High Pressure Phase Transition and Behavior of Proton in Brucite Mg(OH)2

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Various kinds of OH-related chemical species are included even in nominally anhydrous minerals. E v e n i f t h e r e i s a s m a l l a m o u n t o f h y d r o u s c o m p o n e n t i n m i n e r a l s , O H a f f e c t s t h e p h y s i c o c h e m i c a l p r o p e r t i e s o f m i n e r a l s d u r i n g g e o l o g i c a l r e a c t i o n s . I n v e s t i g a t i o n o f O H i n minerals is an important theme for earth science. T h e m a j o r g e o l o g i c a l r e a c t i o n s o c c u r u n d e r co nd it io ns of hi gh te mp er at ur e an d hi gh pr es su re deep under the surface of the Earth, such as at the lower crust and mantle. From the interests of the b e h a v i o r o f O H o r p r o t o n s i n m i n e r a l s u n d e r conditions of high temperature and high pressure, i n f r a r e d ( I R ) m i c r o s p e c t r o s c o p y w i t h a h i g h temperature diamond anvil cell ( HTDAC ) is a useful me th od to in ve st ig at e a sm al l qu an ti ty of hy dr ou s c o m p o n e n t s a n d t h e b e h a v i o r o f p r o t o n s i n minerals. Th e cr us t an d ma nt le of th e Ea rt h co ns is t of rocks of which the chemical components are chiefly S i , M g a n d O . M a g n e s i u m h y d r o x i d e M g ( O H ) 2 (m in er al na me : br uc it e ) is a pr ot ot yp e of hy dr ou s magnesium silicates because of its simple chemical composition and crystal structure. The behavior of bruci te under high tempe ratur e and high press ure h a s b e e n w i d e l y s t u d i e d b y X - r a y a n d n e u t r o n diffraction and Raman and IR spectroscopy. In this ar ti cl e, we re po rt on th e IR ab so rp ti on sp ec tr a of b r u c i t e u n d e r c o n d i t i o n s o f h i g h t e m p e r a t u r e a n d high pressure, measured with an HTDAC and an IR microscope at beamline BL43IR [1]. A lever-type and externally heating HTDAC was settled on a pulse stage of 1 μ m steps under the IR m i c r o s c o p e o f B L 4 3 I R . T h e r u b y f l u o r e s c e n c e technique was used for measuring pressure at an e l e v a t e d t e m p e r a t u r e . A t h e r m o - c o u p l e l o c a t e d very closely to a diamond was used for measuring High Pressure Phase Transition and Behavior of Proton in Brucite Mg (OH) 2 t e m p e r a t u r e . F l u o r o c a r b o n f l u i d ( F l u o r i n e r t ) w a s used as a pressure transmitting medium. A rhenium ga sk et of 25 0 μ m th ic kn es s wa s pr ei nd en te d an d drilled with a YAG laser (1064 nm ). The IR spectra we re me as ur ed wi th an LN 2 co ol ed In Sb de te ct or a n d a B r u k e r F T I R I F S 1 2 0 H R u n d e r t h e con dit ion s of 4 cm -1 res olu tio n and 512 sca ns. A n a t u r a l c l e a r s i n g l e c r y s t a l o f b r u c i t e f r o m Z i m b a b w e w a s p r e p a r e d . T w o s e r i e s o f e x p e r i m e n t s u s i n g ( 0 0 1 ) p l a t e l e t s a m p l e w e r e carried out. The sample was pressurized at 25 C, a n d n o f u r t h e r p r e s s u r e w a s a p p l i e d d u r i n g t h e h e a t i n g o f t h e s a m p l e . B e c a u s e o f t h e r m a l e x p a n s i o n o f H T D A C i n c l u d i n g p r e s s u