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# Quantitative interpretation of the chemical composition of lepidolite and estimation of the fluorine content of aqueous solutions in equilibrium

Bernard Moine, Paul Alexandre, Stefano Salvi, Philippe de Parseval

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Bernard Moine, Paul Alexandre, Stefano Salvi, Philippe de Parseval. Quantitative interpretation of the chemical composition of lepidolite and estimation of the fluorine content of aqueous solutions in equilibrium. 2025. hal-05390839

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1 **Quantitative interpretation of the chemical composition of**
2 **lepidolite and estimation of the fluorine content of aqueous**
3 **solutions in equilibrium**

4 Bernard Moine¹, Paul Alexandre², Stefano Salvi¹*, Philippe de Parseval¹,³

5 ¹ University of Toulouse, CNRS, GET, IRD, OMP, 14 Av. Edouard Belin, 31400 Toulouse,
6 France

7 ² Geology Department, Brandon University, Brandon, MB R7A 6A9, Canada;
8 alexandreP@BrandonU.ca

9 ³ Centre Raimond Castaing, Espace Clément Ader, 31400 Toulouse, France;
10 philippe.deparseval@get.omp.eu

11 * Corresponding author: stefano.salvi@get.omp.eu; orcid: 0000-0003-4962-0668

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13 **Abstract**

14 It is well established that *lepidolites* are mixtures between a di-octahedral pole, i.e.,
15 muscovite (Mus) K(Al₂) [Si₃AlO₁₀] (OH)₂, and two tri-octahedral poles, i.e., trilithionite (Trl)
16 K(Li₁.₅Al₁.₅) [Si₃AlO₁₀] (F, OH)₂ and polylithionite (Pol) K(Li₂Al) [Si₄O₁₀] (F)₂. Due to
17 variations in xF = F/ (F + OH) in the Trl component, four components must be considered:
18 Mus, F-Trl, OH-Trl, F-Pol. Based on 22 analyses from the Tanco pegmatite, the mole
19 fractions (xi) of each of the components were calculated by solving a linear matrix using the
20 EPMA Si-Al-F and ICP-Ms Li values. The values of x(Mus) range from 0.1 to 0.4 and are
21 perfectly correlated with the values of octahedral vacant sites. The values of x(OH-Trl) reach
22 as high as 0.7 with a large proportion in the 0.35-0.42 range. These results demonstrate that
23 all the Li-components are tri-octahedral and that the name “Li-muscovite” is not appropriate.
24 This allows for the calculation of fluoride concentrations in aqueous solutions in equilibrium
25 with *lepidolites* using the HCh software by Yuri Shvarov (Lomonosov Moscow State
26 University). The thermodynamic properties for the F-Trl and F-Pol components are known
27 from experimental work, but those for the OH-Trl component remain unknown. For a
28 preliminary approach, these properties were estimated using those of OH- and F-phlogopite
29 as models. The results of the calculations are presented for 1kb and T = 400 and 500°C. The
30 solutions in equilibrium with *lepidolites* exhibit a high ionic strength (I), leading to log(mF-
31 total) values ranging from ~ -0.7 to ~ -0.4 at 400°C, and from ~ -1.2 to ~ -0.9 at 500°C.
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These high F concentrations are consistent with calculations based on the experimental calibration of the geo-fluorimeter.

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**Keywords** *Lepidolite*; Chemical composition; OH-trilithionite component; Tanco pegmatite; Calculation of aqueous solution composition

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**Acknowledgments:** This research was funded by Brandon University Grant No. 227530

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*(Lithium Ore Mineralogy of the Tanco Pegmatite)* to PA, and by an ISIFOR grant

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*(QuartzEXPLO)* from the Carnot Institute to SS. The authors gratefully acknowledge A. Bayg

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(Brandon University, Canada) for assistance with sample preparation and A. Marquet

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(Geoscience and Environment Toulouse, France) for conducting the laser ablation ICP-MS

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analyses. B. Moine extends his sincere thanks to Y. Shvarov (Lomonosov Moscow State

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University, Moscow, Russia) for his introduction to the use of the HCh software. He also

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wishes to warmly thank Valérie for showing him that solving a matrix can be easily done using functions in EXCEL. The authors would like to thank the editor, associate editor and two reviewers for their positive input which helped improve the final manuscript.

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**Statements and Declarations:** The authors declare no conflicts of interest. The authors have

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no relevant financial or non-financial interests to disclose. The authors have no competing

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interests to declare that are relevant to the content of this article. All authors certify that they

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have no affiliations with or involvement in any organization or entity with any financial

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interest or non-financial interest in the subject matter or materials discussed in this

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manuscript. The authors have no financial or proprietary interests in any material discussed in

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this article.

