Igneous Rocks and Triangle Diagrams

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Overview

In this part of the exercise, you learn the method of point counting to quantify the composition of igneous rocks. Point counting is a much more precise technique than visual estimation for determining composition.

 

Learning Objectives

  • Estimate mineral compositions of rocks by point counting on hand specimens
  • Estimate mineral compositions of rocks by point counting on a thin section
  • Identify felsic igneous rocks accurately using the QAP triangle diagram

 

How Do Geologists Identify Igneous Rocks?

Geologists classify different rocks by emphasizing various aspects of texture (size, shape, and arrangement of grains) and composition (minerals). For igneous rocks, we begin usually by considering mineralogy: What minerals are present? What is the percentage of each mineral? A detailed description of the mineral percentages in a rock might look like the table below. All of these rocks are crystalline; granite and peridotite are coarse grained while basalt is fine grained.

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