# Mineral Chemistry of Li-Bearing Minerals at the Giant Tanco Pegmatite, Canada

Paul Alexandre, Stefano Salvi

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Paul Alexandre, Stefano Salvi. Mineral Chemistry of Li-Bearing Minerals at the Giant Tanco Pegmatite, Canada. Minerals, 2025, 15 (3), pp.221. 10.3390/min15030221. [https://doi.org/10.3390/min15030221]

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Article

# Mineral Chemistry of Li-Bearing
Minerals at the Giant Tanco
Pegmatite, Canada

Paul Alexandre and Stefano Salvi

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Article

# Mineral Chemistry of Li-Bearing Minerals at the Giant Tanco Pegmatite, Canada

Paul Alexandre <fnref n="1" />,* and Stefano Salvi <fnref n="2" /> ID

**Abstract:** The highly fractionated late Archean Tanco pegmatite (Bernic Lake, SE Manitoba, Canada) is a world-class producer of tantalum and cerium but is also a major source of lithium. In order to better understand the major Li hosts and the overall Li budget of the Tanco pegmatite, the lithium-bearing minerals present here were analyzed for major and trace elements by electron microprobe and laser ablation ICP-MS, respectively. The major Li-bearing minerals present in the Tanco pegmatite are eucryptite (approximately 11.0 wt% Li₂O), montebrasite (~11.2 wt%), lithiophilite (9.1 wt%), spodumene (~8.8 wt%), petalite (5.45 wt%), lepidolite (4.36 wt%), and tancoite (5.2 wt%); Li is also present in lithiowodginite, tourmaline, muscovite, beryl, pollucite, and apatite (between 0.1 and 1.3 wt% Li₂O). Most of the Li present in Tanco is contained in petalite (69.4% of all the Li present here), followed by spodumene (11.4%), montebrasite (11.1%), and eucryptite (4.0%); all remaining Li-bearing minerals contain 4.0% of the Li present in the Tanco pegmatite. Overall, the Tanco pegmatite contains approximately 0.71 wt% Li₂O, on par with previous estimates. The major practical implications of these finding are that (1) all Li-bearing minerals have to be considered to properly estimate the Li endowment of any pegmatite; (2) the main Li-bearing mineral is not always spodumene; (3) the exact and detailed Li mineralogy of a pegmatite will directly affect extraction and processing; and (4) a significant proportion of Li in any pegmatite is contained in other minerals than the main one, be it spodumene of petalite.

**Keywords:** Tanco pegmatite; Tanco; pegmatite; lithium; lithium minerals; spodumene; amblygonite; montebrasite; lepidolite; petalite

## 1. Introduction

The highly fractionated late Archean Tanco pegmatite in southeast Manitoba, Canada, belongs to the Bernic Lake pegmatite group hosted by the Bird River Greenstone Belt of the Superior Province (Figure 1). It is exceptionally mineralogically diverse, comprising more than 100 mineral species, mostly silicates, oxides, phosphates, sulfides, and native elements [1]. Four of these minerals have been described at the Tanco pegmatite for the first time [1], underlining its scientific importance.

The Tanco pegmatite is significantly enriched in Ta, Cs, Rb, Be, F, and Li, among others [2], which makes it also economically important. It has been mined for Ta since 1967 and for Cs since 1997, but production is shifting exclusively to Li, due to the extraordinarily strong and growing demand for this commodity currently. Indeed, Li batteries have the highest capacity for electricity storage and therefore are in very high demand and very likely will remain so for the foreseeable future. These considerations are driving a robust

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Academic Editor: Leonid

Dubrovinsky

Received: 8 January 2025

Revised: 12 February 2025

Accepted: 21 February 2025

Published: 24 February 2025

**Citation:** Alexandre, P.; Salvi, S. Mineral Chemistry of Li-Bearing Minerals at the Giant Tanco Pegmatite, Canada. *Minerals* **2025**, *15*, 221.
https://doi.org/10.3390/min15030221

**Copyright:** © 2025 by the authors. License MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">¹ Geology Department, Brandon University, Brandon, MB R7A 6A9, Canada</footnote>

<footnote n="2">² Géosciences Environnement Toulouse, 31400 Toulouse, France; stefano.salvi@get omp.eu</footnote>

<footnote>* Correspondence: alexandrep@brandonu.ca</footnote>
exploration effort for similar Li-endowed pegmatites in the near vicinity of the Tanco pegmatite but also elsewhere in Canada and worldwide.

However, the Li-bearing minerals at Tanco remain relatively insufficiently studied, both for major and for trace elements. While the major Li hosts here are known (specifically, spodumene, amblygonite, eucryptite, and lepidolite; [1]), their precise mineral chemistry remains poorly known. Indeed, the vast majority of mineral chemistry data for the Tanco pegmatite dates from the 1970s (e.g., amblygonite [3], petalite, spodumene [4], eucryptite [5], pollucite [6]) and do not include any trace element data; the one major exception is a more recent study of the major and trace elements of Tanco pegmatite micas [7].

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Location (A) and simplified regional geological background of the Tanco pegmatite (C) within the context of the westernmost portion of the Superior Craton of the Canadian Shield (B). Modified after [1,8] and references within.</figcaption></figure>

The importance of precisely knowing the detailed mineral chemistry of the Li-bearing minerals at Tanco is two-fold. Firstly, there is the more theoretical need to significantly update our knowledge about Li minerals and, by extension, about the formation and evolution of the Tanco pegmatite. The second is more practical: knowing the detailed
mineral chemistry of the major hosts of Li within the Tanco pegmatite would assist with production and processing and will help establish a Li budget for the pegmatite. It is therefore the primary goal of this paper to provide the precise and detailed major- and trace-element mineral chemistry of the Li-bearing minerals at the Tanco pegmatite, using electron microprobe and laser ablation ICP-MS.

Beyond defining the precise mineral chemistry of all Li-bearing minerals present, this study aims to examine the evolution of the Tanco pegmatite, using the trace-element composition of the analyzed minerals. Building on previous work [7], it seeks to confirm earlier findings on the multi-stage evolution of the pegmatite [9,10] and compare Tanco with nearby pegmatites, particularly the Raleigh Lake pegmatite in northwestern Ontario, Canada, a significant Li resource [11].

## 2. Geology of the Tanco Pegmatite

The Tanco pegmatite is located approximately 135 km (as the crow flies) or about 165 km (by road) northeast of Winnipeg, Manitoba. It is hosted by the southern portion of the Bird River Greenstone Belt, itself part of the Archean Winnipeg River sub-province of the Superior Province (Figure 1B). Its local geology and structure have been thoroughly described elsewhere (e.g., [8] and references within); therefore, only the briefest of outlines will be provided here.

The Bird River Greenstone Belt (ca. 2.73 Ga) is bordered by a series of batholiths of intermediate composition dated at ca. 2.78 to 2.67 Ga (Figure 1C); these are followed by numerous peraluminous leucocratic intrusions to which the Tanco pegmatite is related [9]. The Tanco pegmatite is the largest of a pegmatite swarm emplaced ca. 2.64 Ga ago mostly into metagabbros and some amphibolites (Figure 2 [12]).

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Local geological background of the Tanco pegmatite, with a surface projection of the currently known extent of the pegmatite body. The position of the east-west cross-section in Figure 3 is indicated. After [13,14].</figcaption></figure>

The Tanco pegmatite itself is a sub-horizontal body of approximately 2 by 1 km of known lateral extent and a thickness of up to 100 m (Figures 2 and 3). Nine zones have been defined based on mineral composition, texture, and location (Figure 3): the border zone, the wall zone, the aplitic albite zone, the lower intermediate zone, the upper intermediate zone, the central intermediate zone, the quartz zone, the pollucite zone, and the lepidolite zone. The full descriptions of the different zones, including mineral and chemical compositions, are provided in [2,8] and will be briefly summarized here (see also Table 1).
<figure><img src="image_8.png" /><figcaption>**Figure 3.** East-west cross-section through the Tanco pegmatite, showing the different zones defined based on mineral composition, texture and location [14]. The drill holes on which the cross-section is based are shown, as well.</figcaption></figure>

<table><caption>**Table 1.** Volume and main minerals of the different zones of the Tanco pegmatite (from [1,2]). The wall zone, the lower intermediate zone, and the upper intermediate zone are the main ones, combining to approximately 83% of the whole pegmatite.</caption><thead><tr><td>Zone</td><td>Volume</td><td>Main Minerals</td></tr></thead><tbody><tr><td>Border zone</td><td>0.1%</td><td>albite (66%), quartz (29%)</td></tr><tr><td>Wall zone</td><td>30.8%</td><td>albite (41%), quartz (36%), K-feldspar (15%), muscovite (6%)</td></tr><tr><td>Albitic aplite zone</td><td>2.7%</td><td>albite (67%), quartz (27%), muscovite (3%)</td></tr><tr><td>Lower intermediate zone</td><td>28.8%</td><td>albite (25%), quartz (34%), K-feldspar (24%), petalite (10%)</td></tr><tr><td>Upper intermediate zone</td><td>13.1%</td><td>petalite (46%), K-feldspar (25%), quartz (11%), albite (7%)</td></tr><tr><td>Central intermediate zone</td><td>13.6%</td><td>K-feldspar (50%), albite (20%), quartz (15%), muscovite (12%)</td></tr><tr><td>Quartz zone</td><td>7.5%</td><td>quartz (95%)</td></tr><tr><td>Pollucite zone</td><td>1.3%</td><td>pollucite (75%), lepidolite (7%), albite (5%), quartz (5%)</td></tr><tr><td>Lepidolite zone</td><td>2.1%</td><td>Li-muscovite (70%), K-feldspar (10%), quartz (10%), albite (8%)</td></tr></tbody></table>

* The border zone is dominantly a saccharoidal assemblage of albite and quartz along the pegmatite/wall rock contacts and replacing zone; it is very thin (up to 30 cm).

