Salt of the Earth….or at least the sea

Last week I posted about testing milkshakes or salt in the X-Ray Diffraction machine here in the geology dept. Depite the allure of milkshakes, I went with salt just because it was so much easier.

Salt, and by that I mean table salt, is just NaCl. Sodium Choloride. Halite. The salt I tested was sea salt which was created from the evaporation of sea water. The stuff I purchased was made in France on the shore of the Mediterranean Sea. I fully expected the vast majority of what was in the salt to be halite, but I also assumed there were to be some other bits of stuff in it as well. The dissolved components of sea water are mostly sodium and chlorine, but there are lots of other trace things as well (Mg, SO-, Ca, K, etc). As the water evaported, I assumed other minerals would grow in the evaporate, and was what I wanted to determine.

I started with some very coarse sea salt. I had to choose coarse sea salt, because the finer salt has an additive to prevent caking. The additive might have screwed up the reading, so I got the coarse grain salt and ground it up with a mortar and pestle.

The powder is then placed on a special slide with an indentation on it to hold the powder.

Making the powder is important because our XRD machine is a powder XRD not a crystal XRD. With powder XRD is much easier to prepare samples.

The actual XRD unit itself is pretty small, considering how big they used to be:

The slide with the powder is placed in the machine. The machine then shoots a beam of X-Rays at the powder (created from copper) and rotates the slide with respect to the beam over a set angle (in my case 2θ was from 15 to 60….its 2θ, not θ. See Bragg equation.)

The end result, is a series of measurements taken at different angles which measure the intensity of the diffration of the sample. I ended up with a graph that looked like this:

What is interesting about this a) there were no other trace minerals, and b) the peaks were uniformly off from the peaks which were expected from Halite. The current guess is that the other elements or at least some of them, rather than forming new minerals were substituted into the Halite. This increased the spacing in the crystal lattice and increased uniformly the angle at which diffraction occured.

The problem is, if that is the case, that should be the case with most all Halite which is almost always created from evaporating sea water.

I might run some more tests on other samples to try and find out what the deal is.