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2670Acta Cryst. (1974). B30, 2670

The Crystal Structure of H ydromagnesiteB Y M . A K A O , F . M A R U M O A N D S . Iw A I

Research Laboratory o f Engineering Materials, Tok yo Institute of Technology, Oh-okayama 2-12,Meguro-ku, 152 Tokyo, Japan(Received 22 M ay 1974; accepted 27 June 1974)

Crystals of hydromagnesite, 4MgCO3. Mg(OH)2.4H20, are monoclinic, P21/c, with a= 10.11 (1), b=8.97 (1), c=8.39 (1),g,, ,8= 114.6 (9) , Z= 2, and have a strong pseudo-orthorhombic character. Thestructure of the mineral has been determined from three-dimensional X-ray data collected by photo-graphic methods on the assumption of an orthorhombic space group Bbcm (No. 64). The structure wassolved by Patterson and Fourier methods, and refined by the least-squares method to a conventional Rvalue of 0.15 for 282 observed reflexions. Both independent magnesium atoms have octahedral coordina-tions with average Mg-O distances of 2.10 and 2.04 ,~, respectively. The structure is based on a three-dimensional framework of MgO6octahedra and triangular carbonate ions. There are large cavities withdimensions of about 4.6 x 4-6 4.1 /~ in the structure.

In trod u ct ionThe following magnesium carbonates are knownin nature: magnesite (MgCO3), nesquehonite(MgCO3.3H20), lansfordite (MgCO3.5H20), artinite[MgCOa. Mg(OH)2.3H20], hydromagnesite, and dyp-ingite [4MgCO3. Mg(OH)2.5H20]. Among these min-erals hydromagnesite is the only species stable underatmospheric conditions. Two chemical formulae wereproposed tor hydromagnesite:4MgCO3. Mg(O H)2. 4H20 (Winchell & Winchell, 1951)and 3MgCO3.Mg(OH)2.3H20 (Palache, Berman &Frondel, 195 1). Recently the true stoichiometricformula was revealed to be the former on the basis ofa chemical analysis of specimens from Dovez, Minabdistrict, Iran by Bariand, Cesbron, Vachey & Sadr-zadeh (1973). The crystals of hydromagnesite are al-ways finely twinned with twinning planes parallel to(100). The first reliable X-ray study with the single-crystal method was carried out by Murdoch (1954),and showed that the crystal is monoclinic, havingpseudo-orthorhombic nature with cell dimensionsa=18.58, b=9.06, and c=8.42 A. This cell is B-centred and gives a= 10.20, b=9.06, c=8.42, and fl=114 of the monoc linic cell in reduced-cell representa-tion. The more accurate values were recently deter-mined by H. T. Evans Jr., D. Appleman and M. Morse(Hemingway & Robie, 1972) to be a= 10.106, b=8. 951 ,e=8 .388 A, and f l= 114.61 . Bariand et al. (1973) con-firmed these cell dimensions and obtained P21/c forthe space group of this mineral.Little has been known about the structure of hydro-magnesite, although it has considerable industrial im-portance. Its structure determination was undertakento obtain a better knowledge of the crystal chemistryof the magnesium carbonates.

E x p e r i m e n t a lThe specimens used were platy crystals from FortPoint, San Francisco, California. Because of the finelamellar twinning on (100), a single crystal suitable forstructure determination could not be isolated. X-rayexperiments were therefore carried out with a twinnedspecimen of dimensions 0.01 x 0.09 x 0.22 mm. Thecell dimensions and the space group were determinedfrom Weissenberg photograp hs taken with Cu K~radiation. They are in agreement with the previouslyreported results and are given in Table 1 together withother crystallographic data.

