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Clay Minerals (1981) 16, 383-393.
A M E T H O D T O E L I M I N A T E T H E B A C K G R O U N D I N
X R A Y D I F F R A C T I O N P A T T E R N S O F O R I E N T E D
C L A Y M I N E R A L S A M P L E S
S . J . V A N D E R G A A S T A ND A . J . V A A R S
Netherlands Institute for Sea Research, P.O. Box 59, Texel, The Netherlands
Received 15 June 1981)
A BS TR A C T: A m ethod is described for calculating, and then sub tracting, the background from
X-ray diffraction patterns of oriented cla y mineral samp les. Ti-K~ radiation is used and, to
minimize the ab sorption o f this radiation by air, a vacuum and helium-flushed device has bee n
developed. This device can be used with other X-ray sources, offeringa considerable increase of
intensity---e.g. Co-Kc~ adiation is increased by 125 ~. With the back ground -elimina tedpatterns a
better sem i-quantitative estimate of the com position of clay m ineral mixtures is po ssible. Sm all
differences in com position of two sam ple s can be identified by su btracting one o f the
background-eliminated patterns from the other. Using th is method, peak m axima of smectite-
group minerals can also be accurately determined.
I n o r d e r t o e s t i m a t e t h e c la y m i n e r a l c o m p o s i t i o n o f s e d i m e n t o r s o il sa m p l e s , t h e
i n t en s i ti e s o f s o m e l o w - a n g l e b a s a l r e fl e c ti o n s o f c l a y m i n e r a l s o n t h e i r X - r a y d i f f r a c t io n
p a t t e r n s a r e u s u a l ly m e a s u r e d . A t t h e se l o w a n g l e s, t h e r e f le c t io n s a r e s u p e r i m p o s e d o n a
s h a r p l y d e c r e a s in g b a c k g r o u n d t h a t h a s t o b e s u b t r a c t e d f r o m t h e r e fl e ct io n s . T o i m p r o v e
t h is m e t h o d , d e t e r m i n a t i o n o f t h e b a c k g r o u n d is n e c e s s a ry .
I n n o r m a l X - r a y p a t t e r n s m e a s u r e d w i t h a s u i t a b l e d i v e r g e n c e sl it ( w h i c h p r o v i d e s a
c o n s t a n t r a d i a t e d v o l u m e a t al l a n g le s ) , t h e b a c k g r o u n d is c o m p o s e d o f ( F ig . 1 ) - - ( a )
i n c o h e r e n t r a d i a t i o n : C o m p t o n d i f f u s i o n ( 3) , f l u o r e s c e n t r a d i a t i o n ( 4) a n d t e m p e r a t u r e -
d i f f u s e s c a t t e r i n g (3 ) ( C u l l i t y , 1 96 7 ); ( b ) a p p a r a t u s - d e p e n d e n t f a c t o r s : i n c i d e n t b e a m ( 5 )
a n d a i r s c a t t e r ( 6 ); (c ) t o t a l r e f l e c t io n ( 7 ) ( G u i n i e r , 1 9 56 ). T h e r e f l e c t i o n s in t h e X - r a y
p a t t e r n c a u s e d b y c o h e r e n t r a d i a t i o n r e s u l t n o t o n l y f r o m t h e 0 0 l r e f le c t io n s o f th e c l a y
m i n e r a l s b u t a l s o f r o m d i f f r a ct i o n b y s m a l l p a r ti c l e s (1 ), m i c r o - p o r e s (1 ) ( T e h o u b a r &
M 6 r i n g , 1 9 69 ) a n d c r y s t a l d e f e c t s ( 2) ( P o n s , 1 98 0; P o n s
e t a l . ,
1 9 8 0 ) . A l t h o u g h t h e l a s t
t h r e e e ff e ct s c a n n o t s t ri c tl y b e r e g a r d e d a s b a c k g r o u n d , t h e y o f t e n a r e r e g a r d e d a s s u c h
b e c a u s e t h e y c a n n o t b e d i st i n g u is h e d f r o m t h e r e al b a c k g r o u n d . T h e b a c k g r o u n d e f fe c ts
m e n t i o n e d a b o v e a r e a l s o i n f lu e n c e d b y th e L o r e n z - p o l a r i z a t i o n f a c t o r ( 6) , t h e a b s o r p t i o n
f a c t o r ( 4 ) a n d t h e a t o m i c s c a t t e r in g f a c t o r ( 8) . O n X - r a y d i f f r a c t io n t r a c e s , t h e s u m o f t h e
e f f e ct s 1 t o 8 r e s u l t s a t l o w a n g l e s i n a c u r v e m o r e o r l e s s r e p r e s e n t e d b y c u r v e 9 o f F i g . 1.
