Iddingsite - MgO * Fe₂O₃ * 4 H₂O

Named by Andrew Cowper Lawson in 1893 for one of the great American petrologists Joseph Paxson Iddings. Iddingsite is a common alteration of olivine during oxidation, hydrothermal and deuteric processes. It appear as a reddish-brown replacement of olivine. Iddingsite is a pseudomorph, and during the alteration process the olivine crystals had their internal structure or chemical composition changed, although the external form has been preserved. The alteration of olivine to iddingsite occurs in a highly oxidizing environment under low pressure and at intermediate temperatures.

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
Olivine altered by Iddingsite, Patagonia. PPL image, 2x (Field of view = 7mm)	Olivine altered by Iddingsite, Patagonia. PPL image, 2x (Field of view = 7mm)	Olivine altered by Iddingsite, Patagonia. PPL image, 2x (Field of view = 7mm)
Olivine partially altered by Iddingsite. PPL image, 2x (Field of view = 7mm)	Olivine partially altered by Iddingsite. XPL image, 2x (Field of view = 7mm)	Olivine partially altered by Iddingsite. PPL image, 10x (Field of view = 2mm)
Olivine partially altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. PPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. XPL image, 10x (Field of view = 2mm)
Olivine partially altered by Iddingsite. PPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. PPL image, 10x (Field of view = 2mm)
Olivine partially altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine partially altered by Iddingsite. PPL image, 10x (Field of view = 2mm)
Olivine crystals altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. PPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. XPL image, 10x (Field of view = 2mm)
Olivine crystals altered by Iddingsite. PPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. PPL image, 10x (Field of view = 2mm)
Olivine crystals altered by Iddingsite. XPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. PPL image, 10x (Field of view = 2mm)	Olivine crystals altered by Iddingsite. XPL image, 10x (Field of view = 2mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). XPL image, 2x (Field of view = 7mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)
Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)	Olivine crystals altered by Iddingsite. Somalia (Africa). PPL image, 2x (Field of view = 7mm)