Shigaite
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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.
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Shigaite | |
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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] |
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