Shigaite
| Shigaite | |
|---|---|
Reddish-brown shigaite crystal (2 cm across) with pink rhodochrosite from South Africa | |
| General | |
| Category | Sulfate mineral |
| Formula | NaAl3(Mn2+)6(SO4)2(OH)18·12H2O |
| IMA symbol | Sga |
| Strunz classification | 7.DD.35 |
| Dana classification | 31.1.2.1 |
| Crystal system | Trigonal |
| Crystal class | Rhombohedral (3) H-M symbol: (3) |
| Space group | R3 |
| Unit cell | a = 9.51 Å, c = 32.83 Å, Z = 3 |
| Identification | |
| Color | Yellow, burnt orange, brown, black |
| Twinning | On {0001} |
| Cleavage | Perfect on {0001} |
| Tenacity | Moderately flexible |
| Mohs scale hardness | 2 |
| Luster | Vitreous to dull |
| Streak | Very pale yellow to white |
| Diaphaneity | Transparent |
| Specific gravity | 2.32 |
| Optical properties | Uniaxial (−) |
| Refractive index | n = 1.546 |
| Pleochroism | Distinct; O = yellow; E = very pale yellow |
| Ultraviolet fluorescence | Non-fluorescent |
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. The formula was significantly revised in 1996, identifying sodium as a previously unknown constituent.