* The wall zone (up to 35 m thick) contains predominantly large columnar microcline (up to 3 m) in a matrix of quartz, medium-grained albite, and tabular greenish muscovite (up to 10 cm); other types of albite and lithian muscovite are also present in minor amounts.

* The albitic aplite zone forms layers (up to 16 m thick) and also smaller lens-like bodies in the eastern part of the pegmatite; it is situated between the wall zone and the overlying zones. This zone can also be dispersed as a network within the central intermediate zone. The albite is mostly saccharoidal and only rarely medium-grained in texture. This zone is characterized by major Ta and Nb mineralization.

* The lower intermediate zone (up to 25 m thick) is located mostly in the lower central parts of the pegmatite. It is very heterogeneous, with two main assemblages: (1) large microcline-perthite crystals, spodumene + quartz pseudomorphs of petalite (up to 2 m) embedded in medium-grained quartz, albite, and micas; and (2) quartz pods (0.5–2 m), amblygonite-montebrasite, and spodumene + quartz aggregates. The feldspar-rich assemblages are separated from the quartz accumulations by radial cleavelandite and micas rims.
* The upper intermediate zone (up to 24 m thick) is the main source of Li-rich minerals. It evolves gradually from the lower intermediate zone upwards, with almost the total disappearance of albite and micas. The main minerals present are exceptionally large: amblygonite up to 2 m, microcline-perthite up to 10 m, petalite up to 13 m in length. This zone also contains spodumene + quartz pseudomorphs of petalite, amblygonite and quartz; low-temperature mineral assemblages can be locally abundant.

* The central intermediate zone (up to 45 m thick) is divided into two segments situated in the central parts of the eastern and western extremities of the pegmatite; its overall shape approximates the contours of the pegmatite. Its contacts with the neighboring units are usually sharp, with local gradual transitions. The main minerals in this zone are microcline-perthite, quartz, and fine-grained greenish muscovite; it is a main source of Nb and Ta oxides.

* The minor quartz zone, situated on the eastern extremity of the pegmatite, contains only quartz.

* The pollucite zone is made of several lenticular bodies located mainly along the upper contacts of the upper intermediate zone. The largest lens (180 m × 75 m × 12 m) is the only one in the eastern part of the pegmatite. Petrologically it is equivalent to the upper intermediate zone but is defined by the significant extent of some of the pollucite bodies, which can contain 75% pure pollucite and are therefore the main source of Ce. Coarse veins of lepidolite, quartz, and feldspar are common.

* The lepidolite zone (up to 18 m thick) corresponds to two flat-lying, east-west elongated sheets and several smaller bodies within the central intermediate zone, or along its contacts with the spodumene (petalite)-rich zones. Fine-grained (less than 2 mm) lithian muscovite is more common here than lepidolite, mostly intergrown with the microcline-perthite and quartz along the contacts with the central intermediate zone. This zone is economically significant because of the presence of lepidolite and Ta-Nb oxides.

As a whole, the Tanco pegmatite is dominated by quartz (32%), albite (26%), and K-feldspar (22%), with these three minerals constituting 80% of the pegmatite volume. They are followed by petalite (9%), muscovite and Li-muscovite (6%), pollucite (1.3%), spodumene (0.9%), and amblygonite (0.8%); all other minerals combined account for less than 2% [1].

It would seem, based on textural and paragenetic evidence, that the bulk of all nine zones was produced by primary crystallization from a volatile-rich silicate melt, albeit also being affected by metasomatic reactions between zones and with the host rocks [1].

# 3. Analytical Methodology

## 3.1. *Sampling and Sample Preparation*

Extensive sampling was conducted at the Tanco mine in the framework of a larger research program on the mineralogy, petrology, and geochemistry of the Tanco pegmatite. It resulted in the collection of approximately 200 hand samples predominantly from the volumetrically most significant pegmatite zones, specifically the wall, aplitic albite, lower intermediate, central intermediate, upper intermediate, and lepidolite zones (Figures 3 and 4). For the purpose of this particular study, specifically focused on the mineral chemistry of Li-bearing minerals, a subset of approximately 50 samples was selected representing the widest possible range of mineralogy. Of these, 24 samples representing the widest possible variety of Li-bearing minerals were selected for thin section preparation and subsequent analysis; the majority of these samples are nearly monomineralic (Figure 5).
<figure><img src="image_9.png" /><figcaption>**Figure 4.** Typical hand samples of Li-rich minerals collected from the Tanco pegmatites and used in this study.</figcaption></figure>

Polished thin sections were prepared at Brandon University (Canada) and examined by optical microscopy and using the scanning electron microscope (SEM) at Brandon University. The SEM instrument is a JEOL JSM-6390 LV analytical SEM, equipped with an Oxford Instruments ULTIM MAX 100 Energy Dispersive Spectrometer Characteristic X-ray detector and Aztec 4.1 analytical software.
<figure><img src="image_10.png" /><figcaption>**Figure 5.** Optical microscope images (cross-polarized transmitted light) of predominantly monominerallic samples from the Tanco pegmatite used in this study. (A) predominantly lepidolite (Lep); (B) predominantly muscovite (Mu); (C) predominantly spodumene (Spod); (D) petalite (Pet) in contact with fine-grained mixture of quartz and petalite (Q-Pet); (E) a mixture of several minerals such as muscovite, spodumene, sericite (Ser), and albite (Ab); and (F) predominantly amblygonite (Amb) with some quartz inclusions.</figcaption></figure>

## 3.2. EMPA

Major element compositions were measured in thin sections using the electron probe microanalyzer (EPMA) Cameca SXFive at the Raimond Castaing Micro-Characterization Centre (Toulouse, France). The elements analyzed were F, Rb, Al, Si, K, Ca, Cs, Ti, Mn, Fe, Na, Mg, Ba, Cr, and Zn. The standards used are albite, wollastonite, synthetic Al₂O₃, sanidine, synthetic MnTiO₃, synthetic Fe₂O₃, topaz, synthetic MgO, and synthetic Cr₂O₃. Detection limits for major element oxides vary between 0.02 wt% (for Al₂O₃) and 0.15 wt% (for ZnO) for an overall average of 0.06 wt%; the detection limit for F is 0.60 wt%. Typically, between 20 and 25 analytical spots were measured for each mineral in a given thin section, with some minerals (spodumene, amblygonite, pollucite) being measured in more than one thin section. Each analysis consisted of 30 s blank measurement on each side of the peak and a 60 s peak measurement; the operating conditions were accelerating voltage 20 kV, current 10 nA, and beam size 2 µm.
### 3.3. LA-ICP-MS

The lithium and trace elements compositions of the Li-bearing minerals from the Tanco pegmatite were measured using the Thermo Element XR Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) coupled with a Merchantec femtosecond later, located at the Geoscience and Environment Toulouse (France). Analytical spots were placed on the exact same areas as the EMPA ones, or as close as possible. Each ICP-MS analysis, conducted in low resolution mode, consisted of 30 s blank collection followed by 60 s sample signal collection (corresponding to the laser ablation duration) and 40 s wash-out. Within each analysis, 42 cycles were measured, each of which contained 10 measurements per peak for each element; each measurement time was 0.005 sec. The laser settings were as follows: 20% power output, approximately 2.2 J/cm² fluence, repetition rate of 10 Hz, and spot size of 20 µm. All analyses were performed in line mode, with speed of 10 µm/sec; approximately 20 to 50 lines per mineral were carried out. External standards were NIST610 and NIST612; internal calibration based on the concentration of Al obtained by EMPA. The elements analyzed were Li, Be, B, Mg, P, Ca, Sc, Ti, Mn, Fe, Ni, As, Se, Rb, Sr, Y, Nb, Sn, Cs, Ba, Ta, Pb, Th, U, and Rare Earth Elements (REE).

