Natural Alteration of Mica and Reactions Between Released Ions in Mineral Deposits

7/24/2019 Natural Alteration of Mica and Reactions Between Released Ions in Mineral Deposits

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Clays and Clay Minerals Vol. 23, pp. 247 255. Pergamon Press 1975. Printed in Great Britain

N T U R L L T E R T I O N O F M I C N D R E C T I O N S

B E T W E E N R E L E S E D I O N S I N M I N E R L

D E P O S I T S

J. RIMSAITE

Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A OE8, Canada

Received 3 October 1973)

Abstract--Chemical changes during the natural alteration of micas were studied by electron microprobe

and classical chemical analyses of fresh and altered portions of mica flakes from 10 Canadian mineral

deposits. Results of 50 new analyses are discussed in five examples, starting from simple changes

in the interlayer followed by exsolution of titania and ending with complex replacements of anions

and cations in all layers of the mica structure. Alteration of micas starts along 001 cleavage planes

and fractures and gradually extends into the entire flake leaving some or no remnants of the host

mica. The removal of ions from the mica structure and from the flake takes place by gradual depletion,

by exsolution of oxides, and/or by alternating removal and redeposition of a newly-formed oxide,

illustrated in the following example of the removal of Ti;

1) removal of Ti from the mica structure and exsolution of rutile in the parent mica;

2) destruction of rutile and recrystallization as anatase on the surface of the mica; and

3) destruction of anatase, removal of Ti from the surface of the host and crystallization of anatase

away from the parent mica.

Residual minerals replacing the original micas are secondary and recrystallized micas, chlorite, vermi-

culite, serpentine, talc and depleted, optically amorphous flakes. Newly-formed minerals which may

incorporate the released ions into their structures are rutile, anatase, sericite, hydronepheline, chlorite-

serpentine aggregates, goethite and jarosite. The following partly-altered micas are indicative of sulphide

mineralization;

a) bright-green, partly altered phlogopite from- ultrabasic rocks and from Co-Cu-Fe-Ni sulphide

assemblages. The green phlogopite formed by replacement of K, A1 and Ti by Fe, Mg and OH;

b) depleted mica coated with jarosite and Ni-goethite in the oxidation zone of a nickel deposit.

The jarosite formed from released S and Fe from decomposed Fe-Ni sulphides and K released from

the host mica; and

c) chlorite-sericite-biotite alteration zones adjacent to Pb -Z n-Cu -F e deposits.

IN TRO D U CTIO N

With increased information on proton losses from the

mica structure in relation to controlled pH of solutions

used in experimental alterations Mehmel, 1938;

Farmer et al., 1971; Hoda and Hood, 1972; Robert

and Pedro, 1972), it is interesting to compare the

chemical trends of natural mica alteration. Micas are

common hosts for ore minerals, and the mica layer

adjacent to the ore is commonly altered to chlorite.

The replacement of micas by ore minerals is in part

mechanical, where ore minerals grow into and poss-

ibly promote the cleavage an d fractures of mica Fig.

la) and/or chemical, where the silicate mineral is

gradually replaced by ore minerals Figs. lb and If).

Electron microprobe a nd wet chemical analyses of

fresh mica and its alteration products were carried

out on abo ut 50 minerals from 10 mineral deposits

in an attempt to study natural alteration of micas

in mineralized ultrabasic, alealic and granitic rocks.

The ultrabasic rocks from the three Cu-Ni deposits

are partly to completely altered to serpentinites and

characterized by low concentrations of Si, A1 and

alkalis, and the absence of feldspars. I n these rocks,

phlogopits is the principal carrier of AI and alkalis.

basic and grani tic rocks, and two in alkalic rocks a

nepheline syenite and a syenite). Two sulphide depo-

sits occur in granitic gneisses which contain mica

bands in part replaced by Fe-Ni-Cu sulphides. In

these deposits, micas adjacent to sulphides are altered

to chlorite. One granitic gneiss contains disseminated

Pb-Zn-Cu-Fe sulphides and is markedly altered to

chlorite and kaolinite. The alteration of biotite Fig.

ld) is probably hydrothermal, but some kaolinite

might have formed under present or past weathering

conditions. Three of the deposits contain prominent

oxidized areas. The oxidized rocks vary from compac t

to crumbly fragments and d issintegra ting gossan soils.

