Chalk is a familiar substance, recognized globally as the white material used for writing on blackboards or drawing on sidewalks. True chalk is a soft, white, porous sedimentary rock found in the Earth’s crust. Its geological origin and chemical composition set it apart from manufactured products that share its name. This material has played a role in human history, serving as a filler, a source of lime, and a major component of several industrial processes.
The Core Chemical Identity
Geological chalk is defined by its chemical makeup, which is almost entirely calcium carbonate (\(\text{CaCO}_3\)). This compound is a common mineral form of calcite, the primary constituent of limestone and marble. Chalk is a fine-grained variety of limestone, typically containing minor amounts of silt and clay as impurities.
The mineral calcite is an ionic salt that reacts with acid to produce carbon dioxide gas, a characteristic used by geologists to identify the rock. True chalk is a soft, friable rock, meaning it is easily crumbled, which makes it suitable for writing and industrial applications. Its mineralogical purity is high, often consisting of 96% to 99% calcium carbonate.
The Biological and Geological Origin
The formation of natural chalk is a long biological and geological process that occurred over millions of years under marine conditions. The material originates from the microscopic skeletons of single-celled marine algae known as coccolithophores. These planktonic organisms live in the upper, sunlit layers of the ocean and surround themselves with tiny, intricate calcium carbonate plates called coccoliths.
When these organisms die, their minuscule calcareous shells sink to the ocean floor, accumulating as a soft, white mud called calcareous ooze. Over vast stretches of time, this deep-sea sedimentation process leads to the formation of thick layers of sediment. The weight of the overlying water and subsequent sediment compresses and consolidates the ooze into the soft rock known as chalk.
Massive chalk deposits found worldwide, such as the White Cliffs of Dover, are evidence of the immense scale of this biological accumulation. The most extensive chalk formations are from the Cretaceous period, an interval named after the Latin word for chalk, creta. This geological record provides scientists with a window into past oceanic conditions and the global carbon cycle.
Modern Chalk Versus Natural Chalk
Many products commonly referred to as “chalk” today are not made from the geological calcium carbonate rock. The material used in most commercial writing implements, such as classroom and sidewalk chalk, is typically manufactured from gypsum. Gypsum is a different mineral, chemically known as calcium sulfate (\(\text{CaSO}_4\)), which is preferred for its manufacturing properties.
Gypsum-based chalk is softer and produces less fine, airborne dust compared to chalk made from natural calcium carbonate. This characteristic makes the manufactured version less irritating in enclosed spaces like classrooms, which is why it is marketed as “dustless chalk,” even though it still produces residue. Sidewalk chalk is also made from gypsum, often mixed with pigments for color and shaped into larger sticks.
Some anti-dust varieties of blackboard chalk are made from calcium carbonate, but they include binders like fatty acids or starch to reduce the amount of dust released during use. Other specialized products, such as tailor’s chalk, are often made from talc, and the white powder used by gymnasts and climbers is magnesium carbonate. The term “chalk” has become a general name for a variety of compressed mineral powders used for marking or absorbing moisture, even if their chemical identities differ significantly from the original sedimentary rock.