phenacite

or phen·a·kite (-kīt')
n.
A natural beryllium silicate, Be₂SiO₄, occurring as vitreous crystals sometimes used as gems.

[Greek phenāx, phenāk-, impostor + -ITE¹.]

Be
Hexagonal -- Rhombohedral

Environment

A mineral of pegmatites and high-temperature veins.

Crystal description

Almost always in free, well-developed crystals, which range from wedge-edged rhombohedral scales (usually on feldspar), to short, or even long, prisms (with quartz and beryl). Usually small, commonly only a fraction of an inch (0.5 cm) across. Elongated crystals often penetration-twinned, with re-entrants in the tips.

Physical properties

Colorless and white. Luster glassy; hardness 7Ɖ-8; specific gravity 3.0; fracture conchoidal; cleavage poor prismatic. Transparent to translucent.

Composition

Beryllium silicate (45.6% BeO, 54.4% SiO ₂ ).

Tests

Infusible and insoluble in acids. Unlike quartz, usually does not decrepitate in the flame.

Distinguishing characteristics

Crystals provide the best clues; the rhombohedral scales can only be confused with one of the carbonates, which will be acid-soluble and soft. The prismatic crystals often resemble those of quartz, but quartz is striated horizontally on the prisms, whereas phenakite is striated vertically. Topaz has a basal, and better, cleavage. Quartz decrepitates, preventing any effort to fuse it; sturdier beryl whitens and fuses only on thinnest edges.

Occurrence

The best U.S. specimens are from Mt. Antero, Colorado, in pegmatitic pockets in short-prismatic, commonly penetration-twinned crystals, associated with beryl, fluorite, and quartz. Also near Colorado Springs at several localities in the Cheyenne Mountains district in rhombohedral-habit crystals often perched on microcline feldspar, with smoky quartz. Similar crystals are found on Baldface Mountain in New Hampshire on the Maine border; not far away, at Lord's Hill, Maine, prismatic crystals to Ɛ in. (1.5 cm) long grow on smoky quartz. The most attractive specimens are flat crystals up to 2 in. (5 cm) across, with short-prism zones, sometimes grouped in great clusters at São Miguel de Piraçicaba, Minas Gerais, Brazil. The largest crystals are prisms embedded in cleavelandite feldspar or white quartz at Kragerø, Norway. They attain 6 or 8 in. (15-20 cm) in length and are an inch or more (2-3 cm) thick. The original discovery was of 2-3 in. (5-8 cm) crystals in mica, companions of the emeralds of (Sverdlovsk) Ekaterinburg in Russia.

Remarks

Named from a Greek word for "to deceive," because it was long confused with quartz. A crystal was pictured in a British mineralogical work in 1811 (J. Sowerby) and designated as white tourmaline 20 years before it was recognized as a new mineral.

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