## 4. Analytical Results

The chemical compositions of the Tanco pegmatite Li-bearing minerals were obtained by electron microprobe (major elements) and laser ablation ICP-MS (trace elements), according to the analytical procedures outlined above. They are reported here organized by mineral, starting with the two micas (lepidolite and muscovite), followed by spodumene, amblygonite, and petalite.

Albite, fairly abundant at the Tanco pegmatite (25.6 vol% overall; Table 1), beryl (0.4 vol%), and pollucite (1.3 vol%) [1] were also analyzed, both by EMPA and LA-ICP-MS. Their Li concentrations were however fairly low: below detection for albite, an average of 148 ppm for pollucite (ranging from 22 to 229 ppm), and an average of 875 ppm for beryl (ranging from 728 to 1022 ppm). Therefore, albite is not a Li host at the Tanco pegmatite, and beryl and pollucite are only very minor ones: the full analytical data for those three minerals are not reported here, and they will not be discussed further.

### 4.1. Micas

The two micas identified at the Tanco pegmatite by optical and electron microscopy are lepidolite and muscovite. Both were analyzed for major and trace elements (by electron microprobe and laser ablation ICP-MS, respectively) and the results are reported here (Table 2, Figure 6).

Lepidolite shows a significant variation in all major and many trace elements (Figure 4), with a wide compositional range between the two end-members reported in Table 2. SiO₂ varies from 47.10 (end-member 1) to 45.31 wt% (end-member 2; overall average of 46.11 wt%), Al₂O₃ varies from 26.02 to 31.09 (overall average of 28.31 wt%), K₂O varies from 8.73 to 9.48 wt% (overall average of 8.98wt%), F varies from 7.60 to 3.19 wt% (overall average of 5.28 wt%), Li₂O varies from 5.04 to 3.90 wt% (overall average of 4.36 wt%), Rb₂O varies from 3.84 to 2.62 wt% (overall average of 3.37 wt%), and Cs₂O varies from 0.93 to 0.62 wt% (overall average of 0.83 wt%; Table 1). The empirical structural formulas for the two end-members are:

$$K_{0.729}(Al_{1.362}Li_{0.683}Rb_{0.161}Cs_{0.026}Na_{0.016})\sum=1.974(Si_{3.080}Al_{0.920})\sum=4.000O_{10}(F_{1.599}OH_{0.401})$$

and

$$K_{0.791}(Al_{1.088}Li_{0.528}Rb_{0.110}Cs_{0.017}Na_{0.020})\sum=2.037(Si_{2.963}Al_{1.037})\sum=4.000O_{10}(F_{0.672}OH_{1.328})$$
The two lepidolite end-members defined here (Table 1) correspond approximately to trilithionite and to lithium-muscovite (Figure 6).

There are also differences between the two end-members in trace elements, but these are much less pronounced (Table 2). On average, lepidolite trace elements are dominated by B (overall average of approximately 324 ppm), Ta (294 ppm) Sn (290 ppm), P (125 ppm), Nb (68 ppm), As (53 ppm), Ba (31 ppm), and Be (25 ppm; Table 1). The average values for the other trace elements combined are less than 10 ppm; Rare Earth Elements are below the detection limit.

<figure><img src="image_11.png" /><figcaption>**Figure 6.** Chemical composition of the two micas present at the Tanco pegmatite, lepidolite and muscovite, obtained by electron microprobe and laser ablation ICP-MS and plotted in the lower portion of the micas diagram of [7,15]. Overall, the two micas vary between muscovite and trilithionite and can be broadly defined, using their mineral chemistry, into lepidolite and Li-muscovite.</figcaption></figure>

Overall, the lepidolite composition reported here is not too dissimilar from that reported previously at Tanco [7,16] (Figure 6).

Muscovite, on the other hand, shows a much narrower range and no systematic variation (Table 2). On average, it contains 44.65 wt% SiO₂, 32.73 wt% Al₂O₃, 9.81 wt% K₂O, 3.74 wt% FeO, 1.55 wt% F, and a series of major elements present at less than 1 wt% oxide (Na, Mg, Mn, Ti, Rb, Cs, and Zn; Table 2). Li is present at much lower amounts than lepidolite (average Li₂O of 0.35 wt%; Table 2). All these values are very similar to those published, with only K and Rb significantly lower (K₂O of approximately 10.2 wt% and Rb₂O of approximately 1.8 wt% [7,16]). The empirical average structural formula for the Tanco pegmatite muscovite is:

$$ (K_{0.818}Na_{0.091}Rb_{0.009})\sum=0.918(Al_{1.631}Fe_{0.209}Li_{0.096}Mg_{0.040})\sum=1.990(Si_{3.023}Al_{0.977})\sum=4.000O_{10}(OH_{0.676}F_{0.324})_2 $$

The composition reported here is that of lithium-muscovite (Figure 6).

The trace element composition of muscovite is dominated by Cs (average of 373 ppm), Sn (240 ppm), Nb (223 ppm), Ta (144 ppm), B (24 ppm), and Be (17 ppm; Table 1); the averages of all other trace elements combine to 15 ppm; and Rare Earth Elements are below detection limit.
<table><caption>**Table 2.** Summary of the chemical composition of lepidolite and muscovite from the Tanco pegmatite, obtained by electron microprobe and laser ablation ICP-MS (marked by *). Ca, Cl, Ba, and Cr were also analyzed, but were systematically below the detection limits. *Bdl*—below detection limit; *na*—not analyzed.</caption><thead><tr><th rowspan="3">Major Elements, wt%</th><th colspan="5">Lepidolite</th><th colspan="3">Muscovite</th></tr><tr><th colspan="3">Overall</th><th colspan="2">End-Member Means</th><th rowspan="2">Min</th><th rowspan="2">Mean</th><th rowspan="2">Max</th></tr><tr><th>Min</th><th>Mean</th><th>Max</th><th>1</th><th>2</th></tr><tr><td></td><td colspan="3">(n = 23)</td><td></td><td></td><td colspan="3">(n = 21)</td></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>44.95</td><td><strong>46.11</strong></td><td>47.64</td><td>47.10</td><td>45.31</td><td>44.16</td><td><strong>44.65</strong></td><td>45.02</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>24.98</td><td><strong>28.31</strong></td><td>31.49</td><td>26.02</td><td>31.09</td><td>32.28</td><td><strong>32.73</strong></td><td>33.10</td></tr><tr><td>TiO<sub>2</sub></td><td></td><td>na</td><td></td><td></td><td></td><td>0.19</td><td><strong>0.23</strong></td><td>0.29</td></tr><tr><td>FeO</td><td>0.00</td><td><strong>0.02</strong></td><td>0.08</td><td>0.04</td><td>0.01</td><td>3.52</td><td><strong>3.74</strong></td><td>4.06</td></tr><tr><td>MnO</td><td>0.07</td><td><strong>0.21</strong></td><td>0.35</td><td>0.31</td><td>0.12</td><td>0.16</td><td><strong>0.24</strong></td><td>0.32</td></tr><tr><td>MgO</td><td>0.00</td><td><strong>0.01</strong></td><td>0.05</td><td>0.01</td><td>0.00</td><td>0.35</td><td><strong>0.40</strong></td><td>0.46</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.07</td><td><strong>0.13</strong></td><td>0.24</td><td>0.12</td><td>0.16</td><td>0.59</td><td><strong>0.71</strong></td><td>0.79</td></tr><tr><td>K<sub>2</sub>O</td><td>8.47</td><td><strong>8.98</strong></td><td>9.52</td><td>8.73</td><td>9.48</td><td>9.56</td><td><strong>9.81</strong></td><td>10.09</td></tr><tr><td>Li<sub>2</sub>O*</td><td>3.51</td><td><strong>4.36</strong></td><td>5.24</td><td>5.04</td><td>3.90</td><td>0.34</td><td><strong>0.35</strong></td><td>0.37</td></tr><tr><td>Rb<sub>2</sub>O</td><td>2.46</td><td><strong>3.37</strong></td><td>3.97</td><td>3.84</td><td>2.62</td><td>0.08</td><td><strong>0.21</strong></td><td>0.32</td></tr><tr><td>Cs<sub>2</sub>O</td><td>0.54</td><td><strong>0.83</strong></td><td>1.18</td><td>0.93</td><td>0.62</td><td>0.01</td><td><strong>0.04</strong></td><td>0.07</td></tr><tr><td>ZnO</td><td>0.00</td><td><strong>0.05</strong></td><td>0.18</td><td>0.05</td><td>0.00</td><td>bdl</td><td><strong>0.07</strong></td><td>0.19</td></tr><tr><td>F</td><td>2.47</td><td><strong>5.28</strong></td><td>8.48</td><td>7.60</td><td>3.19</td><td>0.88</td><td><strong>1.55</strong></td><td>2.35</td></tr><tr><td>F=O</td><td>1.04</td><td><strong>2.22</strong></td><td>3.57</td><td>3.20</td><td>1.34</td><td></td><td><strong>0.65</strong></td><td></td></tr><tr><td>Total</td><td></td><td><strong>95.51</strong></td><td></td><td>95.93</td><td>95.63</td><td></td><td><strong>94.73</strong></td><td></td></tr><tr><td>APFU</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Si</td><td></td><td><strong>3.015</strong></td><td></td><td>3.080</td><td>2.963</td><td></td><td><strong>3.023</strong></td><td></td></tr><tr><td>Al<sup>IV</sup></td><td></td><td><strong>0.985</strong></td><td></td><td>0.920</td><td>1.037</td><td></td><td><strong>0.977</strong></td><td></td></tr><tr><td>Σ<sub>TET</sub></td><td></td><td><strong>4.000</strong></td><td></td><td></td><td></td><td></td><td><strong>4.000</strong></td><td></td></tr><tr><td>Al<sup>VI</sup></td><td></td><td><strong>1.199</strong></td><td></td><td>1.088</td><td>1.362</td><td></td><td><strong>1.631</strong></td><td></td></tr><tr><td>Fe</td><td></td><td><strong>0.001</strong></td><td></td><td>0.002</td><td>0.001</td><td></td><td><strong>0.209</strong></td><td></td></tr><tr><td>Mn</td><td></td><td><strong>0.012</strong></td><td></td><td>0.017</td><td>0.007</td><td></td><td><strong>0.014</strong></td><td></td></tr><tr><td>Mg</td><td></td><td><strong>0.001</strong></td><td></td><td>0.001</td><td>0.000</td><td></td><td><strong>0.040</strong></td><td></td></tr><tr><td>Li*</td><td></td><td><strong>0.590</strong></td><td></td><td>0.683</td><td>0.528</td><td></td><td><strong>0.096</strong></td><td></td></tr><tr><td>Σ<sub>OCT</sub></td><td></td><td><strong>1.803</strong></td><td></td><td></td><td></td><td></td><td><strong>1.990</strong></td><td></td></tr><tr><td>Rb</td><td></td><td><strong>0.142</strong></td><td></td><td>0.161</td><td>0.110</td><td></td><td><strong>0.009</strong></td><td></td></tr><tr><td>Cs</td><td></td><td><strong>0.023</strong></td><td></td><td>0.026</td><td>0.017</td><td></td><td><strong>0.001</strong></td><td></td></tr><tr><td>K</td><td></td><td><strong>0.750</strong></td><td></td><td>0.729</td><td>0.791</td><td></td><td><strong>0.818</strong></td><td></td></tr><tr><td>Na</td><td></td><td><strong>0.017</strong></td><td></td><td>0.016</td><td>0.020</td><td></td><td><strong>0.091</strong></td><td></td></tr><tr><td>Σ<sub>A</sub></td><td></td><td><strong>0.932</strong></td><td></td><td></td><td></td><td></td><td><strong>0.919</strong></td><td></td></tr><tr><td>F</td><td></td><td><strong>1.112</strong></td><td></td><td>1.599</td><td>0.672</td><td></td><td><strong>0.324</strong></td><td></td></tr><tr><td>Trace Elements*, ppm</td><td colspan="3">(n = 26)</td><td></td><td></td><td colspan="3">(n = 28)</td></tr><tr><td>Be</td><td>21.1</td><td><strong>25.1</strong></td><td>29.9</td><td>25.3</td><td>24.2</td><td>14.33</td><td><strong>17.17</strong></td><td>19.92</td></tr><tr><td>B</td><td>278.8</td><td><strong>323.7</strong></td><td>389.5</td><td>292.1</td><td>345.9</td><td>21.52</td><td><strong>24.32</strong></td><td>26.79</td></tr><tr><td>P</td><td>102.9</td><td><strong>124.7</strong></td><td>535.7</td><td>144.5</td><td>271.7</td><td>33.24</td><td><strong>58.48</strong></td><td>93.59</td></tr><tr><td>Sc</td><td>0.63</td><td><strong>0.95</strong></td><td>1.17</td><td>0.90</td><td>0.97</td><td>1.89</td><td><strong>2.78</strong></td><td>3.51</td></tr><tr><td>As</td><td>2.8</td><td><strong>53.1</strong></td><td>202.7</td><td>80.8</td><td>95.7</td><td></td><td>bdl</td><td></td></tr><tr><td>Sr</td><td>2.58</td><td><strong>4.91</strong></td><td>9.24</td><td>7.2</td><td>5.6</td><td>0.179</td><td><strong>0.641</strong></td><td>2.290</td></tr><tr><td>Y</td><td>0.02</td><td><strong>0.09</strong></td><td>0.25</td><td>0.11</td><td>0.14</td><td>0.010</td><td><strong>0.033</strong></td><td>0.069</td></tr><tr><td>Nb</td><td>56.6</td><td><strong>67.7</strong></td><td>86.6</td><td>82.5</td><td>60.0</td><td>211.50</td><td><strong>223.4</strong></td><td>236.71</td></tr><tr><td>Sn</td><td>269.1</td><td><strong>290.4</strong></td><td>329.3</td><td>305.6</td><td>282.6</td><td>230.74</td><td><strong>239.6</strong></td><td>254.51</td></tr><tr><td>Cs</td><td></td><td>bdl</td><td></td><td></td><td></td><td>339.02</td><td><strong>373.3</strong></td><td>420.50</td></tr><tr><td>Ba</td><td>12.6</td><td><strong>30.5</strong></td><td>61.5</td><td>42.0</td><td>35.1</td><td>4.36</td><td><strong>9.72</strong></td><td>23.19</td></tr><tr><td>Ta</td><td>208.1</td><td><strong>293.8</strong></td><td>384.8</td><td>340.1</td><td>256.1</td><td>139.14</td><td><strong>144.2</strong></td><td>149.68</td></tr><tr><td>Pb</td><td>1.385</td><td><strong>1.970</strong></td><td>3.620</td><td>1.99</td><td>2.43</td><td>0.952</td><td><strong>1.447</strong></td><td>2.210</td></tr><tr><td>Th</td><td>0.004</td><td><strong>0.042</strong></td><td>0.324</td><td>0.07</td><td>0.15</td><td>0.000</td><td><strong>0.002</strong></td><td>0.012</td></tr><tr><td>U</td><td>0.007</td><td><strong>0.021</strong></td><td>0.065</td><td>0.03</td><td>0.04</td><td>0.001</td><td><strong>0.008</strong></td><td>0.031</td></tr></tbody></table>
## 4.2. Spodumene

The Tanco spodumene data form two compositional groups, differentiated mostly by their Li contents (average Li₂O of 9.21 and 8.43 wt% and no overlap between the two groups: Table 3) and their trace elements contents, specifically P, Ti, Mn, Fe, Rb, Sn, and Cs (Table 3). All other major and trace elements are present in similar concentrations in the two groups, with Al₂O₃ of approximately 27.3 wt% and SiO₂ of 65.6 wt%; other major elements are present in very low amounts and combine to 0.26 and 0.34 wt% oxide, on average (Table 3). Based on these, the average structural formulas for the two groups can be written as:

$$ (Li_{1.097}Na_{0.004})\sum_{i=1.101}Al_{0.898}(Si_{1.945}Al_{0.055})\sum_{i=2}O_{6} $$

and

$$ (Li_{1.030}Na_{0.004})\sum_{i=1.034}Al_{0.965}(Si_{1.998}Al_{0.002})\sum_{i=2}O_{6}. $$

Si and Li are present in amounts higher than what had been previously reported for the Tanco spodumene (approximately 63.6 wt% SiO₂ and 7.7 wt% Li₂O [4,16]); apart from that, the compositions reported here correspond to that of a typical spodumene.

The trace elements in spodumene are dominated by Cs (on average 5.9 and 849 ppm for the two groups), Sn (83 and 281 ppm), Mn (99 and 148 ppm), P (99 and 42 ppm), Mg (10.2 and 11.4 ppm), and Rb (1.0 and 31 ppm; Table 3); all other trace elements combine to approximately 18 and 22 ppm. All trace elements combine to 326 and 1421 ppm. Rare Earth Elements were never detected. Importantly, these trace elements concentrations are different from those reported recently for “secondary” spodumene from an unspecified location in the Tanco pegmatite, which are dominated by Na (562 ppm), Fe (336 ppm), and Mn (186 ppm) [10].

