See also:group designation for a
See also:family of igneous rocks whose essential characteristics are that they are of acid composition (containing high percentages of
See also:silica), consist principally of
See also:quartz and
See also:felspar, with some
See also:hornblende or
See also:augite, and are of holocrystalline or " granitoid " structure . In popular usage the
See also:term is given to almost any crystalline
See also:rock which resembles granite in appearance or properties . Thus syenites, diorites, gabbros, diabases, porphyries,
See also:gneiss, and even limestones and
See also:dolomites, are bought and sold daily as "granites." True granites are
See also:common rocks, especially among the older strata of the
See also:earth's crust . They have
See also:great variety in
See also:colour and general appearance, some being
See also:white or
See also:grey, while others are
See also:pink, greenish or yellow: this depends mainly on the state of preservation of their felspars, which are their most abundant minerals, and partly also on the relative proportion in which they contain
See also:biotite and other dark coloured silicates . Many granites have large rounded or angular crystals of felspar (Shap granite, many Cornish granites), well seen on polished faces . Others show an elementary foliation or banding (e.g .
See also:Aberdeen granite) . Rounded or
See also:oval dark patches frequently appear in the granitic
See also:matrix of many Cornish rocks of this group . In the
See also:field granite usually occurs in great masses, covering wide areas . These are generally elliptical or nearly circular and may be 20 M. in diameter or more . In the same
See also:separate areas or " bosses " of granite may he found, all having much in common in their mineralogical and structural features, and such groups have probably all proceeded from the same focus or deep-seated source . Towards their margins these granite outcrops often show modifications by which they pass into diorite or
See also:syenite, &c.; they may also be finer grained (like porphyries) or
See also:rich in
See also:tourmaline, or intersected by many
See also:veins of pegmatite .
See also:main granite dikes or veins often run out into the surrounding rocks, thus proving that the granite is intrusive and has forced its way upwards by splitting apart the strata among which it lies . Further evidence of this is afforded by the alteration which the granite has produced through a zone which varies from a few yards to a mile or more in breadth around it . In the vicinity of intrusive granites slates become converted into hornfelses containing biotite, chiastolite or
See also:sillimanite and a variety of other minerals; lime-stones recrystallize as
See also:marbles, and all rocks, according to their composition, are more or less profoundly modified in such a way as to prove that they have been raised to a high temperature by proximity to the molten intrusive mass . Where exposed in cliffs and other natural sections many granites have a rudely columnar appearance . Others
See also:weather into large cuboidal blocks which may produce structures resembling cyclopean
See also:masonry . The tors of the west of England are of this nature . These differences depend on the disposition of the joint cracks which
See also:traverse the rock and are opened up by the
See also:action of
See also:frost and weathering . The majority of granites are so coarse in
See also:grain that their
See also:principal component minerals may be identified in the
See also:hand specimens by the unaided
See also:eye . The felspar is pearly, white or pink, with smooth cleaved surfaces; the quartz is usually transparent, glassy with rough irregular fractures; the micas appear as shining black or white flakes . Very coarse granites are called pegmatite or
See also:giant granite, while very
See also:fine granites are known as microgranites (though the latter term has also been applied to certain porphyries) . Many granites show pearly scales of white mica; others contain dark
See also:green or black hornblende in small prisms . Reddish grains of
See also:sphene or of garnet are occasionally visible .
In the tourmaline granites prisms of black
See also:schorl occur either singly or in stellate groups . The parallel banded structures of many granites, which may be
See also:original or due to crushing, connect these rocks with the granite gneisses or orthogneisses . Under the microscope the felspar is mainly
See also:orthoclase with perthite or
See also:microcline, while a small amount of
See also:plagioclase (ranging from
See also:oligoclase to
See also:albite) is practically never absent . 'These minerals are often clouded by a deposit of fine mica and
See also:kaolin, due to weathering . The quartz is transparent, irregular in
See also:form, destitute of cleavage, and is filled with very small cavities which contain a fluid, a
See also:mobile bubble and sometimes a minute crystal . The micas,
See also:brown and white, are often in parallel growth . The hornblende of granites is usually
See also:pale green in section, the augite and
See also:enstatite nearly colourless . 'Tourmaline may be brown, yellow or blue, and often the same crystal shows zones of different
See also:colours .
See also:zircon and iron oxides, in small crystals, are always
See also:present . Among the less common accessories may be mentioned pinkish garnets; andalusite in small pleochroic crystals; colourless grains of
See also:topaz; six-sided compound crystals of cordierite, which weather to dark green pinite; blue-black hornblende (riebeckite),
See also:beryl, tinstone, orthite and
See also:pyrites . The sequence of
See also:crystallization in the granites is of a normal type, and may be ascertained by observing the perfection with which the different minerals have crystallized and the
See also:order in which they enclose one another . Zircon, apatite and iron oxides are the first; their crystals are small, very perfect and nearly
See also:free from enclosures; they are followed by hornblende and biotite; if
See also:muscovite is present it succeeds the brown mica .
