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MINERALOGICAL MAGAZINE, DECEMBER 1982, VOL. 46, PP. 469-73

e n e r a l c o m m e n t s o n t h e i d e n t i f i c a ti o n o f c h l o r i t e sin t h i n s e c t i o n s

E. P. S GGERSON ND L. M. TURNERDepartment of Geology, University of Natal, King George V Avenue, Durban, Natal 4001, South Africa

ABSTRACT. Although a chlorite is usually simple torecognize in thin section because of its characteristiccolour, form, and often very low or anomalous inter-ference colours, the identification of the various speciesis more difficult and requires both chemical and X-raywork for accurate determination. Roy and others havedemonstrated that chlorites are polymorphous and occurin two structural groups, the normal 14 A chlorites andthe 7/k septechlorites (Nelson and Roy, 1958). Manychlorite varieties may be found in both structural states,low-temperature end-members and compositionallyextreme end-members favouring the septechlorite struc-ture, especiallyantigorite, amesite, chamosite, greenalite,and cronstedtite. Phillips (1964) subsequently proposedthat six variables, Y, R 3+, R z , Z, H, plus the appropriatestructural designation, must be fixed to define completelya variety of the chlorite group and from this he developeda numerical system of classification. Unfortunately thesespecific chemical and structural properties cannot bedirectly recognized by normal optical methods.CHL ORI T E S a r e Mg, Fe, A1 phyllosil icates with alayered structure similar to the micas, of tabularhabit, occurring in flaky and scaly aggregates. Inthin section they exhibit mottled extinction andrange from colourless to green, Mn and Cr varietiesdisplaying orange, orange-brown, pink, or lavendercolours. Members of the chlorite family are mon o-clinic or triclinic and therefore biaxial, yet becausethe refractive indices of the two rays in (001)sections are so close, the optic axial angle (2V)is very small. For most purposes chlorites may,therefore, be considered to be uniaxial, With posi-tive and negative varieties. The septechlorite, anti-gorite, however, has 2V 20-60 ~ They may o c c u rin all environments and their compositio n is thusclosely related to that of the original rocks ormine rals from which they have formed. Chlor ite ischaracteristic of low-grade metamorphosed peliticsediments and basic igneous rocks or as an altera-tion product of other ferromagnesian minerals. Inigneous rocks it occurs most commonly as alate-stage crystallization product especially inamygdales, whereas in sedimentary rocks it is acommon detrital component, often associated withclay minerals in a mixed-layer structure.

Varietal names are related to chemical composi-tion and structure, the scheme presented by Hey(1954) recognizing the impor tance of the Fe/(Fe +Mg) ratio as well as the substitution of Si for A1in tetrahedral coordination. Many attempts havebeen made to relate the chemical composition ofchlorites to their optical properties, e.g. Winchell(1936), Hey (1954), and Albee (1962). Hey (1954)presented a two-dimensiona l scheme (see fig. 1)showing the relationships between refractive index,birefringence, density and composition, andseparating oxidized chlorites from orthochloriteson the basis of the Fe 20 3 content, which exceeds4 ~ in the former. A third axis to Hey s diagramto indicate Fe 3+ substitution in tetrahedral an doctahedral sites (cf. Nelson and Roy, 1958, p. 721)would demonstrate the relationship between thefour main compositional series listed by Phillipsand Griffen (1981) viz.:

Amesite Antigori te seriesPseudothuringi te Brunsvigite seriesKlement ite Delessite seriesThuringit e Strigovite series.Albee preferred to discard varietal names and tobase routine petrographic identifications on opticsign and interference colours relating these toFe/(Fe + Mg) ratios (Table I). He recognized, how-ever, that chlorites in ultramafic rocks, andamygdales and veins may depart from his general-izations. Deer e t a l (1966) further commented tha tpleochroism is generally exhibited more stronglyby chlorites with higher iron content.A consequence of Hey s scheme is that theincrease in Fe/(Fe + Mg) ratio and oxidat ion stateis accompanied by an increase in refractive indexand a general increase in absorpt ion (pleochroism)from colourless (Mg-Al-rich varieties) to green(Fe-rich members); those displaying anomalousinterference colours being those with extremely lowbirefringencewhere e -~ o. Aluminous chlorites canalso be identified in terms of the scheme. Albee sscheme is useful for general purposes when dealingwith marie and ultr amafic assemblages and though

