Cryptomelane

{{Short description|Oxide mineral}} {{infobox mineral | name = Cryptomelane | boxwidth = | boxbgcolor = | image = Kryptomelan2, Maroko.jpg | imagesize = 260px | alt = | caption = | category = Oxide mineral | formula = K(Mn4+,Mn2+)8O16 | IMAsymbol=Cml<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> | molweight = | strunz = 4.DK.10 | dana = | system = Tetragonal | class = Dipyramidal (4/m) H-M symbol: (4/m) | symmetry = ''I4/m'' | unit cell = a = 9.956 Å, b = 2.8705 Å, c = 9.706 Å; β = 90.95°; Z = 1 | color = Steel-gray to bluish gray; light tan or gray in reflected light | colour = | habit = Rare as subhedral crystals, commonly as compact fine-grained masses, banded colloform, botryoidal, or radial fibrous aggregates | twinning = Twinning developed on (010) and (101) resulting in a pseudotetragonal unit cell. | cleavage = | fracture = Conchoidal | tenacity = Brittle | mohs = 6 – 6.5 | luster = Metallic to dull | streak = Brownish black | diaphaneity = Opaque | gravity = 4.17 – 4.41 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = Tarnishes to dull grayish black | other = | references = <ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/cryptomelane.pdf Handbook of Mineralogy]</ref><ref name=Mindat>[http://www.mindat.org/min-1164.html Mindat.org]</ref><ref name=Webmin>[http://www.webmineral.com/data/Cryptomelane.shtml Webmineral data]</ref><ref name=atlas>[https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Cryptomelane Mineralienatlas]</ref> }} '''Cryptomelane''' (K(Mn4+,Mn2+)8O16) is the potassium endmember of the hollandite group, a family of tectomanganates with a 2 × 2 tunnel structure.

In 1942 the name ''cryptomelane'' was proposed as part of an effort to sort out the manganese oxide minerals referred to as ''psilomelane''. Cryptomelane was identified and defined based on X-ray diffraction studies of samples from Tombstone, Arizona; Deming, New Mexico; Mena, Arkansas; and Philipsburg, Montana.<ref>[http://www.minsocam.org/ammin/AM27/AM27_607.pdf Richmond, W. E., and Fleischer, M., ''Cryptomelane, a new name for the commonest of the "psilomelane" minerals,'' American Mineralogist, 27, 607, 1942]</ref>

Cryptomelane was approved in 1982 by the International Mineralogical Association (IMA). The type locality is the Tombstone District, Cochise County, Arizona, US. The name comes from the Greek for hidden and black, in reference to the confusion and difficulty in recognition of the various black manganese oxide minerals referred to as ''psilomelane'', the collective term for hard manganese oxides.<ref name=HBM/><ref name=Mindat/>

It is of rather common occurrence in oxidized manganese deposits where it occurs as replacements and open space fillings in veins and vugs. It occurs in association with hollandite, pyrolusite, nsutite, braunite, chalcophanite, manganite and various other manganese oxides.<ref name=HBM/>

<gallery widths="200px" heights="200px" perrow="1"> File:Hollandite.jpg|Polyhedral representation of the 2 × 2 tunnel structure of cryptomelane. The black atoms represent K.<ref name=A>Post JE, Von Dreele RB, Buseck P (1982) Acta Crystallographica B38: 1056-1065</ref> </gallery>

==References== {{Reflist}}

Category:Manganese(II,IV) minerals Category:Oxide minerals Category:Tetragonal minerals Category:Minerals in space group 87