Plays Chem Minerals (1992) 18:483-490 PHYSICS CHEMISTRY NMIHERAIS Springer-Verlag1992 Magnesiochloritoid: Compressibility and High Pressure Structure Refinement Paola Comodi 1, Marcello Mellini 2, and Pier Francesco Zanazzi 1 1 Dipartimento di Scienze della Terra, Piazza Universitfi,1-06100 Perugia, Italia e Dipartimento di Scienze della Terra, Via delle Cerchia 3, 1-53100 Siena, Italia Received June 21, 1991 / Accepted November 4, 1991 Abstract. The response of magnesiochloritoid to pressure has been studied by single crystal X-ray diffraction in a diamond anvil cell, using crystals with composition Mgl.3Feo.TA14Si2Olo(OH)4. The unit cell parameters de- crease from a = 9.434 (3), b = 5.452 (2), c = 18.136 (5)/k, /~= 101.42 ~ (2) (1 bar pressure) to a = 9.370 (7), b = 5.419 (5), c = 17.88 (1) A, fi = 101.5 ~ (1) (42 kbar pres- sure), following a slightly anisotropic compression pat- tern (linear compressibilities parallel to unit cell edges: fla= 1.85, fib----1.74, tic= 3.05 x 10 -4 kbar -1) with a bulk modulus of 1480 kbar. Perpendicular to c, the most com- pressible direction, the crystal structure (space group C 2/c) consists of two kinds of alternating octahedral layers connected via isolated SiO 4 tetrahedra. With increasing pressure the slightly wavy layer [-Mgl.3Fe0.7A1Oi(OH)4 ] tends to flatten. Furthermore, the octahedra in this layer, with all cations underbonded, are more compressible than the octahedra in the (A1308) layer with slightly overbonded aluminum. Comparison between high-pres- sure and high-temperature data yields the following equations: ae, r = 9.434 ~ - 174-10 -5 ~kb -1 .P +9.10 -5 ~~ -1 .(T-25 ~ be, r = 5.452 A - 95.10- 5/~kb- 1. p +5.65.10 -5 A ~ -1 .(T-25 ~ ce, r = 18.136/~.-549.10 -5/kkb -1 .P + 16.2- 10 -5 ~ ~ .(T- 25 ~ with P in kbar and T in ~ These equations indicate that the unit cell and bond geometry of magnesiochlori- toid at formation conditions do not differ greatly from those at the outcrop conditions, e.g. the calculated unit- cell volume is 917.3 A 3 at P= 16 kbar and T=500 ~ whereas the observed volume at room conditions is 914.4 ~3. In addition, they show that the specific gravity increases from formation at depth to outcrop at surface conditions. Introduction Chloritoid is an important mineral indicator of meta- morphism of pelitic rocks (Halferdahl 1961). The idea- lized chemical composition of the chloritoid group is: M 2 +A14SizOlo(OH)4, or MO. A1203. SiO2. H20 (MASH) as a useful mnemon- ic code, with M2+= Fe, Mg or Mn, iron usually dom- inating over the other cations. A more sophisticated crystal chemical formula, with some information on the mineral structure, is: [-M2 +A102(OH)4] [A13Os] Si2, where the first bracket gives the composition of the trioc- tahedral brucite-like layer (L1) and the second bracket gives that of the close-packed octahedral layer (L2) with three quarters of its octahedral sites occupied by alumi- num. These two layers alternate along c and are inter- connected through isolated SiO 4 tetrahedra (T), the api- cal oxygen atoms belonging to the first layer L1 and the basal planes to the empty octahedral sites of the second layer L2 (Hanscom 1975, 1980; Ribbe 1982). Renewed interest in the chloritoid group was prompted by the ubiquitous finding of the new mineral magnesiochloritoid (Chopin et al. 1992) as a high-pres- sure phase in metapelites (Chopin 1983), and after the realization of its role, together with magnesiocarpholite, as an index mineral for the blueschist metamorphism of metapelitic terranes (Chopin and Schreyer 1983). Cho- pin and Moni6 (1984) have estimated a temperature close to 500 ~ and a pressure close to 16 kbar as conditions of formation for magnesiochloritoid in the Monte Rosa Massif (western Italian Alps). These specimens have been later studied by Ivaldi et al. (1988), who determined ther- mal expansion and structure at high temperature. Following the latter work, we now report compressi- bility measurements and a crystal structure refinement under high hydrostatic pressure conditions. Finally, an attempt is made at sketching the structure configuration occurring under P - T conditions.