Xrd Pattern

8/11/2019 Xrd Pattern

1/11

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.


6/11

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 '


7/11

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.


8/11

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


9/11

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


10/11

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 .


11/11

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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Xrd Pattern