Mineralogy: Crystal Growth, Defects, Twinning & Phase Diagrams

What is crystal growth?

The process by which atoms, ions, or molecules arrange into an orderly, repeating crystal structure.

What are the three main stages of crystal growth?

Nucleation, growth, and maturation.

What factors affect crystal growth?

Temperature, pressure, chemical composition, available material, cooling rate, space, and supersaturation.

How does cooling rate affect crystal size?

Slow cooling often produces larger crystals, while rapid cooling often produces smaller crystals.

What is chemical zoning?

Changes in a mineral's chemical composition from its core to its rim or between growth sectors.

What is normal zoning?

A compositional change that follows the expected trend as conditions change during growth.

What is reverse zoning?

Zoning in which the rim becomes enriched in a component opposite to the usual growth trend.

What is oscillatory zoning?

Repeated compositional bands caused by fluctuating growth conditions.

What is sector zoning?

Compositional differences between different growth sectors or faces of a crystal.

What is a solid solution?

A mineral structure in which different chemical elements can substitute for one another.

What is simple cationic substitution?

One positive ion replaces another positive ion in a crystal structure.

What is simple anionic substitution?

One negative ion replaces another negative ion in a crystal structure.

What is coupled substitution?

Two or more substitutions occur together to maintain overall electrical neutrality.

What is an interstitial mechanism?

An additional atom or ion occupies a space between normal lattice sites.

What is an omission mechanism?

A lattice site is left vacant to help maintain charge balance.

What must substitutions maintain in a crystal?

Overall electrical neutrality, along with suitable ionic size and coordination.

What is polymorphism?

The ability of the same chemical composition to form different crystal structures.

What is an example of polymorphism?

Diamond and graphite are both carbon but have different crystal structures.

What is a displacive transformation?

A structural change involving small shifts in atomic positions.

What is a reconstructive transformation?

A structural change requiring substantial rearrangement and breaking and reforming of bonds.

What is an order-disorder transformation?

A change in how regularly atoms occupy available crystallographic sites.

What is a binary composition diagram?

A diagram representing a system with two chemical components.

What is a ternary composition diagram?

A triangular diagram showing compositions involving three components.

What do the corners of a ternary diagram represent?

100% of one component at each corner.

What does a P-T diagram represent?

Pressure versus temperature and the stability fields of different phases.

What does a T-X diagram represent?

Temperature versus composition and how phase stability changes with composition.