338
8 ALLOCHTHONOUS SEDIMENTS
mineral to have as a matrix in an oil reservoir. During production, water entering the
reservoir may cause the clay matrix to expand and thus destroy permeability.
8.3.2.4 Chlorite
The chlorites are similar to the clay minerals just described in many ways, but also share
affinities with the mica mineral group. The chlorites are mixed-layer lattice clays with
up to 9% FeO and 30% MgO.
Chlorites occur as an alteration product of primary micas, and are a common accessory detrital mineral in immature sands and mudrocks. Conversely chlorite replaces illite and other clay minerals at the point where diagenesis merges into metamorphism;
it is a characteristic constituent of the microcrystalline matrix of greywackes.
8.3.2.5 Glauconite
The fifth clay mineral to consider is glauconite. This term is used in two ways. It is applied to pretty, rounded green grains commonly seen in marine sediments and to a particular mineral (Plate 3A). Analyses of the former show them to contain a mixture of
clays whose lattices are in various stages of ordering. Glauconite proper is a three-layer
clay mineral containing magnesium, iron, and potassium. Unlike the other clay minerals, glauconite does not form from the hydrothermal or terrestrial weathering of preexisting minerals. Glauconite occurs in dark green amorphous grains seldom larger than
fine-sand grade. It is found both in mudrocks and sandstones. Where especially abundant in sands, the rock is commonly named "greensand." Glauconite formation accompanied by greensand sedimentation occurred at particular times on a worldwide basis,
notably in the Cambrian and through the late Cretaceous and early Tertiary (Pettijohn
et al., 1972, p. 229).
Glauconite grains appear to have formed by the replacement of fecal pellets and by
infilling foraminiferal tests which have subsequently been destroyed. Glauconite also
occurs infilling larger shells and replacing detrital micas. It is a matter of observation
that glauconite occurs in ancient sediments of marine origin. It is easily weathered and,
with one or two exceptions, does not occur in any abundance as a reworked mineral.
There has been considerable debate both as to the nature of the chemical reactions
responsible for glauconite formation and of the parameters that control the reaction.
Glauconite has generally been considered to form by the transformation of degraded
smectitic or illitic-type sheets into more ordered mixed-layer lattices. Several studies,
however, have argued for neoformation of glauconite within voids (Odin, 1972, 1988;
Bjerkli and Ostmo-Saeter, 1973; Chamley, 1989). A polygenetic origin for glauconite
formation seems to be clearly proven.
Geochemical evidence suggests that glauconite formation, by whatever process, occurs in seawater at low temperatures and in an environment that is neither strongly oxidizing nor reducing. Optimum depth for glauconite genesis appears to be between 50
and 1000 m. However, these parameters are hard to define because, once formed, glauconite is stable in seawater and can survive transportation in a marine environment.
Thus glauconite is found disseminated in marine mudrocks, in clean well-sorted cross-
8 ALLOCHTHONOUS SEDIMENTS
mineral to have as a matrix in an oil reservoir. During production, water entering the
reservoir may cause the clay matrix to expand and thus destroy permeability.
8.3.2.4 Chlorite
The chlorites are similar to the clay minerals just described in many ways, but also share
affinities with the mica mineral group. The chlorites are mixed-layer lattice clays with
up to 9% FeO and 30% MgO.
Chlorites occur as an alteration product of primary micas, and are a common accessory detrital mineral in immature sands and mudrocks. Conversely chlorite replaces illite and other clay minerals at the point where diagenesis merges into metamorphism;
it is a characteristic constituent of the microcrystalline matrix of greywackes.
8.3.2.5 Glauconite
The fifth clay mineral to consider is glauconite. This term is used in two ways. It is applied to pretty, rounded green grains commonly seen in marine sediments and to a particular mineral (Plate 3A). Analyses of the former show them to contain a mixture of
clays whose lattices are in various stages of ordering. Glauconite proper is a three-layer
clay mineral containing magnesium, iron, and potassium. Unlike the other clay minerals, glauconite does not form from the hydrothermal or terrestrial weathering of preexisting minerals. Glauconite occurs in dark green amorphous grains seldom larger than
fine-sand grade. It is found both in mudrocks and sandstones. Where especially abundant in sands, the rock is commonly named "greensand." Glauconite formation accompanied by greensand sedimentation occurred at particular times on a worldwide basis,
notably in the Cambrian and through the late Cretaceous and early Tertiary (Pettijohn
et al., 1972, p. 229).
Glauconite grains appear to have formed by the replacement of fecal pellets and by
infilling foraminiferal tests which have subsequently been destroyed. Glauconite also
occurs infilling larger shells and replacing detrital micas. It is a matter of observation
that glauconite occurs in ancient sediments of marine origin. It is easily weathered and,
with one or two exceptions, does not occur in any abundance as a reworked mineral.
There has been considerable debate both as to the nature of the chemical reactions
responsible for glauconite formation and of the parameters that control the reaction.
Glauconite has generally been considered to form by the transformation of degraded
smectitic or illitic-type sheets into more ordered mixed-layer lattices. Several studies,
however, have argued for neoformation of glauconite within voids (Odin, 1972, 1988;
Bjerkli and Ostmo-Saeter, 1973; Chamley, 1989). A polygenetic origin for glauconite
formation seems to be clearly proven.
Geochemical evidence suggests that glauconite formation, by whatever process, occurs in seawater at low temperatures and in an environment that is neither strongly oxidizing nor reducing. Optimum depth for glauconite genesis appears to be between 50
and 1000 m. However, these parameters are hard to define because, once formed, glauconite is stable in seawater and can survive transportation in a marine environment.
Thus glauconite is found disseminated in marine mudrocks, in clean well-sorted cross-