8.3 MUDROCKS
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sediments. Kaolinite is an important authigenic cement in some sandstones (see Section 8.5.3.3.3).
In certain circumstances kaolinite sedimentation is sufficiently abundant to form a
pure claystone termed "kaolin," also known as pipe clay, ball clay, china clay, and fire
clay. The Oligocene Bovey Tracey lake beds of Devon are composed largely of pure
kaolinite derived from the adjacent Dartmoor granite (Edwards, 1976).
Tonsteins are another distinctive rock type, composed largely of kaolinite. These occur intimately associated with coals in the Carboniferous strata of Europe. Individual
beds are thin yet laterally very extensive. While it is possible that tonsteins formed from
the clays of weathered granite, their regional persistence favors an origin due to alteration of volcanic ash falls (Price and Duff, 1969; Bohor and Triplehorn, 1993). Tonsteins
are tough indurated rocks but the younger kaolinite claystones are generally white and
plastic.
Kaolinite is extensively used in the ceramic, paper-making, and pharmaceutical industries. Pure kaolinite rocks are nonmarine in origin because kaolinite quickly transforms to more complex clays in the presence of seawater.
8.3.2.2 Illite Group
The illite clays, sometimes termed the hydromicas, are three-layer aluminosilicates with
up to 8% K20. This potassium may either be present due to the incomplete degradation of potash feldspars to kaolinite, or to diagenesis of kaolinite within a marine environment. Illite is the most abundant clay mineral in sediments but it is less obvious than
kaolinite because it is seldom present in crystals that can be seen with an optical microscope. Even under an electron microscope, illite crystals are smaller and less well developed than those of kaolinite (see Section 8.5.3.3.3).
8.3.2.3 Smectite Group
The third group of clay minerals is the smectites, of which montmorillonite is the chief
example. These are three-layer lattice types which have the unusual property of expanding and contracting to adsorb or lose water. Montmorillonite can contain up 20%
water as well as calcium and magnesium. Mudrocks composed largely of smectite clays
are termed bentonites. Bentonite deposits may be recognized at outcrop by their powdery cauliflower-like surface. Placed in water, montmorillonite lumps expand visibly and
fall apart. Bentonites are formed by the alteration of volcanic ash in place. This may occur in both marine and continental environments. Fragments of detrital glass, frequently
devitrified, are generally present, together with microscopic grains of quartz, micas, feldspars, and heavy minerals.
The type montmorillonite comes from Montmorillon in France. Montmorillonite is
also the chief constituent of the Jurassic and Cretaceous Fuller's Earth in southern England (Robertson, 1986). Bentonites occur in the Upper Cretaceous rocks of Arkansas, Oklahoma, Texas, and Wyoming, notably the Mowry Shale of the Big Horn basin
(Slaughter and Earley, 1965).
Because of their peculiar properties, bentonites are a major constituent of circulating
mud systems used in rotary drilling. Conversely, montmorillonite is an unfortunate clay
337
sediments. Kaolinite is an important authigenic cement in some sandstones (see Section 8.5.3.3.3).
In certain circumstances kaolinite sedimentation is sufficiently abundant to form a
pure claystone termed "kaolin," also known as pipe clay, ball clay, china clay, and fire
clay. The Oligocene Bovey Tracey lake beds of Devon are composed largely of pure
kaolinite derived from the adjacent Dartmoor granite (Edwards, 1976).
Tonsteins are another distinctive rock type, composed largely of kaolinite. These occur intimately associated with coals in the Carboniferous strata of Europe. Individual
beds are thin yet laterally very extensive. While it is possible that tonsteins formed from
the clays of weathered granite, their regional persistence favors an origin due to alteration of volcanic ash falls (Price and Duff, 1969; Bohor and Triplehorn, 1993). Tonsteins
are tough indurated rocks but the younger kaolinite claystones are generally white and
plastic.
Kaolinite is extensively used in the ceramic, paper-making, and pharmaceutical industries. Pure kaolinite rocks are nonmarine in origin because kaolinite quickly transforms to more complex clays in the presence of seawater.
8.3.2.2 Illite Group
The illite clays, sometimes termed the hydromicas, are three-layer aluminosilicates with
up to 8% K20. This potassium may either be present due to the incomplete degradation of potash feldspars to kaolinite, or to diagenesis of kaolinite within a marine environment. Illite is the most abundant clay mineral in sediments but it is less obvious than
kaolinite because it is seldom present in crystals that can be seen with an optical microscope. Even under an electron microscope, illite crystals are smaller and less well developed than those of kaolinite (see Section 8.5.3.3.3).
8.3.2.3 Smectite Group
The third group of clay minerals is the smectites, of which montmorillonite is the chief
example. These are three-layer lattice types which have the unusual property of expanding and contracting to adsorb or lose water. Montmorillonite can contain up 20%
water as well as calcium and magnesium. Mudrocks composed largely of smectite clays
are termed bentonites. Bentonite deposits may be recognized at outcrop by their powdery cauliflower-like surface. Placed in water, montmorillonite lumps expand visibly and
fall apart. Bentonites are formed by the alteration of volcanic ash in place. This may occur in both marine and continental environments. Fragments of detrital glass, frequently
devitrified, are generally present, together with microscopic grains of quartz, micas, feldspars, and heavy minerals.
The type montmorillonite comes from Montmorillon in France. Montmorillonite is
also the chief constituent of the Jurassic and Cretaceous Fuller's Earth in southern England (Robertson, 1986). Bentonites occur in the Upper Cretaceous rocks of Arkansas, Oklahoma, Texas, and Wyoming, notably the Mowry Shale of the Big Horn basin
(Slaughter and Earley, 1965).
Because of their peculiar properties, bentonites are a major constituent of circulating
mud systems used in rotary drilling. Conversely, montmorillonite is an unfortunate clay