American Mineralogist, Volume 63, pages 3l l-315, 1978
Crystalstructurerefinementof lawsonite
WnRNTRH. Baun
Department
of Geological Sciences, Uniuersity o.f Illinois
Chicago, Illinois 60680
Abstract
LawsoniteC
, a 6 r A l ," S i r r lO 7 ( O H ) r . H r O , i s f o u n d i n l o w - t e m p e r a t u r eh,i g h - p r e s s u rm
e etamorphic rocks. It is a hydrous counterpart of anorthite. A specimenfrom the type locality,
T i b u r o n P e n i n s u l aM
, a r i n C o u n t y , C a l i f o r n i a ,w a s u s e df o r a n X - r a y s t u d y :a : 8 . 7 9 5 ( 3 ) , b :
5 . 8 4 7 ( l ) ,c : 1 3 . 1 4 2 ( 6 ) AV
, : 6 7 5 . 3 A , Z - 4 , D c a t . : 3 0 8 8 g . c m - 3 , s p a c eg r o u p C c m m .
Refinement of 865 F"m gave an R : 0.0256.The structure is based on a three dimensional
framework generatedby cross-linkingribbons composedof edgesharing singlechains of Al
coordination octahedra and lateral bridging silicategroups The openingsof the framework
a c c o m m o d a t et h e C a a t o m s a n d t h e w a t e r m o l e c u l e sT. h e o b s e r v e dm e a n d i s t a n c e a
s r eS i ' :
O : 1 . 6 3 3A
, l'
O
1 . 9 1 3a n d C a '
O
2 . 4 2 1 A . T h eh y d r o g e na t o m sp a r r i c i p r r ei n
b e n t a n d b i f u r c a t e dh y d r o g e nb o n d s . I n d i v i d u a l c a t i o n - a n i o nd i s t a n c e sc o n f o r m w e l l t o t h e
exLendedelectrostaticvalence rule.
Introduction
The crystal structure of lawsonite, CaAlrSirO,
( O H ) r . H z O , w a s d e t e r m i n e db y W i c k m a n ( 1 9 4 7 )
in space group C222,. The Si-O bond lengthsin
t h e S i r O , g r o u p a s f o u n d b y W i c k m a n r a n g e df r o m
1 . 5 9 t o 1 . 7 2A . S u b s e q u e n t l yt,h e s t r u c t u r ew a s r e fined by Rumanova and Skipetrova(1959). They
found that it could be describedin the centric space
group Ccmm and that the Si-O distanceswere lying
i n a n a r r o w r a n g e :b e t w e e n1 . 6 5a n d 1 . 6 9 A .R e c e n t l y
P a b s t( 1 9 7 7 )p o i n t e do u t t h a t l a w s o n i t ei s d e n s e rt h a n
a n o r t h i t e ,i t s a n h y d r o u se q u i v a l e n t T
. he volume per
o x y g e na t o m i n l a w s o n i r ei s 1 8 . 7 4 A ,w h i l e i n a n o r t h i t e i t i s 2 0 . 9 4 4 . L a w s o n i t ei s o n e o f s e v e r a le s p e c i a l l y d e n s eh y d r o u ss i l i c a t e sf o u n d i n t h e F r a n c i s c a n
F o r m a t i o n , C a l i f o r n i a .I t i s t y p i c a l o f a l o w - t e m p e r ature, high-pressuremetamorphic paragenesis.
