Magnesiodumortierite, a new mineral from very-high-pressure rocks (Western Alps); II, Crystal chemistry and petrological significance

Magnesiodumortierite occurs as rare inclusions in pyrope megablasts of the coesite-bearing whiteschists of the Dora-Maira massif, W Alps, Italy, together with kyanite, talc, clinochlore, ellenbergerite and rutile. Textural evidence suggests that it formed as a primary phase (together with ellenbergerite) at approximately 25-30 kbar, and also as a somewhat later phase during early decompression. It is isostructural with dumortierite, with Mg as the dominant cation in the single chain of face-sharing M1 octahedra. Chemical variations among eight samples with a range of Ti contents show that, with respect to dumortierite, complete replacement of Al in M1 by Mg and vacancies is charge-compensated by additional protons. Ti incorporation proceeds additionally by the mechanism Ti + Mg for 2 (super [6]) Al, leading to partial replacement of Al by Mg in M4. The resulting compositions range from a Ti-free end-member close to (Mg (sub 1-) (sub x<P6M><$B0><P255MIV>x) )Al (sub 2) Al (sub 4) Si (sub 3) O (sub 17-2) (sub x) (OH) (sub 1+2) (sub x) B (x approximately 1/3) towards (Mg (sub 1/3) Ti (sub 1/3<P6M><$B0><P255MV>1/3) )(Al (sub 0.5) Mg (sub 0.5) ) (sub 2) Al (sub 4) Si (sub 3) O (sub 16) (OH) (sub 2) B. Phase-relation suggests that the substitution leading from dumortierite to Ti-free magnesiodumortierite (Mg + H (super +) for Al in the single chain) is favoured by increasing P, the Mg content in the dumortierite phase being buffered at a maximum value by its coexistence with chlorite + garnet (at fixed H (sub 2) O and B (sub 2) O (sub 3) activities).