Exsolution lamellae in Pyroxene

Exsolution is a process by which a solid solution phase unmixes into two separate phases in the solid state. Exsolution occurs only in minerals whose compositions vary between two or more pure endmember compositions. Many igneous pyroxenes show exsolution textures of thin lamellae of one pyroxene in a host of a different composition. The lamellae occur as oriented intergrowths that display parallel and herringbone textures. These lamellae result from the exsolution of a separate pyroxene phase from a host grain due to subsolidus re-equilibration (that occurs while the mineral is in the solid state) during slow cooling. During slow cooling the composition of both the host crystal and the lamellae change with temperature as the lamellae increase in size. The width of lamellae depends on the original crystal composition and the cooling rate.

Exsolution lamellae are often planar and their orientation is crystallographically controlled. The result of unmixing in the Ca-Mg-Fe-pyroxenes is commonly visble in thin section as set of thin exsolution lamellae. The miscibility of Ca-poor (pigeonite or orthopyroxene) and Ca-rich (augite or clinopyroxene) reduces with lowering temperature, and each may exolve the other. In the case of augite lamellae exsolving from pigeonite (or pigeonite lamellae exsolving from augite) one or two set of lamellae may develop, possibly simultaneous, one within 20° of (001), the other within 20° of (100). The precise orientation is determined by the lattice parameters of the host and lamellae phases, and is a function of temperature and bulk composition (Fig.1).

Fig.1: Orientation of exsolution lamellae in pyroxene. When pigeonite or augite exsolve they may form exsolution lamellae that form parallel to the (001) plane. At lower temperature the exsolution of Opx or augite result in exsolution lamellae that are parallel to the (100) plane. From Stephen A. Nelson

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
Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). PPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). PPL image. 10x (Field of view = 2mm)
Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). PPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)
Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). PPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)
Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a peridotite from Monte Duria (Adula nappe, Central Alps, Northern Italy). XPL image. 10x (Field of view = 2mm)
Pyroxene with exsolution lamellae in a gabbro (Antarctica). PPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a gabbro (Antarctica). PPL image. 10x (Field of view = 2mm)	Pyroxene with exsolution lamellae in a gabbro (Antarctica). PPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite. PPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. PPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. PPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite crystals. PPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)
Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)
Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite crystals. XPL image. 2x (Field of view = 7mm)	Pigeonite exsolution lamellae in augite. XPL image. 10x (Field of view = 2mm)