Kröhnkite

Kröhnkite ( Na₂Cu(SO₄)₂•2H₂O ) is a rare copper sulfate mineral named after B. Kröhnke who first researched it.[2] Kröhnkite may be replaced by Saranchinaite, the anhydrous form of the mineral, if heated to temperatures above 200 °C (392 °F)[5]

Kröhnkite
General
Category	Sulfate mineral
Formula (repeating unit)	Na2Cu(SO4)2•2H2O
Strunz classification	7.CC.30
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 5.78 Å, b = 12.58 Å c = 5.48 Å; β = 108.3°; Z = 2
Identification
Color	Blue, dark sky blue, greenish blue, yellowish green
Crystal habit	Encrustations (on matrix), fibrous, massive
Twinning	Common, sometimes heart-shaped
Cleavage	Perfect {010}, good (011), very imperfect {101}
Fracture	Conchoidal
Mohs scale hardness	2.5 – 3.0
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.92
Optical properties	Biaxial (-)
Refractive index	nα = 1.544 nβ = 1.578 nγ = 1.601
Birefringence	0.057
2V angle	78° measured
Solubility	Readily soluble in water
References	[1][2][3][4]

Crystallography

Kröhnkite has monoclinic symmetry (2/m).[4] 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.[6] Specifically, it belongs to the 2/m symmetry class meaning, the mineral has a 2-fold rotation axis about the b axis.[6] It also has a unique motif of silicon tetrahedra chains and copper octahedra aligned along the c axis and linked together by sodium atoms.[4] Kröhnkite exhibits the optical property birefringence; the difference in the two refractive indices of a mineral.[2] 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.[2]

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.[7]

Discovery and occurrence

Kröhnkite was first researched after an occurrence in the Chuquicamata Mine, Chile, and has been reported from a number of locations in the Atacama region. Associated minerals in the discovery location include atacamite, blodite, chalcanthite, antlerite and natrochalcite. It occurs in the oxidized zone of copper deposits in arid environments.[3]

References

1. "Kröhnkite Mineral Data." http://webmineral.com/data/Krohnkite.shtml. Accessed 28 November 2010.
2. "Kröhnkite Mineral Data." http://www.mindat.org/min-2277.html. Accessed 8 October 2010.
3. "Kröhnkite." http://rruff.geo.arizona.edu/doclib/hom/krohnkite.pdf. Accessed 21 November 2010.
4. Hawthorne, F.C. (1975) Refinement of Crystal Structure of Kröhnkite. Acta Crystallographica, 31, 1753- 1755.
5. https://www.mindat.org/min-2277.html. Accessed 17 February 2019.
6. "Crystallography." Mineralogy Tutorials. John Wiley and Sons, Inc. CD-ROM.
7. Herwig, S. , Hawthorne F.C. (2006) The Topology of Hydrogen Bonding in Brandtite, Collinsite and Fairfieldite. The Canadian Mineralogist, 44, 1181- 1196.