"It was that that caused his resignation."
 
How do members describe the that-clause?  I want to call it a noun clause
functioning as the delayed subject, but you can't turn the sentence
around as you can other sentences with noun clause delayed subjects ("It
is a pity that we missed each other" can become "That we missed each
other is a pity").
 
There's also a definite sense in which "that" is functioning as a
relative pronoun introducing a relative clause modifying the first
"that."
 
Is there some in-between status for "that"?  Is there a name for it?
 
Michael Kischner