Scanning electron micrograph (SEM) of chalk. Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Magnification is 1800x at 12.7cm image size. | |
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Credit: | Science Photo Library / Science Source / Nano Creative |
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