O n X - r a y p a t t e rn s o b t a i n e d w i th C o - K s r a d i a ti o n , t h e t ru e b a c k g r o u n d is o f t e n o n l y
v i s i b le f r o m ,-~ 1 6 ~ 2 0. T o e s t i m a t e t h e b a c k g r o u n d o n t h e l o w - a n g l e s i de o f t h e 0 0 l
r e f le c t io n s o f c l ay m i n e r a l s i n X - r a y d i f f r a c ti o n p a t t e r n s w e c h o s e T i - K a r a d i a t i o n
( 2 = 2 . 7 4 9 73 A ) . M e a s u r e d a t 5 0 ~ r e l a t i v e h u m i d i t y ( r . h . ), t h e 00 1 s m e c t i t e r e f l e c ti o n t h e n
g a v e it s m a x i m u m a t ~ 11 ~ 2 0. T h i s p e a k s h i f t m a k e s p a r t o f t h e b a c k g r o u n d o n t h e
l o w - a n g l e s i d e o f t h e p a t t e r n v i s ib l e . T o o b t a i n a s u i t a b l e d i f f r a c t e d i n t e n s i t y a t t h e a n g l e s
0009-8558/81/1200-0383502.00 9 1981 Bla ckw ell Scientific Pub lication s
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384
S J v a n d e r G a a s t a n d A J V a a rs
Z e 4
FIG. 1. Facto rs contributing to back grou nd in normal X-ray diffraction patterns. It is assumed
that the irradiated vo lume is kept cons tant at all angles and the total reflection peak is overlapped
by the incident beam. (1) small particles, micro-pores; (2) crystal defects of large spacing minerals;
(3) Co mp ton diffusion, te mpe ratu re diffuse scattering; (4) fluorescen t radia tion , absorp tion
facto r; (5) inciden t beam; (6) Lor enz -pol ariza tion factor, ai r scatter; (7) tota l reflection; (8) atom ic
scattering factor; (9) bac kgro und resulting from the combined effects 1-8.
a t which th e c lay miner a l re f lec t ions a re measu red, and a lso to avoid the inf luence of the
inc iden t beam on the pa t t e rn , an au tom a t i c d ive rgence s li t s ys t em (ADSS ) was used. Th i s
dev ice var i e s the d ive rgence con t inuous ly , so tha t , w ha teve r the ang le o f the gon iom e te r ,
the same a rea i s i r radia ted; th is resul ts in a decreas ing i r radia ted vo lum e wi th decreas ing
angle . Th us w i th decreas ing angle there i s: (a ) a progress ive decrease of the tem pera t ure-
di f fuse sca t te r ing; (b) a s lowly increas ing L oren z-po lar iza t ion fa c tor e ffect ; (c ) a decrease
in the abs orp t ion fac to r e ffec t; (d) a s l ight decrease in the a tom ic sca t te r ing fa c tor e ffect .
This AD SS effec t i s a lso accom pan ied by a decrease in the d i f f rac ted in tens i ty f rom the
smal l par tic les , mic ro-p ores and c rys ta l defec ts . With the me tho d descr ibed in th is paper ,
Co mp ton d i f fus ion and f luo re scen t r ad ia t ion we re p reven ted f rom reach ing the coun te r
by us ing a c rys ta l m on oc hr om a to r be tween the s ample and the de tec to r . A i r - s ca tt e r was
avo ided b y f lush ing the spec imen ch amb er w i th he l ium.
To de te rmine the fo rm o f the back gro und cu rve , we ana lysed the X- ray pa t t e rns o f a
kaol in i te , tw o smect i tes of d i f fe rent c rys ta l l in i ty , some s ize f rac t ion s f ro m tw o sedim ent
~ON ~OMATOR CR~ST~L
I ETECTOR
? 2
/
~ C U U M
FiG. 2. Schem atic view of the vacuum -helium device.
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Background elimination in XRD patterns 3 8 5
s a m p l e s a n d a ce r a m i c t i le s a m p l e h o l d e r . U s i n g a c o m p u t e r , t h e b a c k g r o u n d w a s
c a l c u l a t e d f o r a ll p a t t e r n s f r o m t h e f o r m u l a y= Ax B T h e b a c k g r o u n d v a lu e s w e r e
s u b t r a c t e d f r o m t h e o r ig i n a l p a t t e r n a n d t h e b a c k g r o u n d - e l i m i n a t e d p a t t e r n p l o t t e d .
I f t h e p e a k t o b a c k g r o u n d r a t i o is l o w i n n o r m a l X - r a y p a t t e r n s , t h e p e a k v a l u e s a r e
o f t e n s h i f t e d t o h i g h e r A. v a l ue s . W i t h t h is m e t h o d m o r e a c c u r a t e p e a k v a l u e s w e r e
o b t a i n e d .
B e c a u s e o f th e h i g h l ev e l o f a b s o r p t i o n o f T i - K ~ r a d i a t i o n b y a i r , a d e v ic e w a s
d e v e l o p e d i n w h i ch a p a r t o f t h e X - r a y p a t h i s e v a c u a t e d a n d a n o t h e r p a r t i s
h e l i u m - f l u s h e d ( F i g . 2 ). W i t h t h i s d e v i c e th e i n t e n s i t y o f t h e T i - K ~ r a d i a t i o n w a s i n c r e a s e d
2 00 t i m e s. A n i n c r ea s e o f t h is m a g n i t u d e w a s n e c e s s a r y t o o b t a i n a w o r k a b l e p a t t e r n .
