Cryptomelane – K(Mn⁴⁺,Mn²⁺)₈O₁₆ – is a common oxide, the most common mineral in those very fine-grained Mn oxide masses formerly generically called “psilomelane.”

It is difficult to recognize, so much so that its name means “the hidden dark one” (Greek: kryptos = hidden, melano = dark). It was proposed only in 1942 and approved in 1982, so great was the difficulty of separating the various minerals from these black Mn ores. It occurs as a discreet, inconspicuous material. At best, as mammillated or botryoidal masses that, however, also characterize several other Mn and Fe minerals. Macroscopically, it is indistinguishable from hollandite and several other Mn oxides.

A generic mining term for these compact, banded, earthy black masses, formed basically by a mixture of several cryptocrystalline Mn minerals, is “wad.” If the masses are fibrous, it is probably psilomelane or cryptomelane. Coarser grains can be confused with hollandite and coronadite. “Foamy” or “spongy” structures are psilomelane and pyrolusite. Brown iron ore may be extremely similar macroscopically, but under a reflected light microscope, the internal red to brown reflections identify the material as hematite.

Cryptomelane is classified in the Coronadite Group, in the Hollandite Supergroup. It rarely presents well-formed crystals. At most, subhedral crystals of no more than 2 mm in height. To identify the mineral, it is necessary to employ X-ray Diffraction, Infrared Spectroscopy and chemical methods.

In the field, the hydrogen peroxide test (observing the effervescence with a magnifying glass and waiting a few minutes to confirm the effervescence!) identifies these dark materials as Mn oxides/hydroxides.