{{Short description|Rare copper sulfate mineral}} {{Infobox mineral | name = Kröhnkite | category = Sulfate mineral | boxwidth = | boxbgcolor = | image = Krohnkite-21581.jpg | imagesize = 260px | caption = | formula = Na<sub>2</sub>Cu(SO<sub>4</sub>)<sub>2</sub>•2H<sub>2</sub>O | IMAsymbol = Khk<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | strunz = 7.CC.30 | system = Monoclinic | class = Prismatic (2/m) <br /><small>(same H-M symbol)</small> | symmetry = ''P2''<sub>1</sub>/c | unit cell = a = 5.78 Å, b = 12.58 Å <br />c = 5.48 Å; β = 108.3°; Z = 2 | color = Blue, dark sky blue, greenish blue, yellowish green | habit = Encrustations (on matrix), fibrous, massive | twinning = Common, sometimes heart-shaped | cleavage = Perfect {010}, good (011), very imperfect {101} | fracture = Conchoidal | tenacity = | mohs = 2.5 – 3.0 | luster = Vitreous | refractive = n<sub>α</sub> = 1.544 n<sub>β</sub> = 1.578 n<sub>γ</sub> = 1.601 | opticalprop = Biaxial (-) | birefringence = 0.057 | 2V = 78° measured | streak = White | gravity = 2.92 | density = | melt = | fusibility = | diagnostic = | solubility = Readily soluble in water | diaphaneity = Transparent | other = | references = <ref name=Webmin>"Kröhnkite Mineral Data." http://webmineral.com/data/Krohnkite.shtml. Accessed 28 November 2010.</ref><ref name="mindat"/><ref name=rruff/><ref name="hawthorne"/> }} '''Kröhnkite''' ( Na<sub>2</sub>Cu(SO<sub>4</sub>)<sub>2</sub>•2H<sub>2</sub>O ) is a rare copper sulfate mineral named after B. Kröhnke who first researched it.<ref name="mindat">{{mindat|name=Kröhnkite Mineral Data |id=2277|accessdate=8 October 2010}}</ref> Kröhnkite may be replaced by Saranchinaite, the anhydrous form of the mineral, if heated to temperatures above 200 °C (392 °F).<ref name="mindat"/>
==Crystallography== Kröhnkite has monoclinic symmetry (2/m).<ref name="hawthorne">{{cite journal |last1=Hawthorne |first1=F. C. |last2=Ferguson |first2=R. B. |title=Refinement of the crystal structure of kröhnkite |journal=Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry |date=15 June 1975 |volume=31 |issue=6 |pages=1753–1755 |doi=10.1107/S0567740875006048|bibcode=1975AcCrB..31.1753H }}</ref> 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.<ref name="mintuts">{{cite book |last1=Schwarzenbach |first1=Dieter |title=Crystallography |date=1996 |publisher=John Wiley |location=New York |isbn=978-0-471-95598-6}}</ref> Specifically, it belongs to the 2/m symmetry class meaning, the mineral has a 2-fold rotation axis about the b axis.<ref name="mintuts"/> It also has a unique motif of sulfate tetrahedra chains and copper octahedra aligned along the c axis and linked together by sodium atoms.<ref name="hawthorne"/> Kröhnkite exhibits the optical property birefringence; the difference in the two refractive indices of a mineral.<ref name="mindat"/> 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.<ref name="mindat"/>
==Importance== Kröhnkite has the same general formula ( (X<sub>2</sub>M(TO<sub>4</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>) ) 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.<ref name="herwig">{{cite journal |last1=Herwig |first1=S. |last2=Hawthorne |first2=F. C. |title=The topology of hydrogen bonding in brandtite, collinsite and fairfieldite |journal=The Canadian Mineralogist |date=1 October 2006 |volume=44 |issue=5 |pages=1181–1196 |doi=10.2113/gscanmin.44.5.1181|bibcode=2006CaMin..44.1181H }}</ref>
==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.<ref name=rruff>{{cite web |last1=Anthony |first1=John W. |last2=Bideaux |first2=Richard A. |last3=Bladh |first3=Kenneth W. |last4=Nichols |first4=Monte C. |title=Kröhnkite |url=http://www.handbookofmineralogy.org/pdfs/krohnkite.pdf |website=Handbook of Mineralogy |publisher=Mineral Data Publishing |access-date=22 May 2022 |date=2005}}</ref>
==References== {{Reflist}}
{{DEFAULTSORT:Krohnkite}} Category:Sulfate minerals Category:Copper(II) minerals Category:Monoclinic minerals Category:Minerals in space group 14