Extended planar defects and the rapid incorporation of Ti4+ into olivine

• Published in: Contributions to mineralogy and petrology Vol. 166; no. 4; pp. 1223 - 1233
• Main Authors: Burgess, Katherine D., Cooper, Reid F.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2013
• Subjects: Earth and Environmental Science; Earth Sciences; Geology; Mineral Resources; Mineralogy; Original Paper; Olivine; Diffusion in olivine; Ti; Extended defects; TEM
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-013-0918-x

The formation of extended planar defects in minerals such as olivine is related to high point defect concentration and can be driven by large gradients in chemical potential, where the energy of the system is lowered by the ordering of defects along specific planes in the crystal. The presence of extended defects has the potential to create the (apparently) anomalous ionic diffusion in olivine as reported recently (Spandler and O’Neill in Contrib Mineral Petrol 159(6):791–818, 2010 ). High-resolution transmission electron microscopy and energy-filtered imaging were done using experimental samples designed to examine the impact of a TiO2 and f O2 on the potential to form such defects in ferromagnesian olivine. Doped basalt (5 wt% TiO 2 )–olivine reaction couple experiments were run at 1 atm and 1,310 and 1,410 °C for 50 h at various f O2 , ranging from 10 2 below to 10 2 above the quartz–fayalite–magnetite buffer. Our results show that extended planar defects in olivine, parallel to {101} ol and occurring in ordered “clusters” with a prolate spheroid geometry ~5–25 nm across and extending up to 150 nm into the olivine, are present near the olivine–glass interfaces in all of our experimental high-TiO 2 basalt–olivine samples. Increased Ti content in the olivine is associated with the defects; ordering of Ti 4+ and octahedral site vacancies leads to a two- or three-layer superstructure in the olivine. Defect nucleation and growth is driven by the large TiO 2 chemical potential gradient across the phase boundary at the start of the experiments, which provides access to microstructures not otherwise present.