AZURITE

Azurite – Cu3(CO3)2(OH)2 – is a carbonate of copper and has a deep blue color that is unmistakable, extremely characteristic. Integrates oxidized copper ores. In addition, well-formed crystals of azurite fetch very high prices in the collector minerals market.

It forms prismatic crystals that are translucent at the sharp edges. The crystals reach up to 30 cm in length and more than 100 combined forms have been recorded. It has two varieties.

Azurite, once exposed to the atmosphere, is unstable and is often pseudomorphically replaced by malachite.

An important diagnostic feature is its strong effervescence in dilute HCl.

1. Characteristics

Crystal system: Monoclinic prismatic.          

Color: Azure blue, Berlin-blue, blue, very dark to pale blue.     

Habit: Prismatic tabular, stalagtite, massive, equidimensional, earthy, acicular, crusts.       

Cleavage: {011} perfect, but interrupted. {100} fair, {110} poor. Striations // to z, rare.       

Tenacity:  Brittle.         

Twinning: Rare on {-101}, {-102} or {001}.       

Fracture: Very fragile, producing conchoidal fragments.       

Mohs Hardness: 3.5 – 4

Parting: No.         

Streak: Light blue.         

Lustre: Vitreous, subadamantine, dull.          

Diaphaneity: Transparent.           

Density (g/cm³): 3.8

          

2. Geology and Deposits

Azurite forms in the oxidation zone of copper sulphide ores, associated with carbonate rocks, almost always from enargite, famatinite or tennantite-tetrahedrite. The reaction involves CO2 + H2O + copper sulfides. It can also form by copper-containing solutions reacting with carbonates.

Among the secondary copper minerals, azurite is much rarer than malachite.

 

3. Mineral Associations

Azurite associates with primary and secondary copper minerals and with typical secondary minerals from oxidation zones.

Among the primary copper minerals are native copper, chalcocite, bornite, chalcopyrite, famatinite, enargite, and tennantite-tetrahedrite.

Among the secondary copper minerals are a black mineral (tenorite), a red mineral (cuprite), the green minerals (malachite, pseudomalachite, brochantite, antlerite, atacamite, dioptase, botallackite, libethenite, olivenite, duftite and natrochalcite) and the blue minerals (chrysocolla, cyanotrichite, plancheite, sampleite, linarite, caledonite, chalcanite, shattuckite, liroconite, mrázekite, connellite and kröhnkite). Several of the blue minerals can appear green and vice versa.

Other typical oxidation zone minerals are quartz, secondary Fe minerals (“limonite”), secondary lead minerals (cerussite), secondary zinc minerals (smithsonite), manganese minerals (wad), barite and carbonates (calcite, dolomite), forming a very colorful set of minerals.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.730     nβ: 1.758     nγ: 1.838:

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Blue with strong pleochroism.    

Relief: High.           

Cleavage: Perfect cleavage on {011}, a good cleavage on {100} and a cleavage in traces on {110}. Cleavage is usually difficult to impossible to observe. 

Habits: Tends to prismatic and tabular.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors:  Maximum birefringence is 0.108, extremely high, but the interference colors are masked by the intense blue color. It is almost the same color in PPL and CPL.

Extinction:  Oblique.          

Elongation sign: No information available.   

Twins: No.         

Zoning: No.             

CONVERGENT LIGHT

Character:  B(+). The intense blue color makes it difficult or impossible to obtain the figure.         

2V angle: 68o         

Alterations: can alterate to malachite, a green secondary carbonate of Cu.          

May be confused with:  other blue copper secondary minerals. Check the list of associated minerals to see the possibilities.        

Image of a thin section containing azurite, observed with the naked eye. The blue of the azurite continues to stand out, even at the standard thickness of the section, which is only 30 microns..
Azurite 01: Azurite in PPL, associated with a green secondary copper mineral. the lighter blue and the deeper blue are the two colors of pleochroism.
Azurite 01: The same section as of the previous image, now in XPL. The blue colors has not changed much from PPL to XPL, but now some crystals are in the position of extinction and are black.

5. Reflected Light Microscopy

Sample preparation: the polishing of azurite presents no difficulties and is of good quality.

PLANE POLARIZED LIGHT – PPL

Reflection color:  Dark gray.      

Pleochroism: Very faint, masked by internal blue reflections, even in PPL.      

Reflectivity: ~8% (Very low!)        

Bireflectance: Discreet.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong grayscale anisotropy may be masked by internal blue reflections.        

Internal reflections: Strong, intense, generalized, in deep blue.      

May be confused with: the fact that azurite exhibits effervescence with dilute HCl greatly facilitates its recognition in relation to other blue-colored secondary copper minerals.

Connelite forms different crystals and cornetite has a more greenish color.

Cyanothrichite has lighter blue and lirconite, also lighter blue, is softer.

Linarite is softer (2.5) and denser (5.3).

Lazurite and lazulite occur in other environments.

Macro image of a polished section with malachite (green) and azurite (blue). The blue color of azurite stands out and is very diagnostic even to the unaided eye.
Azurite 01: In PPL, azurite. The black portion is a hole in the polished section. Holes are common in oxidized ores. The slight pleochroism is easy to see.
Azurite 01: The same section as of the previous image, now in XPL. Azurite shows strong anisotropy between blue and light gray, with typical deep blue internal reflections. The gangue, to the right of the image, shows colorless reflections.
Azurite 02: Very fine-grained limestone impregnated with secondary copper minerals. In PPL, it is impossible to identify either the carbonate due to the very fine granulation or the copper minerals.
Azurite 02: The same section as of the previous image, now in XPL. The carbonate has colorless internal reflections and marked anisotropy. The greenish internal reflections are from secondary copper minerals such as malachite and several others (unidentifiable mineral). The blue internal reflections are from azurite.
Azurite 03: In PPL, copper ore formed by secondary copper minerals. In blue, covellite. In the center of the image, azurite (lighter gray) and another secondary copper mineral.
Azurite 03: The same section as of the previous image, now in XPL. Covellite shows its typical bright orange anisotropy, azurite shows its internal reflections in blue, and the other secondary copper mineral (chrysocolla?) has yellowish-green internal reflections.