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Article

Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal

by
Beiqi Zheng
1,2,* and
Meihua Chen
3
1
School of Environment, Harbin Institute of Technology, Harbin 150090, China
2
Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055, China
3
Gemmological Institute, China University of Geosciences, Wuhan 430074, China
*
Author to whom correspondence should be addressed.
Crystals 2021, 11(11), 1363; https://doi.org/10.3390/cryst11111363
Submission received: 24 September 2021 / Revised: 1 November 2021 / Accepted: 2 November 2021 / Published: 8 November 2021
(This article belongs to the Special Issue Gem Crystals)
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Abstract

:
Few studies have focused on gem-quality tourmaline acting as a petrogenetic recorder, and the colour genesis of pink elbaite is still controversial. We carry out in situ major, trace element and boron isotope composition analyses on a single tourmaline crystal. This crystal is characterized by sudden transformation from colourless to pink, which can represent full pegmatite magma evolution. According to the analysis results, all spots are divided into alkali groups according to X-site occupancy and subdivided into elbaite series. The pink part accommodates higher concentrations of volatile and incompatible elements. The result is most consistent with successive pegmatite evolution in which the colourless part crystallized from the early stage, while the pink part crystallized from the late stage. The relatively consistent δ11B value between the colourless and the pink part suggests no fluid exsolution occurred during pegmatite evolution. The slight increase of δ11B values within the pink part and the colourless part may be due to mica crystallization. The combination of (Li++Mn2+) (Al3++Xvac)-1 and the exclusive positive linear relationship of Mn2+ vs. Ti4+ indicate that Mn2+ is the main cause of pink, while Mn2+-Ti4+ intervalence charge transfer also plays an important role.

Graphical Abstract

1. Introduction

Tourmaline is distinguished from other gem minerals by its unique splendour and changeable colour, even within individual crystals. It is a borosilicate mineral that has a complicated crystal structure and a complex with the general structural formula XY3Z6(T6O18)(BO3)3V3W, where X = Ca, Na, K, vacancy; Y = Mg, Fe2+, Mn2+, Al, Li, Cr3+, V3+, Fe3+, (Ti4+); Z = Mg, Al, Fe3+, V3+, Cr3+; T = Si, Al, (B); V = OH, O; W = OH, F, O [1]. Elbaite, ideally Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)4, is mostly formed in pegmatite [2]. Some studies have noted that even a single crystal can offer comprehensive information on its formation environment [3,4,5], so finely developed compositional zonation elbaite has been documented to be a petrogenetic recorder of the pegmatite system.
As schorl is the most widespread tourmaline which has a black appearance, quite low percentages of tourmaline can reach gem quality [6]. Thus, it is considered an attractive mineral and collectable gemstone. Other less attractive tourmalines are used to study their colouration mechanism and colour enhancement (e.g., irradiation, heating, diffusion and thermochemical treatment) to increase their economic value. Although numerous studies have unearthed the origin and evolution process of pegmatites [7,8,9,10,11,12,13,14], controversy remains regarding pegmatite evolution, especially crucial ore-forming elements and volatile components. One approach to solve this issue is to conduct detailed analysis on mineral zonation. Tourmaline is verified to precisely record information at very small scales in both time and space [15]. As a common accessory mineral in pegmatite, tourmaline, especially gem tourmaline with zonation, is a better carrier than other rock-forming minerals, such as feldspar and muscovite. Therefore, more research is needed to expand gem tourmaline utility.
Spectroscopic methods were widely used in studying the colour genesis of pink-red tourmaline, e.g., X-ray photoelectron spectroscopy by Hong et al. [16], Li et al. [17]; UV-visible, NIR, IR and Raman spectroscopy by Reddy et al. [18], EPR spectroscopy by Babińska et al. [19]. However, the understanding about pink-red tourmaline remains inconclusive and it still needs further research.
This article focuses on a single colourless-pink tourmaline crystal, and this occurrence provides an opportunity for us to investigate the composition of gem tourmaline in a changing environment. We conducted in situ major, trace and boron isotope analyses to (1) provide detailed insight into how gem quality tourmaline acts as an evolution recorder in pegmatite system; (2) provide a new perspective of colourization mechanism by using chemical composition variation and element substitution.

2. Sample Description

The bi-coloured tourmaline crystal in this study was derived from a miarolitic pegmatite from the Daray-Nur field in the Kunar area. The Kunar area is located in the Nuristan belt of Eastern Afghanistan (Figure 1), which contains the largest rare-metal pegmatites and is famous for outputting polychrome and giant tourmaline.
It exhibits a prismatic morphology nearly 10 mm in length and 3 mm in width (Figure 2a,b). It has a symmetrical prismatic appearance and a spherical cross-section shaped by the alternate crystallization of a trigonal prism and a hexagonal prism, resulting in a R3m syngony. This tourmaline has well-developed gradiant colour zonation intensified in the C-axis direction and splits the tourmaline into two parts: the top quarter of the sample is pink, and the rest is nearly colourless (Figure 2c,d). A small subordinate tourmaline crystal is parallel intergrowth with the prime body crystal. This small crystal also exhibits the same colour zonation. The pink part tourmaline exhibits intense pleochroism from pink to light pink while the colourless part tourmaline shows negliable pleochroism. Very poor cleavage and flawlessness under the 10 magnifying glasses are shown in this entire crystal. It is optically uniaxial and negative.

