Is of mine a prepositional phrase?

"Of mine" is not a prepositional phrase. It's a possessive pronoun acting as an adjective modifying a noun that is understood but not stated.

Here's why:

* Prepositional phrases always begin with a preposition (e.g., "of," "in," "on," "by"). "Of mine" doesn't have a preposition.

* Possessive pronouns show ownership (e.g., "mine," "yours," "hers," "his"). They are used as adjectives to modify nouns.

Example:

"That book is of mine."

Here, "of mine" modifies the understood noun "book." The full sentence could be "That book is one of mine."