slate / slāt/ • n. 1. a fine-grained gray, green, or bluish metamorphic rock easily split into smooth, flat pieces. ∎ a flat piece of such rock used as roofing material. 2. a flat piece of slate used for writing on, typically framed in wood, formerly used in schools. ∎ a list of candidates for election to a post or office, typically a group sharing a set of political views: another slate of candidates will be picked for the state convention. ∎ a range of something offered: the company has revealed details of a $60 million slate of film productions. ∎ a board showing the identifying details of a take of a motion picture, which is held in front of the camera at its beginning and end. 3. [usu. as adj.] a bluish-gray color: suits of slate gray. • v. [tr.] 1. cover (something, esp. a roof) with slates. 2. Brit., inf. criticize severely: his work was slated by the critics. 3. (usu. be slated) schedule; plan: renovations are slated for late June | [tr.] the former brickyard is slated to be renovated. ∎ (usu. be slated) nominate (someone) as a candidate for an office or post: I understand that I am being slated for promotion. 4. identify (a movie take) using a slate. PHRASES: wipe the slate cleansee wipe.DERIVATIVES: slat·y adj.
Slate is a hard, fine-grained metamorphic rock that forms when sedimentary rocks , such as shale and mudstone, are subjected to relatively low temperature and pressure. It occurs chiefly among older rocks. Millions of years of geological compression force the flaky minerals (mica, chlorite, quartz ) within clay sediments to shift perpendicular to the pressure. This pushing alters the material's fundamental structure and creates a new feature known as slaty cleavage. True slate splits easily along this plane into thin, but durable, sheets.
While slate's characteristic color is gray-blue, varieties range from dark gray to black. Organic materials present in the parent rock can create different tinges. Iron oxide creates a reddish purple tinge; chlorite turns slate green. The rock also varies greatly in surface texture and luster; some slates have a dull, matte finish while others can be as shiny as mica.
Better grades of the rock are widely used for roofing, flooring and sidewall cladding. Slate is also used to make blackboards and pool tables. Pennsylvania and Vermont serve as the principal slate producers for the United States, although slate mines can also be found in Maine, Georgia, Lake Superior, and the Rocky Mountains.
See also Bedding; Metamorphism; Sedimentation
slate, fine-grained rock formed when sedimentary rocks such as shale are metamorphosed by great pressure. Slate splits into perfectly cleaved, broad thin layers; this characteristically regular and planar cleavage is called slaty cleavage. In the formation of slate, pressure causes the flaky minerals within the sedimentary rock, such as mica, clay, and chlorite, to be reoriented; the flat faces of the minerals lie at right angles to the source of the pressure, and the planes of easy cleavage are also at right angles to the source of the pressure. The rock is not necessarily compressed in the same direction as the sedimentary layers were originally laid down, and because the compression crumples and deforms the original sedimentary layers, the planes of slaty cleavage usually cut through the old bedding planes. Slate is intermediate in hardness between mica schists and shale; the better grades are used for roofing. Its characteristic color is gray-blue. Slate is mined in Maine, Vermont, Pennsylvania, Georgia, Lake Superior, and the Rocky Mts.