M A T E R I A L S A N D M E T H O D S
T h e b a c k g r o u n d w a s d e t e r m i n e d o n < 2 / ~ m f r a c t i o n s o f a k a o l in i t e f r o m P r o v e n c e ,
F r a n c e , a m o n t m o r i l l o n i t e f r o m W y o m i n g , U S A , a s m e c t i t e f r o m E t h i o p i a , 2 - 0 .5 ,
0 . 5 ~ )- 2 , < 0 -2 a n d < 0. 0 5 m f r a c t i o n s f r o m a s e d i m e n t 5 0 c m f r o m t h e t o p o f a c o r e f r o m
t h e N o r t h A t l a n t i c , < 2 m f r a c t i o n s f r o m t h is s e d i m e n t a n d a l s o f r o m a s e d i m e n t 25 c m
f r o m t h e t o p o f th e s a m e c o r e , a n d , f i na l l y , a c e r a m i c t i le s a m p l e h o l d e r . T h e s e d i m e n t a n d
s m e c t i te f ra c t i o n s w e re C a - e x c h a n g e d . O r i e n t e d s a m p l e s w e r e p r e p a r e d o n p o l i s h e d
p o r o u s c e r a m i c t i le s ( D i i m m l e r & S c h r 6 d e r , 1 96 5) a n d d r i e d a t r o o m t e m p e r a t u r e . X - r a y
r u n s w e r e c a r r i e d o u t a t 5 0 ~ r .h . a n d a l s o a f t e r g l y c o l a t i o n . T h e r . h . w a s k e p t c o n s t a n t
w i t h a h u m i d i t y g e n e r a t o r d e v e l o p e d a t o u r i n s t i t u te . A n X - r a y g e n e r a t o r P W 1 73 0 w i t h
TABLE t. A pp ara tus and experimental conditions.
Bro ad focus Co-tube, Bro ad focus Ti-tube,
X - r ay t ub e P W 2 2 5 6 / 2 0P h i l i p s ) P W 2 2 5 0 / 2 2Philips)
Rat ing
Window tube
Divergence slit
Receiving slit
An ti-scatter sl i t
Mo n o ch r o m a to r
Counter
Counter window
X-ray pa th
Window vac. par t
Specimen chamber
flushed with
Scanning speed
Scanning range
Scanning time
Recording
40-50 kV and 40-50 mA 25 kV and 48 mA
Be, 300 pm Be, 150 m
0.5 ~ AD SS PW1386/50 (Philips)
open at 0.3 ~ 20
0-2 mm 0.2 mm
1o 1o
G r a p hi te , A M R G r a p h it e , A M R
Propor t ional , Propor t ional k ryp ton
PW 1965/60 (Philips) cou nter with a signal
amplifier, 60 x
M ica Be, 150 pm
Vacuum (1 mm Hg) Vacuum (1 mm Hg)
6 m My lar fo i l 6 /~m M ylar fo il
Air , 50~ r .h . , 150 ml Helium, 50 ~ r .h . , 150 ml
min - l rain - 1
0.5 ~ 20 m in - 1 Stepscan, 4 sec 0.02~ 20 l
3-16 ~ 20 0-30 ~ 20
26 m in 140 min
Recorder PM 8 2 0 3 Computer , Tek tron ix 4051 ,
(Philips) f loppy disc unit (File
Manager Tektronix) and plotter
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386 S . J . v a n d er G a a s t a n d A . J . V a a r s
channel control PW 1390 and motor control PW 1394 Philips) was used; apparatus
constants are given in Table 1. The Simple Linear Regression Program S t a t i s t i c s 1 ,
Tektronix) was used to estimate the shape of the background.
RESULTS
Using the reflection method in combination with the ADSS implies that the irradiated
sample volume decreases with decreasing angle. Because small particles, micropores and
crystal defects show their diffraction effects at low angles, they will only slightly affect the
part of the X-ray diffraction pattern studied. To eliminate any interference from these
low-angle diffraction effects, a well-ordered kaolinite sample was chosen as a starting
point for determining the background. Applying the ADSS, the graphite monochromator
and the vacuum-helium device, the background on this sample comprises only the
temperature-diffuse scattering, the absorption factor, the Lorenz-polarization factor and
the total reflection, influenced by the ADSS effect.
G
L 9
FIG. 3. Backgr ound curves calculated fr om X-ray diffraction patt ern values, represented by dots,
comp ared w ith curves calculated fro m the form ula y = A x s. A = kaolinite, Provence; B = ceramic
tile; C=montmor illoni te, Wyoming; D=smec tite , Ethiopia; E= 2-0 .5 m, F=0 .5- 0.2 pm,
G=
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Background e l imina t ion i n XRD pat t e rns
387
F o r s m e c t i t e - c o n t a i n i n g s a m p l e s , t h e r .h . s t r o n g l y i n fl u e nc e s t h e e f fe c ts c a u s e d b y 1 a n d
2 i n F i g . 1. B o t h l o w a n d h i g h h u m i d i t i e s r e s u l t i n a n i n c r e a s e o f t h e d i f f r a c t e d i n t e n s i t y i n
th e 1 - 4 ~ 2 0 r e g io n .
T h i s w a s n o t f o u n d w i t h t h e k a o l i n i te s a m p l e a n d o n l y a sm a l l d e v i a t i o n w a s o b s e r v e d
f r o m 1 - 2 .5 ~ 2 0 w h e n i t w a s g l y c o l a te d . G l y c o l a t i o n o f t h e s m e c t it e s a n d t h e s m e c t i t e -
c o n t a i n in g s e d i m e n t s a m p l e s m a s k e d t h e b a c k g r o u n d b e t w e e n t he 001 p e a k a n d 1~ 20.
