Birthday blogging

I am currently enjoying the final month of my contract—this means trying to accomplish all of the "last things" that have to happen before this research becomes a published paper in between packing my things to go into storage, purchasing tickets for some post-contract travel, and applying for jobs so that I have somewhere to go when the travel phase ends. Needless to say, I haven't been making much time to post blogs, but since I've always taken a random approach to the timing of them anyway, perhaps no one will notice.

Today I get to celebrate my birthday week by playing with expensive toys—more time on the electron microprobe to generate yet more data. I am looking forward to it—there is much fun to be had looking at minerals in that much detail. At this point in my project I've got the routines down to convert the data into useable format, so should be able to have it incorporated with the preceding sessions before the weekend is over. Working on the weekend? You betcha—best way to assure that I am actually done (enough) with my research when the contract ends.

Extrapolation of the likely composition of a mineral from mixed analyses

I have mentioned before the difficulties of using a microprobe to analyze very small phases. The electron beam with which we do the analysis can, with care, be focused to about 1 micron diameter (remember that there are 1,000 microns in every millimeter). However, should the mineral phase of interest be smaller than one micron, in any dimension, the analysis will yield the composition of not just that mineral, but of whatever happens to be next to it as well.

The below photo shows one of my experiments for which this was a problem. As with all back-scatter electron images the amount of brightness or darkness of any given part of the image is based on the composition of the sample in that location. Brighter areas contain more heavy elements, darker areas more light elements. The brightest grains in this image are the large pale grey crystals, which often have dark centers; these are garnets. The dark centers are the pyrope (Mg-garnet) seeds that were included in the experimental powder to give the new garnet, which is much higher in Fe (iron), a place to start growing from. The narrow stick-shaped crystals which occur in a group on the left hand side of the image are chloritoid. Unfortunately, as you can see by the scale bar on the bottom of the image, they are too narrow to obtain a good analysis. Through careful searching of the sample we found a few places where the chloritoid grains were slightly larger than the others—these were the ones we analyzed, in hopes that we would be lucky. Alas, 15 times we tried, and 15 times we failed to obtain an analysis which was only chloritoid, but instead they were "mixed" results of both chloritoid plus another phase.

How do I know for certain that they are mixed? Look at the below graphs and you can see for yourself. The upper graph shows the composition of all of those mixed analyses with respect to how much aluminum and how much silica they contain (blue-green hollow triangles). It also shows the region (grey circle) within which all of the matrix mica in this sample plots, and the location of where kyanite (Al₂SiO₅), also present in this sample, plots. As you can see, there is a clear trend going from the solid green triangle towards the mica, and another trend going from the solid green triangle towards the location of kyanite. The lower graph shows the trends for iron vs aluminum. By plotting this data for a variety of different combinations of elements I have come up with my best guess as to the composition of chloritoid is in this sample (solid bluish-green triangles). Is it as accurate as if I'd been able to get a good measurement? No. Does it give me information I can use when doing other parts of my data analysis? Yes, yes it does. Playing with graphs is one of the fun parts of my job—the information they convey communicates so very clearly.

water is important for growth, even for minerals

One of the joys about the learning to run experiments process is that one gets to learn the results of both intended and unintended phenomena. In my experiments the intention is to seal powder of known composition into gold capsules along with a sufficient H₂O and graphite to ensure that the chemical reactions which take place when we elevate the pressure and temperature (to simulate what happens to rocks buried at great depth) take place in “water-saturated” conditions (which is to say there is enough water available for the growth of minerals which require water as part of their chemical formula, such as the micas). However, learning to weld the capsules is a difficult process (I’ve got a draft post on that topic just waiting for me to get photos that actually display the features I want them to show).

As a result of my welding trials and tribulations I’ve had mixed success in the “sealing” part of the above paragraph. Despite the issues with my first attempts at sealing, we ran my first experiment nonetheless, giving two samples a week and a half at elevated pressure/temperature (in this case 650 C and 25 kbars). Once they were “cooked” we had the gold capsules mounted into small disks of epoxy, then carefully polished the disks until the insides of the capsules were exposed. During the polishing stage we received our first confirmation that they had not achieved the same level of “sealed”. Apparently when properly sealed the presence of water inside the capsules ensures that the pore space in between the grains of powder are occupied, and as a result even the high pressures to which we subject them aren’t enough for the new minerals to properly interlock when they grow. As a result, while there are new crystals present, the texture isn’t very rock-like, and when polishing it is easy to accidentally remove clumps of the sample itself. This is the texture we were anticipating, and, for one of the samples run in the first experiment, this is exactly what happened. In these cases we polish only enough to just expose the inside of the capsule, then add more epoxy, letting it soak down into those pore spaces and let it dry before completing the polishing process without so much risk in losing what we are trying to polish.

