BECKE LINE TEST

 

            Evaluating the mineral sample under plane-polarized light of petrographic polarizing microscope used in optical mineralogy as used in observing relief, the low- or moderate-relief minerals concentrate light as a thin bright white line along their margins. This line, as described in the field of optical mineralogy is called the Becke line, will move inwards or outwards if the mineral grain is brought slowly into or out of focus of the polarizing microscope. The following rule regarding the direction of movement of the Becke line enables the comparison between the refractive index of the mineral and the Canada balsam and that is usually at the edge of the thin section, or the refractive index of two adjacent minerals. The Becke line usually moves as found clearly visible under polarized microscope used in optical mineralogy into the substance of higher refractive index when the distance between objective and section is increased. It is however very important to note that Becke lines cannot be successfully observed with very high-relief minerals.

 

TWINKLING

 

            It has been known in the field of optical mineralogy that some minerals with a high birefringence have one refractive index close to that of Canada balsam and the other one considerably different. Such mineral change markedly in relief on rotation of the stage of petrographic polarizing microscope in plane-polarized light, this phenomenon is called twinkling. One good example of a mineral that twinkles is mineral Calcite.