Thursday, October 6, 2011

Textures

1. HOLOCRYSTALLINE TEXTURE


The term holocrystalline is used when a rock is composed entirely of crystals. This type of texture is characteristic of deep seated or plutonic igneous rock, e.g. GRANITE

2. HEMICRYSTALINE TEXTURE

The term hemicrystalline is used when a rock is composed partly of crystals and partly of glass. This is characteristic of rock which has consolidated on or near the surface e.g. Volcanic rocks.

3. HOLOHYALLINE TEXTURE

The term holohyalline is used when a rock consists wholly of glass. This type of texture is restricted to volcanic and hypabyssal rock occurring at the immediate contact which the country rock, e.g. Obsidian.

4. ALLOTRIOMORPHIC TEXTURE

This is holocrystalline equigranular texture in which the minerals are of approximately same size and majority of the crystal are anhedral in shape, e.g. Aplite.

5. HYPIDIOMORPHIC TEXTURE

This is a type of holocrystalline, equigranular texture in which the minerals are approximately same size and majority of the minerals are sub-hedral e.g. Granite.

6. PANIDIOMORPHIC TEXTURE

This is a type of holocrystalline, equigranular texture in which the minerals are of approximately same size and majority of crystals are euhedral in shape, e.g. Lamprophyre.

7. PORPHYRITIC TEXTURE

This is a type of inequigranular texture (which is characterized by marked difference in grain size of minerals) in which larger crystals, known as phenocrysts, are embedded in a fine – grained ground mass, e.g. Basalt, Andesite, and Rhyolite.

8. FELSOPHYRIC TEXTURE

This is a variety of in-equigranular, porphyritic texture in which larger crystals, known as phenocrysts are embedded in a cryptocrystalline ground mass.

9. VITROPHYRIC TEXTURE

This is variety of inequigranular porphyritic texture in which larger crystals known as phenocrysts are embedded in a glassy ground mass e.g. Basalt.

10. POIKILITIC TEXTURE

This is a type of holocrystalline inequigranular (marked difference in grain size) texture and the term implying the occurrence of number of oriented or un-oriented crystals, totally enclosed within a large crystal.

11. OPHITIC TEXTURE

This is a special case of poikilitic texture in which plates of Augite enclose numerous thin laths of plagioclase. This is the characteristic texture of the rock known as Dolerite.

12. SUB-OPHITIC TEXTURE

This texture is similar to ophitic but in this case fine laths of plagioclase are partially enclosed in Augite crystal.

13. CLASTIC TEXTURE

Clastic texture results from the accumulation of detrital particles. It is characterized by point contents between (grains) and associated intergranular spaces.

14. NON-CLASTIC TEXTURE

Non-Clastic texture are dominated by the crystalline texture results from the crystallization within or at the surface of sediments and are characterized by an interlocking crystal mosaic.

15. SCHISTOSE TEXTURE

The texture is characterized by Prismatic content acicular minerals. Quartz usually is associated with these minerals. All the above minerals show a parallel array among each. Rock other showing schistose texture is called schist.

16. GNEISSOS TEXTURE

This texture is characterized by distinct bandings of light (quartz, feldspars, muscovite etc.) and dark (hornblende, biotite, hypersthenes, garnet) colour minerals. In addition to compositional banding it may contain streaks, lines of quartz and / or plagioclase. Rock exhibiting gneissose texture is called gneiss.

17. GRANULOSE TEXTURE

The texture is characterized by predominance of equidimentional minerals such as Quartz, Feldspar, Pyroxene and Calcite etc.

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