Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962

Phenakite Group

[A NESOSILICATE]


Phenakite

Until the early nineteenth century, certain crystals found with emerald in the mines on the Tokovaya River (east of Sverdlovsk in the Ural Mountains) were presumed to be quartz. In The Illustrated Encyclopedia of Minerals and Rocks, Dr. J. Kourimsky states that, during an inspection of the mines, the Russian inspector and mineral collector, Count Petrovsky, collected several specimens. A closer examination of the material revealed properties different from those of quartz, and further analysis identified beryllium silicate. An unknown mineral was discovered! Phenakite’s resemblance to quartz caused the use of the Greek word meaning “deceiver” to be adopted as the name.

Phenakite is a member of the phenakite group of the nesosilicates. Phenakite develops primarily in pneumatolytic or hydrothermal environments, but the decomposition of beryl can produce it as a secondary mineral. Its short prismatic, acicular or frequently twinned, typically flat rhombohedral crystals develop in the hexagonal crystal system. Fibrous spherulites and granular forms also occur. A single direction of cleavage is indistinct. Although faceted gems seldom exceed the five-carat range, its usual pegmatitic origin does allow the development of rare, large, usually heavily included, transparent to translucent crystals. [The largest known was a 1470-carat pebble found in Sri Lanka. An eye-clean 569-carat faceted oval gem, inhabited by numerous tiny acicular inclusions, plus several smaller gems were cut from this find.] Less common environments for phenakite are mica schists and granites. The best known deposits are located in the Ural mountains, Kragero in Norway, San Miguel de Paracicaba in Brazil, Pala County in California, Lord’s Hill in Maine, Colorado’s Mount Antero and the Pike’s Peak area, Virginia and New Hampshire in the USA, and Italy, Sri Lanka, Tanzania, Namibia, and Zimbabwe. Associated minerals include albite, amazonite, apatite, beryl, chrysoberyl, mica, quartz, fluorite, and topaz. Care must be taken not to mistake it for rock crystal, beryl, topaz, sapphire, or diamond.

Brazil is noted for its large, colorless, eye-clean, cuttable phenakite material. Extremely rare red crystals from the Urals yield unique gems rarely seen and seldom available. Though surface stains are responsible for the pink, pinkish red, brown, and yellow hues, as well as the impurities that produce colors in some crystals, phenakites are remarkably chemically pure. Inclusions of other minerals seldom occur. Numerous needle-like inclusions can produce a translucent crystal that, when properly cut, displays an attractive chatoyancy. Jaroslav Bauer and Vladimir Bouska caution in A Guide in Color to Precious and Semi Precious Stones that the typically delicate colors of phenakite are often light-sensitive and may fade in as little as a few months if frequently exposed to sunlight. Deep-toned phenakite gems usually exhibit strong pleochroism.

The brilliant cut displays phenakite’s attractive gem attributes well. Phenakite is a bright gem, but its weak dispersion limits the fire and scintillation. However, a hardness of 7.5 - 8 and its toughness makes phenakite a durable choice for any type of jewelry for a collector of unusual gems.


    Phenakite
Composition    Be2[SiO4] beryllium silicate
Class    silicates
Group    phenakite
Species    phenakite
Variety    colorless and by color
Crystal System    hexagonal (per Arem and Schumann; trigonal (per Bauer and Bouska)
Habit    crystal-rhombohedral, prismatic, acicular, granular, fibrous spherules
Cleavage    imperfect; in one direction
Streak    white
Fracture    conchoidal; brittle
Fracture Lustre    vitreous
Lustre    vitreous
Diaphaneity    transparent, translucent
Colors    colorless, brown, pink, red, yellow
Phenomena    chatoyancy
Specific Gravity    2.93 t0 3.0
Hardness    7.5 to 8.0
Toughness    good
Refractive Indices    o= 1.654; e= 1.67
Birefringence    0.016
Optic Character    uniaxial positive
Dispersion    0.015
Pleochroism    distinct; colorless: orange-yellow; blue: red-strong blue
Luminescence    pale green or blue, possible light rose in UV light
Spectrum    not diagnostic
Chelsea Filter    not applicable
Aqua Filter    not applicable
Solubility    no information
Thermal Traits    avoid thermal shock; infusible; [Manuel of Mineralogy: produces white enamel when fused with sodium carbonate
Treatments    none known
Inclusions    aikinite crystals, mica in material from Brazil