Field emission scanning electron microscopy a high-resolution ...

Clay Minerals (1997) 32, 197-203
Field emission scanning electron
microscopy a high-resolution technique
for the study of clay minerals in sediments
J. M. HUGGETT AND H. F. SHAW
Department of Geology, Imperial College of Science, Technology and Medicine, London, SW7 2BP, UK
(Received 5 April 1996; revised 23 August 1996)
ABSTRACT: The use of field emission electron microscopy for the study of clay mineral
petrography in mudrocks and sandstones is discussed. The methodology including sample
preparation is outlined and three examples of the application of the technique are described: the
formation of authigenic illite in mudrocks, the multiple generation of authigenic illites in sandstones
and the effects of KC1 drilling muds on shale fabrics. In the study of authigenic illite formation in
Palaeocene mudrocks from the North Sea, the FESEM analyses have demonstrated the formation of
illites with increasing burial depth that conventional SEM and XRD analyses had failed to show. The
FESEM analyses of the authigenic illites in Carboniferous sandstones from the southern North Sea
revealed at least three different habits representing different generation episodes rather than one illite
formation event.This has important repercussions with regard to the interpretation of stable isotope
and dating data for the illites. Significant petrographic changes in shales after treatment with KCI
drilling muds have been observed from FESEM analyses, suggesting reactivity between the shales
and the KC1 muds.
High-resolution field emission scanning electron
microscopy (FESEM) is achieved through use of a
field emission gun (FEG) in place of the more
conventional tungsten hairpin or LaB6 filaments. A
resolution of 0.5 nm has been achieved under ideal
conditions and 5 nm is regularly achieved at 30 kV
with conducting materials (Fig. 1). However, with
insulating materials such as rocks this figure is
closer to 50 nm. The amount of clay data which can
be acquired using back-scatter or conventional
secondary electron imaging techniques is limited
by the resolution of such instruments. For clay
mineral studies the optimal resolution is 0.5 p.m for
back-scattered electron microscopy (BSEM) or 2.5
nm for conventional secondary electron microscopy
(CSEM) compared with 0.5 nm for FESEM (Table
1). Good quality photomicrographs can be obtained
at a magnification of x 2,000 for BSEM, x 5,000
for CSEM and x 30,000 for FESEM. In favourable
circumstances (i.e. observations of electrically
conducting materials rather than rock samples),
9 1997 The Mineralogical Society
photomicrographs of acceptable quality can be
obtained at x 15,000 magnification for CSEM
and 50,000 for FESEM. In view of the high-
resolution imaging offered by FESM observations,
it is surprising that FESEM has not been more
widely adopted for the petrographic analysis of fine
grained rocks.
METHODOLOGY
A field emission gun produces an electron beam
with an extremely high current density, obtained by
applying an intense electric field to a tungsten
single crystal with a needle-shaped tip. Unlike the
conventional thermal emission gun this requires
ultra high vacuum (10 -1~ Torr). The FEG is often
referred to as the high brightness gun because the
emission source for electrons is extremely small
and because the number of electrons emitted per
unit area is much greater than with other types of
electron guns. This, together with the much

198 J. M. Huggett and H. F. Shaw
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FIG. 1. Accelerating voltage vs. resolution for different types of electron source (JEOL, 1994).
narrower range of emitted electron energies than is
obtained with thermal emission guns, permits high
resolution at low kV (Fig. 1). The ability to work at
low kV is particularly valuable for ultra-thin clay
mineral particles which would appear to be
'electron transparent' at high kV. For FESEM
examination it is not always necessary to apply a
conductive coating, particularly if working at very
low kV, but for most clay work 10 kV is about the
optimum and a conductive coating is preferred. The
specimens should have either gold-palladium or
platinum coatings of 30-40 nm thickness as the
gold coating used in conventional SEM is too
coarse and can be observed as a mosaic-like pattern
on the sample surface thus obscuring morphological
features. This is demonstrated by comparing the
micrographs in Figs. 2 and 3. A Polaron 5000 series
coater with a two minute coating time and normal
FIG. 2. FESEM of chlorite-smectite in a Palaeocene mudrock. This sample was coated with gold rather than
platinum and the mosaic-like pattern of the coating is clearly visible on all surfaces.

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200 J. M. Huggett and H. F. Shaw
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Fro. 3. Micrographs of a Palaeocene mudrock succession, central North Sea. (A) BSEM of a mudrock buried to
2 kin. Organic matter (black) and mica (e.g. arrow) define the laminated fabric. The diffuse matrix is
predominantly clay, but individual particles are not resolved; (B) CSEM of same mudrock sample shown in (A);
(C) FESEM image of the same sample as (B); (D) CSEM of mudrock buried to approximately 3 km; (E) FESEM
image of the same sample as (D), showing 'illitic' overgrowths not visible at the lower magnification;
(F) HRTEM lattice fringe image of illite plate with overgrowth shown in (E) nucleated at a dislocation site in the
plate.

