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Chromite is a black sub-metallic mineral consisting of oxide of chromium and iron. It is slightly magnetic and the only ore of chromium. A common constituent of peridotite rocks and the serpentines derived from them. Also associated with corundum. One of the first minerals to separate from a cooling magma. Chromium is widely used in metal plating and in stainless steel. It has the formulae FeCr2O4 and a relative hardness of 6.
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Contact metamorphism refers to metamorphism resulting from the intrusion of magma which takes place at or near the contact point with the molten rock.
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Granodiorite is a plutonic igneous rock formed from cooling magma in deep zones of mountain belts. Granodiorite is a course-grained intermediate igneous rock comprised mainly of feldspar and quartz, and either pink or white in colour depending upon the proportions of feldspar and quartz.
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Hydrothermal refers to the alteration of minerals or rocks by super-heated mineral rich fluids, usually water, within a crystallizing magma.
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Igneous intrusion refers to a body of igneous rock that has made its way into pre-existing rock (known as country rock). Igneous intrusions are emplaced as magma, which is less dense than solid rock and therefore tends to move upwards. It can then force its way through cracks in the rocks and can wedge them apart or, if it is hot enough, it can melt and replace them.
Igneous intrusions can be of a variety of shapes and sizes, ranging from huge batholiths to bodies only one or two meters across. The general term 'pluton' can be applied to any of these. Intrusions may cut across the bedding of the country rock. They are then termed 'discordant' or 'transgressive'; dykes and laccoliths are examples. Intrusions that follow the bedding of the country rock, such as sills, are termed 'concordant'.
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Igneous rock is rock that has originated from molten or semi-molten magma. Igneous rocks are composed almost entirely of silicate minerals. Of many different types, they can be classified in various ways: by composition, crystal size, or mode of occurrence. One method is to divide them according to their silica content into three main groups: acid rocks, basic rocks, and intermediate rocks. There is, however, continuous gradation from acid to basic. Acid rocks are characterized by the presence of 10 per cent or more of quartz, usually accompanied by lighter- coloured minerals such as orthoclase feldspar and muscovite. This acid group includes rocks such as granite and rhyolite. Molten acid lava is very viscous, and its viscosity prevents the molten lava flowing very far and produces tall, conical volcanoes of the Vesuvius type. Intermediate rocks have less than 10 per cent quartz; diorite and andesite are typical of this group.
Basic igneous rocks are characterized by the absence of quartz and the predominance of dark minerals such as ferromagnesian minerals (amphiboles, pyroxenes). Common basic igneous rocks include gabbros and basalts. When extruded on to the Earth's surface these low-viscosity basic lavas form flat plateaux such as the Columbia Plateau, USA. Those igneous rocks that crystallize below the Earth's surface are termed intrusive or plutonic, according to whether they were formed at shallow depths, like dolerite, or deeper down, like granite. All these rocks have relatively large crystals produced by slow cooling of the molten magma. Igneous rocks that are extruded at the Earth's surface are termed extrusive or volcanic: basalt is an example. The rapid cooling that occurs at the surface does not allow large crystals to form, and these rocks are glassy or contain only small crystals.
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Ironstone is a rock containing at least fifteen percent iron. Ironstone forms in igneous rocks through mahmatic differentiation - dense heavy minerals such as magnetite crystallise first and sink to the base of the molten magma to form an iron-rich layer. Ironstone also forms in sedimentary rocks such as limestone and sandstone.
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A laccolith (or laccolite) is a dome of igneous rock between two layers of older sedimentary rock. They are formed by the intrusion of magma, which forces the overlying strata into a dome.
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Leucite is a rather rare mineral occurring only in igneous rocks and usually in recent lavas. It is found in rocks where the amount of silica in the magma was not sufficient to form feldspar. It is not found, therefore, in rocks that contain quartz. It's name is derived from the Greek word for ' white'. It has the formulae KAlSi2O6 and a relative hardness of 6.
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Magma refers to molten rock beneath the earth's crust. It solidifies to form igneous rocks.
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The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
©1993 - 2009 The Probert Encyclopaedia
Southampton, United Kingdom
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