There is also an opinion that it is a water sign. Once the zodiacal wheel is moved, Libra becomes neutral element, Ophiuchus water and Scorpio air. Then everything else remains the same. It makes for many types of symmetry. Look here: http://theabysmal.wordpress.com/2011/01/17/ophiuchus-or-serpentus/
Maybe its an ether sign. Ether, according to the Greeks is the fifth element which they thought that the heavenly bodies were composed.