CHALCANTHITE

Chalcanthite – CuSO4.5H2O – is a relatively common sulfate, one of several secondary copper minerals. It is part of oxidized copper ores in arid climates, but only rarely is or has been the main ore mineral of the deposit.

It has some demand as a collector’s mineral, but needs to be stored in compatible humidity conditions because it dehydrates easily. In addition, it is easy to grow very aesthetic crystals of chalcanthite in the laboratory; therefore, care must be taken with pieces found for sale, as there are synthetic pieces sold as natural.

It is classified in the Chalcanthite Group, constituting the copper analogue of siderotyl (Fe) and pentahydrite (Mg). It may contain Fe, Co, and Mg. There is a variety with Fe.

It is soluble in water and, once exposed to air, dehydrates forming a lighter-colored powder. It has a peculiar, sweet, metallic taste, but this test should be avoided to prevent copper poisoning.

The fact that it is water-soluble makes the production of thin sections and polished sections much more difficult and laborious, as it is necessary to work with mineral oil as a lubricant at all stages (sawing, grinding, polishing). Furthermore, the ore is usually cavernous (drusic), requiring impregnation.

1. Characteristics

Crystal system: Triclinic pinacoidal.          

Color: Light or dark blue, in very strong, intense colors. It can also be green or blue-green, also in strong colors.

Habit: Short prismatic, may be tabular, but crystals (up to 2 cm) are rare. Usually massive, fibrous, granular, crusty, stalactitic.       

Cleavage: {1-10} imperfect, {111} imperfect, {110} in traces.       

Tenacity: Brittle.        

Twinning: Rare, on {010}.       

Fracture: Conchoidal.       

Mohs Hardness: 2.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, resinous.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.28

 

2. Geology and Deposits

Chalcanthite is a secondary copper mineral formed by rapid oxidation in late oxidation zones developed over copper sulfide deposits, most commonly in arid climates where it is not dissolved by groundwater, as it is soluble in water.

It also occurs in abandoned mines, where it crystallizes on the mine walls by the action of acidic surface water on veins containing copper sulfides.

 

3. Mineral Associations

It occurs associated with gangue minerals characteristic of oxidation zones, such as quartz, carbonates (calcite, aragonite), “limonite” (goethite), and gypsum.

It occurs with other hydrated Cu sulfates (brochantite, devilline, connellite, antlerite), with hydrated Fe sulfates (rhomboclase, fibroferrite, rhomerite, copiapite, amaranthite, melanterite), and hydrated Al (+-Fe) sulfates (halotrichite, alunogen, coquimbite).

Also with other secondary Cu minerals such as malachite, azurite, covellite, and native copper.

Also with pyrite and chalcopyrite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα:  1.514      nβ: 1.537     nγ: 1.543

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless to pale blue. 

Relief: Low           

Cleavage: {1-10} imperfect, {111} imperfect and {110} in traces.

Habits: Rare, short prismatic crystals, may be tabular. Generally massive, granular, fibrous, crusty, or stalactitic.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.029, corresponding to colors up to half of the 2nd order: strong, intense colors in orange, red, blue, and green.

Extinction: Probably oblique.           

Elongation sign: No information available.            

Twins: Rare, by {010}, forming crosses.         

Zoning: No.             

CONVERGENT LIGHT

Character: B(-)          

2V angle: 56º         

Alterations: It dehydrates, forming a lighter-colored powder.          

May be confused with:  other blue secondary copper minerals.     

Caption.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying chalcanthite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with chalcanthite, such as pyrite, chalcopyrite, and covellite.

Sample preparation:        

PLANE POLARIZED LIGHT – PPL

Reflection color:        

Pleochroism:       

Reflectivity:         

Bireflectance:        

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:         

Internal reflections:       

May be confused with:        

General Characteristics: 

Caption.