Name: Date: 1. Identify the following rocks based upon their mineral composition

Question

Name: Date: 1. Identify the following rocks based upon their mineral composition. Assume each is a coarse grained rock. Specimen Composition Rock Name K-spar Quartz Biotite Amphibole 30% 5% 70% 20% 5% 5% 50% 30% 20% B Plagioclase Amphibole Biotite Pyroxene Plagioclase Pyroxene Amphibole 2. If specimen C above contained phenocryts and was fine grained, how would you name it? 3. What minerals are likely to form phenocrysts in basalt? 4. Could phenocrysts be found in obsidian (this is a trick question)? Explain. and 5. What is the origin of vesicles in a vesicular basalt?

Explanation / Answer

1) As the rocks are all coarse grained, so the IUGS classification scheme for the classification of plutonic rocks have been used for the present purpose.

Specimen A= Tonalite

Specimen B= Diorite

Specimen C= Gabbro

2) I would have named it as Basalt,as this is the volcanic equivalent of gabbro.

3) Generally basalts contain Olivine and some minor amounts of Plagioclase phenocrysts.

4) Obsidian is a kind of glass and forms by the rapid chilling of a volcanic lava getting out of a volcano and contains minimal amount of crystals. So, the formation of larger sized minerals is not likely in this case. So, phenocrysts are not found in Obsidian.

5) Basaltic lava contain some of the dissolved gaseous materials. When the lava comes to the ground surface the gaseous materials from the lava evaporates out. As a result the surface of the lava become uneven and the rest of the gaseous materials remain within the lava and form the vesicles in later days.

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