Kröhnkite is a rare copper sulfate mineral

Sulfate mineral

The sulfate minerals are a class of minerals which include the sulfate ion within their structure. The sulfate minerals occur commonly in primary evaporite depositional environments, as gangue minerals in hydrothermal veins and as secondary minerals in the oxidizing zone of sulfide mineral deposits...

 named after B. Kröhnke who first researched it.

Crystallography

Kröhnkite has monoclinic symmetry (2/m). Monoclinic symmetry implies that the mineral contains three axes of differing length (typically labeled a, b, and c), two of which intersect each other at 90° and one that intersects at an acute angle. Specifically, it belongs to the 2/m symmetry class meaning, the mineral has a 2-fold rotation axis about the b axis. It also has a unique motif of silicon tetrahedra chains and copper octahedra aligned along the c axis and linked together by sodium atoms. Kröhnkite exhibits the optical property birefringence

Birefringence

Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...

; the difference in the two refractive indices of a mineral. Because this mineral is birefringent, it must be anisotropic. Anisotropic minerals cause the velocity of light to vary depending on the direction of travel through the mineral. Kröhnkite is biaxial negative, which reveals that the mineral has two optic axes.

Importance

Kröhnkite has the same general formula ( (X₂M(TO₄)₂(H₂O)₂) ) as minerals which are found in environments affected by hydrothermal alteration, making it important in identifying where such alterations have occurred. Furthermore, the minerals sharing this composition are organized according to three crystal structure types- one being the unique Kröhnkite structure which is often used to describe minerals exhibiting the same chain-like structure.

Discovery and occurrence

Kröhnkite was first researched after an occurrence in the Chuquicamata

Chuquicamata

Chuquicamata, or "Chuqui" as it is more familiarly known, is by digged volume the biggest open pit copper mine in the world, located in the north of Chile, 215 km northeast of Antofagasta and 1,240 km north of the capital, Santiago...

 Mine, Chile

Chile

Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

, and has been reported from a number of locations in the Atacama region. Associated minerals in the discovery location

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

 include; atacamite

Atacamite

Atacamite is a copper halide mineral: a copper chloride hydroxide with formula Cu2Cl3.It was first described for deposits in the Atacama Desert of Chile in 1801....

, blodite

Blödite

Blödite is a hydrated sodium magnesium sulfate mineral with formula: Na2Mg2·4H2O.The mineral is clear to yellow in color and forms monoclinic crystals. A synonym for the mineral is bloedite ....

, chalcanthite

Chalcanthite

Chalcanthite, whose name derives from the Greek, chalkos and anthos, meaning copper flower, is a richly-colored blue/green water-soluble sulfate mineral CuSO4·5H2O. It is commonly found in the late-stage oxidation zones of copper deposits...

, antlerite

Antlerite

Antlerite is a greenish hydrous copper sulfate mineral, with the formula Cu34. It occurs in tabular, acicular, or fibrous crystals with a vitreous luster...

 and natrochalcite. It occurs in the oxidized zone of copper deposits in arid environments.