8/13/2019 Definition of clay and clay minerals

1/2

Clays and Clay Minerals Vol. 43, No. 2, 255-256, 1995.R E P O R T

D E F IN I T IO N O F C L A Y A N D C L A Y M I N E R A L J O IN T R E P O R T O F T H EA IP E A N O M E N C L A T U R E A N D C M S N O M E N C L A T U R E C O M M I T T E E S

ST PH N GUGGENHEIM*Department of Geological Sciences, University of Illinois at Chicago, 845 W. Taylor St., Chicago, Ill inois 60607

R. T. MARTIN**Ardaman and Associates, Inc., 8008 S. Orange Ave., Orlando, Florida 32859

INTRODUCTIONThe definition of cla y was first formalized in 1546

by Agricola. It has been revised many times since,although the funda menta ls invol ving plasticity, parti-cle size, an d h ardeni ng on firing were r etained by most.For an exhaustive account of the history of the defi-ni tio n to 1963, the reader is referred to Macken zie(1963). More recent developments may be found inWeaver (1989).

The definition of clay raises the i mpor tant issue ofclay constituents and, implicitly, the definition of clayminer al. Mackenzie (1963, p. 15) noted the inappro -priateness of defining clay minera l as any mine ralwhich occurs in clay since, amo ng several reasons, itwould include many accessory minerals that are notcharacteristic of clay. Previous definitions of claymin era l (e.g., Bailey 1980) simp ly equat ed clay mi n-erals to phyllosilicates, thus endorsing the use of a t ermwithout apparent justification. The term clay min-eral, however, is useful when applied in context toclay constituents. The approach of equating clay min-eral to phyllosilicate fails to consider the relationshipof the properties of the bulk clay with the propertiesof the constituen ts of clay. This inconsistenc y is ad-dressed here in part, without making maj or changes tocurrent usage.

Weaver (1989, p. 5) noted an addition al complica-tion: the conceptual prob lem of combini ng particle size

* Chairman of the AIPEA Nomenclature Committee: Theother members are A. Aliett i (Italy), V. A. Drits (Russia), M.L. L. Formoso (Brazil), E. Galhn (Spain), H. M. K6ster (Ger-many), D. J. Morgan (England), H. Paquet (France), T. Wa-tanabe (Japan), and ex officio members D. C. Bain (Editor,Clay Minerals and R. E. Ferrell (Editor, Clays and ClayMinerals .** Chairman of the CMS Nomenclature Committee: Othermembers are D. L. Bish, D. S. Fanning, S. Guggenheim, H.Kodama, F. J. Wicks.C o p y r i g h t 9 1 9 9 5 , T h e C l a y M i n e r a l s S o c i e ty

require ments of clay constituents with m ineralogy. Be-cause the term min era l has a precise definition thatdoes not include particle size, it follows that the defi-nition of a mineral group cannot be based on particlesize.

Both the nomenclature committees of AIPEA andCMS started work on the definitions of clay at the sa metime, and it became clear that a m ore acceptable resultwould be obtaine d if the two commit tees joined efforts.This report represents the recomm endation s of bothcommittees.

CLAYDef in i t ion

The term clay refers to a natural ly occurring ma-terial composed primarily of fine-grained minerals,which is generally plastic at appropriate water conten tsand will harden with dried or fired. Although clay usu-ally contains phyllosilicates, it may conta in other m a-terials that impart plasticity and h arden whe n dried orfired. Associated phases in clay may i nclude materialsthat do not impart plasticity and organic matter.Discuss ion

The natura lly occurring requir ement of clay ex-cludes synthetics. Based on the st andard definition ofmineral, clays are primarily inorganic materials ex-cludin g peat, mu ck, s ome soils, etc. that con tai n largequantiti es of organic materials. Associated phases, suchas organic phases, ma y be prese nt (see below for furtherdiscussion relating to associated phases). Plasticityrefers to the ability of the material to be molde d to an yshape. The plastic properties do not require quantifi-cation to apply the term clay to a material. The fine-grained aspect canno t be quantified, because a specificparticle size is not a pr operty tha t is u niversally ac-cepted by all disciplines. For example, most geologistsand soil scientists use particle size less than 2 /~m,

255

8/13/2019 Definition of clay and clay minerals

2/2

256 G uggenhe im and Ma r t in Clays an d Clay Minera lss e d i m e n t o l o g i s t s u s e 4 # m , a n d c o l l o i d c h e m i s t s u s e 1# m f o r c l a y - p a r t ic l e s iz e . S e d i m e n t o l o g i s t s m a y u s e t h et e r m c l a y a l s o t o d e n o t e g r a i n s iz e o n l y . I t i s m o r ep r e c is e , h o w e v e r , t o g i v e t h e a c t u a l d i m e n s i o n s o f t h ep a r t i c l e s , e . g . , p a r t i c l e s l e s s t h a n 4 ~ z m .

