X-Ray Diffraction Analysis (XRD) is a powerful method used to determine which crystalline substances are present in a sample — in other words, to identify its mineral phase composition.
The method also enables quantitative analysis of the overall mineral composition of the sample — that is, it not only identifies which minerals are present (qualitative analysis), but also determines the proportion of each mineral in the sample.
The method also enables quantitative analysis of the overall mineral composition of the sample — that is, it not only identifies which minerals are present (qualitative analysis), but also determines the proportion of each mineral in the sample.
Theoretical Basis of the Method
One of the defining features of minerals is their crystalline lattice — an orderly internal structure. The type of lattice affects a mineral's properties: strength, shape, hardness, conductivity, and more.
This lattice structure repeats at regular intervals, called interplanar spacings.
XRD identifies substances in a sample by recognizing their unique set of interplanar spacings and the relative intensity of peaks on an X-ray diffraction pattern.
When X-rays hit a crystalline substance, they reflect off atomic planes and interfere with one another, creating a distinct diffraction pattern — essentially, a "fingerprint" of the crystal structure.
It's not just about what elements or compounds are in the material, but what form they take. For example, calcium and carbon dioxide can combine to form the mineral calcite — that’s one phase. But they can also form a completely different mineral, dolomite. Although the chemical makeup may be similar, the crystal structures differ, and this is exactly what XRD detects.
Dolomite has ordered inter-bedding of Ca and Mg atoms layers
How XRD Works in Practice
Let’s say we have a rock sample from a gold deposit. At first glance, it's just a grayish-green stone — but geologists suspect it might contain gold associated with sulfide minerals.
To confirm or reject this hypothesis, they turn to XRD.
Let’s say we have a rock sample from a gold deposit. At first glance, it's just a grayish-green stone — but geologists suspect it might contain gold associated with sulfide minerals.
To confirm or reject this hypothesis, they turn to XRD.
Stages of XRD
1. Sample Preparation
The rock is ground into a fine powder. The more uniform and fine the particles, the clearer the diffraction pattern. However, overgrinding can be counterproductive: particles smaller than 0.1 μm may become X-ray amorphous and not produce a clear pattern. This often happens with soft minerals with perfect cleavage, such as galena, which breaks into tiny cubes.
1. Sample Preparation
The rock is ground into a fine powder. The more uniform and fine the particles, the clearer the diffraction pattern. However, overgrinding can be counterproductive: particles smaller than 0.1 μm may become X-ray amorphous and not produce a clear pattern. This often happens with soft minerals with perfect cleavage, such as galena, which breaks into tiny cubes.
2. Diffraction Measurement
The powdered sample is placed in a diffractometer, which directs X-rays onto it. The rays scatter and reflect off atomic planes within the crystals, producing a pattern of peaks.
3. Database Matching
The resulting spectrum is compared to reference patterns in a database to determine which mineral phases are present. In a gold ore sample, one might find quartz (the main rock-forming mineral), pyrite, arsenopyrite, and sometimes, depending on deposit type: chlorite, sericite, carbonates, orthoclase, jarosite, barite.
4. Interpretation
By identifying the mineral makeup of ore and surrounding rocks, geologists can assess the deposit's potential and decide whether further exploration is worthwhile.
5. Quality check and corrections
Second mineralogical method for phase determination and chemical composition analysis of the sample is used to prove accuracy of the phase identification and quantification.
Can XRD detect gold?
XRD can detect gold minerals directly in relatively high concentrations. Usually its concentration is too low to produce clear peaks in sample without upgrading its concentration.
XRD is excellent at revealing indicator minerals and carriers of gold, such as:
Can XRD detect rare and rare-earth metals?
As with gold, XRD can directly detect minerals of rare metals. Sometimes RE metals tend to be present in trace amounts in other minerals or form accessory or amorphous phases, that is when we need to concentrate them and use micro-analysis to measure RE as impurities in minerals.
However, the presence of certain mineral indicators can hint at them:
Rare metals: tantalum, niobium, zirconium, hafnium, germanium, indium
Rare-earth elements (REEs): lanthanum, cerium, neodymium, ytterbium, etc.
Typical indicator minerals include:
XRD can detect gold minerals directly in relatively high concentrations. Usually its concentration is too low to produce clear peaks in sample without upgrading its concentration.
XRD is excellent at revealing indicator minerals and carriers of gold, such as:
- Pyrite (FeS₂)
- Arsenopyrite (FeAsS)
- Chalcopyrite (CuFeS₂)
- Pyrrhotite (Fe₁₋ₓS)
- Quartz, sericite, chlorite — signs of hydrothermal processes linked to ore formation
Can XRD detect rare and rare-earth metals?
As with gold, XRD can directly detect minerals of rare metals. Sometimes RE metals tend to be present in trace amounts in other minerals or form accessory or amorphous phases, that is when we need to concentrate them and use micro-analysis to measure RE as impurities in minerals.
However, the presence of certain mineral indicators can hint at them:
Rare metals: tantalum, niobium, zirconium, hafnium, germanium, indium
Rare-earth elements (REEs): lanthanum, cerium, neodymium, ytterbium, etc.
Typical indicator minerals include:
- Monazite ((Ce,La,Nd,Th)PO₄)
- Bastnäsite ((Ce,La)(CO₃)F)
- Eucryptite (LiAlSiO₄)
- Columbite, tantalite, xenotime, loparite — depending on the deposit type
- Zircon (ZrSiO₄) — may contain uranium and thorium
Important Note
Since XRD only detects crystalline materials, it cannot identify amorphous phases or provide a full chemical composition of a sample.
In complex or heterogeneous systems, it’s common to perform chemical analysis first, to determine the overall composition. This helps with accurate interpretation of diffraction patterns and prevents mistakes in phase identification.
Since XRD only detects crystalline materials, it cannot identify amorphous phases or provide a full chemical composition of a sample.
In complex or heterogeneous systems, it’s common to perform chemical analysis first, to determine the overall composition. This helps with accurate interpretation of diffraction patterns and prevents mistakes in phase identification.