Optical relief (usually noted as simply relief) is a visually observable property in
optical mineralogy
Optical mineralogy is the study of minerals and Rock (geology), rocks by measuring their optics, optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrog ...
used to identify minerals based on their
refractive index
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
. 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
Canada balsam, also called Canada turpentine or balsam of fir, is the oleoresin of the balsam fir tree (''Abies balsamea'') of boreal North America. The resin, dissolved in essential oils, is a viscous, sticky, colourless or yellowish liquid t ...
. 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 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 ions of calcium (Ca), sodium (Na), iron (Fe ...
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 pr ...
.
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.
Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...
(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 al ...
(Bt).
Polarity
Relief is said to be 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
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