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.^[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	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

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.^[5] Shigaite occurs in metamorphosed deposits of manganese ore^[5] and is the Mn²⁺ analogue of motukoreaite.^[6]

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.^[6]

History

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

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

Shigaite has been found associated with the following minerals:^[5]

Ioi mine, Japan rhodochrosite sonolite manganosite pyrochroite jacobsite hausmannite galaxite

Wessels Mine, South Africa rhodochrosite leucophoenicite gageite caryopilite

Iron Monarch, South Australia arsenoclasite gatehouseite hematite hausmannite manganoan ferroan calcite barite gypsum

Notes

1. Some sources incorrectly list it as the Loi Mine,^[7] presumably because of a mistaken reading of uppercase "i" as lowercase "L".