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Gabbros are basic rocks solidified under great pressure at considerable depths in the earth's crust. They are coarse-grained and consist of plagioclase feldspar with augite. Sometimes the term is used to include similar rocks composed of the same feldspar with other minerals, and according to the minerals contained they are known as norites, troctolites, eucrites, etc.
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Gadolinite, named after the Russian chemist Gadolin, or ytterbite is a naturally occurring complex silicate containing beryllium, iron and many of the rare earth metals, of which the latter is an important source. The principal rare earths that occur in gadolinite are yttrium and erbium, together with smaller amounts of cerium and lanthanum.
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Gahnite (zinc spinel) is a dark green mineral of the spinel group consisting of zinc aluminium oxide. It is named after J G Gahn the Swedish chemist.
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Galena (lead sulphide) or galenite is virtually the only source of lead and an important ore of silver. It has the formulae PbS and a relative hardness of 3. It is a very common metallic mineral. When found in veins that show a connection to igneous rocks, it is frequently found with silver minerals.
Galena is also found in limestones either as veins or as a replacement deposit.
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Gannister is a very siliceous close-grained variety of clay with a poor alkali content found especially under coal seams in the Coal Measures of northern England.
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Garnet is a widely distributed group with several minerals. They are found in both metamorphic and igneous rocks. The chief use of red transparent garnets are as an inexpensive gem stone, however, much is used as an abrasive material. They have the formulae A3B2(SiO4)3 and a relative hardness of 8. The commonest colour of garnet is red and the lustre is vitreous. The dodecahedron and trapezohedron are the common forms. There are also white, green, yellow, brown, and black varieties. The garnet is a silicate, the bases being alumina lime (grossularite, essonite, or cinnamon stone), or alumina magnesia (pyrope), or alumina iron (almandine), or alumina manganese (spessarite), or iron lime (common garnet, melanite, allochroite), or chromium lime (ouvarovite, the colour emerald green). The garnet was, in part, the carbuncle of the ancients. Garnet is a very common mineral in gneiss and mica slate.
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Garnierite is an ore of nickel. It's probably formed as an alteration of nickel bearing peridotites. It has a nice apple green colour and the formulae (Ni,Mg)6Si4O10(OH)8. It has a relative hardness of 4. It is named after the French geologist Jules Garnier.
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The Gault is a series of beds of clay and marl in the South of England, between the upper and lower greensand of the Cretaceous period.
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Gaylussite, named after the French chemist Gay-Lussac, is a yellowish white, translucent mineral, consisting of the carbonates of lime and soda, with water.
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Geanticlinal is an upward bend or flexure of a considerable portion of the earth's crust, resulting in the formation of a class of mountain elevations called anticlinoria.
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The Genesee epoch is a closing subdivision of the Hamilton period in the American Devonian system. It is so called because the formations of this period crop out in Genesee, New York.
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Geochronology is the branch of geology concerned with ordering and dating of events in the earth's history, including the origin of the earth itself.
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Geochronite is a lead-grey or greyish blue mineral with a metallic lustre, consisting of sulphur, antimony, and lead, with a small proportion of arsenic.
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A geode is a round hollow nodule containing earthy matters, sometimes quartz, sometimes agate.
Geodes are found in most volcanic rocks and are formed by water depositing materials in the hollows of these rocks.
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Geodynamics is the branch of geology concerned with the forces and processes, especially on a large- scale, of the earth's interior, particularly as regards their effects on the crust or lithosphere.
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Geology is the scientific study of the origin, history, structure, and composition of the earth.
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geomorphology or geomorphogeny is the branch of geology that is concerned with the structure, origin, and development of the topographical features of the earth's crust.
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Geosynclinal is the downward bend or subsidence of the earth's crust, which allows of the gradual accumulation of sediment, and hence forms the first step in the making of a mountain range.
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A geosyncline is a broad elongated depression in the earth's crust containing a great thickness of sediment.
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Germanite is a mineral consisting of a complex copper arsenic sulphide containing germanium, gallium, iron, zinc, and lead: an ore of germanium and gallium.
