Matt Nowell, EBSD Product Manager, EDAX
Generally, when we visit a beach, we try to bring home a shell or a piece of driftwood. However, when I was putting on my shoes for the first time, I noticed some sand was still present. My last beach trip had been to the Cayman Islands. I immediately noticed that this sand looked much different than the sand at Sunset Beach. I decided to save a little bit of each for some microscopy and microanalysis when I got back home.
When I looked at them both more closely, I saw that the sand from Sunset Beach (SB) on the left was much darker with black flecks, while the sand from Grand Cayman (GC) was much lighter. Thinking about the possible composition of the sand got me thinking about the bladesmithing competition held at the TMS annual meetings. One year, the team from UC Berkeley created a sword using magnetite found at local beaches using magnets. I thought it would be interesting to examine both of these sands with my SEM, EDS, and EBSD tools.
Initially I placed a bit of sand on an aluminum stub for SEM and EDS analysis. To reduce charging effects, I used the Low Vacuum capability of our FEI Teneo FEG-SEM, running at 0.1 mbar pressure. Images were collected using the Annular BackScatter (ABS) detector for atomic number contrast imaging. The sand grains from Sunset Beach were generally a little smaller than the Grand Cayman sand, as expected from visual inspection. Both sands exhibited cracking and weathering, which isn’t surprising in hindsight either. Many grains show flat surfaces, with internal structure visible with ABS imaging contrast.
I followed the imaging work with compositional analysis using EDS. The Sunset Beach sand was primarily composed of silicon and oxygen grains, which I suspect is quartz. The single brighter grain in Figure 3 was composed of an iron-titanium oxide. The Grand Cayman sand was primarily a calcium carbonate (Ca-C-O) material. The more needle shaped grains were primarily sodium and chlorine, which I assume is then salt that has solidified during the evaporation of the water. All this leads me to believe I really didn’t do a good job of cleaning my shoes after Grand Cayman.
While quartz being present in sand wasn’t surprising to me, the observation of calcium carbonate did remind me of some geological homework I did on the island. The water in Grand Cayman was very clear, which made it great for snorkeling. We swam around and saw a coral reef, a sunken ship, lots of fish, and stingrays. To understand why the water was so clear, I read that it was the lack of topsoil, and the erosion and runoff of topsail to the water that was responsible for the clarity. Looking again at this reference, it mentions that the top layer of the island is primarily composed of carbonates. The erosion of this material would explain the primary composition of the beach sand in my shoes.
Of course, the next step now is analyzing these sands with EBSD to determine the crystal structure of the materials. I’ve started the process. I’ve mounted some of the sand in epoxy, and hand polished to get some flat surfaces for analysis. I’m able to get EBSD patterns, but getting a good background is going to be tricky. I think the next step will be to watch my colleague Shawn Wallace’s webinar on Optimizing Backgrounds on MultiPhase samples to be presented on September 27th. You can also register for this here.
In the meantime, I’ll keep the sand samples on my desk to remind me of summer as the colder Utah winters will be approaching. It will be a good reason to stay inside and write the next chapter of this analysis for another blog post.