Cryptomelane (K(Mn⁴⁺,Mn²⁺)₈O₁₆) is the potassium endmember of the hollandite group, a family of tectomanganates with a 2 × 2 tunnel structure.

Cryptomelane
General
Category	Oxide mineral
Formula (repeating unit)	K(Mn4+,Mn2+)8O16
IMA symbol	Cml[1]
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
Identification
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	[2][3][4][5]

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.^[6]

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.^[2][3]

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.^[2]

• 
  Polyhedral representation of the 2 × 2 tunnel structure of cryptomelane. The black atoms represent K.^[7]