Leaving academia for financial reasons - how to communicate the reason of departure in a professional manner?
Express that you have enjoyed working in your department. (Even if this is not entirely true.)
Explain that you are only leaving because of an opportunity "too exciting to turn down." Don't volunteer exactly why it is exciting; if asked, talk about what you'll be doing, not what you'll be paid.
If there's a way you can help the department while you're in industry -- serving on a committee, helping with accreditation, accepting an intern, things like that -- it would be kind of you (though it is not obligatory) to volunteer unasked.
Keep your resignation letter short and cordial.


These tactics should suffice to keep your departure as amicable as it can be, assuming you're doing the normal things to minimize disruption (e.g. not leaving mid-term).