Introduction Together with calcite, a trigonal CaCO 3 , the orthorhombic polymorph of Ca-carbonate, aragonite, is one of the most widespread biominerals, formed during the matrix biomineralization process [1]. Aragonite constitutes an important part of the material for limestone formations. During the transition of carbonate mud into limestone, aragonite transforms into calcite. This process proceeds at low temperatures, consequently, excitation of this transformation should be connected with a nature of bioaragonite. The heating of a biomineral is reported as a good tool for the modelling of fossilization [2]. Thermally induced transformation of Ca-carbonate phases is a well-studied matter in the case of geological aragonites [3] as well as bioaragonites [4]. Considering the thermal behaviour of bioaragonite of bivalves, the main attention has been devoted to the details of aragon- ite–calcite transition [5], especially with respect to the or- ganic compound impact on this process [6]. The shift of the aragonite–calcite transformation temperature from 250 to 500°C has been attributed to cationic substitu- tions [7] and hydration of aragonite [4, 8, 9]. In the orthorhombic structure, ions of Ca, being in the IX coordination, tend easily to be substituted by Sr, a phenomenon that might be treated as having a taxa-specific character. So, the highest Sr content in bioaragonites has been reported in coral biomi- neral [10] with an observable water temperature-de- pendent Ca/Sr ratio [11]. A more moderate Sr content has been reported to be characteristic of molluscan bioaragonite, snails and bivalves [12]. Another important substitution into bioaragonite lattice is a water molecule, assumed by Passe-Coutrin [4] and proved by Verma [13]. Basically, there exist two structural forms of hydrated Ca-carbonates: trigonal (rhombohedral) monohydrocalcite CaCO 3 ·H 2 O [14], and monoclinic ikaite, CaCO 3 ·6H 2 O [15]. In both, the water is involved in a Ca-carbonate structure via hydrogen bonding. The first might be treated as a hydrated modification of calcite, and the second as a hydrated modification of aragonite (with lowered symmetry). Both have been reported to tolerate the substitutions in cation position, such as Sr, Na, Fe and Mg. The recent works of Verma [13] show clearly the unisotropy character of water substitution into bioaragonite lattice. The thermal behaviour of different types of water, bound into minerals, has been well discussed by Földavari [16], revealing that TA is a suitable method to distinguish the different bonding manners. In this work, an attempt to clarify the low-tem- perature changes of bioaragonite with respect to lattice changes, thermal effects and evolved gas com- position is made. Experimental Materials Subfossil shells of Tapes decussatus were collected on the shore of the Irish Sea near Carnarfon, Wales. The shells were cleaned of periostracum by 50% H 2 O 2 , washed in distilled water, dried and shattered into 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2009 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, 2009 TEMPERATURE-INDUCED CHANGES IN CRYSTAL LATTICE OF BIOARAGONITE OF TAPES DECUSSATUS LINNAEUS (MOLLUSCA: BIVALVIA) J. Nemliher 1 , K. Tánsuaadu 2* and T. Kallaste 1 1 Institute of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia 2 Laboratory of Inorganic Materials, Ehitajate tee 5, 19086, Tallinn, Estonia The characteristics of bioaragonite of shells of recent T. decussatus during heating were studied by the means of TG-DTA-EGA (FTIR), XRD, XRF and FTIR. The mass loss recorded up to 2.5% appeared with the higher rates at 110–150, 200–250, 295–300, and 390–415°C at heating of 10°C min –1 up to 500°C. IR analysis of the evolved gases revealed the emission of water and CO 2 . The lattice constants tend to change with anisotropy character (parameters a and c diminish whilst b tends to grows) and with an overall contraction of cell volume (from 227.36 to 226.84  3 ) during heating was established. The peculiarity of bioaragonite was explained by substitution of H 2 O and sulphate ion into the lattice. In spite of those substitutions, bioaragonite reveals an ortho- rhombic structure, which is preserved during the changes up to calcite formation above 380°C. Keywords: bioaragonite, crystal structure change, structural substitutions, TG/DTG/DTA/TG-FTIR * Author for correspondence: kaiat@staff.ttu.ee