Depending on the environment of formation, many minerals can have isomorphic substitution of various elements in the crystall structure which can alter the optical properties. This chemical variation is due to the fact that structurally equivalent positions in the crystallographic framework may be occupied by ions of different elements. Many minerals belong to groups having a solid solution series between two end members, making it often difficult to distinguish a particular member. This effectis often observed by differences in color, change in refractive index, an dtwinning pattern. Crystal size .
Most clay minerals are too small to identify in the petrographic microscope.
Thus, interference colors observed through the microscope may not match the sequence of colors in the order listed in color charts. In such instances, the determination of optic sign or interference figure may be required for mineral identification. Alteration due to physical or chemical weathering . Many minerals may be physically rounded during transport such that crystal faces are no longer present. Chemical weathering may obsure the identity of th eoriginal grain.