MICROPEGMATITE, in petrology, a very fine intergrowth of quartz and alkali felspar, 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 (q.v.) or coarse granitic veins do on a large scale (see Petrology, Pl. 2, figs. 6 and 8); the quartz forms angular patches scattered through a matrix of felspar. 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 felspar 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 felspathic background resembling certain primitive inscriptions. Micropegmatite differs from “graphic granite” only in being so much finer grained that its nature can only be detected with the microscope. The felspar of micropegmatite is usually orthoclase, but sometimes 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, 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 French petrographers). 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 such spherulitic growths of fibrous micropegmatite (see Quartz-Porphyry); in their centres there is often a quartz or felspar 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 felspar, but are less 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-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. Hence 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 felspar which has the lowest fusion point. Eutectics are common in 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 composition is by no means constant (this may perhaps be due to small admixtures of soda and 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 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. F.)