In biomineralization the crystallization of minerals takes place in intimate contact with bio-organic molecules. These influence the lattice constants of the forming mineral. Herein we study this effect in the mineralized attachment organ (byssus) of the marine bivalve Anomia simplex. We compare two approaches to studying the biomolecule-induced lattice deformation: X-ray diffraction of annealed samples and in situ heating X-ray diffraction. In the latter case, the lattice deformation is obtained by extrapolation of the thermal expansion from high to low temperature. A positive lattice deformation was found along all three axes of the orthorhombic aragonite lattice. The microstrain fluctuations were found to be anisotropic and largest in the direction of smallest macrostrain. The all-positive macrostrain is in contrast to previous data from shells, showing that much remains to be learnt about the impact of biomolecules on minerals.