The single-crystal elastic moduli of hedenbergite (CaFeSi2O6) have been measured at 20 ¿C and 1 bar using Brillouin spectroscopy. The moduli are (in gigapascals): C11=222, C22=176, C33=249, C44=55, C55=63, C66=60, C12=69, C13=79, C23=86, C15=12, C25=13, C35=26, C46=-10. The comparison of elastic properties among Mg-Fe-Ca bearing pyroxenes, known as quadrilateral pyroxenes, reveals only weak variations with changes in composition. Of the four quadrilateral pyroxenes, orthoferrosilite has elastic properties distinctive from the others. The principal differences among these pyroxenes are due to subtle structural differences. In particular, the mechanical linkage between the M2 polyhedral chains in clinopyroxenes enhances the importance of the cation in this site. In contrast to the orthopyroxenes, the aggregate shear modulus μ of the calcium-bearing clinopyroxenes (diopside and hedenbergite) exhibits no dependence on the amount of iron (Fe2+) present in the structure, while the ratio K/μ does. As a result, the compressional and shear acoustic velocities of the calcium-bearing clinopyroxenes show a smaller dependency on iron content than do the orthopyroxenes. ¿ American Geophysical Union 1988