There are at the very least several i-adjectives can be used as na-adjectives by dropping the final い and adding な in its place. The most common examples of this, as far as I am aware, are 大きい and ...
Example 悲しさ is sadness, and 悲しみ is sadness too. What is the difference? According to Dictionary of Basic Japanese Grammar, -み "is more emotive and concrete characterization of some state" and -さ ...