Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Schreyer

Is the partitioning of boron between tourmaline and muscovite dependent on the crystallization environment?

Journal of the Czech Geological Society, volume 45 (2000), issue 1-2, 13 - 20

According to classical mineralogical wisdom, tourmaline contains the constant amount of 3.0 boron atoms per formula unit (p. f. u.) located in trigonal (BO₃)-units of the structure, and muscovite is a boron-free mineral, so that a distribution coefficient D_B (=concentration of B in tourmaline / concentration of B in muscovite) is infinity. Recent analytical and experimental results show, however, that both assumptions do not always hold. Reviews are presented on natural and synthetic Al-rich tourmalines (olenites) containing up to 2.27 B p. f. u. of excess boron located in the tetrahedral ring site replacing silicon, and on natural and synthetic micas in the system muscovite - boromuscovite (KAl₂[BSi₃O₁₀](OH)₂). Like for excess-boron olenite, boromuscovite synthesis is favored by high pressures and low temperatures. The muscovite with the highest B-content found thus far coexists with excess-boron olenite (resulting in a D_B-value of 5.62) in a unique pegmatite that may have formed at, or been subjected to, relatively high pressures and low temperatures. However, Al-rich, very Mg, Fe-poor chemical environments alone may also favor tetrahedral boron in both minerals, because - if Mg, Fe²⁺ were present - these two elements would preferentially fractionate into tourmalines of the dravite-schorl series which thus far have always been found to contain only stoichiometric trigonal boron. At the same time, boron seems to fractionate preferably into these Mg, Fe-tourmalines rather than into muscovite, at least under normal crustal pressure conditions. More experimentation is warranted to determine variations of D_B as a function of PTX-conditions, with the possibility that this property be used to evaluate the conditions of formation of tourmaline - muscovite pairs.