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**Introduction**

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*Lepidolite* is a major mineral series in certain evolved granites, LCT (Lithium-Cesium-

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Tantalum) pegmatites, and associated hydrothermal veins. Like spodumene, it is an important

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lithium ore (Bradley et al. 2017).
Early wet chemical analyses on separated minerals established that *lepidolite* can be considered a mixture (possibly a solid solution) between a dioctahedral end-member, muscovite (m or Mus) K(Al₂) [Si₃ AlO₁₀] (OH, F)₂, where F is nearly absent, and two trioctahedral end-members: trilithionite (t or Trl) K(Li₁.5 Al₁.5) [Si₃ AlO₁₀] (F, OH)₂ and polylithionite (p or Pol) K(Li₂ Al) [Si₄ O₁₀] (F, OH)₂, where F >> OH (Stevens 1938; Foster 1960). The abbreviations and symbols used are provided in Table 1.

At this stage, two points of terminology must be clarified: the use of the terms “lepidolite” and “solid solution.”

The commonly used name *lepidolite* does not, in fact, correspond to an IMA-approved mineral species. This mineral is now recognized as a member of the polylithionite–trilithionite solid-solution series. Nevertheless, we will retain the term *lepidolite* throughout this work for two reasons: (1) until relatively recently, *lepidolite* was accepted by the IMA as a valid mineral species (e.g., Rieder et al., 1998), and the term remains widely used among geologists; and (2) all relevant literature concerning the Tanco pegmatite—the source of the sample studied here—uses the term *lepidolite* (e.g., Rinaldi et al. 1972; Van Lichtervelde et al. 2008; Alexandre and Salvi, 2025).

Experimental studies on apparently homogeneous mixtures of dioctahedral and trioctahedral end members have shown that these may represent either true solid solutions (Robert et al. 1993) or mechanical aggregates of micro-domains. In particular, Šulcek et al. (2023) demonstrated that “there is no solid solution between trilithionite and muscovite; instead, Li-muscovite crystals consist of domains differing in the relative proportions of muscovite and trilithionite.” Because these domains are extremely small (< 1 µm), the bulk chemical composition of such crystals can nevertheless be treated as representative of a homogeneous mineral.

Based on synthetic products studied by X-ray diffraction, Munoz (1968, pp. 1502 and 1509) demonstrated that F can be substituted by OH in trilithionite up to xOH = 0.5 without affecting the Li concentration. However, this substitution is very limited in polylithionite. Therefore, when interpreting the chemical composition of the mineral, three Li-bearing components (Pol, F-Trl, and OH-Trl) must be considered alongside muscovite.

In situ EPMA analyses do not directly measure Li. This limitation has led to empirical correlations to estimate Li₂ O wt% based on SiO₂ or F wt% (Tischendorf et al., 1997). However, these estimates are highly imprecise.
In recent years, EPMA analyses have been supplemented by in situ Li measurements (among others) using LA-ICP-MS. As a result, accurate analyses are now available, allowing for the calculation of the proportions of the four components. However, analytical advances have not been accompanied by significant progress in data interpretation, which remains largely descriptive. A common approach involves plotting binary diagrams of Li₂ O wt% versus F wt% to highlight a positive correlation between these elements as an indicator of variations in the proportions of muscovite and Li-F-bearing components (e.g., Foster 1960; Monier and Robert 1986; Robert et al. 1993; Tischendorf et al. 1997; Van Lichtervelde et al. 2008; Breiter et al. 2022). This approach assumes that data trends reflect variations in the proportion of the Mus component, with negligible F and Li, versus the Trl-Pol system with constant Li and F values for the studied site. However, this overlooks the fact that Mus is not the only OH-bearing end-member, as OH-Trl must also be considered (Munoz 1968). Li-micas with low F concentrations relative to Li are often referred to as "Li-muscovite" (Tischendorf et al. 1997; Van Lichtervelde et al. 2008), but this terminology is misleading unless the predominance of the Mus molar fraction is confirmed and its relationship with the number of vacant octahedral sites in the structural formula is verified.

However, calculating the molar fractions of OH-Trl and F-Trl is not merely a semantic issue. Theoretically, it allows for the determination of xF = F/(F+OH) in the Trl component, which is essential for thermodynamic models of fluorine concentration in aqueous solutions (mHFAQ) at equilibrium with lepidolites. This is significant because, for instance, mHFAQ controls the hydrothermal solubility of minerals like columbite-tantalite, which are often associated with Li-micas (e.g., Zaraisky et al. 2010).

Based on chemical analyses of lepidolites from the Tanco pegmatite, this study aims to:

1) Develop a simple Excel file for calculating the molar fractions of the four lepidolite components by solving a 4x4 matrix of linear equations within the Si-Al-F-Li system and verifying the consistency of these calculations with the number of vacant octahedral sites given independently by the structural formulae.

2) Using published or estimated thermodynamic data on these components, propose a preliminary approach for calculating fluorine concentrations in aqueous solutions at equilibrium with lepidolites using the HCh software (Shvarov 2008).
124 Geological Setting

125 The Tanco pegmatite is located in SE Manitoba, Canada, approximately 150 km NE of the
126 city of Winnipeg. It is hosted by the southern portion of the Bird River Greenstone Belt, itself
127 part of the Archean Winnipeg River sub-province of the Canadian Superior Province (Gilbert
128 2006; Gilbert 2008). Given that its local geology, mineralogy, geochemistry, and structure
129 have been thoroughly described elsewhere (e.g., Crouse and Černý 1972; Černý et al. 1998;
130 Kremer 2010; Stilling et al. 2006; Breasley et al. 2022; Breasley et al. 2025; Alexandre and
131 Salvi 2025, and references within), only the briefest of outlines will be provided here.