3. Methods

3.1. Major and Trace Element Analysis

Mineral compositions were analysed with a JEOL JXA-8230 Electron Probe Microanalyzer equipped with five wavelength-dispersive spectrometers (WDS) at the Laboratory of Microscopy and Microanalysis, Wuhan Microbeam Analysis Technology Co., Ltd. The samples were first coated with a thin conductive carbon film prior to analysis. Operating conditions for quantitative WDS analyses involved an accelerating voltage of 15 kV, a beam current of 20 nA and a 10 µm spot size. Data were corrected online using a ZAF (atomic number, absorption, fluorescence) correction procedure. The peak counting time was 10 s for Ca, Mg, K, F, Si, Al, Fe, Na, Ti, and Mn. The background counting time was 1/2 of the peak counting time on the high- and low-energy background positions. The following standards were used: Diopside (Ca, Mg), Sanidine (K), Barium Fluoride (F), Olivine (Si), Pyrope Garnet (Fe, Al), Jadeite (Na), Rutile (Ti), Rhodonite (Mn). Tourmaline structural formulae calculation was based on a normalization to 31 anions apfu (atoms per formula unit) and an optimization of ΣYZTB cations (ideal = 18.000). The occupancy of tourmaline was further calculated by using the Excel program provided in Morgan [21]. Analyses of minerals in thin sections were conducted by the LA-ICP-MS method at the Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. The testing points followed a straight line from the dark part to the light part.
A photon machine analyte HE-193-nm ArF excimer laser ablation system and Agilent 7900 quadrupole ICP-MS were combined for the experiments. This laser ablation system comprised a squid signal smoothing device. The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on a mineral surface with a fluent of approximately 2 J/cm2. The ablation protocol employed a spot diameter of 35 μm at an 8 Hz repetition rate for 40 s after measuring the gas blank for 20 s. Helium was applied as the carrier gas to efficiently transport aerosols to ICP-MS. Standard reference materials, including GCS-1g, BCR-2G and GSE-1g, and NIST 612, were used as external calibration standards. Standard reference materials were run after every 6–8 tourmaline testing points; a calculation was conducted for every element in every plot analysis.
Raw data reduction was performed offline by ICPMSDataCal software using a 96.5%-normalization strategy (assume there was approximately 3.5 wt% H2O in tourmaline) without applying internal standardization [22]. Each spectrum was carefully examined. Segments of the spectrum related to mineral inclusions were removed. Measured values of these reference glasses deviating from preferred values are typically better than ±5% for major elements and ±10% for trace elements.

3.2. Boron Isotope Analyses

Boron isotopic compositions of tourmaline were measured in situ by the LA-MC-ICP-MS method at the Beijing Createch Testing Co., Ltd. The measurements were carried out on a Neptune Plus MC-ICP-MS manufactured by Thermo Scientific and a matching RESOlution SE 193 nm laser ablation system. During the analyses, a beam diameter of 50 μm, frequency of 8 Hz and energy of approximately 7–8 J/cm2 were used. The aerosol produced in ablation was blown out with He as the carrier gas, and after passing a T-cock, it was mixed with Ar gas, and the mixture was loaded into the plasma of the MC-ICP-MS for ionization. 11B and 10B signals were collected statically and simultaneously by Faraday cups. The detailed analytical procedure was similar to that described by Yang and Jiang [23].
11B/10B ratios were obtained for the unknown samples, and two standard samples were used in this experiment, including an international IAEA B4 (δ11B = −8.71‰) [24] as the internal standard and an in-lab standard IMR RB1 (δ11B = −12.96 ± 0.97‰) as the external standard. IAEA B4 was used to correct the instrument before the experiment. One analysis of the standard and one analysis of the sample were performed alternatively and repeatedly. Instrument mass fractionation was calibrated by the standard-sample-standard bracketing method (i.e., SSB method) to calculate the true δ11B value. The IMF in this study (−13.64 ± 0.34; 2SD, N = 4) was consistent with the reported values (−12.96 ± 0.97‰) conducted by Hou et al. [25]. The testing error (2σ) from 0.000130 to 0.000180 during our analysis was within the previously reported analytical error.

4. Results

Ten analysis spots on major elements (Figure 3a,b) and 16 analysis spots on minor elements (Figure 3c) of tourmaline are listed in Table 1 and Table 2, respectively. Tourmalines show compositional variations in SiO2 (37.59–38.36 wt%), TiO2 (0–0.07 wt%), Al2O3 (40.62–42.17 wt%), CaO (0.23–2.25 wt%), MnO (0–0.06 wt%), Na2O (1.34–1.91 wt%) and F (0.71–1.38 wt%), and extremely low contents (~0%) are exhibited in FeO, MgO, and K2O. All tourmaline spots belong to the alkali group according to X-site occupancy (Figure 4a) and are further divided into elbaite series (Figure 4b) [1].
All testing spots exceed peraluminous with the calculated Al in the structural formulae ranging from 8.40 to 8.49 apfu. For trace elements, Li (10,561–12,628 ppm), Be (40.5–61.4 ppm), Ga (306–512 ppm), Sn (31.8–78.2 ppm), Bi (1766–3915 ppm), and Pb (49–1083 ppm) exhibit higher concentrations, while other trace elements are below 10 ppm.
Both major and trace elements show systematic differences between the pink part and the colourless part. Some elements show sudden changes: the pink part of this tourmaline has much higher Li (12,348–12,899 ppm), Pb (711–1083 ppm), and Sum REE (25.34–159.40 ppm) contents than the colourless part (10,561–11,341 ppm, 49–128 ppm, and 0.62–2.25 ppm, respectively). The pink part displays lower Na2O (1.34–1.53 wt%) than the colourless part (1.77–1.91 wt%). Other elements including Al and Ga, exhibit a steady increase, while Mn, Ca, F, Sc, Cu, Sb, and Bi exhibit a steady decrease from the colourless part to the pink part.
Basically, tourmaline shows obvious negative correlations in the plots of (Li++Mn2+) (Al3++Xvac)-1 (Figure 5a), indicating the consumption of Al and Xvac while compensating for Li+ and Mn2+ from the colourless part to the pink part. In addition, spots of Al2O3 vs. SiO2, CaO vs. Na2O, F vs. OH and Al2O3 vs. MnO all show fine linear relationships (Figure 5b–e). An exclusive linear relaitionship of TiO2 vs. MnO is only exhibited in the pink part tourmaline (Figure 5f).
Ten analysis boron isotopic spots data are listed in Table 3 and shown in Figure 6a, b. The tourmalines from the pegmatite exhibit a small variation in boron isotopic compositions, with δ11B ranging from −11.77 to −11.14‰. The δ11B values of the colourless tourmaline vary from −11.73 to −11.24‰, which are similar to those of the pink tourmaline (δ11B = −11.77~−11.14‰).

5. Discussion

5.1. Composition Variations and Substitution Mechanisms

Tourmaline has been known for its complex chemistry for more than a century. Various sites in the crystal structure and diverse ions occupying these sites promote tourmaline production of many end-member species [26]. Most pink to red gem-quality tourmalines belong to elbaite, fluor-elbaite, rossmanite and fluor-liddicoatite species [2]. The average composition of tourmaline in this study varies from a colourless part (Na0.61Ca0.10□X0.29) (Al2.45Li0.55)Al6Si6O18(OH)3(OH)0.53F0.47 to a pink part (Na0.49Ca0.37□X0.14) (Al2.40Li0.60)Al6Si6018(OH)3(OH)0.34F0.66. This composition falls in olenite-elbaite solid solution, which cannot simply be expressed by two ideal constitutions and the same atomic arrangement. Based on a detailed classification diagram of the Li-Fe-Mg dominant occupancy of the Y site [15], tourmaline is subdivided into the elbaite series (Figure 4b). The pink part tourmaline is best classified as fluor-elbaite because more than half of the W site is occupied by F [1]. The pink part and the colourless part generally show similar elemental compositions but separate into two groups with specific elements. As mentioned in Section 4, the colourless part has higher Na2O and Al (Figure 7a,b) and lower Mn, F, Li, Sc, Cu, Sum REE, Pb, and Bi (Figure 7c–j) than the pink part. The F contents (up to 1.38 wt%) in this tourmaline are remarkably higher than those in barren pegmatite [27], which is attributed to the high volatility and low viscosity of fertile pegmatite magma and even higher than some fertile pegmatite [4,11]. Moreover, Bi (1766–3915 ppm) and Pb (49–1083 ppm) exhibit unusually high contents in this tourmaline. Similar high Pb and Bi contents of tourmaline have been reported only in tourmaline from Myanmar (Pb 1640 ppm and Bi 153 ppm) [28], which indicates that these tourmalines are associated with Pb and Bi polymetallic deposits. That is, specific trace elements of tourmaline can assess the prospects for ore exploitation related elements.
In the present study, the major/major ratios of Al2O3 vs. SiO2, CaO vs. Na2O, F vs. OH and Al2O3 vs. MnO. (Figure 5b–e) show a fine linear relationship. This phenomenon indicates that the pre-existing major element influences the major element that enters the crystal structure later. However, whether the relative trace element is controlled by the major element is still debated [29,30]. van Hinsberg [31] published an experimental study of element fractionation between tourmaline and melts and found that tourmaline could not fractionate specific trace elements from melts; this result was further verified [32,33]. If pre-existing elements control the later elements, we can anticipate that spots in this tourmaline crystal have the same slope in one pair of elements (e.g., Al2O3 vs. SiO2). In fact, the pink part and the colourless part separate into two trends when we set up the Na2O vs. Pb, CaO vs. Pb, CaO vs. Bi and MnO vs. Sc relationships (Figure 8a–d). In summary, we suggest that major and trace element compositions can be similarly influenced by local melts or fluid components. It is a passive geochemical monitor that records the information of the pegmatite system rather than causing the changes.

