The effect of magnesium substitution on the hardness of synthetic and biogenic calcite
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esearch Letters
The effect of magnesium substitution on the hardness of synthetic and biogenic calcite Miki E. Kunitake, Shefford P. Baker*, and Lara A. Estroff*, Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 *Address all correspondence to Shefford P. Baker and Lara A. Estroff at [email protected] and [email protected] (Received 2 July 2012; accepted 20 August 2012)
Abstract Biogenic minerals often contain inorganic and organic impurities that are believed to harden and toughen the material. However, because of the complexity of these systems, it is difficult to deconvolute the effect of each of these impurities on the hardness of the material. We have created single-crystal samples with a range of magnesium concentrations and measured their hardness while controlling for orientation. We find that hardness increases linearly with magnesium content and that magnesium impurities could account for ∼20% of the increased hardness in biogenic calcite from the mollusk Atrina rigida when compared with pure geologic calcite.
Many organisms synthesize amazingly hard and tough functional materials by combining inorganic minerals with organic macromolecules in complex, hierarchical structures.[1] For example, sea urchin teeth, which contain high magnesium content polycrystalline calcite (CaCO3) embedded in a proteinbased matrix, are hard and tough enough to grind limestone rocks.[2] Although biogenic calcite has been reported to be harder than geologic calcite,[3–8] the reasons for the increased hardness are not well understood. Changes in hardness have been speculated to arise from differences in magnesium[3–5,7,8] and organic macromolecule content,[4,6,7,9] as well as microstructural features including crystallographic texture,[4,6] crystallite size,[10] and inorganic-to-organic ratio.[3] However, it is not possible to quantify the relative importance of these compositional and microstructural differences from the existing data, because multiple parameters vary simultaneously between hardness measurements of different biologic systems and even within measurements of a single system. [5,8] In previous work,[4] we showed that the hardness of singlecrystal calcite is sensitive to the orientation of the crystal (both the crystal face indented and the azimuthal angle of the indenter with respect to that face), and that the combination of the mechanisms listed above leads to a hardness for single-crystal calcite from the outer shell of the mollusk Atrina rigida that is as much as 70% greater than the hardness of a relatively pure geologic single-crystal calcite, Iceland spar (at the same orientation). To provide more detailed insight into the effect of magnesium incorporation on the hardness of single-crystal calcite, we have produced synthetic samples with excellent control of both composition and microstructure, and investigated their mechanical properties using nanoindentation.
We created samples with a range of Mg concentrations by epitaxially overgrowing synthetic calcite seed cry
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