Acid rain from the Cretaceous-Tertiary (K-T) boundary impact event should have caused significant damage to freshwater life, but only minor extinctions of freshwater species are actually observed. We propose a mechanism to neutralize the acid using larnite (β-Ca2SiO4), produced as a result of the specific lithology at the Chicxulub impact site. The impact vapor plume must have been enriched in calcium from the carbonate-rich target, leading to the crystallization of larnite. The acid-neutralizing capacity of the larnite grains would have been high enough to consume acid produced after the K-T event within several hours, reducing it to a level at which freshwater life would not have been affected, even if all the acid had precipitated instantaneously after the K-T impact. This scenario can explain some of the extinction selectivity at the K-T boundary.