Phenakite

phenakite

bertrandite

chrysoberyl

euclase

Images

Formula: Be2(SiO4)
Nesosilicate (insular SiO4 groups), phenakite group
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).

Localities

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 beryl variety 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).

In New Hampshire, USA, for phenakite, the tabular habit is most common, ranging in size to 2 cm with single prismatic crystals only a few millimeters long. Crystals are colourless, often transparent, and form in the later stages of crystallisation in the miaroles associated with smoky quartz, microcline, albite and topaz. Evidence of late crystallisation is demonstrated by phenakite crystals perched on other crystals in the pocket. Phenakite forms in both lithium-cesium-tantalum (LCT) and niobium-yttrium-fluorine (NYF) pegmatites and is stable under an extensive range of conditions. It is more likely to form if the pegmatite contains significant topaz, which is indicative of high fluorine activity. In New Hampshire, most phenakite occurrences are in NYF pegmatites, the ones that also contain topaz. The few known exceptions are at the Iron Mountain mine in Bartlett, where it is found in veinlets in magnetite masses in granite, and at the Ham and Weeks mine in Wakefield, where it is found as tiny prismatic crystals. North Sugarloaf Mountain also has phenakite and many rare-earth elements but no topaz (R&M 97.3.229-230).

At Spruce Hill, Albany, Carroll County, New Hampshire, USA, prismatic, colourless phenakite crystals were collected, one of them being an exceptional 5 cm long, well terminated, and of gem quality. Prismatic crystals of phenakite are uncommon in New Hampshire, and well-formed ones are generally less than a few millimeters in length (R&M 97.3.229-230).

The South Baldface Mountain Phenakite locality, Chatham, Carroll County, New Hampshire, USA, is the most prolific locality for phenakite in New Hampshire, and topaz also occurs there. A 15 inch pocket was found full of clay with topaz and smoky quartz crystals, together with phenakite crystals generally up to 2.5 cm in size, but one extraordinary crystal measuring 4.5 ×  1.9 cm has been found (R&M 97.3.229-230).

At the North Sugarloaf Mountain locality, Bethlehem, Grafton County, New Hampshire, USA, a flattened tabular crystal of phenakite has been found measuring 3.7 × 3 cm, which is one of the largest phenakites known from the United States (R&M 97.3.229-230).

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 Wasau syenite complex, Marathon county, Wisconsin, USA, phenakite has been found with altered siderite and late bertrandite. Separately, at the Ladick quarry, phenakite crystals to 1 cm have been found in the pegmatite associated with cassiterite (R&M 95.2.163).

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

Alteration

With decreasing temperature, at about 200oC phenakite hydrates to bertrandite (AM 71.277-300).

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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