Cryptomelane
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Cryptomelane 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... Wikipedia
Crystal class: Dipyramidal (4/m); H-M symbol: (4/m)
Crystal system: Tetragonal
Luster: Metallic to dull
Mohs scale hardness: 6 – 6.5
Space group: I4/m
Unit cell: a = 9.956 Å, b = 2.8705 Å, c = 9.706 Å; β = 90.95°; Z = 1