Isotropic vs. Anisotropic minerals

Reading 12

Isotropic minerals

appear black in cross-polarized light, no matter how you rotate the stage.  They do this because they did not alter the polarization of the light as it passed through, so the cross-polarizer destroys all the light. 

Anisotropic minerals

have colors that vary in cross-polarized light, and turn dark at regular intervals as you rotate the stage (extinction). These minerals have color because – somehow – they alter the polarization of light as it passes through them.

distribution of electrons in the mineral

n depends on:

The index of refraction

n is:

slower light

Higher n =

isotropic

All minerals in the cubic system are

optic axis

the path down which a mineral appears isotropic

uniaxial

one optic axis- tetragonal or hexagonal minerals

biaxial

 two optic axes- Orthorhombic, monoclinic, and triclinic minerals

double refracrion

 the beam of light splits into two beams, and they both refract through the mineral

negative

minerals with ω > ε

positive

minerals with ω < ε