Slate Rock Definition, Composition, and Uses

This easily flaked rock has many uses and lends itself to fossil preservation

Slate is a fine-grained, hard metamorphic rock.
Slate is a fine-grained, hard metamorphic rock. niolox / Getty Images

Slate is a metamorphic rock with a dull luster. The most common color of slate is gray, but it can also be brown, green, purple, or blue. Slate is formed when a sedimentary rock (shale, mudstone, or basalt) is compressed. Over time, slate may transition into other metamorphic rocks, such as phyllite or schist. You've likely encountered slate on a building or an old chalkboard. 

Slate is the finest-grained metamorphic rock, which means you have to examine it closely to see its structure. It's also a foliated rock that displays what is called "slaty cleavage." Slaty cleavage occurs when fine clay flakes grow in a plane perpendicular to the compression. Striking slate along the foliation causes it to display fissility, breaking the rock into smooth, flat sheets.

Composition and Properties

Close examination of slate shows its cleavage and structure.
Close examination of slate shows its cleavage and structure. kyoshino / Getty Images

Slate is hard, brittle and crystalline. However, the grain structure is so fine that the crystals are not readily visible to the naked eye. When polished, slate appears dull, but is smooth to the touch.

Like many rocks, slate consists primarily of silicates, which are compounds made of silicon and oxygen. In slate, the elements mainly form the minerals quartz, muscovite (mica), and illite (clay, an aluminosilicate). Other minerals found in slate can include biotite, chlorite, hematite, pyrite, apatite, graphite, kaolinite, magnetite, feldspar, tourmaline, and zircon.

Some samples of slate appear spotted. These spots typically appear when iron is reduced. The spots may be spherical or appear as ovoids when stress deforms the rock.

Where to Find Slate

Penrhyn Slate Quarry, near Bethesda in north Wales, 1857.
Penrhyn Slate Quarry, near Bethesda in north Wales, 1857. Hulton Archive / Getty Images

In Europe, most slate is mined in Spain. It is also mined in the United Kingdom, and parts of France, Italy, and Portugal. Brazil is the second-biggest producer of slate. In the Americas, it's also found in Newfoundland, Pennsylvania, New York, Vermont, Maine, and Virginia. China, Australia, and the Arctic also have large reserves of slate.

The Many Uses of Slate

Slate is the traditional material used to make chalkboards.
Slate is the traditional material used to make chalkboards. ideabug / Getty Images

Most slate mined today is used to produce roofing tiles. Slate is a good material for this purpose because it does not absorb water, survives freezing and thawing well, and can be cut into sheets. For the same reason, slate is used for flooring, decorations, and paving.

Historically, slate has been used to produce writing tablets, whetstones, laboratory bench tops, whetstones, cemetery markers, and billiard tables. Because slate is an excellent electrical insulator, it was used for early electrical switch boxes. The Inuit used slate to make blades for ulus, a multi-purpose knife.

Meanings of the Word "Slate"

As shale is compressed, it becomes slate.
As shale is compressed, it becomes slate. Thiradech / Getty Images

The word "slate" has held different meanings over the years and in various industries. In the past, the terms "slate" and "shale" have been used interchangeably. In the modern usage, geologists say shale is converted into slate. However, if you're looking at a partially metamorphosed rock, it's difficult to say whether it should be categorized as slate or as shale. One way to tell shale and slate apart is to strike it with a hammer. Slate emits a "tink" or a ring when struck. Shale and mudstone produce a dull thud.

A sheet of smooth stone used for writing might be referred to as a "slate," regardless of its composition. In addition to slate, writing boards have been made using soapstone or clay.

American coal miners may refer to the shale forming the floor and ceiling of a mine as slate. Fragments of shale separated from coal during processing may also be called slate. Although technically incorrect, the language is traditional.

Fossils in Slate

Ammonite Fossil in Slate
Ammonite Fossil in Slate. Walter Geiersperger / Getty Images

Compared to other metamorphic rocks, slate forms under relatively low temperature and pressure. This makes it a good for fossil preservation. Even delicate structures may be preserved and readily discerned against the fine grain of the rock. However, the foliation pattern of slate can shear fossils or distort them when the rock cleaves.

Key Points

  • Slate is a fine-grained, metamorphic rock formed by compression of sedimentary shale, mudstone, or basalt.
  • Gray slate is common, but the rock occurs in a variety of colors, including brown, purple, green, and blue.
  • Slate consists mainly of silicates (silicon and oxygen), phyllosilicates (potassium and aluminum silicate), and aluminosilicates (aluminum silicate).
  • The term "slate" also refers to objects made from the rock, such as slate tablets or roofing tiles.
  • The phrases "clean slate" and "blank slate" refer to slate's use in chalkboards.

Sources

  • Albert H. Fay, Slate, A Glossary of the Mining and Mineral Industry, United States Bureau of Mines, 1920.
  • Essentials of Geology, 5th Ed, Stephen Marshak. W.W. Norton and Company, Inc. 2016.
  • R. W. Raymond, Slate, A Glossary of Mining and Metallurgical Terms, American Institute of Mining Engineers, 1881.