Beryl the Varieties of Beryl the Physical Properties of Beryl
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Beryl 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) 4.