Engineering geology of British rocks and soils Mudstones of the ...

Calcium sulphate is commonly present in the Mercia
Mudstone Group as both the hydrous form gypsum
(CaSO 4.2H 2O) and the anhydrous form anhydrite (CaSO 4).
Anhydrite is stable at depth or at the surface at temperatures
above 42C but transforms to gypsum, in the presence
of water, in the near-surface zone if the overlying beds are
removed by erosion. The conversion to gypsum is accompanied
by an increase in volume of 63% (Shearman et al.,
1972) which can produce brecciation and distortion of the
surrounding rock. In general, the transformation is
complete within 50 to 100 m below the surface. Gypsum
may be found as finely disseminated crystals in pores, as a
cement, as nodules or veins and as massive deposits up to 2
m thick that may also contain anhydrite. Gypsum is usually
found in the upper part of the Mercia Mudstone Group particularly
in the Cropwell Bishop Formation and its stratigraphic
equivalents in other basins. Gypsum is often
removed by groundwater in the near-surface zone.
Strontium sulphate (celestite) is present in small quantities
as disseminated crystals or small nodules in the more gypsiferous
parts of the Mercia Mudstone Group. In the Bristol
area the nodular beds are sufficiently rich in celestite for
their extraction to be commercially viable and they were the
world’s main source of this mineral until the late 1970’s.
The celestite-rich beds are probably the same age as the
gypsum rich beds of the East Midlands (Thomas, 1973).
Halite is present in the major basins or at basin margins
of the East Irish Sea Basin, the Cheshire Basin, the
Staffordshire Basin, the Worcestershire Basin, the
Somerset Basin and East Yorkshire. It is present either as
high purity beds with interbedded mudstone or, mixed with
mudstones and siltstones in ‘Haselgebirge facies’. Halite is
highly soluble and is not usually found within 40 to 60 m
of the surface in Britain since it is removed by groundwater.
Its solution results in the collapse of overlying strata
which forms a breccia.
Mica is often present as silt-sized to fine sand-sized
plates usually of muscovite with some biotite. Haematite or
pyrite is present at or below the detection limits of X-ray
diffraction analysis. Where iron is present as Fe 3+ , usually
as haematite, the rock is red and where it is in its reduced
Fe 2+ form, usually as pyrite, the rock is green or grey
(Leslie, 1989).
Other non-clay minerals which may be present as minor
constituents of the Mercia Mudstone Group are feldspar
and heavy minerals such as titanium-iron oxides, apatite,
zircon, monazite, tourmaline, rutile, magnetite, anatase,
barytes, barycelestite, ilmenite, xenotime and chromite.
Several copper minerals and native copper are present in
the mineralised zone at the base of the Weatheroak
Sandstone near Redditch (Old et al., 1991). Pyrite is
commonly overgrown and replaced by later diagenetic
sulphides of copper, zinc, lead, arsenic, cobalt, nickel,
silver and mercury. Anhydrite may be replaced by chalcopyrite,
chalcocite and pyrite in the Cheshire Basin.
Malachite specks are present in a celestite-rich breccia at
Henbury near Bristol (Kellaway and Welch, 1993).
3.3.2 Clay minerals
The major clay minerals of the Mercia Mudstone Group
are illite, chlorite, mixed layer clays (illite-smectite,
chlorite-smectite) and, in some horizons, smectite. The distribution
of clay minerals in the Mercia Mudstone Group is
summarised in Figure 3.3.
The detrital clays illite and chlorite are present throughout
the Mercia Mudstone Group, illite is the major
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component of the clay mineral assemblage and has been
reported as clay-size mica in some reports (Perrin, 1971).
Chlorite is a minor component and is present as detrital,
well-crystallised, silt-sized particles up to 0.05 mm long
and as poorly- to well- crystallised clay-sized particles. In
the lower and the upper parts of the red Mercia Mudstone
and the Blue Anchor Formation they are often the only clay
minerals which have been identified.
The authigenic clay minerals; smectite, palygorskite,
sepiolite and mixed layer clays (chlorite-smectite) are usually
present as clay-sized particles in the middle part of the
Mercia Mudstone Group but are often absent from the lower
and upper strata. Smectite may be present as a minor part of
the clay mineral assemblage at some horizons in the middle
and upper part of the red Mercia Mudstone but it is rare in the
lower part and absent in the Blue Anchor Formation.
Magnesium-rich palygorskite and sepiolite are present in
minor or trace quantities. Sepiolite is often associated with
the Arden Sandstone Member and equivalent strata (Jeans,
1978; Bloodworth and Prior, 1993) and in some skerries and
gypsum bands above and below this member (Taylor, 1982).
Palygorskite is sometimes present in the upper red beds associated
with gypsum. These minerals have not been identified
in the Cheshire Basin. Palygorskite and sepiolite may be
present in concentrations below the detection limit of X-ray
diffraction techniques and are not recorded unless a more
sensitive technique is used such as a scanning electron microscope
with a back scatter facility.
The mixed layer clays can be highly variable in the proportions
of their component clays. The mixed layer clay
chlorite-smectite may be dominated by chlorite or smectite
or may form the regularly interlayered mineral corrensite.
A study of chlorite-smectite in the Nottingham area by
Bloodworth and Prior (1993) found that the proportion of
smectite interlayers increased as the proportion of mixed
layer clays in the clay mineral assemblage increased.
*
Present usually in minor quantities but sporadically distributed
Minor constituent: Carbonate and sulphate minerals 20% of whole
rock or assumed from description
Major or minor constituent
Mineral currently exploited
Major celestite in some parts of Avon previously exploited.
Key to Figure 3.2 (opposite) The distribution of carbonate
and evaporite minerals in the Mercia Mudstone Group.