X-ray diffraction and fluorescence are powerful tools for mineral identification . XRD reveals crystal structures by analyzing how X-rays scatter off atomic planes, while XRF measures elemental composition through characteristic X-ray emissions.
These techniques provide complementary information about minerals. XRD identifies specific mineral phases and polymorphs, while XRF quantifies elemental content. Together, they offer a comprehensive approach to mineral characterization in various geological settings.
X-ray Diffraction for Mineral Identification
Principles of X-ray Diffraction
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X-ray diffraction results from X-rays interacting with periodic atomic structures in crystalline materials, producing constructive interference patterns
Bragg's Law (n λ = 2 d s i n θ nλ = 2d sinθ nλ = 2 d s in θ ) describes diffraction conditions, relating X-ray wavelength to interplanar spacing and diffraction angle
XRD employs monochromatic X-rays generated by sources (copper or molybdenum) to probe mineral crystal structures
Diffraction patterns serve as unique "fingerprints" for mineral identification
XRD provides information on crystal symmetry, unit cell parameters, and atomic positions within crystal lattices
Powder XRD analyzes finely ground samples, identifying multiple mineral phases in mixtures
Applications in Mineralogy
Phase identification determines specific minerals present in a sample
Quantitative analysis measures relative abundances of minerals in mixtures
Crystal structure refinements reveal detailed atomic arrangements
Unit cell parameter determination calculates dimensions of crystal lattices
Crystallinity assessment evaluates the degree of structural order in minerals
Strain analysis measures deformation in crystal structures
Texture analysis examines preferred orientation of crystallites in samples
Interpreting XRD Patterns
XRD Pattern Components
Diffraction peaks represent X-rays plotted against diffraction angle (2θ)
Peak positions correspond to specific lattice planes in crystal structures
Peak intensities relate to atomic scattering factors and unit cell atom arrangements
Background signal results from factors (sample holder, amorphous content)
Peak width influenced by crystallite size and instrumental factors
d-spacings calculated from peak positions using Bragg's Law
Example: A peak at 2θ = 26.6° for Cu Kα radiation corresponds to d = 3.34 Å (characteristic of quartz)
International Centre for Diffraction Data (ICDD) maintains comprehensive XRD pattern database for minerals and materials
Pattern Analysis Techniques
Indexing assigns Miller indices (hkl) to peaks, relating them to crystal planes
Example: For cubic crystals, (100), (110), and (111) are common low-index planes
Determines crystal system and lattice parameters from peak positions and intensities
Rietveld refinement extracts detailed structural information from XRD patterns
Refines atomic positions, occupancies, and thermal parameters
Accounts for factors (preferred orientation, absorption effects)
Considers factors influencing patterns during interpretation
Preferred orientation affects relative peak intensities
Particle size effects broaden diffraction peaks
Strain in crystal lattices causes peak shifting and broadening
X-ray Fluorescence for Elemental Analysis
Principles of X-ray Fluorescence
X-ray fluorescence occurs when atoms excited by high-energy X-rays emit characteristic X-rays
Inner-shell electrons ejected, outer-shell electrons fill vacancies, emitting element-specific X-rays
Emitted X-ray energies enable qualitative identification of elements in samples
XRF performs qualitative and quantitative elemental analysis
Detection limits typically in parts per million (ppm) range for many elements
Energy-dispersive XRF (ED-XRF) uses semiconductor detectors to measure X-ray energies directly
Wavelength-dispersive XRF (WD-XRF) employs crystal diffraction to separate X-rays by wavelength
WD-XRF offers higher spectral resolution but lower sensitivity than ED-XRF
XRF Analysis Considerations
Matrix effects influence XRF measurements
Absorption reduces intensity of characteristic X-rays
Enhancement occurs when one element's fluorescence excites another
XRF provides non-destructive analysis with minimal sample preparation
Rapid elemental analysis ideal for minerals and rocks
Limitations include difficulty detecting light elements (atomic number < 11)
Potential interferences from overlapping spectral lines require careful interpretation
Quantitative analysis requires calibration using standard reference materials
Sample homogeneity crucial for accurate results in bulk analysis
Characterizing Minerals with XRD and XRF
Sample Preparation Techniques
Proper sample preparation crucial for accurate XRD and XRF analysis
XRD sample preparation methods
Powder preparation involves grinding samples to fine, uniform particle size
Oriented mounts used for clay mineral analysis
Single-crystal XRD requires selection of high-quality crystals
XRF sample preparation techniques
Pressed pellets for analyzing major and trace elements in rocks
Fused beads eliminate particle size effects and reduce matrix effects
Loose powder analysis for non-destructive testing of small samples
Integrated Analysis Approaches
Combining XRD and XRF provides complementary structural and compositional information
XRD identifies mineral phases and polymorphs
Example: Distinguishing calcite (CaCO3) from aragonite (CaCO3) based on crystal structure
XRF quantifies elemental data for chemical formulae and compositional variations
Example: Determining Fe/Mg ratio in olivine ((Mg,Fe)2SiO4) solid solution series
Trace element analysis by XRF fingerprints mineral deposits
Example: Using rare earth element patterns to identify specific pegmatite sources
Advanced XRD techniques provide insights into mineral behavior
In-situ high-temperature studies examine phase transitions
High-pressure experiments simulate deep Earth conditions
Micro-XRF and micro-XRD enable spatially resolved analysis of heterogeneous samples
Study mineral zoning in metamorphic rocks
Analyze complex intergrowths in ore deposits
Data integration requires specialized software and mineralogical databases
Example: Rietveld analysis software for quantitative phase analysis
Mineral identification databases (e.g., RRUFF project) for pattern matching