132 The Bird River Greenstone Belt (ca. 2.73 Ga) is bordered by a series of batholiths of
133 intermediate composition dated at ca. 2.78 to 2.67 Ga (Beakhouse 1991). These are followed
134 by numerous peraluminous leucocratic intrusions, to which the Tanco pegmatite is related
135 (e.g. Beakhouse 1991).

136 The Tanco pegmatite is the largest body in a pegmatite swarm emplaced ca. 2.64 Ga ago
137 (Camacho et al. 2012) mostly into metagabbro and some amphibolites (meta-andesites;
138 Crouse and Černý 1972; Černý et al. 1981). It forms a sub-horizontal body of approximately
139 2 by 1 km of known lateral extent and of up to 100 m thickness (Černý et al. 1998). It is
140 characterized by a specific, unusually complex structure, consisting of nine zones that can be
141 defined by their mineral composition, texture, and location within the pegmatite (Crouse and
142 Černý 1972; Stilling et al. 2006). These are (in the likely emplacement order) the border
143 zone, the wall zone, the aplitic albite zone, the lower intermediate zone, the upper
144 intermediate zone, the central intermediate zone, the quartz zone, the pollucite zone, and the
145 lepidolite zone. Full description and mineralogy of these zones can be found in Crouse and
146 Černý (1972) and Stilling et al. (2006). Four of these zones (the wall zone, the lower
147 intermediate zone, the central intermediate zone, and the upper intermediate zone) account
148 for approximately 86% of the whole pegmatite (Alexandre and Salvi 2025, and references
149 within). Based on textural and paragenetic evidence, it was interpreted that the bulk of all
150 nine zones was produced by primary crystallization from a volatile-rich silicate melt, with an
151 important role played by metasomatic reactions among the different zones and between melt
152 and the host rocks (Černý et al. 1998). The Tanco pegmatite has been classified as a relatively
153 high-temperature – low pressure lithium pegmatite with petalite as the dominant Li silicate
154 (Černý and Ferguson 1972).
Taken in its entirety, 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%) and muscovite and Li-muscovite (6%). All other minerals combined account for approximately 5% (Černý et al. 1998; Stilling et al. 2006). *Lepidolite s.s.* is a relatively minor mineral in the Tanco pergmatite, accounting for approximately 0.3 vol% of the whole body (Černý et al. 1998).

The *lepidolite* zone, from which the samples studied in this work come from, is a minor zone of the Tanco pegmatite (approximately 2.1 vol%). It is up to 18 m thick and 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* (~70% combined), mostly intergrown with the microcline-perthite (~20%) and quartz (~10%) along the contacts with the central intermediate zone.

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# Analytical Methodology and Results

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## EMPA Analyses

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Major element compositions were measured in thin sections using the Cameca SXFive electron probe microanalyzer (EPMA) at the Raimond Castaing Micro-Characterization Centre (Toulouse, France). The elements analyzed included F, Rb, Al, Si, K, Ca, Cs, Ti, Mn, Fe, Na, Mg, Ba, Cr, and Zn. The standards used were albite, wollastonite, synthetic $Al_2 O_3$, sanidine, synthetic $MnTiO_3$, synthetic $Fe_2 O_3$, topaz, synthetic MgO, and synthetic $Cr_2 O_3$.

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Detection limits for major element oxides ranged from 0.02 wt% (for $Al_2 O_3$) to 0.15 wt% (for ZnO), with an overall average detection limit of 0.06 wt%. The detection limit for F was 0.60 wt%. Each analysis included a 30-second blank measurement on both sides of the peak and a 60-second peak measurement. The operating conditions were an accelerating voltage of 20 kV, a current of 10 nA, and a beam size of 2 µm.

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As explained by Moine et al. (2019), using a topaz standard instead of a fluorite standard with the PC1 crystal is essential for obtaining precise fluorine measurements. For *lepidolite* samples with fluorine concentrations between 3 and 8 wt%, the standard deviation of the measurements is approximately 2.2%.
187

188 LA-ICP-MS Analyses

189 The lithium and trace element compositions of *lepidolite* from the Tanco pegmatite were
190 measured using a Thermo Element XR Inductively Coupled Plasma-Mass Spectrometer
191 (ICP-MS) coupled with a Merchantec femtosecond laser at Geoscience and Environment
192 Toulouse (France). Analytical spots were placed on the exact same areas as the EPMA
193 measurements or as close as possible.

