Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO2–Al2O3–B2O3–NaCl–H2O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al2O3–H3BO3 solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ([4]B-rich cores, [4]B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is:  
3Al6(BO3)5(OH)3+(122x)SiO2(aq)+2yNaCl(aq)2NayAl3Al6(Si6xBx)O18(BO3)3(O3x,OH1+x)+(4.5x)B2O3(aq)+(2.5x)H2O+2yHCl(aq).
The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with Δ11Bjer-fluid ≈ −2.8‰ at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids δ11B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems.
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