Andalusite schist

The word schist is derived from the Greek word schízein meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie. Schist is a medium-grained strongly-foliated crystalline metamorphic rock, formed by dynamic metamorphism, that can be readily split into thin flakes or slabs due to the well-developed parallelism of more than 50% of the minerals present, particularly those of lamellar or elongate prismatic habit, e.g., mica and amphiboles. Individual mineral grains are discernible by the naked eye, and this property sets it apart from slate. There are many varieties of schist and they are named for the dominant mineral comprising the rock, e.g. mica schist, green schist (green because of high chlorite content), garnet schist, actinolite schist, biotite schist etc.

Schistosity is a type of foliation, characterized by the preferred orientation of elongated or platy mineral grains (which are abundant in schistose rocks). Schistosity is a result of pressure in the crust which forces the grains to align perpendicular to the force applied. This force may be compressive (in mountain ranges) or simply caused by the weight of the overlying rocks. Metamorphic reactions between minerals upon increased burial will lead to the loss of schistosity because feldspar increases in abundance as micas become unstable. This process will lead to the formation of high-grade metamorphic rock gneiss (and gneissose fabric which can be described as a poorly developed schistosity).

The mineral assemblages and textures of the schist change with the temperature and pressure of recrystallization. With increasing metamorphism, the grain size usually increases and, depending on appropriate chemical availability, minerals such as chloritoid, garnet, staurolite, cordierite, andalusite, and kyanite crystallize as large crystals (called porphyroblasts) in a foliated micaceous matrix. Many porphyroblasts contain inclusions, indicating that they crystallized by replacement of some other mineral or rock.

Andalusite porphyroblasts in a schist. Clooney, Donegal. Keele collection. From G. Stimpson.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• 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.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)
Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)
Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 1x (Field of view = 9mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)
Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)
Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (I order gray), partially altered by sericite, within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)	Andalusite porphyroblast (colorless, high relief) within a fine-grained matrix (rich in biotite, muscovite and graphyte). Sardinia, Italy. PPL image, 2x (Field of view = 7mm)