Skeletal Garnet

The abundance of inclusions in Al-silicate porphyroblasts has been attributed to the limited mobility of lions. At greenschist and lower amphibolite facies conditions, Al-ions are far less mobile than Si, Fe, Mg, K or Ca-ions, unless the pH is extremely high or low, or if salinity is extremely high. This means that porphyroblasts of Al-silicates such as andalusite, cordierite, staurolite, chloritoid and garnet can be expected to grow at Al-rich sites such as mica layers and to have difficulty in replacing minerals that lack Al by an intact lattice: such minerals are included instead. This explains why Al-silicate porphyroblasts commonly contain numerous inclusions when they grow in quartz and graphite-bearing pelitic rocks. Such porphyroblasts may even obtain a skeletal shape when they grow into Al-poor domains, for instance quartz-rich layers or strain shadows. They are unable to build a complete lattice at these sites, but rather follow grain boundaries.

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.

Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. XPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)
Skeletal garnet in a quartz-rich layer. XPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. XPL image, 10x (Field of view = 2mm)
Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 20x (Field of view = 1mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)
Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)
Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. XPL image, 10x (Field of view = 2mm)	Skeletal garnet in a quartz-rich layer. PPL image, 10x (Field of view = 2mm)