Shigaite , Rhodochrosite Locality: N'Chwaning Mines, Kuruman , Kalahari manganese fields, Northern Cape Province , South Africa ( Locality at mindat.org ) Size: 2.7 x 2.4 x 1.7 cm. A superb bladed, dark reddish crystal of Shigaite on Rhodochrosite is covered by additional yellow/golden shigaite crystals. This is a super thumbnail from a small and unusual find, according to Charlie. The vitreous luster is fantastic, the combination fascinating. Ex. Charlie Key. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Shigaite
A manganese-aluminium sulfate hydrate forming small hexagonal plates, discovered in 1985 in Japan's Shiga Prefecture.
Intro adapted from Wikipedia (CC BY-SA).
Shigaite is an IMA-recognized mineral species in the sulfate class, with the chemical formula Mn₆Al₃(OH)₁₈[Na(H₂O)₆](SO₄)₂·6H₂O.
It crystallizes in the hexagonal crystal system (space group R-3). It is a member of the Woodwardite, Hydrotalcite structural group.
Key properties: Mohs hardness 2; specific gravity 2.32; Very pale yellow to white streak; Vitreous to dull luster; refractive index 1.546; optically uniaxial.
IMA symbol Sga, IMA status: approved, named in 1984.
Found as a rare secondary mineral in metamorphosed manganese ore deposits.
It is found associated with Rhodochrosite, sonolite, manganosite, pyrochroite, jacobsite, hausmannite, galaxite (Ioi mine, Japan); rhodochrosite, leucophoenicite, gageite.
The name refers to Shiga Prefecture, Japan, within which the original locality, the Ioi mine, is located.