Geology Reference
In-Depth Information
Usually it's inorganic:
Most minerals are
inorganic,
meaning they are not alive or
the remains of a live organism. However, some animals create mineral shells. In
this case, when the animal dies and these shells remain (and form sedimentary
rocks, explained in Chapter 7), the shell materials can be considered minerals.
It has an orderly structure:
When atoms combine to form minerals, they do so in
an organized way that forms a geometric pattern called a
crystal.
Earth materials
that form without this orderly crystalline structure are described as
amorphous
or
glassy
and are not minerals.
It is naturally occurring:
True minerals are found in nature. Humans have the tech-
nology to combine atoms into crystals in a lab, but the manmade crystals are not
considered minerals.
It has a specific chemical composition:
Each mineral has a defined
chemical com-
position:
a combination of elements that creates its particular crystal structure.
Some minerals may have multiple, similar chemical compositions, where similar-
sized atoms of different elements may replace each other. In these cases the small
range of variation is well-defined for each mineral.
Making Crystals
Minerals come in many shapes. The shape of a mineral is determined by how the atoms
creating the mineral are organized. This three-dimensional shape formed by the bonded
atoms is the
crystal structure
(sometimes called the crystal
lattice
) of the mineral. When
the atoms bond together, they arrange themselves in a particular pattern.
No matter how large or small, all the crystals of a particular mineral have the
same crystal structure. Minerals with more than one common crystal shape are
said to have multiple
habits.
For example, the mineral pyrite (called
fool's gold
)
has more than one commonly occurring habit, or crystal shape.
You may have seen large crystals such as the one illustrated in Figure 6-1, and you've un-
doubtedly seen gemstones in jewelry, which are large crystals as well. Large crystals are
minerals, but the minerals in rocks are often much smaller. Here are two examples of
why you find smaller minerals in many rocks:
