LITHIUM IN TOURMALINE–CAN IT BE CALCULATED ACCURATELY?

The light lithophile cation Li can be an important component of tourmaline  [XY₃Z₆(BO₃)₃T₆O₁₈V₃W].  However, the electron microprobe (EMP), which is widely used for tourmaline analysis, lacks the ability to measure Li or H.  Determination of Li can be done by direct measurement using ancillary techniques (wet chemistry, SIMS, ICP) or, with certain assumptions, by stoichiometric constraints using EMP data.  Given the proper normalization scheme, Li, residing in the Y site, is stoichiometrically calculable if all other cations in the octahedral Y and Z sites and tetrahedral T site (sum of sites=15) have been analyzed.  Assuming no octahedral or tetrahedral vacancies, Li can be calculated based on the algebraic expression: Li apfu=15 – (Y site cations + Z site cations + tetrahedral site cations (apfu)).  Because OH contents are commonly variable, a cation normalization scheme is desirable.  Assuming Si=6 as a means of normalization (i.e. no tetrahedral Al) is one possibility.  Testing the Li calculation method based on a 6 Si normalization scheme was carried out using existing data sets with complete analyses (Li obtained through wet chemistry, SIMS and ICP techniques; and normalization of the complete data sets to 15 cations) by comparing analyzed Li values to Li values calculated using the Li calculation method with Si=6 normalization scheme.   In most instances it is possible to accurately calculate Li to within ± 15% of the measured values for tourmalines with > 0.5 Li apfu.  For bulk analytical techniques (wet chemical or ICP), overestimation of the Li values could be due to Si-rich contaminants such as quartz inclusions.  Underestimation of the Li values could be due to the presence of tetrahedral Al or Li-rich contaminants such as spodumene or lepidolite.  The calculated Li values of low-Li tourmalines are commonly low, or negative, relative to the measured Li contents.  This is likely the result of the presence of tetrahedral Al in low-Li tourmalines.  Ultimately, the Si=6 normalization scheme allows a reasonable minimum estimation of Li in tourmalines with > 0.5 Li apfu.