Basal Reflection
Definition
The diffraction peak arising from the repeat distance along the stacking direction of clay layer silicates (the c-axis or 001 reflection). Its d-spacing is the primary diagnostic feature for distinguishing clay mineral groups.
- Also called
- 001 reflection, c-axis reflection
- Origin
- Repeat distance along the layer-stacking direction
- Diagnostic use
- Distinguishing clay mineral groups by d-spacing
- Technique
- X-ray diffraction
Common questions
Why is the basal reflection the first thing examiners check in clay XRD?+
Its d-spacing is directly tied to the thickness of the repeating clay layer unit, which differs consistently between groups such as kaolinite, illite, and smectite. Checking it first quickly narrows the mineral group before finer peaks are interpreted.
Can the basal reflection shift, and does that complicate identification?+
Yes, some clay groups such as smectites expand or contract their basal spacing depending on humidity and interlayer cations, so the same mineral can show a different d-spacing under different conditions. Examiners often run a glycolation or heat treatment to see how the peak shifts, since that behaviour itself is diagnostic.
Related terms
- Bragg's Law
- nλ = 2d sin(θ). A monochromatic X-ray beam diffracted off the parallel lattice planes of a crystal produces constructive interference only at...
- D-Spacing
- The perpendicular distance between adjacent parallel planes of atoms in a crystal lattice, measured in angstroms or nanometres. Each set of planes...
- Mixed-Layer Clay
- A clay mineral in which two different layer types (for example smectite and illite layers) are interstratified in a regular or random...
- Oriented Mount
- A clay sample preparation in which fine clay particles are settled onto a glass slide so that their flat basal planes align...
- Rietveld Refinement
- A full-pattern fitting method for XRD quantification that models the entire diffractogram using crystal structure data for each phase, adjusting phase abundances...