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SPHERULITES (Gr. cchaipa, sphere, Mho...

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Originally appearing in Volume V25, Page 662 of the 1911 Encyclopedia Britannica.
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SPHERULITES (Gr. cchaipa, See also:sphere, Mhos, See also:stone) , in See also:petrology small rounded bodies which commonly occur in vitreous igneous rocks. They are often visible in specimens of See also:obsidian, See also:pitch-stone and See also:rhyolite as globules about the See also:size of See also:millet See also:seed or See also:rice See also:grain, with a duller lustre than the surrounding glassy See also:base of the See also:rock, and when they are examined with a See also:lens they prove to have a radiate fibrous structure. Under the See also:microscope the spherulites are of circular outline and are composed of thin divergent See also:fibres, which are crystalline and react on polarized See also:light. Between crossed nicols a See also:black See also:cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crossed wires; as the See also:stage is rotated the cross remains steady; between the black arms there are four See also:bright sectors. This shows.. that the spherulite consists of radiate, doubly refracting fibres which have a straight extinction; the arms .o£ the black cross correspond to those fibres which are extinguished. The, aggregate is too See also:fine grained for us to determine directly of what minerals it is composed. Spherulites are commonest in See also:acid glassy rocks like ..those above mentioned, but they occur also in basic glasses such as tachylyte. Sometimes they compose the whole See also:mass; more usually they are surrounded by a glassy or felsitic base. When obsidians are devitrified the spherulites are often traceable, though they may' be more or less completely recrystallized or silicified. In the centre of a spherulite there may be a crystal (e.g. See also:quartz or See also:felspar) or sometimes a cavity. Occasionally spherulites have zones of different See also:colours, and while most frequently spherical they may be polygonal, or irregular in outline. In some New See also:Zealand rhyolites the spherulites send branching " cervicorn " processes (like stags' horns) outwards through the surrounding See also:glass of the rock.

The name axiolites is given to See also:

long, elliptical or See also:band-like spherulites. Occasionally spherulites are met with which are See also:half a "See also:inch or more in See also:diameter. If the rock be pounded up fragments of these can be picked out by See also:hand and subjected to See also:analysis, and it is found that from their See also:composition they may be regarded as a mixture of quartz and acid felspar. See also:Direct microscopic See also:evidence as to the presence of these minerals is rarely obtainable. Some authors describe spherulites as consisting of See also:felsite or microfelsite, which also is supposed to be a cryptocrystalline quartzofelspathic. substance. Very large and cavernous spherulites have been called lithophysae; they are found in obsidians at Lipari, the Yellowstone See also:Park, &c. The characteristic radiate fibrous structure is usually conspicuous, but the fibres are interrupted by cavities which are often arranged as to give the spherulite a resemblance to a See also:rose-bud 'with folded petals separated by arching interspaces. Some of these lithophysae are an inch or more in diameter. In the See also:crystallization of a glass there must be contraction, and it is supposed that thus the concentric cavities arise. The See also:steam and other vapours in the magma would fill these empty spaces and exert a powerful mineralizing See also:action on the warm rock. The presence of See also:garnet, See also:tridymite, fayalite and other minerals, very abnormal in rhyolites in these cavities, in the lithophysae is accounted for in this way. The fibres of these coarse spherulites are often broad and seem to belong to See also:alkali felspar (sanidine or anorthoclase) embedded in tridymite and glass; by See also:analogy it is often inferred that the extremely tenuous fibres of See also:ordinary spherulites have the same composition.

Artificial glass which has not the right composition, or is retained for too long a See also:

time in a See also:furnace, sometimes crystallizes, and contains spherulites which may be as large as a See also:marble. As the glass -has little similarity in chemical composition to volcanic obsidians these spherulites when analysed throw little light on the See also:mineral nature of spherulites in rocks. They show, however that in viscous semi-solid glasses near their See also:fusion point crystallization tends to originate at certain centres and to spread outwards, producing spherulitic structures. Many salts and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide. There are many structures in rocks which are allied to spherulites and usually grouped with them, though probably they are not exactly of the same nature. Some are more vitreous, while others are more perfectly crystalline than the true spherulites. Of the former we mention the doubly refracting glassy spheroids See also:common in rhyolites and obsidians. They differ in no respect from the surrounding hyaline base in ordinary light, but between crossed nicols appear as rounded bodies faintly lighted, with a black cross like that of the spherulites. They are portions of the glass which are in a See also:state Qf See also:compression or See also:strain and hence no longer isotropic. In See also:gelatin, celluloid and artificial glasses similar appearances are occasionally seen. See also:Opal, especially the variety known as hyalite, exhibits the same phenomenon. In the See also:group of porphyries known as granophyres crystals of quartz and felspar occur surrounded by a ground-mass which has a radiate fibrous or spherulitic structure.

The fibres consist of quartz and felspar, usually in graphic intergrowth over considerable areas, and often sufficiently coarse to be easily distinguishable by means of the microscope. Often the quartz or the felspar of the spherulite extinguishes simultaneously with a crystal of either of these minerals lying in the centre of the aggregate. Exactly what the relationships of the spherulites are to those of 'the obsidians has never been cleared up; they are probably analogous growths but not identical. The name granospheres has been given to these bodies. Another group of radiate fibrous growths resembling spherulites in many respects consists of See also:

minute feathery crystals spreading outwards through a fine grained or glassy rock. In the' See also:variolites there are straight or feathery felspar crystals (usually See also:oligoclase) forming See also:pale coloured spherulites, a See also:quarter to half an inch in diameter. The same rocks often contain similar aggregates of plumose See also:skeleton crystals of See also:augite. Many volcanic rocks have small See also:lath-shaped crystals of felspar or augite diverging from a common centre. To distinguish these radiate crystal See also:groups from the cryptocrystalline spherulites they have been called sphaerocrystals. They are commonest in those rocks which contain a fine ground-mass and have been rapidly consolidated. Stellate groupings are frequent also in secondary minerals, being very characteristic of See also:natrolite, See also:chlorite and See also:chalcedony; often the component prisms are very narrow and regularly arranged so that in microscopic sections they give a black cross exactly like that of the spherulites. (J.

End of Article: SPHERULITES (Gr. cchaipa, sphere, Mhos, stone)

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