Chemistry:Aenigmatite

Short description: Sapphirine supergroup, single chain inosilicate mineral

Aenigmatite
Aenigmatite from Kangerdluarsuk, Greenland
General
Category	Inosilicates
Formula (repeating unit)	Na2Fe2+5TiSi6O20
Strunz classification	9.DH.40
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 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
Color	Velvet-black
Crystal habit	Poorly developed prismatic crystals, occurring as irregular clusters; 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
|re|er}}	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

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 in March 1980 in South Yemen. Other notable studied occurrences include:

• Narsaarsuk and elsewhere in Greenland.
• The Khibiny and Lovozero alkaline massifs on Kola Peninsula, Russia .
• The Yenisei Range, Krasnoyarsk Krai, Russia .
• The volcanic island of Pantelleria, Italy.
• In the United States, from Granite Mountain, near Little Rock, Pulaski County, Arkansas, and Santa Rosa, Sonoma County, California.
• In Australia, from Warrumbungle volcano, Nandewar volcano, and the Mount Warning complex, New South Wales; and the Peak Range Province, Queensland.
• In Canada, from Mount Edziza, the Ilgachuz and Rainbow Range shield complexes.
• From Logan Point quarry, Dunedin Volcano, New Zealand.

See also

• List of minerals