But is used to combine two contrasting principal clauses. It is placed between the two clauses. It is placed at the beginning of the subordinate clause which can go before or after the principal clause. The two words are placed before the two subjects or complements. They are placed before the double subjects or complements.
Oversaturated rocks can contain neither olivines nor feldspathoids .
For is a coordinating conjuction and cannot technically subordinate one clause to another. However is used to combine a contrasting principal and subordinate clause. It can only be placed between the two clauses, and must be preceded by a comma or semicolon. Either clause can be placed first, but the second clause automatically becomes subordinate. It is more generally used at the beginning of a sentence to introduce a contrasting idea to has gone before.