Of the felspars the plagioclase separates first and forms well- ' shaped crystals of which the central parts may be more basic than the
See also:outer zones . Last come orthoclase, quartz, microcline and micropegmatite, which fill up the irregular spaces
See also:left between the earlier minerals . Exceptions to this sequence are unusual; sometimes the first of the felspars have preceded the hornblende or biotite which may envelop them in ophitic manner . below:- An earlier generation of felspar, and occasionally also of quartz, maybe represented by large and perfect crystals of these minerals giving the rock a porphyritic character . Many granites have suffered modification by the action of vapours emitted during cooling . Hydrofluoric and boric emanations exert a profound influence on granitic rocks; their felspar is resolved into aggregates of kaolin, muscovite and quartz; tourmaline appears, largely replacing the brown mica; topaz also is not uncommon . In this way the rotten granite or
See also:stone, used in pottery, originates; and over considerable areas kaolin replaces the felspar and forms valuable
See also:sources of china
See also:clay . Veins of quartz, tourmaline and
See also:chlorite may traverse the granite, containing tinstone often in workable quantities . These veins are the principal sources of tin in Corn-
See also:wall, but the same changes may appear in the
See also:body of the granite without being restricted to veins, and tinstone occurs also as an original constituent of some granite pegmatites . Granites may also be modified by crushing . Their crystals tend to lose their original forms and to break into mosaics of interlocking grains . The latter structure is very well seen in the quartz, which is a brittle
See also:mineral under stress .
White mica develops in the felspars . The larger crystals are converted into lenticular or elliptical " augen," which may be shattered through-out or may have a peripheral seam of small detached granules surrounding a still undisintegrated core . Streaks of "granulitic " or pulverized material
See also:wind irregularly through the rock, giving it a roughly foliated character . The interesting structural variation of granite in which there are spheroidal masses surrounded by a granitic matrix is known as " orbicular granite." The spheroids range from a fraction of an inch to a
See also:foot in diameter, and may have a felspar crystal at the centre . Around this there maybe several zones, alternately lighter and darker in colour, consisting of the essential minerals of the rock in different proportions . Radiate arrangement is sometimes visible in the crystals of the whole or
See also:part of the spheroid . Spheroidal granites of this sort are found in Sweden, Finland,
See also:Ireland, &c . In other cases the spheroids are simply dark rounded lumps of biotite, in fine scales . These are probably due to the adhesion of the biotite crystals to one another as they separated from the rock magma at an early stage in its crystallization . The Rapakiwi granites of Finland have many
See also:round or ovoidal felspar crystals scattered through a granitic matrix . These larger felspars have no crystalline outlines and consist of orthoclase or microcline surrounded by
See also:borders of white oligoclase . Often they enclose dark crystals of biotite and hornblende, arranged zonally .
Many of these granites contain tourmaline, fluorite and
See also:monazite . In most granite masses, especially near their contacts with the surrounding rocks, it is common to find enclosures of altered sedimentary or igneous materials which are more or less dissolved and permeated by the granitic magma . The chemical composition oY a
See also:Yew granites from different parts of the
See also:world is given below:- SiO2 . Al2O, . FezO3 . FeO . MgO . CaO . Na,O . K2O . I . 74.69 16.21 1.16 0.48 o•28 1.18 3'64 II .
71.33 II .18 3.96 1.45 o•88 2.10 3'51 3'49 IV . 76.12 12.18 1.21 0.72 1.12 1.54 2'55 3.21 V . 73.90 13'65 0.28 0.42 0.14 0.23 2'53 7'99 VI . 68.87 16.62 0.43 2-72 I.6o 0.71 1.80 6.48 I . Carn Brea,
See also:Cornwall (
See also:Phillips) ; II . Mazaruni, Brit .
See also:Guiana (
See also:Harrison); III . Redo, near Alne, Vesternorrland, Sweden (Holmquist) ; IV . Abruzzen, a group of hills in the
See also:Riesengebirge (Milch) ; V . Pikes
See also:Colorado (
See also:Matthews); VI .
See also:Wilson's Creek, near Omeo,
See also:Victoria (Howitt) . Only the most important components are shown in the table, but all granites contain also small amounts of zirconia, titanium
See also:oxide, phosphoric acid,
See also:sulphur, oxides of barium, strontium,
See also:manganese and
See also:water .
These are in all cases less than 1%, and usually much less than this, except the water, which may be 2 or 3 % in weathered rocks . From the chemical composition it may be computed that granites contain, on an
See also:average, 35 to 55 % of quartz, 20 to 30 % of orthoclase, 20 to 30 % of plagioclase felspar (including the albite of microperthite) and 5 to 10 % of ferromagnesian silicates and minor accessories such as apatite, zircon, sphene and iron oxides . The aplites, pegmatites, graphic granites and muscovite granites are usually richest in silica, while with increase of biotite and hornblende, augite and enstatite the analyses show the presence of more
See also:magnesia, iron and lime . In the weathering of granite the quartz suffers little
See also:change; the felspar passes into dull cloudy, soft aggregates of kaolin, muscovite and secondary quartz, while chlorite, quartz and
See also:calcite replace the biotite, hornblende and augite . The rock often assumes a rusty brown colour from the liberation of the oxides of iron, and the decomposed mass is friable and can easily be dug with a
See also:spade; where the granite has been cut by joint planes not too close together weathering proceeds from their surfaces and large rounded blocks may be left embedded in rotted materials . The amount of water in the rock increases and part of the alkalis is carried away in solution; they form valuable sources of mineral
See also:food to
See also:plants . The chemical changes are shown by the following analyses: Analyses of I., fresh grey granite; II. brown moderately
See also:firm granite; III. residual sand, produced by the weathering of the same mass (anal . G . P .
See also:Merrill) . The differences are surprisingly small and are principally an increase in the water and a diminution in the amount of alkalis and lime together with the oxidation of the ferrous oxide . (J .
JAMES GRANGER (1723-1776)
GRANT (from A.-Fr. graunter, O. Fr. greanter for cr...
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