Felsitic texture

Felsitic texture is a microcrystalline texture of the ground-mass of acid extrusive rocks. The groundmass of such rocks consists of minute crystalline formations, such as grains or fibers, and finely dispersed vitreous material. Volcanic glass is unstable and tends to change spontaneously (devitrify) from the glassy to the crystalline state in periods of time that are relatively short by geologic standard. Geologically ancient glasses are therefore very rare, and most glassy rocks are of Paleogene age or younger (less than 65.5 million years old).

Devitrification commonly begins along cracks in the glass or around large crystals and may spread outward until eventually the entire mass has been converted to fine crystals aggregate. Devetrification results in a submicroscopic mixture of various rock mineral and their alteration products. Usually individual mineral species are indistinguishable, but the jumbled aggregate is faintly birefringente. In some cases mineral species can be recognized, and feldspar, clay minerals, and various form of silica have been reordered as devetrification products.

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 crystals in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)	Quartz crystal in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)	Plagioclase crystal in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)
Quartz crystals in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)	Quartz crystals in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)	Quartz crystals in a Rhyolite with felsic (devitrified) groundmass. Antarctica. XPL image, 10x (Field of view = 2mm)
Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)
Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 2x (Field of view = 7mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 2x (Field of view = 7mm)
Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. The green, fibrous crystals are pyroxene. PPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)
Rhyolite with felsic (devitrified) groundmass. The green, fibrous crystals are pyroxene. PPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. PPL image, 10x (Field of view = 2mm)
Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. The green, fibrous crystals are pyroxene. PPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)
Rhyolite with felsic (devitrified) groundmass. The green, fibrous crystals are pyroxene. PPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 10x (Field of view = 2mm)	Rhyolite with felsic (devitrified) groundmass. XPL image, 2x (Field of view = 7mm)
Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. PPL image, 2x (Field of view = 7mm)
Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)
Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)	Dacite with felsic groundmass from Bulgaria. XPL image, 2x (Field of view = 7mm)