Table 1. Crys tal data fo r hydromagnesitePseudo-Monoclinic cel l orthorhombic cellSpace group P21/c Bbcma 10"11 (1) ,~ 18.371 (9)/~b 8-97 (1) 8.961 (9)c 8.38 (1) 8"384 (8)fl 114"6 (9) 90"2 (4) U 690"8/~3 1380 AsZ 2 4

Chemical formula 4MgC03. Mg(0Hh.4H20F.W. 467.6; Dm=2"24, D.,=2"25 g cm -ap(Cu Ks) = 39"7 cm- 1Although the true symmetry is monoclinic, theX-ray photographs showed a strong pseudo-ortho-rhombic character close to the symmetry of the spacegroup Bbcm. The relation of the pseudo-orthorhombicunit-cell vectors (a', b' and c') to those of the true cellis: a' = 2 a - c, b ' = b, and c' = c. The angle f l' is 90.2 .Consequently, most of reflexions overlapped thosefrom the twin counterpart on the Weissenberg photo-graphs. Only reflexions with large l' were separately

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M . A K A O , F . M A R U M O A N D S . I W A I 2671r e c o r d e d ( t h e p r i m e m e a n s t h e i n d e x i s r e f e r r e d t o t h ep s e u d o - o r t h o r h o m b i c c e l l ) . F r o m a c l o s e e x a m i n a t i o no f t h o s e r e f l e x i o n s w i t h l a r g e l ' s , t h e d e v i a t i o n f r o mt h e o r t h o r h o m b i c s y m m e t r y w a s f o u n d t o b e sm a l l.T h e d i f f e r e n c e i n i n t e n s i t i e s b e t w e e n h 'k ' l ' a n d h'k '] 'w a s a b o u t 8 % o n a v e r a g e. B e c a u s e o f t h e s t ro n go r t h o r h o m b i c c h a r a c t e r a s w e l l a s t h e o v e r l a p p i n g o fr e f l e x i o n s f r o m t w i n c o m p o n e n t s , t h e s t r u c t u r e d e t e r -m i n a t i o n w a s c a r r i e d o u t b y a s s u m i n g Bbcm c r y s t a ls y m m e t r y . T h e d e n s i t y o f 2 . 24 g c m - 3 o b t a i n e d b y t h ef l o t a t i o n m e t h o d a g r e e s w e l l w i t h t h e c a l c u l a t e d v a l u e ,i f Z i s a s s u m e d t o b e 4 i n t h e p s e u d o - o r t h o r h o m b i c c e ll .I n t e n s i t y d a t a w e r e c o l l e c t e d o n m u l t i p l e - f i l m e q u i -i n c l i n a ti o n W e i s s e n b e rg p h o t o g r a p h s t a k e n w i t h C u K ~r a d i a t i o n a r o u n d t h e b ' ( = b ) a n d c ' ( = c) a x e s u p tot h e f i f t h l a y e r . T h e y w e r e v i s u a l l y e s t i m a t e d w i t h a ni n t e n s i t y s c a l e , a n d c o r r e c t e d f o r L o r e n t z , p o l a r i z a t i o n ,a n d s p o t - s h a p e f a c t o r s . C o r r e c t i o n f o r a b s o r p t i o nf a c t o r s w a s a l s o m a d e d u r i n g t h e s t r u c t u r e r e f i n e m e n t .W h e n t h e t w o c o m p o n e n t r e f l e x i o n s f r o m t h e t w i ni n d i v i d u a l s d i d n o t o v e r l a p o n t h e f i l m s , i n t e n s i t i e s f o rb o t h r e f l e x i o n s p o t s w e r e m e a s u r e d s e p a r a t e l y a n ds u m m e d . 2 8 2 i n d e p e n d e n t i n t e n s i t i e s w e r e o b t a i n e da n d u s e d f o r th e d e t e r m i n a t i o n o f th e a v e r a g e d s t r u c -t u r e . T h e a c c u r a c y : )f t h e m e a s u r e m e n t w a s lo w o w i n gt o t h e t w i n n i n g .

S t r u c t u r e d e t e r m i n a t i o nT h e a v e r a g e d s t r u c t u r e w i t h o r t h o r h o m b i c s y m m e t r yw a s s o lv e d b y t h e h e a v y - a t o m m e t h o d . O n e o f t h e i n-d e p e n d e n t m a g n e s i u m a t o m s w a s l o c a t e d o n t h e t h r e e -d i m e n s i o n a l P a t t e r s o n f u n c t i o n . T h e p o s i t i o n s o f al lt h e r e m a i n i n g a t o m s o t h e r t h a n h y d r o g e n w e r e f o u n do n F o u r i e r m a p s s y n t h e s i z e d w i t h p h a s e s d e t e r m i n e db y t h e m a g n e s i u m a t o m . T h e s t r u c t u r e w a s r e f i n e d b ya l o c a l v e r s i o n o f t h e f u l l - m a t r i x l e a s t - s q u a r e s p r o g r a mL I N U S ( C o p p e n s & H a m i l t o n , 1 97 0) , e m p l o y i n g i s o -t r o p i c t h e r m a l a n d s e c o n d a r y e x t i n c t i o n p a r a m e t e r s .T h e f i n al R v a l u e w a s 0 . 1 5 f o r 2 8 2 o b s e r v e d r e f le x i o n s .T h e c o r r e c t i o n s f o r a b s o r p t i o n a n d s e c o n d a r y e x t i n c -t i o n f a c t o r s w e r e m a d e b y a s s u m i n g t h a t t h e c r y s t a ls h a p e w a s a r e c t a n g u l a r p a r a l l e l e p i p e d w i t h d i m e n s i o n s0 .0 1 x 0 . 0 9 x 0 - 22 m m . T h e c o m p u t e d e x t i n c t i o n f a c t o rwas 0 . 5 10 -4 a n d t h e l a r g e s t c o r r e c t i o n w a s a b o u t1 7 % o f F . T h e f o l l o w i n g w e i g h t i n g s c h e m e w a s e m -p l o y e d : w = 0 - 3 i f [ F o [ = 0 , w = l . 0 i f 0 < [ F o [_ < 1 0 ,w = 0 . 5 i f 1 0 < [ F o [ < 5 0 , a n d w = 0 . 1 i f 5 0 < ]F o [. T h ea t o m i c s c a t t e r i n g f a c t o r s u s e d w e r e t h o s e f o r n e u t r a la t o m s g i v e n i n International Tables fo r X-ray Crystal-lography ( 1 9 6 8 ) . I n v i e w o f l o w a c c u r a c y o f t h e i n t e n -s i t y d a t a , r e f i n e m e n t w a s t e r m i n a t e d a t t h is p o i n t . T h ef i n a l p o s i t i o n a l a n d t h e r m a l p a r a m e t e r s a r e l i s t e d i nT a b l e 2 . *

* A list of structure factors has been deposited with theBritish Library Lending Division as Supplementary Publica-tion No. SUP 30563 (3 pp.). Copies may be obtained throughThe Executive Secretary, International Union of Crystal-lography, 13 White Friars, Chester CH1 1NZ, England.

T a b l e 2 . Final positional and thermal param etersfor hydromagnesitex y z B [A2Mg ( 1) 0"1726 4) 0"06 68 8) 0"18 25 13) 0"8 (2 )Mg (2) 0"0 0"0 0"0 0"9 (3)C( I) 0"0436 (18) 0-25 0"25 1"3 (6)C(2) 0"2363 (1 3 ) 0"3 29 2 32) 0-0 0"2 (5)OH 0.3867 (1 1) 0.4 77 0 24) 0.0 0.5 (4)

H zO 0 . 3 7 9 9 1 1 ) 0 . 0 9 8 2 2 2 ) 0 . 1 6 9 4 2 8) 2 .9 (4 )O ( 1 ) 0 . 0 0 5 2 1 3 ) 0 . 1 5 1 5 2 4 ) 0 . 1 7 7 9 3 1) 3-8 (5 )0(2) 0. 11 09 13) 0.25 0.25 2.0 (5)0( 3) 0 . 2 14 8 12 ) 0 . 19 60 26) 0 .0 0 .9 ( 4)0( 4) 0 . 2 47 9 9) 0 . 1 03 9 16 ) 0 . 364 6 20) 0 .9 (3 )

