A plutonic igneous rock is composed of 50% orthopyroxene, 45% olivine and 5% clinopyroxene. What is the appropriate name of the rock according to the IUGS classification?
- Norite
- Wehrlite
- Troctolite
- Harzburgite
The Correct Option is D
Solution and Explanation
To determine the appropriate name of the rock according to the IUGS classification, we need to understand the mineralogical composition and their implications.
- The rock in question is a plutonic igneous rock comprising:
- 50% Orthopyroxene
- 45% Olivine
- 5% Clinopyroxene
- According to the IUGS classification for ultramafic igneous rocks:
- Norite is typically dominated by plagioclase and orthopyroxene. Thus, it is not applicable here since olivine is predominant.
- Wehrlite is a rock mainly composed of clinopyroxene and olivine, which doesn't match as orthopyroxene is more abundant here.
- Troctolite consists predominantly of olivine and plagioclase, whereas orthopyroxene has the largest presence in the given composition.
- Harzburgite is identified by a high presence of olivine and orthopyroxene, with very little or no clinopyroxene. The given composition matches this classification since 50% orthopyroxene and 45% olivine are the dominant minerals.
- Therefore, based on the mineral composition of the rock, the correct classification according to the IUGS nomenclature is Harzburgite.
Conclusion: The appropriate name for the rock is Harzburgite, aligning with the given mineral distribution.
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