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CRYSTALLITE

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Originally appearing in Volume V07, Page 569 of the 1911 Encyclopedia Britannica.
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CRYSTALLITE . In See also:

media which, on See also:account of their viscosity, offer considerable resistance to those molecular movements which are necessary for the See also:building and growth of crystals, rudimentary or imperfect forms of See also:crystallization very frequently occur. Such media are the volcanic rocks when they are rapidly cooled, producing various kinds of See also:pitchstone, See also:obsidian, &c. When examined under the See also:microscope these rocks consist largely of a perfectly amorphous or glassy See also:base, through which are scattered See also:great See also:numbers of very See also:minute crystals (microliths), and other bodies, termed crystallites, which seem to be stages in the formation of crystals. Crystallites may also be produced by allowing a See also:solution of See also:sulphur in See also:carbon disulphide mixed with See also:Canada See also:balsam to evaporate slowly, and their development may be watched on a microscopic slide. Small globules appear (globulites), spherical and non-crystalline (so far as can be ascertained). They may coalesce or may arrange themselves into rows like strings of beads—margarites—(Gr. papyapIrgs, a See also:pearl) or into See also:groups with a somewhat radiate arrangement—globospherites. Occasionally they take elongated shapes—longulites and baculites (See also:Lat. baculus, a See also:staff). The largest may become crystalline, changing suddenly into polyhedral bodies with evident See also:double See also:refraction and the See also:optical properties belonging to crystals. Others become See also:long and See also:thread-like—trichites (Gr. Opii , rptx6r, See also:hair)—and these are often curved, and a See also:group of them may be implanted on the See also:surface of a small crystal. All these forms are found in vitreous igneous rocks.

H. P. J. Vogelsang, who was the first to See also:

direct much See also:attention to them, believes that the globulites are pre. liminary stages in the formation of crystals. Microliths, as distinguished from crystallites, have crystalline properties, and evidently belong to definite minerals or salts. When sufficiently large they are often recognizable, but usually they are so small, so opaque, or so densely crowded together that this is impossible. In igneous rocks they are usually See also:felspar, See also:augite, See also:enstatite, and See also:iron oxides, and are found in abundance only where there is much uncrystallized glassy base; in contact-altered sediments, slags, &c., microlithic forms of See also:garnet, See also:spinel, See also:sillimanite, cordierite, various See also:lime silicates, and many other substances have been observed. Their See also:form varies greatly, e.g. thin See also:fibres (sillimanite, augite), See also:short prisms or rods (felspar, enstatite, cordierite), or equidimensional grains (augite, See also:spinet, See also:magnetite). Occasionally they are perfectly shaped though minute crystals; more frequently they appear rounded (magnetite, &c.), or have See also:brush-like terminations (augite, felspar, &c.). The larger microliths may contain enclosures of See also:glass, and it is very See also:common to find that the prisms have hollow, See also:funnel-shaped ends, which are filled with vitreous material. These microliths, under the See also:influence of crystalline forces, may See also:rank themselves .See also:side by side to make up See also:skeleton crystals and networks, or feathery and arborescent forms, which obey more or less closely the See also:laws of crystallization of the substance to which they belong. They See also:bear a very See also:close resemblance to the arborescent See also:frost See also:flowers seen on window panes in See also:winter, and to the stellate See also:snow crystals.

In magnetite the growths follow three axes at right angles to one another; in augite this is nearly, though not exactly, the See also:

case; in See also:hornblende an See also:angle of 57° may frequently be observed, corresponding to the See also:prism angle of the fully-See also:developed crystal. The interstices of the network may be partly filled up by a later growth. In other cases the crystalline arrangement of the microliths is less perfect, and branching, arborescent or feathery groupings are produced (e.g. felspar, augite, hornblende). See also:Spherulites may be regarded as radiate aggregates of such microliths (mostly felspar mixed with See also:quartz or See also:tridymite). If larger porphyritic crystals occur in the See also:rock, the microliths of the vitreous base frequently grow outwards from their faces; in some cases a definite See also:parallelism exists between the two, but more frequently the See also:early crystal has served merely as a centre, or See also:nucleus, from which the microliths and spherulites have spread in all directions. (J. S.

End of Article: CRYSTALLITE

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