Coloured scanning electron micrograph (SEM) of Chalk dust (anti-dust type) from a school chalkboard eraser. Chalk (also known as calcite) is a white, porous sedimentary rock. It is a form of limestone that is chemically called calcium carbonate (CaCO3). Limestone forms in deep marine environments from the slow accumulation of very small calcite plates (coccoliths) produced by certain types of microorganisms called coccolithophorids. The famous White Cliffs of Dover are an example of a natural limestone deposit. Magnification: x2,135 when shortest axis printed at 25 millimetres.
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