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Abstract:
Detailed mineralogical and gemological records were conducted on 340 unheated spinels from the Horana, Eheliyagoda, Ratnapura, and Okkampatiya mining areas in Sri Lanka. The color of Sri Lankan spinel varies greatly: in addition to the mainstream pink to purple pink, green and blue can also be seen. Compared with spinel from other regions such as Myanmar, Vietnam, and Tanzania, Sri Lanka's spinel has more abundant inclusions: several mining areas generally have inclusions such as dolomite, apatite, zircon, and chondrodite. Minerals such as graphite and forsterite are also found in spinel produced in the Horana region; graphite and rutile have been found in spinel produced in the Okkamptiya region. Partially healed fissures are most common in spinel in the Okkampatiya mining area; Unlike Vietnamese spinel, dislocations and growth structures are almost absent in Sri Lankan spinel. The LA-ICP-MS analysis results showed that there were no significant differences among the mining areas. LA-ICP-MS analysis of 5 Sri Lankan cobalt blue spinels showed a variation of 11 to 120 ppm in this chromogenic element. The UV visible absorption spectrum results show that Sri Lankan spinel has a combination spectra with variable ratios of the spectral components Cr3+, V3+ and Fe2+ from pink to red, orange, purple to purple, and blue-green. The results of infrared spectroscopy and laser Raman spectroscopy analysis showed that all samples showed no indications for heat treatment.
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Keywords:
- gemological-mineralogical characteristic /
- spinel /
- Sri Lanka /
- spectroscopy /
- inclusion
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ACKNOWLEDGMENTS: The authors would like to thank the ICA GemLab, for carrying out the analyses, photos and general documentation.
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Figure 1. Crystal structure of ordered spinel (Kefeni & Mamba, 2019)
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Figure 2. (a) Well-developed (octahedral), colorless-transparent spinel crystal hosted by a Horana spinel; (b) Orangey-brown, transparent, irregular-rounded crystal of unknown natural hosted by a Horana spinel; (c)Colorless-transparent, elongated-prismatic, rounded apatite crystal; (d)Colorless-transparent irregular rounded dolomite crystal.
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Figure 3. (a) Partially healed fissure with coarse textures, these mostly composed of isolated/elongated (tube-like) constituents. Typical network- or fingerprint-like patterns are not developed; (b)Partially healed fissure showing the typical spinel pattern. This is composed of innumerable, grayish white, octahedron-like cavities ("negative crystals"). They are arranged in parallel strings that are oriented in different directions. Many cavities are accompanied by tiny tension fissures that are oriented parallel to each other.
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Figure 4. (a) Well-developed, colorless-transparent, octahedron-like negative crystals; (b)Swarm of irregular shaped cavities (negative crystals)accompanied by planar, partially healed fissures; (c) Very common in Okkampitiya spinels are irregularly shaped (often bizarre-looking) cavities, accompanied by unhealed-frosted or poorly healed fissures. These cavities often contain a polycrystalline minerals substance. Also present are small, black-opaque platelets/grains.
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Figure 5. (a) Stringer-like formations are a common inclusion feature in Okkampitiya spinels. They are composed of tiny, grayish pinpoint inclusions and reflective particles; (b) Stringer-like formations are straight and oriented nearly parallel to each other. Also present are delicate planar, highly reflective tension fissures that are oriented parallel to each other and perpendicular to the strings.
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Figure 6. Irregularly shaped, cavity companied by a grayish white aureole and delicate healed fissures
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Figure 8. Correlation diagram Fe2O3 vs. Cr2O3 for purple-violet and blue-green Sri Lanka spinels
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Figure 9. Correlation diagram of Fe vs. V in the concentration range 0-2 wt.% Fe2O3 resp. V2O3 for Sri Lankan spinels
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Figure 10. Representative UV-Vis-NIR spectra of spinels of different colors from Sri Lanka
Table 1 Overview of the mineral inclusions regarding Sir Lanka spinels from different location
Minerals [1]-R [1]-H [1]-O [1]-E [2] [3] Minerals [1]-R [1]-H [1]-O [1]-E [2] [3] Alanite + Hogbomit + Albite + K-feldspa + + Ankerite + Llimonite + Apatite + + + + + "Magnesite/" + Aragonite + Rhodochrosite Biotite + Paragonite + Boehmite + Phlogopite + Calcite + + + + Plagioclase + Chondrodite + + + Pyrite + + Clinochrorite + Pyrrhotite + Diaspore + Quartz + + Diopsite + Rutile + + Dolomite + + + + + Silimanite + Feldspar + Sphene + Florencite + Spinel + Forsterite + + Sulfide * Graphite * * * * + Uraninite + Hematite + Zirco + + + + + Hercynite + Zoisite + [1] This study using Raman Microspectroscopy (*identified by optical microscopy). [2] Gubelin & Koivula (1986 and/or 2005). [3] Kleismantas et al. (2017) using SEM.R = Ratnapura, H = Horana, O = Okkampitiya, E = Eheliyagoda 
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Table 2 Chemical fingerprinting of the spinels examined for this study using EDXRF-data
wt/% Locality Color V2O3 Cr2O3 Fe2O3 ZnO Eheliyagoda Pink-Red 0.04-0.89 0.03-0.66 0.11-0.55 0.04-1.55 Orange 0.10-1.06 0.01-0.17 0.05-0.48 0.02-1.35 Purple-Violet 0.01-0.15 0.01-0.22 0.21-1.33 0.08-0.78 Blue-Green 0.01-0.02 bdl-0.01 0.69-1.18 0.06-0.47 Horana Pink-Red 0.02-1.67 0.01-1.32 0.05-1.57 0.03-3.42 Orange 0.25-0.96 0.01-0.32 0.05-0.60 0.05-1.11 Purple-Violet 0.02-0.08 bdl-0.09 0.21-1.23 0.05-0.97 Blue-Green 0.01-0.07 0.06-0.99 0.21-2.08 0.13-3.92 Okkampitiya Pink-Red 0.01-1.36 0.06-0.99 0.21-2.08 0.02-1.36 Orange 0.23-0.76 0.03-0.17 0.06-0.48 0.05-1.71 Purple-Violet bdl-0.10 0-0.15 0.41-1.92 0.04-0.37 Blue-Green bdl bdl 1.04-3.38 0.47-1.53 Ratnapura Pink-Red 0.01-0.23 0.01-0.20 0.38-0.88 0.05-0.61 Orange 0.21-0.65 0.01-0.08 0.05-0.31 0.05-0.61 Purple-Violet 0.01-0.03 bdl-0.09 0.47-2.35 0.06-0.62 Blue-Green 0.01-0.02 0-0.02 0.60-5.04 0.07-3.54 Ga2O3 content = 0.01-0.13 wt.%, MnO content = bdl-0.11 wt.%
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