Cordierite - (Mg,Fe)₂[Si₅Al₄O₁₈]* nH₂O

Cordierite, which was discovered in 1813, is named after the French geologist Louis Cordier, who first studied this species. Cordierite is always present as a solid solution between Mg-rich cordierite (Mg,Fe)₂Al₃(Si₅AlO₁₈) and Fe-rich sekaninaite (Fe,Mg)₂Al₃(Si₅AlO₁₈). Cordierite structure is it is characterized by hexagonal rings of six silicate tetrahedral (Si₆O₁₈)^12-, where one-half of the oxygen ions of each SiO₄ tetrahedron are bridging oxygens shared with adjoining tetrahedral in the ring. The rings are stacked vertically (parallel to c) and adjoining rings and stacks are united by 3Al³⁺ and 2 Mg²⁺ in tetrahedral and octahedral coordination respectively.

Cordierite is most characteristic of metamorphic rocks formed by high temperature and low stress (confining pressure may range widely) acting on aluminous, often silica-deficient rocks. In thermal contact zones adjoining igneous rocks, cordierite porphyroblasts form in argillaceous sediments in the early stages of heating at the expense of chlorite and is characteristic of the resulting hornfels rocks. With increasing thermal grade, cordierite may well persist as an important mineral in various mineral associations until sediments are literally fused to glass (buchite rocks), in this high-grade rock cordierite is present as indialite (the high grade polymorph) and it is associated with spinel, sillimanite, anorthite, corundum and magnetite. In regional metamorphism, cordierite is formed only in environments where pressures are high and uniform. In these environments, the pre-existing minerals such as kyanite and biotite, become unstable reacting to give cordierite:

1) muscovite + biotite = alkali feldspar + cordierite + garnet
2) biotite + aluminosilicate = cordierite + spinel
3) biotite + aluminosilicate = cordierite + garnet + orthopyroxene

Optical Properties:
• Form: Euhedral to subhedral pseudohexagonal crystals
• Color: Colorless
• Twinning: Cordierite may be untwinned, or may have polysynthetic twinning like plagioclase; however most crystals are complexly twinned by polysynthetic or cyclic forms. Twin lamellae, in basal section, commonly radiate from a common center to yield cyclic star patterns of three, six or twelve rays with composition planes 60° or 30° to position of extinction.
• Birefringence: Usually I order gray-white and never exceed I order red
• Alteration: Cordierite alters rather quickly, beginning on basal parting planes and causing the entire cordierite crystal to assume a yellowish cast. The alteration product is usually a fine-grained, yellow-greenish aggregate (Chlorite+biotite+muscovite) called Pinite.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• 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.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Rounded cordierite crystal in a contact metamorphic rock. Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Rounded cordierite crystal (with cyclic twinning) in a contact metamorphic rock. Sardinia, Italy. XPL image, 10x (Field of view = 2mm)	Rounded cordierite crystals in a contact metamorphic rock. Sardinia, Italy. PPL image, 10x (Field of view = 2mm)
Rounded cordierite crystals (I order gray) in a contact metamorphic rock. Sardinia, Italy. XPL image, 10x (Field of view = 2mm)	Rounded cordierite crystals in a contact metamorphic rock. Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Rounded cordierite crystals (I order gray) in a contact metamorphic rock. Sardinia, Italy. XPL image, 10x (Field of view = 2mm)
Rounded cordierite crystal in a contact metamorphic rock. Sardinia, Italy. PPL image, 10x (Field of view = 2mm)	Rounded cordierite crystal (with cyclic twinning) in a contact metamorphic rock. Sardinia, Italy. XPL image, 10x (Field of view = 2mm)	Rounded cordierite crystals in a contact metamorphic rock. Sardinia, Italy. PPL image, 10x (Field of view = 2mm)
Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)
Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)
Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal in a granulite. XPL image, 10x (Field of view = 2mm)
Cordierite (I order gray) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with polysynthetic twinning) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite enclosed in a Garnet in a granulite. PPL image, 10x (Field of view = 2mm)
Cordierite enclosed in a Garnet in a granulite. PPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with complex twinning) in a granulite. XPL image, 2x (Field of view = 7mm)	Cordierite (I order gray) in a granulite. XPL image, 2x (Field of view = 7mm)
Cordierite (I order gray and with complex twinning) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with complex twinning) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with complex twinning) in a granulite. XPL image, 10x (Field of view = 2mm)
Cordierite (high relief and partially altered along fractures) in a granulite. PPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with polysynthetic twinning) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (high relief and partially altered along fractures) in a granulite. PPL image, 10x (Field of view = 2mm)
Cordierite (I order gray) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray) in a granulite. XPL image, 2x (Field of view = 7mm)	Cordierite (I order gray) in a granulite. XPL image, 10x (Field of view = 2mm)
Cordierite (I order gray and with sericite alteration) in a granulite. XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray) in a granulite. XPL image, 2x (Field of view = 7mm)	Cordierite (I order gray) and bordered by Biotite in a granulite. XPL image, 2x (Field of view = 7mm)
Cyclic twinning in a cordierite crystal. Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal. Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)	Cyclic twinning in a cordierite crystal. Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)
Cyclic twinning in a cordierite crystal. Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with complex twinning). Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)	Cordierite (I order gray and with complex twinning). Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)
Cordierite (I order gray and with complex twinning). Metamorphic xenolith from Vesuvius (Italy). XPL image, 10x (Field of view = 2mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Cordierite - (Mg,Fe)2[Si5Al4O18]* nH2O

Cordierite - (Mg,Fe)₂[Si₅Al₄O₁₈]* nH₂O