5.2. Implications for Growth History and Colour Genesis

Abundant advances manifest composition complexity and boron isotope systematics of tourmaline can be used to decipher and trace geologic processes. The heterogeneous colour and composition distribution in this tourmaline crystal are attributed to a disequilibrium fractional crystallization process. In the hand specimen, the punctuated optical zoning from colourless to pink together with chemical progression (e.g., (Li++Mn2+) (Al3++Xvac)-1) of these two parts is most consistent with successive pegmatite evolution. A normal evolution trend shows that Li is concentrated in the late-stage melt [34]. F also displays an analogous evolution trend [4]. Fractional crystallization of anhydrous minerals leads to water enrichment in the late stage. The pre-existing colourless tourmaline equilibrated with enhydrous residual melt, which was progressively enriched in incompatible elements and volatile components. Higher Mn, F, Li, Sc, Cu, Sum REE, Pb, and Bi contents of the pink part of tourmaline (Figure 7c–j) indicate that it is an advanced product. In the present study, there is a sudden decrease in δ11B values from the colourless part to the pink part with no significant systematic δ11B value differences between these two parts (Figure 6a,b). We attribute it to a sudden change of environment or a pause of tourmaline crystallization. However, δ11B values exhibit increased tendency within the pink part and the colourless part. A previous study showed that residual pegmatite magma was depleted in 10B because of mica crystallization [35]. Combining elements and boron isotope composition, we conclude that the colourless part nucleated at the early stage, while the pink part nucleated at the late stage. Therefore, the composition of the colourless part and the pink part can be representative of the early- and late-stage pegmatite magma, respectively.
Both the colourless part and the pink part followed the exchange vector (Li++Mn2+) (Al3++Xvac)−1, which suggests that Li and Mn substantially increased at the expense of Al and X site vacancies. Crystallization of aluminous minerals in pegmatite systems such as mica and feldspar would result in a less peraluminous magma. The increase in Ca in the late stage is attributed to the F-Ca complex [36]. More occupancy of Ca fills up the X site and leads to fewer X site vacancies. Slight increases in REEs, Sc, Cu, and Sb reflected no other mineral conflicts for these elements and thus were generated at small-scale concentrations of such elements in the late stage. Linear relationships of Na2O vs. Pb, CaO vs. Bi are shown in the colourless part while Pb and Bi are decoupling with Na2O and CaO in the pink part (Figure 8a–c). Even though Pb and Bi were proved to occupy X site recently [37], high Pb content in tourmaline was more recognized as a granitic origin [38] or a product of external fluid derived from sedimentary rock and sulfide deposit [39]. The influx of externally derived fluid components is very common in pegmatite [4,35,40,41,42]. Combing with our discussion in Section 5.1, the sudden increase in Bi and Pb indicates participation of external ore-forming fluid.
In the primary study, Henderson proposed Eu in tourmaline may present in the trivalent state because Sr2+ and Eu2+ have similar geochemical behaviour without correlation in tourmaline [43]. Subsequent studies proved that REE contents do not display obvious composition dependence, while the paragenetic condition is the prime control on REE abundance [44]. Later, experimental studies revealed that tourmaline prefers Eu2+ over Eu3+, because Eu2+ is likely to replace Ca2+ [31]. Jiang emphasized the effects of co-existing minerals that have a marked impact on REE concentrations and patterns in tourmaline, including zircon, xenotime and monazite [45]. It was verified by Zhao et al. [30,46]. The recent study of Vereshchagin et al. [47] confirmed that Eu3+ and Nd3+ occupy the 9-coordinated X-site in the tourmaline structure. In this present study, Eu exhibits extremely low concentrations (avg. 0.03 ppm) and no substitution with CaO (Figure 8e). Besides, Ln exhibits no correlation with elements in X site (e.g., Na2O vs. La (Figure 8f). Consequently, we suggest that tourmaline did not fractionate specific rare earth elements. Instead, it is a passive geochemical monitor. The higher Sum REE contents in the pink part tourmaline reflect a concentration of such incompatible elements in the late-stage magma.
The presence of nearly 3 wt% boron sets tourmaline apart from other silicate minerals. Compared to other common minerals in pegmatite (e.g., mica and feldspar), which contain dozens to hundreds of ppm of boron, tourmaline certainly dominates the evolution of boron and boron isotopes in pegmatite magma. The partition coefficient of boron between fluid and tourmaline is 9, and 11B is preferentially fractionated into fluid [48]. Once fluid escapes from pegmatite magma, the residual magma has lighter δ11B values. The relatively consistent boron isotope composition from the early to the late stage (Figure 6b) indicates that no fluid exsolution occurred in tourmaline crystallization. Furthermore, it is reasonable to deduce that there was no assimilation of external fluids.
The crystal structure of tourmaline can be occupied by almost all transition metals (Fe, Mg, Mn, Cr, Ti, Zn) [49], and natural elbaite colours range from colourless to yellow, pink, blue and green. Knowledge about the colour genesis of pink elbaite is very poor and has long been the subject of discussion. The following mechanisms have been proposed: (1) Mn2+ and Mn3+ alone [50,51]; (2) manganese impurity in which Mn2+ and one broad band in the visible spectrum arises from Mn3+ ions in the high spin state [18]; (3) addition of Cr3+, which is related to a red hue [52]; and (4) irrelevant to Mn2+, Mn3+, Li+, and Cs+ and more likely to the electron-hole colour centre [53]. In this study, the Cr and Cs contents show negligible changes from the colourless part (0–8.25 ppm and 0–0.15 ppm, respectively) to the pink part (0–11.4 ppm and 0–0.51 ppm, respectively), excluding Cr3+ and Cs+ as the colour-causing ions and the third mechanism. Li+ could not be a direct and isolated factor of tourmaline in previous studies [2]. The present pink tourmaline seems irrelevant to Li+ and Cs+. However, the electron-hole colour centre is thought to derive from indirect experiments. An authentic study showed a hole trap of O− but reported only yellow tourmaline. In addition, it interacted with Al nuclei, which is incongruent with our study [54]. Therefore, whether the fourth mechanism of the colour-centre model can interpret the cause of pink colour needs further experiments. The exchange vector (Li++Mn2+) (Al3++Xvac)-1 is controlled in all tourmaline, indicating that increasing Mn2+ plays a crucial role in causing pink tourmaline. In addition, we suggest an ignorable effect of Mn3+ because (1) this tourmaline contains neither detectable Mn3+ nor element substitution of Mn3+ with other trivalent elements (i.e., MnO vs. Al2O3), and (2) a positive linear relationship of Mn2+ and Ti4+ is exclusively exhibited in the pink part (Figure 5f). In this situation, we suggest that the independence of Mn3+ in the first mechanism and the combination of Mn2+ and Mn3+ in the second mechanism are not factors of pink for this tourmaline. Instead, the higher Mn2+ content of pink tourmaline coupled with appreciable Ti4+ indicates possible Mn-Ti interactions. An analogous 325-nm band of Mn2+-Ti4+ intervalence charge transfer has previously been verified on yellow tourmaline [55]. Mn2+ is considered to be responsible for the light pink colour due to the d5 configuration [56]. Based on the data in the present study, we propose that Mn2+ is the main cause of pink and that the Mn2+-Ti4+ intervalence charge transfer occurs in conjunction with Mn2+.

