Beryl Aqua marine Heliodor
Morganite
Emerald Goshenite
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
Interesting history: - Emeralds were mined in ancient Egypt 4000 years ago - 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
1 The chemical properties of beryl
Beryl is a beryllium aluminum silicate Be3Al2(Si 6O18)
Composition: BeO: 14.0%
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 H 2O 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 2+ gadolinite [YFe Be2(SiO4) 2O 2]
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
c a b Beryl tetrahedron
Aluminum polyhedron
Silicon tetrahedron
Oxygen atom
The crystallographic properties of beryl
b a
c Beryl tetrahedron
Aluminum polyhedron
Silicon tetrahedron
Oxygen atom
2 Crystallographic data of beryl
Source: Morosin (1972)
The optical properties of beryl
Beryl is uniaxial negative (oblate indicatrix)
ww = 1.560 – 1.602 ee = 1.557 – 1.599 dd = 0.045 (3rd order interference colors)
Weak to distinct pleochroism May be zoned May be twinned
The thermodynamic properties of beryl
a axis Open circles are for beryl
Closed circles are for emerald
c axis
Both crystals exhibit a negative thermal expansion along the c axis Source: Morosin (1972)
3 Occurrences of Beryl
Igneous rocks In granite, granite pegmatite, rarely in nepheline syenite
Metamorphic rocks 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)
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