B e c a u s e th i s p a r t o f t h e p a t t e r n i s v e r y i m p o r t a n t f o r o b t a i n i n g b a c k g r o u n d v a l u e s , w e
d e c i d e d t o c a r r y o u t t h e m e a s u r e m e n t s f o r e s t i m a t i n g t h e b a c k g r o u n d a t 50Y/o r .h . U n d e r
t h e se c o n d i t i o n s t h e b a c k g r o u n d i s l e a s t o b s c u r e d b y t h e c o h e r e n t r a d i a t i o n c a u s e d b y t h e
c l a y m i n e r a l s . T h e b a c k g r o u n d m e a s u r e m e n t s w e r e s t a r t e d a t 0 ~ 2 0, a l t h o u g h b e l o w 1~ 20
t h e m e a s u r e d v a l u e s a r e i n f lu e n c e d b y i n a c c u r a t e o p e n i n g o f t h e A D S S . I n th e p a t t e r n s o f
t h e > 0. 0 5 m s e d i m e n t f r a c t i o n s i n F i g s 5 , 6 a n d 7 a n d t h a t o f t h e s m e c t i t e i n F ig . 5 , t h e
t o t a l r e f le c t io n p e a k c a n b e s e e n a t a b o u t 1~ 2 0 w h e r e t h e b a c k g r o u n d i s n o t e l i m i n a t e d . I n
s o m e c a s es a p e a k , c a u s e d b y a p a r t o f t h e i n c id e n t b e a m , c a n b e s e e n a t a b o u t 0 .5 ~ 2 0 ( in
F i g . 5, 2 - 0 . 5 m s e d i m e n t f r a c t i o n s ; i n F i g . 6 , < 0 .2 a n d < 0 .0 5 / ~ m s e d i m e n t f r a c t i o n s a n d
t h e E t h i o p i a n s m e c t it e ). T h e p e a k c a u s e d b y t h e t o t a l r e f l ec t io n i s p a r t l y i n c l u d e d i n t h e
b a c k g r o u n d m e a s u r e m e n t s . T h e i n c id e n t b e a m h a d n o i n f lu e nc e o n i t.
T h e f o r m u l a y = A x 8 i n w h i c h x a n d y r e p r e s e n t t h e c o o r d i n a t e s o f th e m e a s u r e d v a l u e s
i n th e d i a g r a m s a n d A a n d B a r e c o n s t a n t s , a p p e a r e d t o d e sc r ib e t he b a c k g r o u n d m o s t
a c c u r a t e ly ( F ig . 3 ) .
sm
~ s ~
sm
l/Mic~ j
995
2 e . . . . .
7 c o K a 2 e . . . . .
i o r c o K m
FIG. 4. X -ray diffraction patterns of the sediment fractions and the Ethiopian sm ectite, run w ith
Co-K ~ radiation at 50~ r.h . (left) and after glycolation (right). Chl = chlorite, K =kaolinite,
I = illite, Sm = smectite.
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3 8 8
S J v a n d er G a a s t a n d A J V a a r s
F i g . 3 , c u r v e A , s h o w s t h e v e r y g o o d f it o f t h e d if f ra c t io n c u r v e o f th e k a o l i n i t e s a m p l e
w i t h t h e c u r v e c a lc u l a t e d f r o m t h e m e a s u r e d v a l u e s . A n e a r p e r f e c t f it w a s a l s o f o u n d f o r
t h e e m p t y c e r a m i c ti le F i g . 3 , c u r v e B ) . I n t h i s c a s e w e w e r e a b l e t o m e a s u r e t h e
b a c k g r o u n d f r o m 0 -7 ~ 2 0 , p r o b a b l y d u e t o t h e v e r y fl at s u r f a c e o f t h e t i le . A t a n g l e s
> 6 ~ 2 0 , t h e b a c k g r o u n d i n c r e a s e d b e c a u s e o f t h e t e m p e r a t u r e - d i f f u s e s c a t t e r in g , t h e
a b s o r p t i o n f a c t o r a n d t h e a t o m i c s c a t te r i n g f a c t o r i n f lu e n c e d b y t h e A D S S e ff ec t . I n th i s
c u r v e w e m e a s u r e d t h e b a c k g r o u n d b e t w e e n 0 .7 ~ a n d 6 ~ 2 0 . T h e s l i g h t b a c k g r o u n d
i n c r e a se w a s a l s o o b s e r v e d i n t h e X - r a y p a t t e r n s o f F i g s 5 , 6 a n d 7 b u t c o u l d b e i g n o r e d i n
I l i i : I
I { 7 , , i ~.i/
/ " ~ J
/ j ' /
574
m
4G6~
C hl 7 4 9 S 5 ~
i I~i
I
I ,I
/
, I
J 4 ~ 8 ~ I t
: : _ j W '
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ackground elimination in XR D patterns 3 8 9
b a c k g r o u n d c a l c u l a ti o n . W i t h d e c r e a s i n g p a r ti c l e si ze a s m a l l e r p a r t o f th e X - r a y p a t t e r n
c o u l d b e u s e d f o r b a c k g r o u n d v a l u es , d u e t o t h e i n c re a s e in w i d t h o f t h e s m e c t it e p e a k a n d
a n i n c r e a s i n g a m o u n t o f t h is m i n e r a l in th e s a m p l e . T h u s o n e b a c k g r o u n d v a l u e b e t w e e n
3 ~ a n d 4 ~ 2 0 o n e a c h o f t h e c u r v e s E , I a n d J F i g . 3 ) s h o w e d a s m a l l d e v i a t i o n b e c a u s e o f
t h i s . T h e c a l c u l a t i o n o f c u r v e H F i g . 3 ) f r o m o n l y t h e f o u r v a l u e s 1 -8 , 2 . 0, 2 -5 a n d 2 6 ~ 2 0 ,
y i e l d e d a v e r y g o o d R 2 v a l u e t h e R 2 v a l u e s r e f l e c t t h e f i t o f t h e c a l c u l a t e d c u r v e t o t h e
m e a s u r e d v a lu e s ) . H o w e v e r t h e X - r a y p a t t e r n f r o m t h i s s a m p l e s h o w s a p e a k a t a b o u t 1 .4 ~
lIMl~
9 9 5
ehl
sm
7 8 4
2 e 2 ~ I o T i K ~ 0 2 ~ 2 2 I 0 T i K ~ 0
FIG. 6. Same samp les as in Fig. 5 afte r glycolation before left) and after right) elimination of the
background. Chl = chlorite, K = kaolinite, I = illite, Sm = sm ectite.
