日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Now this confuses the hell out of me.

To becone silent: しずかになる vs しずかなる
My book (namely, Genki) says you need the に, but google has found much more results without it.
So which one is correct? If both are correct, what's the difference between them?