Chrysoberyl Composition, Crystallization & Structure

Composition. Beryllium aluminate, BeA12O4 = Alumina 80.2, beryllium oxide 19.8.

Crystallization. Orthorhombic. Crystals usually tabular parallel to macropinacoid, which face is vertically striated. Commonly twinned, often in pseudo hexagonal forms.

Structure. Usually in crystals.

Physical Properties. Prismatic cleavage. H. = 8.5 (un usually high). G. = 3.65-3.8. Vitreous luster. Color various shades of green, brown, yellow, sometimes red by transmitted light.

Test. infusible. Insoluble. The finely powdered mineral is wholly soluble in the salt of phosphorus bead (absence of silica). Mineral, moistened with cobalt nitrate and ignited, turns blue (aluminum). Characterized by its extreme hardness, its yellowing to emerald-green color and its twin crystals.

Varieties

1. Ordinary. Color pale green, yellow; sometimes transparent.
2. Alexandrite. Emerald-green variety, but red by transmitted light and generally also by artificial light.
3. Cat’s-eye. Or Cymophane. A variety which when polished shows an opalescent luster, and across whose surface plays a long narrow beam of light, changing its position with every movement of the stone. This effect is known as chatoyancy, and is best obtained when the stone is cut in a oval or round form 8en cabochon). This property of the mineral is thought to be due to numerous minute tubelike cavities, arranged in a parallel position. Chrysoberyl the true cat’s-eye, and Is not to be confused with various other minerals possessing similar properties 8e.g., quartz).

Occurrence. A rare mineral. Occurs in granite rocks and pegmatites and in mica schists. Frequently in river sands and gravels. Found in the alluvial gem deposits of Brazil and Ceylon; the alexandrite variety comes from the Ural Mountains. In the United States it has been found in Oxford Co., and elsewhere in Maine, Haddam, Connecticut; at Greenfield, near Saratoga, New York.

Name. chrysoberyl means golden beryl. Cymophane is derive from two Geek words meaning wave an to appear, in allusion to the chatoyant effect of some of the stones. Alexandrite was named in honor of Alexander II o f Russia.

Use. Serves as a gem stone. The ordinary yellowish green stones are inexpensive, while the varieties alexandrite and cat’s-eye are of considerable value.

Two rare manganese minerals belong in the section of Intermediate Oxides: hausmannite, MnO. Mn₂O₃ and braunite, 3Mn₂O₃ – MnSiO₃ .

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