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390
S J van der Gaast and A J Vaars
I M i c :
9 9 5 ~
c.p
~o41 7~K41
I
I
T 4 ~ e A , ~
/ I
2e 22 io Ti K~ 0 2e 22 I0 TI KO~ 0
FIG. 7. X-ray diffraction patter ns of the < 2 m sediment fractions from 25 and 50 cm depth run
with Ti-Ka radiation an d the va cuum- heliu m device at 50 ~ r.h. Left side before and right side
after background elimination. The vertical scale of the background-cleared patterns was
'equalized' with the 001 kaolinite reflections and subtracted.
2 0 , w h i c h s l i g h t l y o b s c u r e s t h e 1~ 2 0 b a c k g r o u n d v a lu e . T h i s p e a k , c a u s e d b y c o h e r e n t
r a d i a t i o n f r o m s m a l l p a rt ic l es , m i c r o - p o r e s a n d c r y s ta l d e fe c ts , c a n n o t b e r e g a r d e d a s
b e l o n g i n g t o t h e b a c k g r o u n d . I n s u c h a c a s e t h e 1 ~ 2 0 v a l u e h a s t o b e u s e d a s t h i s
a p p r o a c h e s t h e r e a l b a c k g r o u n d m u c h c l o s e r t h a n t h e 1 .8 , 2 .0 a n d 2 .5 ~ 2 0 v a lu e s .
F i g s 5 a n d 6 s h o w t h e p a t t e r n s o f th e s e d i m e n t s a m p l e s a n d t h e E t h i o p i a n s m e c t i t e
o b t a i n e d w i t h T i - K ~ r a d i a t i o n a n d t h e v a c u u m - h e l i u m d e v i c e , w i t h a n d w i t h o u t
b a c k g r o u n d . T h e T i - K ~ r a d i a t io n , g iv e s m u c h b e t t e r p e a k r e s o l u ti o n , a s c a n b e s ee n b y
c o m p a r i n g t h e p a t t e r n s i n F i g . 4 .
T h e r e su l t o f s u b t r a c ti n g t h e b a c k g r o u n d - e l i m i n a t e d p a t t e r n s o f t w o < 2 m s e d i m e n t
f r a c t i o n s i s s h o w n i n F i g . 7 . P e a k v a l u e c o r r e c t i o n s a r e s h o w n i n T a b l e 2 . U s i n g T i - K ~
r a d i a t i o n w i t h t h e w h o l e v a c u u m - h e l i u m d e v i c e, th e i n t e n s i t y w a s i n c r e a s e d 2 0 0 t im e s .
C o - K s r a d i a ti o n , i n c o m b i n a t i o n w i t h t h e v a c u u m p a r t o f t h e d ev i ce , s h o w e d a n i n t en s i ty
i n c re a s e o f 9 0 ~ . W h e n t h e o t h e r p a r t w a s s i m u l t a n e o u s l y f lu s h e d w i t h h e l iu m , t h e
i n t e n s i t y in c r e a s e w a s 1 2 5~ o . T h e i n t e n s i t y o f C o -K / ~ r a d i a t i o n i n c r e a s e d a b o u t 1 0 0 ~
u s in g b o t h v a c u u m a n d h e l iu m . T h e s p e c i m e n c h a m b e r c a n a l s o b e e v a c u a t e d o r f l u sh e d
w i t h o t h e r g a se s, p r o v i d e d t h a t t h e y d o n o t a t t a c k t h e M y l a r w i n d o w s .
T h e s c a n n i n g t i m e f o r o n e s a m p l e is 1 4 0 m i n ( T a b l e 1 ). T h i s l o n g s c a n n i n g t i m e i s d u e
t o : ( 1 ) t h e s t e p s c a n m e t h o d , b e c a u s e f o r c o m p u t e r c a l c u l a t i o n s t h e c o u n t s h a v e t o b e i n
8/11/2019 Xrd Pattern
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Backg round e limina tion in X R D pat t e rns
TABLE 2. A ngstrom values of the smectite peaks from the sediment
samples and the Ethiopian smectite run Co-Ks radiation and Ti-Ke
radiat ion, zx = valu es w it h background, - = valu es without
background.
