An Update on Tourmaline from Luc Yen, Vietnam

Nguy Tuyet Nhung, Le Thi Thu Huong, Nguyen Thi Minh Thuyet, Tobias Häger, Nguyen Thi Le Quyen, and Tran Thi Duyen

ABSTRACT

Since the late 1980s, tourmaline has been collected with ruby, sapphire, spinel, and other minerals from placers along the rivers and streams of Vietnam’s Luc Yen district. In 2004, a body of granitic pegmatite containing tourmaline associated with green feldspar was discovered. Several other sites, spreading over an area of more than 100 km², were later discovered in the area. Tourmaline from Luc Yen may be colorless, but it also is found in a variety of colors, including gray, pink, green, yellow, brown, and purple. Crystals can reach 20 cm in length and may weigh 2–3 kg or more. Most of the material is of the elbaite species; liddicoatite and uvite are less common. Only a small percentage of the tourmaline is suitable for faceting; the rest is carved or kept as mineral specimens in situ for collectors.

INTRODUCTION

Tourmaline is a borosilicate mineral with the extremely complex structural formula XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W, where the most common ions (or vacancies) at each site are X = Na⁺, Ca²⁺, K⁺, and vacancy; Y = Fe²⁺, Mg²⁺, Mn²⁺, Al³⁺, Li⁺, Fe³⁺, and Cr³⁺; T= Si⁴⁺, Al³⁺, and B³⁺; B = B³⁺; V = OH^– and O^2–; and W = OH^–, F^–, and O^2–. Tourmaline’s compositional variability (which mostly occurs at the X, Y, Z, W, and V sites) creates a supergroup of minerals. Gem tourmaline is mined in many places in the world, but is mainly Brazil and Africa. In Vietnam, gem-quality tourmaline (figure 1) has been found with ruby, sapphire, and spinel in the Luc Yen placers since the late 1980s, when crystals were found in the weathering crust by local farmers and construction workers. The first pegmatite body containing colored tourmaline and green feldspar was found at Minh Tien commune and mined in 2004–2005; within a few years, other tourmaline-bearing pegmatite bodies were discovered in Khai Trung (2008–2009), Tan Lap (2009–2010), and An Phu (2011). All of the pegmatite bodies were mined in an unorganized fashion by locals using rudimentary methods. So far, local authorities have banned mining at all the pegmatite bodies, although mining from placers is still permitted (figure 2). A number of published studies have confirmed the presence of gem tourmaline in Luc Yen (see Quoc, 1995; Nhung and Huong, 1996; Ngu, 2001; Nhung et al., 2005, 2010; Blauwet, 2007, 2011; Wilson, 2007; Huong et al., 2012; Long et al., 2013). The material belongs to various species such as elbaite, liddicoatite, dravite, and rossmanite. This paper pre­sents an overview of the characteristics of tourmaline from Luc Yen by combining the data published by other authors with the results of this study.

LOCATION AND ACCESS

The Luc Yen district of Yen Bai Province is located in northwest Vietnam, about 270 km from Hanoi. Luc Yen is most easily accessed by driving north from Hanoi along National Highway 2 and then Highway 70 and Route 152 crossing over the Chay River to Yen The, in the center of Luc Yen district (figure 3). It is possible to access the outcrops using a car or motorbike and then walking about 1–2 km. Highway CT05 connecting Hanoi, Yen Bai, and Lao Cai was completed in late 2014, making travel to Luc Yen more convenient.

GEOLOGY

The Luc Yen district covers an area of more than 800 km². It is located on both sides of the Chay River and distributed in two geological structure zones within the boundary of the Chay River fault: the Red River zone in the southwest and the Lo Gam zone in the northeast (figure 4).

Gem-quality tourmaline occurs in granitic pegmatite outcrops in the Lo Gam area. This area is composed of Proterozoic rocks, including two mica (biotite and muscovite)-quartz schist intercalated with mica (biotite or muscovite)-quartz schist, with marble lenses and quartzite of Thac Ba formation in the lower part. The upper part is An Phu formation composed of calcite marble, calcite-dolomite marble containing ruby, sapphire and spinel. Quaternary sediments are on the top and include alluvium and diluvium (superficial deposits formed by flooding); gemstones are found in the loose sediments. The exposed magmatic blocks present the changing composition from Paleozoic gabbro, syenite, and granosyenite (Nui Chua and Phia Ma complexes) to Triassic granite (Phia Bioc complex). Pegmatite veins are scattered throughout the area, mainly northwest to southeast, with fewer in the north-south direction. Some are said to be the last phase of above magmatic complexes (Xuyen, 2000), while others are of undetermined age. A few pegmatites throughout Luc Yen were found to contain gem-quality tourmaline, and the Minh Tien pegmatite formed 30.58 Ma (Nhung et al., 2007; Huong et al., 2016). Thus, gem-quality tourmaline-bearing pegmatites do not relate to Paleozoic and Triassic magmatic activities in this region but appeared simultaneously with the displacement of the Red River fault during the Tertiary period.

