Laboratory Manual for Introductory Geology 4e

Felsic igneous rocks (from feldspar and silica) have the most silica and the least
iron and magnesium. They contain abundant potassium feldspar (also called orthoclase)
and sodium-rich plagioclase feldspar, commonly quartz, and only sparse ferromagnesian
minerals—usually biotite or hornblende. Like their most abundant
minerals, felsic rocks are light colored and have low specific gravities.
Intermediate igneous rocks have chemical compositions, colors, specific gravities,
and mineral assemblages between those of felsic and mafic rocks: plagioclase
feldspar with nearly equal amounts of calcium and sodium, both amphibole and
pyroxene, and only rarely quartz.
Mafic igneous rocks (from magnesium and the Latin ferrum, meaning iron) have
much less silica, potassium, and sodium than felsic rocks and much more calcium,
iron, and magnesium. Their dominant minerals—calcium plagioclase, pyroxene,
and olivine—are dark green or black and have higher specific gravities than minerals
in felsic rocks. Even fine-grained mafic rocks can therefore be recognized by
their dark color and relatively high specific gravity.
Ultramafic igneous rocks have the least silica and the most iron and magnesium,
but very little aluminum, potassium, sodium, or calcium. As a result, they contain
mostly ferromagnesian minerals such as olivine and pyroxene and very little, if any,
plagioclase. Ultramafic rocks are very dark colored and have the highest specific
gravities of the igneous rocks.
5.3.2 Identifying Igneous Rocks
The name of an igneous rock is based on its mineral content and texture.(FIG. 5.7).
Each of the four compositional groups of igneous rock contains rock types with
coarse, fine, porphyritic, glassy, porous, and fragmental textures. Rocks in a given
column of Fig. 5.7b may have exactly the same minerals, but look so different that
they are given different names. For example, granite and rhyolite are both felsic, but
look different because of their different textures. And although gabbro and granite
have the same texture, they contain different minerals and therefore look different.
Identifying an igneous rock requires no new skills, just a few simple observations
and your ability to identify the common rock-forming minerals. If you are wondering
why there aren’t pictures of all the rock types to help you identify the specimens
in your rock set, it’s because granite may be gray, red, white, or even purple or black
depending on the color of its feldspars. A picture can thus help only if the rock it
shows it is exactly the same as the rock in your set. But if you understand the combination
of minerals that defines granite, you will get it right every time. You will have
a chance to try in Exercise 5.7.
EXERCISE 5.7
Identifying Igneous Rocks
Name:
Course:
Name the specimens in your igneous rock set by following the steps below.
Section:
Date:
Step 1: Place the rock in the correct column in Figure 5.7a by noting its color and heft and estimating mineral abundances
if its grains are coarse enough.
Step 2: Determine the texture of the rock and note which row in Figure 5.7b it corresponds to.
Step 3: The name of the rock can be found in the box at the intersection of the column and row.
Record the names of the rocks on the study sheets at the end of the chapter.
122 CHAPTER 5 USING IGNEOUS ROCKS TO INTERPRET EARTH HISTORY