Xenocryst

Xenocrysts are "foreign" minerals incorporated into the magma during magma scent or during xenoliths fragmentation. Depending on foreign material and magma composition and temperature and available time, the foreign material chemically equilibrates with the melt to varying degree. The thermal and chemical principles of assimilation were enounced by bowen (1928) many decades ago. Assimilation require thermal energy, the source of which can only be the magma itself. Heat from the magma has two sources:

- That released during cooling to lower T
- The latent heat of crystallization

The fate of assimilated crystals depend on their composition and that of the parent melt. Provided sufficient heat is available, a crystalline phase dissolves if the silicate melt is not already saturated with respect to that phase. Thus, quartz xenocrysts can dissolve on a time scale of day in basaltic melts in which the activity of silica in <1, the basalt become so enriched in silica. Crystals react with the magma if they would have precipitated from the magma at higher T.

The presence of xenocrysts or xenoliths in a magmatic rock may suggest they are contaminants, but xenocrysts can also originate by mixing of dissimilar magmas, and foreign material can be incorporated late into the magma with minimal contamination. Strained xenocrysts that show undulatory optical extinction are especially useful in distinguish assimilated solid material from phenocrysts in mixed magmas.

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
Quartz xenocryst (fragmented) with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 2x (Field of view = 7mm)	Quartz xenocryst (fragmented) with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 2x (Field of view = 7mm)	Quartz xenocryst (fragmented) with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 2x (Field of view = 7mm)
Quartz xenocryst (fragmented) with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 2x (Field of view = 7mm)	Quartz xenocryst (fragmented) with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)
Quartz xenocryst with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)
Quartz xenocryst with reaction corona made by clinopyroxene. The brown material is basaltic glass. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. Vulsini volcano (Italy). PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 10x (Field of view = 2mm)
Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)
Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. PPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)
Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. PPL image, 2x (Field of view = 7mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 2x (Field of view = 7mm)	Quartz xenocryst with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)
Quartz xenocrysts with reaction corona made by clinopyroxene. karapinar, Anatolia. XPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. Iblei, Sicily (Italy). XPL image, 10x (Field of view = 2mm)	Quartz xenocryst with reaction corona made by clinopyroxene. Iblei, Sicily (Italy). XPL image, 10x (Field of view = 2mm)
Quartz xenocrysts with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 2x (Field of view = 7mm)	Quartz xenocrysts with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 2x (Field of view = 7mm)	Quartz xenocrysts with reaction corona made by clinopyroxene. Vulsini volcano (Italy). XPL image, 2x (Field of view = 7mm)
Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 2x (Field of view = 7mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). PPL image, 10x (Field of view = 2mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 10x (Field of view = 2mm)
Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 20x (Field of view = 1mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 20x (Field of view = 1mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 20x (Field of view = 1mm)
Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). PPL image, 20x (Field of view = 1mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). PPL image, 10x (Field of view = 2mm)	Olivine xenocryst (with reaction rim made by orthopyroxene and vermicular spinel). XPL image, 10x (Field of view = 2mm)