The mineralogists who work at the museum have been practising mineral identification by eye for many years, but most of the 5,636 known mineral species won’t give us their names that easily. There are two techniques using X-rays that can be employed to give our eyes some help.
Simply put, a mineral is a solid with a long range crystal structure (the way the elements are arranged) and a fixed chemical composition (the ratios of these elements). We can use both of these characteristics for identification.
We can determine the chemical composition of a mineral by collecting X-rays while firing a beam of electrons at it. If we arrange the X-rays by wavelength, we get a spectrum showing which elements are there and, based on the height of the peaks, how much of each there is.
The columbite-tantalite group minerals all look very similar; it is impossible to identify them without knowing their chemistry. There were drawers full of unnamed columbite-tantalite mineral specimens in the museum’s collection. By taking tiny samples—often smaller than a tenth of a millimetre—and analysing their chemistry, we now know the proper names for these minerals for the first time since they were acquired by the museum!
The second technique mineralogists use to identify minerals is X-ray diffraction, also known as XRD. If we hit a sample with a beam of X-rays and record how they are affected by the crystal structure, we can get a “fingerprint” of the mineral.
If we use a database to compare “fingerprints” to other examples, we can identify visually indistinguishable specimens, such as serandite and schizolite.
Knowing the proper names of minerals is important for any museum collection. Chemical analysis and X-ray diffraction are two important ways mineralogists keep our own collection in order and make sure every specimen is properly identified.