Saussurite

Saussurite, named after the Swiss explorer Horace Benedict de Saussure, who discovered it on the slopes of Mont Blanc is a mineral aggregate which is formed as a hydrothermal alteration product of plagioclase feldspar (saussuritization). Saussuritization is a process by which calcium-bearing plagioclase feldspar is altered to a characteristic assemblage of minerals called saussurite; the typical assemblage formed includes zoisite, chlorite, amphibole, and carbonates. Residual fluids present during the late stages of magmatic crystallization can react with previously formed plagioclase feldspar to form saussurite; the saussurite will be spread through the plagioclase or located near its outer margin. Mafic rocks are especially susceptible to saussuritization owing to their high calcium content; the more calcium-rich portions of plagioclase in acidic rocks also are often saussuritized. The process commonly takes place during the low-grade regional metamorphism of gabbros and basalts, both of which contain plagioclase as an essential component.

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
Plagioclase altered by Epidote. PPL image, 2x (Field of view = 7mm)	Plagioclase altered by Epidote. XPL image, 2x (Field of view = 7mm)	Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)
Plagioclase altered by Epidote. XPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. XPL image, 10x (Field of view = 2mm)
Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. XPL image, 10x (Field of view = 2mm)
Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)	Plagioclase altered by Epidote. PPL image, 10x (Field of view = 2mm)