Experimental
A clear, almost colorless specimen of lawsonite
from the type locality, Tiburon Peninsula, Marin
County, California (USNM R3922) was divided into
two parts; one was ground into a spherewith a diameter of 0.04 cm, the other was used for a microprobe
analysis.Data were collectedon an automatic fourcircle diffractometer (trAgKa = 0.560834), using
proceduresdescribedby Baur and Khan (1970).The
0003-004x/78/0304-031
l$0200
3t I
unit cell constantswere refined from the settingof20
reflectionsmeasuredon a singlecrystal X-ray diffractometer. The resulting cell constants agree closely
w i t h t h o s e r e p o r t e db y D a v i s a n d P a b s t( 1 9 6 0 ) .R e ciprocal space was searched in a sphere of radius
sind/tr : 0.844-'. The total number of measured
r e f l e c t i o n sw a s I 1 , 2 6 8 ,w h i c h w e r e a v e r a g e dt o y i e l d
1 , 9 2 8u n i q u e r e f l e c t i o n sO
. f t h e s e 1 , 0 6 3h a d a n i n tensity of less than two sigma, and were not used in
the refinement. The systematic absences(hkl only
presentwith I -t k : 2n; and Oklonly with I : 2n) are
consistent with space group Ccmrn as noted by
Rumanova and Skipetrova. The successfulrefinement confirms this choice. Lorentz-polarizationcorrectionswere applied, but becauseof the small linear
absorption coefficientand the size of the crystal, an
absorption correction was not necessary.Scattering
factors were taken from the Internqtionql Tablesfor
X-ray Crystallography (1974). The refinement of the
865 observedunique structurefactors startedwith the
parameters reported by Rumanova and Skipetrova
( 1 9 5 9 )a n d r e f i n e di n a f e w c y c l e st o a n R ( : > l l F ' o l
- l r c l l / > l r o l ) o f 0 . 0 2 7( w i t h a n i s o t r o p ti ce m perature factors). A difference synthesis revealed
l i k e l y p o s i t i o n sf o r t h e h y d r o g e n a t o m s . U p o n f u r t h e r r e f i n e m e n ti n c l u d i n g t h e h y d r o g e n a t o m s a n
R o f 0 . 0 2 5 6w a s a c h i e v e d .T h e p o s i t i o n a l( T a b l e l )
and the thermal (Table 2) parameterswere used to
BAUR,' REFINEMENT OF LAWSONITE
3t2
T a b l e I L a w s o n i t e ,p o s i t i o n a lp a r a m e t e r s *
Ca
A&
si
4m
81
8m
o( r )
o(2)
o(3)
o(h4)
o(ws)
4m
16
1
8n
8m
4m
H(w)
11(oh)
H(w)geon.
H (oh) el .
8n
8n
8n
8n
0 . 3 3 3 0(ss )
,,
0.98040(s)
0
,4
0
\
0
0.13298(3)
\
0 . 0 4 9 s( 2 )
0
0 , 3 7 8 8 ( 1 ) o . 2 72 6( r )
0.1378(1)
0
0.6391(1)
0
0 . 6 0 9 1( 2 )
0
0.11690(6)
0 . 0 6 s 0 5( 8 )
0 . 0 4 7 9( 1 )
0 . 6 3 9( 4 )
0 . 5 8 7( 6 )
o . 6 73 L
o.5329
0.re4(3)
0 . 0 s 6( 4 )
0,1905
0.0704
0
0
0
0
0.33 TiOr, tracesof MnO and KrO. The sum including 2HrO is 98.54 percent.This corresponds
HrO and
to Caon.,(Al,.ourFeo.o36Tio
or3)Si2.orrOr(OH)r.
shows that the crystal is close to the ideal composit i o n C a A l r S i r O ? ( O H ) r . H r O .T h i s a n a l y s i sc o r r o b o rates observations made on lawsonites from other
localities(seeDavis and Pabst, 1960;Ernst eI al., l97O).
Description of structure
t4
* Estimated staDdard deviations
are listed
in parenThe
theses in units of the least significant
digit.
posj-tions and their
point synnumbers of equivalent
metries are given for every atom. For atons H(w)geom
and H(oh) .
see text.
EI
calculate a final set of structure factors (Table 3').
T h e r o o t - m e a n - s q u a r et h e r m a l d i s p l a c e m e n t sa n d
t h e i r o r i e n t a t i o n sa r e r e c o r d e di n T a b l e 4 .
