Original paper
Magnesiochloritoid, and the Fe-Mg series in the chloritoid group
Chopin, Christian; Seidel, Eberhard; Theye, Thomas; Ferraris, Giovanni; Ivaldi, Gabriella; Catti, Michele
European Journal of Mineralogy Volume 4 Number 1 (1992), p. 67 - 76
39 references
published: Jan 31, 1992
manuscript accepted: May 29, 1991
manuscript received: May 25, 1990
DOI: 10.1127/ejm/4/1/0067
Abstract
Abstract Magnesiochloritoid is the Mg-dominant member of the chloritoid group. With Mg/(Mg+Fe+Mn) up to 0.82, it occurs in high-pressure metamorphic rocks of the eastern and western Alps in association with talc, kyanite, garnet, chlorite, omphacite, phengite, quartz, etc. It is monoclinic, space group C2/c. Magnesiochloritoid from the Monte Rosa massif, Italy, has the formula (Mg0.65Fe0.35)2 Al4O(SiO4)2(OH)4 (from structure refinement) and a = 9.452(3), b = 5.466(2), c = 18.158(6) ., β = 101.47(2)° (powder data), Z = 4 ; Dmeas = 3.25(2) g/cm3 ; biaxial positive, nα = 1.687(2), nβ = 1.690(2), nγ = 1.702(2), 2V = 40-50°, very faint pleochroism (light blue, grey blue, yellowish). The synthetic Mg-end-member grown at 700°C, 33 kbar water pressure shows both the monoclinic and triclinic polytypes ; the former has (powder data) a = 9.427(6), b = 5.442(3), c = 18.140(8) Å, β = 101.12(6)of the latter a = 9.429(5), b = 5.445(2), c = 9.126(4) ., α = 96.36(3), β = 101.12(4), γ = 90.07(4)°, space group Cl. The infrared spectrum is given. Chemical and X-ray data for 23 samples of the Fe-Mg series show that the substitution of Mg for Fe results in a decrease of about 0.8 percent for the a and b parameters, 0.3 for c. The highest substitutions known for Al are 0.3 atom by Fe3+ and 0.6 atom by Cr3+ (pfu.).
Keywords
Keywords magnesiochloritoid • new data • chloritoid group • Fe-Mg substitution