Lexikon

in geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids. The Earth's outer solid layer, the lithosphere, is made of rock. The scientific study of rocks is called petrology. Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic igneous rocks and volcanic rocks. sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). Petrographic descriptions start with the field notes at the outcrop and include megascopic description of hand specimens. The macroscopic characters of rocks, those visible in hand-specimens without the aid of the microscope, are very varied. The geologist in the field depends principally on them and they usually serve for a preliminary classification. However, the most important tool for the petrographer is the petrographic microscope. The detailed analysis of minerals by optical mineralogy in thin section and the micro-texture and structure are critical to understanding the origin of the rock. Characteristics observed under the microscope include colour, colour variation under plane polarised light, fracture characteristics of the grains, refractive index and optical symmetry. The more difficult and skilful part of optical petrography is identifying the interrelationships between grains and relating them to features seen in hand specimen, at outcrop, or in mapping. (http://en.wikipedia.org)

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