Corona texture

Corona texture: several concentric layers of one or more minerals completely encircling an older phase. The layers (which range from one to five in number) represent a sequence of reactions that have taken place (none to completion) to replace the mineral in the core or center of the corona. It is a general term that has been applied to reaction rims.

A change in metamorphic conditions can give rise to porphyroblast growth, or to partial replacement of some minerals by others. Such replacement usually occurs along grain boundaries and causes development of reaction rims. Coronas can be monomineralic or polymineralic and can be divided into several geometric types. If they form closed rings around grains (shells in three dimensions), they are known as coronas. Monomineralic coronas are also known as moats; polymineralic ones composed of an intergrowth of small elongate new grains are known as symplectitic coronas. The structure of lamellar or vermicular fine-grained intergrown material is known as symplectite. Coronas form during both prograde or retrograde metamorphism.

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.

Sapphirine (green) corona on sillimanite (beige), the colorless crystals are cordierite. PPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (high interference colors), the gray crystals are cordierite. PPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (beige), the colorless crystals are cordierite. PPL image, 10x (Field of view = 2mm)
Sapphirine (green) corona on sillimanite (high interference colors), the gray crystals are cordierite. XPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (beige), the colorless crystals are cordierite and the high relief colorless crystal is corundum. PPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (high interference colors), the gray crystals are cordierite and the yellow, anhedral crystal is corundum. XPL image, 10x (Field of view = 2mm)
Sapphirine (green) corona on sillimanite (beige), the colorless crystals are cordierite. PPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (high interference colors), the gray crystals are cordierite. XPL image, 10x (Field of view = 2mm)	Sapphirine (green) corona on sillimanite (beige), the colorless crystals are cordierite. PPL image, 10x (Field of view = 2mm)
Corona of plagioclase + spinel on garnet in an eclogite from Sardinia. PPL image, 10x (Field of view = 2mm)	Corona of plagioclase + spinel on garnet in an eclogite from Sardinia. PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)
Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). PPL image, 10x (Field of view = 2mm)	Yoderite (blue) on kyanite. Mautia Hill (Tanzania). Mautia Hill (Tanzania). XPL image, 10x (Field of view = 2mm)