INCORPORATION OF LARGE CATIONS IN TOURMALINE: INFLUENCES OF P-T-X AND STRUCTURAL CONSTRAINTS (Invited Presentation)

Tourmaline (Tur) is very flexible in terms of cations with variable ionic radii (IR) and ionic charges that can be accommodated in its structure. The longest bond lengths are at the nine-coordinated X-site. In addition to the most common Na and Ca, the X site also accommodates large ions with IR above 1.30 Å. In natural mineral assemblages, these include especially K⁺ (IR=1.55 Å) and Pb²⁺ (IR=1.35 Å), and in trace amounts also Sr²⁺ (IR=1.31 Å) and REE³⁺ (mostly Ce and La, IR~1.2 Å). However, contents of Rb⁺ and Cs⁺ (IR=1.63 and 1.78 Å, respectively) are very low.

High contents of K (up to 0.63 apfu) are known from Tur with significant amounts of Fe³⁺ substituted for Al³⁺ (e.g. oxy-dravite–povondraite series) that formed in a metamorphosed evaporate cap rock breccia in Alto Chapare, Bolivia, at low P-T and high fluid salinity. However, a recent discovery of maruyamaite associated with diamond in UHP rocks of the Kokchetav Massive have high potassium contents in dravitic Tur that may be primarily a function of the P-T conditions of tourmaline origin rather than a restricted composition. The only reported case of elevated Sr (0.06 apfu) in Tur from Austria required a combination of high P and a specific protolith geochemistry.

Elevated contents of Pb are not unusual in pegmatitic Tur formed during late magmatic and early hydrothermal stages of crystallization in pegmatite pockets. However, very high contents of PbO (>10 wt.%) are known from few localities i.e. where Tur develops following breakdown of Pb-bearing minerals. In the amazonite-bearing pegmatite from Minh Tien, Vietnam, Pb-rich elbaite-liddicoatite formed during albitization of amazonite and crystallization from a sodic melt. Later hydrothermal alteration formed a Ca,Pb-tourmaline + quartz assemblage at the expense of early Tur, amazonite and mica. Local extreme Pb contents result from a low-P-T hydrothermal dissolution-reprecipitation of early Pb-rich liddicoatite and immiscibility between fluor-elbaite and Pb-rich tourmaline.

Strikingly different processes during which large cations enter the Tur structure show the efficacy of the “natural laboratory”. The extreme variability of multiple physio-chemical parameters in natural systems allow formation of minerals that would be otherwise very difficult to synthesize without a priori information from nature.