Download Chapter5.pdf

Chapter 5 notes
Minerals
1) Industrial minerals provide the raw materials for the manufacture of building
materials such as sheetrock and concrete
2) Ore minerals are the source of gold, silver, copper, uranium, iron and many other
materials
3) Precious and semi-precious gems are minerals
What is a mineral?
1) Homogeneous – that means it is the same throughout its structure and cannot be
physically broken into smaller components. When you smash a mineral, you just
get smaller fragments of the same mineral.
2) Minerals are naturally occurring and are formed by natural processes. Synthetic
minerals such as cubic zirconia can be created in laboratories but are not true
minerals.
3) A mineral is a solid
4) Minerals are inorganic (do not contain a C-H bond). Minerals are not protein, oil,
plastic, fat, or alcohol.
5) Minerals have a lattice or crystalline structure. A glass, an example is obsidian, a
volcanic glass, has atoms arranged in a random and disorderly manner because it
cooled instantly upon exposure to air. Therefore, obsidian does not meet the
definition of a true mineral because it lacks an orderly crystalline structure.
6) Minerals have a definable chemical composition. For example, quartz is SiO2
(one silicon atom for every two oxygen atoms).
Crystals form in different ways.
1) Solidification of a melt as when water freezes to produce ice crystals.
2) Precipitation from a solution as when salt precipitates out of salt water.
3) Solid-state diffusion when atoms or ions pass through a solid to arrange into a
new crystal lattice (associated with metamorphic rock formation).
?Which is a mineral? Sugar or table salt? Why or why not?
Answer: sugar is not a mineral because it is considered an organic substance (C6H12O6,
note C-H bond). Salt, however, is considered a mineral (NaCl).
•
•
Some mineral formulas are more complex because of ion substitution. That
occurs when ions of about the same size are able to substitute freely for one
another within a crystalline structure. Example: olivine (Fe, Mg)2 SiO4
Zoning within a single mineral is when you have different ionic substitution
within the same mineral. This can happen when a mineral within an igneous rock
•
•
contains different ions, depending upon the cooling rate. Example: a single
crystal of plagioclase feldspar, the crust’s most abundant mineral, can have
mostly calcium (Ca) ions in the center with a gradual change towards sodiun (Na)
ions towards the crystal edges. This substitution occurs when crystallization of
magma began at a high temperature (high temperature favor Ca and low favor Na
in plagioclase).
Minerals make up the rocks and sediments of the Earth.
A crystal is a single, continuous piece of crystalline material bounded by flat
surfaces or faces that formed as the crystal lattice grew.
Physical properties of minerals
There are physical properties of minerals that enable geologists to tell them apart.
1) Color – a useful property but can vary within the same mineral (example: quartz
can be white, clear, pink, black, purple, or yellow).
2) Streak – scraping a mineral specimen along an unglazed procelain plate leaves a
streak (example: hematite leaves a reddish brown streak). Most silicate minerals,
however, are harder than the streak plate and therefore leave no color marking
3) Luster – the quality and intensity of reflected light from a mineral surface. Can
be either metallic or non-metallic. Nonmetallic luster is more common and can be
either glassy (aka vitreous) (feldspars, quartz, micas, pyroxenes, amphiboles) or
earthy (clay minerals).
4) Hardness (Mohs hardness scale) – a scale where 10 representative minerals are
designated as standards of hardness from 1 to 10. Talc, a very soft mineral, is a 1
and diamond, which is the hardest natural substance on earth, is a 10. The Mohs
scale tests the property of “scratchability”. Most geologists don’t carry around
these 10 samples but use other handy items such as their fingernails (2.5), a
copper coin or penny (3-4), a knife or steel nail (> 5), window glass (5-6), a metal
file (carpenter’s file, not a fingernail file) (6-7) to estimate the degree of hardness
of the unknown mineral sample they are trying to define. The hardness of metal
items used depends on manufacturing alloy, of course.
5) Specific gravity – the ratio of a mass of a substance to the mass of an equal
volume of water. Most common silicate minerals weight about 2.5 times as much
as an equal volume of water. Quartz has a specific gravity of 2.65, feldspars range
from 2.56 to 2.76. Need special scales to determine this except for very heavy
minerals such as galena, which has a specific gravity of 7.5.
6) Crystal form of a mineral is a set of faces that have a definite geometric
relationship to one another.
a. In most rocks, minerals don’t develop the shape that what we associate
with a crystal because they are competing for space with other minerals.
b. Most minerals are able to develop crystal faces only if they are surrounded
by a fluid that can be easily displaced as the crystal grows.
c. Steno, a 17th century Danish naturalist, found that the angle between two
adjacent faces of quartz is always exactly the same. Other minerals were
also found to have this regularity. This observation became known as the
law of constancy of interfacial angles.
d. Atoms of different minerals are clustered into geometric forms such as
cubes, bricks, hexagons, etc. and so the shape a crystal has is essentially a
reflection of the orderly three-dimensional stacking of these tiny
geometric forms.
Crystals can precipitate out from a saturated solution. Here is a geode with amethyst
crystals. Because these crystals grew relatively uninhibited by other crystals, they are
called euhedral crystals. Anhedral grains are more common where crystals grow into
one another.
7) Cleavage - Cleavage refers to where a crystal breaks when struck. Crystals break
or split along a preferred direction. This is caused by the weaker bonds of atoms
in that direction. Some break in one direction, some in many directions. Some
minerals have no cleavage (quartz breaks into conchoidal fractures). Do not
confuse cleavage planes and crystal faces. Cleavage planes are repeated while a
crystal face is a single surface.
8) Graphite. Both diamond and graphite have the exact same chemical composition
(pure C) but a different crystal structure. They are polymorphs.
9) Other tests:
a. carbon containing minerals may react with acid to give another identifying
property (example: calcite releases bubbles of CO2 when a weak solution
of HCL is placed on it).
b. Striations or shallow grooves on the flat surface of a cleavage direction are
an indication of plagioclase
c. The mineral magnetite has magnetic properties
10) Silicates: the two most abundant elements of the earth’s crust are silicon and
oxygen. The basic building block for most common minerals is the silicon-oxygen
tetrahedron (SiO4-4).
Oxygen makes up 46.6% of the earth’s crust by weight and 93.8% by volume. Silica is
27.7% by weight and 0.9 % by volume. Next in order of abundance is aluminum, then
iron, calcium, sodium, potassium, and magnesium.
The silicate groups are defined according to how the tetrahedra link together:
•
•
•
•
•
independent tetrahedra (garnet, olivines)
single chains (pyroxenes),
double chains (amphiboles),
sheet silicates (mica, muscovite, biotite)
framework silicates (feldspar [plagioclase, orthoclase), quartz).
Other mineral classes:
•
•
•
•
•
oxides (examples: magnetite and hematite and many ore minerals);
sulfides (galena and pyrite); sulfates (gypsum);
halides (rock salt and fluorite);
carbonates (calcite and dolomite) and
native metals (gold, copper)