Classification of sedimentary rocks |
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Aragonite |
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Composition: Calcium Carbonate (CaCO3). Formation: Formed from a metastable polymorph of calcite. Tested for by means of hydrochloric acid. It is not generally stable in diagenetic conditions and is quickly converted to calcite.
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Calcite |
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Composition: Calcium Carbonate (CaCO3) Formation: Formed mainly from the neomorphism of clay. The crystals of calcite can form many different shapes by combining the basic forms of the positive rhombohedron, negative rhombohedron, steeply, moderately and slightly inclined rhombohedrons, various scalahedrons, prism and pinacoid to name a few of the more common forms. There are more than 300 crystal forms identified in calcite and these forms can combine to produce the thousand different crystal variations.
Tested for by means of hydrochloric acid. There are two varieties: 'High-magnesium calcite' (CaCO3 containing more than 4% MgCO3 ) and 'Low-magnesium calcite' (CaCO3 containing less than 4% MgCO3 ).
This image shows the upper surface of 'beef' obtained from the Shales with Beef division. It is formed from crystals of Calcite exhibiting a cone-in-cone structure.
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Chalk |
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A type of fossiliferous limestone made up entirely of microscopic shells. These tiny shells are coccoliths, the remains of planktonic marine algae called coccolithophores. The texture of chalk is similar to that of micrite or calcilutite, but chalk is white in colour, less dense, and less compact than micrite. Chalk may be distinguished from other white fine-grained sedimentary rocks (such as kaolinite or diatomite) because it fizzes readily in hydrochloric acid. |
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Chert |
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| Composition: Silicon Dioxide (SiO2)
The name Chert is used to describe bedded, massive chalcedony. Formation: A cryptocrystalline variety of microcrystalline quartz. Consists of minutely crystalline silica of chemical or biochemical origin. Some chert contains siliceous skeletons of micro-organisms known as radiolarians, which can be seen in thin section. Other chert forms through the replacement of limestone, often preserving carbonate textures such as oolites, although the rock has been completely altered to silica. Chert can be recognised by its extremely fine grain size, smooth feel, and hardness (scratches glass). Chert varies in colour, and may be black, white, tan, grey, or greenish grey. Flint is a dark variety of chert. |
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Clay |
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| Composition: Sedimentary rock. Crystal system: Hexagonal; commonly prism or pyramid shapes. Cleavage: Three perfect cleavages along which it breaks into rhombohedra. Hardness: Just over 3. Specific Gravity: Around 2.7 Colourless or white. Sometimes red due to presence of iron compounds. Grain size less than 1/256mm and smooth. | ||||||||||||||||||||||||||||||||
Iron Pyrite |
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| Composition: Iron Sulphide (FeS2)
Crystal system: Cubic, commonly in cubes with striated faces, also in dodecahedra with pentagonal faces. Cleavage: None. It breaks with an irregular conchoidal fracture. Hardness: Just over 6. Specific Gravity: Around 5.0
Iron pyrites is brassy yellow or gold in colour, with a metallic lustre. The streak is brownish black. It occurs very widely in many rocks, veins and nodules. |
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Limestone |
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| Composition: Sedimentary rock. Formation: Primarily composed of the minerals calcite and aragonite. Limestones are generally grey (but may be tan, pink, white, black, or other colours). Depending on the amount of iron compounds or non-calcareous matter present, limestone can vary in colour from white to grey, red, yellow or black. | ||||||||||||||||||||||||||||||||
Marl |
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| Composition: Sedimentary rock. Formation: A form of calcareous mudstone. Usually shows only indistinct bedding planes and weathers into small cubical pieces. If the amount of calcareous matter present increases, the marl becomes an earthy (argillaceous) limestone. | ||||||||||||||||||||||||||||||||
Micrite |
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| Micrite is an abbreviation for microcrystalline calcite, made of lime mud. A limestone (carbonate rock) dominated by mud or clay sized particles (also called Calcilutite). Micrite results from the lithification of lime mud, most of which originates from the breakdown of the hard "skeletons" secreted by calcareous algae which live in warm, shallow seas. | ||||||||||||||||||||||||||||||||
Mudstone |
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| Composition: Sedimentary rock.
Formation: A form of clay.
A compacted form of clay which lacks plasticity. Not as fissile as shale (does not split easily). The cemented equivalent of mud. |
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Oolitic limestone |
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| Oolitic limestone contains
sand-grain sized oolites. A fossiliferous limestone (carbonate rock), sometimes called
skeletal limestone, which contains fossil remains of ancient plants or animals. The
organisms which have calcareous shells or skeletons accumulate in lime mud to eventually
form fossiliferous limestone.
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Paper Shale |
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| Composition: Sedimentary rock. Formation: A form of clay. A form of shale that has very regular layers. | ||||||||||||||||||||||||||||||||
Sandstone |
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Colour: Very variable; frequently red, brown, greenish, yellow, grey, white. Texture: Medium-grained. Usually well sorted, the grains are all about the same size; grains sub angular to rounded. Structure: Bedding usually apparent; current bedding and ripple marks are common; graded bedding may also occur. Concretions and fossils may be found. Mineralogy: Quartz is the main component but it is often accompanied by feldspar, mica or other minerals. The grains may be cemented by silica, calcite or iron oxides. |
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Shale |
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| Composition: Sedimentary rock.
Formation: A fissile form of clay.
Equivalent to mudstone. Splits easily along its bedding planes into thin layers, (it is said to be laminated and fissile). |
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Siltstone |
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| Composition: Sedimentary rock. Formation: A sandstone comprising of grains of quartz usually held together by calcium carbonate, where the grains are less than 0.1mm in diameter (1/256 to 1/16 mm). The grains therefore represent a silt rather than a sand. Red coloration may indicate the presence of iron oxides. Greenish or grey colouration may indicate the presence of other compounds of iron. | ||||||||||||||||||||||||||||||||
Zinc Blende |
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| Composition: Zinc Sulphide (ZnS). Crystal system: Cubic: tetrahedra (4-faced solids) and dodecahedra. Often twinned and intergrown. Cleavage: Good: blende broken across its cleavage directions shows a conchoidal fracture. Hardness: About 3.5 Specific Gravity: Around 4.0 There is rarely any difficulty in distinguishing blende owing to its bright lustre. It varies in colour but is mostly dark brown, though sometimes yellow. Usually is transparent or translucent in thin section. | ||||||||||||||||||||||||||||||||
Other Rock & Mineral references |
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