Problemsolving Lab

Interpret Scientific Illustrations

How do you estimate mineral composition?

Igneous rocks are classified by their mineral compositions. In this activity, you will use the thin section in Figure 5.12 to estimate the different percentages of minerals in the sample.

Analysis

1. Design a method to estimate the percentages of the minerals in the rock sample shown in Figure 5.12.

2. Make a data table that lists the minerals and their estimated percentages.

Think Critically

3. Interpret Figure 5.9 to determine where in the chart this rock sample fits.

4. Compare your estimates of the percentages of minerals in the rock with those of your classmates. Why do the estimates vary? What are some possible sources of error?

5. Propose a method to improve the accuracy of your estimate.

122 Chapter 5 • Igneous Rocks

(l)W. K. Fletcher/Photo Researchers, (r)Dave Bartruff/CORBIS

Kimberlites Diamond is a valuable mineral found in rare, ultrabasic rocks known as kimberlites, named after Kimberly, South Africa, where the intrusions were first identified. These unusual rocks are a variety of peridotite. They most likely form deep in the crust or in the mantle at depths of 150 to 300 km, because diamond and other minerals present in kimberlites can form only under very high pressure.

Geologists hypothesize that kimberlite magma is intruded rapidly upward toward Earth's surface, forming long, narrow, pipelike structures. These structures extend many kilometers into the crust, but they are only 100 to 300 m in diameter. Most of the world's diamonds come from South African mines, such as the one shown in Figure 5.15. Many kimberlites have been discovered in the United States, but diamonds have been found only in Arkansas and Colorado. The diamond mine in Colorado is the only diamond mine currently in operation in the United States.

Igneous rocks in construction Igneous rocks have several characteristics that make them especially useful as building materials. The interlocking grain textures of igneous rocks make them strong. In addition, many of the minerals present in igneous rocks are resistant to weathering. Granite is among the most durable of igneous rocks. You have probably seen many items, such as countertops, floors, and statues, made from the wide variety of granite that has formed on Earth. ^

Figure 5.15 Diamonds are mined from kimberlite in mines like this one in Richtersveld, Northern

Figure 5.15 Diamonds are mined from kimberlite in mines like this one in Richtersveld, Northern

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