Key Facts to Know about Shale Rock

How Shale Forms

Shale forms via compaction from particles in slow or quiet water, such as river deltas, lakes, swamps, or the ocean floor. Heavier particles sink and form sandstone and limestone, while clay and fine silt remain suspended in water. Over time, compressed sandstone and limestone become shale. Shale typically occurs in a broadsheet, several meters thick. Depending on the geography, lenticular formations may also form. Sometimes animal tracks, fossils, or even imprints of raindrops are preserved in shale layers.

Composition and Properties

The clay clasts or particles in shale are less than 0.004 millimeters in diameter, so the structure of the rock only becomes visible under magnification. The clay comes from decomposition of feldspar. Shale consists of at least 30 percent clay, with varying amounts of quartz, feldspar, carbonates, iron oxides, and organic matter. Oil shale or bituminous also contains kerogen, a mixture of hydrocarbons from deceased plants and animals. Shale is classified based on its mineral content. There is siliceous shale (silica), calcareous shale (calcite or dolomite), limonitic or hematitic shale (iron minerals), carbonaceous or bituminous shale (carbon compounds), and phospatic shale (phosphate).

The color of shale depends on its composition. Shale with a higher organic (carbon) content tends to be darker in color and may be black or gray. The presence of ferric iron compounds yields red, brown, or purple shale. Ferrous iron yields black, blue, and green shale. Shale containing a lot of calcite tends to be pale gray or yellow.

The grain size and composition of minerals in shale determine its permeability, hardness, and plasticity. In general, shale is fissile and readily splits into layers parallel to the bedding plane, which is the plane of clay flake deposition. Shale is laminated, meaning the rock consists of many thin layers that are bound together.

Read More

Slate Rock Definition, Composition, and Uses

By Anne Marie Helmenstine, Ph.D.

Commercial Uses

Shale has many commercial uses. It is a source material in the ceramics industry to make brick, tile, and pottery. Shale used to make pottery and building materials requires little processing besides crushing and mixing with water.

Crushing shale and heating it with limestone makes cement for the construction industry. Heat drives off water and breaks limestone into calcium oxide and carbon dioxide. Carbon dioxide is lost as a gas, leaving calcium oxide and clay, which harden when mixed with water and dried.

The petroleum industry uses fracking to extract oil and natural gas from oil shale. Fracking involves injection of liquid at high pressure into the rock to force out the organic molecules. High temperatures and special solvents extract the hydrocarbons, leading to waste products that raise concerns about environmental impact.

Shale, Slate, and Schist

Up to the mid-19th century, the term "slate" often referred to shale, slate, and schist. Underground coal miners may still refer to shale as slate, per tradition. These sedimentary rocks have the same chemical composition and may occur together. The initial sedimentation of particles forms sandstone and mudstone. Shale forms when the mudstone becomes laminated and fissile. If shale is subjected to heat and pressure, it can metamorphose into slate. Slate can become phyllite, then schist, and eventually gneiss.

Sources

• Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary and Metamorphic (2nd ed.). Freeman, pp. 281–292.
• H.D. Holland (1979). "Metals in black shales – A reassessment". Economic Geology. 70 (7): 1676–1680.
• J.D. Vine and E.B. Tourtelot (1970). "Geochemistry of black shale deposits – A summary report". Economic Geology. 65 (3): 253–273.
• R. W. Raymond (1881) "Slate" in .A Glossary of Mining and Metallurgical Terms American Institute of Mining Engineers.