Minerals
What is a Mineral
A mineral is a naturally occurring solid, formed by geologic
processes, that has a crystalline structure and a definable
chemical composition.
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To have a crystalline structure it means that the atoms
that make up the mineral are not distributed randomly,
but rather they are arranged in an orderly or specific
manner.
Atomic Structure
An atom consists of a
nucleus surrounded by a
cloud of orbiting
electrons. The nucleus
contains neutrons and
protons.
The atomic number is
the number of protons in
an atom.
Atomic weight refers to
the sum of protons and
neutrons in the nucleus.
An ion is an atom that is not neutral. If there is excess
electrons than it is called an anion (Cl-). When there are
excess protons it is known as a cation (Mg2+).
The type of chemical bonding plays an important role in
determining the properties of the mineral.
An ionic bond is where a cation and anion get close together
and attract (NaCl)
A covalent bond is when the atoms share their electrons to
complete their outer energy shell (e.g. carbon in diamond)
Van der Waals bonds are weak electrical bonds that are
generated due to asymmetry (e.g. carbon in graphite)
Diamond is a mineral made entirely of carbon. These atoms
are covalently bonded and this gives diamond its
characteristic strength. On the other hand graphite is also
composed of carbon atoms but they are bonded by extremely
weak forces. Two different minerals (such as graphite and
diamond) that have the same composition but different crystal
structures are called polymorphs.
Formation of Minerals
Mineral crystals can form in the following ways:
Through the solidification of a liquid. This is what happens
when ice crystal forms as the water is solidified. Many other
minerals are also formed by solidifying molten rock.
Precipitation from a solution, where the dissolved ions in the
solution bond together and form an insoluble compound that
settles out of solution as a solid.
Bio- mineralization. This is where living organisms cause
minerals to precipitate either within or on their body. For
example clams extract ions from water to produce mineral
shells.
Minerals can precipitate directly from a gas. This typically
occurs around volcanic vents or around geysers, where the
volcanic gas and steam enter the atmosphere and cool quickly
or abruptly. Around volcanic regions a bright yellow deposit
can be seen and this is sulfur. It is formed in the following
manner.
When crystals grow there growth may be restricted as existing
crystals acts as obstacles. In this case the mineral grows to fill the
space that is available, and its shape is controlled by the shape of
their surroundings. Minerals without well-formed faces are anhedral
(no clear shape). If the mineral growth is not restricted and it is
able to grow into open space then well-developed crystal faces form
and this is called euhedral (opposite sides are parallel).
Physical properties of minerals
Hardness: The ability of a mineral to resist scratching, and is
dependent on the crystal structure. The degree of hardness is
measured by the observing the comparative ease or difficulty
with which one mineral is scratched by another. Harder
substances will scratch softer ones, whilst the softer
substance will leave a streak on the harder one.
Knife has a
hardness of
5.5-6.0
Fingernail has
hardness of 22.5
Coin has
hardness of 33.5
Cleavage: the
tendency of
minerals to break or split along certain lines of weakness in
their structure. This is a point where the bonds holding the
atoms together in the crystal are weakest.
Fracture: When a mineral is broken, other than along a
cleavage plane, it breaks along an unpredictable, irregular or
curved surface called a fracture. The types of fractures are:
- Conchoidal: fracture with smooth curved surfaces
that resemble the interior of a seashell
- Fibrous: splinters or fibers on the surface
- Hackly: fracture surface is rough and has jagged
edges
- Uneven: rough irregular surfaces
Streak: color of the crushed powder of the mineral.
Determined by scratching it on a white ceramic plate.
Luster: way a mineral surface scatters light. The luster can
be:
- Metallic: best compared to a metallic substance such
as silver, gold
- Adamantine: Diamond like
- Vitreous: glassy
- Sub-vitreous: when less well developed
- Dull: mineral that possesses no luster
- Pearly
- Silky
Diaphaneity: Ability to transmit light.
- Transparent: transmit distinct images
- Translucent: transmits light but not images
- Opaque: no light transmitted
Crystal Habit: shape and character that a crystal is most
likely to form is the habit. The character of the crystal is
described by whether it is crystallized to being amorphous
where there is a complete absence of a crystalline structure.