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 "aenigma" for "riddle", alluding to the enigmatic, uncertain chemical composition of the mineral. Aenigmatite 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

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Aenigmatite crystal in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystals in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystal in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)
Aenigmatite crystal in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. XPL image , 10x (Field of view = 2mm)	Aenigmatite and aegirine crystals in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. XPL image , 2x (Field of view = 7mm)	Aenigmatite crystals in a pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)
Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystals in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)
Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)
Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)	Aenigmatite crystal in a ignimbritic pantellerite (peralkaline rhyolite), Pantelleria island, Italy. PPL image , 10x (Field of view = 2mm)