisition

Accession

2004.1

Source or Donor

Suomynona Mineral Collection

Acquisition Method

Donation

Dimensions

Width

5 in

Depth

3-1/2 in

Length

8 in

Weight

4.13 oz

Dimension Notes

Dimension taken at widest points

Location

Location

Room

Frieda Smith Hall

Building

Crater Rock Museum

Category

Permanent