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Friday, January 14, 2011

ROCKS AND MINERALS

The average composition by weight of chemical elements in the lithosphere is oxygen 47 per cent, silicon 28 per cent, aluminium 8.1 per cent, iron 5.0 per cent, calcium 3.6 per cent, sodium 2.8 per cent, potassium 2.6 per cent, magnesium 2.1 per cent, and the remaining eighty-three elements 0.8 per cent. These elements combine to form minerals. The chief minerals in the lithosphere are feldspars (aluminium silicates with potassium, sodium, or calcium), quartz (a form of silicon dioxide), clay minerals (complex aluminium silicates), iron minerals such as limonite and hematite, and ferromagnesian minerals (complex iron, magnesium, and calcium silicates). Ore deposits consist of common minerals precipitated from hot fluids. They include pyrite (iron sulphide), galena (lead sulphide), blende or sphalerite (zinc sulphide), and cinnabar (mercury sulphide). (Gerrard, 2007)


Rocks are mixtures of crystalline forms of minerals. There are three main types: igneous, sedimentary, and metamorphic. (Gerrard, 2007)


Igneous rocks
Igneous rocks
These form by solidification of molten rock (magma). They have varied compositions (Figure 2.1). Most igneous rocks consist of silicate minerals, especially those of the felsic mineral group, which comprises quartz and feldspars (potash and plagioclase). Felsic minerals have silicon, aluminium, potassium, calcium, and sodium as the dominant elements. Other important mineral groups are the micas, amphiboles, and pyroxenes. All three groups contain aluminium, magnesium, iron, and potassium or calcium as major elements. Olivine is a magnesium and iron silicate. The micas, amphiboles (mainly hornblende), pyroxenes, and olivine constitute the mafic minerals, which are darker in colour and denser than the felsic minerals. Felsic rocks include diorite, tonalite, granodiorite, rhyolite, andesite, dacite, and granite. Mafic rocks include gabbro and basalt. Ultramafic rocks, which are denser still than mafic rocks, include peridotite and serpentine. Much of the lithosphere below the crust is made of peridotite. Eclogite is an ultramafic rock that forms deep in the crust, nodules of which are sometimes carried to the surface by volcanic action. At about 400 km below the surface, olivine undergoes a phase change (it fits into a more tightly packed crystal lattice whilst keeping the same chemical composition) to spinel, a denser silicate mineral. In turn, at about 670 km depth, spinel undergoes a phase change into perovskite, which is probably the chief mantle constituent and the most abundant mineral in the Earth. (Gerrard, 2007)

Sedimentary rocks
Sedimentary Rocks
These are layered accumulations of mineral particles derived mostly from weathering and erosion of preexisting rocks. They are clastic, organic, or chemical in origin. Clastic sedimentary rocks are unconsolidated or indurated sediments (boulders, gravel, sand, silt, clay) derived from geomorphic processes. Conglomerate, breccia, sandstone, mudstone, claystone, and shale are examples. Organic sedimentary rocks and mineral fuels form from organic materials. Examples
are coal, petroleum, and natural gas. Chemical sedimentary rocks form by chemical precipitation in oceans, seas, lakes, caves, and, less commonly, rivers. Limestone, dolomite, chert, tufa, and evaporites are examples.(Gerrard, 2007)

Metamorphic rocks
Metamorphic Rocks
These form through physical and chemical changes in igneous and sedimentary rocks. Temperatures or pressures high enough to bring about recrystallization of the component minerals cause the changes. Slate, schist, quartzite, marble, and gneiss are examples. (Gerrard, 2007)








Sources : Gerrard, John.2007.FUNDAMENTALS OF GEOMORPHOLOGY. New York : Routledge 270 Madison Avenue

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