Calcite (and Aragonite)

Without calcite and aragonite many of Earth’s organisms could not exist. These are the minerals most invertebrate organisms use to construct their shells and hard parts. So if you have ever admired a colorful seashell, you have already paid tribute to the range of colors calcite and aragonite come in and indirectly to the ease with which calcite and aragonite form at the Earth’s surface.

Aragonite and calcite are ‘polymorphs’, minerals that have the same chemical composition but slightly different crystal structures. Both minerals can occur together and are so similar to one another that distinguishing between them is seldom important to anyone but a professional geologist. Calcite is more stable and common than aragonite under the chemistry of our modern seas, but at times in the Earth’s past, aragonite was the more stable and common form. The two minerals are nearly identical in their physical properties, so through the remainder of this website they will be treated together under the name ‘calcite’.

Calcite forms in a range of settings from hot springs and underground caverns to growing coral reefs and seashells. This not only reflects calcite’s abundance, but also the ease with which it forms, dissolves, and reforms. Calcite is almost as common in our society as it is in nature, its uses ranging from medicine and animal feed, to Michelangelo’s ‘Pieta,’ and highway overpasses. To a remarkable degree, human society mimics marine organisms in the sense that we rely on calcite to construct many of the ‘hard’ parts of our society, in the form of marble, plaster, mortar, and cement.

Description and Identifying Characteristics

Calcite occurs in both crystalline and massive forms. Crystals of calcite are usually transparent to translucent, but can exhibit a wide variety of colors if the crystal includes minor impurities. Masses of calcite also tend to be light colored, but as with the transparent crystals, the presence of even minor amounts of impurities can create a wide range of colors.

Since calcite can have so many appearances, the simplest way to determine whether a sample contains calcite is to test its reaction to a dilute acid like household vinegar. Calcite will readily react with acid to ‘effervesce’, producing small bubbles of CO₂ similar to those formed when you open a bottle of soda and pour it into a glass. Soft enough to be easily scratched by a nail, calcite crystals can also be identified by their rhombic cleavage. ‘Rhombic cleavage’ means that calcite crystals break along parallel planes of weakness that meet at the same angles as a rhomb’s sides.

All natural waters contain dissolved calcium and carbon dioxide, and their concentration is especially high in seawater. Many marine animals including corals, snails, clams, algae, and microscopic plankton use calcite and aragonite to form their shells and hard parts. Microorganisms can also indirectly lead to the precipitation of calcite as they alter the chemistry of the fluids in which they live. Once formed, calcite is easily dissolved and its component ions released to precipitate elsewhere.

As a consequence, calcite is not only the main mineral of limestone rocks and marble (metamorphosed limestone), but also a common accessory component of sandstone and siltstone rocks. Calcite forms cave decorations, hot spring travertine, and hydrothermal mineral deposits. Because it is easily precipitated and dissolved at the Earth’s surface temperatures and pressures, calcite is one of the more common fracture-filling vein minerals found in other rocks. Calcite even precipitates in soils, particularly those in arid environments where calcite precipitation can form hard layers called caliche.

The ease with which it dissolves and precipitates is the basis for many of calcite’s uses in human society. Calcite’s name comes from ‘chalix’ or ‘chalx’, which is the Greek word for lime (‘calx’ in Latin). For over 5,000 years, calcite has been used to manufacture lime (CaO, calcium oxide). Although the equipment used to do this has changed across the centuries, the basic process remains the same. Rocks rich in calcium carbonate are heated to high temperatures so the CaCO₃ will burn to form CaO and CO₂ gas. The CaO can then be recombined with water (which always holds some dissolved CO₂) to precipitate CaCO₃ as plaster, mortar, and more recently as cement. Early Roman and Greek frescos attest to the use of lime in art as well.

Although cement production accounts for the bulk of calcite used by our modern society, large amounts are also used in the manufacture of steel and glass. Rocks composed primarily of calcite, such as limestone and marble, are also extensively quarried as decorative building stones and for sculpture.

Because of the ease with which calcite reacts with even weak acids, calcite is used to balance soil acidity, in water treatment, and as a calcium supplement in animal feed. Calcite even has medicinal value as both a supplemental source of calcium and as an antacid. Calcite’s ready reaction with dilute acids not only serves to identify its presence in geological samples, but can neutralize excess stomach acids that cause discomfort!

Even the schoolroom is not devoid of calcite. In geology, ‘chalk’ is the name given to a rock that is composed almost entirely of the calcite shells of microscopic plankton. It is a very soft rock that, when rubbed on a surface, breaks apart to leave white streaks. Although modern schoolroom chalk is often a mixture of anhydrite and calcite, the original chalk for chalkboards was completely composed of the microscopic calcite shells of marine plankton.

One variety of calcite called “Iceland Spar” is clear and colorless with remarkably clear optical properties. Iceland spar was particularly important during World War II because it was used for the sighting equipment of airplane bombardiers and gunners. Today, Iceland spar calcite is still used in some optical instruments such as polarizing microscopes.

