Phenakite - Encyclopedia

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Rhombohedral
    Chemistry : Be2SiO4
    Rarity : Uncommon

Phenakite is a beryllium silicate much rarer than beryl but which is found in the same type of occurrence as the latter : biotite micashists, and especially granitic pegmatites associated with topaz and/or chrysoberyl. It is a mineral that owes its name to the Greek phenax (misleading) because it looks like quartz when it is colorless. Phenakite is transparent to translucent and have a vitreous luster. Most often colorless, it can sometimes be yellowish, bluish or pinkish. Its crystals are flattened according to {001}, generally very rich in faces, or occurs in elongated hexagonal prisms, sometimes longitudinally striated and terminated by a rhombohedron. Phenacite is exceptionally used as beryllium ore, it is also cut for jewelry, but its lack of color makes it not very popular, its crystals are appreciated and sought after by mineral collectors.

2.2 cm phenakite single gemmy crystal from Momeik, Myanmar
Phenakite crystal termination from Momeik, Myanmar
1.11 ct phenakite gemstone from Momeik, Myanmar
1.26 ct phenakite gemstone from Momeik, Myanmar

Phenakite in the World

The best phenakite crystals come from Brazilian pegmatites. Those of Sao Miguel de Paracicaba (Minas Gerais) gave superb colorless tabular crystals assembled in exceptional groups reaching 30 cm, while those of Maraiba and Pica Pau produced magnificent transparent rhombohedral crystals. The pegmatites of Spitzkop (Namibia) gave superb twins. Among the American deposits of Colorado, the famous Pikes Peak pegmatite produced very beautiful clear rhombohedral crystals of several centimeters associated with amazonite. The Malagasy pegmatites specialize more in the elongated crystals : those of Anjanabonoina gave magnificent partially gemmy crystals exceeding 10 cm ! The same goes for the pegmatites from the Momeik region, which provided elongated crystals perfectly gemmy and twinned of more than 2 cm (main photo).

Among the deposits in the micashists, we can point out the 20 cm translucent crystals of Kragerö in Norway and those of Kranobolskaya (Urals, Russia) where crystals of 15 cm are associated with the emerald. Although rare in alpine clefts, 3 cm crystals have been found at Lake Lucendro in Switzerland.

Phenakite in France

In France, small crystals of phenakite are reported in the Alpine clefts of the Plan du Lac in Isere, but it has also been reported by Alfred Lacroix at Framont in Haute-Saône in a cavernous quartz penetrated with limonite, resulting from the decomposition of pyrite.

Twinning and special forms

The penetration twins along the axis [0001] is common for this mineral species.

Fakes and scams

No fake inventoried for this species, however once cut the phenakite gemstones can be confused with the quartz whose optical properties are close.

Hardness : 7.5 to 8
Density : 2.96
Fracture : Conchoidal
Streak : White

TP : Translucent to transparent
IR : 1.650 to 1.670
Birefringence : 0.017
Optical character : Uniaxial +
Pleochroism : Very weak
Fluorescence : None

Solubility : Insoluble

Magnetism : None
Radioactivity : None