Oxidation may have begun along fractures at depth

during hydrothermal alteration and continued after

exposure to surface weathering. All specimens studied

are from compact Precambrian rocks collected

mainly undergro und and from newly made exposures

in the mini ng quarries.

SP~ECIMENS AND ANALYSES

Specimens

Polished thin sections of mica-bearing rocks from

Two deposits are at the contact between altered ultra- mineral deposits were examined under a petrographic

247


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248 J. RIMSAITE

m i c r o s c o p e a n d s p e c i m e n s c o n t a i n i n g p a r t l y a l t e r e d

m i c a s w e r e s e l e c t e d fo r e l e c t r o n m i c r o p r o b e a n a l y s i s

a n d f o r m i n e r a l c o n c e n t r a t io n . M i c a i n t e r g ro w t h s a n d

a l t e r e d m i c a s a l o n g r o c k f r a c t u r e s w e r e s e p a r a t e d

f r o m t h e r e m a i n i n g u n a l t e r e d p o r t i o n s b y c r u s h i n g

a n d h a n d - p i c k i n g u n d e r t h e m i c r o s c o p e . I n c o a r s e -

g ra in ed sp ec i m en s , t h e su r f ace l ay er i n t e rg ro wn o r

c o a t e d w i t h o r e m i n e r a l s w a s s e p a r a t e d b y c l e a v i n g

w i t h a r a z o r b l a d e a n d t h e o r e m i n e r a l s w e r e s e p a r -

a t e d f r o m t h e m i c a l a y e r b y r e p e a t e d c r u s h i n g a n d

s c r ee n i ng . T h e o r e m i n e r a l s b r e a k m o r e e a s i l y t h a n

th e m ica an d w ere co l l ec t ed i n t h e f i n es t f rac t i o n , l e s s

t h an 3 0 /~m d i a . F in a l sep ara t i o n o f d i f f e r en t f r esh

m i c a s p e c ie s fr o m t h e m i c a s i n v a r i o u s s t a g e s o f a l t e r -

a t i o n , a n d f r o m f l a ke s w i t h a d h e r i n g o r e m i n e r a l s w a s

m a d e i n h e a v y l iq u i d s a n d w i t h a F r a n t z i s o d y n a m i c

s e p a r a t o r u s i n g p r o c e d u r e s d e s c r i b e d b y R i m s a i t e

(1 9 6 7 b ) . Al l f r ac t i o n s o f t h e co n cen t r a t es were ex am -

i n e d u n d e r a p e t r o g r a p h i c m i c r o s c o p e a n d f r o m

sev era l h u n d red i n t e rm e d i a t e f r ac t i o n s , t wo t o f i v e

f r a c t i o n s h a v i n g c h a r a c t e r i s t i c h o m o g e n e o u s o p t i c a l

p r o p e r t i e s a n d n a r r o w r a n g e s o f s p e ci fi c g r a v i t y a n d

m a g n e t i c s u s c e p t i b i l i ty w e r e c h o s e n f o r x - r a y a n d

ch em ica l an a l y ses .

X - r a y a n a l y s e s

T o d e t e r m i n e t h e p r o p o r t i o n s o f f r e s h m i c a a n d

r e s i d u a l a l t e r a t i o n p r o d u c t s i n p a r t l y a l t e r e d s a m p l e s ,

a l l co n c en t r a t es were x - r ay ed wi th a P h i l i p s d i f -

f r a c t o m e t e r u s i n g N i - f i l t e re d C u r a d i a t i o n a t 4 5 k V

a n d 1 6 m A o n 0 0 1 - o r ie n t e d p o w d e r s a m p l e s p a c k e d

in to sh a l l o w t r ay h o l d e r s . S i n g l e - c ry s t a l We i s sen b e rg

p h o t o g r a p h s o f f re s h a n d a l t e r e d p o r t i o n s o f m i c a a r e

a l so b e in g m ad e t o d e t e rm i n e s t ru c tu ra l ch an g es .

