The word rarefaction is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 18
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (33 words)
aconite, aerator, airfare, cantref, carinae, carrion, certain, conifer, cornier, crafter, creator, crofter, enactor, erotica, erratic, faction, fainter, fanatic, fancier, fireant, frantic, infarct, narrate, ornater, otarine, ratrace, reactor, recraft, refract, refrain, retrain, terrain, trainer