Twin density of aragonite in molluscan shells characterized using X-ray diffraction and transmission electron microscopy Toshihiro Kogure a,n , Michio Suzuki b , Hyejin Kim a , Hiroki Mukai a , Antonio G. Checa c , Takenori Sasaki d , Hiromichi Nagasawa b a Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan b Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan c Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain d University Museum, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan article info Article history: Received 9 January 2014 Received in revised form 19 February 2014 Accepted 19 March 2014 Communicated by S. Veesler Available online 8 April 2014 Keywords: A1. Biocrystallization A1. Defects A1. {110} twin A1. X-ray diffraction B1. Aragonite B1. Calcium compounds abstract {110} twin density in aragonites constituting various microstructures of molluscan shells has been characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM), to find the factors that determine the density in the shells. Several aragonite crystals of geological origin were also investigated for comparison. The twin density is strongly dependent on the microstructures and species of the shells. The nacreous structure has a very low twin density regardless of the shell classes. On the other hand, the twin density in the crossed-lamellar (CL) structure has large variation among classes or subclasses, which is mainly related to the crystallographic direction of the constituting aragonite fibers. TEM observation suggests two types of twin structures in aragonite crystals with dense {110} twins: rather regulated polysynthetic twins with parallel twin planes, and unregulated polycyclic ones with two or three directions for the twin planes. The former is probably characteristic in the CL structures of specific subclasses of Gastropoda. The latter type is probably related to the crystal boundaries dominated by (hk0) interfaces in the microstructures with preferred orientation of the c-axis, and the twin density is mainly correlated to the crystal size in the microstructures. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Molluscan shells can be called “master” biominerals. Owing to their elaborated and regulated microstructures, molluscan shells attain extremely higher toughness against fracture than the same minerals of geological origin [1,2]. The crystalline phases of calcium carbonate forming molluscan shells are mainly calcite and aragonite. Calcite is thermodynamically the most stable phase at the ambient temperature and pressure [3], but aragonite is also common in many shells, though it has a stability field at a high pressure [4]. It is well known that geological, biogenic and synthetic aragonite crystals contain {110} twins [5,6] which are easily identified using polarized optical microscopy and, if the twins are microscopic, using transmission electron microscopy (TEM). Generally the crystal size of aragonite shells is so small that the observation of the twins has been performed mainly by TEM. The {110} twins were reported in various shell structures consisting of aragonite; in the crossed- lamellar (CL) microstructures [7–14], larval shells [15,16], ligament of shell [17], shell spikes [18], gastropod nacre [19], etc. mainly by using TEM. However, the twinning is not a ubiquitus feature in the aragonite shells. For instance, {110} twins are seldom in aragonite plates of the nacreous layer near the growth front of gastropod shells [20]. Accordingly, quantitative comparison of {110} twin density in aragonite may be informative to consider the formation mechanism of various shell microstructures. For this purpose, probably characterization by only TEM is not sufficient to obtain convincing, quantitative results because the areas observable in TEM are so limited. Recently we proposed a method to evaluate {110} twin density in aragonite from the peak widths of specific reflections in the X-ray diffraction (XRD) pattern and reported the results for several biogenic aragonite specimens [21]. According to computer simulation, existence of {110} twins influences unevenly the peak widths of individual reflections of aragonite. Conversely, the difference of the peak widths between specific reflections can be used to estimate twin density in the specimens. In this study, we focus on molluscan shells consisting of aragonite, estimate the twin Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth http://dx.doi.org/10.1016/j.jcrysgro.2014.03.029 0022-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail addresses: kogure@eps.s.u-tokyo.ac.jp (T. Kogure), amichiwo@mail.ecc.u-tokyo.ac.jp (M. Suzuki), hyejin.kim.fb@hitachi.com (H. Kim), acheca@ugr.es (A.G. Checa), sasaki@um.u-tokyo.ac.jp (T. Sasaki), anagahi@mail.ecc.u-tokyo.ac.jp (H. Nagasawa). Journal of Crystal Growth 397 (2014) 39–46