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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leiteite (lei'-te-ite). A colorless to brown monoclinic mineral: ZnAs3+2O4 .

virgal (vir'-gal). One of an articulated series of more or less rod-shaped or cylindrical ossicles forming a structure that extends outward from an ambulacral plate of certain asterozoans. Pl: virgals or virgalia.

transformation [cryst] (trans-for-ma'-tion). Transformation is remodeling of an existing mineral structure in which major parts of at least the geometry, and usually structural elements of the parent mineral are retained. The change from one crystal polymorph to another, by one of several processes. See: dilatational transformation; displacive transformation; reconstructive transformation; rotational transformation; substitutional transformation; order-disorder transformation. Syn: inversion [cryst]; phase transformation.

granitic layer . A syn. of sial, so named for its supposed petrologic composition. A layer is sometimes called "granitic layer" if it possesses the appropriate seismic velocity (~ 6.0 km/s), although nothing may be known about its composition. Cf: basaltic layer.

sieve tube element . A specialized sieve element.

blanket deposit [ore dep] . A flat mineral deposit, the length and breadth of which are relatively great compared with the thickness. The term has no genetic connotation.

biodegradable (bi''-o-de-grad'-a-ble). Said of materials that can be readily decomposed by organisms.

melanosclerite (mel-a-no-scle'-rite). A problematic organic (chitinous) microfossil with a dense wall and a variety of forms ranging in size from approximately 50 μm to several millimeters. It is present in organic residues from the Paleozoic of Laurentia, Baltica, and Laurasia and thought to have an affinity with hydrozoans (Cashman, 1996).

growth ruga . An irregular ruga or wrinkle on the surface of a bivalve-mollusk shell, having an origin similar to that of a growth line but corresponding to a more pronounced halt in growth.

lattice texture [eco geol] . In mineral deposits, a texture produced by exsolution in which elongate crystals are arranged along structural planes.