Kröhnkite ( Na₂Cu(SO₄)₂•2H₂O ) is a rare copper

sulfate mineral The sulfate minerals are a class of minerals that 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 mine ...

named after B. Kröhnke who first researched it. Kröhnkite may be replaced by Saranchinaite, the anhydrous form of the mineral, if heated to temperatures above 200 °C (392 °F).

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; 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 Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

, 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 ( ; referred to as Chuqui for short) is the largest open pit copper mine in terms of excavated volume in the world. It is located in the north of Chile, just outside Calama, at above sea level. It is northeast of Antofagasta and ...

Mine,

Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...

, and has been reported from a number of locations in the

Atacama The Atacama Desert ( es, Desierto de Atacama) is a desert plateau in South America covering a 1,600 km (990 mi) strip of land on the Pacific coast, west of the Andes Mountains. The Atacama Desert is the driest nonpolar desert in the w ...

region. Associated minerals in the discovery location include

atacamite Atacamite is a copper halide mineral: a copper(II) chloride hydroxide with formula Cu2Cl(OH)3. It was first described for deposits in the Atacama Desert of Chile in 1801 by D. de Fallizen. The Atacama Desert is also the namesake of the mineral. ...

, blodite,

chalcanthite Chalcanthite (, ) is a richly colored blue-green water-soluble sulfate mineral . It is commonly found in the late-stage oxidation zones of copper deposits. Due to its ready solubility, chalcanthite is more common in arid regions. Chalcanthite i ...

antlerite Antlerite is a greenish hydrous copper sulfate mineral, with the formula Cu3(SO4)(OH)4. It occurs in tabular, acicular, or fibrous crystals with a vitreous luster. Originally believed to be a rare mineral, antlerite was found to be the primary o ...

and natrochalcite. It occurs in the

oxidized zone Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the ''classic'' gossan or iron cap all that remains is iron oxides and quartz, ofte ...

of copper deposits in arid environments.

References

{{DEFAULTSORT:Krohnkite Sulfate minerals Copper(II) minerals Monoclinic minerals Minerals in space group 14