Synthesis and stability of werdingite, a new phase in the system MgO-Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -SiO (sub 2) (MABS), and another new phase in the ABS-system

The pure Mg end-member of the newly discovered mineral werdingite with the ideal formula Mg (sub 2) Al (sub 14) Si (sub 4) B (sub 4) O (sub 37) was synthesized with 100% yields at 4-7 kbar, 800-875 degrees C and appears to be a stable phase in the system MgO-Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -SiO (sub 2) . The synthetic Mg-werdingite is triclinic, with a 7.993, b 8.150, c 11.388 Angstrom, alpha 110.45, beta 110.88, gamma 84.62 degrees C. It belongs to a family of phases including grandidierite, Al (sub 8) Si (sub 2) B (sub 2) O (sub 19) (a new phase), mullite and at least two Al-borates that are structural derivatives of sillimanite. Mg- werdingite is stable only at high T (> 800 degrees C) and low to intermediate P (< 10 kbar); at lower T it breaks down in the presence of water to Mg-grandidierite + corundum + the tourmaline phase box Mg (sub 2) Al (sub 7) Si (sub 6) B (sub 3) O (sub 27) (OH) (sub 4) while in near anhydrous conditions at > 10 kbar it forms sinhalite + corundum + dumortierite. This type of stability is in agreement with the occurrences of Fe-bearing natural werdingite in an anhydrous granulite-facies environment. The new synthetic phase Al (sub 8) Si (sub 2) B (sub 2) O (sub 19) could also occur only in natural high-T environments, as an alternative to the assemblage dumortierite + corundum with higher B (sub 2) O (sub 3) contents.