Chapter 5 - Minerals
Minerals
•
Industrial minerals provide the raw materials for the manufacture of
building materials such as sheetrock and concrete
•
Ore minerals are the source of gold, silver, copper, uranium, iron and
many other materials
•
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.
3) Minerals have a definable chemical composition. For example,
quartz is SiO2 (one silicon atom for every two oxygen atoms).
4) 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.
Disordered atoms, as in a glass, do not have a crystal lattice and therefore are not
true minerals.
Crystals possess symmetry as in this example of a halite crystal and a snowflake.
Crystals form in different ways. Solidification of a melt as when water freezes to
produce ice crystals. Precipitation from a solution as when salt precipitates out of
salt water. 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).
Physical properties of minerals (continued)
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 (56), 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.
• 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.
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.
• 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.
• 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.
The angle between adjacent crystal faces is always the same.
halite
garnet
diamond
stibnite
staurolite
calcite
quartz
kyanite
Prismatic crystals
of quartz
exhibiting glassy
luster
7.
Crystal habit – the
general shape or habit
of a crystal or cluster of
crystals. They grow in
forms described as
needle-like, fibrous,
cubic, platy, prismatic,
etc.
8.
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.
Some examples of cleavage planes are shown below.
Example: mica (one direction
of cleavage)
amphibole
Halite (cubic
cleavage)
Pyroxene
Calcite (rhombohedral cleavage)
Diamond
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
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
silicon
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).
End of Chapter 5