472 MICROPALAEONTOLOGICAL TECHNIQUES must be cleaned with a 10% solution of hydrochloric acid and rinsed with distilled water Several methods, each of increasing complexity, can be used for extracting nannofossils from the surrounding matrix The simplest may be done rapidly by making a smear slide This is a small scraping of sediment onto a glass slide; this specimen is then diluted and mixed with a little distilled water The slide is dried on a hotplate and finished by mounting a cover slip with a medium that has a refractive index between the two extremes of calcite, 1.484 and 1.658 Caedax or Cellosize and Elvacite have been used successfully These smear slides are useful for reconnaissance work, but for more detailed analysis, other methods are available The most commonly used technique is separation by centrifuge, which has replaced methods such as wet sieving, gravity settling, and elutriation The object is to prepare slides that contain a high proportion of nannofossil to sediment while retaining the diversity of the original sample To achieve this, the sample is broken down by crushing or shaking in distilled water buffered with sodium carbonate to a pH of around to 8.5 This should then pass through a 45mm sieve Often a water softener such as sodium hexametaphosphate is added to deflocculate the clays, but some authorities avoid this because it has a slightly corrosive effect on the fossils Similarly, ultrasonic vibration is used by some to disaggregate the sample further, but this has been reported to damage some specimens Many variations on the centrifuge speeds have been suggested, but a simple two-stage method involves initially centrifuging with a low speed to settle out particles coarser than 30 mm, retaining the finer material in suspension, then a second stage of rapidly centrifuging the liquid to concentrate the particles down to mm Speeds and times will vary according to the equipment, so the instrument manufacturer’s charts should be consulted for calculating the correct variables Nannofossils may be examined either by light or electron microscopy For light microscopy, a drop of the suspension is placed onto a slide and prepared as for the smear slide A high-powered research-grade optical microscope with magnifications of 800Â or 1200Â (oil immersion lenses) is required Observations are made either by using polarizing attachments and viewing in cross-polarized light or by using phase-contrast methods Though the results obtained in this way are rapid, and often used in industrial applications, more detailed images may be obtained by employing electron microscopy The transmission electron microscope (TEM) has been used to see the structural relationships of the elements of the nannofossils, but the preparation for the TEM requires making carbon Figure Calcareous nannofossils (Calcidiscus) visualized using different methods (A) Phase contrast; (B) scanning elec tron microscopy; scale bar micron; (C) crossed polarized light; (D) gypsum tint plate Photographs by Dr J Young replicas of the specimens, which is a painstaking and skilled operation and is rarely used now A faster and more convenient method is to coat the specimens in a thin layer of gold and observe using scanning electron microscopy (SEM) Figure shows examples of specimens using various observation methods Acid-Insoluble Microfossils Conodonts Conodonts (see Microfossils: Conodonts) are found in marine rock types as diverse as limestones, shales, marls, ironstones, and cherts, and the methods of extracting them from their matrix are just as varied However, unlike the calcareous microfossil groups, the phosphatic nature of conodonts renders them suitable for some acid preparations, and a variety of acids have been used for this purpose Although conodonts may be visible on bedding surfaces in the field, it is far more usual to employ spot sampling at intervals that have been accurately logged A bulk sample size of 1–2 kg should be sufficient to yield many conodonts Because conodonts are durable and quite resistant to weathering and erosion, care needs to be taken to avoid contamination of samples from material washed down slopes Again, only fresh rock should be sampled For all lithologies, samples are pretreated by cleaning the exterior of the bulk sample, oven drying, and then coarsely crushing to centimetre-sized pieces Carbonate-cemented rocks will respond to diluted acids and many methods have been tried The use