Crystal data
C e l l c o n s t a n t s4 : 8 . 7 9 5 ( 3 ) ,b : 5 . 8 4 7 ( l ) , c :
l3.la2(6)A; V : 675.8A3;space group Ccmm, D"^t"
: 3 . 0 8 8 9 . c m ' ; Z : 4 : p ( A g K a ) : 8 . 0 5c m - l ; t r r R:
0 . 1 6 1 . A m i c r o p r o b e a n a l y s i sy i e l d e d ( w e i g h t p e r c e n t ) :3 1 . 0 9A l , O 3 , 3 7 . 7 5S i O , , 0 . 9 0F e , O , , 1 7 . 1 8C a O ,
'To obtain a copy of this table
order DocumentAM78-067
i r o m t h e B u s i n e s sO f f r c e , M i n e r a l o g i c a l S o c i e t y o f A m e r i c a , 1 9 0 9
K S t r e e t ,N . W . , W a s h i n g t o n ,D . C . 2 0 0 0 6 . P l e a s er e m i t $ l 0 0 i n
a d v a n c ef o r t h e m i c r o f i c h e .
The individual non-hydrogenatom positionsdetermined in this refinement differ on the average by
0 . 0 5 A l r o m t h e p o s i t i o n sf o u n d b y R u m a n o v a a n d
Skipetrova(1959).The singlelargestdiscrepancywas
observedfor O(h4) which has shifted in the courseof
t h e r e f i n e m e n tb y 0 . 1 3 4 . T h e a v e r a g ec h a n g ei n t h e
i n d i v i d u a lC a - O , A l - O , S i - O , O ( h 4 ) - O a n d O ( w 5 ) O d i s t a n c e si s 0 . 0 5 A .
The structure can be conveniently described as
c o m p o s e do f e d g e s h a r i n gc h a i n so f A l c o o r d i n a t i o n
o c t a h e d r a( w i t h A l a t y : 0 . 2 5 a n d 0 . 7 5 ) a r r a n g e d
p a r a l l e lt o t h e [ 0 1 0 ]d i r e c t i o n( F i g . l ) . T h e o c t a h e d r a
in these chains share the edgeO(3)-O(ha). The O(2)
vertices(which are not in the shared edges)of neighboring octahedra are laterally bridged by silicate
groups which alternate at heights_t : 0 and 0.5. The
symmetry of this ribbon of octahedra and tetrahedra
i s 2 , a n d i t s c o m p o s i t i o n i s A l r S i O , o ( O H ) r .S i m i l a r
ribbons have been found in vauxite (Baur and
Rama Rao, 1968) where their composition is
A l F e P O , ( O H ) , . G o t t a r d i ( 1 9 6 8 )h a sd e s c r i b e da n a l o gous ribbons in ilvaite, epidote, pumpellyite and
chevkinite. He makes the point that theseoctahedral
chains with lateral bridging tetrahedracan be consid-
T a b l e 2 L a w s o n i t e ,a n i s o t r o p i ct h e r m a l p a r a m e t e r s *f o r n o n - h y d r o g e na t o m s , i s o t r o p i ct h e r m a l p a r a m e t e r s[ o r h y d r o g e na t o m "
Atom
Ca
A,Q,
Si
o (1)
o(2)
o(3)
o(h4)
o(w5)
H(w)
H(oh)
Bt, or B
R
-22
R
0 . 0 0 3 0 0( s )
0 . 0 0 1 s 0( s )
0 . 0 0 1 4 (04 )
o.ooz3(2)
0 . 0 0 2 4( 1 )
0 . 0 0 5 3 0( 1 0 )
0 . 0 0 3 0 9( 1 r )
0 . 0 0 3 4 3( 1 0 )
0 . 0 0 8 1( 4 )
0 . o o 4 7( 2 )
0 . 0 0 1 3 3( 2 )
o . 0 0 1 0 6( 2 )
0 . 0 0 0 9 9( 2)
0 . 0 0 0 9( 1 )
0 . 0 0 1 4( 1 )
0 . 0 0 1 -(71 )
0 . 0 0 1 9( 1 )
