Magnesiodumortierite, a new mineral from very-high-pressure rocks (Western Alps). Part I: Crystal structure
Ferraris, Giovanni; Ivaldi, Gabriella; Chopin, Christian
European Journal of Mineralogy Volume 7 Number 1 (1995), p. 167 - 174
published: Feb 8, 1995
manuscript accepted: Sep 1, 1994
manuscript received: Nov 23, 1993
ArtNo. ESP147050701013, Price: 29.00 €
Abstract Magnesiodumortierite occurs as rare inclusions in pyrope megablasts of the coesite-bearing whiteschists of the Dora-Maira massif, western Alps. It is orthorhombic (a = 11.91(3), b = 20.40(3), c = 4.730(2) Å), s.g. Pmcn) and isostructural with dumortierite, with Mg as the dominant cation in the single chain of face-sharing M(1) octahedra where Ti and vacant sites are also present. An anisotropic refinement of the structure converged to R - 0.031 (1107 reflections and 147 parameters, including some occupancy factors). On the basis of site occupancies and chemical analysis, 2.88 Si + 0.02 P are placed in the tetrahedral sites, leading to the composition [(Mg0.49Ti0.19Fe0.01□0.31 )(Al0.95□0.05)4(Al0.71Mg0.25□0.04)2] [Si(Si0.94P0.01□0.05)2O15.04(OH)2.96]B, with Z = 4 and Dcalc = 3.22 Mg/m3. With respect to dumortierite, some substitution of Al takes also place in the second face-sharing octahedron M(4), but not in M(3) and M(2), which only share corners and edges. One of the H atoms is located as being attached to O(10). The alleged "hexagonal dumortierite" reported from the same area is actually magnesiodumortierite (orthorhombic). The crystal chemistry of the dumortierite group is discussed and a comparison is made with ellenbergerite.