Phenakite

nesosilicate

microcline

Formula: Be2(SiO4)
Nesosilicate (insular SiO4 groups), phenakite group, beryllium mineral
Specific gravity: 2.98
Hardness: 7½ to 8
Streak: White
Colour: Colourless, white, yellow, pale rose
Solubility: Insoluble in acids
Environments:

Pegmatites
Metamorphic environments
Hydrothermal environments

Phenakite occurs in granite pegmatites with microcline, topaz and quartz, in schist as a product of beryl alteration and in greisen (Dana, R&M 84.4.338-344).
Commonly associated minerals include topaz, beryl, chrysoberyl, fluorite, muscovite and quartz (R&M 84.4.338-344, HOM).

At the Morro Agudo and the Talho Aberto pegmatites at São Miguel de Piracicaba, about 50 kilometers east of Belo Horizonte, Minas Gerais, Brazil, phenakite is associated with microcline variety amazonite, beryl, fluorite, tantalite, topaz and schorl (R&M 84.4.338-344).

At the Tait farm occurrence near Bancroft, Ontario, Canada, phenakite crystals reaching 3 cm in length have been found associated with tourmaline (R&M 84.4.338-344).

At Presissac Township, Temiskamingue county, Quebec, Canada, phenakite has been found in a molybdenite-bearing pegmatite associated with beryl and fluorite (R&M 84.4.338-344).

At the Mine Jaune, Vosges Mountains, Alsace, France, phenakite has been found in iron ores associated with scheelite (R&M 84.4.338-344).

At Momeik, Mogok, Sagaing district, Mandalay, Myanmar, phenakite to 3.5 cm in length has been found, rarely associated with danburite (R&M 84.4.338-344).

At Klein Spitzkoppe, Erongo region, Namibia, small crystals of phenakite to 3 mm occur on microcline and quartz at Stiepelmann's pegmatite (Minrec 36.4.326-327).

The type locality for phenakite is the Izumrudnye Kopi area, Malyshevo, Sverdlovsk Oblast, Russia.

At the emerald deposits at Malyshevo, Izumrudnye Kop'I, near Asbest, Central Ural Mountains, Russia, phenakite occurs as single crystals to 13 cm and large groups of crystals in schist associated with both beryl and chrysoberyl (R&M 84.4.338-344).

At Takovaya in the Russian Urals, phenakite occurs in schist with emerald and chrysoberyl (Dana).

At the Volnian Pegmatite, Volodarsk-Volynski, Zhitomir Oblast, Ukraine, phenakite occurs as crystals to 2 cm with albite on orthoclase in a pegmatite, and also inside large smoky quartz crystals (Minrec 40.6.497).

At several localities in Cornwall, England, UK, phenakite occurs, occasionally associated with fluorite, cassiterite, bertrandite, chlorite, and sulphides including arsenopyrite (R&M 84.4.338-344).

At Mount Antero and nearby Mount White, Chaffee County, Colorado, USA, phenakite occurs in pockets as crystals typically less than 2cm, associated with beryl, albite, quartz, fluorite, bertrandite and topaz (R&M 84.4.338-344).

At Crystal Park, El Paso county, Colorado, USA, phenakite crystals to 10cm occur with topaz, zircon, microcline variety amazonite and smoky quartz (R&M 84.4.338-344).

In the Tarryall Mountains, Jefferson county, Colorado, USA, phenakite has been found with albite as inclusions in topaz (R&M 84.4.338-344).

At Harris Park, Park county, Colorado, USA, phenakite has been found with microcline variety amazonite, albite variety cleavelandite, and occasionally topaz (R&M 84.4.338-344).

In the Crystal Peak area of Teller county, Colorado, USA, small phenakite crystals are found associated with microcline variety amazonite, smoky quartz, topaz and albite (R&M 84.4.338-344).

At the Wheeler Peak mine in the Lincoln district, White Pine County, Nevada, USA, phenakite crystals to 1 cm and irregular veins to 3 cm thick occur with associated minerals including beryl, bertrandite, fluorite, allanite, monazite, quartz, scheelite and pyrite in a replacement deposit (R&M 84.4.338-344).

In the Mount Wheeler area, Snakerange, White Pine county, Nevada, USA, phenakite occurs in quartz veins in limestone, associated with muscovite, carbonates and fluorite (AM 48.189-193).

At the Irish Creek tin deposit, Rockbridge county, Virginia, USA, phenakite occurs as irregular grains replacing beryl (R&M 84.4.338-344).

At the Kagem mine, Lufwanyama, Copperbelt, Zambia, phenakite crystals to 1cm occur enclosed in beryl (Minrec 41.1.65).

Alteration

bertrandite to phenakite and H2O
Be4Si2O7(OH)2 ⇌ 2Be2(SiO4) + H2O
Increasing temperature favours the forward reaction (AM 63.664-676).

beryl to chrysoberyl, phenakite and silica (dry)
Be3Al2Si6O18 to BeAl2O4 + Be2(SiO4) + 5SiO2 (dry)
Increasing temperature and pressure favours the forward reaction. At a pressure of 4 kbar the equilibrium temperatute is 1300 deg C (AM 71.277-300).

euclase to beryl, chrysoberyl, phenakite and H2O
20Eu to 3Be3Al2Si6O18 + 7BeAl2O4 + 2Be2(SiO4) + 10H2O
Increasing temperature and decreasing pressure favours the forward reaction. At a pressure of 6 kbar the equilibrium temperature is about 500oC, in the absence of impurities which might be incorporated in the beryl (AM 71.277-300).

euclase to phenakite, chrysoberyl, beryl and H2O
20BeAlSiO4(OH) ⇌ 2Be2(SiO4) + 7BeAl2O4 + 3Be3Al2Si6O18 + 10H2O
Increasing temperature favours the forward reaction (AM 63.664-676).

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