6. Conclusions

The single tourmaline crystal in this study is recognized as a pink part and a colourless part with chemical and boron isotope differences between these two parts. Based on detailed morphology and element and boron isotope analysis, we suggest that this tourmaline can represent an entire evolution process of pegmatite magma. The colourless part was the product of the early-stage magma, while the dark part formed in the late stage. Tourmaline belongs to the alkali group and is subdivided into the elbaite series. Higher Li, Ca, F, Sc, Cu, and Sb and lower Na, Al, and Ga of the pink part of tourmaline are in accordance with the pegmatite evolution trend. Such high contents of Pb and Bi in the pink part of tourmaline may be related to Pb-Bi ore mineralization. The relatively consistent δ11B value reflect no fluid exsolution has happened. The slight increase of δ11B values within the pink part and the colourless part indicates crystallization of mica impact on boron isotope composition in the residual melt. The exchange vector (Li++Mn2+) (Al3++Xvac)-1 and positive linear relationship of Mn2+ and Ti4+ reveal that Mn2+ is the main cause of pink, and the Mn2+-Ti4+ intervalence charge transfer occurs in conjunction with Mn2+.

Author Contributions

Conceptualization, B.Z. and M.C.; methodology, B.Z. and M.C.; validation, M.C.; formal analysis, B.Z.; investigation, B.Z.; resources, M.C.; data curation, B.Z.; writing—original draft preparation, B.Z.; writing—review and editing, B.Z. and M.C.; visualization, B.Z.; funding acquisition, B.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was funded by the author.

Data Availability Statement

Data available on request due to restrictions on privacy. The data provided in this study can be obtained at the request of the corresponding author.

Acknowledgments

Thanks to Wei Gao from Wuhan SampleSolution Analytical Technology Co., Ltd. and Yujun Lei from Wuhan Microbeam Analysis Technology Co., Ltd. for their technical guidance. We are grateful to the two reviewers and the editors for their comprehensive and valuable suggestions.

