A mysterious (to me ;) ) usage of "de"

[georgepepper]

I was given to translate this: Il a de grands chats.
And the correct answer was: He has big cats.

Why "de" and not "des"?

I read French lessons discussing various uses of "de" and couldn't find one that matches. Is it a possesive "de"? Part of an expression? Something else? I'm lost.. :-/

[Remy]

Hi Georgepepper, can you please have a look at this link?

Thanks!

[georgepepper]

I think you forgot to add the link itself...
And thank you very very much for helping create this amazing site :-D

Update: I found the answer. If anyone had the same problem - A "des" (plural article), before an adjective that precedes a (plural) noun, changes to "de".

[Remy]

Exactly, here it is!

http://duolingo.com/#/comment/45416