In this sentence

Plenty of silicon is found on Earth, but it remains a valuable material for the computer industry.
I'm still not sure if the simple subject is plenty or silicon, but I think it's silicon because the second part of the sentence says it.

of silicon is a prepsitional phrase

Anonymous is right; "silicon" is the object of the preposition "of" and cannot be the subject.

The subject is "plenty."

You are correct, the simple subject of the sentence is "silicon." The subject is the main noun or pronoun in the sentence, and in this case, it is "silicon." The word "plenty" is an adjective modifying the subject "silicon," indicating that there is a large amount of silicon.

To determine the simple subject, you can use two steps:

1. First, identify the main verb in the sentence. In this case, the main verb is "is found."

2. Next, ask yourself who or what is doing the action of the main verb. In this sentence, "is found" is a passive construction, which means that silicon is the thing being found, not doing the action of finding. Therefore, silicon is the one being described and is the subject.

So, in this sentence, "silicon" is the simple subject because it is the noun that the verb refers to. The phrase "but it remains a valuable material for the computer industry" refers back to silicon, so it helps to clarify that silicon is the subject in this sentence.