X-ray Diffraction in Mineralogy

X-ray diffraction is the primary method for identifying minerals and determining their crystal structures from the angles and intensities of scattered X-rays.

Definition

The application of X-ray diffraction techniques to identify minerals, measure their unit-cell dimensions, and determine the three-dimensional arrangement of their atoms.

Scope

This topic covers the physics of X-ray scattering by lattice planes, Bragg's law, the distinction between single-crystal and powder methods, the use of d-spacings and reference databases for phase identification, unit-cell refinement, and specialized clay-mineral analysis. It is the bridge between crystallographic theory and practical mineral characterization.

Core questions

• How does Bragg's law relate diffraction angle to interplanar spacing?
• How is a mineral identified from its powder diffraction pattern?
• What is the difference between single-crystal and powder diffraction analysis?
• How are expandable clay minerals distinguished by glycolation and heating?

Key theories

Bragg's law

Diffraction maxima occur when nlambda equals 2d sin(theta), so measuring the angles of scattered X-rays yields the interplanar spacings that fingerprint a mineral and constrain its unit cell.

Powder diffraction fingerprinting

Each crystalline mineral produces a characteristic set of d-spacings and relative intensities; matching a measured pattern against reference databases provides unambiguous phase identification even in fine-grained mixtures.

Clinical relevance

X-ray diffraction is indispensable for identifying fine-grained or intergrown minerals, characterizing clays and ore phases, quantifying mineral proportions in rocks, and is a routine analytical tool in geology, soil science, and materials industries.

History

Von Laue's 1912 discovery that crystals diffract X-rays, followed immediately by the Braggs' formulation of the reflection law and determination of simple structures, founded structural mineralogy. The powder method developed by Debye, Scherrer, and Hull in 1916-1917 made routine mineral identification possible.

Key figures

• William Lawrence Bragg
• William Henry Bragg
• Max von Laue
• Peter Debye

Seminal works

• bragg1913
• klein2007
• cullity2001

Frequently asked questions

What is a d-spacing?

The perpendicular distance between adjacent parallel planes of atoms in a crystal; each set of planes gives a diffraction peak whose position depends on this spacing through Bragg's law.

Can X-ray diffraction identify amorphous materials?

Not directly, because diffraction relies on long-range periodic order; amorphous phases such as volcanic glass produce only broad humps rather than sharp peaks.