VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 Characteristics of corundums from Phuoc Hiep occurrence (Quang Nam Province) Nguyen Ngoc Khoi, Nguy Tuyet Nhung, Nguyen Thi Minh Thuyet*, Phan Van Quynh College of Science, VNU Received 24 June 2007 Abstract. Phuoc Hiep corundum occurrence has been discovered recently in the Central part of Vietnam (Quang Nam Province). Common protoliths in the region are high‐grade metamorphic rocks (typically of amphibolite‐granulite facies), such as garnet‐bearing amphibolites, gneisses and schists, intruded by igneous formations of different composition and age. To date, the rubies and fancy sapphires have been found in secondary, mostly alluvial deposits. The corundum crystals occur as broken, from moderate‐ to well‐rounded, fragments, but remains of the original habits are often present. Their size ranges from 3×5 mm to 20×30 mm, sometimes to 100×120 mm. Most of corundum pieces are of purplish pink to purplish red, with a small proportion of light to dark blue. For the first time, the gemological characteristics of Phuoc Hiep corundums are described in detail in this paper. Common gemological properties of Phuoc Hiep corundums are normal for rubies and sapphires elsewhere in the world. The most notable features are lamellar and polysynthetic (deformation) twinning, low transparency (do to high fracturing along twinning planes), color zoning and secondary, fluid, ʺfingerprintʺ inclusions. Based on the similarity in tectonic settings, geological environments, gemological and geochemical characteristics of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundum deposits in Vietnam, which include Tan Huong and Truc Lau deposits in the North Vietnam. Keywords: Corundum; Geological settings; Crystallography; Gemological properties; Mineral assemblage. 1. Introduction* importance. The main ruby and sapphire mining areas in Vietnam are: • Luc Yen, Tan Huong and Truc Lau in the north; • Quy Chau in the centre; • Dak Ton, Ma Lam, Da Ban in the south. Phuoc Hiep occurrence, which has been discovered recently, is located in Phuoc Son District, Quang Nam Province, approximately 70 km from southwest of Da Nang City (Central Vietnam now is considered as a country with big potential for gemstones. Many kinds of gemstones have been found and mined in different areas of the country. Among these gemstones, ruby and sapphire are of most _ Corresponding author. Tel.: 84‐4‐5587061. E‐mail: nmthuyet@vnu.edu.vn * 152 Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 part of Vietnam). Until now, no any data have been published on the corundums from this area. 153 granulite facies with widespread development of migmatization processes. The magmatic rocks include granitogneisses (Chu Lai Complex ‐ γPR2 cl), apodunites, apoperidotites, apopyroxenites (Hiep Duc Complex ‐ δPZ hd), granodiorites, leucogranites, pegmatites (Dien Binh Complex ‐ γδPZ1 db) and younger formations. The metamorphic rocks of Kham Duc Formation, granitogneisses of Chu Lai Complex and ultramafic rocks of Hiep Duc Complex were intruded by magmatic rocks of Dien Binh Complex and younger magmatic formations, forming metasomatic zones with the characteristic, talc‐ tremolite‐actimolite‐asbestos‐serpentine‐garnet‐ ruby‐sapphire, mineral assemblage (Fig. 1). All the geologic formations in the area belong to Kham Duc zone which extends along east‐west direction and which is influenced by stages of tectonic activities from Late Proterozoic until present. The gemstone occurrence is controlled by main, northwest‐southeast, fault systems (oldest) and meridian fault systems (youngest). 2. Geological settings The locations, where rubies and sapphires were discovered, coincide with the tectonic origin valleys. These valleys occur as depression belts along the fault systems. Investigation works in the area have shown that rubies and sapphires are found in ground, alluvial and first‐order terrace sediments of Thu Bon River and its tributaries. The area of the Phuoc Hiep occurrence is where are distributed metamorphosed volcano‐ sedimentary formations, which include garnet‐ bearing amphibolites, biotite plagiogneisses, quartz‐plagioclas‐biotite schists with disthene, quartz‐feldspar‐biotite‐pyroxene‐garnet, actinolite ‐tremolite, talc‐tremolite schists, tremolite marbles (Kham Duc Formation ‐ PR2 kd). These rocks were metamorphosed to amphibolite‐ u Th n Bo riv er Phuoc Hiep corundum area Mica schist Plagioclasite Phuoc Son town LAO P ok o fa ult + + + + z βQ z + + + Plagioclasite z Mica schist Milonitization zone Sericite schist Fig. 1. Generalized geological map of Phuoc Hiep occurrence. (Chu Lai granite) Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 154 3. Characteristics of Phuoc Hiep corundum The size ranges from 3×5 mm to 20×30 mm, sometimes to 100×120 mm. For implementing this research project, we surveyed Phuoc Hiep occurrence in November of 2005 year. During this time, we had selected corundum samples from Thu Bon River alluvial sediments using a sifting pan and also purchased from local individual gem diggers (Fig. 2). Fig. 3. Corundum crystals from Phuoc Hiep occurrence. Fig. 4. Lozenge‐shaped Phuoc Hiep corundum fragments. Fig. 2. Lots of Phuoc Hiep corundums selected from the gem locality. 