Phenakite

Crystal system Hexagonal
Transparency Transparent
Luster Vitreous
Fracture Conchoidal
Cleavage Indistinct
Specific Gravity 2.95
Hardness 7.5 to 8
Optical Character Uniaxial + ; Double Refractive
Refractive index 1.654-1.670
Birefringence 0.016
Dispersion 0.015
Fluorescence not diagnostic; may be pale green or light blue
Pleochroism Distinct
Chemical Formula
Comments
Streak

Phenakite (FEN-ah-kite) is a rare beryllium-silicate mineral (Be2SiO4) that would be an important gem mineral if the colors were deeper and more material were available. It has sufficient hardness (7 1/2 to 8) and toughness to qualify as a gemstone, but in its usual colorless or light-yellow form it has little appeal from the standpoint of beauty. It may also be light red or brown. The name, which was formerly spelled phenakite, comes from the Greek word meaning "cheat", because it is so similar in appearance to quartz. The mineral occurs as crystal inclusions in some synthetic emeralds.

Phenakite occurs as a pegmatite mineral; also, because of its durability, it is found in alluvial deposits in pegmatite areas. The principal sources are the Ural Mountains in Russia; Minas Gerais, Durango, Mexico; Stoneham, Maine; and Mt. Antero, Colorado.

This rare mineral crystallizes in the rhombohedral division in the hexagonal system. The cleavage is indistinct, the fracture is conchoidal and the specific gravity is about 2.95. The R.I. ranges from a low constant rending of 1.654 to n high of 1.670, with a consequent birefringence of .016. The material is uniaxial with positive sign. The luster is vitreous and the dispersion is .015.

Pleochroism is classed as distinct. No phenomenon is exhibited. Phenakite is infusible and is not attacked by acids.

Phenakite may be distinguished from quartz, topaz or beryl by R.I. or accurate S.G. determination. Optic character or the
lower S.G. of phenakite will separate it from greenish yellow peridot or spodumene.

For most of its history, the largest known cut specimen of phenakite was about 20 carats. Several years ago a student was shown an 1100+ carat lump of "topaz" by his employer, to whom it had been given in Sri Lanka after purchasing many cut stones. The student questioned the identity of the specimen and soon found that it was uniaxial. A check of indices convinced him that the stone was phenakite, and laboratory confirmed his findings. A 500+ carat faceted stone cut from the piece is now at the Smithsonian Museum.

Attractive step or mixed-cut stones can be produced by using 43° crown angles, 39° pavilion angles and Linde A powder on a tin lap. Difficulty is seldom encountered in the fashioning of phenakite.