Petrographical properties: structure, chemistry, mineral

Extrusive rocksIn contrast to plutonic rocks, rocks of this type, e.g. diabase, basalt or rhyolite, form at the transition between the upper mantle (crust) and the surface of the Earth. The relatively fast cooling process leaves these rocks with a fine crystalline structure. Partial melting of neighboring rocks can lead to highly diverse appearances.
Sedimentary rocks Sediments are mainly formed by the weathering, erosion and deposition of older rocks (igneous, sedimentary or metamorphic) which are then transported by water or glacial movements and deposited again in the form of debris, gravel or sand. These rocks frequently contain animal or plant fossils. The pressure of the overlying strata compresses the individual particles of the sediments to form a solid mass, cemented together by water containing binders (e.g. quartz, calcite, clay) circulating in the remaining voids. This process of the solidification of sediments is known as diagenesis. Clastic sediments consist of the mechanically disintegrated parts of the original rock. Depending on the grain size,, we distinguish between conglomerates (≥ 2 mm), sandstones (0.02-2 mm)and siltstones (≤ 0.02 mm). Chemical sediments are "precipitation" from solutions as a result of chemical reactions or biological processes which subsequently solidify under pressure.
Metamorphic rocksMetamorphic rocks are formed from existing rocks and are called orthorocks when formed from igneous rocks or pararocks when the original material is a sedimentary rock. High pressures, high temperatures or chemical influences transform the original rock or even form completely new typess They are usually easily recognized by their dense structure free from virtually all voids, their distinct texture or the clear bedding marks. Their chemical composition, appearance and uses in building vary considerably. Important metamorphic rock types are slate, marble and gneiss.
Granite is probably the best known of the plutonic rocks. Its constituents are feldspar (which determines the color), quartz (responsible for the high mineral hardness) and mica. Granite is weather-resistant, is regarded as the most resistant of rocks, can be used almost without restriction in building work, and is unaffected by airborne pollution. Numerous colors are available: red , pink, yellow, white, grey, blue-green.
Basalt is a dark, usually dark grey to black, extrusive rock with a dense, non-directional structure consisting mainly of feldspar and augite. It exhibits a very high compressive strength, is extremely difficult to work, is weather-resistant, and is ideal for external applications. However, it can become very slippery when smooth. Weathered and aged basalt is also known as diabase. It is formed by the chemical disintegration of the mineral constituents (e.g. chlorite, serpentine).