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Minerals What are minerals? • Rocks are made up of one or more pure, naturally occurring, crystalline materials called minerals • Most minerals are quite rare • Only a few, such as quartz, feldspar and mica are found throughout the Earth’s crust • A mineral can be an element (think periodic table) or a compound (two or more elements) Examples Quartz, a very common mineral, is made of silicon and oxygen Granite rock (shown to the right) is made of an assortment of minerals, including quartz (glassy grains), feldspar (sparkling grains), mica (grey flakes), and hornblende (dark flecks) Ways to Identify Minerals • Mohs Scale of Hardness • You can think of Friedrich Mohs scale as a scratchability test • It helps identify minerals based on their hardness • Check out the scale on the next slide Mohs Scale of Hardness Crystals • There are over 3000 minerals • Crystals are the building blocks of minerals • Crystals occur naturally and have straight edges, flat sides and regular angles • Most minerals grow into specific crystal shapes according to the systems chart on the next slide Crystal Systems Lustre • Some minerals like gold and other metals, appear shiny – another way to identify them • Other minerals like talc, can appear dull • The “shininess” or lustre, of a mineral depends on how the light is reflected from its surface • Shiny surface are said to have “metallic” lustre while dull surfaces are said to have non-metallic lustre Colour • The colour of a mineral can also be used as a clue to a mineral’s identity • However, colour alone cannot identify a mineral • Some minerals like gold and pyrite are both “goldcoloured” • Some minerals are not the same colour all the time • Corundum is white when pure, but blue when it contains iron or titanium (called a sapphire), and when it has chromium it is red (called a ruby) see next slide Misleading Colour Streak • When a mineral is rubbed across a piece of unglazed porcelain tile, it leaves a streak • Streak is the colour of the powdered form of the mineral • Look-alikes, such as gold and pyrite, can be distinguished from each other by their streak – gold leaves a gold streak and pyrite leaves a greenish-black or brown-black streak • Minerals with a hardness greater than the tile will not leave a streak Streak (cont.) Hematite can by dark red, grey or silvery in colour However, its streak is always dark redbrown Cleavage and Fracture • The way a mineral breaks apart can be a clue to its identity • If it breaks along smooth, flat surfaces, or planes, it is said to have cleavage • Mica is a good example of a mineral with cleavage • Minerals that break with rough or jagged edges have fracture • Quartz is an example of a mineral with fracture Cleavage & Fracture (cont.) Mica Quartz