CC

Comprehensive Notes on Minerals and Rocks

Mineral and Rock: Foundational Concepts

Geologic Definition of a Mineral

  • Naturally occurring: Must be formed by natural geological processes.

  • Generally inorganic: Typically does not consist of organic carbon-based molecules.

  • Solid substance: Must exist in a solid state at normal temperatures and pressures.

  • Orderly crystalline structure: Atoms are arranged in a specific, repeating pattern.

  • Definite chemical composition (with some variation): Has a fixed chemical formula, though some element substitution is permissible within structural limits.

Definition of a Rock

  • A solid mass composed of minerals or mineral-like matter that occurs naturally as part of Earth.

  • Most rocks are aggregates of various minerals, where individual mineral properties are retained.

  • Some rocks can consist of only one mineral (e.g., limestone composed primarily of calcite).

  • Some rocks can also be made up of non-minerals (e.g., obsidian, which is volcanic glass and lacks a crystalline structure).

Atoms: Building Blocks of Minerals

  • Atomic Number:

    • The number of protons in the nucleus of an atom.

    • Determines the atom’s chemical nature.

    • Electrons orbit the nucleus and have approximately 1/2000 the mass of a proton.

  • Element:

    • A group of the same kind of atoms.

    • Approximately 90 natural elements exist on Earth, with several more synthesized in laboratories.

    • Elements are organized in the periodic table, grouping those with similar properties.

    • Most elements combine with others to form chemical compounds, which are the basis for minerals.

How Minerals Form

1. Precipitation of Mineral Matter

  • Occurs when dissolved ions in an aqueous solution reach saturation and begin to form crystalline solids.

  • Factors leading to saturation:

    • A drop in temperature of the solution.

    • Loss of water through evaporation (e.g., evaporite deposits like salts).

  • Once saturation is reached, ions bond together to form crystalline solids.

  • Example: Geodes, which form when minerals precipitate from slowly moving groundwater filling fractures and voids within existing rocks.

2. Crystallization of Molten Rock

  • Similar process to water freezing.

  • When magma (molten rock) is hot, atoms are highly mobile.

  • As magma cools, atoms slow down and begin to chemically combine, forming orderly crystal structures.

  • This process generates a mosaic of intergrown crystals, characteristic of igneous rocks.

3. Deposition as a Result of Biological Processes

  • Marine organisms extract calcium (Ca) or silica (Si) from seawater.

  • These organisms then secrete external skeletons composed of either:

    • Calcium carbonate ( ext{CaCO}_3) (used by corals and mollusks).

    • Silica ( ext{SiO}_2) (used by diatoms and radiolarians).

  • Over time, these biological materials can accumulate and form mineral deposits or contribute to sedimentary rocks.

Mineral Structures and Compositions

  • All mineral samples are crystals or crystalline solids, meaning they possess orderly, repeating internal structures.

  • Mineral Structures:

    • Refer to the atomic arrangement that forms the basic building blocks, known as unit cells, of a mineral crystal.

    • Involve the orderly packing of atoms and ions held together by various chemical bonds (ionic, covalent, metallic).

    • Atoms pack in a 3 ext{D} arrangement that minimizes voids, with the shape and symmetry of this packing determining the external crystal shape.

  • Polymorphs:

    • Minerals with the same chemical composition but different internal structures (due to different atomic packing).

    • Example: Diamond and Graphite, both composed purely of carbon.

Polymorphs of Carbon: Diamond vs. Graphite

  • Diamond:

    • Carbon atoms are covalently bonded into a compact, three-dimensional framework.

    • This strong bonding accounts for diamond's extreme hardness.

  • Graphite:

    • Carbon atoms are bonded into sheets.

    • These sheets are joined together by very weak electrical forces.

    • The weak bonds between sheets allow graphite to be soft and easily flake.

Mineral Groups

  • Nearly 4000 minerals have been named, but are typically categorized into two main groups:

    • Rock-forming minerals:

      • Only a few dozen common minerals.

      • Make up most of the rocks of Earth's crust.

      • Primarily composed of the eight elements that constitute the majority of the continental crust.

    • Economic minerals:

      • Less abundant but extensively used in manufacturing products.

      • Not always mutually exclusive from rock-forming minerals (e.g., Calcite is both).

Major Elements in Earth's Crust

  • The eight most abundant elements by weight that make up the vast majority of rock-forming minerals represent more than 98% of the continental crust:

    • Oxygen (O): 46.6%

    • Silicon (Si): 27.7%

    • Aluminum (Al): 8.1%

    • Iron (Fe): 5.0%

    • Calcium (Ca): 3.6%

    • Sodium (Na): 2.8%

    • Potassium (K): 2.6%

    • Magnesium (Mg): 2.1%

    • Total = 98.5% (Note: Transcript states 98.5%, individual percentages sum to 98.5%).

Silicate vs. Nonsilicate Minerals

Silicate Minerals

  • The most common type of minerals on Earth, with over 800 known silicates.

  • Account for >90% of Earth's crust.

  • Basic building blocks are silicon and oxygen.

Nonsilicate Minerals

  • While not as common as silicates, they are economically very important.

  • Include groups such as:

    • Carbonates

    • Sulfates

    • Halides

    • Oxides

    • Sulfides

    • Native elements

The Silicates

  • All silicate minerals contain both oxygen and silicon, the two most abundant elements in Earth's crust.

  • Silicon-oxygen tetrahedron:

    • The fundamental building block of all silicate minerals.

    • Consists of four oxygen ions surrounding a much smaller central silicon ion.

    • Single tetrahedra link together in various ways, forming diverse silicate structures.

