Heavy-Mineral Fraction
Definition
The sub-fraction of soil mineral grains with density above 2.85 g/cm3, including pyroxene, hornblende, garnet, zircon, rutile, and tourmaline; particularly diagnostic of parent-rock geology.
- Density threshold
- Above 2.85 g/cm3
- Example minerals
- Pyroxene, hornblende, garnet, zircon, rutile
- Separated from
- Soil mineral grains
- Diagnostic value
- Parent-rock geology
Common questions
How is the heavy-mineral fraction physically separated from a soil sample?+
A dense liquid such as sodium polytungstate, with a specific gravity around 2.85, is mixed with the crushed and sieved sample so heavy grains sink while lighter quartz and feldspar float, and the two fractions are then decanted apart.
Why do zircon and rutile carry particularly strong geographic information?+
Both minerals resist chemical weathering and physical abrasion far better than most rock-forming minerals, so they survive transport largely unchanged and can sometimes be traced to a specific source rock even after the surrounding sediment has been reworked.
Related terms
- Bromobenzene
- C6H5Br, density 1.50 g/cm3; the lighter component of the forensic density-gradient liquid mixture.
- Bromoform
- CHBr3, density 2.89 g/cm3; the denser component of the forensic density-gradient liquid mixture, used to create the bottom of the density range.
- D-Spacing
- The perpendicular distance between adjacent parallel planes of atoms in a crystal lattice, measured in angstroms or nanometres. Each set of planes...
- Density-Gradient Column
- Glass column filled with stepwise layers of liquids of decreasing density (commonly bromoform 2.89 g/cm3 and bromobenzene 1.50 g/cm3 mixed in known...
- Diatom Frustule
- The siliceous cell wall of a diatom, composed of biogenic opal; species-specific in shape and ornament, and resistant to degradation in soil...
- Forensic Palynology
- The application of pollen and spore analysis to legal investigations. A forensic palynologist identifies the species composition of pollen assemblages from crime-scene...
- Indicator Species (Pollen)
- Pollen taxa with restricted geographic ranges or specific habitat requirements, contrasted with background species that occur everywhere; indicator species provide site-specific geographic...
- Laser Diffraction (PSD)
- A particle-size analysis technique that measures the angular distribution of laser light scattered by suspended particles to produce a continuous grain-size distribution...
- Murray-Tedrow Framework
- The tiered forensic-soil comparison protocol established by Raymond Murray and John Tedrow in their 1992 text 'Forensic Geology,' which organises soil examination...
- Powder XRD
- X-ray diffraction analysis of a finely ground mineral sample, producing a diffractogram with peaks at positions (2θ angles) diagnostic of crystalline mineral...
- Sporopollenin
- The highly chemically inert polymer forming the outer wall (exine) of pollen grains and spores. Resistant to acid, alkali, oxidation, and biological...