C hem ica l ana l y ses

C o m p l e t e c h e m i c a l a n a l y s e s a n d p a r t i a l a n a l y s e s

f o r H 2 0 , F , C 1 , F e O a n d F e 2 0 3 w e r e m a d e o f 1 7

f r e s h a n d a l t e r e d m i c a s u s i n g p r o c e d u r e s d e s c r i b e d

b y M ax w el l ( 1 96 8 ).

E lec t ron m icroprobe ana ly ses

T h e h o m o g e n e i t y a n d d i s t r i b u t io n o f s e l e c t ed e le -

m e n t s i n p a r t l y a l t e r e d m i c a s a n d i n f i n e - g r a i n e d

a l t e r a t i o n p r o d u c t s w e r e e x a m i n e d b y e l e c t r o n s c a n -

n i n g m i c r o g r a p h s .

S e l ec t ed e l em en t s in h e t e ro g en eo u s sp ec im en s were

s e m i - q u a n t i t a t iv e l y a n a l y s e d u s i n g a n e n e r g y d i s p e r -

s iv e s p e c t ro m e t e r d e s c r i b e d b y L a c h a n c e a n d P l a n t

(1 9 7 3 ) . To t a l i r o n d e t e rm in ed b y an e l ec t ro n m ic ro -

p r o b e i s r e p o r t e d a s F e O . W a t e r c o n t e n t , r e p o r t e d

i n b r a c k e t s , w a s o b t a i n e d b y s u b t r a c t i n g f r o m 1 00

t h e s u m o f o xi d e s d e t e rm i n e d b y e l e c t r o n m i c r o p r o b e .

T en ta t i ve s t ruc tura l f o rm u lae

M o s t o f t h e i n t e r m e d i a t e a l t e r a t i o n p r o d u c t s c o n -

s i s t o f s u b - m i c r o s c o p i c i n te r g r o w t h s o r m i x t u r e s o f

f r esh an d a l t e r ed m i n era l s . Th e t en t a t i v e s t ru c tu ra l

fo rm u lae were ca l cu l a t ed fo r t h e es t i m a t ed f r esh an d

a l t e r e d c o m p o n e n t s b y s u b t r a c t i n g p r o p o r t i o n s o f t h e

f r es h c o m p o n e n t f r o m t h e a l t e r e d o n e a n d vice versa.

x - r a y p o w d e r d i f f r a c t i o n r e s u l t s a n d / o r p o t a s s i u m

c o n c e n t r a t i o n s w e r e u s e d t o e s t i m a t e p r o p o r t i o n s o f

f r esh m ica i n a p a r t l y a l t e r ed sp ec im en . Th e p e rcen t -

ag e o f fr esh m ica co m p o n e n t i s g iv en fo r each a l t e r -

a t i o n p r o d u c t a n d w a s s u b t r a c t e d f r o m t h e p a r t l y

a l t e r ed sp ec im en s b e fo re ca l cu l a t i n g s t ru c tu ra l f o rm u -

l ae fo r t h e a l t e r a t i o n p ro d u c t s ; t h u s 'A-3 0 ~o F ' i n d i -

ca t es t h a t 303~o o f f r esh co m p o n en t 'F ' wa s su b t r a c t ed

f ro m th e an a ly s i s t o o b t a in t h e s t ru c tu ra l f o rm u la fo r

t h e a l te r e d c o m p o n e n t ' A ' . B e c a u s e m o s t s p e c i m e n s

r e p r e s e n t in g i n t e r m e d i a t e s t a g e s o f a l t e r a t i o n a r e s u b -

m i c r o s c o p i c m i x t u r e s o f tw o o r m o r e s t r u c t u r a l

p h a s e s , a n d b e c a u s e w a t e r i n s o m e s p e c i m e n s w a s n o t

an a ly sed , t h e s t ru c tu ra l f o rm u lae a r e t en t a t i v e .

T h e s t r u c t u r a l f o r m u l a e a r e b a s e d o n 4 4 p o s i t i v e

an d n eg a t i v e ch arg es (v a l en c i es ) p e r u n i t ce l l fo r m o s t

m icas , v e rm icu l i t e an d fo r t a le , an d o n 5 6 v a l en c i es

f o r re s i d u a l c h lo r i te s . T h a t s o m e a l t e r a t i o n p r o d u c t s

a r e p r o b a b l y m i x t u r e s o f s e v e r al ph a s e s , s u c h a s s e r -

p en t i n e , t a l c , b ru c i t e an d o x id es , i s ev id en t f ro m th e

e x c e s s o f t e t r a h e d r a l s i l i c o n a n d o f o c t a h e d r a l c o n -

ten ts in a few specimens.