esistance at 20 mA, was used to provide the gold-
palladium and platinum coatings.
The electron microscopes used in our studies
were a JEOL 6400F with an Oxford Instruments
high purity germanium energy dispersive X-ray
detector at the Centre for Microscopy and
Microanalysis, University of Queensland, and a
Hitachi $800 (without a microanalyser) at the
Natural History Museum, London.
EXAMPLES
Authigenic illite formation in mudrocks
Until recently, X-ray diffraction (XRD) has been
the principal investigative tool in mudrock diagen-
esis, with the result that identification of authigenic
minerals has often been by inference rather than
observation. Now, FESEM, BSEM, CSEM and
high-resolution transmission electron microscopy
(HRTEM) have been used to examine Palaeocene
mudrocks from a North Sea well, which have been
buried to between 2 and 3 km (see Huggett, 1995).
The XRD analysis of the clay fractions of these
mudrocks indicates that they are composed
predominantly of illite and illite-smectite, with
minor kaolinite and chlorite. The composition of
the illite-smectite is typically 40-50% smectite and
is regularly interstratified. The texture and relation-
ships between the matrix, silt-size grains and
organic matter are best observed in BSEM images
(Fig. 3a). However, from Fig. 3a it is clear that
BSEM is inadequate to observe individual clay
particles. At 2 km depth the clay comprises flakes
with the ragged appearance typical of detrital clay
in both CSEM (Fig. 3b) and FESEM images
(Fig. 3c). At ~ 3 km depth the clay fabric appears
indistinguishable in CSEM images from that seen at
2 km (Fig. 3d). However, in FESEM images of
samples at 3 km depth, wispy overgrowths on illitic
clay particles are a ubiquitous, if minor, component
of the mudrocks (Fig. 3e). The precise chemical
composition of these particles is uncertain as the
particles were too small for collection of X-ray
spectra, though HRTEM lattice fringe images
confirm that they are indeed illite overgrowths on
platy particles (Fig. 3f). The FESEM examination
of samples from intermediate depths indicates that
there is an overall increase in the proportion of
wispy clay overgrowths with depth. The FESEM
analyses have thus revealed the formation of
diagenetic illites in these mudrocks that increases
Field emission electron microscopy 201
with depth, which conventional XRD and SEM
analysis had previously failed to show.
Multiple generations (?) of authigenic fibrous
illite in sandstones
The FESEM examination of authigenic fibrous
illite in Westphalian sandstones from wells in the
southern North Sea has revealed the existence of at
least three morphological types (Figs. 4a & b).
Previous conventional SEM analysis of these
samples had not revealed these variations in illite
morphology. It is possible that these morphological
variations represent different generations of illite
formation. If this interpretation is correct, it could
have important petrographic significance regarding
authigenic illite formation. It could also affect the
interpretation of K/Ar dating and stable isotope
analyses of authigenic illite separates. Such
analyses are made on the basis that the separated
illites represent a single illite formation event in
any one sample. The FESEM observations of the
illites in the Westphalian sandstones show that there
could be several illite formation events.
Effects of KC1 drilling muds on shale fabrics
The effects of KC1 drilling muds on a range of
shales were examined during work carried out
jointly with Eniricerche in Milan, Italy. Figures 5a
and b show samples of a shale composed
predominantly of detrital illite-smectite, examined
before and after being reacted with KC1 drilling
mud at 60~ for two weeks. One effect of the
treatment is shown in Fig. 5b with small fibrous
overgrowths on the platy illite-smectite particles.
The composition of the fibrous overgrowths is
uncertain, as they are too small to be analysed in
situ or to be separated for XRD analysis. Small and
co-workers have demonstrated how fibrous and
platy illite can be grown experimentally in
sandstones in the presence of K-beating solutions
(Small et al., 1992; Small & Manning, 1993). Using
these experimental data as a precedent, it seems
possible that the KC1 muds might have interacted
with the detrital illite-smectite to produce the
fibrous (illitic?) clay overgrowths observed in the
FESEM photomicrographs. However, all that can be
reported here is the observation of these fibrous
overgrowths and any interpretation of the processes
leading to their formation can only be conjectural at
this stage.

202 J. M. Huggett and H. F. Shaw
FIc. 4. FESEM micrographs of illitic clays in Westphalian sandstones from the southern North Sea. (A) Spectrum
of illite morphologies: A -- bladed illite; B -- composite bladed illite; C -- platy illite. Fibrous illite arrowed.
(B) Detail of area adjacent to B in Fig. 4a showing bladed illite (I) and a later generation of cross-cutting fibrous
illites (black arrow) and what appears to be tabular (coalesced plates?) morphologies (arrowed 2).
FIG. 5. FESEM micrographs of a shale (A) before and (B) after treatment with KCI drilling fluid at 60~ for two
weeks.

Field emission electron microscopy 203
CONCLUSIONS REFERENCES
In this paper we have attempted to demonstrate the
usefulness of FESEM for the study of clay fabrics
in sedimentary rocks. The better resolution of the
FESEM gives it significant advantages over
CSEM and BSEM for the detailed observation of
clay fabrics. Apart from adding to our knowledge
of the clay petrography, these more detailed
observations can also influence how we interpret
other types of clay data, e.g. the isotopic and K/At
of illites.
ACKNOWLEDGMENTS
Work using the JEOL 6400F at the University of
Queensland Centre for Microscopy and Microanalysis
was carried out by JMH during a visit funded by the
British Council. Sue Barnes of the Natural History
Museum (London) is thanked for assistance with the
FESEM work done there. We should also like to thank
Dr Luigi Cononico of Eniricerche SpA Milan and Dr
John Berry for allowing us to use photomicrographs
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