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Geyserite is a loose hydrated form of silica, a variety of opal, deposited in concretionary cauliflower-like masses, around some hot springs and geysers.
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Gibbsite is a primarily secondary mineral named after George Gibbs, an American mineralogist. Gibbsite is a hydrate of alumina and gives off a clay-like smell when rubbed or breathed on. It is an important ore of aluminium. Typically it occurs massive, but rarely crystals are found in flattened, tubular, pseudo-hexagonal shapes. It is mostly white, reddish white or colourless with shades of grey, blue and green and a pearly or vitreous lustre.
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Gieseckite, named after Karl Giesecke is a mineral occurring in greenish grey six-sided prisms and having a greasy lustre.
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Girasole is a variety of opal which is usually milk white, bluish white, or sky blue in colour but in a bright light reflects a reddish colour.
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Gismondine, named after it's discoverer, Gismondi, is a native hydrated silicate of alumina, lime, and potash, first noticed near Rome.
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Glacial drift describes earth and rocks which have been transported by moving ice, land ice, or icebergs.
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The glacial epoch was a period during which the climate of the modern temperate regions was polar, and ice covered large portions of the northern hemisphere to the mountain tops.
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Glaciated rocks are those rocks whose surfaces have been smoothed, furrowed, or striated, by the action of ice.
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Glacier theory is the theory that large parts of the frigid and temperate zones were covered with ice during the glacial, or ice, period, and that, by the agency of this ice, the loose materials on the earth's surface, called drift or diluvium, were transported and accumulated.
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Glance is a name given to some sulphides, mostly dark-coloured, which have a brilliant metallic lustre, such as the sulphide of copper which is called copper glance.
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Glauberite, named after the 17th century German chemist Glauber, is a bitter salty tasting mineral that dissolves slowly in water. It is a sedimentary mineral formed by the evaporation of saline water. It slowly alters to gypsum when exposed to air. It has the formulae Na2Ca(SO4)2.
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Glaucodot is a metallic mineral having a greyish tin-white colour, and containing cobalt and iron, with sulphur and arsenic.
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Glauconite is a mineral of marine origin and found in sedimentary deposits of various kinds. It is the green mineral characteristic of the greensand of the chalk and other formations. It is similar to a mineral called celadonite, it is used in the textile, sugar, and brewing industries; as a colouring agent and in the manufacture of fertilizers. It has the formulae (K,Na)(Al, Fe,Mg)2(Al,Si) 4O10(OH)2 and a relative hardness of 2.
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Glaucophane is a dark-blue sodium rich rock forming mineral which, like other amphiboles, is poor in silica. It is of interest to petrologists in helping to define the metamorphic conditions which formed the surrounding rock. It has the formulae Na2(Mg,Fe)3Al2Si8O22(OH)2 and a relative hardness of 7. It often has a blue colour.
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A globulite is a rudimentary form of crystallite, spherical in shape.
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Gmelinite, named after the German chemist Gmelin, is a rhombohedral zeolitic mineral, related in form and composition to chabazite.
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Gneiss refers to a crystalline rock, consisting, like granite, of quartz, feldspar, and mica, but having these materials, especially the mica, arranged in planes, so that it breaks rather easily into coarse slabs or flags. Hornblende sometimes takes the place of the mica, and it is then called hornblendic or syenitic gneiss. Similar varieties of related rocks are also called gneiss.
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Goethite is an ore of iron and a common mineral. Usually formed as a weathering product of iron bearing minerals. The term 'limonite' is commonly used for earthy yellow and brown goethite. It has the formulae FeO (OH) and a relative hardness of 6.
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Gold is a rare element with the formulae Au and a relative hardness of 3 occurring in nature in widely distributed small amounts. The main source of
gold is in gold-quartz veins where gold was deposited from mineral bearing solutions. When the veins weather, the gold is separated from the quartz and mechanically settles on the stream floor as a placer deposit. Gold in jewellery is measured in carats. 24 carat gold is pure, 22 carat gold is 22 parts gold to two parts other metals and so on.
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Gongylite is a yellowish-brown coloured variety of Agalmatolite found in Finland.
