Evaluation of strengthening mechanisms in calcite single crystals from mollusk shells Miki E. Kunitake, Lauren M. Mangano, John M. Peloquin, Shefford P. Baker ⇑ , Lara A. Estroff ⇑ Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA article info Article history: Received 4 May 2012 Received in revised form 21 September 2012 Accepted 24 September 2012 Available online 2 October 2012 Keywords: Biomineralization Mollusks Calcite Nanoindentation Hardness anisotropy abstract Biogenic single-crystal calcite is often reported to be harder and tougher than geologic calcite in the form of Iceland spar. However, the mechanistic origins of the superior mechanical properties of the biogenic materials are still debated. We investigate the hardness and modulus of biogenic calcite from the pris- matic layer of the mollusk Atrina rigida compared with a pure geologic calcite, Iceland spar. On the {0 0 1} face, biogenic calcite is found to be 50–70% harder than geologic calcite. This range is due to the fact that changes in azimuthal angle of the indenter tip lead to a hardness variation of 20% in A. rigida but only 7% in Iceland spar. The higher hardness and increased anisotropy of biogenic calcite could be accounted for by hardening mechanisms based on hindered dislocation motion rather than crack deflection. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Biomineralized tissues are hierarchically organized composites that are often reported to be stronger and tougher than their inor- ganic constituents [1–5]. However, the mechanical properties of these constituents are rarely measured because of their small size, and the measurements that do exist are widely scattered [6–9]. Therefore, understanding the mechanical properties of biogenic composites requires understanding the microstructure, properties and morphologies of the individual constituents at all appropriate length scales. As a model system, we compare the structure and mechanical properties of individual calcite single crystals from the outer shell of the mollusk Atrina rigida with those of a pure geo- logic calcite single crystal, Iceland spar. Biomineralized structures such as mollusk shells are composed of small mineral crystals embedded within a polymer matrix. The mechanical properties of these composites are typically evaluated using a classic engineering approach in which both phases are as- sumed to be homogeneous and the properties of pure single crys- tals are used to describe the mineral [3,4]. However, the individual mineral building blocks in biological tissues are themselves often composites. Analyses of large biogenic crystals have shown up to several weight percent of organic biomacromolecules trapped within the crystals [10–15]. These occluded biopolymers are thought to toughen the biogenic minerals [16]. In addition, multi- ple characterization techniques have revealed, in a range of bio- genic crystals, the presence of a ‘‘granular’’ sub-structure, which may be a remnant of a formation process that begins from an amorphous precursor [17,18]. Despite all of this evidence that indi- vidual biogenic crystals cannot be treated as homogeneous, pure crystals, very little work has been done to quantitatively evaluate their mechanical properties [6,8,19,20]. The focus of the current study is the outer shell of the bivalve A. rigida (Fig. 1a) [12]. The shell of this mollusk is composed of two layers, each containing mineral crystals surrounded by water- insoluble, organic matrices. In the inner, nacreous layer the min- eral is in the form of aragonite ‘‘tablets’’, and in the outer, prismatic layer the mineral is in the form of elongated single-crystal ‘‘prisms’’ of calcite (Fig. 1b). In the prismatic layer, the individual prisms are oriented with their long axes (the crystallographic c- axis) perpendicular to the shell surface (Fig. 1c). Each prism dif- fracts X-rays as a single crystal with coherence lengths similar to those measured for geologic calcite (from hundreds of nanometers to several micrometers) [21,22]. Recent transmission electron microscopy (TEM) studies have further confirmed that Atrina prisms, in contrast to prisms from other mollusk genera, can be treated as single crystals [23]. In addition, compositional analysis reveals that within each prism there are both inorganic impurities, in particular Mg 2+ , and organic macromolecules, including chitin and members of the Asprich protein family [11,12]. Based upon evidence from X-ray coherence length measurements, small-angle X-ray scattering and annular dark-field scanning transmission 1742-7061/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actbio.2012.09.030 ⇑ Corresponding authors. E-mail addresses: spb14@cornell.edu (S.P. Baker), lae37@cornell.edu (L.A. Estroff). Acta Biomaterialia 9 (2013) 5353–5359 Contents lists available at SciVerse ScienceDirect Acta Biomaterialia journal homepage: www.elsevier.com/locate/actabiomat