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Double refraction, or birefringence, is a fascinating optical property in many minerals. Minerals can exhibit this property when crystallizing in shapes other than perfect cubes. The result? A single image viewed through such a mineral will appear doubled!
The best-known double refractive mineral is a transparent specimen of calcite – Iceland spar. Zircon also has strong birefringence; sometimes, its facets are double observed through the stone. Sapphire, ruby, emerald, citrine, topaz, and tourmaline are also double-refractive minerals.
From vibrant rubies to cool emeralds, many beloved gemstones like sapphires, citrines, tourmalines, and topazes possess the captivating power of double refraction. This property even helps experts distinguish between a priceless ruby and a mere spinel-like lookalike.
Minerals like diamonds, garnets, and spinels – with their perfectly cubic crystals – don’t exhibit this fascinating effect. Whether you’re a child marveling at a doubled image through a piece of calcite or a gemologist safeguarding the treasures of a crown, double refraction offers both wonder and valuable knowledge.
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What is Double Refraction in Minerals
Double refraction or birefringence is a common physical property for many minerals. The most common mineral, also remembered while speaking about double refraction, is calcite (Island spar), where double refraction can be observed; however, sometimes it can be observed by specific techniques.
Double refraction in minerals is an optical phenomenon when the light bends or refracts in two different directions, causing light to divide into two rays. This is called double refraction or birefringence and is explained by the extraordinary properties of optically anisotropic crystal structure.
The property called birefringence has to do with anisotropy in the binding forces between the atoms forming a crystal, so it can be visualized as the atoms having stronger “springs” holding them together in some crystalline directions.
A useful situation with mineral crystals occurs when there are two distinct indices of refraction, and they are called birefringent materials. This is associated with uniaxial crystals, which belong to the hexagonal, tetragonal, and trigonal crystal systems.
In a uniaxial crystal, there is one direction such that any light in that direction has the same speed, regardless of its polarization state.
This direction is called the optic axis. The remaining crystal systems (orthorhombic, monoclinic, and triclinic) have two optic axes and are said to be biaxial.
What Does Double Refraction Mean in Rocks
Double refraction of minerals is an important characteristic that can help identify rocks. One of the methods to recognize the rock or mineral professionally is to make a thin section – a thin (0,3 mm) slice of rock glued onto a glass slide.
After that, the slice of rock is checked under a polarizing or petrographic microscope, and double refraction can come in handy with mineral identification.
Rock is a combination of minerals. In most cases, rocks are composed of small crystals or grains of different minerals. Double refraction can help identify the mineral composition of rocks. Some minerals which have double refraction can be identified in thin sections of rocks under a polarizing microscope.
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- Smithsonian Handbooks: Rocks & Minerals
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What Causes Double Refraction in Minerals
Double refraction originates from the crystal structure and chemical composition of minerals. This optical phenomenon’s nature is how atoms are arranged in the crystal structure.
There is an optical anisotropy in crystals, where atoms are arranged non-homogeneously in all directions. It means the crystal has different optical properties in different directions.
The crystal structure of minerals that crystallize in non-cubic crystal systems (tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic) can split the light into two beams. These beams pass the body of the crystal with different velocities, resulting in the doubling of light.
There are two rays of light spotted after refraction in minerals. One ray (called the extraordinary ray [e]) is bent or refracted at a specific angle as it travels through the mineral.
The other ray (the ordinary ray [o]) passes through the mineral unchanged. The greater the difference between rays after the mineral passed, the greater the birefringence index.
TIP: Did you know that one of the ways geologists study rocks is by licking them? Yes, Yes, it’s not a myth. Find out more about licking rocks by geologists in the article below:
Licking Rocks in Geology: Why & How (Answered by Geologist)
What Minerals Have Double Refraction (8 Minerals)
The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, it is strongly displayed in some minerals, such as the Iceland spar variety of calcite.
The double refraction is different in every mineral and can be used to identify gemstones. Double refraction is measured with a refractometer.
All minerals with a non-cubic crystal system have double refraction. The most common is calcite (Iceland spar). Zircon is a well-known double-refractive mineral in gemology mostly. Other minerals that are double refractive are ruby, sapphire, emerald, citrine, tourmaline, and topaz.
Here is the table of minerals that will be discussed further and their double refraction values:
| Mineral | Double Refraction |
|---|---|
| Calcite | 0.172 |
| Zircon | 0.002-0.059 |
| Sapphire (corundum) | 0.008 |
| Ruby (corundum) | 0.008 |
| Emerald (beryl) | 0.006 |
| Citrine (quartz) | 0.009 |
| Tourmaline | 0.014-0.032 |
| Topaz | 0.008-0.016 |
Calcite
The best mineral to observe double refraction is calcite. The transparent variety of calcite is usually called Iceland spar. The double refraction phenomenon can be observed macroscopically without any devices.
How to see double refraction in calcite:
If a pencil mark is drawn upon a sheet of paper and then covered with a piece of glass, only one image will be seen; but if the same paper is covered with a piece of calcite, and the crystal is oriented in a specific direction, then two marks will become visible.
Zircon
Zircon also shows double refraction. It is a bit lower than calcite and can be observed with the help of the lope of a microscope mostly.
Zircon’s double refraction is usually spotted by gemologists in already faceted stones. Zircon’s pronounced double refraction means you can see twice as many facets and twice as much fire.
Corundum (Ruby and Sapphire)
Ruby and sapphire are colored varieties of the mineral corundum. Double refraction is barely noticeable by the human eye; however, it can be spotted by gemological devices.
This property is significant for ruby and spinal separation. Spinel doesn’t have double refraction.