P l a s t i c i t y i s a p r o p e r t y t h a t i s g r e a t l y a f fe c t e d b y t h ec h e m i c a l c o m p o s i t i o n o f t h e m a t e r i a l. F o r e x a m p l e ,s o m e s p e ci e s o f c h lo r i te a n d m i c a c a n r e m a i n n o n -p l a s t i c u p o n g r i n d i n g m a c r o s c o p i c f l a k e s e v e n w h e r em o r e t h a n 7 0 % o f t h e m a t e r i a l i s l e ss t h a n 2 u m e s d( e q u i v a l e n t s p h e r i c a l d i a m e t e r ) . I n c o n t r a s t , o t h e r c h l o -r it e s a n d m i c a s b e c o m e p l a s t i c u p o n g r i n d in g m a c r o -s c o p i c f la k e s w h e r e 3 % o f t h e m a t e r i a l i s le s s t h a n 2# m e s d .

P l a s t i c i ty m a y b e a f f e c te d a l s o b y t h e a g g r eg a t e n a -t u r e o f t h e p a r t i c l e s i n t h e m a t e r i a l . F o r e x a m p l e , m a n y

g l a u c o n i t e s a n d s a r e c o m p o s e d o f ( s a n d s i z e ) g r a i n so f a g g r e g a t e s o f f i n e - g r a i n e d g l a u c o n i t e c r y s t a l l i t e s . A l -t h o u g h t h e m a t e r i a l m a y b e p l a s t ic , t h e y a p p a r e n t l yc o n t a i n n o f in e s b a s e d o n a s t a n d a r d g r a i n s i z e an a l y s i s.C l e a r ly , s u c h g i a u c o n i t e s a n d s a r e c o m p o s e d o f ag g r e-g a t e s o f c l a y . I n c o n t ra s t , m a n y f l i n t c l a y s a r e g r e a t e rt h a n 9 5 % k a o l i n i t e , y e t t h e y a r e n o n p l a s t i c i n t h e i rn a t u r a l s t a t e a n d c o n t a i n a g g r e g at e p a r t i c le s o n l y g r e a t -e r t h a n a b o u t 7 5 ~ zm i n s i z e. A l t h o u g h t h e a g g r e g a t e sa r e c o m p o s e d o f f i n e -g r a i n e d k a o l i n i t e , p la s t i c i t y i sm i n i m i z e d b e c a u s e o f t h e a g g re g a te n a t u re o f t h e k a -o l i n i t e c r y s ta l l it e s . T h u s , f l i n t c l a y s a r e a n a p p a r e n te x c e p t i o n t o t h e p l a s t i c i t y a t t r i b u t e o f th e d e f i n i t i o n .T h e t e r m f i n e - g r a i n e d i n t h e a b o v e d e f i n i t i o n r e fe r st o c r y s t a l l i t e s i z e a n d n o t t o a g g r e g a t e s i z e .A l t h o u g h t h e p r i m a r y m i n e r a l s c o m p o s i n g c l ay s ar ep h y l l o s i l i c a te s , o t h e r m a t e r i a l s t h a t i m p a r t p l a s t i c i tya n d h a r d e n u p o n d r y i n g o r fi ri n g m a y c o m p r i s e c la y s( s ee d e f i n i t i o n f o r c l a y m i n e r a l b e l o w ) . A s s o c i a t e dp h a s e s i n c l a y s ( n o t t o b e re f e r r e d t o as a s s o c i a t e dc l a y p h a s e s , b u t as a s s o c i a t e d p h a s e s i n c l a y ) m a yb e m i n e r a l s s u c h a s q u a r t z , c a l c it e , d o l o m i t e , f e l d s p a rs ,o x i d e s , h y d r o x i d e s , o r g a n i c p h a s e s , e t c. o r n o n c r y s t a l -l i n e p h a s e s , s u c h a s c o l l o i d a l si l i ca , i r o n h y d r o x i d e g e l s,o r g a n i c g e l s , e t c .

