Can a floating shelf be a fireplace Mantel?

Of course a floating shelf can be used as a mantel! The main difference is.. there is no difference.

A floating shelf instantly becomes a mantel as soon as its attached to the wall above a fireplace.

When folks are questioning on this topic the question at hand is more like:

 Does a mantel need to be non-combustible material?

 The answer is no. However, a mantel does need to be a minimum distance above the opening of the firebox. In most situations the minimum distance is at least 12". This is not something to monkey around over. Be sure to check your local code, its often as simple as a google search: (name of your city and state) + "fireplace opening minimum distance to combustibles"

If its a manufactured firebox insert, be sure to check and comply with the firebox manufacturer's specified minimum distance to combustibles.