Glomerophyric Texture

Glomeroporphyritic or Glomerophyric is a term used to describe a porpyritic texture in which phenocrysts are clustered into aggregates called glomerocrysts or crystal clots. Glomeroporphyritic textures are common and often included plagioclase and pyroxenes in basic rocks. They form by a processes known as synneusis, where accumulation of crystals occurs by surface tension and fixing by interpenetration due to crystal growth.

Glomerocrysts are an important consideration in crystal fractionation by crystal settling since the density of the glomerocryst is an average of that of its constituent phases. Formation of glomerocrysts may in part explain the settling of plagioclase in basic intrusions in which plagioclase crystals are less dense than the surrounding magma. The glomerocryst play an important role in the fractionation process, being heavier than the individual crystals of which they consist, they are more prone to sinking in the magma. this is governed by the law of stokes:

V = 2R2 (de - di)g /9η

Where V = sedimentation velocity; R = radius of crystals; di = density of crystals; de = density of magma g = acceleration of gravity. η = viscosity of magma.

In the basalt of Hawaiian lava lake, described by Kirkpatrick (1977), all phenocrysts occurs in glomeroporphyric clots. He ascribed the texture to heterogeneous nucleation so that plagioclase, for example, nucleates on pre-existing plagioclase, pyroxene or olivine.
Helz (1987) suggested, for euhedral crystals of olivine in the kilawea basalt, the mechanism of synneusis; crystals that accidentally collide are likely to stick together if they touch in crystallographic continuity.

It may be difficult to distinguish glomerophyre from xenoliths, especially from cognate xenoliths which represent disrupted cumulates of crystals from the same or closely related magma.

Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.

Photo
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Pyroxene and plagioclase glomerophyre in a Basalt. XPL image , 10x (Field of view = 2mm)
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Pyroxene and plagioclase glomerophyre in a Basalt. XPL image , 10x (Field of view = 2mm)
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Pyroxene and plagioclase glomerophyre in a Basalt. PPL image , 10x (Field of view = 2mm)
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Pyroxene and plagioclase glomerophyre in a Basalt. XPL image , 10x (Field of view = 2mm)