How do we study the Rhynie Chert?
We have already seen in the previous two sections that the Rhynie chert
does not crop out anywhere naturally at the surface and thus there are only three main ways of collecting material and/or seeing
the chert in situ:
Collecting loose blocks of chert brought to the surface by
Taking cores of the bed rock by drilling.
Excavating trenches into the bed
Once the chert is collected, a number of analytical techniques
can be used depending upon the information that the researcher is looking for.
The following describes four of the main methods that have been employed to study, among
other things, the fauna, flora, sedimentology and palaeoenvironments of the
Above: A cut and polished slab of
chert (in this case from the Windyfield site) showing nodular and
brecciated textures (centre) and wavy laminae (top).
One of the first steps, once raw material has been collected, is
to cut the blocks of chert with a rock-cutting saw. Before this is done, the
chert is cleaned and the surfaces examined to identify, if possible, the top and
bottom of the original 'chert bed'. Sometimes examining the surfaces of blocks
alone can reveal important diagnostic structures or 'textures' such as the
splash textures described above (see History of
Research at Rhynie). Usually a series of sequential slices of the chert are cut
perpendicular to the bed and the cut surfaces then polished. This produces clean
surfaces where the internal structures and textures through the entire bed can
be examined in hand specimen (see inset left), such as internal bedding, plants and
or 'way-up' structures (see also the section on Chert textures).
The cut slices of chert can be examined
in more detail under reflected light using a binocular microscope after applying
a thin veneer of microscopy oil to the cut/polished surface. This enables the
identification of, for example, fauna, flora and textures within the chert for
more detailed analysis. Further analysis of such features may involve using thin
sections, acetate peels and SEM (Scanning Electron Microscope) techniques which
are briefly outlined below.
Thin sections of the rock allow
material to be examined under a microscope using transmitted light (i.e.:
light that can pass through the rock) as well as reflected light. They are
prepared by mounting a small cut and polished sample of chert, using a strong
adhesive onto a plate of glass (usually 50x75cm or 25x75cm in size) and then the
exposed surface of chert is gradually ground or 'lapped' down to a certain
thickness, usually somewhere between 100 and 30Ám, depending upon the features
being examined. This slice of rock is then usually thin enough to transmit light, enabling a lot more details
of fossils and textures to be seen. Thin sections may have a
glass cover-slip added for protection of the slide, though alternatively the
'lapped' surface may be polished for examination in back-scattered mode on the
SEM (see below).
|This technique is particularly useful for
examining microscopic details of the plants, animals as well as the sedimentary
textures and mineralogy of the cherts (see inset right). By cutting and mounting sequential thin
sections through a particular chert block, a whole chert bed can thus be examined in
detail (see the section on Chert textures). Because
the thin section of rock on the glass slide has a smooth, flat surface it is
easy to photograph.
Above: Detail of a thin section of Rhynie chert
showing sections through straws of the plant Aglaophyton major.
Geopetal layers of very fine sediment within the straws indicate the 'way up'.
The blue colour in this thin section is stained epoxy resin infilling
pore space (scale bar = 3mm).
Acetate peels are taken by first etching the cut/polished
surface of a chert block with HF (Hydrofluoric acid) for a few seconds. This
technique is done under strict safety conditions in a fume cupboard as HF is
very dangerous. The acid dissolves away a thin layer of silica leaving a slight positive relief of
organic material or any other material that is otherwise insoluble in HF. The
etched surface of chert is then flooded with acetone and a sheet of acetate film
is then carefully placed over the top and allowed to dry for about 10 minutes.
When dry the 'peel' can be pulled from the rock surface and it bears a faithful reproduction of
the 'organic' and textural features left in relief after the etching. The peel
is then mounted onto a glass slide with a cover-slip for study.
analytical method has been used extensively in the study of the Rhynie plants
and their internal anatomy. Successive peels taken through a plant-bearing block
of chert can be helpful in reconstructing the 3D morphology of some of the
plants and their growth habit. However, because the features on the peels are
often very faint and the peels are not always flat, they are not always so conducive to
photography as rock thin sections.
|The SEM or Scanning Electron Microscope allows examination
of rocks at very high magnifications, showing surface textures and details
down to the micron level. This has been a useful technique in looking at
many things from clay minerals and diagenetic features in the cherts and
associated sediments (see inset above right) to the cuticle surfaces of
plants where they have been successfully isolated from the host block of
The back-scattered mode on the SEM is particularly useful for looking
at textures in polished thin sections of chert and sedimentary rock and
highlights differences in mineral chemistries (see inset below right).
Above: SEM image of diagenetic cements
in a sandstone from the Rhynie chert sequence. This shows platy
grain-coating chlorite-smectite clays (left and below) with later
diagenetic quartz crystals (center and above) infilling a void or 'pore
detrital mineral grains.
Above: Back-scattered SEM image of a
polished thin section of chert. The very dark areas are voids in the
sample representing the original cell walls of a plant. The dark grey is
chert that has filled the plant cells. The very
light patches are 'framboids' or clusters of micron-sized crystals of
pyrite (FeS2). The lighter grey fragments below
right of center are probably detrital feldspar grains.
Other Analytical Techniques
We have seen four of the techniques employed to study certain
aspects of the Rhynie chert, but many other analytical methods are also
available to study, for example, the mineralogy and geochemistry of the chert
and its associated rocks. Such techniques may include XRD (X-Ray Diffraction),
XRF (X-Ray Fluorescence), CL (Cathodo Luminescence) and isotope analysis to name
but four. A more detailed review of these and other analytical techniques used
to study sedimentary rocks may be found in Tucker (1988).