Refraction and Double Refraction in Minerals (Plus Refractive Index Chart)

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Identifying minerals can be a challenging task, but understanding the unique properties of different minerals can help us narrow down our options. In this article, we’ll explore the concepts of refraction and double refraction and how they can be used in mineral identification.

What Is Refraction?

Refraction is the phenomenon of light being bent as it passes through different materials. When light travels from one medium to another with a different refractive index, it changes direction due to refraction. The degree to which light is bent depends on the refractive index of the material it is passing through.

The refractive index of a material is a measure of how much the material slows down light as it passes through it. Materials with higher refractive indices will bend light more than materials with lower refractive indices. This is why, for example, when light travels from air into water, it is bent at the surface of the water and takes a new direction within the water.

Some common minerals that exhibit refraction include diamonds and garnets. These minerals are transparent and have high refractive indices, which means that they bend light significantly as it passes through them. This is one of the reasons why diamonds and garnets are prized for their sparkling, brilliant appearance.

Measuring Refractive Index

The refractive index of a mineral is an important characteristic that can help us identify and classify different minerals. Refractive index is a measurement of the speed at which light travels through a particular mineral. This measurement of light can then be used to help in mineral and gemstone identification.

There are various methods for measuring the refractive index of a mineral, but most of them involve using specialized optical instruments and ultimately applying a certain amount of math. fortunately, advances have been made and it’s become much simpler to measure mineral refraction at home with the use of a common instrument called a refractometer.

How a Refractometer Works

To measure the refractive index of a mineral using these instruments, a beam of light is passed through the mineral and the degree to which it is bent is measured. The refractive index is then calculated based on the amount of bending that occurs.

Measuring the refractive index of a mineral can be a complex task that requires specialized training and equipment. However, the work has already been done by competent scientists, and tables of refractive indices for various minerals have been compiled and are widely available. These tables provide values for the refractive indices of different minerals, making it easier to identify minerals based on their refractive properties. I’ve provided a list of common minerals and their refractive indices at the end of this article.

Read More: How To Use a Refractometer to Identify Rocks, Minerals and Gemstones

What Is Double Refraction?

Double refraction, also known as birefringence, is a property that is exhibited by certain minerals when a beam of light is split into two paths as it passes through the mineral. This occurs because the mineral has a different refractive index in different directions, which causes light to be refracted differently depending on its direction of travel.

Minerals that exhibit strong double refraction include peridot, zircon, sphene, epidote, tourmaline, kunzite, ruby, sapphire, topaz, amethyst, quartz, emerald, aquamarine, and chrysoberyl. These minerals can be distinguished from other minerals based on their strong double refraction.

Double refraction can be observed by viewing the edges of the rear facets of a mineral through a lens. When double refraction is present, the edges of the facets will appear double-lined. This simple test can be used to distinguish minerals that exhibit strong double refraction, such as zircon, from minerals that do not, such as diamond.

Using Refraction and Double Refraction in Mineral Identification

Refraction and double refraction are just two of the many tools that can be used to identify minerals. It’s important to use multiple methods and consider all available information when trying to identify a mineral, as no single method is foolproof.

Here are a few tips for using refraction and double refraction in mineral identification:

• Compare the mineral to known specimens: If you have access to a collection of known minerals, you can compare the refractive properties of the mineral you are trying to identify to those of known minerals. This can help you narrow down your options and make an educated guess about the identity of the mineral.
• Use a lens to observe double refraction: As mentioned earlier, minerals that exhibit strong double refraction can be distinguished from other minerals by observing the edges of the rear facets through a lens. If the edges appear double-lined, the mineral is likely one that exhibits strong double refraction.
• Consult tables of refractive indices (below): As mentioned earlier, tables of refractive indices for various minerals are widely available and can be a helpful resource for identifying minerals based on their refractive properties. By comparing the refractive index of the mineral you are trying to identify to the values listed in the tables, you can get a better idea of what the mineral might be.

