Here’s an interesting one! Chalk (Calcium Carbonate – CaCO3) under a Scanning Electron Microscope:
Those little guys are similar to Diatomaceous Earth (Silica) in form, but are actually made from calcium and not silica. From reddit user Foramsgalorams:
Diatoms would be glassy rather than chalky, as their hard parts are made from silica rather than calcite. It would be difficult to tell the difference just by looking at a pile of diatom powder v chalk powder though, check out diatomaceous earth.
Anyway like the other person said, these are coccolithophores. Specifically they are Emiliania huxleyi, the most widespread extant species due to the way they do so well in waters of varying temperature or nutrient content. When you see satellite images of plankton blooms it’s probably E. huxleyi.
Also, these ones seem to be alive, or were very recently. It’s certainly not an image of actual chalk. After death all the coccoliths separate and sink to the seafloor as individual plates. The 100 metre high white cliffs of Dover on the south coast of England are made from countless coccoliths deposited during the Cretaceous.
This next one is climber’s chalk (Magnesite – Magnesium Carbonate)!