Calcite vs Aragonite

You may think of carbon as an element that on Earth is found mainly in living things (that is, in organic matter) or in the atmosphere as carbon dioxide. Both of those geochemical reservoirs are important, of course, but the vast majority of carbon is locked up in carbonate minerals. These are led by calcium carbonate, which takes two mineral forms named calcite and aragonite.

Calcium Carbonate Minerals in Rocks

Aragonite and calcite have the same chemical formula, CaCO₃, but their atoms are stacked in different configurations. That is, they are polymorphs. (Another example is the trio of kyanite, andalusite, and sillimanite.) Aragonite has an orthorhombic structure and calcite a trigonal structure. Our gallery of carbonate minerals covers the basics of both minerals from the rockhound's viewpoint: how to identify them, where they're found, some of their peculiarities.

Calcite is more stable in general than aragonite, although as temperatures and pressures change one of the two minerals may convert to the other. At surface conditions, aragonite spontaneously turns into calcite over geologic time, but at higher pressures aragonite, the denser of the two, is the preferred structure. High temperatures work in calcite's favor. At surface pressure, aragonite can't endure temperatures above around 400°C for long.

High-pressure, low-temperature rocks of the blueschist metamorphic facies often contain veins of aragonite instead of calcite. The process of turning back to calcite is slow enough that aragonite can persist in a metastable state, similar to diamond.

Sometimes a crystal of one mineral converts to the other mineral while preserving its original shape as a pseudomorph: it may look like a typical calcite knob or aragonite needle, but the petrographic microscope shows its true nature. Many geologists, for most purposes, don't need to know the correct polymorph and just talk about "carbonate." Most of the time, the carbonate in rocks is calcite.

Calcium Carbonate Minerals in Water

Calcium carbonate chemistry is more complicated when it comes to understanding which polymorph will crystallize out of solution. This process is common in nature, because neither mineral is highly soluble, and the presence of dissolved carbon dioxide (CO₂) in water pushes them toward precipitating. In water, CO₂ exists in balance with the bicarbonate ion, HCO₃⁺, and carbonic acid, H₂CO₃, all of which are highly soluble. Changing the level of CO₂ affects the levels of these other compounds, but the CaCO₃ in the middle of this chemical chain pretty much has no choice but to precipitate as a mineral that can't dissolve quickly and return to the water. This one-way process is a major driver of the geological carbon cycle.

Which arrangement the calcium ions (Ca²⁺) and carbonate ions (CO₃^2–) will choose as they join into CaCO₃ depends on conditions in the water. In clean fresh water (and in the laboratory), calcite predominates, especially in cool water. Cavestone formations are generally calcite. Mineral cements in many limestones and other sedimentary rocks are generally calcite.

The ocean is the most important habitat in the geological record, and calcium carbonate mineralization is an important part of oceanic life and marine geochemistry. Calcium carbonate comes directly out of solution to form mineral layers on the tiny round particles called ooids and to form the cement of seafloor mud. Which mineral crystallizes, calcite or aragonite, depends on the water chemistry.

Seawater is full of ions that compete with calcium and carbonate. Magnesium (Mg²⁺) clings to the calcite structure, slowing down the growth of calcite and forcing itself into calcite's molecular structure, but it doesn't interfere with aragonite. Sulfate ion (SO₄^–) also suppresses calcite growth. Warmer water and a larger supply of dissolved carbonate favor aragonite by encouraging it to grow faster than calcite can.

Calcite and Aragonite Seas

These things matter to the living things that build their shells and structures out of calcium carbonate. Shellfish, including bivalves and brachiopods, are familiar examples. Their shells are not pure mineral, but intricate mixtures of microscopic carbonate crystals bound together with proteins. The one-celled animals and plants classified as plankton make their shells, or tests, the same way. Another important factor appears to be that algae benefit from making carbonate by ensuring themselves a ready supply of CO₂ to help with photosynthesis.

All of these creatures use enzymes to construct the mineral they prefer. Aragonite makes needlelike crystals whereas calcite makes blocky ones, but many species can make use of either. Many mollusk shells use aragonite on the inside and calcite on the outside. Whatever they do uses energy, and when ocean conditions favor one carbonate or the other, the shell-building process takes extra energy to work against the dictates of pure chemistry.

This means that changing the chemistry of a lake or the ocean penalizes some species and advantages others. Over geologic time the ocean has shifted between "aragonite seas" and "calcite seas." Today we're in an aragonite sea that is high in magnesium—it favors the precipitation of aragonite plus calcite that's high in magnesium. A calcite sea, lower in magnesium, favors low-magnesium calcite.

The secret is fresh seafloor basalt, whose minerals react with magnesium in seawater and pull it out of circulation. When plate tectonic activity is vigorous, we get calcite seas. When it's slower and spreading zones are shorter, we get aragonite seas. There's more to it than that, of course. The important thing is that the two different regimes exist, and the boundary between them is roughly when magnesium is twice as abundant as calcium in seawater.

The Earth has had an aragonite sea since roughly 40 million years ago (40 Ma). The most recent previous aragonite sea period was between late Mississippian and early Jurassic time (about 330 to 180 Ma), and next going back in time was the latest Precambrian, before 550 Ma. In between these periods, Earth had calcite seas. More aragonite and calcite periods are being mapped out farther back in time.

It's thought that over geologic time, these large-scale patterns have made a difference in the mix of organisms that built reefs in the sea. The things we learn about carbonate mineralization and its response to ocean chemistry are also important to know as we try to figure out how the sea will respond to human-caused changes in the atmosphere and climate.