Todorokite - Optical Properties

Optical Properties

Todorokite is biaxial, as are all monoclinic (and orthorhombic) minerals. In the polariscope, and in the polarizing microscope, specimens may be illuminated from below by light that is polarized by the polarizer, and viewed from above through an analyzer that transmits light of only one direction of polarization. When the directions of polarization of the polarizer and analyzer are at right angles, the specimen is said to be viewed between crossed polars. When todorokite is rotated between crossed polars it appears dark and light in turn, being dark when the crystal face or cleavage face is parallel to one direction of polarization. This is called parallel extinction. All uniaxial minerals display parallel extinction, but so do orthorhombic biaxial minerals such as olivine and orthopyroxenes.

The refractive index of todorokite has not been determined, except insofar as it is very high; the original report gave it as greater than 1.74, and a later investigation put it even higher, greater than 2.00. For comparison, diamond has a refractive index of 2.42 and quartz 1.54. A biaxial crystal has three mutually perpendicular optical directions, X, Y and Z, with different refractive indices α, β and γ for light vibrating in planes perpendicular to these directions. The birefringence is the numerical difference between the greatest and the least of these indices; for todorokite it is nearly 0.02. Todorokite is distinctly pleochroic, appearing dark brown when viewed along the X direction, and yellowish brown when viewed along the Z diretion, but the strength of the effect varies from faint to strong in material from different localities. The orientation of optical directions with respect to the lattice parameters is Y parallel to b and Z near or parallel to c.