Understanding Minerals: Composition & Classification

• Rocks and minerals are solids found in the earth's crust, but they differ in composition and structure.

Mineral Definition

• Minerals are naturally occurring, inorganic solids with unique chemical and physical properties.
• Key characteristics: uniform chemical composition, crystalline internal structure.
• Crystalline structure: atoms arrange in a unique, organized manner.
• Minerals can consist of single or multiple elements.
• Magma is not a mineral when liquid, but becomes one upon cooling and hardening into a solid.
• Size ranges from vast (tree-sized) to tiny (grain of sand).

Classifying Minerals

• Minerals are classified by chemical composition and crystalline structure.
• Other characteristics used for classification: color, density, hardness, streak, luster, and solubility.
• Some minerals are specific to certain areas (e.g., pyroxene in meteorites).
• Geologists classify crystal structures by number of sides, angles, cleavage, and fracture.
• Three main crystalline structures: Body-Centered Cubic (BCC), Hexagonal Close-Packed (HCP), and Cubic Close-Packed (CCP).

Crystalline Structures

• Body-Centered Cubic (BCC): A cube with one atom at each corner and one in the center; one central atom surrounded by eight atoms; iron is an example.
• Hexagonal Close-Packed (HCP): A hexagon structure with multiple layers; middle layer holds the central atom surrounded by six atoms; layers above and below each have three atoms; one central atom surrounded by six atoms surrounded by six atoms; titanium is an example.
• Cubic Close-Packed (CCP): A cube similar to the hexagon structure with multiple layers; middle layer holds the central atom surrounded by six atoms, surrounded by three atoms on each side, which then have one atom on the very last outer level; layer one has seven atoms, two layers surrounding the middle layer each has three atoms, the outermost layers each have one atom; gold is an example.

Rock Definition

• Rocks are solid, interconnected collections of two or more crystalline minerals.
• Rocks are composed of mineral grains held together in solid form and can vary in size.
• Petrology - The study of rocks.
• Fine-textured grain rocks are called rhyolite and large-textured grain rocks are called pegmatite.
• Unlike minerals, rocks do not always have a repeating arrangement of atoms (e.g., granite).

Classifying Rocks

• Rocks are classified based on their formation within the rock cycle.
• The rock cycle is a process that either creates new rocks or recycles old ones.
• Three main rock classifications: igneous, sedimentary, and metamorphic.
• The surface of the earth's crust is 75% sedimentary rock and 25% igneous and metamorphic rock.
• The entire crust is 95% igneous and metamorphic rock and 5% sedimentary.

Rock Types

• Igneous: Formed from cooled magma ejected from volcanoes; quicker cooling results in finer grain texture, and slower cooling results in larger grain texture; intrusive (e.g., granite) or extrusive (e.g., basalt).
• Sedimentary: Formed over time by pressure from other rocks; often shows fossil records; used to interpret history; clastic (e.g., shale), biological (e.g., coal), or chemical (e.g., limestone).
• Metamorphic: Formed from other rocks; chemical reactions can alter the minerals; different heat levels can create different rocks from the same minerals; foliated (e.g., slate) or non-foliated (e.g., marble).
• Rock Formation Process: Igneous to Sedimentary occurs through weathering, sedimentation, and lithification; Sedimentary to Metamorphic occurs through heat & pressure, deformation, and recrystallization; Metamorphic to Igneous occurs through melting and crystallization.
• Process: Igneous to Metamorphic occurs through heat & pressure, deformation, and recrystallization; Sedimentary to Igneous occurs through melting and crystallization; Metamorphic to Sedimentary occurs through weathering, sedimentation, and lithification.

Rocks vs. Minerals

• A rock is not a mineral; minerals do not always make up rocks.
• Some rocks (e.g., bioclastic) contain no minerals and are entirely made of dead organic matter.

Examples

• Rocks containing minerals: granite, shale, limestone, marble, sandstone, coal.
• Rocks not containing minerals: peat, chalk, flint.
• Rocks have varied compositions and shapes, while minerals have definite compositions and crystalline structures.
• Fossils are only found in sedimentary rocks.
• Only minerals have nutritional value.