Co-K s radiation Ti-K~ radiation
50~ r .h. glycola ted 50~ r .h . glycolated
A /k /x A
2-0 .5pm 14.66 17 .69 14 .6 6 14 .66 17 .72 17 .72
0.5-0.2/~m 14.64 17.6 9 1 4 .6 4 14.6 4 17-66 17.66
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3 9 2
S J v a n d e r G a a s t a n d A J V a a r s
s h i f te d . T h e s m e c t i t e p e a k a l s o s h i f te d t o w a r d l o w e r a n g le s , c a u s i n g p e a k o v e r l a p w i t h t h e
l o w - a n g l e m a x i m u m .
T h e a d v a n t a g e o f e li m i n a ti n g t h e b a c k g r o u n d o f X - r a y p a t t e r n s is s h o w n i n F i g. 7 . T o
c a l c u la t e t h e d i ff e re n c e i n c l a y m i n e r a l c o m p o s i t i o n , t h e b a c k g r o u n d - e l i m i n a t e d p a t t e r n
o f th e < 2 / ~ m f r a c t io n o f th e s e d i m e n t f r o m 2 5 cm i n th e c o r e w as s u b t r a c t e d f r o m t h a t a t
5 0 c m a n d t h e d i f fe r e n c e c u r v e , s m o o t h e d o v e r fi v e m e a s u r e m e n t s , p l o t t e d ( F i g . 7 ). B e f o r e
s u b t r a c t i o n t h e v e r t ic a l sc a le o f t h e t w o c u r v e s w a s d e t e r m i n e d i n o r d e r t o e q u a l i z e t h e
a v e r a g e v a l u e o f t h e p e a k i n t e r v a l b e t w e e n 2 2 .3 0 a n d 2 2 . 4 2 ~ 2 0 ( t h e 0 0 1 k a o l i n i t e
r e f l e c t i o n ) .
T h e r e s u l t i n g d i f f e re n c e c u r v e , a s s u m i n g e q u a l a m o u n t s o f k a o l i n i t e in b o t h f r a c t i o n s ,
s h o w s t h a t t h e f r a c ti o n f r o m 5 0 c m d e p t h c o n t a i n s m o r e s m e c ti te t h a n t h a t f r o m 2 5 c m
d e p t h . T h i s c u r v e s h o w s a l s o t h a t a s m a l l p e a k r e m a i n s a t t h e l o w - a n g l e s id e o f t h e
k a o l i n i t e a n d i ll i te / m i c a r e f le c t io n s . T h i s i s p r o b a b l y d u e t o a s m a l l d i f fe r e n c e b e t w e e n t h e
p r o p e r t i e s o f t h e se m i n e r a l s i n t h e t w o s a m p l e s . T h e p e a k a t 1 .5 ~ 2 0 in d i c a t e s a s m a l l
d i f f er e n c e i n t h e lo w - a n g l e p h e n o m e n a .
D u e t o t h e r a p i d l y d e c r e a si n g b a c k g r o u n d , a s m e c t it e p e a k s h i f t w a s o b s e r v e d w i t h
d e c r e a s i n g p a r t i c l e si ze in t h e X - r a y p a t t e r n s o f th e s e d i m e n t s a m p l e s o b t a i n e d w i t h
C o - K ~ r a d i a t i o n ( T a b l e 2 ) . T h i s s h i f t w a s a b s e n t i n t h e X - r a y p a t t e r n s o b t a i n e d w i t h
T i - K a r a d i a t i o n . B o t h w i t h C o -K : r a n d T i -K :r r a d i a t i o n t h e E t h i o p i a n s m e c t i te s h o w e d a
p e a k s h if t; t h is w a s c o m p l e t e l y c o r r e c t e d a f t e r b a c k g r o u n d s u b t r a c t i o n o f th e X - r a y
p a t t e r n o b t a i n e d w i t h T i -K ~ r a d i a t io n .
T o c h e c k t h a t t h e a m o u n t o f k a o l i n it e w a s c o n s t a n t i n t h e t w o s e d i m e n t s a m p l es , a n
o r i e n t i n g i n t e r n a l s t a n d a r d w a s a d d e d . W e o b t a i n e d e n c o u r a g i n g r e s u l t s w i t h m o l y b -
d e n i t e , w h i c h w i t h T i - K a r a d i a t i o n g a v e i t s m a i n p e a k a t ~ 2 6 ~ 2 0 . I n t h is c a s e t h e 2 7 - 2 8 ~
2 0 r e g i o n w a s u s e d f o r b a c k g r o u n d c a l c u l a t i o n s . M o l y b d e n i t e l a a s a l s o b e e n u s e d a s a n
i n t e rn a l s t a n d a r d i n q u a n t i t a ti v e X - r a y d i f f r a c ti o n o f cl a y m i n e r a l s b y Q u a k e r n a a t ( 19 7 0)
a n d C o d y T h o m p s o n (1 97 6).
C O N C L U S I O N S
A d e v i c e h a s b e e n d e v e l o p e d t o m e a s u r e , a t a f e w p r e d e t e r m i n e d p o i n t s, t h e b a c k g r o u n d
o n t h e X - r a y p a t t e r n o f a n o r i e n t e d c la y m i n e r a l s a m p le . F r o m t h e m e a s u r e d b a c k g r o u n d
v a l ue s a b a c k g r o u n d c u r v e c a n b e c a l c u l a te d w i t h a f o r m u l a y = A x e a n d t h i s s u b t r a c t e d
f r o m t h e o r i g i n a l X - r a y p a t t e r n . S l i g h t d i f f er e n c e s i n c la y m i n e r a l c o m p o s i t i o n c a n b e
d e t e c t e d b y s u b t r a c t i n g o n e o f th e t w o b a c k g r o u n d - c l e a r e d p a t t e r n s f r o m t h e o t h e r. T h e
a d v a n t a g e s o f th is a p p r o a c h a r e e n h a n c e d w h e n a n o r i e n ti n g in t e r n a l s t a n d a r d , s u c h a s
m o l y b d e n i t e , i s u s e d .
