Information AboutMicropegmatite |
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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. s 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. REFERENCES |
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