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. A hard rain will dissolve this water-soluble mineral and local sewers will return it to the sea where, in a process known as the calcium cycle, it will contribute to dissolved marine calcium. It’s ironic to think that this colorful material is made of the residue of evaporated seawater, from oceans long gone. The sidewalk chalk art pictured above is made from formed, dyed gypsum. A unique occurrence of gypsum as wind-blown sand can be found at White Sands National Monument in New Mexico (USA). Gypsum is the structural component of dry-wall and a thickener in many personal care products.
Gypsum quarried near Paris (France) was used to make plaster (of Paris). As an evaporite mineral, it’s water-soluble. These beds are commonly found in association with other evaporite minerals, particularly halite. Gypsum, calcium sulfate (CaSO4), occurs in thick evaporite beds. Today, sidewalk and blackboard chalk are made from gypsum, as it’s more common and easier to work with than chalk.
In this case, the word organic means something that was once alive. When this sedimentary rock is further compressed and metamorphosed, it may become limestone and then marble. Chalk and coal are both organic sedimentary rocks. Chalk, composed principally of calcium carbonate (CaCO3), formed underwater by slow accumulation and compression of the calcite shells of single-celled coccolithophores. Help them identify numbers where necessary. Ask children if they can put the matching number of rocks in each number’s circle. Blackboard and sidewalk chalk were originally made from the sedimentary rock of the same name a form of soft limestone. Sidewalk chalk Rocks Instructions: Ahead of time, draw 10 circles on the concrete with sidewalk chalk.
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