We wrote <u>in our journals</u> for twenty minutes each day.
Answer:
adverbial phrase
<u>In our journals</u> cannot be a clause because it does not contain a subject and a verb.
It is a prepositional phrase because it contains a preposition, <em>in</em>, an object of the preposition, <em>journals</em>, and a word that modifies the object, <em>our</em>.
<u>In our journals</u> is prepositional phrase that acts as an adverb because it is modifying a verb, <em>wrote</em>. It is answering the question "Where?"
A prepositional phrase that acts as an adverb is an adverbial phrase.