MICROPALAEONTOLOGICAL TECHNIQUES 471 methods are suitable for use across different groups The selection of the method, therefore, will depend as much on the facilities and methods of the laboratory and the experience of the technician as on the ‘correct’ laboratory manual method Calcareous Microfossils Foraminifera (see Microfossils: Foraminifera) and ostracods (see Microfossils: Ostracoda) both secrete calcitic material that is preserved in the fossil record, and are generally of a similar size and are often found together in the same environments There are exceptions to this, but techniques suitable for one group may be applied to the other Calcareous nannofossils, however, by virtue of their much smaller size, require slightly more specialized treatment Foraminifera and ostracods The sample size required is generally between 250 and 500 g; once cleaned, coarsely broken up, and oven dried, the sample is weighed prior to the start of processing Weighing enables the specimen abundance to be estimated on completion of the laboratory work During the drying stage, there is a possibility that any clay in the sample could be baked hard, rendering any further processing more difficult, so drying is carried out at low temperatures for many hours The extraction of foraminifera and ostracods from argillaceous sediments is relatively straightforward and comes close to being a standard technique; this is because both the composition and the grain size differ between fossil and matrix If the sample is poorly consolidated, a simple boiling in distilled water may be sufficient to break down the rock; this can be done in a microwave oven or on a hotplate Stirring the sample is avoided because this can easily damage specimens Various agents can be added to the water if the initial boiling is ineffective A detergent such as Teepol may help, but this has been shown to cause some dissolution of delicate calcitic material One of the problems with clay-rich sediments is that they tend to be sticky and difficult to disperse In these circumstances, a deflocculating agent will help, such as sodium carbonate (washing soda), sodium hydroxide, or sodium hexametaphosphate (Calgon) The ideal result is to turn the rock into a fine soup that may then be sieved to concentrate the residue Better lithified sediments, such as shales, require treatment harsher than simple boiling in a water softener In these cases, a weak solution of hydrogen peroxide (15%) is used On contact with the rock, the effect of the generation of small bubbles of oxygen within the pore spaces breaks the rock apart However, hydrogen peroxide can cause dissolution of the microfossils, so the time spent immersed has to be kept to minimum This is particularly important if the rock contains any pyrite, because reaction with hydrogen peroxide will create sulphuric acid, which further dissolves the microfossils Other similar methods employ sodium hypochlorite (domestic bleach) or sodium hydroxide; these are less destructive but take a longer time to be effective For marly sediments, a very effective method is to use an organic solvent, such as petroleum spirit, paraffin, or white spirit (turpentine substitute) After the rock has been dried and all pore spaces opened, it is immersed in spirit for several hours The soaking time is dependent on the porosity of the rock; for very finegrained sediments, a longer time is required for the spirit to enter the rock The spirit is then decanted off, filtered, and saved for reuse The vessel containing the rock is then topped up with water, which will cause the sample to disintegrate and form a thick slurry The slurry is then boiled in excess water and washed with a detergent before sieving If the rock type is carbonate rich, the difference between the composition of the fossil and the rock is minimal and chemical methods may not be effective In these cases, physical breakdown of the rock is achieved by forces of crystallization within the pore spaces; chalks will respond well to this approach The most commonly used material for this procedure is sodium sulphate decahydrate (glauber salt) in a supersaturated solution After the rock chips have been dried in the oven, the still-warm sample is immersed in the solution and returned to the oven and allowed to soak thoroughly When saturated, the liquid is decanted and the sample is placed in a freezer overnight Rapid freezing has the effect of causing many nucleation sites, allowing fast growth of very small crystals in the pore spaces This is sufficient to break the rock apart and has less destructive power than does slow crystal growth, and therefore gives better fossil recovery Calcareous nannofossils Calcareous nannofossils (see Fossil Plants: Calcareous Algae) as a group are composed of several forms, such as coccoliths, nannoconids, and discoasters They range in size between 0.25 and 30 mm, and, due to this very small size, the number of specimens in fine-grained pelagic sediments can often exceed million cm Therefore, when collecting in the field, only a very small sample size (approximately cm3) is required Contamination must be very carefully avoided in all stages of nannofossil preparation: the surface of the rock must be scraped clean, cleaned tools must be used to extract the sample, and the sample must be placed in a clean, labelled bag In the laboratory, any glassware