SHIGAITE

    Class : Sulfates, chromates, molybdates
    Subclass : Hydrated sulfates
    Crystal system : Trigonal
    Chemistry : Mn6Al3(OH)18Na(H2O)6(SO4)2 6H2O
    Rarity : Very rare


Shigaite is a rare mineral classified in the sulfate family. This mineral is typically a byproduct formed in metamorphosed manganese deposits. It is often associated with minerals such as rhodochrosite or hausmannite, depending on the locality. First discovered at the Ioi mine in Shiga Prefecture, Japan, its name pays homage to this region. Shigaite occurs as hexagonal tabular crystals, sometimes reaching nearly 3 cm, often grouped in lamellar aggregates or rosettes. Cleavage is perfect on {0001}. Shigaite varies in color from pale yellow to bright yellow, golden, or burnt orange, becoming brown to black when weathered. Its luster ranges from vitreous to dull, with a low hardness of about 2 on the Mohs scale. Due to its rarity, shigaite has no significant industrial applications, but it is prized by collectors because of its aesthetic appeal.


Photo : Shigaite on rhodochrosite from N'Chwaning I Mine, Kuruman, Kalahari manganese fields, Northern Cape Province, South Africa

2.8 cm Shigaite from N'Chwaning I Mine, Kuruman, South Africa © Bruce Cairncross
3 cm Shigaite and rhodochrosite from N'Chwaning I Mine, South Africa © Kyle Kevorkian
Shigaite from Iron Monarch, Iron Knob, Australia © Stephan Wolfsried
Shigaite from Homer Mine, Iron County, Michigan, USA © Steve Stuart

Shigaite in the World

The most impressive crystals of shigaite, reaching up to 3 cm, come from the N'Chwaning I mine, near Kuruman, in the Cape Province of South Africa. This locality is renowned for the exceptional quality of its specimens, often associated with minerals such as rhodochrosite. Very good crystals also come from the United States, the Bengal Cannon Mine and the Homer Mine, in Iron County, Michigan, have produced centimeter-sized and high-quality crystals. The type find, however, comes from the Ioi mine, located in Shiga Prefecture, Japan, where shigaite was first described in metamorphosed deposits of manganese in plasters. In Australia, the Iron Monarch quarry, at Iron Knob, is another notable source, with isolated millimeter-sized crystals.

Shigaite in France

This mineral is not present in the French underground.

Twinning

A twin is reported on {0001} for this mineral species.

Fakes and treatments

No fake inventories for this mineral species.


Hardness : 2
Density : 2.32
Fracture : Irregular
Trace : Pale yellow to white

TP : Translucent to transparent
RI : 1.546
Birefringence : Undetermined
Optical character : Uniaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : -

Magnetism : None
Radioactivity : None

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