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Realgar is a sulphide of arsenic found in veins of lead, silver, and gold ores associated with arsenic minerals and stibnite and also occurs as a deposit from hot springs. When mixed with saltpetre and burned it gives a bright white light and was formerly used in fireworks to provide colour. It has the formulae AsS and a relative hardness of 2.
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Red Crag are deposits of fossil remains on the coast of Essex and Suffolk in England used in the manufacture of manure.
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Red Ochre is a soft, earthy variety of haematite mineral.
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Replacement refers to the process by which one mineral is replaced by another and the original physical form is often retained.
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Resinous refers to a lustre with the appearance of resin.
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The Rhaetic beds are a group of strata which occur at the top of the Triassic and at the base of the Jurassic system. They are also known as the White Lias, or the Avicula contorta beds, from one of their best known and commonest fossils. In England they may be traced from the coast of Devon past Bristol to Redcar on the Yorkshire coast, forming a narrow strip on the west side of the Lias shales. They contain comparatively few fossils; but their fauna, though small, is very characteristic, and includes several forms that do not occur elsewhere.
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Rhodochrosite is a carbonate of manganese and an important manganese ore. It occurs in veins of copper, lead and silver that have been altered by hydrothermals, and as a secondary deposit in sedimentary layers of manganese oxide. Rhodochrosite is mainly associated with calcite, but may also occur with quartz. In South America it is used as an ornamental stone. It has a relative hardness of 3.5 to 4.
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Rhodonite is an often pink or red coloured mineral sometimes polished for use as an ornamental stone. Rhodonite is formed mainly in manganese-bearing metamorphic rocks, especially marble and skarn, which were previously limestone. It is also found in sediments that have been altered by metasomatism. It has the formulae (Mn,Fe,Mg)SiO3 and a relative hardness of 7.
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Rhyolite is a fine grained extrusive igneous rock similar to granite. Rhyolite forms from particularly viscous molten lava and is grey, brown or reddish in colour, darkening with age. Rhyolite is essentially composed of feldspar, mica and quartz with numerous accessory minerals present.
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Riebeckite is an accessory mineral of the amphibole group found in granite and syenite and other igneous rocks. Riebeckite is a hydrous silicate of sodium, ferrous iron, magnesium and ferric iron named after the German Emil Riebeck who first discovered it.
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Rock cork is a species of asbestos, resembling cork. It is soft, easily cut and very light.
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A mineral is a homogeneous inorganic natural substance of a definite chemical composition, it is usually a solid, but occasionally a liquid. A few are composed of a single element, but most are formed by the combination of several elements. A product which has developed by a natural process is a natural substance. A mineral is homogeneous physically and chemically when it consists of elements of the same kind throughout. In this a mineral differs from rock, which is also an inorganic natural substance, but one composed of different minerals, and which, in contrast to them, forms large parts of the earth's crust. There are some rocks (for instance sandstone or limestone), which consist almost entirely of one mineral (quartz or calcite for example). We do not classify them as minerals, for they do not have to be homogeneous, as do minerals. The science which studies minerals is called mineralogy. It not only observes and defines the various characteristics of minerals, but also explains their origin and development. Petrography
concentrates on studying rocks. These sciences, together with geology, which studies the origin, composition and evolution of the earth, are not just theoretical sciences. Their findings affect practical life with ever increasing force. The vast and vital role that minerals and rocks play in the modern world is taken for granted. They form the basic raw materials of industry, whether they are metallic minerals extracted from ores, minerals for the chemical, glass or ceramic industry, or others.
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In jewellery, a rose diamond is a diamond that is quite flat underneath, with its upper part cut in diverse little faces, usually triangles, the uppermost of which terminates in a point.
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Rottenstone is a light, porous, somewhat friable, siliceous rock used for polishing steel and other metals. It consists almost entirely of silica, with a small percentage of alumina and other impurities, and is derived from siliceous limestones after the removal of the calcareous matter.
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Rouge roi is a highly decorative fossiliferous marble quarried in Belgium. It varies in colour from deep fawn through to a rich red-brown, the surface being broken with irregular patches of grey and soft white interspersed with strong veins of opaque white.
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Rubasse is a beautiful spangled red variety of quartz, coloured by the presence of minute blood- coloured scales of iron oxide. The natural stone is very rare, but may be found in Brazil.
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Ruby is the red transparent form of corundum containing impurities of chromium and iron. It is a precious stone. Artificial rubies have been manufactured since 1902 from ammonia alum and chrome alum. And in 1960 and artificial ruby was used in the first working laser.
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Rutile is an accessory mineral found in igneous rock and in metamorphic gneiss and schist. It is an ore of titanium, and is a high-temperature form of titanium dioxide. It has the formulae TiO2 and a relative hardness of 6.5. It exhibits a strong lustre and is often found in twinned crystals. Rutile is found in granite, granite pegmatites, gneiss, mica schist, metamorphic limestone and dolomite. It is present as an accessory mineral in the rocks or in quartz veins. The name rutile is derived from the Latin word 'rutilus' meaning red.
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Abbreviations
Rocks & Minerals
