Optical relief
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Optical relief (usually noted as simply relief) is a visually observable property in
optical mineralogy Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope. Opti ...
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.


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 The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe I ...
in the center surrounded by the low relief mineral
plagioclase feldspar Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prop ...
. 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 (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