Magnesiodumortierite, a new mineral from very-high-pressure rocks (western Alps). II. Crystal chemistry and petrological significance
Chopin, Christian; Ferraris, Giovanni; Ivaldi, Gabriella; Schertl, Hans-Peter; Schreyer, Werner; Compagnoni, Roberto; Davidson, Cameron; Davis, Andrew M.
European Journal of Mineralogy Volume 7 Number 3 (1995), p. 525 - 536
published: May 19, 1995
manuscript accepted: Dec 14, 1994
manuscript received: Nov 8, 1994
ArtNo. ESP147050703006, Price: 29.00 €
Abstract Magnesiodumortierite occurs as rare inclusions in pyrope megablasts of the coesite-bearing "whiteschists" of the Dora-Maira massif, western Alps, together with kyanite, talc, clinochlore, ellenbergerite and rutile. Textural evidence suggests that it formed as a primary phase (together with ellenbergerite, at about 25-30 kbar, 700°C) and as a somewhat later phase during early decompression. Magnesiodumortierite is orthorhombic, biaxial (-), α = 1.678(1), ß = 1.700(1), γ = 1.701(1), 2Vα = 38.5 ± 1.0°, X//c. The pleochroism, X pale pink to red, Y and Z colourless, is due to an Fe2+-Ti4+ charge-transfer absorption band at 20,300 cm-1. Cell parameters are a = 11.91(3), b = 20.40(3), c = 4.730(2) Å, space group Pmcn, Z = 4, Dcalc = 3.22 Mg/m3. Magnesiodumortierite is isostructural with dumortierite, with Mg as the dominant cation in the single chain of face-sharing Ml octahedra. Chemical variations among eight samples ranging in Ti content show that, with respect to dumortierite, complete replacement of Al in Ml by Mg and vacancies is charge-compensated by additional protons. Ti incorporation proceeds additionally by the mechanism Ti + Mg for 2 A1, leading to partial replacement of Al by Mg in M4. The resulting compositions range from a Ti-free end-member close to (Mg1-x, Ā x)Al2Al4Si3O17-2x(OH)1+2xB (x ≃ 1/3) toward (Mg1/3,Ti1/3, Ā 1/3) (Al0.5Mg0.5)2Al4Si3O16(OH)2B. Phase-relation analysis suggests that the substitution leading from dumortierite to Ti-free magnesiodumortierite (Mg + H+ for Al in the single chain) is favoured by increasing pressure, the Mg content in the dumortierite phase being buffered at a maximum value by its coexistence with chlorite + garnet (at fixed H2O and B2O3 activities).