<table><caption>**Table 3.** Summary of the chemical composition of spodumene from the Tanco pegmatite, obtained by electron microprobe and laser ablation ICP-MS (*). K, Rb, Ca, Cl, Cs, Ti, Mg, and Ba were also analyzed by electron microprobe but were systematically below the detection limits. Two empirically defined compositional groups are present, mostly differing in Li and some trace elements concentrations. *Bdl*—below detection limit; blank cell—not analyzed.</caption><thead><tr><th rowspan="2">Major Elements, wt%</th><th colspan="3">Group 1</th><th colspan="3">Group 2</th></tr><tr><th>Min</th><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th></tr></thead><tbody><tr><td></td><td></td><td>(n = 39)</td><td></td><td></td><td>(n = 24)</td><td></td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>26.68</td><td><strong>27.31</strong></td><td>27.72</td><td>26.68</td><td><strong>27.02</strong></td><td>27.46</td></tr><tr><td>SiO<sub>2</sub></td><td>64.79</td><td><strong>65.61</strong></td><td>66.65</td><td>64.79</td><td><strong>65.71</strong></td><td>66.65</td></tr><tr><td>Li<sub>2</sub>O*</td><td>8.82</td><td><strong>9.21</strong></td><td>9.72</td><td>8.22</td><td><strong>8.43</strong></td><td>8.72</td></tr><tr><td>MnO</td><td>bdl</td><td><strong>0.03</strong></td><td>0.12</td><td>bdl</td><td><strong>0.02</strong></td><td>0.06</td></tr><tr><td>FeO</td><td>bdl</td><td><strong>0.06</strong></td><td>0.22</td><td>0.02</td><td><strong>0.13</strong></td><td>0.22</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.02</td><td><strong>0.07</strong></td><td>0.15</td><td>0.02</td><td><strong>0.06</strong></td><td>0.15</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>bdl</td><td><strong>0.02</strong></td><td>0.07</td><td>bdl</td><td><strong>0.02</strong></td><td>0.07</td></tr><tr><td>ZnO</td><td>bdl</td><td><strong>0.02</strong></td><td>0.22</td><td>bdl</td><td><strong>0.02</strong></td><td>0.09</td></tr><tr><td>F</td><td>bdl</td><td><strong>0.07</strong></td><td>0.47</td><td>bdl</td><td><strong>0.08</strong></td><td>0.47</td></tr><tr><td>Total</td><td></td><td><strong>102.40</strong></td><td></td><td></td><td><strong>101.50</strong></td><td></td></tr><tr><td>APFU</td><td></td><td><strong>0.26</strong></td><td></td><td></td><td><strong>0.34</strong></td><td></td></tr><tr><td>Si</td><td></td><td>1.945</td><td></td><td></td><td>1.998</td><td></td></tr><tr><td>Al<sup>IV</sup></td><td></td><td>0.055</td><td></td><td></td><td>0.002</td><td></td></tr><tr><td>Σ<sub>TET</sub></td><td></td><td>2.000</td><td></td><td></td><td>2.000</td><td></td></tr><tr><td>Al<sup>VI</sup></td><td></td><td>0.898</td><td></td><td></td><td>0.965</td><td></td></tr><tr><td>Li</td><td></td><td>1.097</td><td></td><td></td><td>1.030</td><td></td></tr><tr><td>Na</td><td></td><td>0.004</td><td></td><td></td><td>0.004</td><td></td></tr><tr><td>Σ<sub>R2+</sub></td><td></td><td>1.101</td><td></td><td></td><td>1.033</td><td></td></tr></tbody></table>
<table><caption>Table 3. *Cont.*</caption><thead><tr><th rowspan="2"></th><th colspan="3">Group 1</th><th colspan="3">Group 2</th></tr><tr><th>Min</th><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th></tr></thead><tbody><tr><td>Trace elements, ppm *</td><td></td><td>(n = 56)</td><td></td><td></td><td>(n = 25)</td><td></td></tr><tr><td>Be</td><td>1.18</td><td><strong>2.42</strong></td><td>8.85</td><td>2.57</td><td><strong>3.56</strong></td><td>5.19</td></tr><tr><td>B</td><td>3.60</td><td><strong>6.29</strong></td><td>12.20</td><td>3.06</td><td><strong>5.27</strong></td><td>11.87</td></tr><tr><td>Mg</td><td>4.41</td><td><strong>10.23</strong></td><td>20.09</td><td>5.74</td><td><strong>11.44</strong></td><td>26.83</td></tr><tr><td>P</td><td>31.97</td><td><strong>99.33</strong></td><td>241.10</td><td>17.15</td><td><strong>41.59</strong></td><td>75.05</td></tr><tr><td>Sc</td><td>0.37</td><td><strong>0.81</strong></td><td>1.36</td><td>0.51</td><td><strong>1.87</strong></td><td>6.24</td></tr><tr><td>Ti</td><td>4.0</td><td><strong>9.9</strong></td><td>41.2</td><td>10.3</td><td><strong>31.8</strong></td><td>76.2</td></tr><tr><td>Mn</td><td>47.8</td><td><strong>99.1</strong></td><td>247.4</td><td>83.9</td><td><strong>148.2</strong></td><td>312.4</td></tr><tr><td>Ni</td><td></td><td>bdl</td><td></td><td></td><td>bdl</td><td></td></tr><tr><td>As</td><td>0.900</td><td><strong>4.520</strong></td><td>14.650</td><td>0.650</td><td><strong>2.643</strong></td><td>7.830</td></tr><tr><td>Se</td><td></td><td>bdl</td><td></td><td>5.560</td><td><strong>5.560</strong></td><td>5.560</td></tr><tr><td>Rb</td><td>0.340</td><td><strong>0.985</strong></td><td>4.740</td><td>0.300</td><td><strong>30.999</strong></td><td>327.790</td></tr><tr><td>Sr</td><td>0.201</td><td><strong>0.540</strong></td><td>0.949</td><td>0.150</td><td><strong>0.302</strong></td><td>0.602</td></tr><tr><td>Y</td><td>0.024</td><td><strong>0.066</strong></td><td>0.511</td><td></td><td>bdl</td><td></td></tr><tr><td>Nb</td><td>0.022</td><td><strong>0.066</strong></td><td>6.990</td><td>0.034</td><td><strong>0.511</strong></td><td>1.638</td></tr><tr><td>Sn</td><td>8.0</td><td><strong>82.6</strong></td><td>324.5</td><td>19.9</td><td><strong>281.2</strong></td><td>1180.9</td></tr><tr><td>Cs</td><td>0.7</td><td><strong>5.9</strong></td><td>46.6</td><td>3.1</td><td><strong>848.8</strong></td><td>4047.6</td></tr><tr><td>Ba</td><td>0.351</td><td><strong>0.930</strong></td><td>1.760</td><td>0.371</td><td><strong>0.850</strong></td><td>1.620</td></tr><tr><td>Ta</td><td>0.167</td><td><strong>2.120</strong></td><td>5.240</td><td>0.106</td><td><strong>6.261</strong></td><td>21.560</td></tr><tr><td>Pb</td><td>0.311</td><td><strong>0.506</strong></td><td>0.775</td><td>0.440</td><td><strong>0.593</strong></td><td>0.820</td></tr><tr><td>Th</td><td>0.004</td><td><strong>0.017</strong></td><td>0.105</td><td>0.001</td><td><strong>0.019</strong></td><td>0.116</td></tr><tr><td>U</td><td>0.001</td><td><strong>0.007</strong></td><td>0.079</td><td>0.001</td><td><strong>0.003</strong></td><td>0.008</td></tr></tbody></table>

### 4.3. Amblygonite

Amblygonite from Tanco also comprises two compositional groups, slightly differing from each other in major and some trace elements; these are observed in all samples and are not functions of sample location within the pegmatite. It is dominated by P (51.99 and 51.61 wt% P₂O₅ on average), Al (35.20 and 35.59 wt% Al₂O₃), Li (10.84 and 11.66 wt% Li₂O), and F (4.24 and 2.88 wt%; Table 4); it is the last of two elements that is mostly differentiated into the two groups. All other major elements analyzed combined are 0.14 and 0.07 wt% oxide. The structural formulas of the two groups can be written as

$$Li_{1.063}Al_{1.008}(P_{1.071}O_{4.248})(F_{0.329}OH_{0.671})$$

and

$$Li_{1.142}Al_{1.019}(P_{1.063}O_{4.252})(F_{0.223}OH_{0.777}).$$

The amounts of Li reported here are significantly higher than previously reported for the Tanco pegmatite (9.9 wt% Li₂O [3]) or compared to pegmatites elsewhere (on the order or 9.8 wt% Li₂O, references in [3] and other data). The Nagatare pegmatite (Fukuoka Prefecture, Japan) and the Chursdorf pegmatite (Mittelsachsen, Germany) are rare types with Li₂O contents systematically exceeding 10 wt%.