Conflicts of Interest

The authors declared that they have no conflict of interest in this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Crystals 11 01363 g001 550
Figure 1. Geological sketch map modified from Rossovskiy and Chmyrev [20]: 1. Nuristan median massif Gneisses, crystalline schists, and marmorized limestones; 2. Lower Cretaceous-Paleogene pegmatite-bearing intrusive Laghman complex. Biotite and two-mica granites, biotite-hornblende granites, granodiorites, and quartz, rliorites; 3. Vakhan zone; 4. Neogene-Quatemary deposits. Sands. gravels, and clays; 5. Baluchistan-Himalayan fold region with a superimposed Alpine geosynclinc; 6. North Pamir Hercynian fold region. Karakorum-South Pamir Cimmerian-Alpine fold region. Belts of rare-metal pegmatites: (I) Nuristan; (II) Hindu Kush. Fields of rare-metal pegmatites: (A) Daray-Nur. (B) Chauki. (C) Parown. (D) Kantiwa. (E) Shahidan. (F) Shamakat. (G) Daram-Daram. (H) Pachagan. (I) Mundol. (J) Nilau-Kulam. (K) Kurgal. (L) Alingar.
Figure 1. Geological sketch map modified from Rossovskiy and Chmyrev [20]: 1. Nuristan median massif Gneisses, crystalline schists, and marmorized limestones; 2. Lower Cretaceous-Paleogene pegmatite-bearing intrusive Laghman complex. Biotite and two-mica granites, biotite-hornblende granites, granodiorites, and quartz, rliorites; 3. Vakhan zone; 4. Neogene-Quatemary deposits. Sands. gravels, and clays; 5. Baluchistan-Himalayan fold region with a superimposed Alpine geosynclinc; 6. North Pamir Hercynian fold region. Karakorum-South Pamir Cimmerian-Alpine fold region. Belts of rare-metal pegmatites: (I) Nuristan; (II) Hindu Kush. Fields of rare-metal pegmatites: (A) Daray-Nur. (B) Chauki. (C) Parown. (D) Kantiwa. (E) Shahidan. (F) Shamakat. (G) Daram-Daram. (H) Pachagan. (I) Mundol. (J) Nilau-Kulam. (K) Kurgal. (L) Alingar.
Crystals 11 01363 g001
Crystals 11 01363 g002 550
Figure 2. (a,b) Hand specimen and its crystal size in length/width; (c) prime crystal and subordinate crystal in this study show transparent appearance, euhedral gradual zonation from colourless to pink; (d) a simplified morphology sketch map of this hand specimen.
Figure 2. (a,b) Hand specimen and its crystal size in length/width; (c) prime crystal and subordinate crystal in this study show transparent appearance, euhedral gradual zonation from colourless to pink; (d) a simplified morphology sketch map of this hand specimen.
Crystals 11 01363 g002
Crystals 11 01363 g003 550
Figure 3. Spot number with its corresponding position analysed by EPMA (a,b), LA-ICP-MS and LA-MC-ICP-MS (c).
Figure 3. Spot number with its corresponding position analysed by EPMA (a,b), LA-ICP-MS and LA-MC-ICP-MS (c).
Crystals 11 01363 g003
Crystals 11 01363 g004 550
Figure 4. (a) Classification diagram for tourmalines based on (a) X-site occupancy; (b) secondary division is made according to the dominant occupancy of the Y site (Mg2+, Fe2+ and 2Li+) [1].
Figure 4. (a) Classification diagram for tourmalines based on (a) X-site occupancy; (b) secondary division is made according to the dominant occupancy of the Y site (Mg2+, Fe2+ and 2Li+) [1].
Crystals 11 01363 g004
Crystals 11 01363 g005 550
Figure 5. Major elements of tourmaline: (a) Li+Mn vs. Al+Xvac; (b) Al2O3 vs. SiO2; (c) CaO vs. Na2O; (d) OH vs. F; (e) Al2O3 vs. MnO; (f) TiO2 vs. MnO.
Figure 5. Major elements of tourmaline: (a) Li+Mn vs. Al+Xvac; (b) Al2O3 vs. SiO2; (c) CaO vs. Na2O; (d) OH vs. F; (e) Al2O3 vs. MnO; (f) TiO2 vs. MnO.
Crystals 11 01363 g005
Crystals 11 01363 g006 550
Figure 6. (a) Frequency histogram of boron isotope compositions of tourmaline; (b) δ11B values from the pink part to the colourless part.
Figure 6. (a) Frequency histogram of boron isotope compositions of tourmaline; (b) δ11B values from the pink part to the colourless part.
Crystals 11 01363 g006
Crystals 11 01363 g007 550
Figure 7. (a–d) show the major elements Na, Al, Mn, F (wt%), and (e–j) show trace element (ppm) concentrations from pink to colourless parts in sequence.
Figure 7. (a–d) show the major elements Na, Al, Mn, F (wt%), and (e–j) show trace element (ppm) concentrations from pink to colourless parts in sequence.
Crystals 11 01363 g007
Crystals 11 01363 g008 550
