NESSE: Read pages 127-130. Uniaxial minerals are minerals with only 2 indices of refraction, or to put it another way, minerals that have a=a for axial relations (hexagonal and tetragonal). These minerals have a single optic axis parallel to the c-direction. All of the other minerals have 3 indices of refraction, and are called biaxial and have 2 optic axes. You can think of this like the difference between a circle and an ellipse. In a uniaxial mineral, the center of the circle is where the c-axis comes thru and the radius to the edge of the circle is the same in all directions. So far so good. In a biaxial mineral a doesn't equal b so the circle is now elongated in either the a or b direction (doesn't matter which one). So the cross section is an ellipse with c coming through the center. However, since this is light, we are really talking about Na and Nb. In order to define an ellipse you have a short axis and a long axis and 2 centers. These are the optical centers or optic axes. In orthorhombic minerals they are symmetrical about the c-axis since the crystallographic axes are still perpendicular. In the Monoclinic and Triclinic Systems, the optic axes can move around since the crystallographic axes are no longer perpendicular. What you need to know: OPTIONAL READING Extinction Accessory Plates