By using these tips and considering all available information, you can increase your chances of accurately identifying minerals using refraction and double refraction.

List of Mineral Refractive Indices and Double Refraction Values

Credit for the following information: International Gem Society

Mineral	Refractive Index	Double Refraction
Hematite	2.940-3.220	0.287
Cinnabar	2.905-3.256	0.351
Proustite	2.881-3.084	0.203
Pyrargyrite	2.880-3.080	0.200
Cuprite	2.849	none
Rutile	2.616-2.903	0.287
Brookite	2.583-2.700	0.117
Anatase	2.488-2.564	0.046-0.067
Diamond	2.417-2.419	anomalous
Fabulite	2.409	none
Stibiotantalite	2.370-2.450	0.080
Sphalerite	2.368-2.371	none
Crocoite	2.290-2.660	0.270
Wulfenite	2.280-2.400	0.120
Tantalite	2.260-2.430	0.160
Linobate	2.210-2.300	0.090
Manganotantalite	2.190-2.340	0.150
Cubic zirconia (CZ)	2.150-2.180	none
Mimetite	2.120-2.135	0.015
Phosgenite	2.114-2.145	0.028
Senarmontite	2.087	none
Boleite	2.030-2.050	0.020
Zincite	2.013-2.029	0.016
Cassiterite	1.997-2.098	0.096-0.098
Simpsonite	1.994-2.040	0.058
Gadolinium gallium garnet (GGG)	1.970-2.020	0.058
Sulfur	1.958-2.245	0.291
Bayldonite	1.950-1.990	0.04
Scheelite	1.918-1.937	0.010-0.018
Andradite garnet	1.880-1.940	none
Anglesite	1.878-1.895	0.017
Uvarovite garnet	1.865	none
Purpurite	1.850-1.920	0.007
Sphene (titanite)	1.843-2.110	0.100-0.192
Yttrium aluminum garnet (YAG)	1.833-1.835	none
Zircon	1.810-2.024	0.002-0.059
Cerussite	1.804-2.079	0.274
Gahnite	1.791-1.818	none
Spessartite garnet	1.790-1.820	none
Painite	1.787-1.816	0.029
Monazite	1.774-1.849	0.049-0.055
Almandine garnet	1.770-1.820	none
Gadolinite	1.770-1.820	0.01-0.04
Ruby (corundum)	1.762-1.778	0.008
Sapphire (corundum)	1.762-1.778	0.008
Benitoite	1.757-1.804	0.047
Shattuckite	1.752-1.815	0.063
Chrysoberyl	1.746-1.763	0.007-0.011
Periclase	1.740	none
Scorodite	1.738-1.816	0.027-0.030
Staurolite	1.736-1.762	0.010-0.015
Grossular garnet	1.734-1.759	none
Chambersite	1.732-1.744	0.012
Hessonite garnet	1.730-1.757	none
Epidote	1.729-1.768	0.015-0.049
Pyroxmangite	1.726-1.764	0.016-0.020
Azurite	1.720-1.848	0.108-0.110
Pyrope garnet	1.720-1.756	none
Hodgkinsonite	1.719-1.748	0.022-0.026
Taaffeite	1.719-1.730	0.004-0.009
Rhodonite	1.716-1.752	0.010-0.014
Gahnospinel	1.715-1.754	none
Spinel	1.712-1.762	none
Kyanite	1.710-1.734	0.015-0.033
Adamite	1.708-1.760	0.048-0.050
Diaspore	1.702-1.750	0.048
Serendibite	1.701-1.743	0.005
Sapphirine	1.701-1.734	0.004-0.007
Aegirine-augite	1.700-1.800	0.030-0.050
