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Formula: Be2(SiO4)
Nesosilicate (insular SiO4 groups), phenakite group,
beryllium-bearing mineral
Crystal System: Trigonal
Specific gravity: 2.96 to 3 measured, 2.96 calculated
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 Mimoso do Sul, Espírito Santo, Brazil, phenakite in colourless rounded crystals has been found with
aquamarine. Other associations include
schorl, feldspar,
quartz and topaz
(Minrec 54.735).
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).
At the Mount Rosa Complex, El Paso County, Colorado, USA, phenakite has been found in two locations in the
complex and is generally associated with pegmatites that
originated in the Pikes Peak granite, that is a salmon-pink granite
that is composed of perthitic microcline and
quartz, with lesser amounts of albite
and biotite. The phenakite occurs both as 1 to 3 mm prismatic crystals
and as flattened lenticular crystals 0.1 to 2 mm in size, but the lenticular habit is much more common. Prismatic
phenakite has been recovered from quartz veins at Bear Creek and also
from a single pegmatite pocket near Helen Hunt Falls.
Lenticular crystals are the common form of phenakite at the Specimen Rock locality where it is found in small
miarolitic cavities in pegmatites.
A unique feature of the Mount Rosa Complex is that
phenakite and bertrandite occur
as primary minerals rather than resulting from
secondary alteration of
beryl; beryl was never a crystallising
phase. This is in contrast to other Colorado pegmatite
localities, such as Mount Antero, where phenakite and
bertrandite usually form as breakdown products of
beryl.
In some granitic rocks the alkali elements, sodium and potassium, occur
in greater molar abundance than aluminium.
Phenakite and bertrandite,
that contain no aluminium in the chemical formulae, may form in these
conditions as
primary minerals, instead of
beryl, because of the lack of
aluminium. In more aluminium-rich
systems, the crystallisation of beryl, an
aluminium-bearing mineral, will be favoured over
bertrandite and phenakite.
An excess of alkali elements could either
(1) react with and alter earlier-formed
beryl to produce
secondary phenakite
and feldspar or
(2) combine the chemical components of beryl in a fluid phase with alkali
elements to directly crystallise primary
phenakite and feldspar from the
fluid. The latter represents the formation of beryllium-bearing minerals in
the Mount Rosa Complex, according to the
equation:
2Be3Al2Si6O18 + 3SiO2 +2K2O (or Na2O)
→ 3Be2SiO4 + 4K(or Na)AlSi3O8
beryl composition + quartz + alkali
oxides →
phenakite + feldspar
The excess alkali elements react with beryl components to form less
common beryllium-bearing minerals. Both
phenakite and bertrandite
require acid (pH 4 to 5) conditions for their formation, although, for
bertrandite, crystallisation can occur up to near neutral (pH = 7)
(R&M 97.5.417).
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).
At the Lake George Area, Park county, Colorado, USA, there was a new find of phenakite in 2023,
comprising about 60 loose, thumbnail-size phenakite crystals and a few small-miniature-size crystal
groups. The phenakite crystals appear just a little bit different from the classic ones from Mount
Antero, 50 miles or so to the west, both localities are situated in alkali
pegmatites in the
granitic Pikes Peak Batholith. The short-prismatic
phenakite crystals, the largest of which approach 3 cm long, are transparent to milky white with
slight tinges of orange. Most examples are loose, doubly terminated single crystals, although a minority
are drill-bit twinned in familiar phenakite fashion. A small number of the crystals show blue
microcrystal inclusions of the very rare
beryllium-scandium
silicate species bazzite, as do a very few Mount Antero
phenakites too
(Minrec 55.1.82).
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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