MAGNESIODUMORTIERITE, A NEW MINERAL FROM VERY-HIGH-PRESSURE ROCKS (WESTERN ALPS) .1. CRYSTAL-STRUCTURE

Magnesiodumortierite occurs as rare inclusions in pyrope megablasts of the coesite-bearing whiteschists of the Dora-Maira massif, western Al ps. It is orthorhombic (a = 11.91(3), b = 20.40(3), c = 4.730(2) Angst rom), s.g. Pmcn) and isostructural with dumortierite, with Mg as the d ominant cation in the single chain of face-sharing M(1) octahedra wher e Ti and vacant sites are also present. An anisotropic refinement of t he structure converged to R = 0.031 (1107 reflections and 147 paramete rs, including some occupancy factors). On the basis of site occupancie s and chemical analysis, 2.88 Si + 0.02 P are placed in the tetrahedra l sites, leading to the composition square(0.05))(4)(Al(0.71)Mg(0.25)s quare(0.04))(2)] i(0.94)P(0.01)square(0.05))(2)O-15.04(OH)(2.96)]B, wi th Z = 4 and D-calc = 3.22 Mg/m(3). With respect to dumortierite, some substitution of Al takes also place in the second face-sharing octahe dron M(4), but not in M(3) and M(2), which only share comers and edges . One of the H atoms is located as being attached to O(10). The allege d ''hexagonal dumortierite'' reported from the same area is actually m agnesiodumortierite (orthorhombic). The crystal chemistry of the dumor tierite group is discussed and a comparison is made with ellenbergerit e.