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MICRONUCLEUS
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MICRONUCLEUS , the smaller See also:nucleus in See also:Infusoria (q.v.). In fission it divides by mitosis, and in conjugation furnishes the pairing or gametonuclei, by whose reciprocal See also:fusion a zygote- nucleus is formed, which gives rise to the meganuclei and micronuclei of the individuals of the next See also:cycle of fission. MICROPEGMATITE, in See also:petrology, a very See also:fine intergrowth of See also:quartz and See also:alkali See also:felspar, occurring as the last product of consolidation in many igneous rocks which contain high or moderately high percentages of See also:silica. It shows the same structure on a See also:minute See also:scale as certain pegmatites (q.v.) or coarse granitic See also:veins do on a large scale (see PETROLOGY, Pl. 2, See also:figs. 6 and 8); the quartz forms angular patches scattered through a See also:matrix of felspar. In polarized See also:light the See also:separate areas of each See also:mineral extinguish at the same See also:time, and this proves that even though apparently discontinuous they have the same crystalline See also:orientation. The felspar may be considered an irregular crystal of spongy structure, the interstices being filled up by another spongy crystal of quartz. This See also:kind of mineral intergrowth is said to be " graphic," because the coarsely graphic veins have triangular quartz areas dotted over a felspathic background resembling certain See also:primitive See also:inscriptions. Micropegmatite differs from " graphic See also:granite " only in being so much finer grained that its nature can only be detected with the See also:microscope. The felspar of micropegmatite is usually See also:orthoclase, but some-times See also:albite, See also:oligoclase or See also:microcline. Occasionally it has crystalline See also:form, and then it has been proved that the quartz may be so disposed that the two minerals have a definite relation between their crystallographic axes (parallel growth). The quartz typically occurs as angular patches; at other times it forms See also:club-shaped, curved or vermiform threads (vermicular micropegmatite, myrmekite), and then some authors consider that the felspar has been corroded and the quartz fills up the spaces thus produced (quartz de corrosion of See also:French petrographers). Micropegmatite is often so fine grained that even in the thinnest sections and with high See also:powers it cannot be resolved into its components. This fine micropegmatite resembles threads, having a divergent arrangement. In some rocks the whole ground See also:mass consists of such spherulitic growths of fibrous micropegmatite (see QUARTZ—See also:PORPHYRY); in their centres there is often a quartz or felspar crystal; the See also:outer boundaries of the See also:spherulites are not usually circular but irregular owing to the interlocking of adjacent spherulites at their margins (" granophyric structure "). Micrographic structures may occur in other minerals, e.g. quartz and See also:garnet, cordierite, See also:epidote or See also:hornblende, See also:augite and felspar, but are less See also:common, and the name micropegmatite is usually reserved for aggregates of quartz and felspar. In rocks where micropegmatite frequently occurs (e.g. granite, porphyry and granophyre, quartz-See also:diorite) it is usually the last product of consolidation, and represents the See also:mother liquor See also:left over after the other minerals had separated out in more or less perfect crystals. Hence it has no definite form of its own, but fills up the irregular interspaces between the earlier crystallizations. For that See also:reason it has been compared to a eutectic, and supposed to be the mixture of quartz and felspar which has the lowest fusion point. Eutectics are common in See also:alloys and often have a very perfect micrographic structure. The eutectic mixture of quartz and orthoclase has been estimated to contain 70–75% of the latter. This theory, however, is not without its difficulties; analyses of micropegmatite prove that its See also:composition is by no means See also:constant (this may perhaps be due to small admixtures of soda and See also:lime felspars); and experimental researches on the fusion points of mixtures of quartz and felspar have not yet shown that there is a definite mixture which melts at a See also:lower temperature than any other. Furthermore micropegmatite is not always the last consolidation product, as a eutectic should be, but may occur as well-shaped phenocysts lying in a felsitic or glassy matrix which solidified at a still later time. Micrographic structures in the minerals of igneous rocks prove only that these minerals crystallized simultaneously. (J. S. Additional information and CommentsThere are no comments yet for this article.
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