GeoWord of the Day

The GeoWord of the Day is a free service of the American Geosciences Institute. All of the terms and definitions are from the Glossary of Geology, 5th Edition Revised.

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run [ore dep] . n. A flat irregular ribbonlike orebody following the stratification of the host rock.

epidote (ep'-i-dote). (a) A yellowish-green, pistachio-green, or blackish-green mineral: Ca2Al2(Fe3+,Al)(SiO4)(Si2O7)O(OH) . It commonly occurs associated with albite and chlorite as formless grains or masses or as monoclinic crystals in low-grade metamorphic rocks (derived from limestones), or as a rare accessory constituent in igneous rocks, where it represents alteration products of ferromagnesian minerals. Syn: pistacite; arendalite; delphinite; thallite. (b) A name for a mineral group with an analogous formula, but with Ca partially replaced by rare-earth elements, and Fe3+ replaced by Al, Mn3+, V3+, Fe2+ or Mg.

cryptocrystalline (cryp''-to-crys'-tal-line). (a) Said of the texture of a rock consisting of crystals that are too small to be recognized and separately distinguished even under the ordinary microscope (although crystallinity may be shown by use of the electron microscope); indistinctly crystalline, as evidenced by a confused aggregate effect under polarized light. Also, said of a rock with such a texture. Cf: microcrystalline; dubiocrystalline. Syn: microaphanitic ; microcryptocrystalline; microfelsitic; felsophyric. (b) Said of the texture of a crystalline rock in which the crystals are too small to be recognized megascopically. This usage is not recommended "since it cannot be known that an aphanitic rock is cryptocrystalline until the microscope has shown that it is actually microscopically crystalline" (Johannsen, 1939, p.206). (c) Descriptive of a crystalline texture of a carbonate sedimentary rock having discrete crystals whose diameters are less than 0.001 mm (Bissell and Chilingar, 1967, p.103) or less than 0.01 mm (Pettijohn, 1957, p.93). Some petrographers use an upper limit of 0.004 mm.

coarse-grained . (a) Said of a crystalline rock, and of its texture, in which the individual minerals are relatively large; specif. said of an igneous rock whose particles have an average diameter greater than 5 mm (0.2 in.). Johannsen (1931, p.31) earlier used a minimum diameter of 1 cm, and referred to igneous rocks having walnut-size to coconut-size grains as "very coarse-grained". Syn: phaneritic. (b) Said of a sediment or sedimentary rock, and of its texture, in which the individual constituents are easily seen with the unaided eye; specif. said of a sediment or rock whose particles have an average diameter greater than 2 mm (0.08 in., or granule size and larger). The term is used in a relative sense, and various size limits have been suggested and used. Cf: fine-grained; medium-grained. (c) Said of a soil in which gravel and/or sand predominates. In the U.S., the minimum average diameter of the constituent particles is 0.05 mm (0.002 in.), or, as used by engineers, 0.074 mm (retained on U.S. standard sieve no.200); the International Society of Soil Science recognizes a diameter limit of 0.02 mm. Cf: fine-grained.

palagonitization (pa-lag''-o-nit''-i-za'-tion). Formation of palagonite by hydration of volcanic (usually basaltic) glass.

homopolar (ho-mo-po'-lar). Of uniform polarity; not separated or changed into ions; not polar in activity.

romanèchite (ro-man'-è-chite). An iron-black to steel-gray monoclinic mineral: (Ba,H2O)2(Mn4+,Mn3+)5O10 . Calcium, potassium, sodium, cobalt, and copper are sometimes present. Romanèchite has a brownish-black streak, and commonly occurs massive, botryoidal, reniform, or stalactitic. It is an important ore of manganese. Syn: psilomelane; black hematite.

deflection angle [surv] . A horizontal angle measured from the forward prolongation of the preceding line to the following line; the angle between one survey line and the extension of another survey line that meets it. A deflection angle to the right is positive; one to the left is negative.

siltite (silt'-ite). A term used by Kay (1951) for a siltstone.

pseudoconformity (pseu''-do-con-form'-i-ty). A term used by Fairbridge (1946, p.88) for a stratigraphic relationship that appears conformable but is characterized by nonaccumulation or deficiency of sediment, such as a slump gap in which an entire formation slipped away off the crest of a rising anticline or in which no trace of a hiatus is immediately apparent from the structure.


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