Compared to published compositions [3], and based on the dominance of estimated amount of OH (on the order of 0.7 APFU) over the measured F (around 0.3 apfu; Table 4; Figure 7), the amblygonite compositions reported here are in fact closer to the montebrasite than to the amblygonite end-member and should be referred to as such: from now on, we will speak of montebrasite.
<table><caption>**Table 4.** Chemical composition of montebrasite from the Tanco pegmatite, as determined by electron microprobe (major elements, with the exception of Li) and laser ablation ICP-MS (*: trace elements and Li), following the analytical methodologies described in the text. Several other elements (Cl, Ni, Se, Y, and the REE) were also measured by LA-ICP-MS but were always below or near their respective quantification limits. Two empirically refined compositional groups are present, differing mostly in Li and some trace elements concentrations. *Bdl*—below detection limit; *na*—not analyzed.</caption><thead><tr><th rowspan="3">Major Elements, wt%</th><th colspan="3">Group 1</th><th colspan="3">Group 2</th></tr><tr><th>Min</th><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th></tr><tr><th colspan="3">(n = 25)</th><th colspan="3">(n = 16)</th></tr></thead><tbody><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>34.93</td><td><strong>35.20</strong></td><td>35.51</td><td>35.28</td><td><strong>35.59</strong></td><td>35.80</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>51.11</td><td><strong>51.99</strong></td><td>52.56</td><td>51.13</td><td><strong>51.61</strong></td><td>52.11</td></tr><tr><td>Li<sub>2</sub>O*</td><td>10.26</td><td><strong>10.84</strong></td><td>10.71</td><td>10.90</td><td><strong>11.66</strong></td><td>12.78</td></tr><tr><td>Na<sub>2</sub>O</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.05</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.03</td></tr><tr><td>MgO</td><td></td><td><strong>bdl</strong></td><td></td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>CaO</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.04</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.03</td></tr><tr><td>MnO</td><td><strong>bdl</strong></td><td><strong>0.02</strong></td><td>0.08</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.06</td></tr><tr><td>FeO</td><td><strong>bdl</strong></td><td><strong>0.02</strong></td><td>0.08</td><td><strong>bdl</strong></td><td><strong>0.02</strong></td><td>0.07</td></tr><tr><td>SrO</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.09</td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>SiO<sub>2</sub></td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.03</td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>K<sub>2</sub>O</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.06</td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>ZnO</td><td><strong>bdl</strong></td><td><strong>0.04</strong></td><td>0.12</td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>BaO</td><td><strong>bdl</strong></td><td><strong>0.01</strong></td><td>0.09</td><td></td><td><strong>na</strong></td><td></td></tr><tr><td>SO<sub>3</sub></td><td></td><td><strong>na</strong></td><td></td><td><strong>bdl</strong></td><td><strong>0.02</strong></td><td>0.10</td></tr><tr><td>F</td><td>3.57</td><td><strong>4.24</strong></td><td>4.92</td><td>2.30</td><td><strong>2.88</strong></td><td>3.45</td></tr><tr><td>Cl</td><td></td><td><strong>bdl</strong></td><td></td><td></td><td><strong>bdl</strong></td><td></td></tr><tr><td>F=O</td><td></td><td><strong>1.79</strong></td><td></td><td></td><td><strong>1.21</strong></td><td></td></tr><tr><td>Total APFU</td><td></td><td><strong>100.63</strong></td><td></td><td></td><td><strong>100.59</strong></td><td></td></tr><tr><td>Al</td><td></td><td>1.008</td><td></td><td></td><td>1.019</td><td></td></tr><tr><td>P</td><td></td><td>1.071</td><td></td><td></td><td>1.063</td><td></td></tr><tr><td>Li</td><td></td><td>1.063</td><td></td><td></td><td>1.142</td><td></td></tr><tr><td>F</td><td></td><td>0.329</td><td></td><td></td><td>0.223</td><td></td></tr><tr><th rowspan="3">Trace Elements, ppm *</th><th colspan="3">(n = 26)</th><th colspan="3">(n = 28)</th></tr><tr><th>Min</th><th>Mean</th><th>Max</th><th>Min</th><th>Mean</th><th>Max</th></tr><tr><td>Be</td><td></td><td><strong>bdl</strong></td><td></td><td>0.660</td><td><strong>1.119</strong></td><td>1.790</td></tr><tr><td>B</td><td>0.350</td><td><strong>1.863</strong></td><td>3.340</td><td>2.170</td><td><strong>6.374</strong></td><td>12.780</td></tr><tr><td>Sc</td><td>0.988</td><td><strong>1.157</strong></td><td>1.385</td><td>2.140</td><td><strong>2.734</strong></td><td>3.270</td></tr><tr><td>Ti</td><td>140.7</td><td><strong>165.9</strong></td><td>190.1</td><td>30.0</td><td><strong>32.9</strong></td><td>35.0</td></tr><tr><td>As</td><td>0.720</td><td><strong>3.581</strong></td><td>21.370</td><td>1.220</td><td><strong>3.543</strong></td><td>5.920</td></tr><tr><td>Se</td><td></td><td><strong>bdl</strong></td><td></td><td></td><td><strong>bdl</strong></td><td></td></tr><tr><td>Rb</td><td></td><td><strong>bdl</strong></td><td></td><td>0.055</td><td><strong>0.369</strong></td><td>0.650</td></tr><tr><td>Sr</td><td>0.134</td><td><strong>0.730</strong></td><td>3.120</td><td>0.214</td><td><strong>0.511</strong></td><td>0.978</td></tr><tr><td>Nb</td><td>0.177</td><td><strong>0.406</strong></td><td>0.696</td><td>0.186</td><td><strong>0.345</strong></td><td>0.715</td></tr><tr><td>Sn</td><td>23.1</td><td><strong>30.7</strong></td><td>46.5</td><td>24.2</td><td><strong>33.2</strong></td><td>43.6</td></tr><tr><td>Cs</td><td>0.120</td><td><strong>1.708</strong></td><td>6.440</td><td>0.816</td><td><strong>4.612</strong></td><td>9.380</td></tr><tr><td>Ba</td><td>0.130</td><td><strong>1.711</strong></td><td>6.590</td><td>0.499</td><td><strong>1.255</strong></td><td>3.180</td></tr><tr><td>Ta</td><td>37.3</td><td><strong>62.6</strong></td><td>87.3</td><td>22.5</td><td><strong>52.6</strong></td><td>127.7</td></tr><tr><td>Pb</td><td>0.017</td><td><strong>0.418</strong></td><td>1.344</td><td>0.365</td><td><strong>0.676</strong></td><td>1.517</td></tr><tr><td>Th</td><td><strong>bdl</strong></td><td><strong>0.009</strong></td><td>0.040</td><td></td><td><strong>bdl</strong></td><td></td></tr><tr><td>U</td><td><strong>bdl</strong></td><td><strong>0.011</strong></td><td>0.110</td><td></td><td><strong>bdl</strong></td><td></td></tr></tbody></table>
<figure><img src="image_12.png" /><figcaption>**Figure 7.** The Tanco pegmatite montebrasite situated within the OH-F amblygonite-montebrasite solid solution series, compared to previous data from Tanco (* [3]*) and to pegmatites elsewhere in the world (** data from [3] plus other studies). The new data indicate that OH dominates F, and thus, this phase should in fact be named montebrasite.</figcaption></figure>

Trace elements are predominantly very low, at the ppm level, with only Ti (166 and 33 ppm), Ta (63 and 53 ppm), and Sn (31 and 33 ppm) exceeding 10 ppm (Table 4). All trace elements combine to 271 and 140 ppm. Rare Earth Elements were systematically below detection.

### 4.4. Petalite

The petalite from the Tanco pegmatite is dominated by Si (an average $SiO_2$ of 77.70 wt%), Al (average $Al_2O_3$ of 16.80 wt%), and Li (average $Li_2O$ of 5.45 wt%; Table 5). Li reported here is somewhat higher than the previously reported ~4.5 wt% $Li_2O$ [4,16]. Na, Mn, Mg, and Rb oxides are low and combine to approximately 0.06 wt%. Based on these numbers, the average structural formula of the Tanco petalite is:

$$(Si_{7.938}Al_{0.062})\sum=8.00Al_{1.930}(Li_{2.235}OH_{0.272})\sum=2.51O_{10}.$$

This chemical composition is very similar to that previously reported, with the exception of Li, which had been reported at approximately 4.55 wt% $Li_2O$ [1].