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Goslarite (white vitriol) is hydrated zinc sulphate. It is a rare mineral precipitated from water seeping through the walls of lead mines and is formed by the decomposition of sphalerite.
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Gossan is decomposed rock, usually reddish or ferruginous (owing to oxidized pyrites), forming the upper part of a metallic vein.
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Granite is a plutonic, igneous, crystalline, granular rock, consisting of quartz, feldspar, and mica, and usually of a whitish, greyish, or flesh-red colour. It differs from gneiss in not having the mica in planes, and therefore in being destitute of a schistose structure.
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Granitel is a variety of grey granite with small crystalline granules of feldspar and quartz. Granitel was worked by the Romans as marble.
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Granodiorite is a plutonic igneous rock formed from cooling magma in deep zones of mountain belts. Granodiorite is a course-grained intermediate igneous rock comprised mainly of feldspar and quartz, and either pink or white in colour depending upon the proportions of feldspar and quartz.
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Granulite is a whitish, granular metamorphic rock, consisting of feldspar and quartz intimately mixed. Granulite is formed deep in the Earth's crust at very high temperature and under high pressures. It is sometimes called whitestone or leptynite.
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Graphic tellurium is an alternative name for Sylvanite.
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Graphite is pure carbon with a relative hardness of 2, often confused with the heavier molybdenite. Formed from organic materials or by the presence of hydrocarbons in a metamorphic region. Used in the manufacture of crucibles, as a lubricant when mixed with oils, as 'lead' for pencils when mixed with clay.
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Gravel is a mixture of coarse sand and small water-worn stones. The term may also be applied to small water- worn stones on their own.
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Greasy refers to a lustre of a mineral which appears to be covered with thin coat of oil.
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Green earth (also known as mountain green) is a variety of Glauconite, found in cavities in amygdaloid and other eruptive rock, and used as a pigment by artists.
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Greenockite, named after Lord Greenock, is the most common mineral containing cadmium but found in few places and usually as an earthy coating on zinc ores. The largest use of cadmium is for electroplating other metals to form chemical resistant coatings. It has the formulae CdS and a relative hardness of 4.
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Greensand is a variety of sandstone, usually imperfectly consolidated, consisting largely of glauconite, a silicate of iron and potash of a green colour, mixed with sand and a trace of phosphate of lime. Greensand is often called marl, because it is a useful fertilizer. The greensand beds of the American Cretaceous belong mostly to the Upper Cretaceous.
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Greisen is a crystalline rock consisting of quartz and mica, common in the tin regions of Cornwall and Saxony.
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Greystone is a greyish or greenish compact rock, composed of feldspar and augite, and allied to basalt.
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Groppite is a red or brownish-red coloured crystalline mineral with a hardness of 2.5, comprised mainly of aluminium, iron and magnesia found in limestone.
Grossularite (grossular) is a translucent garnet of a pale green colour like that of the gooseberry.
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Ground mass is the fine-grained or glassy base of a rock, in which distinct crystals of its constituents are embedded.
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Gummite is a yellow amorphous mineral, essentially a hydrated oxide of uranium derived from the alteration of uraninite.
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Gymnite, so called because it is found in the Bare Hills, Maryland (gymnos being Greek for naked), is a hydrous silicate of magnesia.
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Gypsum is a common mineral distributed in sedimentary rocks, often as thick beds. Gypsum is usually found under beds of rock salt as it's one of the first minerals to crystallize from evaporated salt waters and is also produced in volcanic areas and in rock veins due to the action of sulphuric acid. Gypsum is used in the production of plaster of Paris. It has the formulae CaSO4.2H2O and a relative hardness of 2.
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Gyrolite is a white or colourless, glassy-looking secondary mineral. A silicate of calcium, gyrolite is formed through the alteration of pre-existent calcium silicates and is found mainly in crevices of basalt or as white spherules on a basalt groundmass, rarely larger than 5 cm in diameter. Gyrolite was first discovered in 1851 and was confirmed as a distinct mineral in 1855. Gyrolite aggregates are often accompanied by apophyllite, quartz and zeolites such as analcime.
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Abbreviations
Rocks & Minerals
