Synthesis and stability of werdingite, a new phase in the system MgO - Al2O3 - B2O3 - SiO2 (MABS), and another new phase in the ABS-system
Werding, Günter; Schreyer, Werner
European Journal of Mineralogy Volume 4 Number 1 (1992), p. 193 - 208
published: Feb 13, 1992
manuscript accepted: Jul 17, 1991
manuscript received: Nov 15, 1990
ArtNo. ESP147050401014, Price: 29.00 €
Abstract The pure Mg end member of the newly discovered mineral werdingite with the ideal formula Mg2Al14Si4B4O37 was synthesized with 100 % yields at 4-7 kbar, 800-875°C employing dried B2O3 and a fired gel of the MgAl-silicate composition. Under these conditions it also grew from an anhydrous mixture of synthetic Mg-grandidierite, MgAl3SiBO9, and another new phase having the composition Al8Si2B2O19, thus suggesting that werdingite is a stable phase in the system MgO-Al2O3-B2O3-SiO2. Mg-werdingite is triclinic with a0 = 7.993(3) ; b0 = 8.150(2) ; c0 = 11.388(4) Å ; α = 110.45(2)°, β = 110.88(2)° ; γ = 84.62(2)°. Together with grandidierite, Al8Si2B2O19, mullite, and at least two Al-borates, werdingite belongs to a family of phases that are structural derivatives of sillimanite. Mg-werdingite is stable only at high temperatures (> 800°C) and low to intermediate pressure ( 10 kbar) are sinhalite + corundum + dumortierite. This type of stability is in agreement with the occurrence of Fe-bearing natural werdingite in an anhydrous granulite-facies environment. The other newly synthesized borosilicate phase Al8Si2B2O19 could only occur in natural high-temperature environments as well, as an alternative to the assemblage dumortierite + corundum with higher B2O3 contents.