Laboratory Manual for Introductory Geology 4e

■ Hardness: Use a steel safety pin or the tip of a knife blade and a magnifying
glass to determine mineral hardness in fine-grained rocks. Be sure to scratch a
single mineral grain or crystal, and when using a glass plate, be sure a single grain
is scratching it. Only then are you testing the mineral’s hardness. Otherwise,
you might be dislodging grains from the rock and demonstrating how strongly
the rock is cemented together, rather than measuring the hardness of any of its
minerals.
■ Streak: Similarly, be very careful when using a streak plate because it may not be
obvious which mineral in a fine-grained rock is leaving the streak. Remember, some
common minerals are also harder than the plate. As a result, streak is generally not
useful for identifying single grains in a rock that contains many minerals.
■ Crystal habit: Crystal habit is valuable in identifying minerals in a rock when it
has well-shaped crystals. But when many grains interfere with one another during
growth, the result is an interlocking mass of irregular grains rather than well-shaped
crystals. Weathering and transportation break off corners of grains and round their
edges, destroying whatever crystal forms were present.
■ Breakage: Use a hand lens to observe cleavage and fracture. Rotate the rock
in the light while looking at a single grain. Remember that multiple parallel shiny
surfaces represent a single cleavage direction. Breakage can be a valuable property
in distinguishing light-colored feldspars (two directions of cleavage at 90°) from
quartz (conchoidal fracture), and amphibole (two directions of cleavage not at 90°)
from pyroxene (two directions of cleavage at 90°).
■ Specific gravity: A mineral’s specific gravity can be measured only from a pure
sample. When two or more minerals are present in a rock, you are measuring the
rock’s specific gravity, which includes contributions from all of the minerals present.
However, the heft of a very fine-grained rock can be very helpful in interpreting
what combination of minerals might be in it.
■ Reaction with dilute hydrochloric acid: Put a small drop of acid on a fresh, unaltered
surface. Be careful: a thin film of soil or weathering products may contain calcite
and make a noncarbonate mineral appear to fizz. Use a hand lens to determine
exactly where the carbon dioxide is coming from. Are all of the grains reacting with
the acid, or is gas coming only from the cement that holds the grains together?
Now use these tips to identify minerals in the rock specimens provided.
EXERCISE 4.5
Identifying Minerals in Rocks
Name:
Course:
Section:
Date:
I. Your instructor will provide similar-sized samples of granite and basalt. Granite has grains that are large enough for you
to identify. It forms by slow cooling of molten rock deep underground. Basalt also forms by cooling of molten material,
but it cools faster and has much smaller grains.
(a) Examine the granite. Does it have well-shaped crystals or irregular grains? Suggest an explanation for their shapes.
(continued)
102 CHAPTER 4 MINERALS, ROCKS, AND THE ROCK CYCLE