Geography 111 - Principles of Geology
Notes for the second week of class
Last week we introduced our topic with a glimpse into the abyss of geologic
time, a review of global-scale patterns and processes, and an introduction
to some of the evidence supporting both the uniformitarian (or gradualist)
and the catastrophist perspective on earth history.
This week we focus our attention on the most basic building blocks of
matter in order to understand some of the important characteristics of
earth materials. We will talk briefly about the structure of atom and the
nature of chemical compounds as a prelude to our discussion of minerals,
rocks, and the rock cycle.
TOPICS COVERED IN READINGS:
Chapter 2:
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Defining characteristics of minerals
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Internal structure of atoms: nucleus, electrons and orbital shells
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Atomic number, atomic mass, and isotopes
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Ions and chemical reactions
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Periodic table of the elements
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Types of chemical bonds: ionic, covalent, metallic
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Isotopes and radioactive decay
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Atomic structure
of mineral crystals
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Another page
with crystal structures
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Physical properties of minerals
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Crystal form
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Hardness
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Cleavage
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Fracture
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Color
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Streak
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Luster
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Specific gravity and density
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Polymorphs
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Major classes of rock-forming minerals:
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Silicates, the silica tetrahedron, and major structural types
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Isolated tetrahedra
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Ring structures
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Single chains
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Double chains
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Sheets
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Framework silicates
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Difference between ferromegnesian and nonferromagnesian silicates
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Ion size and cation substitution
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Carbonates
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Oxides
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Sulfides and sulfates
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Other nonsilicate mineral groups
A little more detail on the silicate minerals:
It is useful to review some basic
information about the silicate minerals. Recall that there are several
basic structural
groups, all based on the silica
tetrahedron as a building block, with different degrees of coordination
among tetrahedra within the crystal structure. (While you are at
it, you might also review some of the basic physical
properties of minerals.)
The basic structural groups, with the name of a prominent group of minerals
belonging to each, are as follows:
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Isolated tetrahedra, also known as nesosilicates
(olivines are the most important group for our discussion)
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Ring structures, also known as cyclosilicates
(no dominant group will be discussed in class)
-
Single and double chains, also known as inosilicates
(we will discuss pyroxenes as single-chain and amphiboles
as double-chain)
-
Sheet structures, also known as phyllosilicates
(we are primarily concerned with the micas and the clay minerals)
-
Framework silicates or tectosilicates
(we are primarily concerned with the feldspars and quartz)
With the increasing degree of coordination among tetrahedra as you move
from the top to the bottom of this list, there is an increase in the relative
importance of silicon compared to other elements. We will also find that
the olivines, pyroxenes, amphiboles, and some of the micas (e.g. biotite)
contain a higher proportion of iron and magnesium, and to some extent calcium
as well; whereas the framework silicates and some of the micas (e.g. muscovite)
contain more silicon, aluminum, potassium, and sodium. You can see
this if you look at some of the typical chemical formulas (each group actually
contains many different minerals with some variation in the chemical formulas):
| Structure |
Mineral group |
Sample Chemical formulas |
| Isolated tetrahedra |
Olivine |
(Mg or Fe)2SiO4 |
| Single chain |
Pyroxene |
(Mg or Fe)SiO3 or CaMgSi2O6 |
| Double chain |
Amphibole |
Ca2(Mg or Fe)5Si8O22(OH)2 |
| Sheet |
Mica (biotite) |
K(Mg or Fe)3(Al or Fe)Si3O10(OH
or F)2 |
| Sheet |
Mica (muscovite) |
KAl2(AlSi3O10)(OH)2 |
| Framework |
Feldspar (plagioclase) |
NaAlSi3O8 or CaAl2Si2O8 |
| Framework |
Feldspar (potassium feldspar) |
KAlSi3O8 |
| Framework |
Quartz |
SiO2 |
We will make use of the information provided here as we discuss the
formation of different kinds of igneous rocks next week.