Shigaite

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Shigaite
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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]
Strunz classification 7.DD.35[2]
Dana classification 31.1.2.1[2]
Crystal symmetry Trigonal - rhombohedral
H-M symbol (3)
Space group: R3[3]
Unit cell a = 9.51 Å, c = 32.83 Å,[2] Z = 3[4]
Identification
Color Yellow, burnt orange, brown, black[4]
Crystal system Trigonal[2]
Twinning On {0001}[4]
Cleavage Perfect on {0001}[4]
Tenacity Moderately flexible[4]
Mohs scale hardness ~2[4]
Luster Vitreous to dull[2]
Streak Very pale yellow to white[4]
Diaphaneity Transparent[2]
Specific gravity 2.32[3]
Optical properties Uniaxial (-)[2]
Refractive index n = 1.546[3]
Pleochroism Distinct; O = yellow; E = very pale yellow[4]
Ultraviolet fluorescence Non-fluorescent[3]

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.[2] The formula was significantly revised in 1996, identifying sodium as a previously unknown constituent.

Description

File:Shigaite-Rhodochrosite-201388.jpg
Dark-red and yellow shigaite on pink rhodochrosite

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.[4] Shigaite occurs in metamorphosed deposits of manganese ore[4] and is the Mn2+ analogue of motukoreaite.[5]

Structure

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.[5]

History

Shigaite was discovered in 1985 in the Ioi Mine,[lower-alpha 1] Shiga Prefecture, Japan.[2] The original study, published in the journal Neues Jahrbuch für Mineralogie, Monatshefte,[1] identified the formula as Al4Mn7(SO4)2(OH)22·8H2O.[6] The formula was significantly revised in 1996 using a sample from the N'Chwaning Mine, South Africa.[5] 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.[7]

Distribution

As of 2012, shigaite is known from the following sites:[2]

  • 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.[4]

Association

Shigaite has been found associated with the following minerals:[4]

Notes

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References

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Bibliography

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Further reading

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