Saturday, 8 December 2012

Crushing rock – part 2

In this short post I will continue to explain the process of rock crushing. The final stage is to take the millimetre size rock pieces and to crush them up into micron size pieces, a process called milling. We use a machine called a Planetary Mill, which you can see in the photo below. There are four receptacles with balls inside that spin round and crush the rock into small grains. The result is a fine rock powder, which feels quite like flour. We use this rock powder for various geochemical analyses such as determining the carbon isotopic composition of the rock.

For my rock samples this is a fairly quick process, as sedimentary rocks are not very hard compared to igneous and metamorphic rocks. However, a lot of time has to be spent on cleaning the receptacles and machine between each batch, to avoid contamination. 

Top: The Planetary Mill. Bottom left: one receptacle containing rock pieces and balls to crush the rock. Bottom right: the resulting powder produced by milling.

Wow! Of the week
When I found this rock while sawing last week, it was a total wow! moment. As before, with other wow! specimens, on the outside the rock looked very non-descript, just a sandy lump. But once I sawed into the rock I saw all these holes throughout the rock that I haven’t seen before. I tested to rock with some dilute acid and discovered that calcite exists in the holes, while the rest of the rock is made from sand. I think that these holes were formed by gypsum crystals that have now dissolved, as some of the holes are long and thin like the crystal habit of gypsum.

This is the first direct evidence I have found of gypsum from the site I am working on. Its very exciting as it means that this rock formed from the evaporation of water, maybe in a lagoon that dried up. And it might indicate that seawater inundated the area prior to its drying out. In the bigger picture, its one more piece in the puzzle of understanding the environment the tetrapods lived in, fantastic!

Until next time

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