Practical Igneous Rocks 1 2 nd year Geology

Practical Igneous Rocks 1 2 nd year Geology students By Ahmed Refaee

Outlines Igneous Rock Textures ﻣﺠﻤﻮﻋﺔ ﺍﻟﺘﺤﺒﺐ B. Inequigranular textures ﺍﻻﻧﺴﺠﺔ ﺍﻟﻐﻴﺮ ﻣﺘﺴﺎﻭﻳﻪ ﺍﻟﺘﺤﺒﺐ ﻋﺔ C. Directive textures D. Intergrowth/ Exsolution textures ﻟﻤﺰﺩﻭﺝ E- Reaction textures ﺍﻻﻧﺴﺠﺔ ﺍﻟﺘﻔﺎﻋﻠﻴﺔ A. Equigranular textures ﺍﻻﻧﺴﺠﺔ ﺍﻟﻤﺘﺴﺎﻭﻳﻪ

A- Equigranular textures 1 - If most of the grains are - that is they are bounded by well-formed crystal euhedral faces. The texture is said to be idomorphic. 1 2 - If most of the grains are subhedral - that is they bounded by only a few wellformed crystal faces, the texture is said to be hypidiomorphic (Granitic). ﺍﻟﻨﺴﻴﺞ ﻧﺎﻗﺺ ﺍﻟﺸﻜﻞ ﺍﻟﻨﺴﻴﺞ ﻛﺎﻣﻞ ( ﺍﻟﺸﻜﻞ )ﺍﻟﺠﺮﺍﻧﻴﺘﻰ 2 most crystals euhedral most crystals subhedral

3 - If most of the grains are anhedral - that is they are generally not bounded by crystal faces, the texture is said to be allotriomorphic. ( ﺍﻟﻨﺴﻴﺞ ﻋﺪﻳﻢ ﺍﻟﺸﻜﻞ )ﻋﺪﻳﻢ ﺍﻻﻭﺟﺔ ﺍﻟﺒﻠﻮﺭﻳﺔ most crystals anhedral 3

B- Inequigranular textures 1 - Porphyritic Large crystals (phenocrysts) in a finer grain or glassy matrix( Groundmass). 2 - Glomerophyric Phenocrysts occur in separated clusters. Glomerophyric tex. Porphyritic tex.

3 - Poikilitic Smaller crystals of one mineral are completely enclosed in larger crystals of another mineral. 4 - Ophitic Poikilitic tex. Laths of plagioclase in a coarse grained of pyroxene crystals, wherein the plagioclase is completely surrounded by pyroxene grains – very common in mafic rocks. 5 -Sub-ophitic As above but pyroxene only partially encloses plagioclase laths. Ophitic tex.

C- Directive textures These textures produced as a result of flow of lavas during their consolidation 1 - Trachytic A texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material Trachitic tex.

D-Intergrowth/ Exsolution textures Intergrowth textures are when a crystal of one mineral appears to be completely embedded within a crystal of another mineral. Granophyric Graphic intergrowth of quartz and alkali feldspar Granophyric tex. Myrmekitic Graphic intergrowths of plagioclase and quartz Myrmekitic tex.

D-Intergrowth/ Exsolution textures Parallel lamellae, or trains of blebs, of one mineral, and all of the same optical orientation, are enclosed in a single “host” crystal of another mineral. Perthitic Parallel streaks of sodium -rich feldspar (albite) within a host of potassium-feldspar (orthoclase or microcline). Very common in some granites Antiperthitic Parallel streaks of potassiumfeldspar (orthoclase or microcline) within a host of sodium -rich feldspar (albite). Antiperthitic tex.

D-Intergrowth/ Exsolution textures Radiate textures are those in which elongate crystals diverge from a common nucleus. Spherulitic Sperulites are approximately spheroidal bodies in a rock, composed of an aggregate of fibrous or elongate crystals of one or more minerals radiating from a nucleus, with glass or crystals in between. Spherulitic tex.

E-Reaction textures 1. Reaction Rims/ Coronas Individual crystals of one mineral completely surrounded by crystals of a different mineral. Indicates disequilibrium crystallization involving two members of a discontinuous reaction series.

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