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Chapter 30 Minerals and Their Formation Background Rocks are made up of minerals like how atoms make up molecules Most minerals are compounds of different elements Most minerals are not perfect like we will see in class…they are just blobs in rocks because of where they are formed Physical Properties of Minerals (KNOW!) Mineral appearance Crystal form and color Hardness Luster Specific gravity Streak Cleavage and fracture Crystal Form Crystal Form: orderly arrangement of atoms in a crystal (what a crystal looks like is its arrangement) Example: Quartz = six-sided prisms and asbestos = narrow fibers When two or more minerals contain the same elements in the same amounts but they are arranged different it is a POLYMORPH Ex. Graphite and diamond Quick Question We can identify minerals by their physical characteristics but why is it hard to identify minerals solely by their crystal form? Hardness A measure of how easily a mineral can be scratched or how resistant it it to being scratched Ex. Diamond can scratch glass Hardness Cont. Why is a diamond so much harder than another mineral? The bonds that hold the atoms together are basis for the minerals strength. Gold has larger atoms and is loosely packed so it has weaker bonds (24K gold is VERY soft) Diamonds are made of little carbon atoms so it is tightly packed (a diamond is very hard) Another look Mohs Hardness Scale Mineral Rating Testing Method Talc 1 Softest known mineral. It flakes easily when scratched by a fingernail. Gypsum Calcite 2 A fingernail can easily scratch it. 3 A fingernail cannot scratch it, but a copper penny can. Fluorite Apatite Feldspar 4 A steel knife can easily scratch it. 5 A steel knife can scratch it. 6 Cannot be scratched by a steel knife, but it can scratch window glass. Quartz Topaz Corundum Diamond 7 Can scratch steel and hard glass easily. 8 Can scratch quartz. 9 Can scratch topaz. 10 Hardest known mineral. Diamond can scratch all other substances. Cleavage Cleavage is the way that mineral breaks, its planes of weakness. Minerals that break along smooth, flat surfaces have cleavage. Ex. Mica has cleavage and Fracture!... Mineral that breaks uneven, rough, or jagged surfaces have fracture. Quartz has fracture quartz Luster The way a mineral reflects a light luster. Does not depend on color! Either metallic or nonmetallic Streak While colors helps us identify minerals it isn’t super reliable When a mineral is rubbed across a piece of porcelain tile a streak of powdered mineral is left behind. Streak: color given off in its powered form (important for minerals with a metallic or semimetallic luster Specific Gravity The specific gravity of a mineral is the ratio of its weight compared with the weight of an equal volume of water. Gold has specific gravity of 19 It means gold is 19 times heavier than water. 19 times heavier Quick Question Why is panning for gold possible? Chemical Properties Taste test and Acid Test Taste test helps identify salts Never taste minerals without being an expert some minerals are actually poisonous Some minerals react with acid and “fizz” Carbonate minerals react 30.2 Minerals into Rocks Minerals make up rocks and minerals are made of elements that are in the earth 1000’s of minerals and only about 24 common minerals The 24 minerals are made up of mainly 8 elements (out of all 88 found on earth) 8 elements are 98% of the crust About ½ or 50% of the crust is oxygen 5 groups of rock-forming minerals Silicates 90% of earths crust (oxygen and silicon bond to make quartz) Oxides Oxygen with one or more metals (great economic importance) 5 groups cont. •Carbonates •Limestone, carbon and oxygen bonded •Sulfides and Sulfates •Sulfur as main element combined with metallic elements •Fools gold 30.3 On the Way to Rocks Minerals are formed by crystallization Growth of a solid from atoms that form together in a certain arrangement Overall minerals are formed from the crystallization of magma and from the precipitation or evaporation of water REVIEW SLIDES An illustration appearance:luster,color and streak An illustration cleavage and fracture