D u e t o t h e r a p id l y d e c re a s i n g b a c k g r o u n d o f X - r a y p a t t e r n s o b t a i n e d w i t h C o - K ~
r a d i a t i o n t h e s m e c t i t e p e a k s o m e t i m e s s h i f t s t o h i g h e r v a l u e s . T h i s e f fe c t i s p r e v e n t e d b y
u s i n g t h e b a c k g r o u n d e l i m i n a t i o n m e t h o d o n t h e X - r a y p a t t e r n s o b t a i n e d w i t h T i - K ~
r a d i a t i o n .
T h e l o n g w a v e l e n g t h T i - K ~ r a d i a t i o n a l s o gi v es b e t t e r r e s o l u t i o n . F o r a l l X - r a y s o u r c e s
a c o n s i d e r a b l e g a i n i n i n t e n s i t y is o b t a i n e d w h e n t h e v a c u u m - h e l i u m d e v i c e i s u s e d .
K f l
l in e s f r o m X - r a y s o u r c e s c a n b e u s e d i f t h e K ~l~ 2 d o u b l e t h i n d e r s a c c u r a t e m e a s u r e m e n t s
o f t h e X - r a y r e f le c t io n s .
T h i s m e t h o d a n d d e v i c e o f fe r i n t e r e s t i n g p o ss i b il it i es t o c l a y m i n e r a l o g i s t s , s e d i m e n t o -
l o g is t s a n d s o il s c ie n t is t s i n t e r e s te d i n i m p r o v i n g t h e i r q u a n t i t a t i v e a n a l y t i c a l t e c h n i q u e s .
8/11/2019 Xrd Pattern
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Background elimination in XRD patterns 3 9 3
A C K N O W L E D G M E N T S
T h a n k s a r e e x t e n d e d t o M r F . E i j g e n r a a m f o r d e v i si n g t h e c o m p u t e r p r o g r a m ; M r F . S c h i l li n g f o r a s si s t an c e i n
c o n s t r u c t i n g t h e h u m i d i t y g e n e r a t o r ; D r D . S p i t z e r f o r h i s h e l p i n s o l v i n g t h e m a t h e m a t i c a l p r o b l e m s ; M r B .
V e r s c h u u r f o r d r a w i n g t h e f ig u r es a n d D r s D . T c h o u b a r a n d C . H . P o n s o f t h e U n i v e r s i ty o f O r l r a n s , F r a n c e , f o r
t h e i r s t i m u l a t i n g d is c u s s i o n s . T h e m a n u s c r i p t w a s c r it i c al l y r e a d b y D r s J . H . F . J a n s e n a n d D . E i s m a a n d t y p e d
b y M r s J . H a r t . T h e a u t h o r s a r e e s p e c ia l l y a p p r e c i a t i v e o f th e h e l p f u l c o m m e n t s o f t h e s t a f f o f t h e X - r a y
a p p l i c a t i o n d i v i s i o n , P h i l ip s , A l m e l o a n d o f D r W . H . D i e m e r o f I & S , P h i li p s , E i n d h o v e n .
R E F E R E N C E S
C O D Y R . D . & T H OM P SO N G . L . ( 1 9 7 6 ) Clays Clay Miner. 2 4 , 2 2 4 .
CUL L Ia 'Y B . D . ( 1 9 6 7 ) Elements of X-ray Diffraction. A d d i s o n - W e s l e y , R e a d i n g , M a s s a c h u s e t t s . 5 1 4 p p .
Df3MMLER H . & SCHROEDER D. (1965 ) Z. Pflanzenernaehr. Dueng. Bodenk. 109, 35 .
G u N m g A . (1 95 6)
Th~orie et Technique de la Radiocristallographie. Dunod
P a r i s .
P ON S C . H . ( 1 98 0 ) P h D t h e s i s, O r l r a n s U n i v e r s i t y , F r a n c e .
PONS C.H . , BEN BRAI-I IMJ . , Y f 2CE L A. , T CHOUBARD. & T CHOUBAR C . ( 1 9 8 0 ) Clay Miner. 15, 111.
QUAKERNAATJ . ( 1 9 7 0 ) J. sedim. Petrol. 40, 506 .
T CHOUBAR D. & M~RI NG J . ( 1 9 6 9 ) J. Appl. Cryst. 2, 128.
