minerals

Mineral Definition and Composition

• Minerals are naturally occurring, solid crystalline substances not liquid, with a specific chemical composition and are generally inorganic.

  • Naturally Occurring, Solid, Crystalline, Generally Inorganic, Specific Chemical Composition.

• Calcite (CaCO₃) is an exception to the general rule that minerals are inorganic; calcite is often formed organically by sea creatures.

  • Chemical formula: ext{CaCO}_3

• Ionic Bonding: ions of opposite charge attract; ionic bonding is the most common bonding in minerals.

• Covalent Bonding: atoms share electrons; covalent bonds are typically stronger than ionic bonds (e.g., in diamond).

  • Key idea: electrons shared between atoms; network of strong bonds gives high hardness for covalent minerals.

• Crystallization and Crystal Growth:

  • Minerals form by crystallization: growth of a crystal from liquid or gas.

  • Atoms or ions must come together in correct proportions and structural arrangement for the crystal to grow.

  • Atoms or ions attach themselves to growing crystal faces; crystallization is a process of ordering that takes time.

• Crystal Forms:

  • Halite forms cubic crystals; Quartz forms hexagonal crystals.

  • Some mineral specimens retain their crystallographic shape even when other properties change.

• Notable example: Cave of Crystals in Naica Mine (giant gypsum crystals, selenite).

  • Related resources mention the Naica Project and related images of massive crystal formations.

• Polymorphism: a mineral can share the same chemical composition with another mineral but have different atomic arrangements.

  • Example concept: same composition, different structure leads to different mineral species.

• Mnemonic/Note on pressure: slide content states “YOUR DAD'S BEEN UNDER A LOT OF PRESSURE LATELY.” as a thematic hint to metamorphic settings where pressure drives mineral/formational changes.

Major Classes of Rock-Forming Minerals (by Anion)

• Sulfide anion: ext{S}^{2-} ; Sulfides

• Sulfate anion: ext{SO}_4^{2-}; Sulfates

• Oxygen anion: ext{O}^{2-}; Oxides

• Carbonate anion: ext{CO}_3^{2-}; Carbonates

• Silicate anion: ext{SiO}_4^{4-}; Silicates

• Each class is defined by its characteristic anion and includes common minerals (see examples below).

Carbonates

• Carbonate minerals share the carbonate anion ext{CO}_3^{2-}. Examples include:

  • Calcite: ext{CaCO}_3

  • Siderite: ext{FeCO}_3

  • Malachite: Cu₂CO₃(OH)₂

• Calcite, Siderite, and Malachite illustrate carbonate chemistry and varied cations.

• Calcite is environmentally important and commonly found in carbonate rocks; calcite forms limestone and marble and weathers relatively easily.

Silicate Minerals and the Silica Tetrahedron

• Silicate minerals are the most important in geology because most rocks are made of them.

• Silicate minerals contain the silica anion ext{SiO}_4^{4-}, commonly called the silica tetrahedron: a 4-sided shape with oxygen at the corners and silicon at the center.

  • Representation: ext{SiO}_4^{4-}

• Abundance in Earth's crust: silicates dominate; several major silicate minerals are abundant in common rocks.

• Silicate structure is built by sharing corner oxygens between tetrahedra, forming various frameworks.

Most Abundant Minerals in Earth’s Crust (approximate proportions)

• Silicates dominate the crust; non-silicates are less abundant.

• General order (by abundance among crustal minerals):

  • Plagioclase Feldspar ≈ 39%

  • Quartz ≈ 12%

  • Alkali Feldspar ≈ 12%

  • Pyroxenes ≈ 11%

  • Micas ≈ 5%

  • Amphiboles ≈ 5%

  • Clays ≈ 5%

  • Other Silicates ≈ 3%

  • Nonsilicates ≈ 8%

Silicate Structures: How SiO₄ Tetrahedra Bond

• Silica tetrahedra can bond by sharing corner oxygen atoms to form extended networks.

• Key phrase: “Silica tetrahedra can bond together by sharing their corner oxygen atoms.”

• Structural classification depends on how many oxygens are shared between tetrahedra:

  • Isolated tetrahedra: no oxygen atoms are shared (e.g., olivine).

  • Single chains: two shared oxygens per tetrahedron (e.g., pyroxene).

  • Double chains: 2–3 shared oxygens (e.g., amphibole).

  • Sheets: 3 shared oxygens (e.g., mica like Biotite, Muscovite).

  • Framework: all four oxygens shared (e.g., feldspar, quartz).

Silicate Groups in Granite (typical minerals)

• Granite commonly contains:
  1) Quartz (framework silicate)
  2) Feldspar (framework silicate)
  3) Muscovite (sheet silicate)
  4) Biotite (sheet silicate)
  5) Amphibole (double chain silicate)

• Note: Granite is the most abundant rock in the continental crust.

Other Rock-Forming Mineral Groups (by anion)

• Carbonates: as above (Calcite ext{CaCO}_3 )

• Oxides: minerals with oxygen bonded to other element cations (e.g., Hematite Fe₂O₃)

• Sulfides: sulfide anion ext{S}^{2-} (e.g., Pyrite ext{FeS}_2)

• Sulfates: SO₄²⁻ (e.g., Gypsum CaSO₄·2H₂O; Anhydrite CaSO₄

Crystal Properties and Identification

• Hardness: resistance to abrasion or scratching; governed by bond strength. Covalent minerals tend to be harder than ionic minerals.

• Cleavage: plane(s) along which a mineral tends to break; related to bond strengths in crystal structure.

