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.

Learning to create capsules for experiments

Having recently started a position as a post-doc with an experimental petrology team it is now time for me to make the transition from reading a huge stack of background material to actually doing experiments. This requires learning a whole new skill set, like welding. Here follows the notes I’ve made on the first few steps of creating sealed capsules full of material to be taken to elevated temperatures and pressures during my forthcoming experiments. This is mostly for my own information, but someone out there might find it interesting or useful (or have helpful comments they can add), so I’ll share it here.

*Step one: Prepare the holder in which the capsule will be placed while filling it

During the filling process we use small metal disks into which holes of varying diameter have been drilled as a holder for the capsules (different sized holes are needed because different experiments use different sized capsules). First find a disk which has a hole with the correct diameter (or make a new hole in a disk if necessary). It needs to be just big enough to insert the tube into, without being loose. Then use fine sandpaper to carefully polish the metal around that hole so that when you get to step 6 you will have an easier time of filling the capsule.

*Step two: cut the tube for the capsule

For my experiments I will be using capsules made of gold tubing that is 2 mm outside diameter (1.4 mm inside diameter).

Obtain the correct diameter and metal tube (our lab uses both gold and platinum/gold alloys in a variety of sizes, I’m to use gold for mine) and then cut off a ~7 mm length from one end. To cut the tubing place it on a metal plate, then place an x-acto blade upon the tube and use the blade to exert a gentle pressure to roll the tube back and forth until the blade cuts through without squishing the tube. The back-and-forth motion is essential. This is not “sawing”, which uses a serrated blade to tear chunks out of a material which is stationary, but rather the tube itself rolls during the process as the blade slowly cuts into it.

*Step tree: Pinch closed one end of the tube

To seal the first end of the tube use pliers which have slightly rounded side edges. It is important to use the correct tool, as the sharp sided pliers can pierce a hole in the gold between the unpinched and pinched portions of the tube. (However, one of my colleagues uses the straight-sided ones, but he is very, very careful.) We use a three-part closure, which looks much like a symmetrical peace symbol. In order to create this do it in stages, don't try to squeeze it to final tightness on the first go, that won't work. Instead do a little at a time, slightly pinching on one side, turn the tube 1/3 of a rotation and pinch again, repeating around the tube, tightening the pinches a bit more each pass. Ideally, one wants it to be closed all the way to the center without leaving a hole at the triple junction. Unfortunately, this is difficult to achieve when working on such a small tube with pliers which are so much larger than the tube. Once the end is pinched shut trim the three edges using a cutting tool with beveled edges. The reason for wanting beveled edges to the cutter is so that the place where the two edges of gold meets is as narrow as possible, which will make welding easier. Hold the cutter at a slight angle so that once trimmed the triple point at the junction is slightly higher than the far ends of each of the three seams. (This photo, above left, taken through the microscope, shows a crimped tube held in place in the clamp and ready to weld.)

*Step four: Weld the pinched end shut

The voltage necessary for welding will change based on a variety of factors, including the diameter and length of the capsule, the thinness of the seam, the sharpness and length of the graphite in the welding tool, and what, if anything, you use to cool the capsule as you work. Unfortunately, our

welder isn't very precise and it can be difficult to adjust it to the perfect voltage for any given job. For this size we tried a variety of settings between 25 and 30 V, the 25 V was clearly too low--the welder left it looking "dirty" and coated with black, which is graphite from the welder being left on the gold. At 30 V it was too high; there is too much melting. In between that range the exact value was hard to find, and as variables change, so does the perfect voltage for the task. One variable which can make a huge difference is the sharpness of the graphite point. We have two different sharpeners, one of which makes a sharper point than the other. Using a “point” created by the duller of the two sharpeners at a voltage which isn’t high enough for that point and then switching to a point created by the sharper of the two sharpeners without turning down the voltage will result in the entire end of the tube melting.

The bit of advice I obtained the next day seems to have made a difference—don’t try to touch the gold with the graphite point, but rather hold it just barely close enough to cause an arc between them, and then try to draw that arc along the length of the seam. This isn’t easy, but I did wind up with useable results. Alas, the photo to the upper left doesn't show the welding very clearly--gold is just too darn shiny to photograph well through a microscope with a cheap camera when resting in a brass holder (this is after adjusting the brightness/contrast/intensity to make it visible at all).

I was also told that when welding I should try to start at the outer edge and draw the graphite point towards the middle, which brings excess gold from the edge towards the center to fill the small hole at the triple-junction. The goal is also to wind up with a flat bottom after welding.

*Step five: Prepare the welded tube for filling

After welding the tube it is necessary to re-shape the tube so that the capsule will have properly rounded/curving edges. We have a form (photo to the left) into which the tube is placed carefully so that the widest parts will be inside the form and not pinched between the two halves of the form (one chooses the correct diameter chamber within the form for the tube in question, of course). Once it is positions correctly the form is closed, and the tube is pressed back into a cylindrical shape. Once it has been re-shaped in the form it is put it into the holder (prepared in step one, and resting on a metal plate) and insert into the gold tube a small rod which has a diameter which just fits into the tube (in this case the rod needs to fit into a space 1.4 mm wide). Gently tap the rod with a mallet so that the bottom of the capsule flattens against the underlying metal plate and spreads out to match the curve of the sides of the hole in which the tube rests (take care as to not strike it so hard as to tear a hole in the gold tube!).

After much effort I now have three small capsules with one end of each sealed and flattened, and the other end still open and ready to fill. Stay tuned for steps 6 and 7 once I get them working. Having finally managed to get the tubes ready to fill, I chose to rest on my laurels and call it good for the day.