The trace elements in petalite, determined by LA-ICP-MS, are dominated by Ca (387 ppm on average), P (109 ppm on average), Cs (63 ppm on average), Fe (43 ppm on average), and B (36 ppm on average). All other trace elements combine to a total of about 70 ppm; Table 5); all trace elements combine to 2318 ppm.
<table><caption>**Table 5.** Chemical composition of petalite from the Tanco pegmatite, as determined by electron microprobe (major elements) and laser ablation ICP-MS for trace elements and Li (*), following the analytical methodologies described in the text. Several other elements (Fe, K, Cs, Sc, Sr, Y, Sn, Ba, U, Th, and the REE) were also measured by EMPA or LA-ICP-MS but were always below or near their respective quantification limits. *Bdl*—below detection limit; blank cell—not analyzed.</caption><thead><tr><th>Major Elements, wt%</th><th>Min</th><th>Mean<br>(n = 79)</th><th>Max</th></tr></thead><tbody><tr><td>Na<sub>2</sub>O</td><td><i>bdl</i></td><td><b>0.01</b></td><td>0.06</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>16.61</td><td><b>16.80</b></td><td>17.00</td></tr><tr><td>SiO<sub>2</sub></td><td>76.32</td><td><b>77.70</b></td><td>79.05</td></tr><tr><td>MnO</td><td><i>bdl</i></td><td><b>0.01</b></td><td>0.05</td></tr><tr><td>MgO</td><td><i>bdl</i></td><td><b>0.02</b></td><td>0.07</td></tr><tr><td>Rb<sub>2</sub>O</td><td><i>bdl</i></td><td><b>0.03</b></td><td>0.07</td></tr><tr><td>Li<sub>2</sub>O*</td><td>4.22</td><td><b>5.45</b></td><td>6.29</td></tr><tr><td>H<sub>2</sub>O<sub>Estimated</sub></td><td></td><td><b>0.47</b></td><td></td></tr><tr><td>Total</td><td></td><td><b>99.57</b></td><td></td></tr><tr><td><b>APFU</b></td><td></td><td></td><td></td></tr><tr><td>Na</td><td><i>bdl</i></td><td><b>0.002</b></td><td>0.012</td></tr><tr><td>Al</td><td>0.200</td><td><b>2.017</b></td><td>2.042</td></tr><tr><td>Si</td><td>7.827</td><td><b>7.938</b></td><td>8.045</td></tr><tr><td>Li</td><td>1.731</td><td><b>2.235</b></td><td>2.579</td></tr><tr><td>OH<sub>estimated</sub></td><td></td><td><b>0.267</b></td><td></td></tr><tr><td><b>Trace Elements, ppm*</b></td><td></td><td>(n = 118)</td><td></td></tr><tr><td>Be</td><td>5.50</td><td><b>11.49</b></td><td>24.00</td></tr><tr><td>B</td><td>9.0</td><td><b>36.0</b></td><td>320.0</td></tr><tr><td>P</td><td>28.0</td><td><b>108.9</b></td><td>302.0</td></tr><tr><td>Ca</td><td>80.0</td><td><b>386.7</b></td><td>980.0</td></tr><tr><td>Ti</td><td>3.40</td><td><b>8.84</b></td><td>18.00</td></tr><tr><td>Mn</td><td>4.8</td><td><b>13.7</b></td><td>48.0</td></tr><tr><td>Fe</td><td>13.0</td><td><b>43.7</b></td><td>170.0</td></tr><tr><td>Ni</td><td>1.70</td><td><b>6.94</b></td><td>15.20</td></tr><tr><td>As</td><td>2.40</td><td><b>12.41</b></td><td>70.00</td></tr><tr><td>Nb</td><td>0.10</td><td><b>0.52</b></td><td>1.50</td></tr><tr><td>Ta</td><td>1.01</td><td><b>2.75</b></td><td>6.00</td></tr><tr><td>Se</td><td>1.0</td><td><b>16.5</b></td><td>42.0</td></tr><tr><td>Cs</td><td>20.5</td><td><b>62.9</b></td><td>300.0</td></tr><tr><td>Ba</td><td><i>bdl</i></td><td><b>1.75</b></td><td>17.00</td></tr><tr><td>Pb</td><td>0.61</td><td><b>1.65</b></td><td>3.90</td></tr></tbody></table>

# 5. Discussion

## 5.1. *The Major Li Hosts in the Tanco Pegmatite*

The question of exactly which minerals contain the bulk of Li at the Tanco pegmatite has been previously addressed, albeit somewhat superficially, with silicate (muscovite, lepidolite, petalite, spodumene, eucryptite, pollucite, tourmaline), phosphate (amblygonite-montebrasite, tancoite, lithiophilite, apatite), and oxide (lithiowodginite) minerals identified as the main Li hosts [1,17]. The new data presented here allow us to provide a more detailed and precise appraisal of the Li budget at the Tanco pegmatite.

Volumetrically, the main Li-bearing minerals (when considering the totality of the Tanco pegmatite as known today) are petalite (9.03 vol%) and muscovite (2.97 vol%), followed by spodumene (0.92 vol%), montebrasite (0.77 vol%), tourmaline (0.38 vol%), beryl (0.37 vol%), lepidolite (0.27 vol%), eucryptite (0.26 vol%), and apatite (0.15 vol% [1,14]).

In terms of Li contents, the most significant minerals are eucryptite (11.03 wt% Li<sub>2</sub>O [16]), montebrasite (10.2 wt% Li<sub>2</sub>O; Table 2), lithiophilite (9.13 wt% Li<sub>2</sub>O [16]), spo-
dumene (8.8 wt% Li₂O; Table 3), petalite (5.45; Table 5), lepidolite (4.36 wt% Li₂O; Table 2), and tancoite (5.20 wt% Li₂O [18]), followed by lithiowodginite, tourmaline, muscovite, beryl, pollucite, and apatite (between 0.1 and 1.3 wt% Li₂O).

Considering these volume proportion and Li content data together, and accounting for mineral density, it becomes apparent that by far most the important Li-bearing mineral at the Tanco pegmatite is petalite. Containing 5.45 wt% Li₂O and accounting for 9.03 vol% of the pegmatite, this mineral contains 69.4% of all the Li present at Tanco. Petalite is followed by spodumene (11.4% of Li in the pegmatite), montebrasite (11.1%), and eucryptite (4.0%): these four minerals contain 96% of all Li present at the Tanco pegmatite. All remaining Li-bearing minerals account for the remaining 4.0%.

## 5.2. Li Budget of the Tanco Pegmatite

Another difficult question is the overall concentration of Li at the Tanco pegmatite, made that much more challenging by the mineralogical and chemical variability between the different Tanco pegmatite zones (Table 1; [8]). This topic has been addressed in the past [2], using the average Li contents and the volume proportion of the individual zones described earlier (Section 2; [2,19]).

Here, we use a different approach, using the volume proportion of all the individual Li-bearing minerals for the totality of the Tanco pegmatite as it is known today (Table 1; Section 5.1; [1,19]) and the Li contents of each of these minerals, as summarized above. This approach allows us to calculate the overall Li contents of the Tanco pegmatite, to approximately 0.71 wt% Li₂O.

This concentration is marginally lower than the previous estimate of 0.76 wt% Li₂O [2]; the two estimates, based on different approaches, do not disagree to the extent of invalidating one or the other but rather confirm each other. The discrepancy between them is likely due to mineral chemistry variations for the individual Li-bearing minerals between the pegmatite zones and the sometimes-difficult estimation of the chemical composition of these zones [2]. On the basis of these two estimates, it can be concluded that the average Li contents of the Tanco pegmatite is in the order of 0.73 wt% Li₂O.

This estimate is on the same order as other pegmatites in the same part of the Canadian Shield. For instance, the Raleigh pegmatite in northwestern Ontario has been estimated to contain approximately 1.47 wt% Li₂O [20]; pegmatites elsewhere in the world range from 0.10 to 1.41 wt% Li₂O [1].

## 5.3. Li Deportment as Function of Pegmatite Differentiation

Rare earth element pegmatites crystallize from highly differentiated, flux-rich, and nearly water-saturated melts [21]. Fractional crystallization is not the only mechanism that may be evoked to explain the complex zoning observed at the Tanco pegmatite (Section 2, Figure 3; [1]). Processes such as supercooling (i.e., strong undercooling below liquidus [22,23]); the presence of boundary layers [20]; multi-melt immiscibility [24]; endometasomatism by a late, strongly fractionated melt [7]; and post-crystallization alterations, have all been demonstrated to play an important role in the formation of a complex pegmatite body such as the Tanco pegmatite. The complex zoning observed at Tanco (Figure 3) and its fairly complex mineralogy [25] necessarily required a complex set of processes, even though fractional crystallization would have played a central role. Here, we will use differentiation indicators such as the Nb/Ta ratio to study Li deportment in the minerals of the Tanco pegmatite as function of differentiation. We will also compare these findings to the Raleigh Lake pegmatite in northwestern Ontario, a simpler body that nevertheless contains two distinct zones of varying mineralogy [20]. Comparing the Tanco pegmatite to the Raleigh Lake one is pertinent not only because they have similar
geotectonic settings, are in relatively close proximity, and are of a similar age, but also
because the Raleigh Lake pegmatite is the only one nearby to have trace elements analyzed
for all Li-bearing minerals [20]. Incidentally, this underlines the necessity to analyze
trace elements—or radiogenic isotopes—as proxy for differentiation; something that is
excessively rarely carried out.