Onyx is a term used for both a layered variety of quartz, as well as a layered variety of calcite, so don’t be confused by the term’s double use. The layered calcite (often known as Mexican Onyx) can be distinguished because it is softer and is easily carved into different shapes. From 1200 to 300 B.C., the Olmec of central Mexico carved figurines from calcite onyx that were widely traded from Guatemala to Costa Rico, a tradition that has been continued by other people to the present day.

Aragonite has almost the same uses as calcite, but with one notable addition. Mollusks are the only family of marine organisms that tend to precipitate shells almost entirely out of aragonite, rather than calcite or a mixture of calcite and aragonite. The iridescent inner layer of these shells, often called mother-of-pearl, is quite beautiful and was once widely used for buttons and decorative jewelry. Natural pearls, which form in mollusks and are widely used in jewelry, are also composed of aragonite.

Carbon dioxide is a greenhouse gas that traps heat radiated from the Earth to warm the Earth’s surface. Natural emissions of carbon dioxide from volcanoes and plants warm the Earth so that it is not a frozen planet. However, as human activities increase the levels of CO2 in the atmosphere, we run the risk of changing the Earth’s climate by increased greenhouse warming. Although the burning of fossil fuels is the best-known source of increased atmospheric CO₂, the production of lime and concrete is another important contributor to greenhouse warming.

In the Upper Midwest region, calcite and dolomite are the most important minerals that comprise the carbonate rock layers that cover the region’s southeastern extent. These rocks are quarried for use as building stones and as gravel for road construction. Marl, a mixture of calcite and clay minerals, forms beneath many of the state’s lakes and wetland areas. Vein deposits of calcite occur in many regional rock units, but are particularly common in the basalts and gabbros that form the North Shore of Lake Superior.

Calcite and aragonite also form most of the cave decoration found in caverns present within the carbonate rock units across the southern part of this region. In Upper Midwest caves, calcite is the primary mineral component of cave formations such as stalactites and stalagmites, although many of the more delicate cave features are composed of aragonite.

On a more domestic front, calcite is the mineral that dissolves to form the ‘hard’ water (water with high concentrations of dissolved ions) present in many Upper Midwest communities. In homes without water softeners, calcite can precipitate from calcium-rich groundwater to plug household plumbing or form a crust below dripping faucets.

Since calcite and aragonite can exhibit so many appearances, there are a number of minerals with which it may be mistaken. Calcite and aragonite, however, are the only common minerals that react vigorously with dilute acid to produce bubbles of CO₂.

Aragonite:

Aragonite is a polymorph of calcite, a mineral that has the same chemical composition as calcite, but has a slightly different crystal structure. For most purposes aragonite and calcite can be considered to be the same mineral. In its crystal form, aragonite tends to have needlelike crystals and lacks calcite’s rhombohedral cleavage. In massive form, it is difficult to distinguish the two minerals in the field, as both react vigorously with cool dilute acid. Calcite is the more stable form and is far more abundant, and aragonite tends to convert to calcite over time. So in general a sample that reacts vigorously with acid is more likely to be calcite, rather than aragonite.

Dolomite:

Dolomite is a mineral that most often forms as an alteration of calcite, as magnesium replaces much of the calcium in the crystal structure. Consequently, it is not unusual for the two minerals to occur together. Both minerals exhibit the same well-developed rhombohedral cleavage and have roughly the same hardness. The easiest way to distinguish the two is that dolomite will not readily react with dilute acid at room temperature. Dolomite only reacts if it is scratched or crushed to produce a powder. The greater surface area of the powder allows the reaction to occur quickly enough to be seen.

Gypsum:

Gypsum can also form translucent crystals and may be associated with calcite, but gypsum only has one very well developed cleavage plane, while calcite has three that form well-developed rhombs. Unlike calcite, gypsum does not react with dilute acid to form bubbles, although if the gypsum is crushed into a powder it may simply dissolve.

Quartz:

Quartz crystals and masses may have the same luster as calcite, but quartz does not exhibit cleavage and will not react with dilute acid. Quartz is also much harder than calcite. It cannot be easily scratched by a nail and quartz will easily scratch a glass plate.

Chert:

Chert is a rock composed of very small microscopic quartz crystals. It can be distinguished from calcite because it is much harder than calcite. Chert cannot be easily scratched by a nail and will easily scratch a glass plate. It also will not react with dilute acid as calcite does.

Potassium or Plagioclase Feldspar:

The feldspar minerals may have the same color and luster as some calcite varieties, but they can be easily distinguished on the basis of their hardness. Unlike calcite, feldspar minerals are harder than glass and cannot be easily scratched by a nail. Cleavage faces in feldspar minerals also meet at right angles, unlike calcite’s rhombohedral cleavage. Finally, feldspar minerals will not react with dilute acid as calcite does.

Limestone:

Confusing limestone and calcite is not an error in identification, but is simply a minor error in terms. Limestone is the name for a rock that is primarily composed of the mineral calcite. Distinguishing between rock and mineral terms is important though, as a limestone rock may contain other minerals in addition to its calcite.