Transcript minerals
Geology Topic Native Elements, Minerals Rocks & Ores Native Elements Only a few minerals occur as pure elements in the earth’s crust. Gold Graphite Au C Silver Ag Sulfur Diamond Copper Platinum S C Cu Pt Most are found as chemical compounds called minerals Beryl Be Al Silicate Quartz SiO2 Calcite CaCO3 Rocks are mixtures of minerals and elements. Lapis lazuli • Lazulite • Calcite • Pyrites Boulder Opal opal in cracks of boulder Unakite quartz,feldspar epidote CHEMISTRY • An element contains only one type of atom • A compound contains two or more elements chemically combined • A mixture contains elements and or compounds that are not chemically combined GEOLOGY • A Native Element contains only one type of atom and is found naturally on earth in its pure form. • A mineral contains two or more elements chemically combined • A rock contains a mixture of elements and compounds ORES are rocks containing valuable minerals • Hematite Iron Ore Gold in Quartz Malachite Copper Galena Lead ORES • Gold in W.A. is mined from tellurides, chlorites, quartz veins and in copper ores at Telfer • Gold nuggets are native elements. • Copper is found in cuprite, malachite and azurite • Native copper is rare • Diamond in W.A. is mined from volcanic plugs called • Iron ores consist of haemetite, Kimberlite pipes. magnetite, pyrite and limonite Crystalline Haemetite is used to make jewellery ORES • Only a few minerals are rock forming and most rock is made from a combination of the commonest of these such as feldspars, quartz, mica, olivine, calcite, pyroxene and amphiboles Physical Properties of Minerals • COLOUR • HARDNESS • CLEAVAGE • SPECIFIC GRAVITY • STREAK • CRYSTAL FACES Colour...................... Minerals tend to occur in a range of colours, and colour patterns which help to identify them It is chromium which produces the intense red of ruby and the brilliant green of emerald. Most minerals are coloured by a limited number of metals present as impurities. The most common elements affecting colour are: chromium, iron, manganese, titanium and copper. Quartz displays a profusion of colours, patterns and optical effects unsurpassed by any other gem colourless is rock crystal purple quartz is amethyst yellow is citrine brown is smoky quartz pink is rose quartz. black is morion Hardness................... • Hardness depends upon the forces holding the atoms of the mineral together. • In 1812, a scientist, F. Moh devised a scale of hardness into which all minerals can be placed. • He selected ten minerals and arranged them in order so that any one mineral could be used to scratch only minerals which are less. • Diamond is the- hardest natural material, 140 times harder than corundum. Cleavage................ Distinct cleavage planes can be illustrated in this box mica specimen The manner in which a mineral splits is an aid to identification. These planes are related to the lattice work of the mineral's atomic structure. Streak………… • When minerals are scratched, the powder that is made by the scratch is called the streak. • Sometimes the colour of the streak can be used to identify the mineral. • Eg. Haemetite a black mineral has a red streak. No other black mineral has a red streak. Specific Gravity................... • It was Archimedes who first worked out the principal of specific gravity or relative density. Specific gravity is defined as the ratio of the weight of a substance compared to that of an equal volume of water. • For example, a piece of galena (lead ore), with a specific gravity of 7.4 will feel much heavier than a piece of quartz of a similar size but with specific gravity of 2.65, reflecting the way the atoms are packed together. Crystals Crystals are minerals or elements that have been cooled slowly enough for crystals to have formed. Such conditions occur most often in pegmatites or pipes. Crystals can be classified by their shapes into seven groups. What makes a mineral a Gemstone? • There are over two hundred recognised gemstones. Most are minerals and are cut from naturally occurring crystals. • To qualify as a gem, the specimen should be hard and tough to resist scratching and have beautiful clarity, colour or 'fire'. Value is based on rarity and rare quality. Next slides are advanced or extension only ……. Optical effects………. In diamond the 'fire' and sparkle is caused by dispersion as the gemstone acts like a prism. The brilliant colours displayed by opal are caused by the light being scattered as it passes through planes of regularly spaced voids between the transparent silica spheres. Double Refraction ………. Double refraction can be seen in a piece of Calcite. Light separates into two rays which makes images seen through the crystal appear to be doubled. Refraction.................... • This is a measure of the mineral's ability to bend a beam of light. This ability is called refractive index. Some gemstones can be identified by the spectral colours they produce. Ruby coloured by chromium Almandine garnet coloured by iron -Measuring refraction using an optical instrument called a Refractometer. What makes a mineral a Gemstone? • There are over two hundred recognised gemstones. Most are minerals and are cut from naturally occurring crystals. • To qualify as a gem, the specimen should be hard and tough to resist scratching and have beautiful clarity, colour or 'fire'. Value is based on rarity and rare quality.