Optical relief (usually noted as simply relief) is a visually observable property in optical mineralogy used to identify minerals based on their

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

. Relief is determined by observing the degree to which grains stand out from a mounting medium of known refractive index Nesse, W. D., 1991, Introduction of Optical Mineralogy, 2nd edition, p. 25.,usually either oil or Canada Balsam. Relief is an important part of the

Becke line test The Becke line test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direct ...

Magnitude

Minerals that stand out significantly (have a difference in refractive index of .12 or more) have high or strong relief, and will have very sharp boundaries between itself and the material it is next to. Intermediate is .04 to .12, and low or weak is less than .04. Low relief materials have boundaries that are hard to distinguish from each other. File:Igneous rock gabbro. Photomicrograph with one polar (cropped to specific cpx and plag).jpg, Thin section under plane-polarized light with the high relief mineral clinopyroxene in the center surrounded by the low relief mineral plagioclase feldspar. File:Grtrelief.jpg, Thin section under plane-polarized light with the high relief mineral

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

(Grt) next to the intermediate relief mineral

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

(Bt).

Polarity

Relief is said to positive when the refractive index of the mineral is higher than the refractive index of the medium, and negative when the opposite occurs. In regular observations only the magnitude of relief can be seen (minerals with positive and negative relief will look the same) and other techniques (such as the Becke Line Method) must be employed in order to determine if a mineral has positive or negative relief.

References

*Nesse, W. D., 1991, Introduction of Optical Mineralogy, 2nd edition. Optical mineralogy {{petrology-stub