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Unit 5 - Rocks & Minerals
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In class notes from the fifth unit of grade 12 earth and space science, rocks and minerals.
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Unit 5 - Earth & Space Science Notes, Minerals
● There are over 4000 minerals found on Earth, however, only a few dozen are common. All
minerals share five general characteristics:
○ 1. Naturally occuring (i.e., “minerals” formed in the laboratory are not considered to be
true minerals because they are artificially created)
○ 2. Inorganic (coal is not a mineral because it is formed from the decay of plant [organic]
matter)
○ 3. A specific chemical composition (either an element [e.g., sulphur] or a compound
[e.g., NaCl])
○ 4. Crystalline Solids (water is not a mineral because it is a liquid at room temperature)
○ 5. Atoms are arranged in a repeated pattern
Minerals Are Composed of What Elements?
● Eight different elements make up 98% of the Earth’s crust:
○ Oxygen - 46.6%
○ Silicon - 27.7%
○ Aluminum - 8.1%
○ Iron - 5.0%
○ Calcium - 3.6%
○ Sodium - 2.8%
○ Potassium - 2.6%
○ Magnesium - 2.1%
○ Other - 1.5%
● From the above table, 74.3% of the Earth’s crust is oxygen and silicon. As a result, most rock
forming minerals are silicates (rocks composed of silicon, oxygen, and another element). The
two classes of silicates are the ferromagnesium silicates (minerals that contain magnesium
and/or iron) and the non-ferromagnesian silicates (minerals that don’t have iron or
magnesium).
Crystal Formation and Structure
● Crystals form from magma (either on or below the surface of the Earth), as the heat is lost and
the atoms and ions come closer together to form bonds in different compounds. Crystals will
also form in solutions. As the liquid evaporates the elements or compounds in the solution will
precipitate (solidify).
● The unique properties of minerals are based on the arrangement and the different types of
bonding between atoms. Ionic bonding between positive Na and negative Cl ions in halite (is
the naturally occurring salt NaCl) results in a weak crystal structure compared to the strong
covalent bonds between C atoms in a diamond. A third interaction, the weak electrostatic
interactions called Van der Waals interactions, also holds the minerals together.
● When carbon atoms are arranged in a tetrahedral structure, it forms the hardest natural
mineral, a diamond.
● When the carbon atoms are arranged in sheets, the weak Van der Waal bonds hold the sheets
together forming a soft mineral called graphite.
● When some crystals form in open spaces, the clear crystalline shapes can be seen. However,
when they are formed in confined spaces, the smooth crystal faces are not produced, although
the same repeated arrangement of atoms occurs.
, Mineral Crystal Shapes
● Cubic
○ All 3 axis are the same length and they meet at 90 degrees, for example halite
● Tetragonal
○ 3 axis meet at 90 degrees but only two are of the same length, for example wulfenite
● Orthorhombic
○ 3 axis meet at 90 degrees and all are a different length, for example topaz
● Monoclinic
○ 3 axis of different lengths, 2 meet at 90 degrees and the third at an oblique (not 90)
angle, for example gypsum
● Triclinic
○ 3 axis of different lengths meet at oblique angles, for example kyanite
● Hexagonal
○ 3 axis of equal lengths meet at 60 degrees, 4th axis is vertical and can be a different
length, for example quartz
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