Identifying Minerals by X-Ray Diffraction

Identifying a mineral can be intellectually stimulating; it’s a good exercise for one to try to identify a mineral one self. However one should realize that sometimes nature poses problems that require the help of instrumentation.

Mineral textbooks and the internet can help in the ID of minerals. These resources show how characteristics such as appearance, hardness, crystal morphology, cleavage and specific gravity can help find out what a mineral is. However with there being more than 4800 known minerals it is understandable that these resources may be insufficient in which case the only option is to send that mineral to a laboratory with special instruments. The best method for mineral identification used by laboratories is X-Ray Diffraction (XRD).

In XRD a small sample of the mineral the size of a grain of rice or smaller, is sent to a laboratory. The mineral is powdered and placed on a special plate that resembles a microscope slide. It is then put into a machine as big as a refrigerator. A beam of x- rays hits the sample at a very slowly increasing angle while an electronic detector rotates around it and records the x-rays bouncing off of the sample. This process produces an x-ray diffraction pattern which appears on a computer screen as a series of peaks of different height. These peaks correlate mathematically with atomic spacings between planes of atoms in the mineral crystal and are a direct result of the crystalline structure. The computer stores this pattern.

Over the last 70 years geologists and mineralogists have recorded the patterns of all known minerals. Before computers one used to look up lines on chart paper against figures in reference books. The process could take days to match the main lines if one had no idea what one was dealing with. The age of computers has allowed for the creation of databases which store all of these mineral patterns making mineral identification much easier.

As may now be apparent after a pattern has been established by x-ray diffraction as mentioned the computer then sifts through all of the patterns in the database to find the correct match to that of the sample. One might think that the mineral has now been correctly identified and that no further work is required but that is not the case. While computers are wonderful tools human intelligence is still needed as two samples may have similar structures and the operator will have to deduce which structure is the best match by performing physical or chemical tests to distinguish one mineral from another.

Once the correct mineral pattern has been determined from x-ray diffraction a report is generated as a computer file which is then emailed to the customer in an easy to read format. Because expensive instrumentation and a trained individual are involved in the process there is a cost for analysis. XRD is the method of choice with professionals and serious mineral collectors. It is based on a scientifically rigorous method of probing the crystalline structure with accurate x-ray measurement of the distances between atomic planes of a mineral.

A customer does not need to understand the physics behind XRD or the complex mathematics; all they should know is that the results are unparalleled for reliability and ease of being understood. It is good practice for a person interested in using XRD for the identification of their minerals to do some research and find out in advance how long it takes for different labs to provide clients with results and how much they charge. Sometimes collectors use free non-commercial XRD facilities at universities or museums but this may not be the best idea. These institutions are sometimes overwhelmed with requests and often take months to send results. They may not even do the work at all if it does not fall within the research interests of the department. A person interested in the service may be better off going with a private company that offers much faster service.