Granoblastic Polygonal Texture

A granoblastic texture is an equigranular texture in which crystals adopt a polygonal morphology with grain triple junctions of approximately 120 degrees. The formation of granoblastic textures occurs to minimise the combined surface energy of phases within a rock.

Granoblastic textures are often seen in the products of thermal metamorphism such as hornfels, and in high grade granulites.

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.

Bibliography

• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005)

Photo
Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 2x (Field of view = 7mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 2x (Field of view = 7mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 2x (Field of view = 7mm)
Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)
Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)	Hornblende crystals with Polygonal Texture in an Amphibolite. XPL image, 10x (Field of view = 2mm)
Polygonal texture in quartz. XPL image, 2x (Field of view = 7mm)	Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)	Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)
Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)	Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)	Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)
Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)	Polygonal texture in quartz. XPL image, 10x (Field of view = 2mm)