0.0040(2)
2.6(O.9)
7.0(1.6)
0.oo42(2)
o . 0 0 4 8( 2 )
0.0226(7)
0 . 0 0 1 3( 1 )
0 . 0 0 1 8( 1 )
0 . 0 0 1 4( 1 )
* The anisorropi.c
rhermal paramerers are defined
by exp[-(Brrt.2
R
-12
0
- 0 . 0 0 0 1 2( s )
0
- 0 . 0 0 0 8( 1 )
0
0
0
R
R,
"23
IJ
0
- 0 . 0 0 0 0 7( 2 )
o . 0 0 0 0 2( 2 )
0
- 0 . 0 0 0 3( 1 )
0
0 . 0 0 0 0 3( 4 )
0
0
o . 0 0 0 4( 1 )
0 . 0 0 0 3( 1 )
0 . 0 0 0 4( 1 )
0
0
0
0
+ g2|k2 a B3zL + 2ltZhk
+ 2Btgfrl + }lzSkl)]
BAUR, REFINEMENT OF LAWSONITE
Table 4. Lawsonite,thermal ellipsoidsx
Angle
Atom
Axis
[1oo]
2
3
1
2
3
0 . 0 9 6( r )
0 . 1 0 8( 1 )
0 . r 0 9( 1 )
0 . o 72 ( r )
0 . 0 77 ( 1 )
0 . 0 9 7( 1 )
900
90
090
6 4( r o )
rs4(9)
e6(2)
2
3
1
2
3
0 . 0 7 4( 1 )
0.077(1)
0.093(r)
0.091(3)
0 , 0 9 5( 3 )
0.rr8 (3)
2(2)
900
8 8( 2 )
90
090
900
1
2
3
1
2
3
0.081(2)
0.095(2)
0 . 1 1 9( 1 )
0.078(2)
0.08s(2)
0 . 1 0 8( 2 )
5 3( 4 )
r 2 7( 4 )
r2t-(2)
1 6( 3 )
900
74(3)
1
2
3
1
2
3
0.083(2)
0 . 0 9 r( 2 )
0.L28(2)
0 . 1 1 1( 4 )
0.125(3)
0.198(3)
rs(2)
900
75(2)
90
090
900
caI
A.t
si1
o(1)
o(2)
o(3)
o(h4)
O(w5)
with
axis
Displacement
* Root-mean-square
therDal
axes and their orientations
[oro]
90
2 6( 1 0 )
6 s( 1 0 )
88(2)
90
90
90
3 7( 4 )
62(5)
6 7( 2 )
90
90
90
90
90
[oo1]
90
0
90
8 9( 2 )
9 7( 2 )
7( 2 )
92(2)
90
2(2)
0
90
90
8 9( 4 )
1 3 0( 3 )
4 0( 3 )
r 0 6( 3 )
90
1 6( 4 )
1 0 5( 2 )
90
ls (2)
0
90
90
displacements
along principal
relative
to a, b and c.
313
mined here, with those reported by Haga and Takeu c h i ( 1 9 7 6 )f o r i l v a i t e ( i n p a r e n t h e s e s 8
) :. 7 9 5 ( 8 . 8 1 8 ) ,
5 . 8 4 7 ( 5 . 8 5 3a) n d 1 3 . 1 4 21( 3 . 0 0 5) 4 . S t r u n z c o n c l u d e d
that lawsoniteand ilvaite are isostructuraldespitethe
fact that their spacegroups are different (Pcmn for
ilvaite versus Ccmm for lawsonite).The similarity in
the b cell constant is due to the length of the repeat
unit of the octahedral-tetrahedralribbons. It corresponds to the height of two octahedra sharing an
edge. Similar repeat units ranging in length from 5.6
to 5.9A can be observedin epidote,pumpellyite,perrierite and chevkinite.The similarity of the other two
cell constants,however, is fortuitous becausethe arrangement of the ribbons relative to each other is
clearly different in lawsonite and ilvaite as has been
shown by Gottardi (1968).
The relatively high density of lawsonite as compared to its anhydrous counterpart anorthite (Pabst,
1977) is related to the fact that Al is octahedrally
coordinatedin lawsonitebut only four coordinatedin
anorthite. Analogous changes in density accomp a n y i n g c h a n g e si n c o o r d i n a t i o n n u m b e r a r e w e l l
known from many pairs of low-pressurehigh-pressure phases(for instance quartz and stishovite).
