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

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), s = 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 Fe-Ti charge-transfer absorption band at 20,300 cm. Cell parameters are a = 11.91(3), b = 20.40(3), c = 4.730(2) A, space group Pmcn, Z = 4, Dcaic = 3.22 Mg/m. 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 (Mgi-x, • x)Al2Al4Si3Oi7-2x(OH)i+2xB (x « 1/3) toward (Mgi/3,Tii/3,U 1/3) (Alo.5Mgo.5)2Al4Si3Oi6(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). Key-words: magnesiodumortierite, new mineral, high-pressure phase, Dora-Maira massif, dumortierite group, boron. Introduction analyse (Schaller, 1905). Classical analyses usually have low contents of boron (Claringbull & The orthorhombic aluminium-borosilicate Hey, 1958), while boron and water are ignored dumortierite, ideally (Ali-x,CHx)Al2Al4Si3BOi8-3x in electron-microprobe analyses (Alexander (OH)3x (Moore & Araki, 1978), was described by et al, 1986; Schertl et al, 1991; Visser & Senior, Bertrand (1880) and is notoriously difficult to 1991; Taner & Martin, 1993). Recent complete 0935-1221/95/0007-0525 $ 2.75 © 1995 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart