Gravity Separation Technique

When large number of sedimentary samples are to be investigated for microfossil contents in a short time as done by palynologists of oil and coal exploration laboratories, the gravity separation technique is most suitable.

As the matrix, organic matter, spores and pollen grains have different specific gravities, different portions can be separated by floating the processed samples in heavy liquids of about 2 specific gravity. Generally, spores and pollen grains range between the specific gravity 1.3 to 1.7. Funkhouser and Evitt (1959) have suggested the use of following:

1. Saturated water solution of zinc chloride whose specific gravity could be taken to 1.96. The basic ingredient here is cheap and need not be reclaimed.

2. Where the heavy liquid of higher specific gravity is required the mixture of zinc iodide, potassium iodide and cadmium iodide may be used mostly, retaining its specific gravity to 2.3. The ingredients are costly and can be reclaimed without any apparent loss and could be used again and again.


Heavy minerals

Undigested mineral matter is removed primarily by differential density centrifugation (heavy liquid separation). The commonly used heavy liquids are solutions of zinc chloride or zinc bromide, or a bromoform/alcohol mixture. Variations of density separation technique are outlined by Gray (1965a) and Traverse (1988). Some eastern European laboratories reportedly now use a 'potassium cadmium mixture' If opaque sulphide minerals are present, samples should be cleaned with an oxidizing agent before heavy liquid treatment. White (1988) indicated that the specific gravity of palynomorphs increases with the degree of carbonization, which may impact density separation. A survey of the literature indicates that specific gravities of heavy liquids used by palynologists range from 1.65 to 2.0. The specific gravity must be appropriate for the research goal. For example, most forensic studies require a 2.0 s.g. because it is less likely to be challenged in court. Many investigators of recent pollen advocate that lower specific gravity should be used. Litwin and Traverse (1989) recommended that pyrite be removed prior to density separation.

Fine minerals (clays)

Fine inorganic material can be removed either chemically or mechanically before or after heavy mineral separation. Sieving and swirling may also be helpful. A surfactant detergent will deflocculate and suspend clays during short or differential centrifugation (Gray 1965 a, b; Barss and Williams 1973; Litwin and Traverse 1989). The clay then can be decanted with the supernatant. The most widely used surfactants are Quaternary-O, Darvan No. 4 (Hills and Sweet 1972) and Brij 35. Algonox, and Alcojet (Peterson et al., 1983). Surfactants may damage some microfossils (e.g. may etch Foraminifera), but there is no evidence that they harm organic-walled microfossils (Hodgkinson 1991).

Finally the standard extraction techniques of palynomorphs from rock samples are summarized in the following flow chart.

Raw sample

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