Grain Mount
Definition
A microscope slide prepared by dispersing sand-grade grains in a mounting medium of known refractive index. Standard forensic grain mounts use Meltmount or Canada balsam (RI ~1.52-1.54). The mounting medium RI affects the contrast of grain edges under PLM.
- Sample type
- Sand-grade mineral grains
- Common media
- Meltmount or Canada balsam
- Medium RI
- Approximately 1.52 to 1.54
- Viewing method
- Polarised light microscopy (PLM)
Common questions
Why does the mounting medium's refractive index matter?+
Under PLM the visibility of a grain's edge (the Becke line) depends on the refractive-index contrast between the grain and the surrounding medium, so a medium chosen close to common mineral RIs like quartz can be used deliberately to test whether a grain's RI is above or below that reference value.
How does a grain mount differ from a thin section?+
A grain mount disperses loose, already-separated grains in a liquid or resin on a slide for quick optical comparison, while a thin section is a solid rock or sediment sample ground to a fixed 30-micron thickness that preserves the original spatial relationships between grains.
Related terms
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Extinction Angle
- The angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal...
- Pleochroism
- The property of some minerals (hornblende, tourmaline, biotite) to show different colours when the plane of polarised light rotates relative to the...
- QEMSCAN / MLA
- Automated mineralogy systems (QEMSCAN is from FEI/Thermo Fisher; MLA is the Mineral Liberation Analyser) that combine a scanning electron microscope with automated...
- SEM-EDX
- Scanning electron microscopy with energy-dispersive X-ray spectroscopy. The electron beam excites characteristic X-rays from each element present in the target grain, giving...