Shale can be found in many parts of the world and is an important rock type for the oil and gas industry as well as construction applications. There are several ways to identify shale rock in the field.
Color and Texture
Shale comes in a wide variety of colors including black, brown, red, green, gray and yellow. The color depends on the mineral composition as well as any organic material that is present. Freshly exposed shale will appear more vibrant in color while weathered outcrops appear dull and earthy toned. The texture of shale is smooth and fine-grained. It has a "splintery" fracture with thin layers that break into shard or plate-like pieces. The finer clay-rich varieties feel slippery to the touch when wet.
Layering
One of the defining characteristics of shale is its visible layering. The rock forms distinct thin beds or laminations reflecting seasonal variations or changes in depositional environments. The laminations are less than a couple centimeters thick. Shale may contain occasional thicker sandy layers or lenses as well but will be dominantly composed of the thin mudstone beds. The layering often leads to a platy, fissile structure that causes the rock to split into thin sheets.
Reaction to Acid
Shale contains a significant amount of clay minerals like illite, smectite and kaolinite. These clays contain silicate minerals that will react with dilute hydrochloric acid. Placing a drop of 10% HCl on a shale sample will cause fizzing or bubbles, unlike quartz-rich sandstone or limestone. The rate of reaction depends on the clay content.
Hardness and Density
Shale has a low hardness of 1 to 4 on the Mohs Scale. It can be easily scratched with a knife blade or copper penny. Shale also has a relatively low density compared to other sedimentary rocks, typically in the range of 2.3 to 2.8 g/cm3. Pure clay minerals have even lower densities around 1.5 to 2.6 g/cm3.
Environment of Deposition
Shale forms in calm, quiet water settings like lakes, deltas, floodplains and deep ocean basins where very fine-grained clay and silt deposits can accumulate over time. The minerals settle from suspension in slow moving or still water. This depositional environment contrasts with sandstones and conglomerates that form closer to shorelines and reflect higher energy conditions.
So, the fine grain texture, splintery fractures, fissile layering, clay mineral composition, reaction to acid and low hardness/density along with the lacustrine or marine origin can be used together to identify shale rock in the field. With experience, geologists can readily distinguish shale from other sedimentary rocks.
