CH.3.5 Geoscience Basics: Minerals and Properties

Understanding Minerals

🌎 Mineral Formation

Minerals are naturally occurring, inorganic solids with a definite chemical composition and a highly ordered atomic arrangement (crystal structure). They are not ephemeral; they have finite lifespans, but these can be very long.

Processes of Mineral Formation:

• Precipitation from Solution: Minerals form when dissolved substances in water (groundwater, surface water, hydrothermal fluids) become supersaturated and precipitate out of solution. Advanced Concept: This process is influenced by factors like temperature, pressure, pH, and the presence of other ions.

• Sublimation: Minerals form directly from a gas phase, often seen near volcanic vents where gases cool and deposit minerals. Advanced Concept: The composition of the gas and the rate of cooling affect the resulting minerals.

• Crystallization from a Melt: Minerals crystallize from molten rock (magma or lava). Advanced Concept: The cooling rate and the chemical composition of the melt determine the sequence and types of minerals that form (Bowen's Reaction Series).

• Solid-State Growth: New minerals grow from the constituents of pre-existing minerals, often during metamorphism. Advanced Concept: This involves complex chemical reactions and diffusion processes within the solid state.

• Solid-Liquid or Solid-Gas Reactions: Atoms are exchanged between solid minerals and a liquid or gas phase, producing new minerals (weathering, hydrothermal alteration, metamorphism). Advanced Concept: These reactions are governed by thermodynamic principles and reaction kinetics.

💎 Crystal Habits

Crystal habit refers to the characteristic shape of a mineral crystal. It reflects the internal arrangement of atoms.

Individual Crystal Habits:

• Equant: Roughly equal dimensions in all directions.

• Tabular: Flattened, like a tablet.

• Platy: Very thin and sheet-like.

• Prismatic: Elongated, like a prism.

• Bladed: Flattened and elongated, like a blade.

• Acicular: Needle-like.

• Capillary/Filiform: Hair-like.

Crystal Aggregates: Minerals often grow together to form aggregates.

• Granular: Composed of many small, roughly equant crystals.

• Lamellar/Foliated: Crystals arranged in parallel layers.

• Fibrous: Crystals arranged in parallel fibers.

• Radiating: Crystals radiating outward from a central point.

• Divergent: Crystals diverging from a common area.

• Reticulated: Crystals forming a network.

• Roseiform: Petal-like arrangement of crystals.

• Drusy: Surface covered with small crystals.

• Dendritic: Tree-like branching.

• Massive: No distinct crystal shape.

• Banded: Layers of different colors or compositions.

• Concentric: Layers around a central point.

• Botryoidal/Colloform: Rounded, grape-like clusters.

• Oolitic: Small, spherical grains.

• Pisolitic: Larger, spherical grains.

• Amygdaloidal: Filling of gas vesicles in volcanic rocks.

• Geodes: Hollow, spherical cavities lined with crystals.

• Concretions: Solid, spherical masses formed by precipitation within a rock.

🪨 Macroscopic Mineral Properties

These properties can be observed with the naked eye or a hand lens.

Hardness: A mineral's resistance to scratching. The Mohs Hardness Scale (1-10) is a relative scale; diamond (10) is not 10 times harder than talc (1). Knoop Hardness is an absolute scale. Hardness depends on bond strength and bond density.

Density & Specific Gravity: Density is mass per unit volume (g/cm³). Specific Gravity is the ratio of a mineral's density to the density of water (dimensionless). Density depends on atomic mass and atomic packing.

Tenacity: How a mineral responds to stress.

• Malleable: Can be hammered into sheets.

• Ductile: Can be drawn into wires.

• Brittle: Breaks easily.

• Elastic: Returns to original shape after deformation.

• Flexible: Bends but doesn't return to original shape.

• Sectile: Can be cut into shavings.

Crystal Faces: The external surfaces of a crystal.

• Euhedral: Well-formed, perfectly geometric faces.

• Subhedral: Some faces, but not perfectly formed.

• Anhedral: No visible crystal faces.

Cleavage: The tendency of a mineral to break along flat planes. Recognized by flat, planar surfaces, strong light reflection, and repetition of surfaces. The number and angles of cleavage planes reflect the crystal structure.

Fracture: The way a mineral breaks when it doesn't cleave.

• Conchoidal: Smooth, curved surfaces (like glass).

• Irregular: Uneven, rough surfaces.

• Splintery: Fibrous, splintery surfaces.

• Hackly: Jagged, sharp edges.

Parting: Breaking along planes of weakness due to twinning or stress.

Striations: Parallel lines on crystal faces.

🔬 Optical and Electromagnetic Properties

These properties describe how minerals interact with light and other electromagnetic radiation.

Diaphaneity: How much light passes through a mineral.

• Opaque: No light passes through.

• Transparent: Light passes through clearly.

• Translucent: Some light passes through, but the image is blurry.

Luster: The appearance of a mineral's surface in reflected light.

• Metallic: Shiny, like a metal.

• Non-metallic: Various descriptions (vitreous, adamantine, silky, waxy, resinous, greasy, pearly, satiny).

