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Chapter 14 Rocks and Minerals 14.1 Identifying Earth Materials Geologist – a scientist who identifies, classifies, and studies rocks. Minerals – a solid inorganic material occurring in nature. Most are chemical compounds. (What’s a compound?) • Quartz is a form of silicon dioxide (SiO2) • Sulfur often appears in its elemental form • Copper also. Rock – a combination of two or more minerals that can occur in many different proportions. Examine granitewhite or pink Æ feldspar glassy crystals Æ quartz flakes Æ mica Identifying Minerals Colour – (Duh!) problem with this is that many minerals appear in different colours. Hardness – (Don’t test by biting, okay? Your dentist won’t appreciate it). We use the Mohs Hardness Scale . We use this by doing a scratch test…scratch one mineral with another. 1 1 2 3 4 5 6 7 8 9 10 Talc Gypsum Calcite Fluorite Apatite Feldspar Quartz Topaz Corundum Diamond softest hardest Lustre – how a mineral reflects light. Metallic Glassy Oily Streak – this refers to what colour of a mark is produced when you scratch a piece of porcelain tile with the mineral. Magnetism – you can test this with a magnet or a compass. Unfortunately there are only four minerals (elements in this case!) that have magnetic properties. Iron Nickel Cobalt Gadolinium 2 Usually, a mineral that has a magnetic property will have iron or maybe nickel. Cobalt and gadolinium are quite rare. Fluorescence – the ability to absorb and emit light. Simply, this means a rock will glow with a different colour when under ultraviolet light. Fluorite Æ blue or purple Calcite Æ red, pink, yellow Crystals – minerals can be classified according to the shape of their crystals. Salt, for example, always forms cubes. Large crystals are generally rare in nature. Small crystals which can be seen under magnification are very common. Cleavage and fracture – some rocks will split apart certain ways if you hit it with a hammer or drop it on the ground. Slate and shale split in sheets. Obsidian flakes apart. Cleavage - when rocks to produce smooth surfaces. Mica Galena Feldspar Fracture – when rocks break into rough chunks. Quartz 3 Classifying Rocks There are essentially 3 classes of rocks: Igneous Sedimentary Metamorphic Thickness of earth’s crust Beneath oceans – 5 km Beneath continents – up to 50 km Igneous “born of fire” – produced when molten rock (called MAGMA) from deep in the earth reaches the earth’s crust. From there it has three choices. 1) Recycle back to the center of the earth. 2) Intrusive – where it forces its way into cracks, but never comes out of the ground. This is the cause of ‘veins’ of rock. 3) Extrusive - It breaks through the surface as LAVA out of volcanoes, where it cools. 4 Special note: the appearance of igneous rocks depends on many things, like mineral content and how quickly the lava cooled. If it cools slow, crystals will form…if cooling is too fast Æ no crystals! Sedimentary Means “born of water” – the cementing together of rock pieces of various sizes. This normally involves a water event such as a flood or water erosion plus pressure. Rivers often deposit huge amounts of 5 silt and other sediments. Overlayering can produce great pressure. Other minerals can act like glue. These eventually form sediment ‘beds’. • Mudstone , shale - from fine clay or mud. • Sandstone – from sand • Conglomerate – larger chunks 6 Metamorphic Metamorphic means “new body” – where sedimentary or igneous rocks are put under great temperature and pressure and change into a new form. A certain ‘start’ rock can turn into different types of metamorphic rock depending upon the amount of pressure and heat which it experiences. However, the new rock is usually: • harder • more brittle How are heat and pressure are applied? 1) downward pressure of overlayering (burial) by sedimentary rock 2) upward pressure of magma 3) lateral tectonic pressure at fault lines 4) heat from contact, or near contact with magma 7 The Rock Cycle The rock cycle represents the many pathways that rocks can take during its “lifetime”. For example a metamorphic rock can be weathered to sedimentary rock or pressurized to metamorphic or even recycled to igneous again. See how many pathways you can suggest for a rock to take…endless, isn’t it? Chapter Review 8