ORIGINAL PAPER Chul-Min Chon Æ Chul-Kyoo Lee Æ Yungoo Song Sin Ae Kim Structural changes and oxidation of ferroan phlogopite with increasing temperature: in situ neutron powder diffraction and Fourier transform infrared spectroscopy Received: 11 December 2004 / Accepted: 15 October 2005 / Published online: 19 May 2006 Ó Springer-Verlag 2006 Abstract The thermal response of the natural ferroan phlogopite-1M, K 2 (Mg 4.46 Fe 0.83 Al 0. 34 Ti 0.22 )(Si 5.51 Al 2. 49 )O 20 [OH 3.59 F 0.41 ] from Quebec, Canada, was studied with an in situ neutron powder diffraction. The in situ temperature conditions were set up at 263, 25, 100°C and thereafter at a 100°C intervals up to 900°C. The crystal structure was refined by the Rietveld method (R p =2.35–2.78%, R wp =3.01–3.52%). The orientation of the O–H vector of the sample was determined by the refinement of the diffraction pattern. With increasing temperature, the angle of the OH bond to the (001) plane decreased from 87.3 to 72.5°. At room tempera- ture, a = 5.13 A ˚ , b = 9.20 A ˚ , c = 10.21 A ˚ , b= 100.06° and V(volume) = 491.69 A ˚ 3 . The expansion rate of the unit cell dimensions varied discontinuously with a break at 500°C. The shape of the M-octahedron underwent some significant changes such as flattening at 500°C. At temperatures above 500°C, the octahedral thickness and mean <M–O> distance was decreased, while the octahedral flattening angle increased. Those results were attributed to the Fe oxidation and dehydroxylation processes. The dehydroxylation mechanism of the fer- roan phlogopite was studied by the Fourier transform infrared spectroscopy (FTIR) after heated at tempera- tures ranging from 25 to 800°C with an electric furnace in a vacuum. In the OH stretching region, the intensity of the OH band associated with Fe 2+ (N B -band) begun to decrease outstandingly at 500°C. The changes of the IR spectra confirmed that dehydroxylation was closely related to the oxidation in the vacuum of the ferrous iron in the M-octahedron. The decrease in the angle of the OH bond to the (001) plane, with increasing tem- perature, might be related to the imbalance of charge in the M-octahedra due to Fe oxidation. Keywords Ferroan phlogopite Æ Neutron powder diffraction Æ FTIR Æ Oxidation Æ Dehydroxylation Introduction Recent studies for trioctahedral micas have been focused on Fe oxidation effect for the three-dimensional cation ordering and the orientation and dehydroxylation of OH group with increasing temperature (Pavese et al. 1997, 1999, 2000; Mookerjee et al. 2001; Chon et al., 2003). For this, in situ high-temperature neutron dif- fraction study with several supplementary equipments, such as Fourier transform infrared spectroscopy (FTIR), mo¨ssbauer spectroscopy, should be employed. Since the first study on the structural changes of mica at high temperature by Takeda and Morosin (1975), various high-temperature structural studies have been conducted. Guggenheim et al. (1987) studied the dehydroxylation of the muscovite at high temperature, and Russell and Gu- genheim (1999) along with Tutti et al. (2000) reported the crystal structures of phlogopite at high temperature. All these studies were performed by X-ray diffraction (XRD). Because X-rays are scattered by electrons, H and/or D atoms cannot always be observed even at high resolution with X-ray analyses. On the other hand, neutrons scat- tered by atomic nuclei can directly provide a large quan- tity of information about light atoms, and then the neutron diffraction has many advantages over XRD, such as its weak absorption, high penetration and angle-inde- pendent scattering length. Rayner (1974) and Joswig (1972) studied the crystal structure of the phlogopite at C.-M. Chon Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Korea C.-K. Lee Æ Y. Song (&) Department of Earth System Sciences, Yonsei University, 134, Shinchondong Seodaemungu, 120-749 Seoul, Korea E-mail: yungoo@yonsei.ac.kr Tel.: +82-2-21232671 Fax: +82-2-3926527 S. A. Kim Korea Atomic Energy Research Institute, Daejeon 305-353, Korea Phys Chem Minerals (2006) 33: 289–299 DOI 10.1007/s00269-005-0045-y