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.


ectooecium (ec'-to-oe'-ci-um). The outer, generally calcified part of the body-wall fold forming an ovicell in cheilostome bryozoans. Cf: entooecium.

wittichenite (wit'-ti-chen-ite''). A metallic steel-gray to tin-white orthorhombic mineral: Cu3BiS3 .

PDB standard . Carbon dioxide prepared from belemnites ( Belemnitella americana ) collected from the Peedee Formation (Upper Cretaceous) of South Carolina. It is used as a standard of comparison in determining the isotopic composition of carbon and of oxygen. It was first used by H. C. Urey in 1951 to determine paleotemperatures on the basis of the fractionation of oxygen isotopes. The initials stand for "Peedee belemnite".

chocolate tablet structure . A type of boudinage where a relatively rigid layer is stretched in all directions, resulting in crossing boudin necks. The surface of the layer resembles that of a sheet of chocolate tablets.

travertine (trav'-er-tine). (a) Biotically and/or abiotically precipitated calcium carbonate (predominantly calcite and aragonite) from spring-fed, heated and/or ambient-temperature waters (Chafetz and Folk, 1984). The spongy or less compact variety is tufa. See also: onyx marble. Syn: calcareous sinter; calc-sinter. (b) A term sometimes applied to any cave deposit of calcium carbonate. (c) A term used inappropriately as a syn. of kankar (Viles and Goudice, 1990). Etymol: Italian "travertino" from Latin "(lapis) tiburtinus" = (stone) of Tibur (Tivoli).

needle [cryst] (nee'-dle). A needle-shaped or acicular mineral crystal.

melanophlogite (mel''-a-no-phlo'-gite). A colorless, white, yellow, reddish, or brown tetragonal mineral consisting of silicon dioxide (SiO2) and containing carbon and sulfur. It was formerly believed to be a partly oriented pseudomorph of alpha quartz after cristobalite containing H2SO4.

solution plane . A plane containing lines of chemical weakness in a crystal, along which solution tends to occur under certain physical circumstances, e.g. great pressure.

outer lip . The abaxial (lateral) margin of the aperture of a gastropod shell, extending from the suture to the foot of the columella. Cf: inner lip.

homogeneous [geophys] . Said of a material whose properties are uniform throughout. Not to be confused with isotropic . Cf: heterogeneous [geophys].

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