If you read my bio, you will see that I list one of my areas of study as experimental petrology, and I’m sure plenty of people wonder what that even means, I know I didn’t really understand what it was when I first arrived at RSES to undertake an honours project doing experimental petrology. So what is it? Firstly, ‘petrology’ is the study of rocks, specifically their origin, composition, structure and distribution. The word itself comes from the Greek ‘petra’ – rock and ‘logos’ –study. The experimental in ‘experimental petrology’ is because we do good old fashioned experiments to understand how the rocks form, varying composition, temperature, pressure and a less well known variable ‘oxygen fugacity’. In this post I will try and explain it simple terms, feel free to ask questions via the comments section.
So what do we use in these experiments?
We start by mixing up powders with a composition similar to naturally occurring rocks, such as basalts or peridotites; this is a rather simple process of weighing out powders and grinding them together with a mortar and pestle and then drying them out. The next step involves creating a capsule to contain the powder, precious metals such as gold, platinum and palladium are our favourites, as they are easy to work with, relatively inert and don’t melt at the conditions of the experiment. I take a tube of platinum, cut a short piece and then weld one end closed, before filling it with powder and sealing the other end. At least they are the simple ones, we often use a second capsule to help contain a buffer material, which will help control the oxidation state of the elements within, sometimes this is another metal capsule on the outside or other times the inner capsule will be made of graphite. This capsule is then mounted in an assembly that includes a graphite cylinder that acts as a heater as well as salt and pyrex to transfer pressure to the capsule.
What allows us to reach high pressure and temperature?
The machines we use are known as piston-cylinder apparatus and are rather simple in their operation; basically they contain a small piston (typically ½ inch in diameter) which is connected to a large pump. The capsule and experimental assembly is placed inside a cylindrical hole in a circular pressure vessel and the whole assembly is confined by being pushed up against a steel plate. The small piston is then driven into the hole in the pressure vessel and squashes the assembly creating the high pressure we desire.
The high temperatures are created by forcing a high current through the graphite heater that is part of the assembly. This allows us to perform experiments up to around 1500 °C and at pressures up to 40 kilobar (40,000 times higher than atmospheric pressure), roughly equivalent to 120 km in depth. We then run these experiments for as little as a few hours or as long a few days, depending on what we want to produce. Longer times produce larger crystals so can be useful for analysing on some instruments whilst the short times allow us to examine processes such the diffusion of elements into crystals.
Once this is finished we quench the experiment, extract and analyse the products, and I’ll write about that in the near future.