The multi-stage formation of the Tanco pegmatite has been previously documented
and is based on the compositional and spatial relationships of the different pegmatite zones
described earlier and informed by experimental studies ([12] and referenced within). The
border, wall, and lower intermediate zones (1, 2, and 4) seem to have formed first, followed
by the aplitic and upper intermediate zones (3 and 5); the central intermediate zone and
the quartz zone (6 and 7) appear to have formed last [9,11,13]. Even if this sequence
is somewhat tentative and thus cannot be considered with absolute certainty, it clearly
points to an ongoing differentiation of the late-magmatic silicate melts that produced the
Tanco pegmatite [9]. The new mineral chemistry data reported here support this ongoing
differentiation, even though the other processes mentioned earlier would have also affected
the Tanco pegmatite and modified its zonal and mineralogical composition.

The Nb/Ta ratio, for instance, can be a particularly sensitive indicator of differentia-
tion extent, with lower values corresponding to higher degrees of differentiation [26–28].
When the Nb/Ta ration is considered in conjunction with Li contents, the latter a highly
incompatible element itself, a clear trend becomes apparent of increasingly Li-rich minerals
with strongly decreasing Nb/Ta ratio, corresponding to a progressively higher degree of
differentiation of the Tanco pegmatite (Figure 8). This trend is mostly defined by mus-
covite (Nb/Ta ~1.5, Li₂O of 0.35 wt%), lepidolite (Nb/Ta ~0.23, Li₂O of 4.35 wt%), and
montebasite (Nb/Ta ~0.007, Li₂O of 10.84 wt%; Figure 5).

<figure><img src="image_13.png" /></figure>

**Figure 8.** The Nb/Ta ratio of the minerals analyzed and the micas reported by \* [7] plotted as function of their Li contents. The major Li-bearing minerals from the Raleigh Lake pegmatite, northwestern Ontario, Canada, are plotted for comparison (\*\* data from [20]). The progressive differentiation trend of lower Nb/Ta ratios and higher Li contents are clearly visible, corresponding to the crystallization sequence reported for the Tanco pegmatite [9,11,12]. The Raleigh Lake pegmatite minerals show a very similar behavior, suggesting similar multi-stage emplacement of that pegmatite. Please note that petalite is absent from this diagram, as its Nb was below detection limit.
Considering that the partition coefficients of Nb and Ta are very similar for the different Li-bearing minerals discussed here (with a slight preferential partitioning towards the white micas), we can observe that the minerals containing the most Li crystallized in the most differentiated silicate melts of the Tanco pegmatite, corresponding to the pegmatite zones that formed the latest, specifically the central-intermediate and the quartz zones [9]. This trend is not applicable to spodumene (Figure 8), which is, however, ubiquitous in all the Tanco pegmatite zones and formed throughout the pegmatite crystallization sequence: this statement is confirmed by its Nd/Ta ratio varying between that of the early muscovite (~1.5) and the late montebrasite (~0.007: Figure 8). A very similar trend, although based on fewer minerals and not as well defined, is observed for the Raleigh Lake pegmatite Li-bearing minerals (muscovite, cookeite, and spodumene (Figure 8): the muscovite values overlap those of the Tanco muscovite, and spodumene demonstrates a similar Nb/Ta ratios and lower Li concentrations.

In other words, the trace element data reported here not only confirm—and visualize—the multi-stage crystallization sequence suggested for the Tanco pegmatite and the associated continuous differentiation [9,11,12] but also clearly indicate that the minerals that are richest in Li, particularly montebrasite, can be found in the latest and most differentiated zones of the Tanco pegmatite. Such an observation, however, must be moderated by the likely existence of other differentiation processes mentioned above, and by the varying solubility of the different Li-bearing minerals studied here, which will depend on the varying pressure and temperature conditions and the varying composition of the remaining silicate fluid during the formation of the Tanco pegmatite.

5.4. *Practical Implications*

As this work demonstrates and had been previously known, pegmatites are potentially a significant Li resource: they can contain up to approximately 1.5 wt% Li₂O [1,20]. It is generally accepted that spodumene is the main Li-bearing mineral in rare-metal pegmatites such as Tanco, but our new findings demonstrate that most—two-thirds—of Li is in fact contained in petalite here (accounting for approximately 9 vol% of the whole pegmatite [1] and containing approximately 5.5 wt% Li₂O: Table 5). Even in those numerous cases where spodumene is the main Li-bearing mineral, it is very rarely the only one: for instance, about a fifth of Li at the Raleigh pegmatite, is contained in micas (muscovite) and clays (cookeite) [20]. These observations have several practical implications:

* Spodumene is not the only potential major Li-bearing mineral in prospective pegmatites and exploration efforts should not discount those that do not contain much of it. From this point of view, pegmatites of different types may also represent a viable Li resource.

* The full Li budget of any Li-rich pegmatite should take into account all Li-bearing minerals and not only the main one: if only the main Li-bearing mineral is considered, the overall Li contents of a pegmatite would be under-estimated.

* Accounting for all Li-bearing minerals will have strong implications for ore extraction and processing, as it will strongly inform the mining processes and milling techniques. Knowing the exact Li mineralogy of a pegmatite will necessarily have a strong effect on maximizing Li extraction.

Another practical implication concerns the method used to estimate the overall chemical composition of a pegmatite, and the Li concentration in particular, a difficult proposition due to their persistent mineralogical variability and heterogeneity. There seem to be two different ways to do that: the first is based on the average composition of the different pegmatite zones and their relative volumes [2], and the second is based on the chemical composition of all Li-bearing minerals and their relative amounts, as in this contribution.
Importantly, these two methods seem to result in very comparable results, which means that bulk estimation can be safely carried out either way, based on the type of data available.

**6. Summary of Findings**

The new data provided here, combined with literature data [1,16,19,24] provide a clear account of the lithium mineralogy of the highly fractionated late-Archean Tanco pegmatite. In terms of Li contents, the most significant minerals are montebrasite (10.2 wt%), lithiophilite (9.1 wt%), spodumene (8.8 wt%), petalite (5.5 wt%), lepidolite (4.4 wt%), and tancoite (5.2 wt%); Li is also present in lithiowodginite, tourmaline, muscovite, beryl, pollucite, and apatite (between 0.1 and 1.3 wt% Li₂O).

On the basis of these compositional data and the relative abundance of the Li minerals present here [1,2], it was possible to establish the major hosts of Li at the Tanco pegmatite. By far most of the Li present at Tanco is contained in petalite (69.4%), which is followed by spodumene (11.4%), montebrasite (11.1%), and eucryptite (4.0%); all remaining Li-bearing minerals contain 4.0% of the Li available at the Tanco pegmatite. These data also allowed us to estimate the overall Li content of the Tanco pegmatite at approximately 0.71 wt% Li₂O, confirming the previous estimate of 0.76 wt% Li₂O [2].

The trace element data reported here, and in particular, the Nb/Ta ratio (Figure 8), clearly illustrate the multi-stage crystallization sequence of the Tanco pegmatite [12] and the associated continuous differentiation and clearly indicate that the more Li-rich minerals, particularly montebrasite, are to be found in the latest and most differentiated zones of the pegmatite. The progressive differentiation trend is similar to, and better defined than, that of the Raleigh Lake pegmatite in northwestern Ontario, Canada (Figure 8 [20]), suggesting that it may be a common occurrence in more complex pegmatite bodies.

**Author Contributions:** Conceptualization, P.A. and S.S.; sampling and methodology, P.A. and S.S.; formal analysis, P.A.; investigation, P.A. and S.S.; data treatment, P.A. and S.S.; writing—original draft preparation, P.A. and S.S.; visualization, P.A.; project administration, P.A.; funding acquisition, P.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Brandon University, Grant # 227530, Lithium ore mineralogy of the Tanco pegmatite, granted 16 January 2022.

**Data Availability Statement:** All raw data are available upon request to P.A.

**Acknowledgments:** The authors would like to acknowledge the assistance of A. Bayg (Brandon University, Canada) with sample preparation, A. Marquet (Geoscience and Environment Toulouse, France) with laser ablation ICP-MS analyses, P. de Parseval (Raimond Castaing MicroCharacterization Centre, Toulouse, France) with electron microprobe analyses, and M. Van Lichtervelde with valuable suggestions and comments.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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