Hydrogen atom positions
The distance O(ha)-H(oh) based on the refined
c o o r d i n a t e so f H ( o h ) i s o n l y 0 . 4 7 ( 5 ) 4 . T h e c o r r e -
ered as the building blocks of structuressuch as lawsonlte.
Within one of its repeat units each of the octahedral-tetrahedral ribbons is linked through four O(3)
atoms to neighboring ribbons by connectingthe silicate tetrahedra directly with neighboring octahedra
and through two O(l) atoms which connect two each
of the tetrahedrainto an Si2O7group. Therefore the
fusion of the ribbons results in a three dimensional
framework of composition AlrSirO?(OH)r. This
framework is sufficiently open to accomodate one
calcium atom and one water molecule per formula
unit thus resulting in the overall composition
CaAlrSi,O,(OH)r.H,O.
The Ca atom has six near neighbors at 2.400 to
2.494A in the form of a distorted octahedron (Table
5). Two additional O(3) atoms at a distanceof
2.976A are too far removed to be counted into the
coordination sphere. Consequently the formula of
l a w s o n i t e s h o u l d b e w r i t t e n a s C a L B r A l rSl .i,r ' oO '
(oH),.H,O.
Strunz (1937) has pointed out that the cell constantsof lawsoniteand ilvaite are very similar. This is
true when we compare the cell parameters deter-
Fig l. Lawsonite, view parallel [010]. Next neighbors are
indicated by broken lines for one each of the Ca, H(w) and
H(oh) atoms. The heights of some atoms are indicated in l00y
3t4
BAUR.' REFINEMENT OF LAWSONITE
T a b l e 5 L a w s o n i t e , i n t e r a t o m i c d i s t a n c e s( A ) a n d a n g l e s ( . ) *
3. Ca
si-o(2)
si-0(3)
si-o(r)
nean
r.6L6(2)
r.647(1)
r,554(1)
I.633
o(2)-o(2)
0(2)-o(3)
0(2)-0(r)
o(3)-0(l)
Eeans
2.659(1) 110.7(1)
2.724(1,) rL3.2(7)
2 . 6 6 1( 1 ) 1 0 9 . 0 ( r )
2.552(L) r0r,3(t)
2.664
109.4
4
I
1
2
o_si_o
si-0 (1) -si
2. At
Ar.-o(h4)
A!.-O(2)
A!.-o(3)
Eean
I.867(1)
1.913(1)
1.960(1)
1,913
0-A.4,-0
0(h4)-0(2)
o(h4)-o(2)
o(h4)-o(3)
0(h4)-0(3)
0(2)-o(3)
0(2)-0(3)
neans
2.652(1) 89.1(1)
2 . 6 9 4 ( 1 ) 9 0 .9 ( r )
2 . 4 6 0 ( 2 ) 8 0 . 0( r )
2 . 9 3 2 ( r ) 1 0 0 .0 ( 1 )
2 . 7 4 0 o - ) 9 0 .0 ( 1 )
2 . 7 3 8 ( r ) 9 0 . 0( 1 )
2,703
90.0
ca-o(2)
Ca-o(w5)
ca-o(1)
ca-o(3)
nean
of
6
nean
of
8
2 . 4 0 0( 1 )
2.433(2)
2.494(2)
2.976(7)
2,560
H(oh)-0(h4)
H(oh)-o(h4)
0(h4)-0(h4)
o (h4)-H (oh)-o (h4)
[0.e80]
2.169
2.752(2)
116.7
H(oh)-o(2)
o (h4) -o (2)
o (h4) -I1 (oh) -o (2)
H(oh)-H(oh)
H(oh)-H(w)
2 .1 8 0
2 . 9 3 3( 1 )
r32.6
1.939
2 .0 0 3
H(w)-o(ws)
Ir(w)-0 (ws)-H(w)
s(w)-H(w)
tt(w) -0 (ha)
o(w5)-o(h4)
0(v5)-H(w)-0(h4)
H(u)-0(2)
0(ws)-o(2)
0(w5)-H(w)-0(2)
[ 0 .e 6 0 ]
[loe .o]
1r.564)
1.898
2.669(I)
r35.4
2.44s
3.229(2)
138.8
* The values in square brackeEs have been preset by assuning a
knom geometry for the OH and H.0 groups.