T h e w a t e r m o l e c u l e w a s d i s t i n g u i s h e d f r o m t h e h y -d r o x y l g r o u p b y c o n s i d e r a t i o n o f t h e n u m b e r o f m o l -ecu les in the un i t ce l l .D i s c u s s i o n

T h e p r o j e c t i o n o f t h e s t r u c t u r e a l o n g t h e c ' a x i s i ss h o w n i n F i g . 1 . T h e s t r u c t u r e i s b a s e d o n a t h r e e -d i m e n s i o n a l f r a m e w o r k o f M g O 6 o c t a h e d r a a n d t ri -a ng [.e s o f c a r b o n a t e i o n s . M g ( 1 ) O 6 o c t a h e d r a a r e a r -r a n g e d i n s t r i n g s p a r a l l e l t o t h e c ' a x is b y s h a r i n g e d g e s .F o u l s u c h s t r i n g s r u n t h r o u g h t h e B - c e n t r e d u n i t c e l l .T h e s t r i n g s a r e f u r t h e r c o m b i n e d i n t o r u g g e d s h e e t sp a r a l l e l t o ( 10 0 ) s h a r i n g a p i c e s o f t h e o c t a h e d r a . T h eM g ( 2 ) O 6 o c t a h e d r a j o i n t h e s e l a y e r s in t h e a ' d i r e c t i o nb y s h a r i n g a p i c e s w i t h t h e M g ( l ) O 6 o c t a h e d r a . A l l t h eo x y g e n a t o m s i n t h e c r y s t a l p a r t i c i p a t e i n t h e c o n s t r u c -t i o n o f t h e f r a m e w o r k ; t w o t h i r d s o f th e m b e l o n g t oc a r b o n a t e i o n s . T h e f r a m e w o r k i s r e i n f o r c e d b y t h es t r o n g C - O b o n d s o f t h e c a r b o n a t e i o n s . T h e r e a rel a r g e c a v i t i e s a r o u n d ( 0 , 0 , ) a n d i t s e q u i v a l e n t s . T h ed i s t a n c e s f r o m ( 0 , 0 , ) t o t h e s u r r o u n d i n g a t o m s a r eg i v e n i n T a b l e 3 . I f t h e v a n d e r W a a l s r a d i u s o f a no x y g e n a t o m i s a s s u m e d t o b e 1 - 4 0 / ~ , t h e s i ze o f t h ecav i t y i s abo ut 4 - 6 4 . 6 x 4 .1 ~ .T h e b o n d d i s t a n c e s a n d a n g l e s a r e g i v e n i n T a b l e 3 .T h e M g ( 1 ) a t o m i s s u r r o u n d e d b y f o u r o x y g e n a t o m sf r o m c a r b o n a t e i o n s , o n e h y d r o x y l g r o u p a n d o n ew a t e r m o l e c u l e , w h i l e t h e M g ( 2 ) a t o m i s s u r r o u n d e db y fo u r o x y g e n a t o m s f r o m c a r b o n a t e i o n s a n d t w oh y d r o x y l g r o u p s . A s s e e n i n T a b l e 3 , t h e M g ( 1 ) O 6o c t a h e d r o n i s m o r e d i s t o r t e d t h a n t h e M g ( 2 ) O 6 o c t a -h e d r o n o w i n g t o t h e e d g e - s h a r i n g . I n v i e w o f t h er a t h e r l a r g e s t a n d a r d d e v i a t i o n s o f p o s i t i o n a l p a r a m -e t e r s , b o t h i n d e p e n d e n t c a r b o n a t e i o n s c a n b e c o n -s i d e r e d t o h a v e a n o r m a l s h a p e . T h e C ( 1 ) O 3 i o n i s a p -p r o x i m a t e l y p a r a l l e l t o ( 0 1 1 ) o r ( 0 1 T ) o f t h e p s e u d o -o r t h o r h o m b i c c e ll . T h e p l a n e o f t h e C ( 2 )O 3 i o n i s p a r a l l e lt o t h e c ' a x i s a n d m a k e s a n a n g l e o f 1 5 w i t h ( 1 0 0) .F r o m t h e in f r a r e d a b s o r p t i o n s p e c t r a o f h y d r o m a g -n e s i t e W h i t e ( 1 9 7 1 ) c o n c l u d e d t h a t d i s t i n c t k i n d s o fc a r b o n a t e i o n s s h o u l d b e c o n t a i n e d i n t h e m i n e r a l .A c t u a l l y , t h e C ( 1 ) O 3 i o n h a s a q u i t e d i f f e r e n t e n v i r o n -m e n t f r o m t h a t a r o u n d t h e C ( 2 ) O 3 i o n . A l l o x y g e na t o m s i n t h e C ( 2 ) O 3 i o n c o o r d i n a t e t o t w o M g a t o m s ,

