Overview
This lecture explains the main processes by which minerals form, including crystallization, deposition, and mineral replacements, and introduces key concepts like pseudomorphs, polymorphs, and crystal twinning.
Main Processes of Mineral Formation
- Minerals are typically formed as part of the rock formation process.
- Sublimation from volcanic gases deposits minerals directly from gas to solid, such as native sulfur near volcanoes.
- Crystallization from lava or magma forms minerals as molten rock cools, with different minerals forming at different temperatures.
- Deposition from aqueous solutions occurs when dissolved minerals precipitate out of hot water during cooling, forming veins or coatings in rocks.
- Evaporation of mineral-rich water leaves minerals behind, as seen in the formation of rock gypsum in drying basins.
Mineral Alterations and Special Forms
- Some minerals can be replaced by other minerals due to environmental changes, a process called pseudomorphism.
- Pseudomorphs ("false forms") contain characteristics of both the original and the replacing mineral, such as malachite replacing azurite.
- Polymorphs are minerals with the same chemical composition but different atomic structures, like graphite and diamond.
Crystal Twinning
- Crystal twinning occurs when mineral crystals grow together in a symmetrical pattern due to growth errors.
- Contact twins share only one plane, and penetration twins share more than one plane between crystals.
- Calcite is an example of contact twinning; staurolite is known for penetration twinning.
Key Terms & Definitions
- Sublimation — transition directly from gas to solid.
- Crystallization — process of a substance solidifying from a liquid or gas into a crystal structure.
- Deposition — minerals precipitating out of solution and depositing as solids.
- Pseudomorph — a mineral replacement where the new mineral retains the form of the original.
- Polymorph — minerals with identical chemical formulas but different internal atomic arrangements.
- Crystal Twinning — symmetrical intergrowth of two or more crystals of the same mineral.
Action Items / Next Steps
- Review the different mineral formation processes.
- Be prepared to learn about the physical properties of minerals, starting with hardness, in the next lecture.