Chemistry:Psilomelane

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Psilomelane
Mineraly.sk - psilomelan.jpg
General
CategoryOxide minerals
Formula
(repeating unit)
The general formula
Ba(Mn2+)(Mn4+)
8
O
16
(OH)
4
or as (Ba,H
2
O)
2
Mn
5
O
10

Barium Manganese Oxide Hydroxide
Crystal systemMonoclinic
Identification
Formula mass590.03 g/mol
Colorblack with gray pyrolusite bands
Crystal habitBotryoidal, Mammillary, Reniform
Cleavagenone
Fractureconchoidal and uneven
Mohs scale hardness5.0 - 6.0
|re|er}}Sub-Metallic, Dull
Streakbrownish black
DiaphaneityOpaque
Specific gravity3.7 - 4.7
Polish lustervitreous to subadamantine
Solubilityin hydrochloric acid
Other characteristicshard 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.

Formula

Generalized formula may be represented as Ba(Mn2+)(Mn4+)
8
O
16
(OH)
4
or as (Ba,H
2
O)
2
Mn
5
O
10
. 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 has no definite chemical composition 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. The mineral often contains admixed impurities, such as iron hydroxides. It is soluble in hydrochloric acid with evolution of chlorine gas.

History and occurrence

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

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, Hout Bay near Cape Town, and Crimora in Augusta county, Virginia. With pyrolusite it is extensively mined in Vermont, Virginia, Arkansas, and Nova Scotia.

Image gallery

See also

Other manganese oxides:

References