194 Each ICP-MS analysis, conducted in low-resolution mode, included a 30-second blank
195 collection, followed by a 60-second sample signal collection (corresponding to the laser
196 ablation duration) and a 40-second wash-out phase. Each analysis consisted of 42 cycles,
197 with 10 measurements per peak for each element; each measurement lasted 0.005 seconds.
198 The laser settings were: 20% power output; approximately 2.2 J/cm² fluence; repetition rate
199 of 10 Hz; spot size of 20 µm. All analyses were performed in line mode with a speed of 10
200 µm/sec, covering approximately 20 to 50 lines per mineral. Internal calibration was based on
201 the aluminum concentration obtained by EPMA. The elements analyzed included Li, Be, B,
202 Mg, P, Ca, Sc, Ti, Mn, Fe, Ni, As, Se, Rb, Sr, Y, Nb, Sn, Cs, Ba, Ta, Pb, Th, U, and Rare Earth
203 Elements (REE). External standards NIST610 and NIST612 were used, with internal
204 calibration based on the Al concentration determined by EPMA. For *lepidolite* samples with
205 Li₂O concentrations between 3.5 and 5 wt%, the standard deviation of the measurements is
206 approximately 4%.

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208 Analytical Results

209 The chemical compositions and structural formulae of the 22 analyzed *lepidolites* are
210 presented in Table 2. These compositions closely align with the Si-Al-Li-(K, Rb, Cs)-F-O-H
211 system, with average and maximum concentrations of FeO, MnO, and MgO of 0.02-0.06,
212 0.21-0.35, and 0.01-0.05 wt%, respectively. This characteristic is crucial for the
213 interpretation we propose.

214 The average concentrations of Rb₂O (3.37 wt%) and Cs₂O (0.83 wt%) are relatively high.
215 However, these elements do not influence our calculations because the sum of the alkalis in
216 the interfoliar site is close to 1 (0.96 to 1.02, average of 0.99).

217 The important point concerns the relative variations of the percentages of F and Li₂O. Those
218 of F vary from 2.47 to 8.48 (Δ = 6.01) while those of Li₂O vary only from 3.51 to 5.24 (Δ =
219 1.73). In the commonly used diagram Li₂O versus F, the correlation is poor (R² = 0.79) with a
220 slope of 0.28 and an intercept of 2.85. This high intercept value means that for F = 0
221 *lepidolite* would be a mixture of ~50% OH-trilithionite (at Li₂O = 5.67 wt%), which already
222 shows the importance of this component.

223

224 Calculation of the Mole Fractions of the Different Components

225 An overview of the main trends in the mole fractions of *lepidolites* from Tanco is presented in
226 the ternary Li/Al vs. F/Al diagram in Figure 1. Most of the samples deviate from the Mus-
227 Pol-Trl triangle for xF-Trl = 1; sample No. 3, with xF-Trl = 0.51 (see below), is indeed
228 located near the Mus-Trl line for xF-Trl = 0.5. The mole fractions of muscovite (m), F-
229 trilithionite (tF), OH-trilithionite (tH), and polylithionite (p) were calculated by solving a 4×4
230 matrix of linear equations in the Si-Al-F-Li system (see EXCEL file in Table A, in the
231 Appendix). The steps for solving this matrix are outlined below.

232 Taking sample No. 3 from Tanco as an example, the wt% concentrations of SiO₂, Al₂O₃, F,
233 and Li₂ O are 44.95, 30.50, 3.98, and 4.278, respectively. Dividing these values by the
234 respective molar masses of these oxides gives the atomic proportions of Si, Al, F, and Li as
235 0.748, 0.598, 0.209, and 0.286. Based on the structural formulae of the four components, the
236 following matrix can be written:

$$
237 \text{(a) } \mathrm{Si} = 3\mathrm{m} + 3\mathrm{tF} + 3\mathrm{tH} + 4\mathrm{p} = 0.748
$$

$$
238 \text{(b) Al} = 3\text{m} + 2.5\text{tF} + 2.5\text{tH} + \text{p} = 0.598
$$

$$
239 \text{(c) } F = 0\text{m} + 2\text{tF} + 0\text{tH} + 2\text{p} = 0.209
$$

$$
240 \text{(d) } 2\text{Li} = 0\text{m} + 3\text{tF} + 3\text{tH} + 4\text{p} = 0.573
$$

241 Solving for m:

$$
242 m = (a - d) / 3 = 0.058
$$

243 Substituting m into (b):

$$
244 b_1 = b - 3m = 0.423
$$

245 This results in a 3×3 matrix [b₁, c, d]:

246 Multiplying (b₁) by 3:

247 $b_2 = 3b_1 = 1.298$
248 Multiplying (d) by 2.5:

$$
249 \quad d_{1} = 2.5d = 1.432
$$

250 Solving for p:

$$
251 \quad p = (d_1 - b_2) / 7 = 0.023
$$

252 Solving for tF:

$$
253 \quad tF = (d_1 - 2p) / 2 = 0.081
$$

254 Solving for tH:

$$
255 \quad tH = (d_1 - 2.5tF - p) = 0.078
$$

256 The final mole fractions sum to 1:

$$
257 \quad m = 0.242, p = 0.097, tF = 0.337, tH = 0.324
$$

258 We verified that solving the matrix using Excel's DETERMAT, INVERSEMAT, and
259 PRODUCTMAT functions yields the same results as our line-combination method (see Table
260 B in the Appendix).

