Question

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?

• A. Norite
• B. Wehrlite
• C. Troctolite
• D. Harzburgite

Answer 1

The Correct Option is D

To determine the appropriate name of the rock according to the IUGS classification, we need to understand the mineralogical composition and their implications.

1. The rock in question is a plutonic igneous rock comprising:
   • 50% Orthopyroxene
   • 45% Olivine
   • 5% Clinopyroxene
2. 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.
3. 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.