Get to know dolomite, Illinois' state rock
You know what rocks? Having a state rock! Dolomite was chosen as Illinois’ state rock by students in 2022.
Unlike Illinois’ colorful state mineral, fluorite, dolomite is a little understated. In fact, it is such a common sight that you might have seen it and not even known it.
Before we get into that, let’s clear something up. Minerals and rocks are both hard substances from the earth, but they are not the same thing. Minerals are the “ingredients” that make up rocks. Think of a rock like a chocolate chip cookie. Cookies are made of sugar, flour, eggs and probably a few other things. In nature’s case, different minerals make up different rocks.
What is even more confusing is that there is a mineral called dolomite AND a rock called dolomite. If you are asking yourself if the dolomite mineral is an ingredient in the dolomite rock, you’re onto something. The answer is yes!
Confused? It’s OK. Sometimes geologists, or scientists who study rocks and the earth, get a little mixed up when talking about the two as well. To avoid confusion, the rock dolomite is often referred to as dolostone.
Dolostone (the rock dolomite) is a sedimentary rock that contains a high percentage of the mineral dolomite. Dolomite the mineral is a combination of calcium, magnesium, carbon and oxygen. Its recipe, or chemical makeup, is written as CaMg(CO3)2.
The combination of calcium and carbon, also known as calcium carbonate, makes up other recognizable things found in nature: chalk, marble, eggshells, pearls and the shells of marine animals.
What does dolomite (or dolostone) look like?
Dolostone often fades to a dull yellowish color. It also often gets mistaken for another similar-looking rock, limestone. If you were to look at the two rocks individually, you probably would not be able to see the difference.
Words to know
Neutralize: To make chemically neutral. Precipitate: A substance separated from a solution. Sedimentary: Formed by or from deposits of sediment.
The difference, like with cookies, is just a tiny ingredient change. Dolostone contains more magnesium than limestone. Sometimes, it is even formed from limestone!
According to the Illinois Department of Natural Resources, many dolostones in the state were originally limestones in which the calcite mineral, or a calcium and oxygen combination, was replaced with dolomite as magnesium-filled water moved through the rock.
Under the sea
Both, however, are formed in similar environments, which brings us back to why dolostone might be considered Illinois’ state rock. Both rocks formed in warm, shallow marine environments where calcium carbonate accumulated in the form of shell debris, fecal material, coral pieces and carbonate precipitates.
During the Silurian period, about 430 million years ago, a warm, shallow sea covered what today is Will County. Swimming in this sea were animals such as snails, trilobites, gastropods and corals — lots of animals with hard calcium carbonate exteriors that have left their mark in the stone in the form of fossils.
Partially because of the remnants of these prehistoric shelled creatures, dolostone can be found in many parts of the state. It is mined in quarries and was once a very popular building material! Today it can be used for many things, like road construction, removing pollutants in industrial settings and neutralizing soil on farms. Sometimes, it is lurking just below the surface!
Go exploring! Which forest preserve has a building made out of dolomite?
