Chapter 2 Atoms, Elements, and Minerals

Minerals Mineralogy: study of minerals Mineral: naturally occurring, crystalline (solid), inorganic substance that possesses a fairly definite chemical composition and a distinctive set of physical properties – Most are composed of 8 elements (silicon, oxygen compose 75% of the crust) – Over 4500 are known – 10 account for more than 90% of the crust

Atoms & Elements Atoms: Smallest, electrically neutral assemblies of energy & matter Electrons: Negative charged particles that orbits the nucleus Nucleus: Core of the atom containing neutrons and protons Protons: Positive charged particles Neutrons: Neutrally charged particles Element: Defined by the number of protons (atomic number) Atomic Mass Number: Number of protons + neutrons Isotopes: Atoms containing different number of neutrons (same number of protons) Atomic Weight (Mass): Weight of an average atom

Helium and Neon Atoms

Fig. 2.6 Diagrammatic representation of (A) sodium and (B) chlorine ions

Bonding Ionic Bonding: Electrons exchanged Covalent Bonding: Electrons shared, e.g. Carbon & Silicon Metallic Bonding: e.g. gold & iron, electrons free to move throughout the crystal Polymorphs: Different crystal structures with same chemical composition

Fig.2-3 Model of the crytstalline structure of the mineral natrolite Small spheres: Na Large spheres: H 2 O Pyramids: SiO 4 tetrahedrons

Figure 2.10 (A) The silicon-oxygen tetrahedron (B) The silicon-oxygen tetrahedron showing the corners of the tetrahedron coinciding with the corners of oxygen ions ASiO 4 –4 B Atomic framework for the most common minerals

Figure 2.12 Common silicate structures. Arrows indicate directions in which structure repeats indefinitely Si 2 O 7 –6

Figure 2.13 Diagram of the crystal structure of olivine, as seen from one side of the crystal

Fig Single-chain Silicate Structure (A) Model of a single- chain silicate mineral (B) The same chain silicate shown diagrammatically as linked tetrahedrons

Fig (B) Mica Sheet silicate

Silicates Quartz: SiO 2, 3-D framework tetrahedra Feldspar: most abundant mineral in crust – Plagioclase feldspar  O, Si, Al, and Ca or Na – Potassium (orthoclase) feldspar  O, Si, Al, and K Single independent tetrahedron  olivine Single chain  pyroxene Double chain  amphibole (e.g. hornblende) Sheet  mica (e.g. muscovite & biotite) 3-D framework  quartz

Physical Properties of Minerals (Identification of Minerals) Classified using physical & chemical properties – usually, only physical properties are used 1. Crystal form: size & shape assumed by crystal faces when crystal has time & space to grow – External Crystal Form: set of faces that have a definite geometric relationship to one another. – Steno’s law: The angle between two adjacent faces in a mineral are always the same. 2. Hardness: resistance to scratching – Mohs’ hardness scale: Fingernail  2.5 Penny  3 Window glass or knife blade  5.5 Steel file  6.5

Moh’s Hardness Scale 1. talc 2. gypsum 3. calcite 4. fluorite 5. Apatite 6. orthoclase feldspar 7. quartz 8. topaz 9. corundum 10. diamond

Physical Properties 3. Cleavage: tendency to break along definite planes Fracture: way a mineral breaks – Splinter – Conchoidal fractures 4. Color: reflecting light, not a reliable property because of impurities 5. Streak: color of powder of mineral 6. Luster: appearance of mineral’s surface in reflected light – Metallic – Nometallic Vitreous (glassy) Pearly Greasy Earthy/dull

Physical Properties 7. Specific gravity: ratio of mineral’s weight to an equal volume of water 8. Taste: Not recommended 9. Double (refraction) imaging: e.g. calcite 10. Reaction to HCl: calcite & dolomite – CaCO 3 + 2HCl  CaCl 2 + H 2 O + CO 2 (g) 11. Magnetism: 12. Striations: Straight parallel line on flat surfaces, e.g. plagioclase feldspar

Conditions of Mineral Formation Geological – Precipitate from molten rock (magmas & lavas) – Precipitate in ocean water – Precipitate in springs, caves, lakes – Precipitate due to evaporation – Sublimation at volcanic vents from gases Biological – Calcite in coral reefs – Magnetite in skulls – Sulfate minerals from bacteria