Synthesis, stability and breakdown products of the hydroxyl end member of jeremejevite in the system Al2O3-B2O3-H2O
Stachowiak, Anja; Schreyer, Werner
European Journal of Mineralogy Volume 10 Number 5 (1998), p. 875 - 888
published: Oct 5, 1998
manuscript accepted: Feb 5, 1998
manuscript received: Feb 14, 1996
ArtNo. ESP147051005004, Price: 29.00 €
Abstract Jeremejevite-OH, Al6[BO3]5(0H)3, here abbreviated as Jer, the hydroxyl end member of the natural fluorine-dominant mineral, was synthesized hydrothermally at 0.3 GPa and 400°-450°C from various oxide and hydroxide mixtures in the system AI2O3-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 starting material, the stability field of Jer for this bulk composition was determined up to 5.0 GPa. At 0.3 GPa and about 575°C Jer decomposes to AI4B2O9 and H2O-B2O3 fluid, whereas at 3.5 GPa, 800°C and at 5 GPa, 750°C AI3BO6 + AIBO3 + fluid form as a result of the instability of AI4B2O9 at high pressure. Jer containing planar BO3-groups in its structure is remarkably stable at high pressure. In run products containing two or three solid phases, the compositions of the coexisting binary fluids could be approximated. In bulk compositions with B2O3/H2O 1, Jer as the only solid may coexist with fluids (melts) having XB2O3 up to 1.0. Jer is not expected to occur as a natural mineral, 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.