Sphere

A magic's sphere refers to its type of magic. There are three known spheres: Elemental, Mental, and Spiritual. Each individual spell circle draws its influences from one or more spheres. For instance, the Cleric Base circle is spiritual in nature, whereas the Sorcerer Base is a hybrid of both spiritual and elemental magics.

Spell Circle	Elemental	Mental	Spiritual
Minor Spiritual			X
Major Spiritual			X
Cleric Base			X
Minor Elemental	X
Major Elemental	X
Ranger Base			X
Sorcerer Base	X		X
Wizard Base	X
Bard Base	X	X
Empath Base		X	X
Savant Base		X
Minor Mental		X
Major Mental		X
Paladin Base			X

Resources

http://www.play.net/gs4/info/magic-guide.asp