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Asked by anon-255461 to Matthew on 21 May 2020.
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Matthew Thirlwall answered on 21 May 2020:
Hi, great question! Depends on what method you use. In most cases we have to physically separate individual minerals from a rock. Some uranium-rich minerals can be dated directly using a laser coupled to a mass spectrometer. For most other cases you have to dissolve the rock or mineral first, which requires hydrofluoric acid to attack Si-O bonds. This requires great care, as HF is very toxic. Once it’s dissolved, you have to chemically separate the elements of interest from the rest of the sample. This usually uses some version of ion exchange, a bit like water softening which separates calcium from hard water. Sometimes we might use co-precipitation, and a type of distillation for the rare volatile metal osmium.
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Comments
Fred commented on :
As Matthew says the chemical method rock dating is done by some sort of radioisotope dating, this involves dissolving a bit of the rock up see above then using a mass spectrometer (a machine that works out the amount of each atomic isotope in a material) to look at the the amount of a radioactive isotope eg U234 tor in younger rocks the amount of C14. When the rock formed it had the natural distributiopn of these elements and after that the radioactive isoptope amount will decrease at a fixed rate , so the ratio of the radioactive isotope to a more stable isotope can be used to date the rock.