Aenigmatite

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Aenigmatite
Aenigmatite - Mineralogisches Museum Bonn2.jpg
Aenigmatite from Kangerdluarsuk, Greenland
General
Category Inosilicates
Formula
(repeating unit)
Na2Fe2+5TiSi6O20
Strunz classification 09.DH.40
Crystal symmetry Triclinic pinacoidal
H-M symbol: 1
Space group: P1
Unit cell a = 10.415(1) Å, b = 10.840(1) Å, c = 8.931(1) Å;
α = 105.107(4)°, β = 96.610(5)°, γ = 125.398(4)°;
Z = 2
Identification
Color Velvet-black
Crystal habit Poorly developed prismatic crystals, occurring as irregular clusters
Crystal system Triclinic - pseudomonoclinic
Twinning Complex by rotation perpendicular to (011) or about [010] of the pseudomonoclinic cell; polysynthetic
Cleavage Good on {010} and {100}
Fracture Uneven
Tenacity Brittle
Mohs scale hardness 5.5
Luster Vitreous to greasy
Streak Reddish brown
Diaphaneity Translucent to opaque
Specific gravity 3.81
Optical properties Biaxial (+)
Refractive index nα = 1.780 - 1.800 nβ = 1.800 - 1.820 nγ = 1.870 - 1.900
Birefringence δ = 0.090 - 0.100
Pleochroism X = yellow brown; Y = red-brown; Z = dark brown to black
2V angle Measured: 27° to 55°
Dispersion r < 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]