Manganite

Manganite
Manganite, Ilfeld, Thuringia, Germany (size: 5.2×4.1×2.7 cm)
General
Category	Oxide mineral
Formula (repeating unit)	MnO(OH)
Strunz classification	4.FD.15
Dana classification	06.01.03.01
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 5.3 Å, b = 5.278 Å, c = 5.307 Å; β = 114.36°; Z = 4
Identification
Color	Dark steel-gray to black, reddish brown in transmitted light, gray-white with brownish tint, with blood-red internal reflections in reflected light
Crystal habit	Slender prismatic crystals, massive to fibrous, pseudo-orthorhombic
Twinning	Contact and penetration twins on {011}
Cleavage	{010} perfect, {110} and {001} good
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	4
Luster	Sub-metallic
Streak	Reddish brown to nearly black
Diaphaneity	Opaque, transparent on thin edges
Specific gravity	4.29–4.34
Optical properties	Biaxial (+)
Refractive index	nα = 2.250(2) nβ = 2.250(2) nγ = 2.530(2)
Birefringence	δ = 0.280, Bireflectance: distinct in grays
Pleochroism	Faint
2V angle	Small
Dispersion	Very strong
References

Manganite is a mineral composed of manganese oxide-hydroxide, MnO(OH), crystallizing in the monoclinic system (pseudo-orthorhombic). Crystals of manganite are prismatic and deeply striated parallel to their length; they are often grouped together in bundles. The color is dark steel-grey to iron-black, and the luster brilliant and submetallic. The streak is dark reddish brown. The hardness is 4, and the specific gravity is 4.3. There is a perfect cleavage parallel to the brachypinacoid, and less-perfect cleavage parallel to the prism faces. Twinned crystals are not infrequent.

The mineral contains 89.7% manganese sesquioxide; it dissolves in hydrochloric acid with evolution of chlorine.

Manganite occurs with other manganese oxides in deposits formed by circulating meteoric water in the weathering environment in clay deposits and laterites. It forms by low temperature hydrothermal action in veins in association with calcite, barite, and siderite. Often associated with pyrolusite, braunite, hausmannite and goethite.

Manganite occurs in specimens exhibiting good crystal form at Ilfeld in the Harz Mountains of Germany, where the mineral occurs with calcite and barite in veins traversing porphyry. Crystals have also been found at Ilmenau in Thuringia, Neukirch near Sélestat in Alsace (newkirkite), Granam near Towie in Aberdeenshire, and in Upton Pyne near Exeter, UK and Negaunee, Michigan, United States, and in the Pilbara of Western Australia. Good crystals have also been found at Atikokan, Ontario and Nova Scotia, Canada. As an ore of manganese it is much less abundant than pyrolusite or psilomelane.