Shale

Sedimentary rock
Shale
Composition
Clay minerals and quartz

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable.^[1] Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility.^[1] Mudstones, on the other hand, are similar in composition but do not show the fissility.

Texture[link]

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.^[1] Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystone (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.^[1] Shale is the most common sedimentary rock.^[2]

Sample of drill cuttings of shale while drilling an oil well in Louisiana. The sand grain is 2 mm in diameter.

Composition and color[link]

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.^[1] Black shale can also be referred to as black metal.^[3] Red, brown and green colors are indicative of ferric oxide (hematite – reds), iron hydroxide (goethite – browns and limonite – yellow), or micaceous minerals (chlorite, biotite and illite – greens).^[1]

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 (except quartz) minerals found in shales and other mudrocks.^[1]

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 (Fe²⁺) and sulfide (S^2-). Pyrite and amorphous iron sulfide along with carbon produce the black and purple coloration.^[1]

Formation[link]

Limey shale overlaid by limestone, Cumberland Plateau, Tennessee

The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain suspended in water long after the larger and denser 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. This process could have taken millions of years to complete.

'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.^[4][5][6] 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.^[5][7][8]

Splitting shale with a large knife to reveal fossils

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[link]

References[link]

Tonio Borg (born May 12, 1957) is a Maltese politician who has been Deputy Prime Minister of Malta since 2004. He was Minister of Home Affairs from 1995 to 1996 and again from 1998 to 2008; since 2008, he has been Minister of Foreign Affairs (Maltese: Ministru għal l-Affarijiet Barranin). Borg is a Member of Parliament for the Nationalist Party.^[1]

Political career[link]

Borg, first elected to the House of Representatives in the 1992 general election, was Minister of Home Affairs from April 1995 to October 1996 and then, while the Nationalist Party was in opposition, Party Spokesman for Home Affairs from October 1996 to September 1998. Following the 1998 election, he returned to his position as Minister of Home Affairs on September 8, 1998. He became Minister of Justice and Home Affairs in April 2003, following that year's election. Borg was elected as the deputy leader of the Nationalist Party in March 2004, and he became Deputy Prime Minister, while remaining Minister of Justice and Home Affairs, when the previous Deputy Prime Minister, Lawrence Gonzi, succeeded Edward Fenech Adami as Prime Minister. Borg also served as Leader of the House of Representatives at that time.^[1]

On March 12, 2008 following the Nationalist Party's victory in the March 2008 parliamentary election, Borg was moved to the position of Minister of Foreign Affairs whilst remaining Deputy Prime Minister.^[2] He was also reappointed Leader of the House of Representatives, a post he held until 6th January 2012.

References[link]

v t e Maltese Government 2008-2013 Jason Azzopardi · Tonio Borg · Joe Cassar · Dolores Cristina · John Dalli · Mario de Marco · Giovanna Debono · Tonio Fenech · Mario Galea · Austin Gatt · Lawrence Gonzi · Carmelo Mifsud Bonnici · Clyde Puli · George Pullicino · Chris Said

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