Monday, 28 April 2014

X-rays and minerals

This week I have finished my analysis of samples from the core using x-ray diffraction (XRD). Basically, I have been zapping rocks with X-rays, fantastic! XRD is used to find out what minerals are present in a sample. I am interested in samples that may contain calcite or dolomite, because this can influence the organic carbon isotope work we are doing. I am also looking at the mineralogy of some interesting fossil-bearing units, to further understand how they formed.
How does it work? A rock sample is ground to a powder and prepared in a sample holder ready to be analysed. The sample is mounted in the machine, which uses an automated system to move the samples around and analyse them – great to watch! But it is so you can remain safely protected from X-rays. X-rays are fired at the flat surface of the sample. Those x-rays then bounce off the crystal lattices present within the mineral fragments, and the angle of the returning x-ray beam (theta) is recorded. The angle is different for different minerals because they have variable thickness of the planes within the crystal lattice, which is controlled by the minerals chemistry and structure. The results are presented as a graph of intensity versus diffracted angle, which gives peaks that are distinct for individual minerals. In essence, it’s a relatively quick and concise way to find out the mineral composition of a material (geological or otherwise).

Samples prepared in their holder and ready to be analysed
The XRD machine            

Each mineral phase has three peaks at different angles that represent its crystal structure. For example, muscovite, a silicate mineral, with a platy crystal shape has a different signature to pyrite, an iron-sulphide, with a cubic crystal shape. The data shown below is from a sandstone sample that is composed of quartz, micas, dolomite, feldspars and clay minerals.

The data from a sandstone sample, with mineral peaks marked in red

In other news, this coming weekend (2nd to 4th May 2014) I will be at The Lyme Regis Fossil Festival, helping on the Microfossil Discovery stall run by The MicropalaeontologicalSociety. Do come along for a great day out of science fun!

Until next time