How do we study the Rhynie Chert?

 

Introduction

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 ploughing.

  • Taking cores of the bed rock by drilling.

  • Excavating trenches into the bed rock.

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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 cherts.

 

Cut blocks

Polished slab of Windyfield Chert

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 geopetal 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

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 30µm, 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.

 

 

 

Thin section of Rhynie Chert

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

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.

This 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.

 

SEM

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 chert.

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).

Diagenetic minerals in a Rhynie sandstone under SEM

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 space'  between detrital mineral grains.

Back-scattered SEM image of chert sample

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).