Most of the slate mined throughout the world is used to produce roofing slates. Slate performs well in this application because it can be cut into thin sheets, absorbs minimal moisture, and stands up well in contact with freezing water. A disadvantage is the cost of the slate and its installation in comparison with other…

Foliation in slate is caused by the parallel orientation of platy minerals in the rock, such as microscopic grains of clay minerals and mica. These parallel mineral grain alignments give the rock an ability to break smoothly along planes of foliation. People exploit this property of slate to produce thin sheets of slate that are…

The word “slate” has not been used consistently over time and in some industries. Today most geologists are careful not to use the word “slate” when talking about “shale.” However, in the past the word slate was often used freely in reference for shale. This confusion of terms partially arises from the fact that shale…

The tectonic environment for producing slate is usually a former sedimentary basin that becomes involved in a convergent plate boundary. Shales and mudstones in that basin are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone. Foliation develops at right angles to the compressive…

Most slates are gray in color and range in a continuum of shades from light to dark gray. Slate also occurs in shades of green, red, black, purple, and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock.

Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals.

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.