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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jacquesdietrichite . A vitreous blue orthorhombic mineral: Cu2BO(OH)5.

agpaitic (ag-pa-it'-ic). (a) Said of crystallization in the presence of an excess of alkali (esp. sodium), so that the amount of aluminum oxide is insufficient for the formation of aluminum silicates (Thrush, 1968, p.18). (b) Said of rocks with an agpaitic coefficient > 1.

pholad (pho'-lad). Any bivalve mollusk belonging to the family Pholadidae, characterized by an equivalve shell of variable size, commonly gaping open at the posterior end.

stone reef . A longshore bar whose upper 3-4 m has been solidly cemented by calcium carbonate derived from organic material. Examples occur off the coast of Brazil near Recife.

parasitic fold . A fold on the limb or hinge of a larger fold with which it is congruent. The small fold is said to be parasitic on the larger. Syn: subsidiary fold.

deviation [drill] (de-vi-a'-tion). (a) The departure of a drilled hole from its straight pathway. The hole may be either vertical or inclined and the departure may be in any direction. Deviation may be intentional, as in directional drilling, or undesirable as in crooked hole. Syn: deflection [drill]. (b) In more general use, the angle of departure of a wellbore from the vertical, without reference to direction. Cf: inclination [drill].

lawsonite-albite facies . A term introduced by Winkler (1967) for rocks formed by burial metamorphism at the same temperatures as those of the lawsonite-glaucophane-jadeite facies but at lower pressures, e.g., 600-750 MPa. Obsolete.

prebiotic . Originating before life began.

disjunct (dis'-junct). (a) Said of a rhombiferan echinoderm pore rhomb in which externally visible slits forming parts of the rhomb are separated by solid areas of plates. Cf: conjunct. (b) Said of the apical system of an echinoid whose anterior part is separated from its posterior part.

white mica . A light-colored mica; specif. muscovite.

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