CALCIUM CARBONATE
Calcite as Limestone and Marble
Limestone is a sedimentary rock that is composed primarily of calcite. It forms from both
the chemical precipitation of calcium carbonate and the transformation of shell, coral, fecal
and algal debris into calcite during digenesis. Limestone also forms as a deposit in caves
from the precipitation of calcium carbonate.
Marble is a metamorphic rock that forms when limestone is subjected to heat and
pressure. A close examination of a broken piece of marble will usually reveal obvious
cleavage faces of calcite. The size of the calcite crystals is determined by the level of
metamorphism. Marble that has been subjected to higher levels of metamorphism will
generally have larger calcite crystals
What is calcium carbonate?
Calcium carbonate accounts for more than 4% of the earth’s crust. As a result, the
three calcium carbonate minerals – calcite, aragonite and vaterite – are among the
most important rock-forming minerals. Rocks are not the only calcium carbonate
deposits in nature – almost all stretches of water and countless plants and animals
contain huge amounts of calcium carbonate as well. These natural resources are
linked by the calcium carbonate cycle.
Plants and animals absorb calcium carbonate in water, where it usually exists
dissolved in the form of calcium hydrogen carbonate Ca(HCO3)2, and use it to build
up their skeletons and shells. After their death, mussels, coccoliths, algae and corals
form sedimentary deposits on sea beds and the rock-forming process is set in
motion.
The first stage is the sedimentation process, from which chalk and limestone originate.
Chalk is a poorly compacted sedimentary calcium carbonate rock whose diagenesis is
incomplete
Physical Properties of Calcite
Chemical Classification carbonate
Color usually white but also colorless, gray, red, green, blue, yellow, brown,
orange
Streak white
Luster vitreous
Diaphaneity transparent to translucent
Cleavage perfect, rhombohedra, three directions
Mohs Hardness 3
Specific Gravity 2.7
Diagnostic Properties rhombohedra cleavage, powdered form effervesces
weakly in dilute HCl, curved crystal faces and frequent twinning
Chemical Composition CaCO3
Crystal System hexagonal
Uses acid neutralization, a low hardness abrasive, soil conditioner, heated for the
production of lime
The construction industry is the primary consumer of calcite in the form of
limestone and marble. These rocks have been used as dimension stones and in
mortar for thousands of years. Limestone blocks were the primary construction
material used in many of the pyramids of Egypt and Latin America. Today, rough
and polished limestone and marble are still an important material used in prestige
architecture.
Modern construction uses calcite in the form of limestone and marble to produce
cement and concrete. These materials are easily mixed, transported and placed in
the form of a slurry that will harden into a durable construction material. Concrete
is used to make buildings, highways, bridges, walls and many other structures.
