Minerals

1. Minerals This is not a spider web. It is a fractal; a repeating geometric design

2. What is a mineral • A mineral is a natural, inorganic, crystalline solid • Ask these questions: • Is it organic? (is it alive or was it once alive) • Coal is made from dead plants and is organic • Does it occur naturally ? • Steel is man-made and is not a mineral • Does it have a crystalline form? • Petroleum occurs naturally and is inorganic but is not a solid so it has no crystalline form and is not a mineral • Does it have a definite chemical composition? • Mixtures are not minerals; compounds can be minerals

3. Types of Minerals • The most common minerals are rock-forming • All minerals fall into two categories: • Silicate minerals • Non-silicate minerals

4. Silicate Minerals • All silicate minerals contain silicon (Si) and Oxygen (O) in varying combinations • Quartz contains only Si and O • Feldspar is the most common silicate mineral • Different types of Feldspar are formed when other elements (K, Na, Ca, etc) are combined with the Si and O • Feldspar and quartz make up 50% of the earth’s crust • Silicate minerals make up 96% of the Earth’s crust

5. Non-silicate Minerals • Only 4% of the Earth’s crust • 6 groups: • Sulfides • Sulfates • Oxides • Halides • Carbonates • Native Elements

6. Crystalline structure • A crystal is a natural solid with a definite shape • A large mineral crystal will display the characteristic geometry of its crystalline structure • Crystals have a specific geometry of atoms that repeats (like a trelis) • If a mineral is allowed to develop unrestricted then it will form one, huge crystal

7. Crystal Shapes • Draw table 9.3

8. Mineral identification • A Mineralogist is a scientist who identifies minerals • Characteristics: • Color • Luster • Streak • Cleavage/fracture • Hardness • Crystal shape • Density

9. Color • This is probably the worst characteristic to classify minerals because it is inconsistent • Some minerals have distinctive colors (Sulfur is yellow, azurite is blue, serpentine is green) • Many minerals are similar in color • Very small amounts of certain element can greatly affect the color • Corundum is a colorless mineral made from Aluminum and Oxygen but with a little bit of chromium it forms a ruby • Sapphire is corundum with traces of cobalt and titanium • Amethyst is quartz that has a purple color because of small amount of Manganese and Iron

10. Luster • Light reflected off the surface of a mineral is called luster • Minerals will either have metallic luster (shiny) or nonmetallic luster (not as shiny) • Minerals that have no shine have a dull, Earthy luster

11. Streak • The color of the mineral in its powdered form is called the streak • This is fairly reliable for identification • Metallic minerals have a dark streak • Ex. Pyrite (fool’s gold) has a black streak • Nonmetallic minerals have a streak that is colorless or a very light shade of the mineral’s normal color

12. Cleavage and Fracture • Cleavage: when a mineral splits evenly along flat surfaces • Ex. Micas which split in sheets • The cleavage runs parallel to a plane in the crystal where the bonding is weak • Fracture: when a mineral breaks unevenly • Curved surfaces are called conchoidal • Fibrous surfaces look like broken wood • Rough surfaces are called uneven or irregular

13. Hardness • Hardness is the ability of a mineral to resist scratching • Ex. A diamond is extremely hard but can easily be split along cleavage planes • The hardness of an unknown mineral can be determined by scratching it against other minerals of a known hardness on the Mohs hardness scale • The hardness of the mineral determines the strength of the bonds between the atoms • Graphite and diamonds are made from carbon. Graphite has no crystal structure so it is not as hard as diamonds

14. Crystal Shape • A certain mineral always has the same general shape because the atoms or ions that form its crystals always combine in the same geometric pattern

15. Density • Density = Mass ÷ Volume • The density depends on what type of atoms the mineral contains and how closely they are packed together • Most of the common minerals have densities between 2 and 3 g/cm3 • Heavy metals (gold, uranium, lead) can have densities from 7 to 20 g/cm3

16. Special Properties • Magnetism • Magnetite is the most common • Lodestone is a type of magnetite that acts like a magnet • Double Refraction • Clear minerals still fracture light that passes through them creating double images • Radioactivity • Some atoms have unstable protons and neutrons. This nuclear decay releases energy • Uranium and Radium are some radioactive elements that occur in minerals • Pitchblende is the most common uranium containing mineral

17. Special Properties • Florescence (ability to glow under UV light) • Calcite is white under normal light but red under UV light • Florescent minerals absorb UV light and then produce visible light of various colors • Phosphorescence (ability to glow under UV light even after UV light is shut off)