Opacitic rim

Opacitic rim or reaction rim is a common structure of hydrated mineral in volcanic rocks such as biotite or amphiboles. The amphiboles, during magma ascent, will decompose in response to the decrease in water pressures in the melt during ascent. Usually the reaction rims on amphiboles consist of a complex assemblage of minerals like clinopyroxene, plagioclase, magnetite and ilmenite. The process of opacitic rim formations can be described as devolatilization process. The amphiboles or biotite crystals release its structurally bound water to the magma in response to the decrease in water pressure in the magma as it ascends to the surface.

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
Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)	Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)	Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)
Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)	Hornblende crystal with opacitic rim. Andesite from Bulgaria. PPL image , 10x (Field of view = 2mm)	Hornblende crystal with opacitic rim. Andesite from Bulgaria. PPL image , 10x (Field of view = 2mm)
Hornblende crystal with opacitic rim. Andesite from Bulgaria. PPL image , 10x (Field of view = 2mm)	Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)	Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)
Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)	Hornblende crystals with opacitic rim. Andesite from Bulgaria. PPL image , 2x (Field of view = 7mm)