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In geology, felsic is an adjective describing igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.
Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments (clasts) or organic material.
Schist is a medium-grade metamorphic rock formed from mudstone or shale. Schist has medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.
Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.
Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Ancient Greek for "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.
Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. It is the volcanic equivalent of syenite. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present.. Biotite, clinopyroxene and olivine are common accessory minerals.
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grain) to porphyritic. Its intrusive equivalent is nepheline syenite.
In petrology, spherulites are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.
Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classification of rocks is based on the information acquired during the petrographic analysis. Petrographic descriptions start with the field notes at the outcrop and include macroscopic description of hand specimens. 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. Electron microprobe or atom probe tomography analysis of individual grains as well as whole rock chemical analysis by atomic absorption, X-ray fluorescence, and laser-induced breakdown spectroscopy are used in a modern petrographic lab. Individual mineral grains from a rock sample may also be analyzed by X-ray diffraction when optical means are insufficient. Analysis of microscopic fluid inclusions within mineral grains with a heating stage on a petrographic microscope provides clues to the temperature and pressure conditions existent during the mineral formation.
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, for example plutons, batholiths, dikes, sills, laccoliths, and volcanic necks. Some geologists use the term plutonic rock synonymously with intrusive rock but other geologists subdivide intrusive rock, by crystal size, into coarse-grained plutonic rock and medium-grained subvolcanic or hypabyssal rock.
Tachylite is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is very brittle and occurs in dikes, veins and intrusive masses. The word originates from the Ancient Greek takhus meaning swift.
Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.
Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.
In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica. Micropegmatite is an outmoded terminology for micrographic texture.
Poikilitic texture refers to igneous rocks where large component crystals contain smaller crystals of other minerals within them. In poikilitic rocks, the contained smaller crystals are termed 'chadacrysts' and the larger phenocrysts are termed 'oikocrysts'. A poikilitic texture is most easily observed in petrographic thin sections.
The Piégut-Pluviers Granodiorite is situated at the northwestern edge of the Variscan Massif Central in France. Its cooling age has been determined as 325 ± 14 million years BP.
Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.
References
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- Dictionary of Mining, Mineral, and Related Terms, U. S. Bureau of Mines, 1996
- Williams, Howel; Turner, Francis J.; and Gilbert, Charles M.; 1954; Petrography; W. H. Freeman.
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