SYNTHESIS, STABILITY AND BREAKDOWN PRODUCTS OF THE HYDROXYL END-MEMBER OF JEREMEJEVITE IN THE SYSTEM AL2O3-B2O3-H2O

Jeremejevite-OH, Al-6[BO3](5)(OH)(3), here abbreviated as Jer, the hyd roxyl end member of the natural fluorine-dominant mineral, was synthes ized hydrothermally at 0.3 GPa and 400 degrees-450 degrees C from vari ous oxide and hydroxide mixtures in the system Al2O3-B2O3-H2O. In the run products Jer usually coexisted with traces of Al4B2O9, probably as minute inclusions. Using synthetic Jer (60 mol%) + H2O (40 mol%) as s tarting material, the stability field of Jer for this bulk composition was determined up to 5.0 GPa. At 0.3 GPa and about 575 degrees C Jer decomposes to Al4B2O9 and H2O-B2O3 fluid, whereas at 3.5 GPa, 800 degr ees C and at 5 GPa, 750 degrees C Al3BO6 + AlBO3 + fluid form as a res ult of the instability of Al4B2O9 at high pressure. Jer containing pla nar BO3-groups in its structure is remarkably stable at high pressure. In run products containing two or three solid phases, the composition s of the coexisting binary fluids could be approximated. In bulk compo sitions with B2O3/H2O < 1, the fluids coexisting with Jer and one or t wo other solids become increasingly hydrous with rising pressure and d ecreasing temperature, so that at 5.0 GPa, similar to 740 degrees C th eir X-B2O3 = B2O3/(B2O3+H2O) lies near 0.07 and at 0.3 GPa, 450 degree s C near 0.01. Thus Jer as a highly B-rich solid may crystallize under such PT-conditions from very B-poor fluids. In bulk compositions with B2O3/H2O > 1, Jer as the only solid may coexist with fluids (melts) h aving X-B2O3 up to 1.0. Jer is not expected to occur as a natural mine ral, because with ubiquitous SiO2 present Al-borosilicates would form. However, at high fluorine activities F-dominant jeremejevite is more stable than these Al-borosilicates and coexists with free SiO2.