chapter 2 atoms, elements, and minerals. minerals mineralogy: study of minerals mineral: naturally...
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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