On the crystal structure and compressional behaviour of talc : a mineral of interest in petrology and material science

The crystal structure of a natural triclinic talc (1Tc polytype) [with composition: (Mg2.93Fe0.06)(I 2.99)(Al0.02Si3.97)(I pound 3.99)O-10(OH)(2.10)] pound has been investigated by single-crystal X-ray diffraction at 223 and 170 K and by single-crystal neutron diffraction at 20 K. Both the anisotropic X-ray refinements (i.e. at 223 and 170 K) show that the two independent tetrahedra are only slightly distorted. For the two independent Mg-octahedra, the bond distances between cation-hydroxyl groups are significantly shorter than the others. The ditrigonal rotation angle of the six-membered ring of tetrahedra is modest (alpha similar to 4A degrees). The neutron structure refinement shows that the hydrogen-bonding scheme in talc consists of one donor site and three acceptors (i.e. trifurcated configuration), all the bonds having O center dot center dot center dot O a parts per thousand currency sign 3.38 , H center dot center dot center dot O similar to 2.8 , and O-H center dot center dot center dot O similar to 111-116A degrees. The three acceptors belong to the six-membered ring of tetrahedra juxtaposed to the octahedral sheet. The vibrational regime of the proton site appears being only slightly anisotropic. The elastic behavior of talc was investigated by means of in situ synchrotron single-crystal diffraction up to 16 GPa (at room temperature) using a diamond anvil cell. No evidence of phase transition has been observed within the pressure range investigated. P-V data fit, with an isothermal third-order Birch-Murnaghan equation of state, results as follows: V (0) = 454.7(10) (3), K (T0) = 56(3) GPa, and K' = 5.4(7). The "Eulerian finite strain" versus "normalized stress" plot yields: Fe(0) = 56(2) GPa and K' = 5.3(5). The compressional behavior of talc is strongly anisotropic, as reflected by the axial compressibilities (i.e. beta(a):beta(b):beta(c) = 1.03:1:3.15) as well as by the magnitude and orientation of the unit-strain ellipsoid (with epsilon (1):epsilon (2):epsilon (3) = 1:1.37:3.21). A comparison between the elastic parameters of talc obtained in this study with those previously reported is carried out.