Ion i c percen tages

Becau se m icas an d ch lo r i t e s h av e d i f f e r en t sp ec i f i c

g r a v i t ie s a n d d i ff e r en t n u m b e r s o f c a t i o n s a n d a n i o n s

p er u n i t ce ll , a d i r ec t co m p ar i so n b e twee n t h ese

m i n e r a l s c a n n o t b e m a d e f r o m t h e i r c h e m i c a l a n a l y s e s

o r s t ru c tu ra l f o rm u lae . Th e i o n i c p e rcen t ag es , h o w-

e v e r, p r o v i d e t h e l e a s t d i s t o r t e d i m a g e o f c h e m i c a l

t r e n d s d u r i n g a l t e r a t i o n r e a c t i o n s i n v o l v i n g m i n e r a l s

wi th d i f f e r en t c ry s t a l s t ru c tu res an d sh o w d i r ec t l y

l o s se s a n d g a i n s o f a n i o n s a n d c a t i o n s i n t h e i n t e r-

l a y e r , i n t h e t e t r a h e d r a l a n d o c t a h e d r a l l a y e r s , a n d

i n th e O , O H - s h e e t s . E x a m p l e s o f n a t u r a l a n i o n i c a n d

c a t i o n i c c h a n g e s d u r i n g a l t e r a t i o n o f m i c a s a r e g i v e n

i n T a b l e 1 . T a b l e s c o n t a i n i n g e l e c t r o n m i c r o p r o b e

a n a l y s e s, s t r u c tu r a l f o r m u l a s o f m i c a s a n d i n t e r m e -

d i a t e a l te r a t i o n p r o d u c t s a n d c a l c u l a t e d i o n ic p r o p o r -

t i o n s a r e a v a i l a b l e f r o m t h e a u t h o r u p o n r e q u e s t.

E X M P L E I

L osses o f i n t e r layer ca t i ons and a l t e ra t i on o f b io t i t e

to c hlo rite alorgt, O01-fi aetures

S p ec im en s were co l l ec t ed f ro m a b recc i a t ed o x i -

d a t i o n z o n e i n a C o - N i d e p o s i t n e a r t h e s o u t h e a s t e r n

e n d o f t h e W e r n e r L a k e , c l o se t o th e O n t a r i o - M a n i -

t o b a b o u n d a r y . T h e W e r n e r L a k e m i n i n g a r e a , d e -

sc r i b ed b y Ca r l so n (1 9 58 ), i s in t h e S u p er io r P ro v in ce

o f t h e C a n a d i a n S h i el d . M a s s i v e a n d d i s s e m i n a t e d F e -

C u - C o - N i s u lp h i de s a n d c o b a l ti t e o c c u r a t t h e c o n -

t a c t b e t w e e n s e r p e n t i n i z e d u l t r a b a s i c r o c k s a n d p a r -

a g n e i ss e s a n d i n b r e c c i a t e d z o n es . T h e l o w - t e m p e r a -

t u re a l t e r a t i o n , se rp en t i n i za t i o n , ch lo r i t i za t i o n , an d

o x i d a t i o n o f t he s e A r c h e a n r o c k s s t a r t e d p r o b a b l y u n -

d e r h y d r o t h e r m a l c o n d i t i o n s , b u t t h e e x t e n s i v e o x i -

d a t i o n a n d f o r m a t i o n o f g o s s a n s a l o n g r o c k f r a c t ur e s

a n d d i s i n t e g r a ti o n o f t h e o x i d i z e d f ra g m e n t s i n t h e

s o i l i s a t t r i b u t e d t o w e a t h e r i n g. T h e m i n e r a l i z e d p a r -

ag n e i s s se l ec t ed fo r t h is s t u d y co n s i s t s o f q u ar t z , g a r -


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Natural alteration of mica 249

Table 1. Examples of chemical changes during alt%ation o f biotite.

