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 & ElementsAtoms: 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: H2O

• Pyramids: SiO4 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

• A SiO4 –4 B

Atomic framework for the most common minerals

Figure 2.12 Common silicate structures. Arrows indicate directions in which structure repeats indefinitely

• Si2O7 –6

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

Fig. 2.14 Single-chain Silicate Structure

• (A) Model of a single-chain silicate mineral

• (B) The same chain silicate shown diagrammatically as linked tetrahedrons

Fig. 2.22 (B) Mica

• Sheet silicate

Silicates

• Quartz: SiO2, 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

– CaCO3 + 2HCl CaCl2 + H2O + CO2 (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