A native sample of psilomelane
|The general formula
Ba(Mn2+)(Mn4+)8O16(OH)4 or as (Ba,H2O)2Mn5O10
Barium Manganese Oxide Hydroxide
|Formula mass||590.03 g/mol|
|Color||black with gray pyrolusite bands|
|Crystal habit||Botryoidal, Mammillary, Reniform|
|Fracture||conchoidal and uneven|
|Mohs scale hardness||5.0 - 6.0|
|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 is a group name for hard black manganese oxides including hollandite and romanechite. Psilomelane consists of hydrous manganese oxide with variable amounts of barium and potassium. Psilomelane is erroneously, and uncommonly, known as black hematite, despite not being related to true hematite, which is an iron oxide.
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.
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
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.
Other manganese oxides: