Mineralogy and Crystal Structure

Study Notes

Graphite is comprised of sheets of carbon atoms bonded by weak van der Waals forces.
The weak bonding in Graphite leads to the mineral being soft and greasy.
Diamond is made up of carbon atoms bonded to each other by strong covalent bonds.
Diamonds are hard and have high melting points due to strong covalent bonding.

Isostructural minerals share the same crystal structure arrangement.
Halite (NaCl) and Galena (PbS) are an example of isostructural minerals.
Both minerals have cubic crystals, perfect cleavage in three directions, and similar crystal shapes.

An isostructural group consists of minerals that share the same crystal structure and are chemically related via a common anion or anionic group.
The Calcite group of carbonates consists of minerals that are isostructural, including:
- Calcite (CaCO3)
- Rhodochrosite (MnCO3)
- Smithsonite (ZnCO3)
- Magnesite (MgCO3)

Polymorphism refers to the ability of a chemical compound to crystallize in more than one structure.
The different crystal structures of a compound are known as polymorphic forms or polymorphs.
Minerals with the same chemical composition but different structures form a polymorphic group.
Polymorphism is caused by different thermodynamic conditions (temperature and pressure) that favor different crystal structures.

Minerals are naturally occurring, solid substances that have a crystal structure.
Minerals have a well-defined chemical composition that can be represented by a chemical formula.
Most minerals are inorganic.

Cations in a crystal lattice can be interchangeable, leading to compositional variations known as solid solutions.
The range of composition produced by a solid solution in a given mineral is called a substitution series.
The compositional extremes of a substitution series are called end members.

Substitutional Solid Solution:
- Simple Substitution: One cation is replaced by another of similar size and charge (e.g., Olivine, between Fe and Mg).
- Coupled Substitution: Charge balance is maintained by coupling substitutions that increase and decrease charge (e.g., Plagioclase).
Omission Solid Solution: Involves the omission of ions from a crystal lattice.
Interstitial Solid Solution: Ions are placed into vacant sites in the crystal lattice, maintaining charge balance. (e.g., Beryl).