Geology Quick Reference: Matter, Minerals, and Silicates

Minerals and Matter

• Minerals: naturally occurring, inorganic solids with ordered internal structure and definite chemical composition.
• Matter basics needed for minerals: atoms, bonds, and crystal structures.

Atomic Structure and Key Terms

• Atom composition: nucleus (protons + neutrons) and electron cloud.
• Atomic number $Z$: number of protons (identifies the element).
• Atomic mass $A$: sum of protons and neutrons in the nucleus.
• Isotopes: same $Z$, different $A$ due to different numbers of neutrons (e.g., $^{16}\mathrm{O},\;^{17}\mathrm{O},\;^{18}\mathrm{O}$).
• Valence electrons: electrons in the outer shell available for bonding.
• Ions: cations (positive) and anions (negative).

Ionic, Covalent, and Metallic Bonding

• Ionic bonding: between cations and anions (e.g., $\mathrm{NaCl}$).
• Covalent bonding: electrons are shared; strongest bond type (e.g., C–C in diamond; Cl–Cl in Cl2).
• Metallic bonding: delocalized electrons; explains malleability of metals.
• Polymorphs: same chemical formula, different structure (e.g., diamond vs graphite).

Key Chemical Concepts for Minerals

• Silicon–oxygen tetrahedron: $\mathrm{SiO_4^{4-}}$; building block for silicates.
• Bonding and crystal habits influence mineral properties and shape.

Physical Properties of Minerals

• Crystal form: external expression of internal atomic arrangement.
• Luster: reflection of light (metallic vs non-metallic).
• Color: unreliable diagnostic (impurities alter color).
• Streak: color of mineral in powdered form.
• Hardness: Mohs scale (1–10); diamond $= 10$, talc $= 1$; relative scratch resistance.
• Cleavage: breaks along planes of weak bonds; flat, well-defined surfaces.
• Fracture: break not along cleavage planes; types include conchoidal (quartz/obsidian), fibrous (asbestos), irregular.

Silicate Minerals and Structures

• Silicates dominate crustal rocks; all share $\mathrm{SiO_4^{4-}}$ tetrahedra that polymerize into structures.
• Five silicate structures (from building block to 3D network):
  1) Isolated tetrahedra (olivine) – SiO$4$ units; example: olivine. 2) Single-chain silicates (pyroxene) – chains of SiO$4$ tetrahedra.
  3) Double-chain silicates (amphibole) – two linked chains.
  4) Sheet silicates (mica, clay) – 2D sheets with weak interlayer bonds.
  5) Framework silicates (feldspar, plagioclase) – 3D framework.
• Polymerization: linking tetrahedra to form larger structures.
• Example minerals:
  • Olivine: isolated tetrahedra.
  • Pyroxene: single chain.
  • Amphibole (hornblende): double chain.
  • Mica (muscovite): sheet silicate.
  • Feldspar & Plagioclase: framework silicates.
• Graphite as a sheet-like example (carbon) to illustrate sheet structure; not a silicate mineral.

Common Silicate Minerals and Textures

• Olivine: mantle mineral; dense; Mg–Fe in between tetrahedra.
• Pyroxene: single-chain structure; cleavage at approximately 90°.
• Amphibole (hornblende): double-chain structure; cleavage at ~60° and ~120°.
• Mica (muscovite): sheet silicate; perfect cleavage into sheets.
• Feldspar & Plagioclase: framework silicates; common in continental crust.

Abundances and Crustal Composition

• Continental crust: oxygen and silicon dominate; major elements also include Al, Fe, Ca, Na, K, Mg.
• Most common rock types in crust:
  • Granitic (continental crust)
  • Basaltic (oceanic crust)

Earth’s Densities and Typical Rocks (review context)

• Mantle density at surface: $\approx 3.1\ \text{g cm}^{-3}$
• Continental crust density: $\approx 2.7\ \text{g cm}^{-3}$ (typical rock: granite)
• Oceanic crust density: $\approx 3.0\ \text{g cm}^{-3}$ (typical rock: basalt)

Quick Recall and Connections

• Ionic vs covalent vs metallic bonds: identify by bonding partner and properties.
• Isotopes: same $Z$, different $A$ due to neutrons.
• Cleavage vs fracture: planes of weak bonds vs break across bonds.
• Silicate structure shapes help predict crystal habit and cleavage (e.g., pyroxene with 90° angles, amphibole with 60/120° angles).
• Five silicate structures determine mineral classes and their properties.