1/12/21 9:52 AM | 5 Min Read

Mineral and Sample Analysis using XRF and XRD

Posted By
Carl Sorrell
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Mineral and Sample Analysis using XRF and XRD

Sometimes Half a Loaf is Worse than No Loaf

IntoCeramics is highly experienced in the characterization of minerals and waste streams for use in ceramics. We analyze hundreds of samples in our labs and in partnership with Clemson University’s Bishop Materials Research Center.

When considering minerals for use in a formulation, it is not enough to just know the elements that are present. Carbon black and diamond are chemically identical, but nobody would mistake one for the other. It is the difference in crystal structure that determines whether elemental Carbon (C) will go on a finger or end up in ink.

More commonly, sand is chemically identical to fused silica (SiO2) but naturally occurring quartz and man-made fused silica have vastly different physical and thermal properties. And fused silica is one heck of a lot more expensive that sand.

Yet, in both examples, a chemical analysis would show SiO2 or C. And that’s all it would show.

 

The First Half of the Loaf: X-Ray Fluorescence Chemistry

Led Zeppelin posterX-ray fluorescence (XRF) is one of the most common chemical analysis methods, especially for inorganic solids.

Just as your wall poster glows (fluoresces) when you turn on a blacklight, so do atoms when they are bombarded with X-rays.

Each element emits X-rays at a unique wavelength, allowing for highly accurate determination of chemical composition be interpretation of the emission spectrum and relative intensities at different wavelengths.

The most accurate X-ray procedure uses the fusion method. A ground powder sample is fused at 1,000°C with low melting point lithium borate to produce a homogenous sample.

Fusion removes variation in XRF data caused by particle size and the packing differences that occur when analyzing pressed powder samples.

XRF by fusion is quantitative and IntoCeramics uses this method almost exclusively.
The only downside is that samples which may contain naturally occurring lithium or boron must be analyzed in a different way.

You may ask: what happens if we don’t know that the sample might contain lithium or boron? We’ll get to that in just bit.

XRF data are expressed as weight percentages of oxides - Na2O, Al2O3, SiO2, and so forth. This is very useful for calculations related to ceramics where metal oxides and mixed metal oxides are our stock in trade.

But let’s say you have mineral sample that contains silica, alumina, soda, and potassia. You know that these are unlikely to occur as discrete oxides in a mineral sample - it’s clay! Or, at least, that’s what it looks like.

This brings us to the second half of the loaf.

 

The Second Half of the Loaf: X-Ray Diffraction Mineralogy

We learn as kids that white light is made up of multiple wavelengths of visible light, from red to violet. We enjoy playing with glass prisms that split the light into a rainbow of colors.

Prism splitting light

Over one hundred years ago, Max von Laue discovered that the distinctive mineral structure spacing within crystals acts like a prism (or, more correctly, a diffraction grating) to X-rays.

This discovery won him the 1914 Nobel prize in physics - and revolutionized mineralogy. Each crystal produces a unique spectrum and can thus be identified.

The sample is first ground to a powder and then bombarded with X-rays while being rotated. A pattern of peaks is produced that identify the mineral composition.

XRD spectrum

It sounds simple, but a hundred years later, X-ray diffraction mineralogy is one of the measurement techniques that requires a high degree of interpretive skill.

In a mixed mineral sample – the most common situation – peaks may overlap, obscuring each other and making interpretation difficult. For this reason, XRD is considered semi-quantitative.

The degree of error can be influenced by

  • How well the minerals are crystallized (e.g., carbon black is poorly crystallized while diamond is highly crystalized)
  • The degree of background amorphous (glass) in the material
  • The similarity of peak patterns in many minerals.

Dr. Charles Sorrell at the University of New South Wales told us the story of a very excited undergraduate student in his crystallography class who reported a clear indication of the mineral oxygen in his pattern. Indeed, solid oxygen at room temperature would have been one heck of a find!

 

Baking the Loaf

Using XRD, we can identify the individual minerals, and with XRF we know the individual oxides. Coupled with the weight loss (LOI) determined during XRF fusion, this lets us accurately characterize the sample using molecular weights and a rational analysis.

In answer to our earlier question, detection of lithium and boron minerals by XRD will alert us that the XRF analysis should be run using a method other than fusion.

Putting all these methods together completes the bread loaf and readies the IntoCeramics team to develop the minerals – and the waste stream from which they are sampled - into a useful product.

The mineral composition is critical to determining what forming, firing, and finishing methods might be needed to make that valuable product.

In a future blog, we will discuss other physical analyses and processing analysis techniques including porosimetry, differential thermal analysis, and strength measurement.

 

Topics: Technology, Analysis

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