A soft, porous form of limestone, composed
mainly of calcium carbonate (CaCO3, see calcium)
with varying amounts of silica, quartz,
feldspar, or other impurities. Chalk is
generally gray-white or yellow-white and is derived chiefly from the remains
of small marine organisms. Embedded flint
nodules are commonly found in chalk beds. Chalk is widely used in lime and
cement manufacture and as a fertilizer.
It is also used in cosmetics, crayons, and oil paints; school chalk is today
usually made from chemically-produced calcium carbonate.
| Chalk cliffs, Dorset, England
AND PLANETARY SCIENCE