Page 92 - Laboratory Manual in Physical Geology

ACTIVITY made up of one color of mineral crystals is usually made
up of one kind of mineral, and a rock made of more
3.2 Mineral Properties than one color of mineral crystals is usually made up
of more than one kind of mineral. However, there are
THINK | How and why do people study exceptions, like the agate in FIGURE 3.1D . It has many
colors, but they are simply varieties (var.)— different
About It minerals?
colors—of the mineral quartz. This means that a min-
eral cannot be identified solely on the basis of its color.
OBJECTIVE Analyze and describe the physical and The mineral’s other properties must also be observed,

chemical properties of minerals. recorded, and used collectively to identify it. Most
PROCEDURES minerals also tend to exhibit one color on freshly broken
surfaces and a different color on tarnished or weathered
1. Before you begin , read the following background surfaces. Be sure to note this difference, if present, to aid
information. This is what you will need :
your identification.
____ Activity 3.2 Worksheets (pp. 102 – 103 ) and Mineral crystals may vary in their clarity : degree
pencil of transparency or their ability to transmit light. They
____ set of mineral samples (obtained as directed may be transparent (clear and see-through, like window
by your instructor) glass), translucent (foggy, like looking through a steamed-
____ set of mineral analysis tools (obtained as up shower door), or opaque (impervious to light, like
directed by your instructor) concrete and metals). It is good practice to record not
____ cleavage goniometer cut from GeoTools only a mineral’s color, but also its clarity. For example,
Sheet 1 at the back of the manual the crystals in FIGURE 3.1B are purple in color and have

2. Then follow your instructor’s directions for transparent to translucent clarity. Galena mineral crystals
completing the worksheets. ( FIGURE 3.2 ) are opaque.
Crystal Forms and Mineral Habits. The geometric shape
of a crystal is its crystal form . Each form is bounded by
flat crystal faces that intersect at specific angles and in
What Are a Mineral’s Chemical and
symmetrical relationships ( FIGURE 3.1A and B ). The crystal
Physical Properties? faces are the outward reflection of the way that atoms or
The chemical properties of a mineral are its characteristics groups of atoms bonded together in a three- dimensional
that can only be observed and measured when or after it pattern as the crystal grew under specific environmen-
undergoes a chemical change due to reaction with another tal conditions. There are many named crystal forms
material. This includes things like if or how it tarnishes ( FIGURE 3.4 ). Combinations of two or more crystals can

(reacts with air or water) and whether or not it reacts with also form named patterns, shapes, or twins (botryoidal,
acid. For example, calcite and other carbonate (CO - dendritic, radial, fibrous: FIGURE 3.4 ). A mass of mineral
3
containing) minerals react with acid, and native copper crystals lacking a distinctive pattern of crystal growth is
tarnishes to a dull brown or green color when it reacts with called massive .
air or water.
The physical properties of a mineral are its characteris- Development of Crystal Faces. The terms euhedral,

tics that can be observed (and sometimes measured) without subhedral, and anhedral describe the extent to which a
changing its composition. This includes things like how it crystal’s faces and form are developed. Euhedral crystals
looks (color, luster, clarity) before it tarnishes or weathers have well developed crystal faces and clearly defi ned
by reacting with air or water, how well it resists scratching and recognizable crystal forms ( FIGURE 3.1A ). Th ey
(hardness), how it breaks or deforms under stress (cleavage, develop only if a mineral crystal is unrestricted as it
fracture, tenacity), and the shapes of its crystals. For example, grows. This is rare. It is more common for mineral

quartz crystals are hard to scratch, glassy, and transparent, crystals to crowd together as they grow, resulting in a
while talc is easily scratched, opaque, and feels greasy. massive network of intergrown crystals with deformed
In this activity, you will use the properties of color and crystal faces and odd shapes or imperfect crystal forms
clarity (before and after tarnishing), crystal form, luster ( FIGURE 3.1B ). Subhedral crystals are imperfect but have
(before and after tarnishing), streak, hardness, cleavage, and enough crystal faces that their forms are recognizable.
fracture to describe mineral samples. Additional properties— Euhedral crystals have no crystal faces, so they have no
such as tenacity, reaction with acid, magnetic attraction, recognizable crystal form ( FIGURE 3.1C ). Most of the
specific gravity, striations, and exsolution lamellae—can also laboratory samples of minerals that you will analyze do
be helpful in analyzing particular minerals. not exhibit their crystal forms because they are small
broken pieces of larger crystals. But whenever the form
Color and Clarity. A mineral’s color is usually its most or system of crystals in a mineral sample can be detected,
noticeable property and may be a clue to its identity. then it should be noted and used as evidence for mineral
Minerals normally have a typical color, like gold. A rock identifi cation.

Mineral Properties, Identification, and Uses ■ 77