Understanding Minerals and Rocks: Composition and Formation
Minerals: The Building Blocks of Rocks
Minerals are solid substances formed by the combination of chemical elements found in the Earth’s crust. Rocks are composed of minerals. To be classified as a mineral, a substance must possess the following characteristics:
- Be solid
- Be naturally occurring
- Have an inorganic origin
- Have a homogeneous chemical composition
Minerals are pure substances, each with a unique chemical composition. This composition determines many of its properties. However, impurities within a mineral’s composition can alter some of its properties, such as color. These variations are known as mineral varieties.
The external appearance of a mineral is determined by the arrangement of its components. There are two main types of minerals:
- Amorphous minerals: Minerals with a disordered arrangement of components, such as agate.
- Crystallized minerals: Minerals with an orderly arrangement of components, resulting in a crystalline structure.
Silicates and Non-Silicates
Most minerals contain the two most abundant elements in the Earth’s crust: oxygen and silicon. These are known as silicates. Silicates are a group of minerals that contain silicon and oxygen as their main components. Some abundant silicates in the Earth’s crust include:
- Quartz: A characteristic mineral of granitic rocks, known for its hardness and resistance to water alteration.
- Feldspars: Common components of crustal rocks like granite and basalt.
- Micas: Abundant in rocks such as granite and shale.
Non-silicates are a group of minerals that do not contain silicon in their composition.
Mineral Formation
For a mineral to form, its components must be present in sufficient quantity and at the appropriate temperature. Minerals can originate in three different scenarios:
- Within a mass of molten rock located inside the crust.
- In solid rock under high pressures and temperatures within the crust.
- On the surface, due to the crystallization of dissolved substances in water.
Properties of Minerals
Key properties used to identify minerals include:
- Color: The type of light a mineral reflects when illuminated with white light.
- Luster: How a mineral reflects light.
- Hardness: The resistance of a mineral to being scratched.
- Streak: The color of the powder produced when a mineral is scratched across a surface, which may differ from the mineral’s surface color.
- Exfoliation (Cleavage): The property of a mineral to fracture into fragments with flat faces.
Rocks: Aggregates of Minerals
Rocks are composed of mineral grains bound together. If all the grains are of the same mineral, it is called a monomineralic rock. There are three main types of rocks: sedimentary, igneous, and metamorphic.
Sedimentary Rocks
Sedimentary rocks often exhibit layering, known as strata. There are four types of sedimentary rocks:
- Detrital rocks
- Chemical rocks
- Organic rocks
- Evaporitic rocks
Igneous Rocks
Igneous rocks, also called magmatic rocks, are formed when magma solidifies. Magma inside the crust can undergo two different processes, giving rise to two types of igneous rocks:
- Plutonic rocks
- Volcanic rocks
Metamorphic Rocks
When a rock is subjected to high pressures and temperatures without melting, the minerals that compose it undergo a series of changes, resulting in a metamorphic rock. Metamorphic rocks can be classified into two types based on their external appearance:
- Layered rocks
- Crystalline rocks