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Beryl. Aqua marine. Heliodor. Morganite. Emerald. Goshenite. Beryl. The varieties of beryl. Beryl: golden or red variety. Emerald: green variety. Aquamarine: blue variety. Morganite: pink variety . Heliodor: greenish-yellow variety. Goshenite: colourless variety.

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Beryl

Aqua marine

Heliodor

Morganite

Emerald

Goshenite

Beryl


The varieties of beryl

Beryl: golden or red variety

Emerald: green variety

Aquamarine: blue variety

Morganite: pink variety

Heliodor: greenish-yellow variety

Goshenite: colourless variety

- Emeralds were mined in ancient Egypt 4000 years ago

Interesting history:

- In the 1600’s, high quality emerald reached Europe.

- The Spaniards seized the emeralds from the Pre-Colombian people.

- Mining in Colombia is still going on today, and often associated with criminal activities.


The physical properties of beryl

Group: beryl is a cyclosilicate

Luster: vitreous, transparent to translucent

Cleavage: imperfect in one direction, conchoidal fracture

Hardness: 7.5 - 8

Specific gravity: 2.6 – 2.9 on average

Crystal habit: hexagonal prism with pincoid termination


The chemical properties of beryl

Beryl is a beryllium aluminum silicate Be3Al2(Si6O18)

BeO: 14.0%

Composition:

Al2O3: 19.0%

SiO2: 67.0%

The vertical hexagonal channels, which are normally vacant, can be occupied by alkali elements such as Li, Na, and Rb or neutral molecules such as H2O or CO2.

This image shows the hexagonal structure along the c-axis of beryl

Silica tetrahedra (upper layer)

Silica tetrahedra (lower layer)

Beryllium tetrahedra

Aluminum polyhedron

Similar but rare species include Euclase [BeAl(SiO4)(OH)] and gadolinite [YFe2+Be2(SiO4)2O2]


c

a

b

The crystallographic properties of beryl

Crystal system: hexagonal

Point Group: 6/m 2/m 2/m

Unit cell parameters:

a = 9.23 Å

c = 9.19Å

Z = 2

Space group: P6/mcc

Beryl tetrahedron

Aluminum polyhedron

Silicon tetrahedron

Oxygen atom


b

a

c

The crystallographic properties of beryl

Beryl tetrahedron

Aluminum polyhedron

Silicon tetrahedron

Oxygen atom


Crystallographic data of beryl

Source: Morosin (1972)


The optical properties of beryl

Beryl is uniaxial negative (oblate indicatrix)

 = 1.560 – 1.602

 = 1.557 – 1.599

 = 0.045 (3rd order interference colors)

Weak to distinct pleochroism

May be zoned

May be twinned


a axis

c axis

The thermodynamic properties of beryl

Open circles are for beryl

Closed circles are for emerald

Both crystals exhibit a negative thermal expansion along the c axis

Source: Morosin (1972)


Igneous rocks

Metamorphic rocks

Occurrences of Beryl

In granite, granite pegmatite, rarely in nepheline syenite

In low to high temperature hydrothermal veins

Associated minerals include micas, quartz, euclase, calcite, tourmaline, lepidote, spodumene, cassiterite topaz, and feldspars

Locations on Earth:

Colombia, Brazil, Madagascar, Zimbabwe, Pakistan, Afghanistan, Russia, United States (Ca, Co, Id, Ut, NC)