The nunbers befote tne
atom designations
are nultiplicities
per polyhedron.
A11 distances
and angles involving
hydrogen atoms are based on H(")*"o..
u.d
H(oh)e1.
(Table 1).
p o s i t i o n ,c a l l e d H ( o h ) " ' i n T a b l e l , p l a c e sH ( o h ) a l most equidistantly between H(w) and a centrosymmetrically related H(oh). It also placesH(oh) within
2.18A of two O(2) atoms thus creating a bifurcated
h y d r o g e n b o n d ( s e e B a u r , 1 9 6 5 ) .A c t u a l l y H ( o h ) i s
even closer (2.169A) to an O(h4) atom, but sincethe
a n g l eO ( h 4 ) - H ( o h ) - O ( h a ) i s I 1 6 . 7 ' i t i s a s s u m e dt h a t
this contact does not representa hydrogen bond.
The hydrogen atom positions as located by X-ray
diffraction differ by 0.3A [distance from H(w) to
H(w)*"o-l and by 0.5A [from H(oh) to H(oh)",]from
the calculated positions. The true hydrogen atom
positions as they could be determined by neutron
diffraction are expectedto be closerto the calculated
values than to the experimental X-ray values. The
hydrogen bonds formed in lawsonite are long, bent,
and bifurcated. therefore the thermal motion of the
hydrogen bonds must be large, thus making their
localization by X-ray diffraction difficult.
The hydrogen positions derived here by geometric
and electrostaticarguments are consistent with the
infrared measurementsand their interpretation rep o r t e d b y L a b o t k a a n d R o s s m a n( 1 9 7 4 )o n l a w s o n i t e . T h e s e a u t h o r s c o n c l u d e df r o m t h e p l e o c h r o i s m
of absorption spectra that all oxygen-hydrogen
b o n d s i n l a w s o n i t ea r e l o c a t e d i n p l a n e sp a r a l l e l t o
( 0r 0) .
s p o n d i n g O ( w 5 ) - H ( w ) d i s t a n c em e a s u r e s0 . 7 8 ( 4 ) 4 ,
Bond distancesand balance of valences
and the angle H(w)-O(w5)-H(w) is l4l(4)'. While
there is no doubt that the hydrogen atoms are in the
T h e m e a n d i s t a n c eS i - O o f 1 . 6 3 3 4 ( T a b l e 5 ) i s
generalvicinity of thesepositions,it is also clear that
close to the value of 1.630A calculated from the rethese refined positions are not as accurate as their g r e s s i o n e q u a t i o n ( S i - O ) m e a n
1.621 +
p r e c i s i o nm i g h t i m p l y .
