💎Mineralogy Unit 14 – Minerals in Rocks and Sediments

What Are Minerals?

• Naturally occurring inorganic solids with a definite chemical composition and ordered atomic structure
• Formed through various geological processes (igneous, metamorphic, sedimentary)
• Composed of one or more chemical elements combined in a specific ratio
• Characterized by their unique physical, chemical, and optical properties
• Play crucial roles in Earth's processes and human activities (construction, technology, jewelry)
• Differ from rocks, which are aggregates of minerals and/or other materials
• Classified based on their chemical composition and crystal structure (silicates, oxides, sulfides, carbonates)

Mineral Formation and Growth

• Minerals form under specific geological conditions (temperature, pressure, chemical environment)
• Primary minerals crystallize from magma or lava during igneous processes
  • Magmatic differentiation leads to diverse mineral assemblages
  • Pegmatites form from residual magmas enriched in volatile components and rare elements
• Secondary minerals form through weathering, alteration, or metamorphism of pre-existing minerals
• Crystal growth occurs as atoms arrange themselves in a repeating pattern
  • Influenced by factors such as temperature, pressure, and availability of chemical components
• Hydrothermal fluids transport dissolved minerals and precipitate them in veins or cavities
• Evaporites form through precipitation from evaporating saline water (gypsum, halite)

Crystal Structures and Systems

• Minerals are classified into six crystal systems based on their symmetry and arrangement of atoms
  • Isometric (cubic), tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic
• Crystal structure refers to the unique arrangement of atoms within a mineral
• Unit cell is the smallest repeating unit that defines the crystal structure
  • Described by lattice parameters (lengths and angles of the unit cell)
• Polymorphism occurs when minerals have the same chemical composition but different crystal structures (graphite and diamond)
• Twinning results from intergrowth of two or more crystals sharing a common plane or axis
• Point groups and space groups further describe the symmetry elements within a crystal structure

Physical Properties of Minerals

• Physical properties are observable characteristics used for mineral identification
• Hardness measures a mineral's resistance to scratching (Mohs scale)
  • Ranges from 1 (talc) to 10 (diamond)
• Cleavage is the tendency of a mineral to break along smooth, flat surfaces
  • Described by the number and orientation of cleavage planes (cubic, octahedral, prismatic)
• Fracture describes the appearance of broken surfaces not along cleavage planes (conchoidal, uneven, splintery)
• Luster refers to the way a mineral reflects light (metallic, vitreous, resinous, pearly)
• Streak is the color of a mineral's powder when scratched on a porcelain plate
• Specific gravity compares a mineral's density to that of water
• Other properties include magnetism, radioactivity, and reaction to acids

Optical Properties of Minerals

• Optical properties describe how minerals interact with light
• Color is the most obvious optical property, but can be variable within a mineral species
  • Caused by the presence of chromophore elements or structural defects
• Transparency refers to the ability of light to pass through a mineral (transparent, translucent, opaque)
• Pleochroism is the change in color when viewed from different directions in polarized light
• Refractive index measures the bending of light as it passes through a mineral
  • Influenced by the mineral's composition and structure
• Birefringence is the difference in refractive indices for light vibrating in different planes
• Extinction occurs when a mineral appears dark under crossed polars in a polarizing microscope
• Interference colors are displayed by minerals in thin section due to birefringence and thickness

Common Rock-Forming Minerals

• Silicates are the most abundant minerals in Earth's crust
  • Characterized by silicon-oxygen tetrahedra (SiO4) linked in various arrangements
  • Include quartz, feldspars, micas, amphiboles, and pyroxenes
• Carbonates contain the carbonate ion (CO3) and are common in sedimentary rocks
  • Examples include calcite, dolomite, and aragonite
• Oxides are composed of metal cations bonded to oxygen anions
  • Occur in igneous, metamorphic, and sedimentary rocks (magnetite, hematite, rutile)
• Sulfides are formed by the combination of metal cations with sulfur anions
  • Often associated with ore deposits (pyrite, galena, sphalerite)
• Native elements occur in their pure form without combining with other elements
  • Examples include gold, silver, copper, and sulfur

Minerals in Sedimentary Environments

• Sedimentary minerals form through weathering, erosion, transportation, and deposition of pre-existing minerals
• Clastic sedimentary rocks contain mineral grains derived from the mechanical breakdown of source rocks
  • Quartz, feldspars, and clay minerals are common in clastic sediments
• Chemical sedimentary rocks form through precipitation of minerals from aqueous solutions
  • Evaporites (gypsum, halite) and carbonates (limestone, dolostone) are examples
• Authigenic minerals crystallize in situ within sedimentary rocks during or after deposition
  • Include glauconite, pyrite, and various clay minerals
• Diagenetic processes alter the mineralogy and texture of sedimentary rocks after deposition
  • Compaction, cementation, and recrystallization modify the original mineral assemblage

Mineral Identification Techniques

• Hand sample identification relies on observing physical properties (color, luster, hardness, cleavage)
  • Mineral identification keys and charts aid in the process
• Optical microscopy uses polarized light to study mineral properties in thin section
  • Allows determination of optical properties (refractive index, birefringence, extinction)
• X-ray diffraction (XRD) analyzes the crystal structure and lattice parameters of minerals
  • Compares the diffraction pattern to a database of known mineral structures
• Scanning electron microscopy (SEM) provides high-resolution images and chemical analysis of mineral surfaces
  • Energy-dispersive X-ray spectroscopy (EDS) determines the elemental composition
• Electron probe microanalysis (EPMA) quantitatively measures the chemical composition of small mineral grains
• Raman spectroscopy identifies minerals based on their unique vibrational modes
  • Non-destructive technique applicable to small samples and in situ analysis

Economic Importance of Minerals

• Minerals are essential raw materials for various industries and applications
• Metallic minerals are sources of valuable metals (iron, copper, gold, silver)
  • Extracted through mining and processed for use in construction, transportation, and technology
• Industrial minerals are used for their physical and chemical properties rather than metal content
  • Examples include gypsum (plaster, drywall), kaolin (paper, ceramics), and talc (cosmetics, plastics)
• Energy minerals are used for fuel or energy production
  • Coal, uranium, and oil shale are examples of energy minerals
• Gemstones are minerals valued for their beauty, rarity, and durability
  • Used in jewelry and decorative objects (diamonds, rubies, sapphires, emeralds)
• Mineral exploration and mining have significant economic, social, and environmental impacts
  • Sustainable practices and responsible resource management are crucial for long-term benefits