A term originally applied only to minerals consisting largely of CaCO3, such as Portland stone, dolomite, marble, and chalk, now used indiscriminately to designate technical and agricultural grades of calcium carbonate. Review: R. S. Boynton in Kirk-Othmer Encyclopedia of Chemical Technology vol. 14 (Wiley-Interscience, New York, 3rd ed., 1981) pp 343-382.