3.1. Crystallography and visual appearance Most of the corundum pieces appeared rounded or tumbled, with no indication of the original crystal form. The crystals occur as broken fragments, the original habits rarely are observed which resemble short hexagonal prisms (Fig. 3). Some pieces are lozenge‐ shaped, broken fragments, formed parallel to rhombohedral faces of the crystals (due to deformation) (Fig. 4). Fig. 5. Different color varieties of Phuoc Hiep corundums Majority of stones, which are selected from this locality, are pink to light pink with violetish or brownish overcastted colorations. Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 The main color varieties are red to pink, blue to light blue, some stones show irregular color distributions (color zoning, color spots, etc, ). Other colors are grey, colorless or smoky (Fig. 5). Almost all stones show low to medium transparency, although transparent pieces are found during the trip to this area. Low transparency is due to high fracturing of stones. 3.2. Gemological Properties For the first time, gemological properties of two main color varieties of Phuoc Hiep corundums are given (Table 1). They were found to be consistent with corundum in general [3, 4, 6, 7]. 3.2.1. Microscopic Features a. Color zoning Color zoning is not a typical feature of Phuoc Hiep corundums. In some pieces it is observed in the form of irregular color distribution (color patches, color spots, etc., see Fig. 5). Color of these patches may be blue to bluish, grey to colorless. b. Twinning Deformation twinning (Fig. 6), which occurred after corundum crystals had formed, is very characteristic for corundums from this area. This phenomenon is the main cause of high fracturing (and low transparency) of these corundums where fractures occur along twinning planes parallel to rhombohedral faces of the crystals after their formation. This feature is very similar to that of corundums from Tan Huong and Truc Lau deposits in North Vietnam [6, 7]. c. Inclusions Due to low transparency of most corundum pieces, it was very difficult to identify all kinds of inclusions. Nevertheless, mica and chlorite inclusions (Fig. 7) have been identified in some corundum crystals. Frequently encountered are secondary fluid inclusions along cracks (ʺfingerprintsʺ), and ʺiron stainsʺ as golden‐ brownish rusted patches. 3.2.2. Chemical Properties Qualitative X‐ray fluorescence analysis of some corundum samples from Phuoc Hiep area has revealed the presence of Cr, Fe and Ti as the main coloring trace elements. Because in Vietnam we have no facility for quantitative EDXRF analysis, their contents for different color varieties have been analyzed by EPMA method (Table 2). As it can be seen from this table, the pink to red variety contains generally higher content of chromium (0.002 ‐ 0.276, av. 0.136wt%), some samples contain high content of iron, whereas Ti, V, Mn are found in minor amounts. The blue to light blue variety appears to contain higher iron content (0.090 ‐ 0.964, av. 0.327wt%). Table 1. Standard gemological properties of Phuoc Hiep corundums Property Pleochroism Fluorescence Long‐wave UV Short‐wave UV Absorption spectra Refractive index no n e Birefringence Specific gravity 155 Color Variety Pink to light red Definite, yellowish pink / violetish pink Definite, light red Inert to very weak Chromium 1.768 ‐ 1.774 1.760 ‐ 1.766 0.008 ‐ 0.010 3.90 ‐ 4.03 Blue to light blue Definite to weak, blue / greenish blue Inert Inert Can not be evaluated 1.770 ‐ 1.772 1.762 ‐ 1.764 0.008 ‐ 0.009 3.92 ‐ 4.02 Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 156 Table 2. Average contents of trace elements in Phuoc Hiep corundums by EPMA analysis Color variety Trace elements Pink to light red (27 points / 3 samples) FeO Ga2O 0.087‐0.175 Mean: 0.125 0‐0.027 0.006‐0.043 0‐0.087 0.002‐0.276 Mean: 0.003 Mean: 0.020 Mean: 0.005 Mean: 0.136 Ti2O V2O5 Cr2O3 MnO 0‐0.021 Mean: 0.006 Blue to light blue 0.090‐0.964 0‐0.021 0.001‐0.482 0.001‐022 0.006‐0.528 0‐0.011 (11 points / 3 samples) Mean: 0.327 Mean: 0.005 Mean: 0.096 Mean: 0.006 Mean: 0.177 Mean: 0.007 Science, VNU). The characteristic UV‐VIS absorption spectra of different color varieties of Phuoc Hiep corundums are shown in Fig. 8‐10. Fig. 8. UV‐VIS‐NIR absorption spectra obtained from red to pink variety (sample PH4). Fig. 6. Deformation twinning in Phuoc Hiep corundum (nicol: +; d = 1.2 mm) mica chlorite Fig. 9. UV‐VIS‐NIR absorption spectra obtained from red to pink variety with blue spot (sample PH6). Fig. 7. Chlorite and mica inclusions in Phuoc Hiep corundum (nicol: +; d = 1.2 mm). 