Heavy media separation and analysis

Simplified heavy media separation method

Heavy media separations (HMS) are commonly used in the geosciences to divide crushed rocks or soils into their respective light and heavy specific gravity fractions e.g. gold panning where water provides the medium and the panning action separates the denser gold particles from the lighter, gangue minerals. In the laboratory, HMS are often employed to separate mineral grains prior to further analysis.

The separated heavy minerals provide important indicators of sedimentary rock provenance, sediment pathways and palaeogeography and the degree of diagenesis that sediments have undergone. They may also enable stratigraphic correlation and are particularly useful where biostratigraphic information is absent.

Separation

Traditionally, the heavy media employed were halogenated organic solvents (e.g. bromoform, methylene iodide and tetrabromoethane) but these are highly toxic, require strict Health and Safety protocols and are increasingly difficult to obtain.

BGS have therefore researched and replaced halogenated organic solvents with lithium polytungstate (LST) media which offer the following advantages:

  • non-toxic
  • can be used in open vessels without the need for forced extraction or specialised personal protective equipment
  • cheaper, as LST is 99% recoverable
  • specific gravity can be simply adjusted by adding or removing deionised water

Analysis

Following the isolation of heavy and light fractions, initial analysis is usually undertaken by inspection using optical microscopy or by employing X-ray diffraction techniques. This may be followed by more detailed morphological assessment and micro-geochemical analysis using either scanning electron microscopy or the electron microprobe.


Optical photomicrograph showing bladed, birefringent sillimanite (Sil) and rounded zircon (Zrn) grains following heavy mineral separation of a Nigerian stream sediment
Optical photomicrograph showing euhedral zircon (Zrn) grains following heavy mineral separation of a Nigerian stream sediment

Contact

Please contact Simon Kemp for further information