Laboratory Manual for Introductory Geology 4e

When a mineral grows without interference from other minerals, it will
develop smooth, flat surfaces and a symmetric geometric shape that we call a
crystal. When a mineral forms in an environment where other minerals interfere
with its growth, it will have an irregular shape but will still have the appropriate
internal (crystal) structure for that mineral. An irregular or broken piece of a
mineral is called a grain, and a single piece of a mineral, either crystal or grain,
is called a specimen.
In the geosphere, most minerals occur as parts of rocks. It is important to know
the difference between a mineral specimen and a rock. A rock is a coherent, naturally
occurring, inorganic solid consisting of an aggregate of mineral grains, pieces of older rocks, or
a mass of natural glass. Some rocks, such as granite, contain grains of several different
minerals; others, such as rock salt, are made of many grains of a single mineral.
Still others are made of fragments of previously existing rock that are cemented
together. And a few kinds of rock are natural glasses, cooled so rapidly from a molten
state that their atoms did not have time to form the grid-like crystal structures
required for minerals. Exercise 3.2 helps you practice this terminology with specimens
provided by your instructor.
EXERCISE 3.2
Is It a Mineral or a Rock?
Name:
Course:
Section:
Date:
(a) Based on the definitions of mineral and rock, determine which specimens used in Exercise 3.1 are minerals, which
are rocks, and which, if any, are neither minerals nor rocks. Write the specimen numbers in the appropriate columns
in the following table.
Minerals Rocks Other
(b) Choose one of the rock specimens and look at it carefully. How many different minerals are there in this rock?
(c) How do you know? What clues did you use to distinguish a mineral from its neighbors?
(d) Describe up to four of the minerals in your own words.
Mineral 1:
(continued )
56 CHAPTER 3 MINERALS