Figure 8. Relationship between major and trace elements: (a) Na2O vs. Pb; (b) CaO vs. Pb; (c) CaO vs. Bi; (d) MnO vs. Sc; (e) CaO vs. Eu; (f) Na2O vs. La.
Figure 8. Relationship between major and trace elements: (a) Na2O vs. Pb; (b) CaO vs. Pb; (c) CaO vs. Bi; (d) MnO vs. Sc; (e) CaO vs. Eu; (f) Na2O vs. La.
Crystals 11 01363 g008
Table
Table 1. Microprobe data of tourmalines from the pink part to the colourless part.
Table 1. Microprobe data of tourmalines from the pink part to the colourless part.
TypePinkColourless
Spots1231234567
SiO237.59237.77837.74338.05338.21938.35937.97037.95538.11738.144
TiO20.0060.0030.0020.0010.0000.0000.0000.0000.0010.000
Al2O340.62240.87240.72541.23041.45041.68941.65241.84941.97642.167
FeO0.0100.0040.0060.0070.0000.0000.0220.0000.0000.000
MnO0.0610.0270.0180.0000.0080.0060.0000.0000.0000.012
MgO0.0000.0010.0000.0000.0000.0000.0020.0000.0010.001
CaO2.0982.2481.5690.8900.6380.2250.2630.3960.5260.715
Na2O1.5011.3371.5251.8111.8151.8711.9121.7961.8281.768
K2O0.0110.0070.0070.0110.0180.0190.0080.0170.0130.002
F1.3831.1061.1110.9700.9610.7980.7120.8020.8690.884
Li2O *2.4922.4142.3902.3392.3162.2502.2392.2562.2842.292
B2O3 *11.10911.09111.04211.10511.13611.12511.05511.09111.15911.197
H2O *3.1773.3023.2833.3723.3873.4603.4773.4463.4383.444
subTotal100.061100.19099.42299.78999.94799.80299.31299.608100.213100.625
O=F−0.582−0.466−0.468−0.408−0.405−0.336−0.300−0.338−0.366−0.372
Total99.47999.72498.95499.38199.54299.46699.01399.27199.847100.253
Structural formula based on 31 total anions
Si5.9055.9185.9465.9595.9695.9875.9575.9405.9345.918
B3.0122.9983.0023.0013.0012.9962.9932.9952.9982.998
Ti0.0010.0000.0000.0000.0000.0000.0000.0000.0000.000
Al7.5227.5477.5627.6107.6317.6697.7037.7207.7037.711
Fe0.0010.0010.0010.0010.0000.0000.0030.0000.0000.000
Mn0.0080.0040.0020.0000.0010.0010.0000.0000.0000.002
Mg0.0000.0000.0000.0000.0000.0000.0000.0000.0000.000
Ca0.3530.3770.2650.1490.1070.0380.0440.0660.0880.119
Na0.4570.4060.4660.5500.5500.5660.5820.5450.5520.532
K0.0020.0010.0010.0020.0040.0040.0020.0030.0030.000
F3.3290.5480.5540.4800.4750.3940.3530.3970.4280.434
Al(T)0.0830.0820.0520.0400.0290.0130.0430.0600.0660.082
Al(Y)1.4391.4651.5101.5711.6011.6561.6601.6591.6371.629
Al(Z)6.0006.0006.0006.0006.0006.0006.0006.0006.0006.000
Li1.5741.5211.5151.4741.4551.4121.4131.4201.4301.430
X-site vacancy0.1870.2150.2680.2990.3400.3920.3730.3850.3580.349
sum(Z+T+Y+B)18.02317.98918.02818.04518.05718.06618.07018.07518.06618.059
* The structural formulae are calculated on the basis of an optimization of 18 cations in sites (Z+T+Y+B) of the tourmaline.
Table
Table 2. LA-ICP-MS data of tourmaline from the pink part to the colourless part (major elements reported in wt% and trace elements reported in ppm).
Table 2. LA-ICP-MS data of tourmaline from the pink part to the colourless part (major elements reported in wt% and trace elements reported in ppm).
TypePinkColorless
Spots123451234567891011
SiO238.2638.1837.8038.4238.2739.4139.5839.5339.5339.3439.6339.3139.4439.5039.4939.58
TiO20.060.040.020.010.010.000.000.000.000.000.000.000.000.000.000.00
Al2O343.9444.3444.5344.0844.0443.7443.5044.1544.3244.4944.2644.6244.4644.4544.5144.31
FeO0.030.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
MnO0.270.120.090.070.040.000.000.000.000.000.000.000.000.000.000.00
MgO0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
CaO2.322.132.322.232.290.910.780.310.160.220.250.260.370.370.440.52
Na2O1.741.771.701.721.691.751.821.901.941.931.931.901.901.881.861.85
K2O0.020.030.010.020.020.010.010.010.010.010.010.010.020.010.000.00
P2O50.020.030.010.020.020.000.000.020.050.020.010.030.030.050.040.04
B2O39.849.809.969.9910.2111.1211.1711.1311.0611.1211.0611.0610.9910.9610.9110.93
Li2O2.712.762.752.682.652.432.412.332.302.262.272.272.282.272.272.29
Total99.2199.2099.2099.2499.2499.3899.2999.3799.3799.3999.4299.4599.4899.5099.5199.54
Be61.4154.5653.2053.2256.5758.0364.8164.3863.5573.3059.3953.2052.1548.3654.8240.46
Sc19.3713.9810.398.115.973.362.111.211.080.871.300.961.490.871.721.37
V0.790.030.090.050.040.070.010.000.030.040.000.270.350.010.150.28
Cr11.360.440.560.000.005.601.131.882.990.004.030.000.000.588.253.88
Co0.280.290.130.070.320.170.000.0031.620.000.0018.810.000.14103.080.00
