A SOFT, FINE-GRAINED, easily pulverized, white to grayish variety of limestone, chalk is composed of the calcite shells of minute marine organisms. Small amounts of other minerals are commonly present, such as glauconite, apatite, and clay minerals. Silica can also be present in small quantities from sponge spicules, diatom and radiolarian skeletons, and nodules of chert and flint. In some localities, the beds of flint nodules are thick enough to have been mined in ancient times. Extensive chalk deposits were formed during the Cretaceous Period (142 to 65 million years ago), the name being derived from the Latin creta, meaning “chalk”.
- Rock type Marine, organic, sedimentary
- Fossils Invertebrates, vertebrates
- Major minerals Calcite
- Minor minerals Quartz, glauconite, clays
- Color White, gray, buff
- Texture Very fine, angular to rounded
Like other limestones, chalk is used for making lime and cement and as a fertilizer. It is used as a filler, extender, or pigment in a wide variety of materials, including ceramics, putty, cosmetics, crayons, plastics, rubber, paper, paints, and linoleum. Modern-day blackboard chalk is a manufactured substance rather than natural chalk. In past times chalk was mined extensively to be burned for making quicklime for mortar. There are buildings lime-mortared 500 years ago that are still standing today.
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