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Common Igneous Structural and Textural Terms

Common Igneous Structural and Textural Terms - 9 Wfrdlerd...

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Unformatted text preview: 9/ Wfrdlerd/ Z77 J $%(/ M‘Cc/fl/M COMMON IGNEOUS STRUCTURAL AND'TEXTURAL TERMS This list contains some of the more common terms used to describe structures and textures of igneous rocks. In general, textures are the small scale features of a rock (visible in thin section) whereas structures are larger scale features (such as layering or pillows). Obviously some characteristics of a rock may be both, and then the term fabric may be useful as it includes both textures and small structures. Many of the terms on this list are considered textures by some workers and structures by others. holocrystalline hypocrystalline hypohyaline holohyaline eguigranular ineguigranular aphanitic (aphyric) cryptocrystalline microcrystalline phaneritic fine-grained medium—grained coarse~grained aplitic pegmatitic porphyritic (1M, 1033) microporBhyritig megacrysts, insets microlite CRYSTALLINITY consisting entirely of crystals containing both crystals and glass consisting mostly (60—90%) of glass consisting entirely of glass (=glassy, vitreous) GRAIN SIZE constituent grains are of nearly the same size constituent grains vary considerably in size a fine—grained or glassy texture in which the constituents are too small to be seen by the naked eye aphanitic texture in which the constituents are too small to be distinguished with a microscope the crystalline texture can be distinguished with a micro— scope but not with the naked eye (or hand lens) rock in whicn the constituents can be distinguished with the naked eye average diameter of the crystals is less than 1 mm average diameter of the crystals is between 1 and 5 mm average diameter of the crystals is greater than 5 mm fine—grained, equigranular, xencmorphic texture which is typical of aplites (=sugary, saccharoidal) a very coarse—grained texture in which the crystals are greater than 1—2 cm in size. TEXTURES_QE INEQUIGRANULAR ROCKS texture of an igneous rock in which larger crystals (pheno- crysts) are set in a significantly finer—grained or glassy matrix (groundmass, mesostasis) porphyritic texture discernable only under the microscope non—genetic terms for the larger crystals in a porphyritic rock; includes phenocrysts, porphyroblasts, xenocrysts tiny needle—like or lath—like crystals for which at least some optical properties are detectable with microscope ...
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