Calcium carbonate can be found in many things, from chalk, mollusk shells, and rocks to pearls, eggshells, and coal balls. There’s a variability to the strength the chemical compound exhibits. And material scientists from the Pacific Northwest National Laboratory (PNNL) are looking into what gives calcium carbonate crystals their strength.

Publishing in Nature Communications, scientists have found that clumps of soft biological matter, incorporated into the atoms in the crystals via chemical reactions, help give the material strength. By learning the mechanisms underlying the formation of this natural material, scientists hope to develop new materials for a sustainable energy future.

“The strength of a material depends on how easy it is to disrupt its underlying crystal matrix. If a material is compressed, then it becomes harder to break the matrix apart. Proteins trapped in calcium carbonate crystals create a compressive force—or strain—within the crystal structure,” according to PNNL. “Unlike the strain that makes muscles sore, this compressive strain is helpful in materials, because it makes it harder to disrupt the underlying crystal structure, thereby adding strength.”

The research team used atomic force microscopy to observe how calcium carbonate incorporates proteins and other strength-giving particles into its structure. During the experiment, they used a concentration of calcium carbonate which naturally forms the crystalline mineral calcite. During its formation, calcite creates uneven surfaces, an array of steps and terraces, as the laboratory described it. Spheres of organic molecules, called micelles, were added as the inclusion material.

The scientists observed that the micelles weren’t randomly distributed on the calcium carbonate. Rather, they stuck to the edges of the steps, “The step edge has chemistry that the terrace doesn’t,” said Jim De Yoreo, a material scientist with PNNL. “There are these extra dangling bonds that the micelles can interact with.”

The micelles were subsequently enveloped by the forming crystal. A mathematical simulation confirmed the micelles were compressed like springs, creating “strain in the crystal lattice between the micelles, leading to enhanced mechanical strength,” according to PNNL.

According to De Yoreo, the new work may provide scientists with ideas regarding how to store carbon dioxide in useful materials, or for incorporating light-responsive particles into crystals for solar energy applications.