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Double refraction or birefringence is a typical physical property for a lot of minerals. Minerals and gemstones which crystalize in any other crystal system except cubic are double refractive. Some minerals have so strong birefringence that the picture observed through the crystal will be doubled.
The best known double refractive mineral is a transparent specimen of calcite – Iceland spar. Zircon also has strong birefringence, and sometimes its facets are doubled observed through the stone. Sapphire, ruby, emerald, citrine, topaz, and tourmaline are also double refractive minerals.
Rubies, sapphires, emeralds, citrines, tourmalines, and topazes are gemstones that can be identified due to double refraction. Double refraction is a key factor for discriminating between ruby and spinel. Minerals that crystallize in a cubic crystal system (diamond, garnets, spinels) don’t have double refraction.
You are welcome to discover this magnificent optical property, which can be used to entertain children with calcite or to identify spinel instead of ruby in priceless crown jewels.
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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 which is 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 in the crystal has the same speed, regardless of its state of polarization.
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 in rocks identification. 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.
Do you want to experience how geologists see rocks through a polarized microscope? Here is an interesting link for you to check how various crystallized samples, including Appolo 12 moon rock, look like.
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- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
What Causes Double Refraction in Minerals
Double refraction originates from the crystal structure and chemical composition of minerals. The nature of this optical phenomenon is the way atoms are arranged in the crystal structure.
In crystals, where atoms are arranged non-homogeneously in all directions, there is an optical anisotropy. 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 (called 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:
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, in some minerals, such as the Iceland spar variety of calcite, it is strongly displayed.
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 which 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:
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 also shows double refraction. It is a bit lower than that of calcite and can be observed with the help of the lope of 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, can be spotted by gemological devices.
This property is significant for ruby and spinal separation. Spinel doesn’t have double refraction.
Emerald – the best-known variety of beryl mineral also has double refraction. The other varieties of beryl – aquamarine, heliodor, goshenite, morganite also have the same value of the double refraction as the crystal structure of all minerals is similar.
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 – a unique mineral that occurs in a rainbow of color and any degree of transparency, 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’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 is a phenomenon mineral that displays freely visible double refraction. Is there something extraordinary in the calcite structure or chemical composition, which makes so special double refraction?
Calcite and its transparent variety Iceland spar is a classical example of double refraction in minerals, which can be observed by a naked eye. Calcite double refraction (0,172) is one of the highest among minerals. Double refraction can be spotted by placing a crystal on any image. It will seem doubled.
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 their size, explained by the size of molecules, the light acts unusually and creates so distinct double refraction.
When light enters along with 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 situated behind the crystal.
FAQ About Mineral’s Double Refraction
Sill did not find the answer to your answers about double refraction? Find frequently asked questions in the section below:
Does Halite Have Double Refraction?
Halite, which is commonly known as table salt, has no double refraction. Halite crystallizes in cubic crystal symmetry and is optically isotropic. That is why it doesn’t have double refraction at all. This can be a good diagnostic feature that will help you to differentiate between calcite and halite.
Does Quartz Have Double Refraction?
Quartz and all its varieties (amethyst, citrine, smoky quartz, rose quartz, and rock quartz) have double refraction (0,009), but it is too low to be spotted by a naked eye. Quartz double refraction can be measured by refractometer only. The high hardness of quartz (7) will help to define the mineral.
Does Sulfur Have Double Refraction?
Sulfur has double refraction (0.291), which is twice higher than that of calcite. However, double refraction of sulfur is rarely discussed as it hardly ever form transparent crystals big enough to observe double refraction. Also, sulfur is easy to identify by other properties like color and hardness.
Does Gypsum Have Double Refraction?
Theoretically, gypsum has double refraction (0,009), but it is too low compared with calcite’s values (0,172). Double refraction in gypsum can be measured by a refractometer. No doubling can be observed with a naked eye. The absence of birefringence in the gypsum can distinguish it from calcite.
Does Calcite Have a Double Refraction?
Calcite and its transparent variety Island spar have the highest double refraction (0,172), which can be observed by the naked eye. You can easily check this optical phenomenon by placing a newspaper under the crystal. Words and signs will be doubled.
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
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:
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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