A C 30B - 10

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2672 T H E C R Y S T A L S T R U C T U R E O F H Y D R O M A G N E S I T E

Fig. 1. Crystal structure o f hydromagnesite viewed along thec' axis. Heights of atoms from the a ' - b ' plane are indicatedas percentages of the c' length. Only atoms between z=0and are shown.T a b l e 3 . I n t e r a t o m i c d i s ta n c e s ( A ) a n d b o n d a n g l e s( ) w i th the i r e s t imated s tandard dev ia t ions inp a r e n t h e s e s

Mg(1)-OH n 2"04 (2)Mg (I)-H20 ii 2"16 (2)Mg(1)-O(2) 2"0 7 2)Mg(l)-O(3) 2"07 2)Mg(1)-O(4 ~) 2"0 9 (2)Mg (1)-O(4") 2"15 (2)Mg(2)-OH *n 2 .09 (3)Mg(2)-O(1 ~) 2. 02 (3)C(1)--O (1 ~) 1 .28 (3)C (1 ) - - 0 ( 2 ) 1 .24 4 )C (2 ) - -O (3 ) 1 .26 4 )C(2 )--O (4 ~) 1 .30 (2)H 2 O I I - - H 2 0 v i 2"84 (3)H2 OI I - H2 0 TM 3"04 (3)X H2 0 "t 2"77 (2)X -O(1 ~) 3"0 2 2)(x ; o ,o ,):Symmetry code

( i ) x y z (v ) x( i i) - x f i - z ( v i ) - x(iii) x - y + z ( vi i) - x(iv) x --y -z

O H *l l -Mg(1 ) -H 20 n 85 .6 8 )OH*n-Mg(1)-O(2) 103"0 8)OHV "-Mg(1)-O(3) 82"3 (8)OH *"-M g(I) -O( 4 I) 86"3 (8)HzO n-M g(l)-O (2) 98"1 (7)HzO t t -Mg( l ) -O (4 i ) 85"5 (7)O(3)--M g(1)-O(4") 101"8 (8)0(4 ~) -M g(l) -O (4 l l) 78 .3 (7)OHVU -Mg(2)-O(1 l) 91-1 (8)O( lt)-M g(2 )-O (1 v) 95.1 (9)0(1 ~)-C(1)--O(I~D l l3 ( l )0(1 ~) -C (1 )-- O (2 ) 123 (1)0(4~ D-C (2)~ 0(3) 119 (1)0(41 ~)-C (2)--0 (4 "i) 121 (2)H20 "I -H2O U-H 20 " it 116 (1)HzOV'-X -O(1 l) 119 "6 6)

yp + z + y z

w h e r e a s i n t h e C ( 1 ) O 3 i o n o n e o x y g e n a t o m c o o r d i n -a t e s t o t w o M g a t o m s b u t t h e r e m a i n i n g t w o o x y g e na t o m s c o o r d i n a t e t o o n e M g a t o m .