Idocrase (vesuvianite)	1.700-1.723	0.002-0.012
Tanzanite	1.691-1.700	0.009
Neptunite	1.690-1.736	0.029-0.045
Willemite	1.690-1.723	0.028-0.033
Rhodizite	1.690	none
Triphylite	1.689-1.702	0.006-0.008
Lithiophilite	1.680-1.700	0.01
Dumortierite	1.678-1.689	0.015-0.037
Legrandite	1.675-1.740	0.060
Hypersthene	1.673-1.731	0.010-0.016
Parisite	1.671-1.772	0.081-0.101
Clinozoisite	1.670-1.734	0.010
Sinhalite	1.665-1.712	0.036-0.042
Lawsonite	1.665-1.686	0.019-0.021
Diopside	1.664-1.730	0.024-0.031
Bustamite	1.662-1.707	0.014-0.015
Kornerupine	1.660-1.699	0.012-0.017
Hiddenite	1.660-1.681	0.014-0.016
Kunzite	1.660-1.681	0.014-0.016
Boracite	1.658-1.673	0.010-0.011
Axinite	1.656-1.704	0.010-0.012
Malachite	1.655-1.909	0.254
Sillimanite	1.655-1.684	0.014-0.021
Jadeite	1.652-1.688	0.20
Peridot	1.650-1.703	0.036-0.038
Ludlamite	1.650-1.697	0.038-0.044
Enstatite	1.650-1.680	0.009-0.012
Euclase	1.650-1.677	0.019-0.025
Phenakite	1.650-1.670	0.016
Dioptase	1.644-1.709	0.051-0.053
Jet	1.640-1.680	none
Eosphorite	1.638-1.671	0.028-0.035
Spurrite	1.637-1.681	0.039-0.040
Jeremejevite	1.637-1.653	0.007-0.013
Barite	1.636-1.648	0.012
Siderite	1.633-1.875	0.242
Danburite	1.630-1.636	0.006-0.008
Clinohumite	1.629-1.674	0.028-0.041
Apatite	1.628-1.649	0.002-0.006
Andalusite	1.627-1.649	0.007-0.013
Friedelite	1.625-1.664	0.030
Smithsonite	1.621-1.849	0.228
Datolite	1.621-1.675	0.040-0.050
Celestite	1.619-1.635	0.010-0.012
Tourmaline	1.614-1.666	0.014-0.032
Actinolite	1.614-1.653	0.020-0.025
Hemimorphite	1.614-1.636	0.022
Lazulite	1.612-1.646	0.031-0.036
Prehnite	1.611-1.669	0.021-0.039
Gaspéite	1.610-1.810	0.22
Turquoise	1.610-1.650	0.040
Topaz	1.609-1.643	0.008-0.016
Sugilite	1.607-1.611	0.001-0.004
Sogdianite	1.606-1.608	0.002
Brazilianite	1.602-1.623	0.019-0.021
Rhodochrosite	1.600-1.820	0.208-0.220
Odontolite	1.600-1.640	0.010
Nephrite	1.600-1.627	0.027
Pectolite (larimar)	1.595-1.645	0.038
Montebrasite	1.594-1.633	0.22
Phosphophyllite	1.594-1.621	0.021-0.033
Meliphanite	1.593-1.612	0.019
Eudialyte	1.591-1.633	0.003-0.010
Chondrodite	1.592-1.646	0.028-0.034
Catapleiite	1.590-1.629	0.039
Wardite	1.590-1.599	0.009
Herderite	1.587-1.627	0.023-0.032
Colemanite	1.586-1.615	0.028-0.030
Howlite	1.586-1.605	0.019
Zektzerite	1.582-1.585	0.003
Amblygonite	1.578-1.646	0.024-0.030
Ekanite	1.572-1.573	0.001
Anhydrite	1.570-1.614	0.044
Augelite	1.570-1.590	0.014-0.020
Emerald (beryl)	1.565-1.602	0.006
Aquamarine (beryl)	1.564-1.596	0.004-0.005
Variscite	1.563-1.594	0.031
Beryl (precious)	1.562-1.602	0.004-0.010