261 This corresponds to a *lepidolite* where xOH = 0.49 and xF = 0.51 in the Trl component. The
262 muscovite fraction represents only 24 mol%. According to Tischendorf (1997, Fig. 7 and
263 Table 3), for (Mg - Li) = -2.37 apfu, this would classify as a Li-muscovite near the boundary
264 (at 2.4) of the trilithionite domain. However, Tischendorf's diagram does not account for F,
265 preventing clear identification of the OH-Trl component. For all analyzed samples, Table 2
266 provides the calculated mole fractions and xOH-Trl values, with three analyses showing
267 slightly xOH-Trl < 0 (xF-Trl > 1).

268 Figure 3 is a plot of the Mus mole fraction (m) vs. the number of octahedral vacancies,
269 calculated from structural formulae with 22 valences. The correlation is strong (R² = 0.993)
270 with a slope of 0.983 and an intercept of 0.012, which is remarkable given that the matrix
271 calculations do not explicitly account for these formulae. Comparable correlations are
272 observed for calculated vs. measured Li₂O (ICP-MS) and for calculated vs. measured F
273 (EPMA), as expected since these measured values were inputs in the matrix calculations. No
274 correlation is observed for Li₂O in Trl vs. xF-Trl. However, this diagram is not meaningful
275 since Li₂O and F were part of the matrix input. Li₂O-Trl variations are directly related to
276 total Trl mole fractions, with Li₂O-Trl values normalized to x-Trl = 1 being nearly constant
277 (mean = 5.634 wt%, standard deviation = 0.008). Figure 4 shows the distribution of xOH-Trl
278 values for 19 samples, ranked in decreasing order. Ten samples have xOH-Trl between 0.35 and 0.42, and three between 0.5 and 0.7. Other Tanco *lepidolite* analyses show high xOH-Trl values (e.g., samples No. 253 and 258 from Van Lichtervelde et al. (2008) have 0.47 and 0.35, respectively).

282 To our knowledge, this study is the first to demonstrate the presence of OH-trilithionite in proportions comparable to F-Trl. This result confirms and refines the experimental predictions of Munoz (1968). The OH-Trl component remains trioctahedral, meaning that *lepidolite* containing it should not be classified as "Li-muscovite".

## 287 Thermodynamic Approach to the Composition of Aqueous Solutions in 288 Equilibrium with *Lepidolites*

289 The HCh software developed by Yuri Shvarov (Shvarov 2008, 2015; Shvarov and Bastrakov 1999) is particularly well-suited for calculating the composition of aqueous solutions in equilibrium with a mineral assemblage. The UNITHERM program serves as a thermodynamic database that can be easily modified and expanded.

293 The MAIN program defines the elements, mineral species, and aqueous species relevant to the problem using successive files: ST (System Menu), BL (Blank Menu), and IN (Input Menu). Based on these files, especially the final IN file, the GIBBS Menu calculates the composition of fluids and solid solutions at equilibrium by employing the free energy minimization technique.

299 **Thermodynamic Data Used**

300 To our knowledge, the only thermodynamic data available for *lepidolites* have been obtained from experimental studies conducted by Russian authors (Ogorodova et al. 2005, 2006; Paukov et al. 2007) on natural minerals. These were followed by applications of the Additivity Rule (e.g., Leitner et al. 2003) to calculate the thermodynamic properties of theoretical fluorinated endmembers. The results of these studies were compiled in the paper by Ogorodova et al. (2010) and are presented in Table 3, where we introduced some modifications to the $\Delta G^{298}$ values to obtain mineral mole fractions through the GIBBS menu that closely match those calculated from the chemical compositions of *lepidolites*. For
308 trilithionite, this modification is minimal ($\Delta G^{\circ}_{298}$ = -5905 kJ instead of -5909), whereas for
309 polylithionite, it is more significant ($\Delta G^{\circ}_{298}$ = -5767.8 kJ instead of -5717). The OH-bearing
310 poles of *lepidolites* are unknown under natural conditions, and no experimental studies exist
311 for OH-endmembers of trilithionite. Based on the suggestions of Munoz (1968, 1971), we can
312 assume as a first approximation that the F-OH exchange in this mineral is comparable to that
313 in phlogopite. This assumption allows us to calculate a $\Delta G_{298}$ value for OH-trilithionite using
314 the relation $\Delta G_{298}(\text{OH-Trl} - \text{F-Trl}) = \Delta G_{298}(\text{OH-Phl} - \text{F-Phl})$, based on the data from Zhu and
315 Sverjensky (1992).

316 Figures 5a and 5b show, for trilithionite at 1 kbar and at 400°C and 500°C, respectively, the
317 variations of log(mHF) as a function of xF of the component. When estimating fluoride
318 concentrations in aqueous solutions, it is essential to distinguish between HF°(aq), the neutral
319 species with an activity coefficient of 1, and the F⁻ ion, whose activity coefficient is lower
320 than 1, making m(F⁻) predominant under low-temperature and/or high ionic strength (I)
321 conditions. Therefore, mF(total) values are the most representative of the fluoride
322 concentration in solution and are directly comparable to concentrations obtained from
323 experimental studies (Aksyuk and Zhukovoskaya 1994). The Figures show that
324 log(mF(total)) values set by trilithionite are significantly higher than log(mHF°) values.

