Aenigmatite

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Aenigmatite
Aenigmatite - Mineralogisches Museum Bonn2.jpg
Aenigmatite from Kangerdluarsuk, Greenland
General
CategoryInosilicates
Formula
(repeating unit)
Na2Fe2+5TiSi6O20
Strunz classification9.DH.40
Crystal systemTriclinic
Crystal classPinacoidal (1)
(same H-M symbol)
Space groupP1
Unit cella = 10.415(1), b = 10.840(1)
c = 8.931(1) [Å]; Z = 2
α = 105.107(4)°
β = 96.610(5)°
γ = 125.398(4)°
Identification
ColorVelvet-black
Crystal habitPoorly developed prismatic crystals, occurring as irregular clusters; pseudomonoclinic
TwinningComplex by rotation perpendicular to (011) or about [010] of the pseudomonoclinic cell; polysynthetic
CleavageGood on {010} and {100}
FractureUneven
TenacityBrittle
Mohs scale hardness5.5
LusterVitreous to greasy
StreakReddish brown
DiaphaneityTranslucent to opaque
Specific gravity3.81
Optical propertiesBiaxial (+)
Refractive indexnα = 1.780 - 1.800 nβ = 1.800 - 1.820 nγ = 1.870 - 1.900
Birefringenceδ = 0.090 - 0.100
PleochroismX = yellow brown; Y = red-brown; Z = dark brown to black
2V angleMeasured: 27° to 55°
Dispersionr < v; very strong
References[1][2][3]

Aenigmatite, also known as Cossyrite after Cossyra, the ancient name of Pantelleria, is a sodium, iron, titanium inosilicate mineral. The chemical formula is Na2Fe2+5TiSi6O20 and its structure consists of single tetrahedral chains with a repeat unit of four and complex side branches. It forms brown to black triclinic lamellar crystals. It has Mohs hardness of 5.5 to 6 and specific gravity of 3.74 to 3.85. Aenigmatite forms a solid-solution series with wilkinsonite, Na2Fe2+4Fe3+2Si6O20.

Aenigmatite is primarily found in peralkaline volcanic rocks, pegmatites, and granites as well as silica-poor intrusive rocks. It was first described by August Breithaupt in 1865 for an occurrence in the Ilimaussaq intrusive complex of southwest Greenland. Its name comes from αίνιγμα, the Greek word for "riddle".

It was also reported from the Kaidun meteorite, possibly a Mars meteorite, which landed on March 1980 in South Yemen. Other notable studied occurrences include:

References[edit]