Mixed-Layer Clay
Definition
A clay mineral in which two different layer types (for example smectite and illite layers) are interstratified in a regular or random sequence along the c-axis. The interlayering produces broad, shifted d-spacings that are diagnostic of specific weathering or burial conditions.
- Structure
- Two or more layer types interstratified along the c-axis
- Example pairing
- Smectite and illite layers
- Diagnostic feature
- Broad, shifted d-spacings in XRD
- Indicates
- Specific weathering or burial conditions
Common questions
Why does mixed-layer clay produce broad, shifted XRD peaks instead of sharp ones?+
A regular pure clay mineral has a consistent repeat spacing between layers, producing a sharp diffraction peak at a defined angle. When two different layer types interstratify irregularly, the repeat distance varies across the crystal, spreading the diffracted intensity over a range of angles and shifting the apparent peak position.
How can identifying a mixed-layer clay help link a soil sample to a specific location?+
The specific ratio and ordering of layer types in a mixed-layer clay reflects the particular weathering history, parent material, and burial environment at a location, so matching this signature between a questioned soil sample and a control sample from a scene can support a geographic association.
Is glycolation or heating typically needed to properly characterise a mixed-layer clay by XRD?+
Yes, standard practice for expandable clay identification, including mixed-layer types involving smectite, involves ethylene glycol solvation and sometimes controlled heating to distinguish expandable from non-expandable layers, since the peaks of these clays shift diagnostically under these treatments in ways non-expandable clays do not.
Related terms
- Basal Reflection
- The diffraction peak arising from the repeat distance along the stacking direction of clay layer silicates (the c-axis or 001 reflection). Its...
- 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...
- 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...