AURICHALCITE
Aurichalcite – (ZnCu)5(CO3)2(OH)6 – is a carbonate typical of the oxidation zones of Cu and Zn deposits, being one of the many secondary copper ore minerals found in oxidized ores.
The mineral never forms large crystals; it always occurs as thin coatings, thick crusts, or divergent tufts of small, fragile acicular crystals that can form complete spheres. The maximum crystal size is 3 cm (exceptional), and its sky-blue color with a greenish tint is highly characteristic.
The fragility of the crystals means that even simply washing the specimen with water can damage them.
Aurichalcite typically contains some calcium. It effervesces in dilute HCl and closely resembles rosasite.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudo-orthorhombic.
Color: Pale green, blue-green, sky blue.
Habit: Crusts, clusters of delicate crystals; rarely columnar, lamellar, or granular.
Cleavage: Perfect {010} and {100}, indistinguishable Striations parallel to [001] .
Tenacity: Something flexible and sectile.
Twinning: Observed in X-ray patterns.
Fracture: Irregular to fibrous.
Mohs Hardness: 1 – 2
Parting: No.
Streak: Pale blue-green to colorless.
Lustre: Pearly to silky.
Diaphaneity: Transparent.
Density (g/cm³): 3.64 – 3.9
2. Geology and Deposits
Aurichalcite is typical of oxidation zones in Cu and Zn deposits.
3. Mineral Associations
It is associated with quartz, calcite, goethite, cerussite, Cu minerals (malachite, azurite, cuprite, linarite, rosasite), and Zn minerals (smithsonite, hemimorphite, hydrozincite, austinite, and adamite), among many others.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.655 nβ: 1.740 nγ: 1.744
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to pale blue or pale green. Weak pleochroism between colorless and blue-green or pale green.
Relief: Relatively high.
Cleavage: No information available.
Habits: Crusts, clusters of delicate crystals; rarely columnar, lamellar, or granular.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.089: interference colors up to the 5th order, with pink and green predominating.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No information available.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: 1-4º (looks uniaxial!).
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
There is no information in the literature regarding the characteristics of aurichalcite under reflected light. The crystals generally form delicate tufts on cavity walls and cannot be polished.
As a transparent mineral, it should exhibit dark gray colors and low reflectivity under plane-polarized light. Under crossed polars, it displays widespread internal reflections in pale blue and colorless hues, corresponding to its macroscopic color.