Example lib

RT-18 biotite

Jarisite

Weight Fresh coated

(%) (A) (Y)

Example IVa Example V

R M- I 0 biotite-sagenite-residual chlorite Phlogopite-vermiculite

Sagenitic

Fresh biotite biotite Residual chlorite Phlogopite Vermiculite Tale

A (C.A .) (E.M.A.) B (E.M.A.) C (C .A .) (E.M .A.) (Ph) (V) (T)

SiO 2 36 9 69 5

AI203 17'8 1'8

TiOz 2 5 0 4

FezO ~ 2 0 7 5

FeO 13 8 2 9

MgO 13 0 2 0

MnO 0.2 0.0

Cr2Os 0'3 0'0

NiO 0.1 0.0

CaO 0 1 0'2

Na 20 0.5 0.1

K20 9-6 0.6

H2 0 2'7 15 9

F 0 32 0 05

CI 0 01 0 05

Rb ppm 500 40

Sr ppm


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250 J. R1MSAII E

o r l e s s l o s ses o f Rb f ro m th e i n t e r l ay er , an d o f S i

an d A1 f ro m th e t e t r ah ed ra l l ay e r . Th e a l t e r ed su r f ace

l a y e r g a i n e d w a t e r, i r o n a n d a p p a r e n t l y s o m e m a g n e -

s iu m. T h e h em at i t e - r i ch c ru s t s co n t a in 5 0 p e r cen t

m o r e N i a n d Z n t h a n t h e h o s t m i c a. T h is e x a m p l e

p r o v i d e s e v i d e n c e o f th e r e m o v a l o f c a t i o n s f r om t h e

i n t e r la y e r a n d f r o m t h e t e t r a h e d r a l a n d o c t a h e d r a l

l a y e r s , a c c o m p a n i e d b y a n a p p a r e n t i n c r e a s e o f F e

a n d M g , a n d o n t h e l o s s o f f l uo r i ne f r o m t h e O , O H -

s h e e ts d u r i n g a l t e r a t i o n o f p h l e g e p it e u n d e r t h e h e m a -

t i t e c r u s t . T h e r e p l a c e m e n t o f t h e p h l o g o p i t e b y

h emat i t e s t a r t ed o n t h e su r f ace an d a lo n g t h e 0 0 1

c l e a v a g e p l a n e s . M e c h a n i c a l s e p a r a t i o n o f t h e t w o

mi n e ra l s i s n o t p o ss ib l e . Th e s t ro n g ad h e ren ce o f t h e

i ro n o x id e (o r h y d ro x i d e) t o t h e su r f ace o f ch l o r i t i zed

m i c a l a y e r m a y b e m e c h a n i c a l a n d / o r c h e m i c a l , a n d

ma y p o ss ib ly i n v o lv e io n i c b o n d i n g b e t we en t h e i r su r -

face layers .

l ib . A l tera t ion o f b io t i te under jarosi te and Ni-

9oethite crusts. M i n e r a l i z e d b i o t i t e - r i c h l a y e r s i n

s c h is t s o f t he T h o m p s o n N i c k e l B e l t, M a n i t o b a , ( Z u r -

b r i g g , 1 9 6 3) , p ro v i d e an i n t e r es t i n g ex amp l e o f a l t e r -

a t i o n o f b i o t i t e t o o p t i c a l l y - a m o r p h o u s f la k e s a n d o f

b i o t i te r e p l a c e m e n t b y j a r o s i t e a n d ' N i - g o e t h i t e ' (F i g .

l f , RT -1 8 ) . I n f r esh p o r t i o n s o f t h e ro ck , t h e b io t i t e

i s en t i r e l y f r esh ( 'A ' i n T ab l e 1 ); o n l y i n o x i d i zed p o r -

t i o n s o f t h e ro ck d o es t h e b io t i t e ex h ib i t v a r i o u s

d eg rees o f a l t e r a t i o n , t h e reb y su g g es t i n g t h a t d ec o m -

p o s i t i o n a n d o x i d a t i o n o f s u l p h id e s a n d a l t e r a t i o n o f

t h e b i o t i t e a r e r e l a t ed . I n o x id i zed a r eas o f t h e sch i s t,

t h e b io t i t e co n s i s t s o f a l t e rn a t i n g f r esh an d a l t e r ed

b a n d s , l o c a l l y r e p la c e d b y ' g o e t h i te ' a n d j a r o s i te .

