Diaplectic changes in minerals result from passage of high-pressure shock waves through rocks. These changes are an important source of information concerning impact processes. To the author’s knowledge, the decomposition of mafic minerals due to shock has not been the subject of detailed studies until recently. Aggregates that form as the result of such changes of mafic minerals include feldspar (alkali feldspar or plagioclase, more rarely both of them), orthopyroxene with elevated alumina content, a mineral of the spinel group (magnetite or ilmenite or hercynite), amphibole, and clinopyroxene. The study of biotite, staurolite, garnet, amphibole, and clinopyroxene in impactites of the Janisjarvi, Popigay and Puchezh-Katunky astroblemes shows that new minerals are formed during selective melting. During such a process the rock, as a whole, remains solid. Recrystallization is practically instantaneous, as the secondary aggregates are very Fine grained (a few micrometers in size). It has been established, by using a scanning electron microscope, microprobe analyses, and the Mössbauer spectroscopy, that this process was accompanied by intensive shock diffusion of material, resulting in an exchange of chemical components between new minerals and the host rocks. This process is accompanied by an increase in oxygen activity and iron oxidation. Superheated vapor-water fluid is very important in this process. The temperature of crystallization varies considerably from one mineral to another and within one grain the temperature may have exceeded the average residual temperature of the whole specimen.