Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rārē-făcĭo (written also separately rārē făcĭo), fēci, factum, 3, v. a. [rarus], to make thin or rare, to rarefy (Lucret.): rarefieri, Lucr. 1, 648: rarefacta, id. 2, 1139; 3, 442: rareque facit, id. 6, 233; 870.