Powder XRD
Definition
X-ray diffraction analysis of a finely ground mineral sample, producing a diffractogram with peaks at positions (2θ angles) diagnostic of crystalline mineral identity via Bragg's law.
- Sample form
- Finely ground mineral powder
- Output
- Diffractogram of intensity versus 2-theta angle
- Principle
- Bragg's law of diffraction
- Application
- Soil mineralogy comparison in trace evidence
Common questions
Why must the sample be ground to a fine powder before analysis?+
Grinding randomises crystal orientation so that every possible diffracting plane is represented in the beam path regardless of how the original grains were oriented, which is necessary to get a complete and reproducible diffraction pattern rather than one biased by a few large crystal faces.
What can powder XRD tell a soil examiner that microscopy alone cannot?+
Microscopy identifies mineral grains by shape and optical properties, but powder XRD gives each crystalline phase a peak pattern specific to its atomic structure, allowing confident identification of minerals that look similar under a microscope or that are present as fine, texturally indistinct particles.
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...
- Heavy-Mineral Fraction
- 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...
- 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...
- Sporopollenin
- The highly chemically inert polymer forming the outer wall (exine) of pollen grains and spores. Resistant to acid, alkali, oxidation, and biological...