r e t r a n s m i t t i n g m e d i u m , t h e i n t e r n a l p r e s s u r e w a s increased by heating with a fixed load. The crys tal stru ctur e of bruc ite is a CdI 2 type s t r u c t u r e w i t h M g O 6 o c t a h e d r a l l a y e r s ( b r u c i t e layer) stacked along the c-axis and the OH dipole oriented normal to the brucite layers ( Fig.1 ). H H' (001) (110) (a) (b) Hydroxyls Mg Fi g. 1. Sc he ma ti c cr ys ta l st ru ct ur e of br uc it e. (a) shows (001) view of brucite layer where an u p p e r a n d l o w e r h y d r o x y l s a r e t h i c k a n d t h i n l i n e s . ( b ) s h o w s a ( 1 1 0 ) h o r i z o n t a l v i e w o f layers. The original OH dipole is vertical to a brucite, and represented as an O-H dipole. The O’ -H ’ di po le in di ca te s a pr es su re in du ce d OH dipole in brucite. H' O H O' 56 Absorbance 0 1 2 3 4 5 2500 3000 3500 4000 4500 Wavenumber ( cm -1 ) 0 . 1 M P a 2 5 C 2 . 6 G P a 2 5 C 3 . 6 G P a 2 5 C 6 . 7 G P a 6 0 C 7 . 3 G P a 1 0 0 C 7 . 7 G P a 1 6 0 C 8 . 2 G P a 2 2 0 C 7 . 9 G P a 3 0 C (a) Absorbance 0 1 2 3 4 5 0.1 MPa 25 C after HT HP experiments 2500 3000 3500 4000 4500 Wavenumber ( cm -1 ) (b) 1 6 . 5 G P a 3 6 0 C 1 6 . 4 G P a 3 0 0 C 1 4 . 4 G P a 2 0 0 C 1 7 . 6 G P a 2 3 C 8 . 5 G P a 2 0 C Fig. 2. IR absorption spectra of (001) platelet single cr ys ta l of na tu ra l br uc it e fr om Zi mb ab we up to 8. 2 G P a 2 2 0 C ( a ) , a n d 1 6 . 5 G P a 3 6 0 C ( b ) . A n abs or pti on pea k at 370 0 cm -1 is due to the or igi nal OH stretching motion and the pressure induced peak appears in the lower wavenumber side of the original. Although X-ray studies showed no phase transition i n v o l v i n g M g - O s u b s t r u c t u r e u n d e r h i g h p r e s s u r e u p t o 3 0 G P a , a p r e s s u r e i n d u c e d I R a b s o r p t i o n p e a k a t 3 6 5 0 c m -1 w a s o b s e r v e d a t 1 0 G P a i n a D A C s t u d y [ 2 ] . S h i n o d a & A i k a w a [ 3 ] a l s o observed the pressure induced absorption peak by DAC and polarized IR spectroscopy, and assigned it to a new OH dip ole und er pre ssu re, sug ges tin g t h a t a p h a s e t r a n s i t i o n o f b r u c i t e i n v o l v e d o n l y protons. Figure 1 shows a schematic drawing of a new proto n site of bruci te under pressu re. Figur e 2(a) shows the IR absorption spectra of a (001) platelet single crystal of brucite from 0.1 MPa, 25 C to 8.2 GP a, 22 0 C. An ab so rp ti on pe ak of th e or ig in al OH dipole is observed at 3700 cm -1 under 0.1 MPa, 2 5 C . A p r e s s u r e i n d u c e d a b s o r p t i o n p e a k appeared at 3650 cm -1 over 3 GPa at 25 C. The sample was pressurized up to 3.6 GPa and heated. A b s o r b a n c e o f t h e p r e s s u r e i n d u c e d p e a k i n c r e a s e d o n c o m p r e s s i o n a n d d e c r e a s e d o n h e a t i n g . T h e position of the pressure induced peak shifted to a l o w e r w a v e n u m b e r o n c o m p r e s s i o n , s u g g e s t i n g enhanced hydrogen bonding between an OH dipole a n d t h e n e i g h b o r i n g O . I n h e a t i n g , t h e p r e s s u r e induced peak continued to be observed at 7.7 GPa, 160 ∞ C and disappeared at 8.2 GPa, 200 C. After cooling, the internal pressure remained at 7.9 GPa an d th e in te ns e pr es su re