Emerald the green variety of Beryl (Be3Al2Si6O18) is one of the oldest sought after gem in the world. Like Diamonds, Rubies and Sapphire - emeralds is beautiful and rare. The rarity is due to low crustal abundance of Beryllium (Be) which according to Emsley (2001 cited in Zwaan,2006) is the 47th most abundant element. The other reason for emeralds rarity is the green colour which is caused by small amounts of Chromium and / Vanadium. To understand the physical and chemical properties of emeralds we first need to understand the silicate mineral beryl. Colourless beryl is composed of SiO2-67 wt%, Al2O3-18.9 wt%, BeO-14.1 wt%. The different colour shades in beryl family are caused by small amount of impurities present in the structure. Goshenite is the colourless variety of beryl, blue colour is due to charge transfers of Iron (Fe2+ to Fe3+) in Maxixe and Aquamarine, or metal ions of Manganese as well as Iron ( Mn2+, Mn3+, Fe3+) in cases of Morganite, Red Beryl, Heliodor and Golden Beryl (Nassau, 1976 and 1980). The cause of green hues ranging from vivid to blue or yellow-green in emeralds depend on the concentration of chromophoric impurities of Chromium (Cr3+) and / or Vanadium (V3+) replacing the Aluminium (Al3+) in the crystal lattice through crystal field reaction which also causes the red fluorescence. The fluorescence increases the luminosity of the blue-green shade but if Iron (Fe3+) is present, this phenomena is suppressed (Nassau, 1980). The other green variety in this group is called the V-emerald or Green Beryl, the cause of colour for this variety is Vanadium (V) only. More discussion over the impurities and their distinct absorption spectra of the visible region in differentiating between green beryl and emerald is concluded in Schwarz and Schmetzer( 2002).
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Beryl has a hexagonal crystal structure which crystallises in point group 6/m2/m2/m and space group P6/m2/c2/c. The projection of the crystal structure in Fig 1 is perpendicular to c-axis. The Al or Y site is surrounded by six Oxygen (O) atoms (AlO6) in octahedral coordination, and both the Beryllium (Be) and Silicon (Si) sites by four O atoms (BeO4 and SiO4) in tetrahedral coordination. The SiO4 tetrahedra polymerize to form six-membered rings parallel to (001); stacking of the rings results in large channels parallel to c-axis creating vacancy sites. The rings are referred to as the 2a (at 0,0,0.25) and 2b (at 0,0,0) positions (Groat et al, 2008). Beryl forming in alkali- and water-rich environment has H2O and the larger alkali atoms (Caesium (Cs), Rubidium (Rb), Potassium (K)) occupying the ample 2a site with Sodium (Na) occupying the smaller 2b site. But if crystallisation occurs in alkali- and water-poor environment both H2O molecules and Na atoms occupy the 2a site with 2b site being empty (Deer et al, 1992). Other minor substitution in the structure may occur with Lithium (Li) replacing Be, Scandium (Sc) and Fe with Al in the structure (Deer et al, 1992).
Beryl: Red - O; Silver - Si; Blue - Al; Green - Be; Yellow - Na;
Purple - Cs
Emerald formation necessitates Be which is rare and is usually concentrated in granites, pegmatites, black shales or their metamorphic equivalent of about 1.4 ppm in lower crust, 2.29 ppm in middle crust, 2.1 ppm in the upper continental crust (Rudnick et al, 2003 cited in Groat et al, 2008). The more common Cr and V (92 and 97 ppm) are usually concentrated in dunites -peridoties -basalts of the oceanic crust -upper mantle and their metamorphic equivalents in the upper continental crust (Rudnick et al, 2003 cited in Groat et al, 2008). Studies by Schwarz et al (2002) has shown high concentrations of Cr and / V can also occur in sedimentary rocks, particularly black shales. Be apart from being rare is also an incompatible element due to its small ionic radius (0.3 Angstrom as BeO) therefore excluded by many minerals during crystallization thus will stay in the melt as long as possible, -Fluorine (F) -Lithium (Li) -Boron (B) and -Phosphorous (P) also encourages the retention of Be in the melt during crystallisation. Though Be solubility can be increased by high levels of aSiO2 and aAl2O3. In hydrothermal fluids the complexing agents that control Be solubility are F, CO3, OH, F-CO3 and F-OH. Most Be deposits are associated with F phases suggesting an important role for F in Be solubility. Being incompatible, it will concentrate in the late stage, water rich melt from which pegmatites crystallise. It may fractionate into the late stage hydrothermal fluids - in quartz-rich veins. This makes emerald quite rare as unusual geological and geochemical processes are required for Be, Cr and / V to meet.
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Emeralds are found at a variety of geographical localities, world-wide such as Afghanistan, Australia, Austria, Brazil, Canada, Columbia, Egypt, Pakistan, Madagascar, Nigeria, Norway, Russia, South Africa, USA, Zambia and Zimbabwe. The petrogenesis of emerald deposits is diverse around the globe but is somewhat unique to its locality. Several petrogenetic models of emerald deposit have been put together by a number of researchers (such as Dereppe et al 2000, Schwarz and Giuliani 2001, Schwarz et al 2001, Barton and Young 2002, and Sabot 2002 cited in Groat et al, 2008; Arif et al, 2010; Moroz et al, 2001) classifying emeralds with or without pegmatitic hydrothermal intrusion, tectonism, isotopic geochemical data but according to Zwaans (2006) argument none actually best fit and the diversity of emerald formation cannot be classified unambiguously.
The discovery of emeralds in Swat Valley, Mingora, North West Frontier Pakistan (NWFP) is relatively new compared to the Columbian deposit. The first discovery from Swat Valley was reported by Davies (1962), and (in 1968, 1982) GÏ‹belin mentioned that emeralds from this deposit has the saturated green shade, vividness and transparency which can fairly rival the Columbian emeralds (Kazmi et al, 1989). Presently the supply of this deposit is only a fraction in contrast to the Columbian rival but this could in future be one of the primary suppliers in the gem world.
It is crucial to conclusively determine the provenance of a gemstone, as prices from different localities vary from few dollars per carat (ct) to few thousand per carat. Columbia is known to provide the world with its finest emeralds for centuries. Regardless of the rumours that Columbian deposit are depleting (Groat et. al, 2008). Emeralds are priced for their colour and Columbian emeralds like Sapphire from Kashmir or Burmese Ruby have always been traded at high prices; in 2000 a stunning premium of $1,149,850 was paid for a 10.11 ct Columbian emerald (Zachovay, 2002).
Many studies have been conducted on different deposit after findings to analyse and to better understand the geological and geochemical mechanism of emerald formation, making distinctions leading to the origin and geological petrogenesis. The main purpose of this dissertation is to compare the analysis of alkali concentration, mineral and fluid inclusions which will be compared with literature based on the genetic model suggested by Schwarz and Giuliani (2001 and Schwarz et al (2001) to conclude a distinction that can differentiate emeralds from Swat deposit to other major deposits. Techniques applied for fluid and mineral inclusion analysis includes Optical microscopy, microthermometrical microscopy and Laser Raman spectroscopy. Chemical component analyses (by Electron Microprobe and Laser Ablation - Inductively Coupled Plasma - Mass Spectrometer) were conducted for the alkali concentration.
Barton, M. D., and Young, S., 2002, Non-pegmatitic deposits of beryllium: Mineralogy, geology, phase equilibria and origin: Reviews in Mineralogy, v. 50, 591-691.