MATERIALS AND METHODS

The 152 tourmalines from Luc Yen in this study included 90 rough and 62 polished samples. The 90 rough samples, including 30 in situ pegmatite pieces and 60 single crystals, were taken from pegmatite bodies and placers by the authors during different field visits in 2004, 2011, 2012, and 2013. Forty-four polished samples were purchased from local people, and the other 18 were loaned by local gem dealers. Refractive index (RI) was measured for 40 samples, and hydrostatic specific gravity (SG) was measured with an electronic balance for 62 samples. UV fluorescence for 40 samples was analyzed using a standard 4-watt long-wave (365 nm) and short-wave (254 nm) UV lamp. Internal features were observed using a standard binocular gemological microscope; solid inclusions were identified in thin sections under a polarizing microscope. Elemental composition of inclusions was analyzed with a JEOL JSM-7600 energy-dispersive spectroscope (EDS) with an integrated Oxford ISIS microanalyzer. These analyses were performed at the Hanoi University of Natural Science at Vietnam National University, the Gemmological Center of the Vietnam Gemstone Association, and the Institute of Geological Sciences, Vietnam Academy of Science and Technology in Hanoi.

Quantitative chemical analyses were carried out at the Institute of Geosciences at Johannes Gutenberg University in Mainz, Germany. Analyses were performed on seven samples, including three mono-color (green, brown, and pink), three bicolor (light green and light pink; light pink and green; colorless and grayish blue), and one tricolor sample (orange, light yellow, and brown-green). In the four color-zoned samples, chemical analysis was conducted for each color zone, yielding a total of 12 data sets for the seven samples. The chemical analyses were carried out with electron microprobe for Si and laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS) for all other elements.

Electron microprobe analyses were performed with a JEOL JXA-8900RL instrument equipped with wavelength-dispersive spectrometers, using 20 kV acceleration voltage and 20 nA filament current. The measurements were calibrated with wollastonite as the standard for Si. LA-ICP-MS analysis for all elements except Si and B (including Li, Be, Na, Mg, Al, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Ge, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, La, and Ta) was conducted using an Agilent 7500ce ICP-MS system in pulse counting mode. Ablation was achieved with a New Wave Research UP-213 Nd:YAG laser ablation system, using a pulse repetition rate of 10 Hz, an ablation time of 60 seconds, a dwell time of 10 millisecond per isotope, and a 100 μm crater diameter. On average, three laser spots were measured for each zone of the color-zoned samples. NIST 612 glass was used as a standard. B and H₂O contents were calculated by CLASTOUR software (Yavuz, 2002), which is used to identify varieties of tourmaline on the basis of the classification scheme proposed by Hawthorne and Henry (1999).

MINERALOGICAL AND GEMOLOGICAL CHARACTERISTICS

Crystal Morphology and Structure. Tourmaline from Luc Yen is found in the form of single crystals (figure 5, left) or as multi-crystal aggregates (figure 5, center). Prismatic crystals usually show a combination of hexagonal and trigonal prisms terminated with a pyramid or pinacoid face (figure 5, left). Color-zoned crystals usually have more complex habits, with many prism faces combining to create multiple stripes parallel to the c-axis. Pink tourmaline crystals can form columnar aggregates (figure 5, center) or radiate from a central point (figure 5, right). Individual crystals can reach up to 20 cm in length (Long et al., 2013).

Lattice parameters of 14 different-colored Luc Yen tourmalines were determined (Nhung et al., 2005; 2010) as follows: a = 15.824–15.994 Å, c = 7.091–7.208 Å, c/a = 0.445–0.453 Å. Notably, three of the samples, which had a uniform dark green and dark brown color, gave the high value for the c lattice parameter (7.190–7.208 Å) and the high c/a ratio (0.451–0.453) that are characteristic for dravite and uvite. The 11 remaining samples, including particolor and homogeneous colored stones, had the lower values (c = 7.091–7.130Å, c/a = 0.445–0.448) characteristic of elbaite.

Visual Appearance and Gemological Properties. Luc Yen tourmalines come in many colors, including pink, green, yellow, orange, red, gray, brown, and black (figure 6), commonly with a mixing of hues (e.g., greenish yellow and brownish red) and a variety of tones and saturations. Many are particolor, with color zoning distributed along the c-axis (figure 6D) or from the center to the periphery (figure 7). Colorless zones are frequently found in particolor material. Sometimes the zoning creates fancy patterns, as seen in figure 8. The color distribution does not follow a predictable pattern; it may be black in the center and pink in the margin, but may also be red or green in the center and green or black in the periphery. Notably, tourmaline from Minh Tien and An Phu has a wider range of color, while tourmaline from Khai Trung and Tan Lap is mainly pink or purple.

