MICROSTRUCTURE OF EXPERIMENTALLY OXIDIZED OLIVINE FROM A MANTLE NODULE: I. MODES OF FE3+ AND OH- OCCURRENCE

Abstract

Olivine (Fa 8.2%) from a mantle nodule in kimberlite (Udachnaya pipe, Yakutia) was heated in air at 700°C for 9 h and then examined by optical spectroscopy (IR and UV spectroscopy), electron microprobe analysis (EMPA), and transmission electron microscopy (TEM) using the method of electron energy-loss spectroscopy (EELS). Ultraviolet spectra recorded along the [001] plane showed the inward decrease of Fe3+. Three zones can be distinguished: (1) an oxidized rim containing Fe3+ as Fe3+-bearing phases and point defects within the olivine matrix; (2) a zone containing on by point Fe3+-bearing defects; and (3) an unoxidized core. Hydroxyl OH-absorption bands in the IR spectrum are typical of mantle olivine and are normally assigned to point OH-defects. However, the TEM-EELS study of the oxidized rim (with an electron beam diameter of 4 nm) established that OH- does not occur as point defects, but interacts with Fe3+ to form iron hydroxides (feroxyhyte, FeOOH, and bernalite, Fe(OH)3), which compose polyphase inclusions several hundred nanometers in size. Our investigations showed that IR spectroscopy combined with a TEM study is required to determine the modes of OH- occurrence in olivines and to correct the interpretation of OH- absorption bands in the IR spectra. Our data also indicate that hydroxyl-bearing olivine is principally oxidized differently to anhydrous olivine. In this case, instead of the typical oxidation products (laihunite, magnetite, hematite), Fe3+ and OH- defects interact to form iron hydroxides.