Spherulite

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Spherulites, in petrology, are small, rounded bodies that commonly occur in vitreous igneous rocks. They are often visible in specimens of obsidian, pitchstone and rhyolite as globules about the size of millet seed or rice grain, with a duller luster than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.

Under the microscope the spherulites are of circular outline and are composed of thin divergent fibers that are crystalline and react on polarized light. Between crossed nicol prisms, a black cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crossed wires; as the stage is rotated the cross remains steady; between the black arms there are four bright sectors. This shows that the sphcrulite consists of radiate, doubly refracting fibers that have a straight extinction; the arms of the black cross correspond to those fibers thar are extinguished. The aggregate is too fine-grained to directly determine what minerals it is composed of.

Spherulites are commonest in acid glassy rocks like those above mentioned, but they also occur in basic glasses such as tachylyte. Sometimes they compose the whole mass; more usually they are surrounded by a glassy or felsitic base. When obsidians are divided the spherulites are often traceable, though they may be more or less completely recrystallized or silicified. In the center of a spherulite there may be a crystal (e.g. quartz or feldspar) or sometimes a cavity. Occasionally spherulites have zones of different colors, and while most frequently spherical, they may also be polygonal or irregular in outline. In some New Zealand rhyolites the spherulites send branching cervicorn processes (like stags horns) outwards through the surrounding glass of the rock. The name axiolites is given to long, elliptical or band-like spherulites.

Occasionally spherulites are found that are half an inch or more in diameter. If the rock is pounded up, fragments of these can be picked out by hand and subjected to analysis, and it is found that from their composition they may be regarded as a mixture of quartz and acid feldspar. Direct microscopic evidence as to the presence of these minerals is rarely obtainable. Some authors describe spherulites as consisting of felsite or microfelsite, which is also supposed to be a cryptocrystalline quartzofelspathic substance.

Very large and cavernous spherulites have been called lithophysae; they are found in obsidians at Lipari, in Yellowstone Park and other places. The characteristic radiate fibrous structure is usually conspicuous, but the fibers are interrupted by cavities that are often arranged as to give the spherulite a resemblance to a rosebud with folded petals separated by arching interspaces. Some of these lithophysae are an inch or more in diameter. In the crystallization of glass there must be contraction, and it is supposed that thus the concentric cavities arise. The steam and other vapors in the magma would fill these empty spaces and exert a powerful mineralizing action on the warm rock. The presence of garnet, tridymite, fayalite and other minerals, very abnormal in rhyolites in these cavities, in the lithophysae is accounted for in this way. The fibers of these coarse spherulites are often broad and seem to belong to alkali feldspar (sanidine or anorthoclase) embedded in tridymite and glass; by analogy it is often inferred that the extremely tenuous fibers of ordinary spherulites have the same composition.

Artificial glass that doesn't have the right composition or is retained too long in a furnace sometimes crystallizes and contains spherulites that may be as large as a marble. As the glass has little similarity in chemical composition to volcanic obsidians, these spherulites when analyzed throw little light on the mineral nature of spherulites in rocks. They show, however, that in viscous semi-solid glasses near their fusion point crystallization tends to originate at certain centers 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 they are probably 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 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 state of compression or strain and hence no longer isotropic. In gelatin, celluloid and artificial glasses similar appearances are occasionally seen. Opal, especially the variety known as hyalite, exhibits the same phenomenon.

In the group of porphyries known as granophyres, crystals of quartz and feldspar occur surrounded by a ground-mass that has a radiate fibrous or spherulitic structure. The fibers consist of quartz and feldspar, 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 feldspar of the spherulite extinguishes simultaneously with a crystal of either of these minerals lying in the center 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 minute feathery crystals spreading outwards through a fine grained or glassy rock. In the variolites there are straight or feathery feldspar crystals (usually oligoclase) forming pale colored spherulites a quarter to half an inch in diameter. The same rocks often contain similar aggregates of plumose skeleton crystals of augite. Many volcanic rocks have small lath-shaped crystals of feldspar or augite diverging from a common center. To distinguish these radiate crystal 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 natrolite, chlorite and 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.

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