Pleochroism was observed in all samples but was most intense in green, brown, and purple stones (see table 1). The material ranged from transparent to translucent to opaque for some black, brown, and green samples. RI measurements indicated values of n_ε = 1.618–1.628 and n_ω = 1.635–1.645, with a birefringence of 0.016–0.023. SG values ranged from 3.05 to 3.20. Yellow and green tourmalines measured from 3.11 to 3.20, while pink, red, orange, brown, colorless, grayish blue, and other green samples showed values from 3.05 to 3.10. These last green samples were uvite species.

All of the tourmalines were inert to long-wave UV radiation. The pink, red, brown, and black samples were inert to short-wave UV as well, though the green, yellow, and brownish yellow specimens fluoresced yellowish green (see figure 9 and table 1). No phosphorescence was observed in any of the samples.

Internal Features. Most of the Luc Yen tourmalines contained inclusions, such as the gas-filled mirror-like fractures described by Liddicoat (1990). Two-phase (gas + liquid) inclusions were the most common (figure 10, left). Growth tubes were also encountered frequently (figure 10, right). A few pink tourmaline samples contained abundant small solid inclusions of albite and tourmaline (figure 11), with albite being more common. Albite inclusions often showed twinning under a polarizing microscope with crossed polarizers. Tourmaline inclusions had needle or rod forms (figure 11, right). In one faceted green tourmaline sample, we observed reddish brown granular inclusions, presumed to be xenotime or monazite, surrounded by halo tension cracks that might have been caused by radioactive elements within the inclusions. Monazite inclusions were previously reported in a pale pink California tourmaline (see Gübelin and Koivula, 1992). Apatite, quartz, and diopside were also found in Luc Yen tourmaline (Huong et al., 2012).

Table 1 compares the gemological properties of Luc Yen tourmaline from this study with those published elsewhere in the literature.

CHEMICAL COMPOSITION

According to the classification of Hawthorne and Henry (1999), tourmaline-group minerals can be divided into three principal groups based on the dominant occupants at the X-site: alkali tourmalines (Na or K), calcic tourmalines (Ca), and X-site-vacant tourmalines (vacancy). These groups are further divided initially based on the W-site occupancy, and then by the V-site, Y-site, and Z-site occupancies (actual or inferred).

Chemical analyses of seven Luc Yen tourmaline samples ranging from nearly colorless to green, yellow, orange, brown, pink, and grayish blue are shown together with data from the literature (Dirlam et al., 2002; Wilson, 2007; Huong et al., 2012) in table 2 and presented in five ternary composition diagrams (figure 12). The results reveal that all of the samples contained Li. The Li₂O content was quite high (up to 2.58 wt.% in the green zone of sample 2) and was lowest (0.006–0.002 wt.%) in two samples with homogeneous color: sample 3 (brown) and sample 4 (green). Five of the samples (1, 2, 5, 6, and 7) contained high amounts of Li and Na, and the ratio of (Na + K)/(Na + K + Ca) ranged from 0.56 to 0.94, indicating that all of them belonged to the elbaite species (alkaline group). One exception was the pink zone of sample 2, which showed a high amount of Ca with a (Na + K)/(Na + K + Ca) ratio of 0.48, indicating that the pink zone is liddicoatite, a calcium-tourmaline group member. The two specimens with the lowest Li concentrations (samples 3 and 4) were rich in Mg and Ca and therefore classified as uvite, also a calcium-rich tourmaline. Samples 5 and 7 contained notably high Mn (on average 5.17 and 6.49 wt.% MnO, respectively). All samples were in the hydroxyl subclass.

APPLICATIONS

Only 5–10% of the Luc Yen tourmaline production is transparent enough to be faceted for jewelry (figures 6 and 13). The rest is carved (figure 14), taking into consideration the color distribution during processing. Some of the material is large, with beautiful color and relatively high transparency.

Tourmalines with beautiful color but significant fractures or small sizes are often kept in the host rock as mineral specimens (again, see figure 5, left). The tourmaline-bearing pegmatite blocks range from a few kilograms to several hundred kilograms and may contain green feldspar or pink-violet lepidolite (figure 15).

