Earth Materials, Geologic Time, and Geologic Processes - Investigations in Environmental Geology - page 4

perfect when well developed, or indistinct when
poorly developed. Some examples of different types of
cleavage are:

Flexible will bend but does not selenite

return to original gypsum

position when force
is released
Malleable can be hammered into native

Basal (1 plane or direction) mica and selenite

gypsum

thin sheets without copper

breaking

Cubic (3 planes at right halite and galena

Brittle does not bend but quartz

angles)
Rhombic (3 planes not at

shatters when
sufficient pressure
is applied

calcite

right angles)

The breaking of a mineral along directions other than
cleavage planes is termed fracture. Some common
types of fracture are conchoidal (quartz), irregular
(pyrite), and fibrous (asbestos).

Other Mineral Properties

Magnetism Magnetite is the only common

mineral to show obvious
magnetic attraction.
Halite has a salty taste,
which is a definite property of
this mineral.
The clay minerals (kaolinite, etc.)
smell earthy, especially when
breathed upon.
Some minerals have a distinctive
feel (e.g., talc feels soapy).
Calcite will effervesce (fizz)
when cold, dilute hydrochloric
acid (HC1) is applied. Dolomite
reacts with dilute HC1 if it is
powdered.

Taste

LUSTER The luster of a mineral is the appearance of a
fresh surface in reflected light. There are two major types
of luster: metallic and nonmetallic. Galena and pyrite
have a metallic luster; nonmetallic lusters include glassy
or vitreous (quartz), resinous (sphalerite), pearly (talc),
and dull or earthy (kaolinite).

Odor

Feel

COLOR Color is one of the most obvious properties
of a mineral but in general it is of limited diagnostic
value. For example, quartz may vary from colorless to
white (milky), gray, rose, purple (amethyst), green,
and black. In contrast, galena is a distinctive gray and
the micas can often be separated on the basis of color,
which results from different chemical compositions.

Chemical
Reaction

QUESTIONS 1, PART A
1. Examine each mineral specimen that your instructor selects
for this exercise. Determine and record in Table 1.1 all of the
properties you test and/or observe. This includes determina-
tion of luster, color, cleavage, fracture, and specific gravity; and
testing for hardness, streak, and other properties.

STREAK The color of a mineral's powder is its streak.
It is obtained by rubbing a mineral specimen across a
streak plate (a piece of unglazed porcelain). Streak is
commonly a diagnostic characteristic of metallic
minerals. Color may vary greatly within one mineral
species but the streak is generally constant (e.g.,
hematite has a reddish streak).

2. After you have recorded the observed physical proper-
ties, with the aid of earlier information and the descriptive
information about each mineral found in Table 1.2, deter-
mine the names of the minerals. Record the chemical com-
position of each mineral and note the information
regarding their geologic, environmental, and economic sig-
nificance. As you will see, some of these minerals are of
particular importance because of the way they influence
the environment when not properly used or when their
presence is not considered.

RELATIVE DENSITY OR SPECIFIC GRAVITY Specific
gravity (s.g.) is the ratio between the weight of a
mineral and the weight of an equal volume of water.
Although s.g. can be determined accurately, it is
sufficient for general laboratory and field purposes to
observe the "heft" of a mineral simply by handling it.
Galena (7.5) and pyrite (5) are heavy; the common rock-
forming minerals (calcite, quartz, and the feldspars) are
of medium weight (2.5 to 2.8); and halite is light (2 .1).

PART B. COMMON ROCKS

TENACITY Tenacity is the resistance of a mineral to
being bent or broken. It should not be confused
with hardness. Some of the terms used to describe
tenacity are:

A rock is an accumulation or aggregate of one or more
minerals, although some nonmineral substances such
as coal or volcanic glass are also considered rocks.
Rocks are generally classified into three major cate-
gories: igneous, sedimentary, and metamorphic. The
interrelations among the three groups of rocks are
shown by the rock system (Figure 1.1). Note that you
need to add another arrow from metamorphic rocks.

Elastic will bend and

return to original
position when
force is released

mica