DIOPTASE
Dioptase – CuSiO3.H2O – is a very rare cyclosilicate, a secondary Cu mineral that can be found in some oxidized ores of Cu, Pb, and Zn. Aesthetic pieces fetch very high prices in the collector’s mineral market. It is sometimes used as a gemstone, and its powder provides a green pigment for paints. Famous occurrences include those from Namibia, the Republic of Congo, and Kazakhstan.
It was initially confused with green beryl (emerald), but the differentiation is simple due to the large contrast in hardness (emerald has a hardness of 8). In addition, its extreme fragility is characteristic, as dioptase does not even withstand cleaning in an ultrasonic bath.
1. Characteristics
Crystal system: Trigonal rhombohedral.
Color: Emerald green to blue-green.
Habit: Short prismatic, pseudohexagonal, up to 5 cm. Can be long prismatic or rhombohedral. Massive, granular, etc.
Cleavage: {10-11} perfect. (cleavage in 3 directions).
Tenacity: Brittle. Very fragile!
Twinning: On {10-11}, rare.
Fracture: Irregular, conchoidal.
Mohs Hardness: 5
Parting: No.
Streak: Green.
Lustre: Vitreous, sub-vitreous, a sub-adamantine.
Diaphaneity: Transparent.
Density (g/cm³): 3.28 – 3.35
2. Geology and Deposits
Dioptase is one of the many green secondary minerals that form in oxidized zones of copper ores. It is not a common mineral and is found only in oxidized ores in desert areas.
In addition to a desert climate, the presence of carbonates is necessary to neutralize the highly acidic solutions (sulfuric acid) resulting from the oxidation of copper sulfides. Once these prerequisites are met, small amounts of silica can react with the copper to form chrysocolla and dioptase.
3. Mineral Associations
It is associated with common minerals such as quartz, fluorite, and carbonates (calcite, dolomite).
The specific paragenesis of oxidation zones is composed of many dozens of minerals, including secondary minerals of:
– Fe (goethite (limonite)),
– Pb (wulfenite, mimetite, duftite, and cerussite),
– Zn (hemimorphite and smithsonite-(Cu)), and
– Cu (chrysocolla, malachite, plancheite, and shattuckite).
4. Transmitted Light Microscopy
Refraction indices: nω: 1.652 – 1.658 nε: 1.704 – 1.710
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Green, with weak pleochroism.
Relief: High.
Cleavage: {10-11} perfect (cleavage in 3 directions).
Habits: Modified and elongated rhombohedral crystals. Short to long prismatic, up to 5 cm. Massive, granular.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.052, corresponding to intense colors of up to the 3rd order, but masked by the intense color of the mineral itself.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: On {10-11}, rare.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(+)
2V angle: No.
Alterations: No information available.
May be confused with: Other green Cu minerals that occur in association, especially if the material is massive or fine-grained.
5. Reflected Light Microscopy
Reflected Light Microscopy is obviously not the recommended analytical method for identifying dioptase. However, it is important to create a slide or polished section to identify the opaque minerals that occur associated with dioptase, such as Pb, Zn and Cu sulfides.
Sample preparation: Polishing dioptase is simple and results in good quality.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray color with a creamy tone, much lighter than the gangue.
Pleochroism: Distinct in shades of gray, clearly visible if the crystals are large.
Reflectivity: Low (<10%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy towards a brown tone, best seen in XPL+2º. When rotating the stage, two positions of intense green reflections alternate with two positions where the brown color predominates.
Internal reflections: Abundant in various shades of green.
May be confused with: Other secondary green copper minerals, such as malachite, antlerite, atacamite, paratacamite, libethenite, chalcanthite, and brochantite, are also present. Pleochroism and anisotropy, however, are quite diagnostic.
