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Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
- IIT JAM
- Geology (GG)
Please answer with detail. pegmatites are igneous rocks having extremely coarse grain size, the term does not have any compositional connotation.
please answer with detail. Pegmatites are igneous rocks having extremely coarse grain size, the term does not have any compositional connotation. What is the unifying cause for such extremely large grains? (A) Liquid immiscibility (B) Slow cooling at lower crustal depths (C) Volatile-aided crystallization (D) Early magmatic preferential growth of specific mineral phases
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Sushmita Best Answer
Answer:C (Volatile aided crystallization) Pegmatite forms during final(late) stage of magma crystallization. Large crystals of pragmatite formed in magma that was extra rich in dissolved water. And H2O increase the diffusion rate & thus inhance growth of crystal so, extremely large crystals are formed. Thus , correct option should be C.
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why not option B
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Sushmita
Igneous rock (pegmatite) with more than 3 cm large crystals, this texture/ structure is found in intrusive rocks. The extra large size does not mean that they cooled extra slowly(large crystals in igneous rocks are usually attributes to slow rate of crystallization but pegmatite is quite different). Instead, the large crystals of a pegmatite formed in magma that was extra rich in dissolved water.