The granite family is by far the largest division of the plutonic rocks, for it comprises all rocks of deep-seated origin with over 66 per cent, of silica and is a common rock type. In general terms, the members of this family may be characterized as consisting of a holocrystalline, and as a rule coarsely crystalline, aggregate of quartz, two or more feldspars, and one or more varieties of mica, hornblende or augite. The feldspars can be recognized by their color and cleavage. Frequently the orthoclase is colored flesh-color or red, while the soda-lime feldspar is usually white. The quartz is recognized by its glassy luster and conchoidal fracture. It is usually white or smoky-gray in color and is found in irregular grains filling up the interstices between the other minerals. The mica, which may be either muscovite or biotite, is to be recognized by its cleavage. Accessory minerals are numerous, including apatite, zircon, sphene, orthite, rutile, garnet, topaz, cordierite, cassiterite, tourmaline, fluorite, molybdenite, hematite, magnetite and iron pyrites. Minerals resulting from the decomposition of the original constituents (and especially of the feldspar) are sericite, talc, kaolin, epidote, calcite, chlorite and serpentine.
The essential minerals can be easily distinguished in the hand specimen: the quartz by its vitreous luster, pellucidity, conchoidal fracture and absence of cleavage; the feldspars by their large and lustrous cleavage surfaces, their twinning and their opaque white or pink color; and the micas by their brilliant luster, platy cleavage, and characteristic color, silvery white for muscovite, brownish-black for biotite. Hornblende, when present, is a black or dark green variety, with the usual prismatic cleavage, the cleaved surfaces meeting at an angle of 124 degrees.
With regard to structure, the granitic structure is typically one in which the component minerals are equidimensional and devoid of crystal faces, each grain having common boundaries with its neighbors, as if formed by simultaneous crystallization. Frequently, however, some of the feldspar occurs as large, well shaped phenocrysts embedded in a holocrystalline ground mass of quartz, feldspar and mica (porphyritic, granite). The size of the individual grain varies considerably. Often granite is coarse-grained in the interior of the mass, and fine-grained in its marginal portions, where cooling has progressed more rapidly. A parallel arrangement of the constituents, especially when the feldspars are idiomorphic, or are developed porphyritically, betrays flow-movements in the partially consolidated granite magma, and is sometimes accompanied by protoclastic granulation; but such flow-structures must be distinguished from the foliation produced by differential movements due to earth stresses after consolidation. In such foliated granites (gneissose granites, augen-gneiss), "nuclei" of the original quartz or feldspar are surrounded by granular aggregates of cataclastic quartz and feldspar, the whole forming eye-like masses wrapped round by fibrous and ribbon-like laminae of white muscovite mica.
A common feature is the presence of dark patches or "heathen," as they are termed by the quarrymen. These patches, which are mostly of irregular shape, are composed of ferromagnesian minerals, iron-ores, and a little plagioclase feldspar. Their origin has been the subject of some controversy. J. A. Phillips considered them to have arisen by a segregation of the more basic constituents of the granite; but it might be in some cases that cases they are fragments (xenoliths) of pre-existing rocks caught up and enclosed by the granite magma. The spheroidal aggregations of plagioclase, dark mica, hornblende and magnetite round a central nucleus, with both radial and concentric structure, have, however, undoubtedly been formed by concretion during the consolidation of the granite magma. The spheroid-granite of Mullaghderg in County Donegal is a well-known example.
Granite occurs as batholith, laccoliths, and stocks, often forming the central core of a mountain range, and surrounded by an aureole of rocks which present unmistakable evidence of the metamorphism produced by heat, or in some cases by the fluoric and boric emanations accompanying the intrusion. Within this contact zone, which, in the case of a large granite massif intruded into folded rocks, may have the width of a mile or more, the rocks are crumpled, baked and altered into hornstone. In slates, the characteristic contact-minerals are developed, including biotite, muscovite, chlorite, ottrelite, tourmaline, chiastolite, andalusite, corundum, kyanite, cordierite, sillimanite, and staurolite. The development of these minerals in the portion of the contact zone furthest removed from the the granite often assumes the character of spots and knots in the slates. The metamorphism round the Leinster, Cornish and Skiddaw granites, and that produced by the intrusion of the newer granites of the Highlands may be cited. In the case of calcareous rocks, limestones are converted into marble, with development of lime-silicates, as for example, the minerals wollastonite, idocrase, diopside and grossularite; while calcareous shales yield lime-silicate rocks (the kalksilikat-hornfels of the Germans). The Coniston limestone, where it is in contact with the Shap granite at Wasdale Head, affords a good example of this kind of contact-metamorphism.Similar phenomena may be seen around the Dartmoor, St. Austell and Bodmin Moor granites.
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