Geoscience Reference
In-Depth Information
13.1.3 Samples for geochemical analysis
A 200 g sample is usually suffi cient for a range of major and
minor element, trace element and isotopic analyses, except for
very coarse-grained or heterogeneous rocks, where about 1 kg
may be required. Particular care needs to be taken to ensure
that the sample is fresh. This might mean that weathered
material needs to be removed fi rst. If possible it is better to
remove the weathered material in the fi eld to ensure that
what you take back is a fresh sample, and also because it is
easier to allow for the extent of the weathering in the fi eld.
Colour changes are a good indication of weathering, but
also the fracture pattern and hardness of the rock tend to
change. In addition some rocks contain minerals that are the
product of weathering (e.g. weathered mudstones often
develop gypsum crystals). Some caution may also need to be
exercised if the sample is being analysed for metal isotopes
in order to avoid contamination from metal chisels and
hammers.
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13.1.4 Samples for mineral extraction
The amount of sample that needs to be collected for mineral
analysis depends on the minerals to be extracted and the
composition of the rock. For heavy minerals such as zircon
that are present only in low abundance, 1-2 kg of rock may be
required. For volcanic glass and feldspars for 40 Ar- 39 Ar and
K-Ar dating, generally at least 1 kg of rock is required. For
pyroclastic and sedimentary rocks it is particularly important
to collect from the base of the bed as the coarser-grained
minerals that are more easily picked out will be most
abundant there.
13.1.5 Samples for fossils
Samples for megafossil analysis: Megafossil samples and their
supporting rock material tend to be large. They are best
wrapped in paper. If they are fragile or the rock needs to dry
out slowly, wrap them in cling fi lm/plastic food wrap and
then paper. They commonly have commercial value and
therefore special permission may be needed to remove and/
or ship them out of the locality or country where they were
collected. Important specimens should be donated to a
museum after analysis and their sample (or museum
acquisition) number referred to in any subsequent
publication.
Samples for microfossil analysis: The size of samples for
microfossil analysis depends on the likely abundance of
fossils within the sample. For an average abundance of
foraminifers 200 g should be suffi cient; for nanofossils and
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