325 At Tanco, analyses of the montebrasite (LiAlPO₄ OH) – amblygonite (LiAlPO₄ OF) series
326 also provide mF values, which are worth comparing with those obtained for *lepidolites*
327 (Alexandre and Salvi 2025). The curves for this series are also plotted in Figures 5, using the
328 thermodynamic data of Vyatkin et al. (2024). These curves are very narrow due to the
329 extremely low I values and are close to those of Trl for log(mF(total)), but they show a
330 steeper slope.

331 The thermodynamic data for muscovite come from SUPCRT (Johnson et al. 1992), while
332 those for phlogopite are from Zhu and Sverjensky (1992). The data on aqueous species are
333 primarily based on the HKF model (Helgeson et al. 1981; Shock and Helgeson 1988; Shock
334 et al. 1997; Tagirov and Schott 2001; Tagirov et al. 2002; Akinfiev et al. 2020). The
335 Rhyzenko model (Rhyzenko 2008; Rhyzenko et al. 1991) is applied for Si (OH)₂ F₂
336 (Kuyunko et al. 1984) and KF(aq) (Akinfiev et al. 2020). According to this model, Akinfiev
337 (2020) provides values for LiF(aq), but they were not retained as they led to excessively high
338 molality values for this species.
The activities of the OH and F poles of Trl are calculated using the relation $a(i) = x(i)^2$, while those of Pol and Mus are $a(i) = x(i)$. The activity coefficients of charged aqueous species are computed in HCh using the extended Debye-Hückel equation (Helgeson 1969).

## 343 Results of the Thermodynamic Approach

The calculations using HCh were performed on four analyses from Table 2: numbers 2, 3, 16, and 21, with xOH-Trl values of 0.691, 0.491, 0.187, and 0.064, respectively. The molar fractions previously calculated for the 4 components of each sample were entered into an IN file, where they were associated with 1 kg of H₂O. To prevent the aqueous solution from affecting the absolute values of the mineral components, concentrations in the system, the molar fractions were recalculated for a sum equal to 15 instead of 1 (i.e., to perform calculations for a suitable mineral/solution ratio). Tables 4a and 4b provide the results from the GIBBS menu calculations for the components of the lepidolites and aqueous solutions at P = 1 kb and T = 400 and 500°C, respectively. Note that the molar fractions of the minerals (for a sum equal to 1) are slightly different from those calculated from the Si-Al-F-Li matrix (above) because GIBBS recalculates them for a Si-Al-K matrix.

The most important result is the calculation of values and variations of log(mHF). The ionic strengths (I) are very high (0.27 to 0.38) at 400°C, which explains the large difference between mHF° and mF-total ($\Delta\log = 1.70$ to 3.05); at 500°C, the ionic strengths are lower (0.06 to 0.96) and this difference is less pronounced ($\Delta\log = 0.8$ to 2.0). Note that the molalities of Li follow those of F-total; they are comparable to those obtained by Lagache and Sebastian (1991) in their experimental study of the spodumene-albite-quartz equilibrium in NaCl solutions.

Since xF-Pol is constant (= 1), polylithionite is in equilibrium with muscovite by a reaction of the following type:

$$ \mathrm{Mus} + 2\mathrm{Li}^+ + 4\mathrm{HF}^\circ + \mathrm{SiO}_2 = \mathrm{F-Pol} + 2\mathrm{Al}(\mathrm{OH})_2\mathrm{F} + 2\mathrm{H}^+ $$

From this, a value of log(mHF°) is calculated, which is also directly defined by the equilibrium:

$$ \mathrm{F-Trl} + 2 \mathrm{H}_2\mathrm{O} = \mathrm{OH-Trl} + 2\mathrm{HF}^\circ $$

It is verified that these equilibria give the same values of log(mHF°).
Figures 5a and 5b show the concentrations of F in aqueous solutions at equilibrium with the studied *lepidolites* (Tables 4a and 4b) at T = 400 and 500°C, respectively. Reference values for ionic strength (I) are also indicated. The values of log(mHF°) fall on the curve calculated for the Trl component. The values of log(mF-total) are very high, clearly above those for this pure component. At 400°C, log(mF-total) ranges from ~-0.69 to -0.43; at 500°C, log(mF-total) ranges from ~-1.24 to -0.92. At 500°C, the log(mF-total) values for Tanco-2 and Tanco-3 are identical to those calculated from experimental data by the geofluorimeter of Aksyuk and Zaraisky (2005). For Tanco-16 and Tanco-21, they are significantly lower (-0.97 vs -0.66 and -0.92 vs -0.34, respectively), but remain of the same order of magnitude.

The addition of 0.5m KCl to the aqueous solution does not affect the molar fractions of the minerals or the values of log(mHF°). The main effect is an increase in ionic strength (I), which leads to increases in mF-total: at 400°C, log(mF-total) ranges from ~-0.51 to -0.33; at 500°C, this value ranges from ~-0.80 to -0.60. Despite the addition of LiCl to Li⁺, the molality of Li-total remains at the level of F-total.