DISCUSSION

The results of 23 chemical analyses—twelve from this study, four from Huong et al. (2012), six from Wilson (2007), and one from Dirlam et al. (2002)—are presented in table 2 and figure 12. Tourmaline from Luc Yen has many different colors that may be homogeneously or heterogeneously distributed in the crystals. Most of them were of the elbaite species, but a few were liddicoatite and uvite. Most of the elbaite and liddicoatite samples were multicolored. Color zoning may appear perpendicular to the c-axis, but in many cases, color zoning is parallel to the c-axis and prism faces, giving different color zones from core to rim. This color zoning distribution is typical for elbaite. A number of studies have confirmed the presence of elbaite in Luc Yen (Nhung et al., 2005; Wilson, 2007; Huong et al., 2012). We also recognized the presence of uvite and liddicoatite. Unlike liddicoatite, uvite has not been reported for watermelon tourmaline samples. In Dirlam et al. (2002) and Laurs et al. (2002), the authors examined the same sample, reportedly from Vietnam. Although no particular locality is indicated, we presume the sample is from Luc Yen, where similar-looking watermelon tourmaline is abundant. The coexistence of elbaite and liddicoatite in one specimen reported by Laurs et al. (2002) is consistent with our study. The species of tourmaline from Luc Yen is determined by its chemical composition (Hawthorne and Henry, 1999). The results of the present study and those from other researchers presented in classification diagrams are quite similar. The difference is the presence of rossmanite reported in Wilson (2007) and uvite in this study. According to the classification of Hawthorne and Henry (1999), sample 3’s (Na + K)/(Na + K + Ca) ratio of 0.49 indicates uvite with a high dravite component. This supports the likelihood of a dravite-containing uvite in Luc Yen.

Although we did not detect rossmanite, one of our samples had a high proportion of vacancies (0.366) in the X-site. Thus, the presence of rossmanite in Luc Yen tourmaline is entirely possible (see Wilson, 2007). Transition metal elements such as Fe, Cr, Ti, V, and Mn cause a rich diversity of color in tourmaline (see Fritsch and Rossman, 1987, 1988a, 1988b). Tourmaline from Luc Yen has a wide variety of colors, and their relationship with the presence and concentration of transition metal elements should be recognized. The green elbaite samples contained more iron (up to 4.88 wt.% FeO) than the pink elbaite (maximum 0.21 wt.%), which always contained more manganese than iron (see green samples 1a, 2b, 9, 15, and 16 and pink samples 1b, 2a, 6, 8, 12, 17, and 18 in table 2). Orange and yellow elbaite (sample 7) had high manganese content (6.47 and 6.57 wt.% MnO, respectively), similar to the Mn-rich tourmaline from Zambia (Laurs et al., 2007). However, high amounts of manganese were also seen in colorless (sample 5a), grayish blue (sample 5b), and gray (sample 14) tourmaline. Green uvite (sample 4), in contrast to green elbaite, contained lower iron (0.01 wt.% FeO), higher chromium (0.68 wt.% Cr₂O₃), and higher vanadium (0.15 wt.% V₂O₅). Brown uvite (sample 3) contained the highest titanium (0.8 wt.% TiO₂) among all tourmaline samples.

Lepidolite and amazonite are found in association with elbaite, and rarely with liddicoatite-elbaite, in the Luc Yen pegmatites. These minerals contain many rare elements such as Li, Rb, Cs, Ta, and Nb. This suggests that gem tourmaline–bearing pegmatite from Luc Yen might be classified as “rare-element” using the classification by Cerny and Ercit (2005). The ages of feldspar (30.58 Ma) from the Minh Tien tourmaline-bearing pegmatite (Nhung et al., 2007) and of phlogopite (33.6 Ma) associated with ruby from Minh Tien and An Phu marble (Garnier et al., 2002) show that these gemstones were formed by the Cenozoic magmatism related to the India-Eurasia collision in northern Vietnam.

So far, gem-quality green uvite/dravite samples have only been collected from placers in Minh Tien and An Phu, and not from pegmatite host rocks. It is more likely they formed during a metasomatic process when pegmatites were injected into carbonate rocks.

CONCLUSIONS

The study results show that most tourmalines from Luc Yen are elbaite; only a small number are liddicoatite and uvite (dravite). The presence of rossmanite in Luc Yen is also possible, as we found some samples showing a high proportion of vacancies in the X-site. Elbaite and liddicoatite-elbaite have been found in alluvium as well as in pegmatite bodies, while the uvite/dravite has been found only in alluvial placers. The transparent green uvite/dravite has been found in placers from Luc Yen, but its appearance is rare. Tourmalines from this area are rich in color but often contain two-phase (gas + liquid) inclusions, growth tubes, and fractures. Therefore, transparent samples suitable for faceting or cutting are rare. Instead, there are a large number of samples with attractive colors and relatively high transparency that are large enough (up to ten centimeters in some cases) for carving. The gem tourmaline-bearing pegmatite bodies in various parts of Luc Yen indicate considerable potential.