3.2.3. Spectroscopic Features To reveal spectroscopic properties of Phuoc Hiep corundums, we have investigated their absorption spectra using Shimadzu UV‐VIS spectrophotometer (model UV‐2450) supplied by Center for Material Sciences (College of Fig. 10. UV‐VIS‐NIR absorption spectra obtained from blue spot (sample PH6). Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 Pink to red variety shows various shades of red, pink, violetish to orangey pink that cause slightly different absorption patterns. Sample PH4 (pure pink) appears to show absorption peaks caused only by Cr3+ at 412, 556 and 694 nm (Fig. 9), while sample PH6 (light red with blue spot) shows peaks caused both by Cr3+ (412, 556 and 694 nm) and Fe3+ (388 nm, see Fig. 9). With the same sample PH6 we also obtained spectrum for blue spot (Fig. 10). Besides the absorption pattern which contains bands and peaks typical for blue sapphires from elsewhere (Fe2+/Ti4+ band at 565 nm, Fe3+/Fe3+ peak at 450 nm) we can see also an absorption peak caused by Cr3+ at 412 nm. 157 sapphires from Phuoc Hiep occurrence are red to blue color, low to medium transparency, medium to big size, lamellar and polysynthetic (deformation) twinning and secondary, fluid, ʺfingerprintʺ inclusions. 3.3. Mineral assemblages Until now, no any outcrops of corundum‐ bearing hard rocks have been found in this area. Nevertheless, we have purchased some samples of corundums in matrix from the local gem dealers (Fig. 11). The typical mineral assemblage, which we could determine in these specimens, is corundum, plagioclase and mica (Fig. 11, 12). Fig. 12. Corundum, plagioclase assemblage (d = 0.6 mm, nicol: +). Based on the characteristics in tectonic settings, geological environment and properties of corundums, Phuoc Hiep occurrence can be grouped into the metapelite‐hosted type of corundum deposits in Vietnam [1, 2, 5, 8]. Although Phuoc Hiep ruby and sapphires have low to medium quality, which are suitable mostly for cabochons, but with big size and quantity, they can be considered as a potential source of gem material for Vietnam, especially when appropriate heat treatment technology is applied [9]. Our results are only initial. Systematic and comprehensive investigation should be done in a near future. Fig. 11. Corundum, muscovite, plagioclase assemblage (d = 1.5 cm). 4. Discussion and conclusion The most prominent features of rubies and Acknowledgements This paper presents the results of implementation of the Projects within the framework of the Fundamental Research Program funded by Vietnam Ministry of Science and Technology, and the Key Project Nguyen Ngoc Khoi et al. / VNU Journal of Science, Earth Sciences 23 (2007) 152‐158 158 QGTD 05.01 ʺEstablishment of some gemstone deposit types having industrial potential in Vietnamʺ, funded by Vietnam National University, Hanoi. References [1] P.G. Cooray, P.S. Kumarapeli, Corundum in Biotite‐sillimanite Gneiss from near Polgahawela, Ceylon, Geological Magazine XCVII (1960) 480. [2] Ch.A. Hauzenberger, T. Hager, W. Hofmeister, V.X. Quang, G.W.A. Rohan Fernando, Origin and formation of gem quality corundum from Vietnam, Proceedings of the 2nd International Workshop on Geo‐ and Material‐Science on Gem‐ minerals of Vietnam, Hanoi, 2003. [3] R.W. Hughes, Corundum: ruby and sapphire, White Lotus, 1991. [4] R.E. Kane et al, Ruby and fancy sapphire from Vietnam, Gem & Gemology 27 (1991) 136. [5] Nguyen Viet Y et al, On the forming origin of sapphire and ruby in Vietnam, Journal of Geology B/23 (2004) 110 (in Vietnamese). [6] Pham Van Long, Nguyen Ngoc Khoi, Tran Kim Hai, Typomorphic characteristics of Vietnam rubies and sapphires, Final report of the Scientific Project, Vietnam Ministry of Industry, 2001 (in Vietnamese). [7] Pham Van Long, Research on the crystallo‐ mineralogical and gemological characteristics of rubies and sapphires from Luc Yen and Quy Chau deposits, PhD. Thesis, College of Science, VNU, 2003 (in Vietnamese). [8] G.J. Simandl, S. Paradis, Corundum in Alumina‐rich Metasediments, in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, 1999. [9] T. Themelis, The heat treatment of ruby and sapphire, Gemlab Inc., USA, 1992. ... Fig. 1. Generalized geological map? ?of? ?Phuoc? ?Hiep? ?occurrence. (Chu Lai granite) Nguyen Ngoc Khoi et al. / VNU Journal? ?of? ?Science, Earth Sciences 23 (2007) 152‐158 154 3.? ?Characteristics? ?of? ?Phuoc? ?Hiep? ?corundum ... Fig. 4. Lozenge‐shaped? ?Phuoc? ?Hiep? ? corundum fragments. Fig. 2. Lots? ?of? ?Phuoc? ?Hiep? ?corundums? ?selected from? ?the gem locality. 3.1. Crystallography and visual appearance Most of? ? the corundum ... rhombohedral faces of? ? the crystals (due to deformation) (Fig. 4). Fig. 5. Different color varieties of? ?Phuoc? ?Hiep? ?corundums Majority? ?of? ?stones, which are selected? ?from? ? this locality,