Polarised Light Microscopy (PLM)
Definition
A microscopy technique in which polarised light is passed through a thin section or grain mount, revealing optical properties (birefringence, extinction angle, pleochroism) that allow mineral identification. One of the core analytical tools in forensic geology since Walter McCrone's era.
- Field
- Forensic geology, mineralogy
- Properties revealed
- Birefringence, extinction angle, pleochroism
- Historical figure
- Walter McCrone
Common questions
What sample preparation does PLM typically require?+
Mineral grains are usually examined as loose grain mounts in an immersion oil of known refractive index, or as a thin section ground to a standard thickness, both of which allow light to pass through and reveal the optical properties under crossed or single polarizers.
Why has PLM remained central to forensic geology despite newer instrumental methods?+
It is fast, needs minimal sample, is non-destructive to grain-sized particles, and gives a trained examiner enough diagnostic optical properties to narrow a mineral's identity before deciding whether more expensive instrumental confirmation is warranted.
Related terms
- Comparative Mineralogy
- The systematic comparison of mineral assemblages from a questioned sample and a reference sample to assess whether they share the same geographic...
- Forensic Geology
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- Geo-Provenance
- The determination of geographic origin of a geological material based on its physical, mineralogical, or geochemical characteristics. The same concept that geologists...
- Heavy-Mineral Suite
- The assemblage of high-specific-gravity minerals (greater than about 2.85 g/cm3) present in a sediment sample, typically including zircon, tourmaline, garnet, and hornblende....
- Trace Evidence
- Small quantities of material transferred between people, objects, and locations during contact. Forensic geology focuses on the geological subset of trace evidence:...