I was taking photos of newly forming Sodium Sulphate crystals when I discovered that within a couple of minutes all the crystals were being completely covered by these brown spots. Once covered there was no way to see the internal structure of the crystals anymore. They grew very quickly, and it almost seems as if they pop in and out of existence, like bubbles in champagne.
My guess would be that it has something to do with a reaction with oxygen, but I would love to hear a better explanation:
and a 100% crop:
Images made through a microscope. All subject types.