Summary
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.
| 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] |
Description edit
Shigaite occurs as hexagonal tabular crystals up to 2 cm (0.79 in) in size or as thin films and coatings. The mineral can be yellow, burnt orange, brown or black in color.[5] Shigaite occurs in metamorphosed deposits of manganese ore[5] and is the Mn2+ analogue of motukoreaite.[6]
Structure edit
Shigaite consists of oxycation sheets of [AlMn2+2(OH)6]1+ intercalated with oxyanion sheets of [Na(H2O)6{H2O}6(SO4)2]3−. Linkage between the sheets and within the oxyanion sheet results largely through hydrogen bonding.[6]
History edit
Shigaite was discovered in 1985 in the Ioi Mine,[a] Shiga Prefecture, Japan.[3] The original study, published in the journal Neues Jahrbuch für Mineralogie, Monatshefte,[1] identified the formula as Al4Mn7(SO4)2(OH)22·8H2O.[7] The formula was significantly revised in 1996 using a sample from the N'Chwaning Mine, South Africa.[6] Sodium, discovered to be a component of shigaite, was not identified in the original study. However, an unidentified volatile had been noted that presumably was a sodium-containing complex.[8]
Distribution edit
As of 2012[update], shigaite is known from the following sites:[3]
- Iron Monarch open cut, South Australia, Australia
- Poudrette quarry, Quebec, Canada
- Ioi mine, Shiga Prefecture, Japan
- Wessels Mine, Northern Cape Province, South Africa
- N'Chwaning Mine, Northern Cape Province, South Africa
- Homer Mine, Michigan, United States
- Bengal Mine, Michigan, United States
The type material is housed in the National Museum of Natural History in Washington, D.C. as sample 122089.[5]
Association edit
Shigaite has been found associated with the following minerals:[5]
|
Ioi mine, Japan
|
Wessels Mine, South Africa
|
Iron Monarch, South Australia
|
Notes edit
References edit
- ^ a b Nickel, Ernest H. "IMA/CNMNC List of Mineral Names" (PDF). Materials Data, Inc. Retrieved May 6, 2012.
- ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
- ^ a b c d e f g h i j "Shigaite". Mindat. Retrieved April 20, 2012.
- ^ a b c d "Shigaite Mineral Data". Webmineral. Retrieved May 20, 2012.
- ^ a b c d e f g h i j k l "Shigaite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved April 20, 2012.
- ^ a b c Cooper, p. 91.
- ^ a b Hawthorne, Frank C.; et al. (November–December 1986). "New Mineral Names" (PDF). American Mineralogist. 71 (11 & 12): 1546. Retrieved May 17, 2012.
- ^ Cooper, p. 96.
Bibliography edit
- Cooper, Mark A.; Hawthorne, Frank C. (February 1996). "The crystal structure of shigaite, (AlMn (super 2+)2 (OH)6)3 (SO4)2 Na(H2O)6 {H2O}6, hydrotalcite-group mineral" (PDF). The Canadian Mineralogist. 34 (1): 91–97. ISSN 0008-4476. Retrieved May 23, 2012.