in du ce d pe ak ap pe ar ed again. Figure 2(b) shows IR absorption spectra up to 16.5 GPa, 360 C. In the higher pressure region, t h e p r e s s u r e i n d u c e d p e a k w a s o b s e r v e d a t t h e h i g h e r t e m p e r a t u r e . T h e p r e s s u r e i n d u c e d p e a k can be observed under 320 C 16.3 GPa, it then disappeared at 340 C 16.0 GPa. The tiny peaks at 16.5 GPa 360 C are interference fringes between c u l e t f a c e s o f d i a m o n d s . A f t e r c o o l i n g , p r e s s u r e r e m a i n e d a t 1 7 . 6 G P a a n d t h e i n t e n s e p r e s s u r e in du ce d pe ak ap pe ar ed ag ai n (1 7. 6 GP a, 23 C) . A f t e r d e p r e s s i n g , t h e p r e s s u r e i n d u c e d p e a k d i s a p p e a r e d a n d t h e o r i g i n a l p e a k s t i l l r e m a i n e d . 57 0 5 10 15 20 0 50 100 150 200 250 300 350 400 Temperature (˚C) Pressure (GPa) Keiji Shinoda Osaka City University E-mail: shinoda @ sci.osaka-cu.ac.jp As shown by the in-situ observations, the onset of t h e p r e s s u r e i n d u c e d p e a k i s a r e v e r s i b l e a n d u n q u e n c h a b l e p h e n o m e n o n a n d i n d i c a t e s a h i g h p r e s s u r e p h a s e t r a n s i t i o n i n v o l v i n g a n e w p r o t o n site. The formation process of the new OH dipole can be proton transfer between brucite layers. At am bi en t pr es su re , th er e is no in te ra ct io n be tw ee n t h e O H i n a b r u c i t e l a y e r a n d t h e O ’ o f t h e n e x t l a y e r . B y c o m p r e s s i o n , t h e O · · · O ’ d i s t a n c e i s shortened, and the energy levels of two neighboring OH groups overlap. T h e a c t i v a t i o n e n e r g y o f p r o t o n t r a n s f e r t o References [1] S. Kimura et al. , Nucl. Instrum. Meth. A (2001) 893. [2 ] M. B. Kr ug er et al . , J. Ch em . Ph ys . 91 (1 98 9) 5910. [ 3 ] K . S h i n o d a a n d N . A i k a w a , P h y s . C h e m . Minerals 25 (1998) 197. [4] K. Shinoda, M. Yamakata, T. Nanba, H. Kimura, T. Moriwaki, Y. Kondo, T. Kawamoto, N. Niimi, N. Miyoshi, N. Aikawa, Phys. Chem. Minerals (2002) - in press. Fig. 3. The circles and squares represe nt two series of HT and HP experim ents with (001) platelet single crystals of brucite. The points where the pressure induced peak was observed are represented by solid marks and the open ones show points where there was no pressure induced peak. A thick line indicates a phase boundary of brucite between the two proton and one proton states. a d j a c e n t o x y g e n i s l o w e r e d b y s h o r t e n i n g t h e O · · · O ’ d i s t a n c e p r o t o n t r a n s f e r i s e n h a n c e d , a n d t h e r e s u l t a n t O ’ H ’ d i p o l e i s f o r m e d b y p r o t o n t r a n s f e r b e t w e e n d o n o r O H a n d t h e n e a r e s t neighboring acceptors O’ ( Fig. 1 ). In Fi g. 3, so li d ci rc le s an d sq ua re s sh ow th e p r e s e n c e o f t h e p r e s s u r e i n d u c e d p e a k , a n d t h e o p e n o n e s d o n o t s h o w t h e p r e s e n c e o f t h e p r e s s u r e i n d u c e d p e a k . T h e r e f o r e , a b o u n d a r y betw een the open and soli d mark s in Fig. 3 must b e a p h a s e b o u n d a r y o f b r u c i t e . T h e t w o p r o t o n state is stable in the high pressure region [4]. 58