C opyr igh t the M iner a log ica l Soc ie t y

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470

Fe TotalF e M g

1 . 0

0 . 8

0 - 6

0 4

0 - 2

E . P . S A G G E R S O N A N D L . M . T U R N E RO x i d i s e d C h l o r i t e s

l

e ,~ s

0

p s S Ss

5 0

o . _p sOg _

c

5 6

w C .\ / 2 :

= \~ / ~ _ \ \, 0 ~ lk 0 0~ _ g \ x . ~ ?

6.9 7 0 8.0

1 2

1 0

8

6

0 04 0

t~k kmOF--

SiFIG. 1. Relationship between op tical prope rties and chemical comp osit ion in th e chlorites ( f rom Hey, 1954).

h e r e c o g n iz e s th e i m p o r t a n c e o f t h e A I c o n t e n t o fc h l o r i t e s a s a r e f l e c t io n o f t h e i r r o c k a s s o c i a t i o n ,i t i s n o t p r o v i d e d f o r i n h i s g e n e r a l i z e d o p t i c a ls c h e m e ( T a b l e I ), w h e r e o n l y a m e d i a l v a l u e o f 2. 6t o 2 . 7 a t o m s p e r 1 0 c a t i o n s f o r A1 i s u s e d . I n o r d e rt o a c c o m m o d a t e t h i s l i m i t a t i o n , w e h a v e a t t e m p t e dt o c o m b i n e t h e m a i n f e a t u r es o f b o t h H e y ' s a n dA l b e e ' s s c h e m e s w i t h t h e r e v i se d n o m e n c l a t u r e o fP h i l l i p s a n d G r i f f e n (1 98 1) i n t o f ig . 2 a n d T a b l e I If r o m w h i c h t h e f o l l o w i n g g e n e r a l i z a t i o n s c a n b em a d e .1 . C o l o u r l e s s c h l o r i t e s w i l l b e M g - r i c h v a r i e t ie s .G r e e n c h l o r i t e s w i l l b e F e - r i c h v a r i e t ie s .2 . C h l o r i t e s w i t h v e r y l o w r e l i e f w i l l b e M g - r i c h ,A l - p o o r v a r i e t i e s .

C h l o r i t e w i t h r e l a t i v e l y h i g h r e l i e fw i l l b e F e - r i c hva r i e t i e s .3 . G r e e n c h l o r it e s w i t h a n o m a l o u s b r o w n i n t e r-f e re n c e c o l o u r s a r e l i k e l y t o b e p y c n o c h l o r i t e( b a s i c r o c k s ) o r p r o c h l o r i t e ( r i p i d o l i t e ) ( p e l i ti crocks) .4 . G r e e n c h l o r it e s w i t h a n o m a l o u s b l u e i n t e r -f e re n c e c o l o u r s a r e l i k e l y t o b e a n t i g o r i t e( t a l c - c h l o r i t e ) , t a l c - c h l o r i t e , d i a b a n t i t e , o rb r u n s v i g i t e .5 . C h l o r i t e s w i t h h i g h f i r s t - o r d e r i n t e r f e r e n c ec o l o u r s w i l l b e M g - A l - r i c h v a r ie t i e s .6 . A s s t a t e d b y D e e r e t a l (1966, p. 237) , ' thed i v i s io n o f t h e m a i n t y p e s o f c h l o r it e a c c o r d -i n g t o o p t i c s i g n p u t s p e n n i n e , c l i n o c h l o r e ,