For the Tanco pegmatite, there is a significant increase in log(mHF°) between analyses Tanco-2 and 3 and Tanco-16 and 21, in relation to the increase in xF of the mineral. Although this gradient seems less clear for log(mF-total) values in Figures 5, it is an effect of the logarithmic scale. In terms of molalities, the ratio of mF-total between these analyses is 1.6 and 2.1 at 400 and 500°C, respectively.

The compositions of montebrasite fall between xF = 0.22 and 0.33 (Alexandre and Salvi 2025). Representative points are plotted in Figures 5. The values of log(mF-total) are close to those for trilithionite, but much lower than those for the *lepidolites* due to very low ionic strengths (≤ 0.002). However, it can be concluded that the compositions of the montebrasite-amblygonite (Mtb-Amb) series align with those of *lepidolites* with high OH-Trl, such as Tanco-2.

## Discussion and Conclusion

### OH-Trilithionite Component of *Lepidolites*

The first step of this study was to identify a major OH-Trl component in the Tanco *lepidolites*. Our calculations were initially based on a fully hydrated muscovite; however, at ~500°C, the xF is expected to be around 0.1 (Muñoz 1984). Accounting for this adjustment
400 leads to a slight increase in the calculated molar fraction of OH-Trl, thereby reinforcing our findings. For instance, in analysis 3 (Table 2), x(OH-Trl) increases from 0.49 to 0.53 as xF(Mus) increases from 0 to 0.1.

403 This result is confirmed by the treatment of comparable data on lepidolites from the Argemela granite (Portugal), published by Breiter et al. (2023). From a set of 21 analyses, the molar fraction values of OH-Trl were as follows: 8 between 0.8 and 0.7; 7 between 0.65 and 0.35; 6 between 0.3 and 0.1. As for the Tanco samples, a plot of Mus mole fraction vs. the number of octahedral vacancies shows a strong correlation (R² = 0.98) with a slope of 1.02 and an intercept of 0.035. It is worth noting that the lepidolites from Argemela contain significant amounts of FeO, MnO, MgO, and ZnO, with the sum averaging 2 wt%. This moderate deviation from the very pure compositions seen in Tanco lepidolites does not appear to have significantly affected the results of our calculations. The occurrence of x(OH-Trl) values greater than 0.7 is very interesting but it requires verification by a few new EPMA analyses because it could be due to an underestimation of F (see Moine et al., 2019).

414 Our calculations on Tanco pegmatite lepidolites focused on minerals in which the molar fraction of muscovite does not exceed 0.4. It would be of interest to extend these calculations to lithiniferous muscovites, where x(Mus) is expected to be largely predominant and the designation "Li-muscovite" is clearly warranted. As a preliminary exploration, we conducted calculations on a set of muscovites. In the studied site, 21 analyses of muscovites show fluorine concentrations ranging from 0.88 to 2.35 wt% (with an average of 1.55 wt%), while Li₂O remains nearly constant between 0.34 and 0.37 wt% (Alexandre and Salvi 2025).
420 These muscovites could serve as a good example, though they exhibit high total contents of Fe, Mn, and Mg (~4.4 wt% as oxides), which may be distributed between a zinnwaldite component (Monier and Robert 1986) and a F-bearing phengite.

424 Similarly, among the eight analyses of Li-muscovite reported by Van Lichtervelde et al. (2008) from Tanco, four show slightly negative values of x(OH-Trl), while the remaining four display highly variable x(OH-Trl) values between 0.3 and 0.9. These correspond to x(Mus) values between 0.84 and 0.96, but the very low F concentrations (0.1 to 0.8 wt%) suggest that analytical uncertainty may account for much of this variation.

429 In another dataset of 19 muscovite analyses from Argemela (Breiter et al. 2023), structural formulae indicate that seven samples possess more than 0.5 vacant octahedral sites and can thus be classified as Li-muscovites. Our calculations show that two of these have x(OH-Trl)
432

values ranging from 0.1 to 0.9, while the other five range from 0.71 to 0.74, with F

433

concentrations of 1.5 to 2 wt% and Li₂ O values between 2 and 2.9 wt%. As in the case of

434

Tanco *lepidolites* (Fig. 3), we observe a strong correlation between the mole fraction of

435

muscovite (m) and the number of vacant octahedral sites, confirming that the Li components

436

in muscovites are, as in *lepidolites*, trioctahedral.

437

Interestingly, preliminary results from matrix-based calculations applied to the chemical

438

compositions of zinnwaldites also reveal frequent x(OH-Trl) values between 0.3 and 0.6

439

(work in progress).

440

441

## Composition of Aqueous Solutions

442

The second step, “Thermodynamic Approach of the Composition of Aqueous Solutions in

443

Equilibrium with *Lepidolites*,” should be considered preliminary. The HCh software is well-

444

suited for this approach. The thermodynamic data for Li-components are subject to

445

uncertainties. The data for F-Trl are probably the most reliable, as they are derived relatively

446

directly from experimental data. Data for F-Pol are more uncertain, as we made significant

447

modifications to $\Delta G^{\circ}_{298}$ as provided by Ogorodova et al. (2010). In the absence of

448

experimental studies, the most hypothetical values are those used for the theoretical OH poles

449

of these components, assuming that the $\Delta G^{\circ}_{298}$ difference between the F and OH poles for Li-

450

micas is the same as for the F and OH poles of phlogopite (Table 3). To obtain better values

451

for these OH poles, experimental data on intermediate terms, like those from Tanco (whose

452

size already reaches 0.5 mm), would be necessary.