R I~ S U M I~ : O n d r c r i t u n e m & h o d e d e c a l c u l d u f o n d c o n t i n u q u e l ' o n p e u t e n s u i t e s o u s t r a i r e d u
c l ic h 6 d e d if f r a ct i o n X d ' u n 6 c h a n t i l l o n d ' a r g i l e o r i e n t r e . O n u t i l i s e l e r a y o n n e m e n t T i - K e a i n s i
q u ' u n e e n c e i n t e 6 t a n c h e s o i t s o u s v i d e s o it b a l a y r e p a r l ' h ~ l i u m a f in d e d i m i n u e r l ' a b s o r p t i o n d e c e
r a y o n n e m e n t p a r l ' a i r. C e t a p p a r e i l l a g e p e u t ~ t r e u ti l is 6 av e c d ' a u t r e s s o u r c e s d e r a y o n s X
p e r m e t t a n t u n a c c r o i s s e m e n t i m p o r t a n t d e l ' i n t e n s i t 6 d i ff r a c tr e r e c u e il l ie , p a r e x e m p l e l ' e ff ic a ci t~
d u r a y o n n e m e n t C o K e e s t a cc r u e d e 1 2 5 ~ . L e s d i a g r a m m e s c o r ri g ~s d e l eu r f o n d c o n t i n u
p e r m e t t e n t u n e m e i l l eu r e e s t i m a t i o n q u a n t i t a t i v e d e l a c o m p o s i t i o n d e m ~ l a n g e s d ' a rg i l e s. D e s
p e t i t es d i f f rr e n c e s d e c o m p o s i t i o n d e d e u x 6 c h a n t i l l o n s p e u v e n t ~ t r e i d en t i f ir e s e n s o u s t r a y a n t l ' u n
d e s d i a g r a m m e s a i n s i c o r r i g r s d e l ' a u tr e . E n u t i l i s a n t c el le m & h o d e l e s m a x i m a d e s ra i e s d e s
m i n r r a u x d ' u n g r o u p e d e s m e c t it e s p e u v e n t 6 tr e d & e r m i n r s a v e c p rr c i si o n .
K U R Z R E F E R A T : E s w i rd e in e M e t h o d e z u e E r m i t t lu n g u n d a n s c hl i eg e n d er S u b t ra k t io n d e s
U n t e r g r u n d e s v o n R 6 n t g e n d i a g r a m m e n o r ie n t ie r te r T o n m i n e r a l p r o b e n b e s c h ri e b e n. A n g e w e n -
d e t w i r d T i - K a S t r a h l u n g , u n d u m d i e A b s o r p t i o n d i es e r S t r a h l u n g d u r c h L u f t z u m i n i m i e re n ,
w u r d e e i n e v a k u u m u n d h e l iu m g e s p f il t e A p p a r a t u r e n t w i c ke l t. D i e s e k a n n m i t a n d e r e n R 6 n t g e n -
s t r a h l u n g s q u e l l e n b e n u t z t w e r d e n , w o b e i e i n b e tr f i ch t l i c h er I n t e n s it f i t s g e w i n n a u f t r i t t - - z . B . w i r d
C o K e - - S t r a h l u n g u m 1 2 5 v e rs t~ i rk t . M i t t e l s u n t e r g r u n d b e r e i n i g t e r D i a g r a m m e i s t e i n e b e s s e r e
h a l b q u a n t i t a t i v e A b s c h i i t z u n g d e r Z u s a m m e n s e t z u n g v o n T o n m i n e r a l g e m i s c h e n m 6 g l i c h .
K l e i n e U n t e r s c h i e d e i n d e r Z u s a m m e n s e t z u n g z w e ie r P r o b e n k 6 n n e n d u r c h d i e S u b t ra k t i o n e i n es
u n t e r g r u n d b e r e i n i n g t e n D i a g r a m m s v o n a n d e r e n e r k a n n t w e rd e n . D i e A n w e n d u n g d ie s er M e t h -
o d e e r m r g l i c h t d i e g e n a u e B e s t i m m u n g d e s P e a k m a x i m u m s v o n M i n e r a l e n d er S m e k t i t g r u p p e .
R E S U M E N : S e d e s cr ib e u n m r t o d o p a r a c a l c u la r y p o s t e r i o r m e n t e s u s tr a e r e l f o n d o d e l os
d i a g r a m a s d e d i f ra c c i 6 n d e m u e s t r a s o r i e n t a d a s d e m i n e r a l e s d e la a rc i ll a . S e u s a r a d i a c i 6 n T i - K a ,
y p a r a m i n i m i z a r la a b s o r c i 6 n d e e s t a r a d i a c i 6 n p o r e l a i re , s e u t i l i za s u m o n t a j e a v a c i o y c o n f l u j o
d e h e l io . E s t e m o n t a j e p u e d e s e r u s a d o c o n o t r a s f u e n t e s d e ra y o s X , o f r e c i e n d o u n c o n s i d e r a b l e
a u m e n t o d e i n t e ns i d a d , p .e ., l a ra d i a c i6 n C o - K s a u m e n t a s u i n t e n s id a d e n u n 1 2 5 ~ . C o n e l f o n d o
e l i m i n a d o e s p o s i b l e m e j o r a r l a s e s t i m a c i o n e s s e m i c u a n t i t a t i v a s s o b r e l a c o m p o s i c i 6 n d e l a s
m e z c l a s d e m i n e r a l e s d e l a a r ch i l la . P e q u e f i a s d i f e r e n c ia s e n c o m p o s i c i 6 n d e d o s m u e s t r a s p u e d e n
s e r d e t e r m i n a d a s p o r d i f e r e n c i a e n t r e l o s d o s d i a g r a m a s e l i m i n a n d o p r e v i a m e n t e e l f o n d o .
U s a n d o e s te m & o d o l o s m f ix i m o s d e d i f r a cc i 6 n d e l o s m i n e r a le s d e s g r u p o d e l a s e s m e c t i t a s
p u e d e n s e d e t e r m i n a d o s c o n p r e c i s i6 n .
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