7.1 Mineral Formation and Properties

Minerals are the building blocks of Earth's crust, forming through various processes. From magma crystallization to precipitation and metamorphism, these processes shape the diverse mineral world we see today. Understanding mineral formation is key to grasping Earth's geological history.

Minerals have unique physical and chemical properties that help us identify them. Hardness, cleavage, luster, and chemical composition are just a few characteristics that set minerals apart. These properties not only aid in identification but also determine a mineral's usefulness in various applications.

Mineral Formation Processes

Crystallization from Magma

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• Occurs as magma cools and solidifies, allowing minerals to form and grow into crystals
• Rate of cooling and composition of magma determine mineral types and crystal sizes
• Slow cooling magma allows for larger crystal growth (granite), while rapid cooling results in smaller crystals or glassy texture (obsidian)
• Composition of magma influences which minerals crystallize first, according to Bowen's reaction series

Precipitation from Aqueous Solutions

• Minerals dissolve in water and precipitate out of solution due to changes in temperature, pressure, or chemical conditions
• Can occur in surface environments (evaporite deposits like halite and gypsum) and groundwater environments (formation of cave structures like stalactites and stalagmites)
• Precipitation can also occur in hydrothermal systems, where hot, mineral-rich fluids cool and deposit minerals in veins or around hot springs (quartz, gold, silver)
• Chemical weathering of rocks can lead to the dissolution and subsequent precipitation of minerals

Metamorphism

• Transformation of existing rocks and minerals due to changes in temperature, pressure, and/or chemical environment
• Minerals can recrystallize, grow larger, or change composition without melting during metamorphism
• Regional metamorphism occurs over large areas due to tectonic forces (burial or compression), resulting in foliated rocks like gneiss and schist
• Contact metamorphism occurs when magma intrudes into surrounding rock, causing localized changes in texture and mineralogy (hornfels)
• Hydrothermal metamorphism involves the interaction of hot, mineral-rich fluids with rocks, leading to metasomatism and alteration of mineral compositions

Mineral Physical Properties

Hardness, Cleavage, and Fracture

• Hardness is a mineral's resistance to scratching, measured using the Mohs scale (1-10)
• Harder minerals can scratch softer minerals (diamond can scratch quartz)
• Cleavage is the tendency of a mineral to break along smooth, flat surfaces parallel to its crystal planes
• Quality and number of cleavage planes can be diagnostic (mica has perfect cleavage in one direction, while halite has perfect cleavage in three directions)
• Fracture describes how a mineral breaks when it lacks well-defined cleavage planes (conchoidal fracture in quartz, uneven fracture in pyrite)

Luster, Specific Gravity, and Other Properties

• Luster is the appearance of a mineral's surface when it reflects light
• Types include metallic (pyrite), submetallic (magnetite), non-metallic (vitreous in quartz, resinous in sulfur, pearly in talc, silky in asbestos, dull in kaolinite), and adamantine (diamond)
• Specific gravity is the ratio of a mineral's density to the density of water
• Useful in distinguishing minerals with similar appearances (gold has a higher specific gravity than pyrite)
• Other properties include color, streak (color of powdered mineral), magnetism (magnetite), and reaction with acid (calcite effervesces with dilute hydrochloric acid)

Mineral Chemical Properties

Composition and Classification

• Minerals are classified based on their chemical composition
• Main categories include silicates (quartz, feldspar, mica), carbonates (calcite, dolomite), oxides (hematite, magnetite), sulfides (pyrite, galena), sulfates (gypsum, barite), halides (halite, fluorite), and native elements (gold, silver, copper)
• Composition determines a mineral's chemical properties and influences its physical properties
• Isomorphous substitution can occur, where ions of similar size and charge substitute for each other in a mineral's crystal structure (magnesium substituting for iron in olivine)

Crystal Structure and Its Influence

• Crystal structure refers to the ordered, repeating arrangement of atoms in a mineral
• Seven crystal systems describe the symmetry and geometry of the crystal lattice (cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic)
• Chemical bonds between atoms influence a mineral's hardness and cleavage (strong covalent bonds in diamond result in its extreme hardness)
• Polymorphism occurs when a chemical compound exists in more than one crystal structure (graphite and diamond are both composed of carbon but have different structures and properties)
• Formation process can affect a mineral's composition and structure (high-pressure metamorphism can cause minerals to recrystallize with different structures, like the transformation of graphite into diamond)