Quantitative Interpretation of the Chemical Composition of “Lepidolite” and Estimation of the Fluorine Content of Aqueous Solutions in Equilibrium

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