Shigaite

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.

Shigaite
Reddish-brown shigaite crystal (2 cm across) with pink rhodochrosite from South Africa
General
CategorySulfate mineral
Formula
(repeating unit)		NaAl3(Mn2+)6(SO4)2(OH)18·12H2O[1]
Strunz classification7.DD.35[2]
Dana classification31.1.2.1[2]
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)[2]
Space groupR3[3]
Unit cella = 9.51 Å, c = 32.83 Å,[2] Z = 3[4]
Identification
ColorYellow, burnt orange, brown, black[4]
TwinningOn {0001}[4]
CleavagePerfect on {0001}[4]
TenacityModerately flexible[4]
Mohs scale hardness2[4]
LusterVitreous to dull[2]
StreakVery pale yellow to white[4]
DiaphaneityTransparent[2]
Specific gravity2.32[3]
Optical propertiesUniaxial (-)[2]
Refractive indexn = 1.546[3]
PleochroismDistinct; O = yellow; E = very pale yellow[4]
Ultraviolet fluorescenceNon-fluorescent[3]

Description
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
  1. Some sources incorrectly list it as the Loi Mine,[6] presumably because of a mistaken reading of uppercase "i" as lowercase "L".

References
  1. Nickel, Ernest H. "IMA/CNMNC List of Mineral Names" (PDF). Materials Data, Inc. Retrieved May 6, 2012.
  2. "Shigaite". Mindat. Retrieved April 20, 2012.
  3. "Shigaite Mineral Data". Webmineral. Retrieved May 20, 2012.
  4. "Shigaite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved April 20, 2012.
  5. Cooper, p. 91.
  6. Hawthorne, Frank C.; et al. (November–December 1986). "New Mineral Names" (PDF). American Mineralogist. 71 (11 & 12): 1546. Retrieved May 17, 2012.
  7. Cooper, p. 96.

Bibliography

Further reading
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