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Classifying Igneous Rocks

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CLASSIFYING IGNEOUS ROCKS An igneous rock is a rock formed by the cooling and crystallization of magma intrusively in the crust or extrusively on the surface. The location and speed of cooling and the type of magma determine which igneous rock is formed. Igneous rocks can be classified according to their colour, texture and density, the minerals present, and the chemical composition of the rock. COLOUR Colour is useful as a diagnostic tool for course-grained igneous rocks but not fine grained. The colour index of a rock indicates the amount of dark ferromagnesian minerals that are present. These include olivine, pyroxene and biotite mica. They are rich in iron and magnesium and usually associated with constructive plate margins. ...read more.


* Fine-grained igneous rocks have grain sizes of less than 0.25mm. (the crystals are not distinguishable with the naked eye and a hand lens is needed for identification). These cool quickly on the surface (extrusive) and are known as volcanic rocks. They commonly form in lava flows. An example of a fine grained igneous rock is basalt * Medium grained rocks have a grain size of 0.25-2mm. (The crystals can be seen with the naked eye but a hand lens is needed for identification). They commonly form in dykes and sills. These rocks are known as hypabyssal. Eg. Dolerite * Course grained igneous rocks have a grain size greater than 2mm. ...read more.


* Flow-banded: These rocks show a rough banding of lava constituents. The bands are highly contorted and continuous for several cms. Flow-banded rocks are commonly associated with viscous lavas. * Amygdaloidal: when vesicles in the rock have been filled in by later generations of crystals. MINERAL COMPOSITION The main minerals in igneous rocks are: Olivene, pyroxene, amphibole, biotite and muscovite micas, quartz, plagioclase and orthoclase feldspars. Which minerals are present in a particular rock depend on temperature, viscosity and plate tectonic context. Recurring mineral groupings are called mineral associations. Studying these enables us to predict which minerals may occur together in a rock and which are mutually exclusive. The main minerals present in an igneous rock are known as the essential minerals of that rock. These have a common temperature crystallisation range and so are commonly found together. Minerals present in small amounts are called accessory minerals. ...read more.

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