Ni1.030.000.002.190.000.710.000.001.120.000.000.870.000.000.000.37
Cu10.787.878.107.216.676.275.644.525.244.594.273.884.984.065.374.99
Zn6.545.211.811.332.391.300.090.080.251.131.150.002.190.002.340.66
Ga305.56351.32357.40359.74369.02431.49443.30459.19472.39477.68486.51487.66507.12502.34509.08511.81
Rb0.470.250.390.000.550.000.000.000.260.000.000.280.030.000.000.10
Sr40.3214.709.556.085.071.181.160.490.300.360.260.290.300.320.440.43
Y3.122.333.652.583.451.322.392.773.814.244.904.244.564.964.854.92
Zr0.220.000.000.000.480.000.000.000.000.000.210.310.110.210.000.00
Nb1.571.811.992.211.930.911.181.031.121.511.562.052.432.733.353.72
Mo0.000.150.280.000.000.120.000.120.590.000.260.000.390.000.120.13
Ag0.000.000.040.000.080.120.000.000.020.000.080.090.180.000.510.03
Cd2.703.994.493.865.638.666.127.1210.147.876.686.295.192.106.875.23
Sn64.0578.1866.8261.2351.8934.4231.7635.1435.3334.1342.0038.0839.8842.1846.0748.66
Sb5.915.955.885.565.453.372.841.551.471.380.931.001.010.781.151.49
Cs0.510.120.000.030.000.000.040.080.150.000.000.000.000.070.000.00
Ba0.740.870.780.160.410.130.220.000.290.000.000.000.000.240.000.00
La27.303.763.583.082.750.150.030.000.040.000.020.010.000.020.050.04
Ce65.359.519.668.779.020.300.150.030.030.000.000.000.000.050.010.04
Pr8.811.511.421.251.510.030.040.040.030.020.010.000.000.020.000.02
Nd37.806.216.045.086.910.570.090.050.000.000.000.000.000.100.000.00
Sm13.203.965.054.465.380.620.030.160.160.110.000.000.170.020.000.06
Eu0.150.020.010.010.000.040.060.000.040.000.050.020.000.000.030.00
Gd5.321.962.632.043.120.180.290.000.000.100.000.260.220.210.170.00
Tb0.310.160.170.180.220.080.040.030.070.050.120.100.090.220.180.29
Dy0.980.300.450.410.450.090.290.280.500.430.380.570.690.440.440.96
Ho0.080.000.000.010.020.020.020.020.020.040.000.030.020.030.030.03
Er0.070.070.020.040.000.040.000.020.020.000.050.000.000.010.000.00
Tm0.000.000.000.000.000.000.000.000.000.000.000.000.000.010.000.01
Yb0.030.000.000.000.000.090.100.000.000.140.000.000.040.050.110.07
Lu0.010.010.000.000.010.030.000.000.000.000.010.050.000.000.020.02
Hf0.000.050.000.000.000.160.020.050.000.000.000.110.000.000.080.00
Ta1.416.868.8812.1513.061.603.072.372.783.714.736.396.398.259.0610.53
W0.030.000.030.030.000.000.030.000.000.070.000.000.000.000.040.12
Hg0.000.000.000.000.000.000.000.000.000.000.000.000.000.000.000.00
Tl0.020.050.080.060.010.010.000.000.000.000.090.000.000.000.070.00
Bi3914.983888.803775.333652.653658.223194.143880.303275.293211.213038.842791.762513.502285.252080.231850.241765.72
Pb711.22964.531082.71910.99865.68127.72110.3557.5748.8450.4161.5971.5881.8893.98104.58118.57
Th91.7469.0063.4454.7347.8022.4938.4227.7839.7848.2752.2559.9959.5358.8757.3155.86
U2.420.900.720.400.310.030.110.030.050.050.060.010.070.000.010.06
SUM REE159.4027.4629.0325.3429.412.251.140.620.910.880.641.051.221.191.041.53
Li+Mn1.301.311.291.261.241.131.131.091.081.061.061.061.061.061.061.07
Al+Xvac2.622.702.752.662.672.782.742.862.902.922.872.932.892.892.892.85
Table
Table 3. Boron isotope compositions (given as δ11B values) of tourmalines from pink part to colourless part.
Table 3. Boron isotope compositions (given as δ11B values) of tourmalines from pink part to colourless part.
TypeSpotδ11B (‰)1SD (‰) a
pink1−11.140.1
2−11.240.2
3−11.410.1
411.530.3
5−11.770.1
colourless1−11.240.2
2−11.440.2
3−11.730.1
4−11.430.3
5−11.390.2
a Internal precision in per mil for a single analysis is calculated from about 100 cycles (standard deviation/mean) × 1000 during each analysis. External precision is better than 0.5‰.
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Zheng, B.; Chen, M. Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal. Crystals 2021, 11, 1363. https://doi.org/10.3390/cryst11111363

AMA Style

Zheng B, Chen M. Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal. Crystals. 2021; 11(11):1363. https://doi.org/10.3390/cryst11111363

Chicago/Turabian Style

Zheng, Beiqi, and Meihua Chen. 2021. "Gem Elbaite as a Recorder of Pegmatite Evolution: In Situ Major, Trace Elements and Boron Isotope Analysis of a Colour-Zoning Tourmaline Crystal" Crystals 11, no. 11: 1363. https://doi.org/10.3390/cryst11111363

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