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Kryptomelan2, Maroko.jpg
Category Oxide mineral
(repeating unit)
Strunz classification 4.DK.10
Crystal system Tetragonal
Crystal class Dipyramidal (4/m)
H-M symbol: (4/m)
Space group 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
Crystal 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.
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 6 – 6.5
Luster Metallic to dull
Streak Brownish black
Diaphaneity Opaque
Specific gravity 4.17 – 4.41
Alters to Tarnishes to dull grayish black
References [1][2][3][4]

Cryptomelane is a potassium manganese oxide mineral with formula K(Mn4+,Mn2+)8O16.

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.[5]

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.[1][2]

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 pyrolusite, nsutite, braunite, chalcophanite, manganite and various other manganese oxides.[1]