A Second Look at Tiger’s Eye

I have a tumbled piece of tiger’s eye that sits on my desk. As I move it in the sunlight, I can easily lose myself in the dance of light, shifting bands of darkness and illumination that ripple across its surface. This phenomenon is called chatoyancy.

Chatoyancy refers to the reflection of light from bands of parallel fibers, which often impart a silky appearance to a mineral. When skillfully cut in a cabochon, a single band of light flows parallel to the orientation of the fibers. The name comes from the French for cat, chat, and eye, oeil. The characteristic flash of light is like that from a cat’s eye.

For the past 125 years, textbooks and museum displays have relied on the beauty of tiger’s eye to add interest to an often dry description of pseudomorphism, a term that simply means the replacement of one mineral by another while preserving the form of the original mineral. Mineralogists once thought that the stunning play of light emerging from tiger’s eye resulted from the infiltration and dissolution of the fibrous mineral crocidolite, a deep blue, asbestos like silicate, by hot waters rich in dissolved silica. Tiger’s eye was viewed as a spectacular form of chalcedony and is often included as a variety of quartz in mineral guides.

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A new study shows that the museums and textbooks were incorrect. Recently, researchers at Pennsylvania State University employed sophisticated analytical techniques to unravel the complex story of the mineral’s formation. They discovered that embedded within the rippling chatoyancy lies a record of the tectonic movements that shaped the Earth. Pseudomorphism is an important, but poorly understood, phenomenon. It plays a key role in the preservation of fossils, in which the original form of organic materials is conserved through replacement by minerals.

Professor Peter Heaney began his research into tiger’s eye because he wanted to learn more about pseudomorphism, but the results pointed in a completely different direction.

Heaney and his colleague, Donald Fisher, looked deep within the structure of the reflective bands in tiger’s eye using electron microscopy to examine the structure more closely and X ray diffraction to identify which minerals were present. They discovered that tiger’s eye contains quartz and crocidolite, but there was no chalcedony and no evidence of mineral replacement.

They found that the chatoyancy is not dependent on the quartz pseudomorphs at all, but rather on microscopic fibers of crocidolite encapsulated within the quartz. The tiger’s eye did not form by the replacement of an existing rock but by the creation of a new one, a vein.

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The shimmer of light records a series of fractures that formed in the rock in response to tectonic stresses related to mountain building events. As a fracture opened, crocidolite fibers grew within the open space while quartz was deposited by circulating mineral laden waters. As the vein became crowded with new mineral growth, the crocidolite fibers were engulfed and encapsulated by the quartz. Subsequent earth movements created new fractures and the process was repeated over and over until layered bands of undulating fibrous intergrowths resulted.

This process is called crack seal vein filling. It offers geologists a graphic record of the changing directions of tectonic forces that acted on a rock through time. It’s much like a treasure map. Once they understand the story of one vein’s formation, they know where to look for more.

In effect, each band of glowing color represents a different crack that opened and was filled. Each band contains a new orientation of embedded crocidolite fibers that record minute rotations of the rock as it formed.

As I move my stone in the sunlight, bands are illuminated as the crocidolite fibers come into a reflective orientation with my eyeball, when rotated into a non reflective orientation the bands become dark. This is the secret of the chatoyancy. It is the orientation of the fibers within tiger’s eye that holds the possibility for the magical dance of light. It is the skill of the craftsman that sets it free. The cutter’s understanding of the fibers is what makes the difference between a stone that makes light shimmy and one that is dull and lifeless.

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The museum displays and textbooks now must be revised. Tiger’s eye can no longer hold its esteemed place as a captivating example of pseudomorphism. Like a celebrity endorsing a product, it has found one that is in better alignment with its innate talents. It has now moved into the position of supermodel for the crack seal vein filling process. While the mechanism of pseudomorphism remains a mystery, tiger’s eye will teach children and rockhounds about movements of the earth that capture light and preserve it in stone.

By Anne Sasso - © Colored Stone - July - August 2003
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