Extrinsic and intrinsic mode of hydrogen occurrence in natural olivines: FTIR and TEM investigation

Olivine crystals from two mantle nodules in kimberlites (pipe Udachnaya and pipe Obnazennaya, Yakutiya, Siberia) were investigated using EMP, TEM, AEM and FTIR techniques to determine the mode of hydrogen occurrence in olivin e. Olivine contains three types of nanometer-sized inclusions:''large" incl usions of hexagonal-like shape up to several hundred nm in size (1), lamell ar defects (2) and small inclusions of hexagon-like shape up to several 10 nm in size (3). Lamellar defects and small inclusions are considered to be a "hydrous" olivine. All three types of inclusions contain OH- or water, bu t they are different with respect to their phase composition. In "large'' i nclusions (1) hydrous magnesium silicates, such as serpentine + talc ("kero lite"?) and 10-Angstrom phase + talc were identified. Lamellar defects (2) and small inclusions (3) are depleted in ME and Fe compared to the olivine matrix, while the silica content is the same as that of olivine. Modulation s in the periodicity of the olivine structure are observed in SAED patterns and HREM images of (2) and (3). The superperiodicity can be referred to OH --bearing point defect ordering in the olivine structure. If this is the ca se, the material of both lamellar defects and small inclusions can be assum ed to be a "hydrous olivine" Mg(2-x)v(x)SiO(4)H(2x) with a cation-deficient olivine crystal structure. Thus. both an extrinsic mode of hydrogen occurr ence in olivine, such as nanometer-sized inclusions of OH--bearing magnesiu m silicates, and an intrinsic mode of hydrogen incorporation into the olivi ne structure, such as "hydrous olivine" in itself, were found. The data obt ained here show that the OH absorption bands observed in olivine spectra at 3704(3717) and 3683(3688) cm(-1) can be unambiguously identified with serp entine; the band at 3677(3676) cm(-1) can be associated with talc. The abso rption bands observed at 3591 and 3660 cm(-1) in olivine match those of the 10-Angstrom phase at 3594, 3662 and 3666 cm(-1).