Geology Reference
In-Depth Information
EXERCISE 1
Earth Materials, Geologic Time,
and Geologic Processes
of atoms and/or molecules that make up a mineral.
Minerals may exhibit many different characteristic
external forms, for example, sheets (mica), cubes (halite
and galena), rhombohedrons (calcite), and hexagons
(rock crystal quartz).
INTRODUCTION
One of the necessary phases of geologic studies is
gaining knowledge of the materials that make up
the Earth's crust. The ability to identify these earth
materials (rocks and minerals) provides one with an
appreciation of the natural environment and
supplies earth scientists with a tool that may aid in
geologic and environmental studies of an area.
The minerals (Part A) and rocks (Part B) studied in
this exercise are the most commonly occurring types
and those of interest because of their importance in
environmental considerations. Over geologic time
(Part C), these mineral and rock materials are
modified by geologic processes (Part D) acting on
and within the Earth.
HARDNESS
The resistance that a mineral offers to
abrasion is its hardness. It is determined by scratching
the surface of a mineral with another mineral or
material of known hardness. The Mohs Scale of
Hardness consists of
10
minerals ranked in ascending
order with diamond, the hardest known mineral,
assigned the number
10.
This scale, together with
several common objects of known hardness, is shown
below and should be referred to in determining the
hardness of unknown minerals. Because weathering
may affect hardness, it is important to make tests on
fresh surfaces.
MOHS SCALE OF
HARDNESS
1. talc
2. gypsum
3. calcite
4. fluorite
5. apatite
6. feldspar
7. quartz
8. topaz
9. corundum
10. diamond
PART A. MINERALS
A mineral is a naturally occurring inorganic sub-
stance that has an orderly internal structure and char-
acteristic chemical composition, crystal form, and
physical properties. Minerals possess many funda-
mental characteristics that are external evidence of an
orderly internal arrangement of atoms, le physical
properties of minerals reflect their internal structure
and provide clues to their identity. Many chemical
properties also aid in mineral identification. For
instance, the carbonate minerals calcite and dolomite
(the main constituents of limestone and dolostone)
effervesce with application of dilute (5 percent)
hydrochloric acid.
COMMON OBJECTS
2-2.5 fingernail
3-3.5 copper penny
5-5.5 knife blade,
5.5-6 glass plate
6.5
steel file
CLEAVAGE AND FRACTURE
The bonds that hold
atoms together in a crystalline structure are not
necessarily equal in all directions. If definite planes of
weakness exist, a mineral will tend to break along
these cleavage planes. Cleavage is described with
reference to the number of cleavage planes and their
angles of intersection, and may be further qualified as
Physical Properties
CRYSTAL FORM
Crystal form refers to the orderly
geometric arrangement of external planes or surfaces
that are controlled by the orderly internal arrangement
