Minerals and Igneous Rocks
Why Study Minerals?
Importance of Minerals: Rocks are composed predominantly of minerals. Understanding minerals is fundamental to geology.
Requirements to be a Mineral
Solid at Room Temperature: Must maintain a solid state at ambient temperatures.
Inorganic: Cannot be a product of living organisms (e.g., shells cannot qualify).
Naturally Occurring: Must form by natural processes without human intervention.
Crystalline Structure: Atoms are arranged in a systematic, repeating pattern.
Chemical Compound: Must have a defined chemical formula.
Example of Minerals
Halite: Commonly known as salt, is a mineral which evaporates naturally from dry lake beds.
Sugar: Not classified as a mineral, even though it has a crystalline structure, since it is organic in origin.
Definitions and Concepts
Glass: A solid with disordered atoms, not fulfilling the crystalline requirement for minerals.
Mineral Characteristics:
Vitreous: Glass-like appearance.
Waxy: Appearance resembling soap.
Olivine: Always exhibits a green hue.
Color Variability: Trace elements can alter the color of a mineral without changing its overall compound.
Mineral Streak
Definition: The color of a mineral's powder when crushed.
Observation: The streak color may differ from the original mineral color.
Ionic and Covalent Bonds
Anions: Negatively charged ions, typically larger than cations.
Cations: Positively charged ions. The sizes of cations and anions influence ionic bonding shapes.
Mineral Cleavage: Refers to how a mineral breaks along flat planes reflecting light (indicating cleavage) as opposed to jagged fractures.
Mineral Classes
Silicates: The most prevalent mineral class; form a large part of both oceanic and continental crusts, primarily containing the silicate ion SiO_4.
Feldspar: The most abundant group of silicates; quartz (SiO_2) is the most durable mineral.
Non-Silicates
Sulfides: Compounds where sulfates bond with metals (e.g., galena).
Oxides: Minerals consisting solely of metals.
Halides: Salts, such as halite (NaCl).
Carbonates: Compounds containing CO_3.
Importance of Halite
NaCl Compound: Essential for human health, aids fluid retention, nerve transmission, used as a de-icer.
Calcite
Chemical Formula: CaCO_3 (calcium carbonate).
Properties: Exhibits double refraction; commonly used in cement production, neutralizing acids, and found in dietary supplements and makeup.
Quartz
Chemical Formula: SiO_2 (silicon dioxide).
Uses: Employed in computer chips and timepieces.
Bonding Types
Ionic Bonds: Formed through electron transfer between oppositely charged ions.
Covalent Bonds: Involve electron sharing, resulting in stronger bonds.
Van der Waals Bonds: Result from electrical attraction between charged particle sheets.
Distinction Between Minerals and Rocks
Minerals: Composed of specific compounds/elements, possessing identifiable properties.
Rocks: Composed of multiple minerals, which may also include organic compounds.
Methods for Identifying Minerals
Reactions: Reactivity to acids.
Taste: Salty if it’s halite.
Smell: For identification.
Magnetism: Some minerals exhibit magnetic properties.
Cleavage: Observing surfaces after breakage.
Crystal Form: The geometric shape of the mineral.
Streak: Assessing the mineral powder.
Hardness: Resistance to scratching.
Luster: Reflective quality of the mineral surface.
Fracture: The texture of the surface created when a mineral breaks (jagged vs. smooth).
Rock Cycle Overview
Processes Involved:
Melting: Transition from solid to liquid rock.
Weathering/Erosion: Breaking down of rocks into sediment.
Deposition: Accumulation of sediment.
Temperature/Pressure Changes: Conditions leading to metamorphic rock formation.
Cooling/Crystallization: Solidification of magma or lava.
Rock Types in the Cycle
Igneous: Formed from cooling magma.
Sedimentary: Formed from lithified sediments.
Metamorphic: Formed under pressure and heat from existing rocks.
Magma Characteristics
Definition: Molten rock containing crystals and gases, can host xenoliths (fragments of other rock).
Types of Magma:
Extrusive: Cools rapidly on the surface, resulting in smaller crystals.
Intrusive: Cools slowly beneath the surface, allowing larger crystals to form.
Types of Melts and Their Sources
Ultramafic: High in iron and magnesium.
Mafic: Still predominantly iron and magnesium; derived from mantle.
Felsic: Richer in silica and feldspar; derived from crust.
Factors Influencing Melt Composition
Source Rock Composition: Initiates magma characteristics.
Partial Melting: Only parts of a rock melt, generally yielding silica-rich magma.
Assimilation: Interaction between ascending magma and surrounding rocks.
Magma Mixing: Blending of different magmas alters composition.
Fractional Crystallization: Sequential formation and settling of crystals as magma cools.
Bowen’s Reaction Series
A historical classification based on temperature and chemical composition influences during crystallization of magmas, indicating that olivine and quartz cannot coexist in the same rock due to differing crystallization temperatures.
Igneous Rock Types
Extrusive vs. Intrusive
Extrusive: Rapid surface cooling, small crystals, forms volcanic rocks.
Intrusive: Slow cooling, larger crystals, forms plutonic rocks.
Textural Classifications
Aphanitic: Fine-grained texture due to rapid cooling.
Phaneritic: Coarse-grained texture from slow cooling.
Porphyritic: Characterized by large crystals embedded in a finer matrix; indicates differing cooling rates.
Glassy: No crystals, results from very rapid cooling.
Vesicular: Contains small holes from gas bubbles.
Pegmatitic: Features crystals larger than 2 cm; typically found in intrusive environments.
Volcanic Hazards
Lava Flows: Movement of molten rock, with factors influencing eruption type being composition, temperature, and gas content.
Pyroclastic Debris: Ejecta from explosive eruptions that can flow and damage environments.
Ash Fallout and Lahars: Ash clouds can damage health; lahars are mudflows caused by volcanic activity melting snow.
Volcanic Benefits
Geothermal Energy: Potential for power generation from volcanic activity.
Nutrient Contribution: Volcanic eruptions can enrich soils, benefiting agriculture.
Conclusion
Understanding minerals and rocks, their properties, and interactions is crucial for both geological studies and environmental applications.