14.6 Å and a = 24.38 Å, having the same b and c values as above. Intergrowths of elongated crystallites resembling the twinned variety of acicular rutile have been observed by electron microscopy in todorokites from an iron-manganese concretion from the Pacific Ocean (a = 14.6 Å) and from the Bakal deposit (a = 14.6 Å and 24.4 Å). Todorokites with around 25 Å have been found in samples from Sterling Hill and the Takhte-Karacha deposit. Appearance Todorokite occurs as spongy banded and reniform (kidney-shaped) aggregates composed of minute lathlike crystals. The crystals are flattened parallel to the plane containing the a and c crystal axes, and elongated parallel to the c axis. Minerals of the hollandite-cryptomelane and romanèchite groups also have fibrous or acicular habits and two perfect cleavages parallel to the fiber axis. Todorokite is dark brown to brownish black in color and brown in transmitted light. It has a black to dark brown streak and the luster is metallic to dull, but silky in aggregates. It is opaque in all but the thinnest slivers, which are transparent. 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