C L A Y M I N E R A LD e f i n i t i o n

T h e t e r m c l a y m i n e r a l r e fe r s t o p h y l l o s i l i c a t e m i n -e r a ls a n d t o m i n e r a l s w h i c h i m p a r t p l a s t i c i t y t o c l a ya n d w h i c h h a r d e n u p o n d r y i n g o r fi ri n g.

D i s c u s s i o nC u r r e n t l y , m i n e r a l s k n o w n t o p r o d u c e t h e p r o p e r t y

o f p l a s t i c i t y a r e p h y l l o s i l ic a t e s . B e c a u s e m i n e r a l s a r en o t d e f i n e d b a s e d o n t h e i r c r y s t a ll i t e s i ze , a p p r o p r i a t ep h y l l o s il i c a te s o f a n y g r a in s i ze m a y b e c o n s i d e r e d c l a ym i n e r a l s . L i k e w i s e , c l a y m i n e r a l s a r e n o t re s t r ic t e d ,b y d e f i n i ti o n , t o p h y l l o s i li c a t e s . I f r e s e a r c h r e v e a l s t h a ta n o n - p h y l l o s i l i c a t e m i n e r a l i m p a r t s p l a s t i c i t y t o a c l a ya n d h a r d e n s u p o n d r y i n g o r f i ri n g , t h i s m i n e r a l i s a

c l a y m i n e r a l . F o r e x a m p l e , if a n o x y - h y d r o x i d e m i n -e r a l i n a c l a y s h o w s p l a s t i c i t y a n d h a r d e n s u p o n d r y i n go r fi r in g , i t m a y b e p r o p e r l y r e f e r r e d to a s a c l a ym i n e r a l . T h u s , a c l a y i s n o t r e q u i r e d to b e p r e d o m -i n a n t l y c o m p o s e d o f p h y l l o s i l i c at e s . M i n e r a l s t h a t d on o t i m p a r t p l a s t i c it y t o c l a y a n d n o n - c r y s t a l l i n e p h a s e s( r eg a r d le s s i f t h e y i m p a r t p l a s t i c i ty o r n o t ) a r e e i t h e r

a s s o c i a t e d m i n e r a l s o r a s s o c i a t e d p h a s e s , r e s p e c -t i v e l y .T h i s d e f i n i t i o n d e p a r t s f r o m p r e v i o u s d e f i n i t i o n s (e .g . ,B a i l e y 1 9 8 0 ) o f c l a y m i n e r a l s , w h e r e c l a y m i n e r a l s w e r ee q u a t e d t o p h y l l o s i l ic a t e s . T h e c u r r e n t d e f i n i t i o nb r o a d e n s t h e s c o p e o f p o s s i b l e m i n e r a l s d e f i n e d a s c la ym i n e r a l s . I n a d d i t i o n , i t s p e c i f ic a l l y r e l a te s t h e p r o p e r t yo f p l a st i c i t y a n d h a r d e n i n g u p o n d r y i n g o r f i ri n g , w h i c hi s b a s ic t o t h e d e f i n i t i o n o f c la y , t o t h e d e f i n i t i o n o fc l a y m i n e r a l . T h e d e f in i t i o n o f c l a y m i n e r a l i n c l u d e sa l l p h y l l o s i li c a t e m i n e r a l s p r e v i o u s l y r e g a r d e d a s c l a ym i n e r a l s a n d p o s s i b l e n e w c l a y m i n e r a l s a s d i s c u s s e da b o v e .

R E F E R E N C E SB a i le y , S . W . 1 9 8 0 . S u m m a r y o f r e c o m m e n d a t i o n s o f A I -P E A N o m e n c l a t u r e C o m m i t t e e . Clays Clay Miner . 28:73- 78 .M a c k e n z ie , R . C . 1 9 6 3 . D e Natura Lutorum . Clays and ClayMinerals, v. X I , P r oceedings of the E leventh N a t iona l Co n-f e rence on Clays and C lay Miner a l s , Pe r gamon P r es s , 1 1-28.Weaver , C .E . 1989. Clays , Muds, andShales . A m s t e r d a m :Elsevier , 819 pp.