Analytical Methods:Heavy Minerals

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Heavy Mineral Analysis
Heavy mineral analysis involves comparison of the composition, diversity and concentration of 'heavy minerals' (mineral types whose denisity is greater than 2.9 g cm-3) in soils and sediments. These include zircon, tourmaline, iron oxides, garnet, pyroxenes and amphiboles.

Questions
The heavy mineral assemblage varies according to parent material; hence soils and sediments from areas with different parent material may have different heavy mineral assemblages. Assumming of course that the local geology contains a significant proportion of heavy minerals in the first place.

This can be used to help:
 * Provenence materials
 * Identify patterns of erosion and deposition within geologicaly diverse catchments
 * Look at the degree of mineral weathering in soils

Sampling and samples
Bulk samples of known volume are required for heavy mineral analysis. Representative samples that do not cross context boundaries are required. Samples may be required from potential source materials as well as from deposits, and replicate sampling from source materials can be invaluable for determining variability in heavy mineral compositions that could hinder interpretation. It is recommended that you consult with a specialist before sampling.

Samples may be air-dried and sieved to <2 mm for storage.

Analysis
Sample are sieved to leave only a particular size fraction (typically the fine to medium sand fraction). The heavy minerals are then seperated from the lighter ones such as quartz and feldspar either by suspending them in a dense liquid such as sodium polytungstate. The heavy minerals sink and the light one float.

The heavy minerals are then mounted on a glass slide and examined either using a petrological microscope to identify the minerals or an SEM-EDX x-ray microanalysis system to identify the chemical composition and identify the mineral.

Identification of minerals is a specialist procedure.

Data and interpretation
The number of grains of each mineral type are counted, and the assemblages from different samples are compared to identify differences and similarities.

If replicate samples have been analysed samples can be compared statistically to show whether differences between them are real or just an artefact of variation in composition. If materials from more than one source have become mixed, mixing lines may help to identify the relative contribution of the two, or in some cases multiple, sources.

Related techniques

 * X-ray diffraction
 * Microscopy
 * Micromorphology
 * X-ray microanalysis (SEM-EDX)

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