  • Strong bonds = poor cleavage; weak bonds = good cleavage.

  • Covalent bonds often yield poor or no cleavage; ionic bonds yield good cleavage.

• Fracture: how a mineral breaks when it does not cleave; e.g., conchoidal fracture in quartz.

• Luster: how light reflects from a mineral surface; metallic vs non-metallic.

  • Non-metallic lusters include adamantine, vitreous (glassy), dull, earthy, greasy, pearly.

• Color and Streak:

  • Color is the mineral’s surface color; streak is the color of the powder when rubbed on a porcelain plate (often more diagnostic for a mineral).

• Density: depends on atomic weights and packing; iron-rich minerals tend to be denser; covalent minerals are typically less dense due to open packing.

• Crystal Habit: characteristic external shapes (e.g., cubic, hexagonal, fibrous, botryoidal, radiating, stalactitic, mammillary).

Mohs Scale of Hardness (examples and references)

• 1 Talc

• 2 Gypsum

• 3 Calcite

• 4 Fluorite

• 5 Apatite

• 6 Orthoclase

• 7 Quartz

• 8 Topaz

• 9 Corundum

• 10 Diamond

• Common reference items:

  • Fingernail ≈ 2.5

  • Copper coin ≈ 3

  • Knife blade ≈ 5.5

  • Window glass ≈ 5.5

  • Steel file ≈ 6.5

Cleavage in Selected Minerals

• Halite (NaCl) shows three planes at 90° (three cleavage directions).

• Muscovite (mica) shows one plane of cleavage (perfect basal cleavage).

• Calcite shows three planes not at 90° (rhomb or rhombohedral cleavage).

• Quartz shows no cleavage; exhibits conchoidal fracture.

• Cleavage is the plane where a mineral will break when stressed.

Crystal Habit and Visual Descriptors

• Common crystal habits include:

  • Geode (geode-like interior cavities lined with crystals)

  • Stalactitic, Botryoidal, Mammillary, Radiating, Fibrous, Geode-like, Conical, etc.

• Habit is influenced by crystal structure and growth conditions.

Notable Minerals and their Roles

• qaurtz amphibole feldspar make granite!

• Quartz: framework silicate; chemical formula {SiO}_2 ; hardness 7; color varies; often used as gemstones.

• Feldspars: aluminosilicates; principal rock-forming minerals; two major series:

  • Alkali feldspars:(K,Na)AlSi₃O

  • Plagioclase: NaAlSi₃O₈ – CaAl₂Si₂O₈

  • Important for ceramics and glass; weather chemically to form clay minerals (hydrated aluminosilicates).

• Mica group: sheet silicates; Muscovite (white mica) and Biotite (dark mica); Muscovite has perfect basal cleavage and is used in applications like windows and oven doors.

• Ferromagnesian minerals: silicates with iron and/or magnesium; generally dark in color; weather easily to oxides such as hematite (limonite).

• Olivine: (Mg,Fe)₂SiO₄; part of basaltic rocks.

• Pyroxene:XY(Si,Al) ₂O ₆; double-chain silicate group in many rocks.

• Amphibole: double-chain silicate; typically darker and forms in metamorphic and igneous rocks.

• Calcite (carbonate): {CaCO}_3 environmentally important carbonate mineral; forms limestone and marble; weathers easily.

• Hematite: Fe₂O₃, α-Fe₂O₃; common oxide mineral; gives red to reddish colors in rocks.

• Pyrite (FeS₂): sulfide mineral; known as “fool’s gold”; can pose environmental concerns when oxidized.

• Native elements: Au (gold), Ag (silver), Cu (copper), C (diamonds) occur in native form.

• Ore minerals: economically important minerals that are mined for metals and other resources.

The Rock Cycle and Major Rock Types

• Three primary rock types:

  • Igneous rocks: formed by melting and crystallization of magma or lava.

  • Example: coarsely crystallized granite (granitoid textures).

  • Sedimentary rocks: formed by weathering, erosion, transport, deposition, burial, and lithification. formed by sediment ation and compaction of mineral particles from pre-existing rocks or organic material. Example: sandstone, which is primarily composed of quartz grains.

  • Metamorphic rocks: formed by recrystallization of existing rocks under high temperatures and pressures in the deep crust/upper mantle.

• The Rock Cycle summarizes how rocks transform through heating, melting, weathering, burial, and metamorphism:

  • Metamorphic rocks arise from burial/heat/pressure, then may melt to form magma, which crystallizes into igneous rocks; weathering and erosion of rocks produce sediments that lithify into sedimentary rocks; and all rocks may be subjected again to metamorphism.

• The crust: Most of the crust is made of igneous and metamorphic rocks overall, but sedimentary rocks are the most abundant rocks at the Earth's surface.

Rock Identification and Practical Notes

• A rock is an aggregate of one or more minerals.

• Identification of minerals relies on multiple properties: color, streak, density, cleavage, hardness, crystal form, luster, and habit.

Quick Reference: Common Rock-Forming Silicates and Related Notes

• Silicate minerals include the core rock-formers: Quartz, Feldspar group, Ferromagnesian group (and related mica, pyroxene, amphibole, olivine).

• Silicate chemistry and structure are central to explaining the variation in rock types.

Special Mentions and Environmental/Practical Implications

• Calcite and other carbonates weather relatively easily, influencing rock weathering and soil formation.

• Ferromagnesian minerals weather to oxides and iron oxides (e.g., limonite), contributing to red-brown soils and sediments.

• Oxide, sulfide, and sulfate minerals have environmental and economic significance due to ore occurrences and weathering products.