453

The approximations made for our calculations using HCh appear to be validated by

454

comparison with the calculations from the geofluorimeter established by A.M. Aksyuk and

455

G.P. Zaraisky (2005), which is probably based on the same experimental works we used.

456

However, it is unfortunate that we only have a summary of the original work by Aksyuk,

457

which is not accessible.

458

The quality of the calculation of mineral-water equilibria for the Mtb-Amb series is limited

459

because, at present, we only have estimates for $\Delta G^{\circ}_{298}$ and S°, without data on Cp values. The

460

curves drawn in Figures 5 are therefore presented as purely indicative. Despite these

461

restrictions, the thermodynamic approach to the composition of aqueous solutions in

462

equilibrium with *lepidolites*, enabled by the initial calculation of the molar fractions of

463

*lepidolite* components, is significant for several points:
464 1) The confirmation of very high fluorine concentrations in fluids at equilibrium with *lepidolites*;
465

466 2) The identification of gradients in these compositions, which are essential for studying the
467 mechanisms of deposition of hydrothermal minerals, whose solubility strongly depends on
468 the concentration of fluorine in solutions, such as columbo-tantalite (Zaraisky et al. 2010;
469 Akinfiev et al. 2020);

470 3) The confirmation of very high Li concentrations (Li⁺ and LiCl°) in solutions at equilibrium
471 with Li minerals, as established for spodumene (Stewart 1978; Lagache and Sebastian 1991).

472 In conclusion, it should be noted that this study proposes an original approach to the
473 interpretation of the chemical compositions of *lepidolites*, which led to the identification of
474 the role of the hydrated pole of trilithionite, a component that has seemingly been overlooked
475 for over fifty years. This approach should be improved, especially for calculating the molar
476 fractions of the different constituents, in order to process a larger number of analyses
477 simultaneously. The preliminary approach to the composition of aqueous solutions in
478 equilibrium with *lepidolites* can also be improved with the publication of new
479 thermodynamic data, particularly concerning the OH-Trl pole. Despite these reservations, the
480 method presented should encourage a more in-depth interpretation of the very large amount
481 of data obtained in recent years on the chemical composition of *lepidolites*.

482

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634

## Figure Captions

636 **Fig. 1** Plot of the Tanco lepidolites in a Li/Al vs F/Al ternary diagram. Note that most samples plot within the triangle Mus-Pol-Trl with xF-Trl = 0.5. Sample Nb3, with xF-Trl = 0.51, plots near the Mus-Trl tie-line for xF-Trl = 0.5

639
640

**Fig. 2** A tetrahedral diagram calculated using the EXCEL file from Shimura and Kemp

641

(2015). The primary trend follows OH-Trl and F-Trl mole fractions, while Mus and Pol

642

fractions show relatively minor variation

643

**Fig. 3** A plot of Mus mole fraction (m) vs. the number of octahedral vacancies, calculated

644

from structural formulae with 22 valences. The correlation is strong (R² = 0.993) with a slope

645

of 0.983 and an intercept of 0.012

646

**Fig. 4** The distribution of xOH-Trl values for 19 samples, ranked in decreasing order. Ten

647

samples have xOH-Trl between 0.35 and 0.42, and three between 0.5 and 0.7

648

**Fig. 5 (a)** Variations of Log(mHFaq) in aqueous solutions as a function of xF-trilithionite for

649

P = 1kb and T = 400°C. Values of ionic strength (I) are shown for Trl and *lepidolite* solid

650

solutions. (b) Variations of Log(mHFaq) in aqueous solutions as a function of xF-trilithionite

651

for P = 1kb and T = 500°C. The values of ionic strength (I) are shown for Trl and *lepidolite*

652

mixtures. Diamonds labelled “Ak” were calculated using the Aksyuk geofluorimeter

653

(http://geo.stbur.ru/info/granites/st/st000.html). The montebrasite-amblygonite solid solution

654

curves were also drawn.

655

656

## Table captions

657

**Table 1** Abbreviations, formulae and symbols

658

**Table 2** Representative analyses of *lepidolites* from the Tanco pegmatite

659

**Table 2** Representative analyses of *lepidolites* from the Tanco pegmatite (continued)

660

**Table 3** Thermodynamic properties of *lepidolite* and montebrasite components

661

**Table 4a** Composition of mineral components and fluid at 400°C

662

**Table 4b** Composition of mineral components and fluid at 500°C

663

664

## Appendix Table captions

665

**Table A** EXCEL file for calculation of the mole fractions of the four *lepidolite* endmembers

666

(analysis Tanco-3 data)

667

668

**Table B** Resolution by EXCEL functions of the matrix system AX = B for analysis Tanco-3