THE PROPERTIES OF PLEOCHROISM, ASTERISM, PHOSPHORESCENCE, THERMOLUMINESCENCE, AND TRIBOLUMINESCENCE




THE PLEOCHROIC MINERALS

TANZANITE

blue to purple
IOLITE

colorless to blue/purple
TANZANITE

purple to gray
IOLITE

colorless/gray to purple

Pleochroic minerals are minerals that show different colors depending on what direction the viewer is observing the crystal. The effect is sometimes quite dramatic. Many minerals are technically pleochroic, but more often than not the color change is so small that it requires optical instruments to detect it. However there are some minerals that show an incredible color change. The greatest change is limited to three colors and is called trichroic. A two color change is called dichroic. Pleochroic, which means "many colors", is often the term used to cover both. Many times the color change is limited to shade changes such as from pale pink to dark pink. In order to view pleochroism you need an individual transparent crystal.

Pleochroism is caused by the absorption of different wavelengths of light travelling through different directions in the crystal. If in one direction, all wavelengths but yellow and blue are absorbed then the crystal will be green (yellow and blue make green). If in another direction all wavelengths are absorbed but yellow, then the crystal will appear yellow. If the crystal is turned from the first direction to the other, then it will change its color from green to yellow. Isometric minerals can not be pleochroic since they have the same structure and thus the same light absorbing capabilities in all directions. Tetragonal, trigonal and hexagonal minerals can only be dichroic since they have one unique structural direction along the major symmetry axis and one direction in every other direction. Only orthorhombic, monoclinic and triclinic minerals can be trichroic since they have three unique axes of symmetry and therefore three unique directions that can absorb light in three different ways. Although pleochroism is a rare phenomena to easily observe, it can be diagnostic.

Some of the best examples of pleochroism are found in specimens of:


*ASTERISM AND THE CAT'S EYE EFFECT

Asterism is a well known light effect in some gemstones. Asterism, which gets it name from aster the Greek word for star, is responsible for the lovely star sapphires, star rubies, star rose quartz and other gemstones. The effect is caused by minute acicular (needle-like) crystals of probably rutile or sometimes other minerals that are included in the host mineral. These minute crystals are microscopic, but there are thousands of them and their combined effect is to diffract light into these bands that appear as rays of light. In trigonal minerals such as corundum, the tiny crystals are aligned along crystallographic axes and this causes the six pointed stars. Although the rutile crystals diminish the clarity of the stone, their star effect more than makes up for this. Star gemstones are really wonderful to look at as the star is responsive to the viewer and moves across the surface of the stone as it is moved and rotated or as the observer moves to a different viewing direction. Some stars even seem to be hovering over the stone, an optical illusion. Some star effects can be seen in some minerals only when light is viewed through the crystal (such as in phlogopite).

Cat's eyes are similar to asterisms and are caused by the same inclusions of minute crystals. But in this case the band on light is limited to one band that shimmers from the top to the bottom of the stone and appears like a glowing cat's eye. Andalusite is the premier cat's eye stone, but the effect is found in other minerals as well. The techincal term for the cat's eye effect is chatoyancy which is translated from the French words "chat" and "oeil" which mean cat and eye respectively. Good quality cat's eye stones have a similar glow and hover effect as the star gemstones mentioned above.


PHOSPHORESCENCE

Many minerals that fluoresce will at times also phosphoresce. Phosphorescence is the ability of a mineral to glow after the initial activating ultraviolet light is removed. The electrons in the mineral have essentially stored the energy of the initial activating light and then re-emit the phosphorescent light on a delayed basis. The reason for the delay is because a certain amount of the electrons are prevented from returning to their lower energy states quickly, at least not as quickly as they were initially energized by the UV light. Most children are familiar with "glow-in-the-dark" toys, t-shirts and stickers that make use of phosphorescent chemicals. It is quite a unique thing to see this occur in a natural mineral. Some phosphorescent intensity is very low and actually beyond the capabilities of the human eye to detect. But specimens that have been exposed to UV light and then placed in front of sensitive photographic film can be reveal to be phosphorescent for some time after the initial activation, by producing an exposure on the film. In some extreme cases for years afterward. This is not a consistent property to observe and is not very diagnostic for this reason.


THERMOLUMINESCENCE

Thermoluminescence is a property of some minerals to glow when they are heated. The minerals contain chemical bonds that emit light when thermal energy (heat) is applied to them. It is well know that steel becomes luminescent when it has been heated and is being worked. Some minerals glow when they are subjected to temperatures of between 50 and 475 degrees C. Activator elements must be present in these minerals just like in UV fluorescence. In some minerals, they will glow only once during heating and then never glow again. This is because the excited electrons were trapped in a higher state during crystallization and only when heated were they allowed to lower their energy state by emitting the photons of light.

Minerals that have been known to show this unusual property are:


TRIBOLUMINESCENCE

Triboluminescence is a property of some minerals to glow when they are crushed, struck, scratched or even rubbed in some cases. The minerals contain chemical bonds that emit light when even mechanical energy is imparted to them. This is not a reliable property as it is not generally consistent from specimen to specimen although sphalerite has shown this property very well. This property must be tested in the dark.

Minerals that have been known to show this unusual property include:

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OTHER PROPERTIES:

Color | Luster | Diaphaneity | Crystal Systems | Technical Crystal Habits | Descriptive Crystal Habits | Twinning | Cleavage | Fracture | Hardness | Specific Gravity | Streak | Associated Minerals | Notable Localities | Fluorescence | Phosphorescence | Triboluminescence | Thermoluminescence | Index of Refraction | Birefringence | Double Refraction | Dispersion | Pleochroism | Asterism | Chatoyancy | Parting | Striations | Radioactivity | Magnetism | Odor | Feel | Taste | Solubility | Electrical properties | Reaction to acids | Thermal properties | Phantoms | Inclusions | Pseudomorphs | Meteoric Minerals
 

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