All Minerals

Limestone

Formula: CaCO₃

Limestone forms by sedimentation, of coral and shells (biological limestone) or by the precipitation of calcium carbonate (as calcite and aragonite), in marine waters. It comes in many forms (e.g. chalk, coquina, fossiliferous, lithographic, oolitic, travertine, tufa). It can be crystalline, clastic, granular or massive. The production of calcium oxide by roasting limestone is one of the oldest chemical transformations done by man, it is the basis of mortar and cement (calcium oxide then reacts with carbon dioxide to regenerate as calcium carbonate).

Materials are considered limestone if they have 50%+ CaO (this comes close to the ideal formula of CaCO₃). Manufacturers blend to produce a consistent product that is close to the ideal (with minimal impurities).

There are thousands of companies around the world that grind raw limestone into powders and aggregates. The powder is also called GCC (ground calcium carbonate), or just calcium carbonate. In ceramics, it is also commonly called "whiting".

GCC has countless applications, but in ceramics it is mainly used as a source of CaO (in glazes, frits, etc).

Related Information

Idea: Gather rocks and fire them. Unexpected results!

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I gathered these downstream from the Oldman River dam in southern Alberta, not far from where the river exits the Rocky Mountains (it cuts through many different rock strata upstream, including both limestone and dolostone, as well as quartz sandstone and siltstone). Upper left: The gravel bar where I collected them. Upper right: The rocks as gathered. Lower left: After firing to cone 04 (about 1900°F). Lower right: After I poured water on them!

Seven disintegrated before my eyes—with sound effects, intense heat and even steam! That identifies them as carbonate-rich rocks, likely limestone or dolostone. Firing drove off their CO2 and caused considerable weight loss, converting the calcium carbonate to calcium oxide—quicklime. Dolostone also produces magnesium oxide. When water was added, the CaO rapidly hydrated to calcium hydroxide, releasing enough heat to break the rocks apart. Two others cracked badly just from the firing. The survivors appear to be more silicate-rich siltstones and sandstones—and some of them can withstand considerably higher temperatures.

The exciting part of this: A river gravel bar is really a randomly assembled collection of ceramic raw materials. Next project is to fire more rocks to cone 6 and 10.

When both mineralogy and chemistry are shown on a data sheet

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Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.

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