Beryl (Emerald)
Emerald, the most famous beryl variety, exhibits double refraction. Other common beryl varieties, such as aquamarine, heliodor, goshenite, and morganite, share a similar crystal structure. This leads to comparable double refraction values (typically around 0.005 – 0.009).
Quartz (Citrine)
Citrine – a yellow-colored variety of quartz the same as its siblings (smoky quartz, crystal quartz, rose quartz, amethyst) has low double refraction. However, even low values of double refraction can help to identify minerals from their imitations.
Tourmaline
Tourmaline is a unique mineral that occurs in a rainbow of color and any degree of transparency, and it also shows double refraction.
Interestingly, it’s impossible to observe birefringence macroscopically; however, they have another phenomenon seen with unarmed eye – pleochroism. Pleochroism is an optical phenomenon when different colors are observed from different sides of the mineral.
Topaz
Topaz’s double refraction value is quite low and ranges between 0.008-0.016; however, it is still helpful to differentiate the precious mineral from their possible imitations.
TIP: Beryl and Emerald are very similar minerals and people always have problems identifying them. Check out the differences between these two beautiful minerals in the article below:
Beryl vs. Emerald: 7 Key Differences (Are They The Same?)
Why Does Calcite Have Double Refraction
Calcite, a truly extraordinary mineral, reveals the phenomenon of double refraction with stunning clarity. What secrets lie within its structure that give it this remarkable optical power?
Its transparent form, known as Iceland spar, became famous for its visible double refraction. Calcite boasts one of the highest levels of double refraction (0.172) among all minerals. Want to see it in action? Simply place a calcite crystal over an image and watch it magically split into two!
The optic axis of a calcite crystal (doubly refracting crystal) is defined by the symmetry of the crystal lattice. In calcite compounds or CaCO3, the CO3 (Carbon trioxide) forms a triangular cluster, and the optic axis lies perpendicular to this.
Because of the cluster’s location and size, explained by the size of molecules, the light acts unusually and creates such a distinct double refraction.
When light enters along the optic axis of the crystal, nothing happens, and the light comes out unpolarized. However, when the light enters at a certain angle to the optic axis, the asymmetry of the lattice splits the ray into two, creating a double image of anything behind the crystal.
FAQ About Mineral’s Double Refraction
Sill did not find the answer to your questions about double refraction. Find frequently asked questions in the section below:
Does Halite Have Double Refraction?
You won’t see double refraction with ordinary table salt (halite). Unlike calcite, its cubic crystal structure makes it optically isotropic. This lack of double refraction is a helpful clue: it’s one way to easily tell the difference between halite and calcite.
Does Quartz Have Double Refraction?
While quartz and its beautiful varieties (like amethyst, citrine, smoky quartz, rose quartz, and rock crystal) possess double refraction, the effect is too subtle to see with just your eyes (0.009). To measure it, you’d need a special tool called a refractometer. But don’t worry – there’s another easy way to identify quartz: its impressive hardness (7)!
Does Sulfur Have Double Refraction?
Sulfur might surprise you! Despite being opaque in its usual form, it has a higher degree of double refraction (0.291) than even calcite. Light rays passing through a clear sulfur crystal would split more dramatically. Unfortunately, such transparent crystals are very rare in nature. Luckily, identifying sulfur is usually quite straightforward, thanks to its distinct yellow color and characteristic brittleness.
Does Gypsum Have Double Refraction?
Gypsum exhibits very weak double refraction (birefringence), with a value of 0.009. This is significantly lower than calcite (0.172), making calcite appear to split light into two distinct rays. While a refractometer can measure gypsum’s double refraction, the effect is too subtle to see with the naked eye. This lack of noticeable birefringence helps distinguish gypsum from calcite.
Does Calcite Have a Double Refraction?
Calcite, including its transparent variety Iceland spar, is known for exhibiting the strongest birefringence (double refraction) observed in a common mineral, with a value of 1.72 x 10^-1. This means light rays entering the crystal are split into two separate paths, causing a single object viewed through the crystal to appear doubled. This effect is easily observable by placing a piece of paper with text under the calcite crystal.
TIP: You can find fake calcites when buying on the internet. Do you know how to identify real calcite? Find out the differences between real and fake calcite in the article below:
Real vs. Fake Calcite: Focus on These 6 Differences
Conclusion
Double refraction in minerals is an optical phenomenon typical for a lot of minerals. Anisotropic minerals of the non-cubic crystal system have double refraction or birefringence.
The most popular mineral to display the double refraction phenomenon is the transparent variety of calcite – Iceland spar.
Double refraction in calcite can be observed when placing a crystal on a printed word. You will see the doubled word through the crystal. When the crystal is rotated, the doubling effect will rotate, too.
Some other minerals also have double refraction. They are:
- zircon
- tourmaline
- sapphire
- ruby
- emerald
- citrine
- topaz.
In most cases, you will not be able to observe doubling in the minerals above by the naked eye. Sometimes you can spot doubled facets in faceted zircon when viewed through the stone under a magnifying glass. To spot the double refraction and to measure it precisely, the refractometer should be used.
Double refraction is a critical characteristic for mineral identification. Tiny grains of calcite can be easily spotted in rocks under the polarizing or petrographic microscope because of calcite’s high value of double refraction (0,172).
Double refraction is an essential method for precious stone identification in gemology. Bloody-red similar-looking cut crystals of ruby and spinel can be easily differentiated because of double refraction.
Spinel is an isotropic mineral and crystallizes in the cubic crystal system. That is why it will not have double refraction. Diamond, garnets (spessartine, hessonite, pyrope, almandine), fluorite, opal, and spinels have no double refraction.
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