Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes
of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The
ratio of clay to other minerals is variable. Shale is characterized by breaks along thin
laminae or parallel layering or bedding less than one centimeter in thickness, called fissility.
Mudstones, on the other hand, are similar in composition but do not show the fissility.
Historical mining terminology Before the mid 19th century, the terms slate,
shale and schist were not sharply distinguished. In the context of underground coal mining,
shale was frequently referred to as slate well into the 20th century.
Texture Shale typically exhibits varying degrees of
fissility breaking into thin layers, often splintery and usually parallel to the otherwise
indistinguishable bedding plane because of parallel orientation of clay mineral flakes.
Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are
described as mudstones or claystone. Rocks with similar particle sizes but with less
clay and therefore grittier are siltstones. Shale is the most common sedimentary rock.
Composition and color Shales are typically composed of variable
amounts of clay minerals and quartz grains and the typical color is gray. Addition of
variable amounts of minor constituents alters the color of the rock. Black shale results
from the presence of greater than one percent carbonaceous material and indicates a reducing
environment. Black shale can also be referred to as black metal. Red, brown and green colors
are indicative of ferric oxide, iron hydroxide, or micaceous minerals.
Clays are the major constituent of shales and other mudrocks. The clay minerals represented
are largely kaolinite, montmorillonite and illite. Clay minerals of Late Tertiary mudstones
are expandable smectites whereas in older rocks especially in mid to early Paleozoic
shales illites predominate. The transformation of smectite to illite produces silica, sodium,
calcium, magnesium, iron and water. These released elements form authigenic quartz,
chert, calcite, dolomite, ankerite, hematite and albite, all trace to minor minerals found
in shales and other mudrocks. Shales and mudrocks contain roughly 95 percent
of the organic matter in all sedimentary rocks. However, this amounts to less than one percent
by mass in an average shale. Black shales which form in anoxic conditions contain reduced
free carbon along with ferrous iron and sulfur. Pyrite and amorphous iron sulfide along with
carbon produce the black coloration and purple. Formation
The process in the rock cycle which forms shale is called compaction. The fine particles
that compose shale can remain suspended in water long after the larger particles of sand
have deposited. Shales are typically deposited in very slow moving water and are often found
in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands.
They can also be deposited on the continental shelf, in relatively deep, quiet water.
'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Common
in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing
environments, such as in stagnant water columns. Some black shales contain abundant heavy metals
such as molybdenum, uranium, vanadium, and zinc. The enriched values are of controversial
origin, having been alternatively attributed to input from hydrothermal fluids during or
after sedimentation or to slow accumulation from sea water over long periods of sedimentation.
Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved on
shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite,
or various carbonate minerals. Shales that are subject to heat and pressure
of metamorphism alter into a hard, fissile, metamorphic rock known as slate. With continued
increase in metamorphic grade the sequence is phyllite, then schist and finally to gneiss.
See also
Bituminous shale Oil shale
Burgess shale Bakken formation
Barnett Shale Bearpaw Shale
Mazon Creek fossil beds Shale gas
Shale gas in the United States Wianamatta shale
Wheeler Shale References