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C H L O R I T E I D E N T I F I C A T I O N 471p r o c h l o r i t e ( r i p i d o l i t e ) , s h e r i d a n i t e , a n dc o r u n d o p h i l i t e a s p o s i t iv e , a n d d i a b a n t i t e ,b r u n s v i g i t e , t h u r i n g i t e , c h a m o s i t e , a n d d e l e s -s i t e a s n e g a t i v e .I n t h e w r i t e r s e x p e r i e n c e , c h l o r i t e s d i s p l a y i n g

a n o m a l o u s b l u e i n t er f e re n c e c o l o u r s a re c o m m o n l yc l a s s i f i e d a s p e n n i n e . F i g . 2 i n d i c a t e s t h a t t h i s i s

n o t n e c e s s a r i l y t h e c a s e, a s c h l o r i te s o f v a r i o u sc o m p o s i t i o n s m a y e x h i b i t t h is o p t ic a l p h e n o m e n o na n d t h i s i s s u b s t a n t i a t e d b y E P M A a n a l y s i s ( p er s .c o m m . f r o m r e f e r e e ) . F r o m o u r o w n t h i n - s e c t i o ns t u d i e s i t h a s t h u s b e e n p o s s i b l e t o e s t i m a t e t h a ta c h l o ri t e w i t h a n o m a l o u s b r o w n i n t e r f er e n c ec o l o u r s , o p t i c a l l y p o s i t i v e s i g n R . I . > 1.6 3 a n d

T A B L E I . Optical properties and chemical compo sition o f thechlorites after Albee, 1962)C o m p o s it io n M g M g F e F e - M g F e

n o r m a l a n o m a l o u s a n o m a l o u s n o r m a lb r o w n b l u e2 V ( + ) ( + ) ( - - ) ( - - )Orie nta t ion fast fast s low slowR.I. 1.56-1.60 1.60-1.63 1.63-1.65 1.65-1.69

T A B L E I I . Optical properties, chemical compo sition, and roc k associations o f the chlorites based onHey, 1954; Albee, 1962; Deer et a l . 1966)Colour R . I. Or i en t a t i on 2V In t e rfe rence 5 Com pos i t i on Co mm on rockcolours associa t ions

2o

e~

..=

r

t

O

1.65-1.73 slow (-- ) norm al > 002 Si ,AI Fe

s l o w - )s l o w - )

norma l 002-007 S ia n o m a l o u sb lue1.62 1 .65 < 002 A1 Fe /M g

1.59-1.63

fast ( + ) anom alousb r o w nfast ( + ) norm als lo w ( - ) a n o m a l o u sb luefast ( + ) anom alousb r o w nfas t (+ ) no rm a ls low ( - - ) anoma lousbluefast ( + ) anam olousb r o w nfast ( + ) nor ma ls lo w ( - ) n o r m a l

20 60~

Fe-sediments,ironstones, ores,pegmatites, veins,amygda l e s

1.56-1.60

1.55-1.59

002-010

Fe-pel i tes , grani toids,quar tzofe ldspathicgneisses

< 002

002-010

Si

A1 M g / F e Basic, peli t ic

< 002 Si

002-016 AIM g

Ult rabasic , carbonates,calc-silicates

Basic000-008

S i M gUlt rabasic , carbonates,calc-silicates

s lo w ( - ) a n o m a l o u s < 0 02b lue

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472

Fe2 0 3> 4 %

1 0

Fe

Total FeF e + M g F e / M g

0-5M g / F e

M g

E . P . S A G G E R S O N A N D L . M . T U R N E RO x i d i s e d C h l o r i t e s

C r o n s t e d l

Thuringite

\ 0 ~

K

. X O

30

~ . ' 0 9

\ \f~

0 I -34 5-0 5 6 6-2 7.0 8.0Si /AI in te t ra hed ra l s i tes

EcoOx..

