Refinement of the Nacrite S T R U C T U R E

Nacrite crystals from a vug within a matrix ofdickite at Red Mountain near Silverton, Colorado, have a = 8.906(2), b = 5.146(1), c = 15.664(3)/~,,/3 = 113.58(3) ~ V = 657.9(3)/~3, and space group Cc. The structure was solved by direct methods to determine phase angles, followed by electron density maps to locate all atoms. Refinement by least-squares ceased at R = 4.5%. Each 7/~ layer has structural detail very similar to those of dickite and kaolinite, although nacrite stacking is based on a / 3 interlayer shifts along the 8.9 ]k axis (with octahedral cations alternating between the I and II sites in successive layers), whereas dickite and kaolinite are based on shifts of -a /3 along the 5.1 ~ axis (with octahedral cations in the same set of sites in each layer). The angle of tetrahedral rotation is 7.8 ~ and the octahedral counterrotations are 7.6 ~ and 8.1~ The H + protons were located on DED maps. The inner 0..H1 vector points exactly toward the vacant octahedron and is depressed -18 .6 ~ away from the level of the octahedral cations. All three surface OH groups have 0...H vectors at 50 ~ to 66 ~ to (001), although OH2 may not participate in interlayer hydrogen bonding. All three interlayer OH-H-O contacts are bent to angles between 132 ~ and 141 ~ and form contacts between 2.94 and 3.12/~,. The interlayer separation of 2.915 /k is slightly larger than in dickite, interpreted as due to a less favorable meshing of the oxygen and hydroxyl surfaces in nacrite--a direct consequence of layer shifts along the 8.9/~ axis. Key Words--Bond lengths, Crystal structure, Hydrogen bonds, Nacrite, Refinement, Tetrahedral rotation. I N T R O D U C T I O N Nacr i t e is the ra res t o f the kaol in po lymorphs . Bailey a n d Ty le r (1960) l is ted 16 occur rences k n o w n to t h e m at t ha t t ime, no t all o f wh ich had been con f i rmed by Xray study. Since then at least 20 add i t i ona l occurrences, wi th Xray con f i rma t ion , h a v e been repor ted in the l i tera ture . Nacr i t e usual ly is cons ide red as a h igh t e m p e r a t u r e kao l in p o l y m o r p h , a n d m o s t occur rences suppo r t a h y d r o t h e r m a l or p n e u m a t o l y t i c origin. Mar u m o (1989) col lected oxygen i so tope da ta t ha t indi ca ted increas ing t e m p e r a t u r e o f f o r m a t i o n in the o rder kaol in i te , dickite, nacr i te , a n d pyrophyl l i t e for clays in K u r o k o depos i t s in n o r t h e r n Japan . U s h a t i n s k i y et al. (1973) a n d B i i h m a n n (1988); however , desc r ibed nacr i te in sed imen t s , where an au th igen ic or lowtempe ra tu re ( < 80~ h y d r o t h e r m a l or igin was pos tu la ted . P e r m y a k o v (1936) syn thes ized nacr i te at 300~176 a n d 2 8 5 3 0 0 a tmosp he r e s . Th i s is the only successful syn thes i s o f nacr i t e k n o w n to us t ha t has been conf i rmed by Xray s tudy o f the product . H e n d r i c k s (1939) was the first to d e t e r m i n e the correct s tacking o f kaol in layers in nacri te . He used single crysta ls o f nacr i te f rom Pike ' s Peak, Colorado , to out l ine a s ixlayer cell w i th c -~ 43 ~ a n d a p p r o x i m a t e s y m m e t r y R 3c. T h i s m e a n t t ha t the a t o m i c coocdina tes in the u p p e r five layers cou ld be de r ived f rom those o f the first layer by ope ra t i on o f the r h o m b o h e d r a l symme t ry e lements . The ac tual s y m m e t r y was lowered to m o n o c l i n i c Cc due to par t ia l filling o f the 18 general pos i t i ons o f R 3 c by only 12 oc tahedra l A1 ions, a n d the cell was sl ightly d i s to r t ed to give a 3 angle o f 90~ '. Copyright 9 1994, The Clay Minerals Society T h e s t ruc ture f o r m e d in th is m a n n e r has the X a n d Y axes in t e rchanged re la t ive to the i r usual des igna t ions in layer silicates, and every layer is sh i f ted by a/3 in the same d i rec t ion ( + or ) a long the 8.9 A repeat . Every o the r layer is ro ta t ed by 180 ~ H e n d r i c k s po in t ed ou t t ha t the layers were b o n d e d across the in te r layer space by " h y d r o x y l b o n d s " (now t e r m e d long hydrogen bonds ) in which surface O H groups pa i red closely wi th basal oxygens o f the nex t layer, a n d he e m p h a s i z e d the i m p o r t a n c e o f such b o n d i n g in the p o l y m o r p h i s m of the kaol in minera ls . T h i s s t ruc ture is qui te different f rom the fourlayer s t ruc ture with/3 = 91~ ' p roposed by G r u n e r (1933) for nacr i te f rom Brand , Saxony, on the basis o f Xray p o w d e r data . G r u n e r recognized the 180 ~ ro t a t i on o f a l te rna te layers, bu t no t the interchange o f the X a n d Y axes. N e w n h a m (1961) recognized t ha t there were on ly two different k inds o f layers in the H e n d r i c k s ' s t ruc ture o f nacr i te a n d tha t it cou ld be desc r ibed as a twolayer s t ruc tu re wi th a larger/3 angle o f 100 ~ He de r ived 36 ways to s u p e r i m p o s e two kaol in layers to give long in te r layer hydrogen bonds ; 12 o f these assemblages h a v e the least ca t ion -ca t ion repu l s ion across the in ter layer space. But the twolayer s t ruc tures o f d icki te a n d nacr i te were j u d g e d the m o s t s table because they were the only ones in wh ich the g rooves a n d ridges o f the layer surfaces due to d ioc t ahed ra l d i s to r t ions would m e s h across the in te r layer space. Kao l in i t e has a less f avorab le in ter face be tween layers. R a d o s l o v i c h (1963) po in t ed ou t t ha t the dicki te s t ruc ture shou ld be m o r e s table t h a n t ha t o f nacr i te because it m i n i m i z e s the