Micropegmatite
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In petrology, micropegmatite is a very fine intergrowth of quartz and alkali feldspar, occurring as the last product of consolidation in many igneous rocks which contain high or moderately high percentages of silica. It shows the same structure on a minute scale as certain pegmatites or coarse granitic veins do on a large scale; the quartz forms angular patches scattered through a matrix of feldspar. In polarized light the separate areas of each mineral extinguish at the same time, and this proves that, even though apparently discontinuous, they have the same crystalline orientation. The feldspar may be considered an irregular crystal of spongy structure, the interstices being filled up by another spongy crystal of quartz. This kind of mineral intergrowth is said to be "graphic", because the coarsely graphic veins have triangular quartz areas dotted over a feldspathic background which vaguely resemble handwriting.
Micropegmatite differs from graphic granite only in being so much finer-grained that its nature can only be detected with a microscope. The feldspar of micropegmatite is usually orthoclase, but can also be albite, oligoclase or microcline. Occasionally it has crystalline 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 club-shaped, curved or vermiform threads (vermicular micropegmatite, or myrmekite); it has been suggested that the feldspar "corrodes away" and the quartz "fills the empty space".
Micropegmatite is often so fine-grained that, even in the thinnest sections and with high powers, it cannot be resolved into its components. This fine micropegmatite resembles threads, having a divergent arrangement. In some rocks the whole ground mass consists of spherulitic growths of fibrous micropegmatite; in their centres there is often a quartz or feldspar crystal; the outer boundaries of the 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 garnet, cordierite, epidote or hornblende, augite and feldspar, but are less common, and the name micropegmatite is usually reserved for aggregates of quartz and feldspar.
In rocks where micropegmatite frequently occurs (e.g. granite, porphyry and granophyre, quartz-diorite) it is usually the last product of consolidation, and represents the mother liquor left over after the other minerals had separated out in more or less perfect crystals. Therefore, it has no definite form of its own, but fills up the irregular interspaces between the earlier crystallizations. For that reason, it has been compared to a eutectic, and supposed to be the mixture of quartz and feldspar which has the lowest fusion point. 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 composition is by no means constant (this may perhaps be due to small admixtures of soda and lime felspars); and, as of 1911, experimental researches on the fusion points of mixtures of quartz and feldspar have not yet shown that there is a definite mixture which melts at a lower temperature than any other. Furthermore, micropegmatite is not always the last consolidation product, as a eutectic should be: it may occur as well-shaped phenocrysts 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.
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