D i f f e r e n c e m a p s b a s e d o n t h e f i n a l a t o m i c p a r a m -e t e rs w e r e s y n t h e s i z e d i n o r d e r t o e x a m i n e t h e d e v i a t i o no f t h e r ea l s t r u c t u r e f r o m t h e a v e r a g e d o n e . T h e m a p sv e r i f i e d t h a t t h e s t r u c t u r e o b t a i n e d w a s a f a i r l y g o o da p p r o x i m a t i o n t o t h e r e a l s t r u c t u r e . P r o m i n e n t d e v i a -t i o n s a r e o b s e r v e d f o r M g ( 2 ) , O ( 1 ) , a n d C ( 1 ) a n d f o rt h e w a t e r m o l e c u l e . T h e s h i f ts a r e a p p r o x i m a t e l y r e -s t r i c t e d to the x z p l a n e .

A s a l r e a d y s t a t e d , t h e s t r u c t u r e i s b a s e d o n a s t a c ko f l a y e r s p a r a l l e l t o ( 1 0 0 ). T h e l a y e r s h a v e m i r r o rp l a n e s p e r p e n d i c u l a r t o t h e c ' a x i s i n t h e a v e r a g e ds t r u c t u r e . T h e m i r r o r p l a n e s a r e , h o w e v e r , n o t e x a c t b u tq u a s i s y m m e t r y p l a n e s i n t h e re a l s tr u c t u r e . A c c o r d -ing ly , t he re i s a sma l l d i f f e re nc e be tw e e n the a tomica r r a n g e m e n t i n a l a y e r a n d t h a t i n i t s m i r r o r i m a g e .W i t h i n a s i n g le c r y s t al o f t h e m i n e r a l t h e l a y e r s s h o u l db e s t a c k e d i n a d e f i n i t e w a y . H o w e v e r , t h e m i r r o ri m a g e o f a l a y e r c a n a l s o b e s t a c k e d o n t h e p r e c e d i n gl a y e r w i t h o u t i n t r o d u c i n g l a r g e s tr e s se s . T h u s t h e o b -s e r v e d p o l y s y n t h e t i c t w i n n i n g o n ( 1 0 0 ) c a n b e e a s i l yu n d e r s t o o d f r o m t h e p s e u d o - o r t h o r b o m b i c n a t u r e an dt h e l a y e r c h a r a c t e r o f t h e s t r u c t u r e .I t is o f i n te r e s t t o c o m p a r e t h e s t r u c t u r e o f h y d r o -m a g n e s i t e w i t h t h a t o f n e s q u e h o n i t e , s i n ce n e s q u e -h o n i t e s l u g g i s h l y c o n v e r t s t o h y d r o m a g n e s i t e w i t hl i b e r a t i o n o f w a t e r a n d C O 2 a t r o o m t e m p e r a t u r e . I nt h e n e s q u e h o n i t e s t r u c t u r e , t h e c o o r d i n a t i o n o c t a -h e d r o n a r o u n d M g i s s t r o n g ly d e f o r m e d o w i n g toe d g e - s h a r i n g w i t h a c a r b o n a t e i o n ( S t e p h a n & M a c -G i l l a v r y , 1 9 7 2 ) . T h e s h a r i n g a l s o c a u s e s a d e f o r m a t i o n

o f t h e c a r b o n a t e i o n . O n t h e o t h e r h a n d , t h e M g O 6o c t a h e d r a s h a r e o n l y a p i c e s i n h y d r o m a g n e s i t e , a n dc o n s e q u e n t l y t h e d e f o r m a t i o n s a r e m u c h s m a l l e r . T h i ss t r u c t u r a l d i f f e r e n c e is p r o b a b l y r e s p o n s i b l e f o r t h ed i f fe re nc e be tw e e n th e s t a b i l i t ie s o f t he tw o m ine ra l s .

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