Streak: The color of a mineral's powder when scratched on a streak plate. Useful for identifying minerals, especially opaque ones.

Color: The overall color of a mineral.

• Idiochromatic: Color is inherent to the mineral's composition.

• Allochromatic: Color is due to impurities or defects.

Play of Colors: Color changes with the angle of light.

• Asterism: Star-like pattern.

• Chatoyancy: Cat's-eye effect.

• Iridescence: Rainbow-like colors.

• Opalescence: Milky or pearly iridescence.

Luminescence: Emission of light when exposed to energy.

• Fluorescence: Light emitted only while exposed to radiation.

• Phosphorescence: Light emitted after radiation is removed.

• Thermoluminescence: Light emitted when heated.

• Triboluminescence: Light emitted when crushed or rubbed.

Magnetism: A mineral's response to a magnetic field.

• Diamagnetic: Slightly repelled by a magnet.

• Paramagnetic: Weakly attracted to a magnet.

• Ferromagnetic/Ferrimagnetic: Strongly attracted to a magnet.

🌳 Silicate Minerals

Silicates are the most abundant minerals in Earth's crust. They are based on the silica tetrahedron (SiO₄)⁴⁻, a structure of one silicon atom surrounded by four oxygen atoms. The way these tetrahedra link determines the silicate group.

Silicate Mineral Groups:

• Nesosilicates (Orthosilicates): Isolated tetrahedra (e.g., olivine, garnet).

• Sorosilicates (Disilicates): Paired tetrahedra (e.g., epidote).

• Cyclosilicates: Ring structures (e.g., beryl, tourmaline).

• Inosilicates: Chain structures (e.g., pyroxenes, amphiboles).

• Phyllosilicates: Sheet structures (e.g., micas, clays).

• Tectosilicates: Framework structures (e.g., quartz, feldspars).

🪨 Non-Silicate Minerals

These minerals are classified by their dominant anion (negatively charged ion) other than silicate.

Non-Silicate Mineral Groups:

• Native Elements: Single elements (e.g., gold, sulfur, diamond).

• Halides: Contain halogen anions (F⁻, Cl⁻, Br⁻, I⁻) (e.g., halite, fluorite).

• Sulfides: Contain sulfide (S²⁻) anions (e.g., pyrite, galena).

• Oxides: Contain oxygen anions (O²⁻) (e.g., hematite, magnetite).

• Hydroxides/Oxyhydroxides: Contain hydroxyl (OH⁻) and/or oxygen anions (e.g., bauxite, goethite).

• Carbonates: Contain carbonate (CO₃)²⁻ anions (e.g., calcite, dolomite).

• Borates: Contain borate anions (e.g., borax).

• Sulfates: Contain sulfate (SO₄)²⁻ anions (e.g., gypsum, anhydrite).

• Phosphates: Contain phosphate (PO₄)³⁻ anions (e.g., apatite, monazite).

• Tungstates: Contain tungstate (WO₄)²⁻ anions (e.g., scheelite).

• Molybdates: Contain molybdate (MoO₄)²⁻ anions (e.g., wulfenite).

Mineral Identification Flowchart Example

This is a simplified example. A complete flowchart would be much larger.

Is the mineral metallic?
    Yes --> Is it magnetic?
        Yes --> Magnetite (likely)
        No -->  Further tests needed (color, streak, hardness)
    No --> Is it light or dark colored?
        Light --> Is there good cleavage?
            Yes -->  Further tests needed (cleavage angles, hardness)
            No --> Quartz (possibly)
        Dark --> Is it hard?
            Yes --> Garnet (possibly)
            No --> Further tests needed (streak, hardness)

Facts to Memorize:

1. Mineral Definition: Minerals are naturally occurring, inorganic solids with a definite chemical composition and crystal structure.

2. Mineral Formation Processes: Precipitation, sublimation, crystallization from melt, solid-state growth, solid-liquid/gas reactions.

3. Crystal Habit: The external shape of a mineral crystal, reflecting its internal structure.

4. Mohs Hardness Scale: A relative scale (1-10) measuring a mineral's resistance to scratching.

5. Specific Gravity: The ratio of a mineral's density to the density of water.

6. Tenacity: A mineral's response to stress (malleable, ductile, brittle, elastic, flexible, sectile).

7. Cleavage: Breaking along flat planes due to weak bonding in the crystal structure.

8. Fracture: Breaking along irregular surfaces.

9. Diaphaneity: Transparency of a mineral (opaque, transparent, translucent).

10. Luster: The appearance of a mineral's surface in reflected light (metallic, non-metallic).

11. Streak: The color of a mineral's powder.

12. Color: Can be idiochromatic (inherent) or allochromatic (due to impurities).

13. Silicate Structure: Based on the silica tetrahedron (SiO₄)⁴⁻.

14. Major Silicate Groups: Nesosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates, tectosilicates.

15. Non-Silicate Classification: Based on the dominant anion (halide, sulfide, oxide, etc.).

16. Examples of Common Minerals: Quartz, feldspar, mica, calcite, dolomite, pyrite, hematite, etc. (Know at least 5-10 examples and their key properties).

17. Economic Importance of Minerals: Many minerals are essential resources for various industries.