Tremolite	1.560-1.643	0.017-0.027
Vivianite	1.560-1.640	0.050-0.075
Serpentine	1.560-1.571	0.008-0.014
Larbradorite	1.559-1.570	0.008-0.010
Hambergite	1.553-1.628	0.072
Pyrophyllite	1.552-1.600	0.048
Muscovite	1.552-1.618	0.036-0.043
Beryllonite	1.552-1.561	0.009
Charoite	1.550-1.561	0.004-0.009
Amethyst (quartz)	1.544-1.553	0.009
Aventurine (quartz)	1.544-1.553	0.009
Rose quartz	1.544-1.553	0.009
Citrine (quartz)	1.544-1.553	0.009
Prasiolite (quartz)	1.544-1.553	0.009
Smoky quartz	1.544-1.553	0.009
Rock crystal (quartz)	1.544-1.553	0.009
Andesine	1.543-1.551	0.008
Oligoclase	1.542-1.549	0.007
Talc	1.540-1.590	0.050
Scapolite	1.540-1.579	0.006-0.037
Petrified Wood	1.540	none
Jasper	1.540	none
Amber	1.539-1.545	none
Ivory	1.535-1.570	none
Apophyllite	1.535-1.537	0.002
Tiger’s eye	1.534-1.540	none
Aragonite	1.530-1.685	0.155
Agate	1.530-1.540	0.004-0.009
Chalcedony	1.530-1.540	0.004-0.009
Chrysoprase	1.530-1.540	0.004-0.009
Moss agate	1.530-1.540	0.004-0.009
Sepiolite	1.530	none
Witherite	1.529-1.677	0.148
Milarite	1.529-1.551	0.003
Nepheline	1.526-1.546	0.0004
Sunstone	1.525-1.548	0.01
Cordierite	1.522-1.578	0.008-0.018
Iolite (cordierite)	1.522-1.578	0.008-0.018
Amazonite	1.522-1.530	0.008
Pearl	1.520-1.690	0.156
Ammolite	1.520-1.680	0.155
Strontianite	1.520-1.670	0.150
Gypsum	1.520-1.529	0.009
Orthoclase	1.518-1.530	0.008
Sanidine	1.518-1.530	0.008
Moonstone	1.518-1.526	0.008
Pollucite	1.517-1.525	none
Carletonite	1.517-1.521	0.004
Stichtite	1.516-1.544	0.026
Thomsonite	1.515-1.542	0.006-0.025
Magnesite	1.509-1.717	0.022
Scolecite	1.509-1.525	0.007-0.012
Leucite	1.504-1.509	0.001
Mesolite	1.504-1.508	0.001
Dolomite	1.502-1.698	0.185
Petalite	1.502-1.519	0.012-0.017
Lapis lazuli	1.500	none
Haüyne	1.496-1.510	none
Tugtupite	1.496-1.502	0.006
Cancrinite	1.495-1.528	0.024-0.029
Celluloid	1.495-1.520	none
Ulexite	1.491-1.520	0.029
Yugawaralite	1.490-1.509	0.011-0.014
Whewellite	1.489-1.651	0.159-0.163
Kurnakovite	1.488-1.525	0.036
Inderite	1.486-1.507	0.017-0.020
Calcite	1.486-1.658	0.172
Coral	1.486-1.658	0.160-0.172
Moldavite	1.48-1.540	none
Natrolite	1.480-1.493	0.013
Sodalite	1.480	none
Analcime	1.479-1.489	none
Thaumasite	1.464-1.507	0.036
Creedite	1.461-1.485	0.024
Chrysocolla	1.460-1.570	0.023-0.040
Obsidian	1.450-1.550	none
Gaylussite	1.443-1.523	0.080
Glass	1.440-1.900	none
Fluorite	1.434	none
Sellaite	1.378-1.390	0.012
Opal	1.370-1.520	none
Villiaumite	1.327	anomalous

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