Eur. Phys. J. B 73, 167–175 (2010) DOI: 10.1140/epjb/e2009-00423-9 Piezoelastic properties of retgersite determined by ultrasonic measurements D. Arbeck, E. Hauss¨ uhl, L. Bayarjagal, B. Winkler, N. Paulsen, S. Hauss¨ uhl and V. Milman