Sidewalk Chalk and the Calcium Cycle
October 19, 2009
Photographer: Rob Sheridan
Summary Author: Rob Sheridan
Blackboard and sidewalk chalk were originally made from the sedimentary rock of the same name; a form of soft limestone. Chalk, composed principally of calcium carbonate (CaCO3), formed underwater by slow accumulation and compression of the calcite shells of single-celled coccolithophores. When this sedimentary rock is further compressed and metamorphosed, it may become limestone and then marble.
Today, sidewalk and blackboard chalk are made from gypsum, as it’s more common and easier to work with than chalk. Gypsum, calcium sulfate (CaSO4), occurs in thick evaporite beds. These beds are commonly found in association with other evaporite minerals, particularly halite. As an evaporite mineral, it’s water-soluble. Gypsum has many uses. Gypsum quarried near Paris (France) was used to make plaster (of Paris). Gypsum is the structural component of dry-wall and a thickener in many personal care products. A unique occurrence of gypsum as wind-blown sand can be found at White Sands National Monument in New Mexico (USA).
The sidewalk chalk art pictured above is made from formed, dyed gypsum. It’s ironic to think that this colorful material is made of the residue of evaporated seawater, from oceans long gone. A hard rain will dissolve this water-soluble mineral and local sewers will return it to the sea where, in a process knows as the calcium cycle, it will contribute to dissolved marine calcium. Continuing the cycle, these calcium salts may then contribute to new coccolithophores or other marine shells, or remain dissolved, ultimately creating new limestone, chalk, or gypsum beds, millions of years from now. Photo taken in Cambridge, Massachusetts in May 2009.