In which I try to explain X-Ray Diffraction (XRD), as I understand it. Or, Look! I’m doing real science! This is what I’ve been focusing on the last few weeks, and therefore haven’t got the energy to come up with something else to talk about. Our XRD machine looks like this:
XRD is a tool used to determine the identity of minerals in a sample (of sandstone, for example) by way of bombarding a sample with X-Rays to determine the crystalline materials (in my case, minerals) present. Due to the mineral’s distinct crystal structure, the X-rays (not light) will diffract, or bounce off, of the different minerals at different angles, producing a diffractogram. This pattern, which is usually a mix of a bunch of different minerals in one rock, is recorded with XRD software, and then the composition of the sample can be interpreted from the diffractogram (there’s a photo of this below).
But before you can do all that, you have to prepare the sample. You don’t need much, about 3 grams will do.
Then you have to put the sample through a ball mill to CRUSH it into POWDER! The ball mill is basically just a zirconia vial with two little zirconia balls inside. You put the sample inside with the balls, strap it into this contraption:
close the lid, and turn it on. The whole thing will rattle around inside for a few minutes and then your sample is powder. The actual crushing of the rock only takes a few minutes, but the cleaning process (which is basically the same, but instead of putting your rock sample in the vial, you put some boring silica sand inside) takes just as long if not longer. Plus, once it’s clean and you have to move on to the next sample, you must “season” the vial with very tiny pieces of the next sample. Luckily you can just do other stuff while the machine is running, but it’s still quite time consuming.
At any rate, once your sample is crushed into powder, you have to very, VERY carefully put it in a sample holder for the XRD so that the powder has a perfectly flat surface.
See? Look at that beautiful smooth surface! I did that. All by myself. Apologies for not having a photo of the tools I used to make this happen, but there were several. And if this surface has even one crack in it? You have to start over. Luckily you can reuse the sample over and over again, so it’s not destructive, just extremely time-consuming.
So once all the samples are in their holders, they go in this fancy rack thing.
The rack gets locked into the XRD machine (see above, it goes right inside the door), and then you program it to run. The machine does it’s thing for a while, and the data is collected. The resulting diffractogram looks something like this:
The peaks correspond to characteristic patterns of different minerals. Determining which they are require a little knowledge about the rock sample or an educated guess about what the minerals might be. This is the part I haven’t learned yet, but it boils down to a lot of trial and error.
There you have it, my quick and dirty summary of what X-Ray Diffraction is, and how you prepare samples for analysis. XRD machines do a lot more than what I’ve described here, but this is what I’m using it for. It’s been fun, but a little exhausting. So with that, I’m going to bed.
** Thank you to my fellow grad student for correcting my errors in this post – you know who you are! :) **