Earth materials encompass minerals, rocks, and other natural substances. This topic dives into their classifications, exploring the distinctions between minerals and rocks, and how they form. It's crucial for understanding the building blocks of our planet.
We'll examine mineral classification based on chemistry and structure, and rock categorization by formation processes. We'll also look at non-mineral materials, both natural and human-made, to get a complete picture of Earth's composition.
Minerals vs Rocks vs Earth Materials
Defining Characteristics and Compositions
- Minerals consist of naturally occurring, inorganic solids with definite chemical compositions and ordered internal structures
- Examples include quartz (SiO2) and calcite (CaCO3)
- Rocks form aggregates of one or more minerals or mineraloids through various geological processes
- Examples include granite (composed of quartz, feldspar, and mica) and limestone (primarily composed of calcite)
- Earth materials encompass a broader category including minerals, rocks, and other naturally occurring substances
- Additional examples include soil, water, and organic matter
- Distinguish minerals from rocks based on homogeneity, chemical composition, and internal structure
- Minerals have uniform composition throughout, while rocks can be heterogeneous
- Non-mineral Earth materials include both natural and anthropogenic substances
- Natural examples: obsidian (volcanic glass), coal (organic sedimentary rock)
- Anthropogenic examples: concrete (mixture of cement and aggregates), brick (fired clay)
- Minerals form through crystallization from solutions or melts, or by precipitation from fluids
- Examples: halite (NaCl) crystallizes from evaporating seawater, diamonds form under high pressure deep in the Earth
- Rocks undergo formation through three main processes
- Igneous: cooling and solidification of magma or lava (granite, basalt)
- Sedimentary: deposition and lithification of sediments (sandstone, shale)
- Metamorphic: transformation of pre-existing rocks due to heat and pressure (marble, slate)
- Earth materials classification considers origin, composition, and physical properties
- Natural vs. anthropogenic
- Organic vs. inorganic
- Solid vs. liquid vs. gas
Mineral Classification by Composition and Structure
Chemical Composition Classification
- Mineral classification systems primarily based on chemical composition
- Dana system: widely used in North America
- Strunz system: preferred in Europe and internationally
- Major chemical classes of minerals include
- Native elements (gold, silver)
- Sulfides (pyrite, galena)
- Oxides (hematite, magnetite)
- Halides (halite, fluorite)
- Carbonates (calcite, dolomite)
- Nitrates (saltpeter)
- Borates (borax)
- Phosphates (apatite)
- Sulfates (gypsum, anhydrite)
- Silicates (quartz, feldspar, mica)
- Silicate minerals, most abundant group, further classified based on silica tetrahedra arrangement
- Nesosilicates (olivine)
- Sorosilicates (epidote)
- Cyclosilicates (beryl)
- Inosilicates (pyroxenes, amphiboles)
- Phyllosilicates (micas, clay minerals)
- Tectosilicates (quartz, feldspars)
Crystal Structure and Variations
- Crystal structure classification based on arrangement of atoms within mineral
- Seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal
- Isomorphous substitution in minerals leads to solid solution series
- Chemical composition varies continuously between two end members
- Example: plagioclase feldspar series from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8)
- Polymorphism occurs when minerals with same chemical composition crystallize in different structures
- Example: diamond and graphite (both composed of carbon)
- Other examples: calcite and aragonite (both CaCO3), pyrite and marcasite (both FeS2)
Rock Categorization by Formation and Content
Igneous Rocks
- Form from cooling and solidification of magma or lava
- Classified as intrusive (plutonic) or extrusive (volcanic) based on cooling environment
- Intrusive examples: granite, diorite, gabbro
- Extrusive examples: rhyolite, andesite, basalt
- Further classified using QAPF diagram based on mineral content
- Q: quartz
- A: alkali feldspar
- P: plagioclase feldspar
- F: feldspathoids
- Texture determined by cooling rate
- Slow cooling (intrusive) produces coarse-grained texture
- Rapid cooling (extrusive) results in fine-grained or glassy texture
Sedimentary Rocks
- Form through deposition and lithification of sediments
- Classified as clastic, chemical, or biochemical based on formation mechanism and composition
- Clastic: formed from rock fragments (sandstone, shale)
- Chemical: precipitated from solution (rock salt, gypsum)
- Biochemical: formed by organic processes (limestone, coal)
- Further classified based on grain size, composition, and texture
- Examples: conglomerate (large clasts), siltstone (fine-grained), oolitic limestone (spherical grains)
- Carbonate classification for sedimentary rocks (Folk or Dunham classification)
- Based on ratio of grains to matrix and depositional texture
- Result from transformation of pre-existing rocks due to changes in temperature, pressure, or chemical environment
- Classified based on texture (foliated or non-foliated) and grade of metamorphism
- Foliated examples: slate, schist, gneiss
- Non-foliated examples: marble, quartzite, hornfels
- Metamorphic grade indicates intensity of metamorphism
- Low-grade: slight changes in mineralogy and texture
- High-grade: significant recrystallization and new mineral formation
- Protolith identification important for understanding metamorphic rock origin
- Example: marble forms from limestone, quartzite from sandstone
Common Non-Mineral Materials and Significance
Natural Non-Mineral Earth Materials
- Soil consists of complex mixture of mineral particles, organic matter, water, and air
- Crucial for supporting terrestrial ecosystems and agriculture
- Classified based on texture, composition, and horizon development
- Natural glasses form by rapid cooling of magma, lacking ordered internal structure
- Obsidian: volcanic glass used for tools by ancient civilizations
- Tektites: formed by meteorite impacts
- Coal forms from accumulation and transformation of plant material
- Major fossil fuel source
- Types include lignite, bituminous, and anthracite, based on carbon content and heat value
- Petroleum and natural gas form from decomposition of organic matter
- Essential for global energy production
- Found in sedimentary basins, trapped in porous rocks
Anthropogenic and Extraterrestrial Materials
- Anthropogenic materials widely used in construction and infrastructure development
- Concrete: mixture of cement, aggregates, and water
- Brick: fired clay product
- Asphalt: petroleum-based material used for road construction
- Meteorites provide valuable information about composition and formation of solar system
- Types include iron, stony, and stony-iron meteorites
- Chondrites: primitive meteorites containing early solar system material
- Ice, while meeting most criteria for minerals, often considered separately
- Unique properties and significance in Earth's climate and hydrological systems
- Forms various structures like glaciers, sea ice, and ice sheets