0.0037CNM - 0.0046NC, where CNM is the mean
The point symmetry of the site of the oxygen atom c o o r d i n a t i o nn u m b e r o f a l l o x y g e na t o m s i n t h e s i l i of the water molecule is mm. Assuming that the hy- cate tetrahedron and 1/C is the number of bridging
drogen atom positions of the water molecule are not
oxygen atoms per tetrahedron(Baur, l976). The indidisordered there are only four different orientations vidual Si-O bond lengthsconform reasonablywell to
possiblefor the molecule.In two of thesethe hydrothe extendedelectrostaticvalencerule (Baur, 1970)as
gen atoms would be close to the Ca atom to which can be seenfrom the calculation shown in Table 6. It
O ( w 5 ) i s b o n d e d . T h i s i s a n u n l i k e l y p o s s i b i l i t y is apparent that the variations in Si-O bond lengths
(Baur, 1972). Of the other two orientations one reflect the variations in bond strengths caused by
w o u l d b r i n g H ( w ) w i t h i n 2 . 5 6 A o f a C a a t o m r e - different coordinations of the individual oxygen
m o v e d b y 7 z bf r o m O ( w 5 ) w h i l e t h e r e m a i n i n go n e i s a t o m s . A n a l o g o u s c a l c u l a t i o n so f t h e o t h e r b o n d
t h e m o s t l i k e l y c h o i c e( F i g . I ) . I n t h i s p o s i t i o n ,c a l l e d l e n g t h sy i e l d f o r A l - O ( h 4 ) 1 . 8 8 0 ,A l - O ( 2 ) 1 . 9 0 0 ,A l H ( * ) * " o - i n T a b l e l , H ( w ) i s l o c a t e d 1 . 8 9 8 A f r o m O(3) 1.960, Ca-O(2) 2.393,Ca-O(w5) 2.421 and CaO(h4). This distancecould correspond to a bent hy- O ( l ) 2 . 5 3 1 4 . T h e l a r g e s td i s c r e p a n c yo c c u r sf o r C a drogen bond, since O(w5)-O(ha) measures2.669A O ( l ) ( 0 . 0 3 7A ; , w t r i c t r i s n o t s u r p r i s i n g s i n c e p r e a n d t h e a n g l eO ( w 5 ) - H ( w ) - O ( h a ) i s 1 3 5 . 4 '( s e eB a u r , dicted bond lengths for atoms with a low formal
l 9 6 s) .
c h a r g eu s u a l l yg i v e l e s sa c c u r a t er e s u l t s( B a u r , 1 9 7 0 ) .
The hydrogen atom of the OH group was located
The shared edge O(3)-O(h4) between the two Al
by calculatingits position of leastelectrostaticenergy coordination octahedra is much shorter (2.460A\
e m p l o y i n g t h e p r o g r a m M e N r o c ( B a u r , 1 9 6 5 ) .T h i s than the averageof the octahedral edgesaround Al.
BAUR: REFINEMENT OF LAWSONITE
T a b l e 6 . L a w s o n i t e ( r ) P a u l i n g b o n d s l r e n g t h s ,p ( O ) , r e c e i v e d
b y t h e o x y g e n a t o m s . ( b ) O b s e r v e da n d c a l c u l a t e db o n d l e n g t h s
si- o*
A,(,
o(1)
o(2)
o(3)
o(h4)
O(w5)
2/6
2/6
3/6
2 x 3 l6
2x3/6
H(d)
t=p(o)
I/2xI/6
tl6
5/ 6
2x5/6
A p( o )
2.333v.u.
1.916
1.916
2,L67
2.083
H(a)
2x4/4
4/4
4/4
2/6
p(o)
Si-o(1)
si-o(2)
si-o(2)
si-o(3)
Dean
si
0.250v.u.
-o.t67
-0.L67
0.083
0
d.
caac
r.6534
r.615
1.615
1 .638
1.630
2 , 3 3 3 v .u .
r.916
2.167
1.833
2.000
d.
obs
1.6548
1.616
I.616
r.647
1.633
lAdl
0,0014
0.001
0,001
0. 009
0.003
* l{(d)
refers
to hydrogen
bond donors,
H(a)
to hydrogen
bond acceptors.
The calculation
of bond lengths
is
=
based on d-_.d_--_ + 0.091Ap(O), Baur (1970).catc
mean
valence
units:
v.u.;
observed si-o
distance:
dob"i
lnAl
- d
= la .
of Lhe
l; Ap(o) is the deviarion
' oos
cafc'
p(O) fron
the polyhedron.
Ehe Eean p(O) for
individual
Other shared edges do not occur in this structure
unlessone considersO(3) to be coordinatedto the Ca
atom.
Acknowledgments
I thank the Computer Center of the University of
Illinois for computer time, Mr. R. Heyman for the
m i c r o p r o b ea n a l y s i s ,D r . A . A . K h a n f o r c o l l e c t i n g
t h e i n t e n s i t y d a t a , a n d t h e S m i t h s o n i a nI n s t i t u t i o n
f o r a s a m p l eo f l a w s o n i t e .
References
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ft?
Manuscript receiued,July 18, 1977; accepted
.for publication, Nouember 25, 1977.
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