Granoblastic texture

Granoblastic texture is the typical equigranular texture formed in metamorphic rocks when grains mutually adjust their boundaries in the solid state in an attempt to achieve textural equilibrium. The minerals do not show a strong tendency to develop their characteristic crystal shape, but are equant, with fairly uniform grain size and straight or smoothly-curved boundaries. In granoblastic texture crystals boundaries intersect at 120 degree triple junctions under ideal conditions. Granoblastic texture is typical of quartzite, marble (also called saccaroid) and other non-foliated metamorphic rocks like hornfels. A rock that has a granoblastic texture can be termed a granofels.

When thermal metamorphism is not associated with any deformation, the mineral grains are randomly oriented, resulting in either granoblastic or hornfelsic textures. The following are some of the most common types of granoblastic textures:

1- Granoblastic polygonal: where the equidimensional grains may have well developed crystal faces resulting in straight grain boundaries, and where triple junctions are common.
2- Granoblastic interlobate: where the grain boundaries are somewhat irregular.
3- Granoblastic amoeboid: where all the grains have irregular outlines, and all the minerals are anhedral.
4- Granoblastic decussate: where the interlocking randomly oriented crystals are somewhat elongate, prismatic or subidioblastic. Usually applied to rocks with one or two mineral species only. Triple junctions are common.
5- Nodular: results from the growth of oval - shaped porphyroblasts of such minerals as cordierite or scapolite in association with other randomly oriented minerals.

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.

Granoblastic polygonal texture (saccaroid) in a marble. XPL image, 10x (Field of view = 2mm)	Granoblastic polygonal texture (saccaroid) in a marble, Carrara, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)	Granoblastic polygonal texture (saccaroid) in a marble, Carrara, Tuscany, Italy. XPL image, 10x (Field of view = 2mm)
Granoblastic polygonal texture (saccaroid) in a marble, Carrara, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Granoblastic polygonal texture (saccaroid) in a marble, Carrara, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)	Granoblastic polygonal texture (saccaroid) in a marble, Carrara, Tuscany, Italy. XPL image, 2x (Field of view = 7mm)
Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)
Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)
Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)	Granoblastic texture (amoeboid) in quartz. XPL image, 10x (Field of view = 2mm)
Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. PPL image, 2x (Field of view = 7mm)	Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. XPL image, 2x (Field of view = 7mm)	Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. PPL image, 2x (Field of view = 7mm)
Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. XPL image, 2x (Field of view = 7mm)	Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. PPL image, 2x (Field of view = 7mm)	Rounded cordierite porphyroblasts (nodular granoblastic texture) in a fine-grained slate. Lozio slate, Bazena, Brescia province, Italy. XPL image, 2x (Field of view = 7mm)
Glaucophane and stilpnomelane crystals in a granofels with granoblastic decussate texture. PPL image, 2x (Field of view = 7mm)	Glaucophane and stilpnomelane crystals in a granofels with granoblastic decussate texture. XPL image, 2x (Field of view = 7mm)	Glaucophane and stilpnomelane crystals in a granofels with granoblastic decussate texture. PPL image, 2x (Field of view = 7mm)
Plagioclase (colorless), corundum (high relief, colorless) and spinel (deep-green) in a hornfels with granoblastic texture. Montecristo Island (Italy). PPL image, 10x (Field of view = 2mm)	Plagioclase, corundum (I order yellow) and spinel (isotropic) in a hornfels with granoblastic texture. Montecristo Island (Italy). XPL image, 10x (Field of view = 2mm)	Plagioclase (colorless), corundum (high relief, colorless) and spinel (deep-green) in a hornfels with granoblastic texture. Montecristo Island (Italy). PPL image, 10x (Field of view = 2mm)