However, in the other of the two samples run in the first experiment I must not have done the final welding properly, because the contents of the capsule were much harder, and held together better, meaning that the pore space was not held open with fluid when the minerals were growing. This was obvious during the polishing process, so I was able to go quite a bit deeper into the capsule (remember these are only 2 mm in diameter and about 5 mm long so “deeper” is only a relative term) before needing to add the additional epoxy.

Today we got to look at these samples in the microprobe, and as expected from the difference in their textures noted while polishing them, they are rather different from one another. The one wherein I had issues with the welding did contain some water; we know this because there are very small grains of mica present. However, neither was it water-saturated, so it lost some due to the poor seal of the capsule. It contains many, many very tiny grains of garnet (~1 micron diameter; remember that there are 1000 microns in every millimeter) which nucleated on their own, and very thin rims of garnet on the “seeds” which had been included in the powder to encourage garnet growth. The rest of the sample is even finer grained “matrix” minerals, which are going to be difficult to analyze. The other, water saturated, sample contains fewer, larger, grains of garnet, and the rims of new garnet growth on the “seeds” are much thicker than in the first sample. While it, too, is generally fine-grained, it will be easier to find single crystals large enough to get a good analysis of their compositions (which we need if we are going to accomplish our goals).

Having had this first look at the samples we’ve set the probe to create “element maps”, pretty full-colour pictures showing which areas are high (warm colours) and which areas are low (cool colours) in specific elements. Once we have these maps, we will use them to select the grains for the detailed compositional analysis. But even before we do that, I now have a better understanding of the difference between water-saturated and water-under saturated environments in terms of the ease at which minerals grow.

Some differences between analyzing minerals in natural rocks and minerals grown in experiments

For my PhD research I worked with Tasmanian metamorphic rocks. The schists I studied had major minerals in the 2 mm to 2 cm size range (though, of course, most of them were closer to 2 mm than 2 cm in size). As a result of these large sizes the approach to microprobe time for mineral analysis was to find sets of minerals near one another (in textural equilibrium) and select points for analysis. I’d take photographs of the selected minerals, make marks upon the printouts showing the locations of the selected points, and then leave the microprobe to do all of the analyses after I’d gone home for the day. Then I’d use ArcMap to align the photographs with the x-y coordinates from the microprobe so that I’d have a good record of where each analysis point was located.

Now that I’ve started my first post-doctoral position things will be somewhat different; I’ve signed on for an experimental position. This means that instead of analyzing mineral assemblages in natural rocks and doing calculations to try to determine at what temperature and pressure they must have formed, I shall be growing minerals at specific temperatures and pressures and analyzing them to see what their compositions are at those temperatures. (The results of these experiments will, in theory, be used by others who wish to determine the temperatures and pressures at which their minerals in natural rocks formed.)

I have not yet gotten to the point of being able to do my own experiments, but today I have spent some time in the microprobe lab with one of my colleagues as she analyzed the minerals which she grew in one of her experiments. The capsule in which she grew the minerals is only a few millimeters long, and the minerals present are all very small. The microprobe here is set up to use a 1 micron beam size, so the minerals need to be at least 1 mm in diameter in order to obtain analyses which are from a single mineral (the one I used in Australia was set up to analyze minerals that are at least 10 microns in diameter—remember that there are 1000 microns in every millimeter!).

Because the minerals here are so small they don’t tend to use the microprobe in automatic mode—instead one stays with the machine while it does the analyses. The procedure is to find a mineral you wish to analyze, analyze it, look at the results, and if it is “good” (has an appropriate proportions of elements for that mineral) move on to a new location to begin the next analysis. If it is “bad” (contains other elements than should be in that mineral and/or contains the correct elements in an unexpected proportion) it is mostly likely due to contamination from a neighboring mineral—sometimes the grain isn’t large enough and the electron beam also analyses the grain next to it, sometimes the grain isn’t thick enough, and the analysis includes the underlying grain. In this case you also move on to the next analysis, but you keep track of the number of “good” vs “bad” data, so that you can obtain sufficient “good” results to be able to state with confidence the composition of each of the minerals in the sample.

The approach is very different to what I have been used to, and requires one to become accustomed to looking at the raw microprobe data (hitherto I simply took the report generated by my microprobe operator and ran the results through the program Ax to convert it to number of cations per oxygen, and then dealt with the numbers only in that form. I am very much looking forward to starting my own experiments—I think that the procedures used here will help me to better understand the microprobe and how it operates, and it all sounds like fun. But first, there is much background reading to do…