OO

13

Ot3~E Tota l Fe

noc tahedra l~ s i t e s

.m

0

0-80

FlG. 2. Relationship between optical properties and chemical composition in the chlorites after Hey, 1954, Albee,1962, and Phillips an d G riffen (1981). A = Albee s medial value of 2.6 to 2.7 atoms per 10 cations fo r A1 used. S hadedarea - field of anomalous interference colours. Note that chlorites in the field to the left of the line e- o = 0 arepositive and those to the right are negative.

s t rong g re e n p l e oc h ro i sm i s mos t p roba b l y a nF e -Al - r i c h c h l o r i t e (a s ma y be found i n a s soc i a t i onwi th ga rne ts , cordie r i te s , and bio t i te s in pe l i t i croc ks ) a nd not a Mg-F e c h l o r i t e a s sugge s t e d byAl be e s s c he me (Ta b l e I ). On t he o t he r ha nd i nt e rms o f fi g. 2 , me t a mo rph ose d u l t r a ma f i c roc ksm a y b e e x p e c t e d t o c o n t a i n M - p o o r , M g - S i - r i c hc h l o r i te s wi t h a no ma l o us b l ue i n t e r fere nc e c o l ou r ,nega t iv e s ign and R. I . < 1 .60, whereas M g-A l-r ichchlor i te s show higher in te rfe rence colours . Di ffe r-ences in R.I. (re l ief) bet we en 1.65 and 1.55 (M ~ L)can be d i s t inguished in th in sec t ion wi th a t ra inede ye, a s c a n t he i n t e ns i t y o f g re e n c o l ou r a ndp l e oc h ro i sm. F o r i n t e rme d i a t e va l ue s a nd t he ox i -

d i z e d e qu i va l e n t s o f o r t ho ( fe rrous ) -c h l o ri t e s t hed i s t i nc t i on i s more d i f f i c u l t a nd re c ourse t o moresoph i s t i c a t e d me t hods ma y be ne c e s sa ry . Add i -t i ona l p rope r t i e s suc h a s po l ysyn t he t i c t wi nn i ngand inc l ined ext inc t ion may a l so be use ful inde l imi t ing chlor i te va r ie t ie s , e .g . c l inochlore .The re i s no s i mp l e wa y o f i de n t i fy i ng t he c h l o r i t e sa nd t he u se o f s i mp l e op t i c a l me t hods i s f r a ugh twi t h unc e r t a in t y . T he r e c ogn i t i on o f t he a pp rox i -mat e chem ica l va r ie t ie s of chlor i te and c lass ify ingt he m by na me c a n be a c c ompl i she d p rov i d i ng c a rei s g iven to th e reco gni t io n o f colour , re lie f, andbi re fr ingence , a s we l l a s the rock and minera la s soc ia t i on . Al be e s s c he me ba se d on Mg / F e ra t i o s

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CHLORITE IDE NTI FIC ATI ON 473is useful for general purposes when dealing withmetamorphosed basic rocks bu t the impo rtance ofA1/Si ratios as given by Hey are neverthelessrelevant and cannot be ignored in the opticalidentification of common chlorites.

REFERENCES

Hey, M. H. 1954)Miner a l . Mag . 30, 277-92.Nelson, B. W., and Roy, R. 1958) Am. Miner a l . 43,707-25.Phillips, W. R. 1964)Miner a l . Mag . 33, 1114-24.- - a n d Griffen, D. T. 1981)Opt ica l Miner a logy . Free-man, San Francisco, 677 pp.Winchell, A. N. 1936)Am. Miner a l . 21, 642-51.

Albee, A. L. 1962)Am. Miner a l . 47, 851-70.Deer, W. A., Howie, R. A., and Zussman, J. 1966) A nIn t r oduc t ion to the Rock For ming M iner a l s . Longrnans,London, 528 pp. [Manuscr ipt rece ived 10 September 1981;rev ised 6 November 1981]