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Formula: Be4Si2O7(OH)2
Sorosilicate (Si2O7 groups), beryllium-bearing mineral
Specific gravity: 2.59 to 2.6
Hardness: 6 to 7
Streak: White
Colour: Colourless, pale yellow
Solubility: Soluble in hydrogen fluoride, HF, sulphuric acid and sodium hydroxide. Partially soluble
in hydrochloric acid (Dana).
Common impurities: Al,Fe,Ca
Environments:
Plutonic Igneous Environments
Pegmatites typical
Metamorphic Environments
Hydrothermal Environments
Bertrandite is commonly found in beryllium-bearing
pegmatites, (Webmin). and also in fissures in
granite and in miarolitic cavities in
greisen.
It is commonly an alteration product of beryl, more rarely a
primary mineral.
Associated minerals include beryl,
phenakite, herderite,
tourmaline, muscovite,
fluorite and quartz
(HOM).
It may also occur in hydrothermal veins as a late-stage product of beryl
resorption and alteration, and it may form pseudomorphs after
beryl
(Dana).
Localities
At Mica creek, Queensland, Australia, bertrandite occurs in a
beryl-bearing
pegmatite, associated with
beryl,
mica, quartz,
albite and limonite. Most
of the bertrandite shows marginal replacement by mica
(AM45.1300).
There are two type localities, Barbin quarry and Petit-Port, both at Nantes, Pays de la Loire, France.
At the Tae Hwa mine, Chung Cheong North Province, South Korea, bertrandite occurs in vein deposits associated
with beryl, scheelite,
wolframite (mineral intermediate between hübnerite and
ferberite) and dolomite
(Dana).
At Spitkopje, Erongo Region, Namibia, bertrandite occurs in a
pegmatite, where it is the last
beryllium-bearing
mineral to crystallise
(MinRec 36.4.323).
At Mount Antero, Chaffee county, Colorado, USA, bertrandite is associated with
beryl and phenakite
(Dana).
At the Mount Rosa Complex, El Paso County, Colorado, USA, bertrandite crystals are present throughout the
complex, mostly associated with pegmatites. At Stove Mountain
bertrandite is common in miarolitic cavities in
pegmatites. Near the Eureka Tunnel at St. Peters Dome, it
occurs in vugs within rocks that were hydrothermally altered by exsolution of late-stage fluids and is often
associated with fluorite and a number of accessory minerals including
genthelvite, zircon and
cassiterite. The bertrandite occurs as thin, tabular to needlelike
orthorhombic crystals that range in size from about 100 microns to 2 mm. Some of the bertrandite is
twinned forming heart-shaped crystals.
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 here 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
Similarly, bertrandite may form rather than
beryl in an aluminium-depleted
system with the excess alkali elements being accommodated by the formation of either
feldspar or muscovite. In all of
these cases, 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 97.5.416-417).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, bertrandite is widespread where
beryl has been corroded by late-stage fluids. It tends to be associated with
fluorapatite. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.504).
In New Hampshire, USA, bertrandite is found as a late-stage mineral in both LCT and NYF
pegmatites, and at many localities throughout the state. It
forms elongated thin tabular colourless crystals generally less than 2 mm long, and less commonly acicular crystals
that rarely exceed 5 mm. Bertrandite in LCT pegmatites
is an alteration product of beryl, and in NYF
pegmatites it can form from the alteration of
phenakite. When phenakite is not
present, bertrandite may be associated with other beryllium minerals
such as danalite
(R&M 97.3.213).
At Folsom Gulch, Carroll County, New Hampshire, USA, bertrandite occurs on
microcline
(R&M 93.2.161).
At North Sugarloaf Mountain, Bethlehem, Grafton County, New Hampshire, USA, spectacular groups of bertrandite
to several mm have been found in association with danalite.
Bertrandite has also been observed in corroded or etched danalite,
which may indicate that bertrandite can form from the dissolution of
danalite, as well as from beryl or
phenakite
(R&M 97.3.213).
At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the
pegmatites are beryl-type
rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most
complex zonation and diverse mineralogy of any of the Keyes
pegmatites. Six zones are distinguished, as follows, proceeding
inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase
border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite
wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite
outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite
middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to
1.2 meters in size that were altered to vuggy
albite-muscovite with
fluorapatite crystals. This unit presumably was the source of the
albite, muscovite,
fluorapatite, quartz and other
crystallised minerals found in pieces of vuggy albite
rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including
tourmaline, graftonite,
triphylite, vivianite,
pyrite, pyrrhotite, and
beryl crystals to 30.5 cm long and 12.7 cm across.
Bertrandite occurs as colourless microcrystals at the Keyes Number 1 mine on the surface of
fluorapatite crystals
(R&M 97.4.311).
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).
bertrandite, euclase and quartz to
beryl and H2O
Be4Si2O7(OH)2 + 8BeAlSiO4(OH) + 14SiO2 ⇌
4Be3Al2Si6O18 + 5H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite and kaolinite to euclase,
beryl and H2O
4Be4Si2O7(OH)2 + 7Al2Si2O5(OH)4 ⇌
10BeAlSiO4(OH) + 2Be3Al2Si6O18 + 13H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite and kaolinite to euclase,
quartz and H2O
Be4Si2O7(OH)2 + 2Al2Si2O5(OH)4 ⇌
4BeAlSiO4(OH) + 2SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite, kaolinite and quartz to
beryl and H2O
3Be4Si2O7(OH)2 + 4Al2Si2O5(OH)4 +
10SiO2 ⇌ 4Be3Al2Si6O18 + 11H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
chrysoberyl, bertrandite and kaolinite to
euclase and H2O
2BeAl2O4 + 2Be4Si2O7(OH)2 +
3Al2Si2O5(OH)4 ⇌ 10BeAlSiO4(OH) + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase to bertrandite, chrysoberyl,
quartz and H2O
8BeAlSiO4(OH) ⇌ Be4Si2O7(OH)2 + 4BeAl2O4 + 6SiO2
+ 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
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