Examples of Silicate Minerals:

  • Light Silicate Minerals (generally lighter in color and lower in density):

    • Feldspars (e.g., Orthoclase ext{KAlSi}3 ext{O}8, Plagioclase ext{Ca}( ext{Al}2 ext{Si}2 ext{O}8), ext{Na}( ext{AlSi}3 ext{O}_8), percentages: Plagioclase 39%, Potassium feldspars 12%, Quartz 12%, Micas 5%, Clays 5%, Other silicates 3%%).

      • Silicate Structure: Three-dimensional networks.

      • Cleavage: Two planes at 90^ ext{o}.

    • Micas (e.g., Muscovite ext{KAl}2( ext{AlSi}3 ext{O}{10})( ext{OH})2).

      • Silicate Structure: Sheets.

      • Cleavage: One plane.

    • Quartz ( ext{SiO}_2).

      • Silicate Structure: Three-dimensional networks.

      • Cleavage: None (fractures).

  • Dark Silicate Minerals (generally darker in color and higher in density, containing iron and magnesium):

    • Olivine ( ext{Mg}, ext{Fe})2 ext{SiO}4 (percentages: Olivine 11%, Pyroxenes 11%, Amphiboles 5%).

      • Silicate Structure: Single tetrahedron.

      • Cleavage: None.

    • Pyroxene group (e.g., Augite ( ext{Mg}, ext{Fe}) ext{SiO}_3).

      • Silicate Structure: Single chains.

      • Cleavage: Two planes at right angles.

    • Amphibole group (e.g., Hornblende ext{Ca}2( ext{Fe}, ext{Mg})5 ext{Si}8 ext{O}{22}( ext{OH})_2).

      • Silicate Structure: Double chains.

      • Cleavage: Two planes at 60^ ext{o} and 120^ ext{o}.

    • Biotite (a dark mica: ext{K}( ext{Mg}, ext{Fe})3 ext{AlSi}3 ext{O}{10}( ext{OH})2).

      • Silicate Structure: Sheets.

      • Cleavage: One plane.

  • Most silicates form from molten rock cooling and crystallizing.

Nonsilicate Minerals

  • Divided into groups based on the negatively charged ion or complex ion they share.

  • Make up approximately 8% of Earth's crust.

  • Common groups and examples include:

    • Carbonates ( ext{CO}_3^{2-}): Calcite ( ext{CaCO}_3) (Portland cement, lime), Dolomite ( ext{CaMg}( ext{CO}_3)_2) (Portland cement, lime).

    • Halides ( ext{Cl}^- ext{, F}^- ext{, Br}^-): Halite ( ext{NaCl}) (common salt), Fluorite ( ext{CaF}_2) (used in steelmaking), Sylvite ( ext{KCl}) (fertilizer).

    • Oxides ( ext{O}^{2-}): Hematite ( ext{Fe}_2 ext{O}_3) (ore of iron, pigment), Magnetite ( ext{Fe}_3 ext{O}_4) (ore of iron), Corundum ( ext{Al}_2 ext{O}_3) (gemstone, abrasive), Ice ( ext{H}_2 ext{O}) (solid form of water).

    • Sulfides ( ext{S}^{2-}): Galena ( ext{PbS}) (ore of lead), Sphalerite ( ext{ZnS}) (ore of zinc), Pyrite ( ext{FeS}_2) (sulfuric acid production), Chalcopyrite ( ext{CuFeS}_2) (ore of copper), Cinnabar ( ext{HgS}) (ore of mercury).

    • Sulfates ( ext{SO}_4^{2-}): Gypsum ( ext{CaSO}_4 ext{· } 2 ext{H}_2 ext{O}) (plaster), Anhydrite ( ext{CaSO}_4) (plaster), Barite ( ext{BaSO}_4) (drilling mud).

    • Native elements (single elements): Gold ( ext{Au}) (trade, jewelry), Copper ( ext{Cu}) (electrical conductor), Diamond ( ext{C}) (gemstone, abrasive), Graphite ( ext{C}) (pencil lead), Sulfur ( ext{S}) (sulfadrugs, chemicals), Silver ( ext{Ag}) (jewelry, photography).

  • Nonsilicate minerals provide most of the mineral resources important to global economies.

Physical Properties of Minerals

  • Minerals have definite crystalline structures and chemical compositions, which endow them with unique physical and chemical properties.

  • These properties are primary diagnostic tools for identifying hand samples of minerals, often determined by observation or simple tests.

  • Key physical properties include:

    • Crystal form: The external expression of a mineral's orderly internal arrangement of atoms.

    • Luster: The appearance of a mineral in reflected light (e.g., metallic, glassy, dull).

    • Color: Often the most conspicuous property, though can be highly variable and unreliable due to impurities.

    • Streak: The color of a mineral's powder, obtained by rubbing it across an unglazed porcelain plate.

    • Hardness: A mineral's resistance to scratching, typically measured on the Mohs scale of mineral hardness.

    • Cleavage/Fracture: The tendency of a mineral to break along planes of weakness (cleavage) or to break irregularly (fracture).

    • Taste: Some minerals have a characteristic taste (e.g., halite is salty).

    • Smell: Some minerals emit a distinctive odor when scratched or heated.

    • Feel: The tactile sensation of a mineral (e.g., soapy for talc, greasy for graphite).

    • Magnetism: Whether a mineral is attracted to a magnet (e.g., magnetite).

    • Double Refraction: The optical property where a single beam of light splits into two upon entering the mineral, causing a double image (e.g., calcite).

    • Reaction to hydrochloric acid: Some minerals effervesce when dilute hydrochloric acid is applied (e.g., calcite, due to ext{CO}_2 release).

    • Specific gravity: The ratio of the weight of a mineral to the weight of an equal volume of water; essentially its density.