L1 - Mineralogy & Crystallography

Most rocks are combinations of various minerals, with their composition, structure, abundance, orientation, and shape reflecting the rock's history and governing its bulk properties. Understanding the origin and behavior of minerals is crucial to understanding rocks.

Key questions to consider include:

• What is a mineral?

• How do minerals behave?

• How do minerals form?

• Why does a specific set of minerals form?

• Why do they behave in a particular way?

A mineral is defined as a naturally occurring homogenous solid with a definite chemical composition and an ordered, repeating physical structure at the atomic or molecular scale.

Breaking down the definition:

• Definite Chemical Composition: Minerals can be expressed as a combination of elements, but this composition doesn't have to be fixed. For example, olivine has the formula $(Mg, Fe)Si_2O_4$, representing a solid solution where magnesium (Mg) and iron (Fe) can substitute for each other in the crystal structure. The end-members of this solid solution are fayalite ($Fe_2SiO_4$) and forsterite ($Mg_2SiO_4$).

• Ordered, Repeating Physical Structure at the Atomic or Molecular Scale: The atoms in a mineral arrange themselves in a repeating pattern, creating a crystal structure with characteristic periodicity. If atoms are randomly arranged due to rapid cooling, the resulting substance is called a glass. Polymorphs are minerals with the same chemical composition but different atomic arrangements (e.g., diamond and graphite).

Chemical Composition of the Earth

There are over 7000 known minerals, each forming in a specific environment and providing information about a rock's history. However, most rocks are composed of a small group of common minerals. The abundance of elements throughout Earth determines which minerals are likely to be prevalent. These elements are: Oxygen (O), Silicon (Si), Iron (Fe), Magnesium (Mg), Aluminum (Al), Calcium (Ca), Sodium (Na), and Potassium (K).

This elemental abundance is determined through the study of:

• Meteorites: Fragments of different planetary bodies.

• The Sun's Composition.

• Xenoliths: Direct samples of the Earth's mantle brought to the surface.

The composition of the Earth's core is uncertain due to its inaccessibility and the potential incorporation of light elements (O, C, H, Si, S).

• Core: Predominantly iron (Fe) at ~80-85%, nickel (Ni) at ~5-10%, and light elements at ~10%.

• Mantle: Predominantly silicon (Si), magnesium (Mg), and oxygen (O), with some iron (Fe), calcium (Ca), and aluminum (Al).

• Crust: Predominantly silicon (Si) and oxygen (O), with accessory aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), and potassium (K).

The crust is readily accessible for sampling and studying its minerals.

Mineral Classification

Minerals are broadly classified based on the type of anion in their structure, as minerals with the same anions tend to exhibit similar properties.

Primary mineral types:

• Oxides $(O^{2-})$: e.g., Magnetite ($Fe3O4$), rutile($TiO_2$)

• Hydroxides $(OH^-)$ : e.g., Goethite $(FeOOH)$

• Sulphides $(S^{2-}, S^-)$: e.g., Pyrite $(FeS_2)$, Galena $(PbS)$, Sphalerite $(ZnS)$

• Halides $(Cl^-, F^-)$: e.g., Halite $(NaCl)$, Fluorite $(CaF_2)$

• Carbonates ($CO3^{2-}$) e.g. Calcite ($CaCO3$), Dolomite
  ($CaMg(CO_3)_2$)

• Phosphates ($PO_4^{3-}$) : e.g. Apatite ($Ca_{10}(PO_4)_6(OH^-, F^-, Cl^-, Br^-)_2$)

• Sulphates ($SO4^{2-}$) : e.g. Gypsym ($CaSO_4.2H_2O$)

• Silicates $(SiO_4^{4-})$: The most abundant mineral type on Earth, and the primary focus for the rest of the course.

Common Silicates