Aenigmatite - Na2Fe52+TiO2[Si6O18]

Aenigmatite, also known as Cossyrite after Cossyra, the ancient name of Pantelleria, is a the only member of the amphibole group of minerals that has triclinic symmetry.
It was first described by August Breithaupt in 1865 for an occurrence in the Ilimaussaq intrusive complex of southwest Greenland. Its name comes from the Greek word for "riddle".

The chemical formula is Na2Fe52+TiO2[Si6O18] and its structure consists of single tetrahedral chains with a repeat unit of four and complex side branches.

Aenigmatite is confined to igneous rocks containing a high percentage of soda and it is primarily found in peralkaline volcanic rocks, pegmatites, and granites as well as silica-poor intrusive rocks.

Optical Properties:
• Relief: High
• Color: Brown or black
• Interference colors: High interference color, but masked by the color of the mineral
• cleavage: Aenigmatite is usually found showing perfect cleavage in one direction which can be seen more clearly exhibited when viewed with the aid of polarizing light
• Form: most commonly found forming lamellar crystal forms or thin laminae producing a lamellar structure

Bibliography


• Klein Cornelis: Mineralogia. Zanichelli 2004
• W. A. Deer, R. A. Howie, J. Zussman (1994): Introduzione ai Minerali che costituiscono le rocce. Zanichelli editore.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
Stephen. A. Nelson


Photo
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Aenigmatite crystal in a Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystals in a Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a Pantellerite (peralkaline rhyolite). XPL image , 10x (Field of view = 2mm)
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Aenigmatite and Aegirine crystals in a Pantellerite (peralkaline rhyolite). XPL image , 2x (Field of view = 7mm)
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Aenigmatite crystals in a Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystals in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)
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Aenigmatite crystal in a ignimbritic Pantellerite (peralkaline rhyolite). PPL image , 10x (Field of view = 2mm)