Shigaite

Shigaite
Reddish-brown shigaite crystal (2 cm across) with pink rhodochrosite from South Africa
General
Category	Sulfate mineral
Formula (repeating unit)	NaAl3(Mn2+)6(SO4)2(OH)18·12H2O[1]
IMA symbol	Sga[2]
Strunz classification	7.DD.35[3]
Dana classification	31.1.2.1[3]
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)[3]
Space group	R3[4]
Unit cell	a = 9.51 Å, c = 32.83 Å,[3] Z = 3[5]
Identification
Color	Yellow, burnt orange, brown, black[5]
Twinning	On {0001}[5]
Cleavage	Perfect on {0001}[5]
Tenacity	Moderately flexible[5]
Mohs scale hardness	2[5]
Luster	Vitreous to dull[3]
Streak	Very pale yellow to white[5]
Diaphaneity	Transparent[3]
Specific gravity	2.32[4]
Optical properties	Uniaxial (−)[3]
Refractive index	n = 1.546[4]
Pleochroism	Distinct; O = yellow; E = very pale yellow[5]
Ultraviolet fluorescence	Non-fluorescent[4]

Shigaite is a mineral with formula NaAl₃(Mn²⁺)₆(SO₄)₂(OH)₁₈·12H₂O that typically occurs as small, hexagonal crystals or thin coatings. It is named for Shiga Prefecture, Japan, where it was discovered in 1985.^[3] The formula was significantly revised in 1996, identifying sodium as a previously unknown constituent.