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



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
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Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(3).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(4).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(5).jpg

Hornblende crystal with Opacitic rim. PPL image , 10x (Field of view = 2mm)
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Hornblende crystal with Opacitic rim. PPL image , 10x (Field of view = 2mm)
bordoopaciticoanfibolo(7).jpg

Hornblende crystal with Opacitic rim. PPL image , 10x (Field of view = 2mm)
bordoopaciticoanfibolo(9).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(10).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(11).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)
bordoopaciticoanfibolo(12).jpg

Hornblende crystals with Opacitic rim. PPL image , 2x (Field of view = 7mm)