Psilomelane

Psilomelane
A native sample of psilomelane
General
Category	Maganese oxides
Formula (repeating unit)	The general formula Ba(Mn2+)(Mn4+)8O16(OH)4 or as (Ba,H2O)2Mn5O10 Barium Manganese Oxide Hydroxide
Identification
Formula mass	590.03 gm
Color	black with gray pyrolusite bands
Crystal habit	Botryoidal, Mammillary, Reniform
Crystal system	monoclinic
Cleavage	none
Fracture	conchoidal and uneven
Mohs scale hardness	5.0 - 6.0
Luster	Sub-Metallic, Dull
Streak	brownish black
Diaphaneity	Opaque
Specific gravity	3.7 - 4.7
Polish luster	vitreous to subadamantine
Solubility	in hydrochloric acid
Other characteristics	hard black manganese oxides such as hollandite and romanechite

Psilomelane, also known as black hematite, is a group name for hard black manganese oxides such as hollandite and romanechite. Psilomelane consists of hydrous manganese oxide with variable amounts of barium and potassium.

Formula

Generalized formula may be represented as Ba(Mn²⁺)(Mn⁴⁺)₈O₁₆(OH)₄ or as (Ba,H₂O)₂Mn₅O₁₀. It is sometimes considered to be a hydrous manganese manganate, but of doubtful composition. The amount of manganese present corresponds to 70-80% of manganous oxide with 10-15% of available oxygen.

Characteristics

Psilomelane is amorphous and occurs as botryoidal and stalactitic masses with a smooth shining surface and submetallic lustre. The mineral is readily distinguished from other hydrous manganese oxides (manganite and wad) by its greater hardness 5 to 6; the specific gravity varies from 3.7 to 4.7. The streak is brownish black and the fracture smooth. Owing to its amorphous nature, the mineral often contains admixed impurities, such as iron hydroxides. It is soluble in hydrochloric acid with evolution of chlorine gas.

History and occurrence

Dendritic psilomelane on a boulder

The name has reference to this characteristic appearance, from the Greek for (naked, smooth) and (black); a Latinized form is calvonigrite, and a German name with the same meaning is Schwarzer Glaskopf.

It is a common and important ore of manganese, occurring under the same conditions and having the same commercial applications as pyrolusite. It is found at many localities; amongst those which have yielded typical botryoidal specimens may be mentioned the Restormel iron mine at Lostwithiel in Cornwall, Brendon Hills in Somerset, Hoy in Orkney, Sayn near Coblenz, and Crimora in Augusta county, Virginia. With pyrolusite it is extensively mined in Vermont, Virginia, Arkansas, and Nova Scotia.

Polished specimen of manganese ore from the Batesville, Arkansas district. Slab of manganese ore showing an intimate mixture of hausmannite and psilomelane in a roughly zonal arrangement and a radiating mass of white barite at the center. The light steel-gray mineral and the black mineral immediately adjacent to the barite are psilomelane. The rest of the black mineral is hausmannite. Natural size.

See also

Other manganese oxides:

• Birnessite
• Pyrolusite

References

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 
• Mindat with locality data
• Webmineral
• Mineral galleries