S m a l l t a b l e t s a n d s p e c k s o f j a r o s i t e ( 1 - 3 # m d i a .) c r y s-

t a l l iz e d o n t h e s u r f a c e a n d a l o n g b a s a l c l e a v a g e

p l an es o f t h e a l t e r ed b io t i t e . S o me o f t h e b io t i t e f l ak es

h a v e u n d e r g o n e a l m o s t c o m p l e t e s t r u c t u r a l d e s t r u c -

t i o n ; t h e y a p p e a r d i s c o l o u r e d a n d a m o r p h o u s u n d e r

th e mi c ro sco p e . Ch emi ca l an a l y s i s (T ab l e 1 , RT-1 8

' Y ' ) o f t h e a m o r p h o u s f l a k e s i n d i c a t e s a p p a r e n t

en r i ch m en t o f S iO2 ( ab o u t 7 0 p e r cen t ) , an d d ep l e t i o n

o f th e o th e r co n s t i t u en t s . S in g l e - c ry s t a l W ei s sen b e rg

p a t t e rn s , x - r ay ed 5 0 0 h r , co n t a in v e ry weak (2 2 1 ) - r e -

f l ec t i o n s i n f e r r i n g s t r c tu ra l d es t ru c t i o n o f t h e a l t e r ed

f lakes .

T h e d e s t r u c t i o n a n d r e p l a c e m e n t o f b i o t i t e i n t h e

sch i s t h as t ak en p l ace i n s t ag es . A t t h e i n i t i a l s t ag e

o f a l t e r a t i o n , t h e b i o t i t e c o n s i s t s o f b r i g h t ( f resh) an d

d a r k ( a l te r e d ) b a n d s , w i t h t h e l a t t e r c o a t e d o r r e -

p l aced b y ' g o e th i t e ' . Th e ' g o e t h i t e ' co n t a i n s 1 4 ~o S iO2 ,

4~,0 A1 2 Os, an d 1 -8 ~o Ni ad j a cen t t o t h e r e mn an t b io -

t i t e (R imsa i t e , 1 9 73 a). Th e j a ro s i t e r e t a i n s m o s t o f t h e

p o t a s s i u m ( 8% K 2 0 ) a n d 5 0 ~ o o f t he r u b i d i u m

(30 0 p p m ) r e l eased f ro m th e a l t e r ed b io t i t e h o s t . T h e

r e p l a c e m e n t o f b i o t i t e b y j a r o s i t e a n d ' N i - g o e t h i t e '

s t a r t s a t t h e su r f ace an d a l o n ~ f r ac tu res o f t h e b io t i t e

g r a d u a l l y s p r e a d i n g i n t o a d j a c e n t p o r t i o n s o f a f la k e

t il l t h e m i c a i s e n t ir e l y r e p l a c e d b y c l a s ti c j a r o s i t ~

'g o e th i t e ' ag g reg a t es . T h e a l t e r ed ro ck c ru mb l es t o

smal l f r ag men t s an d d i s i n t eg ra t es i n t h e so i l . Th e

b r ig h t y e l l o w an d r ed p a t ch es i n t h e d i s i n t eg ra t i n g

r o c k a n d s o il , a n d r e d - s t a i n e d a r e a s w i t h o x i d e - c o a t e d

m i c a b a n d s a t t h e s u r f a c e a r e g o o d i n d i c a t o r s o f t h e

o ccu r r en ce o f su lp h id e min era l i za t i o n b e lo w th e su r -

face.

IIc . Iron -rich co atings o n the surface (?f phlogopite,

and incorporation o f the iron into the in terlaye r of resi-

dual chlorite. T h e a d s o r b e d a n d / o r b o n d e d h y d r o xi d e s

o n t h e su r f ace an d b e tween t h e 0 0 1 c l eav ag e p l an es

o f K - d e p l e t e d m i c a s a r e o f g r e a t i n t e r e s t in c o n n e c -

t i o n wi th t h e c ry s t a l l i za t i o n o f i r o n - r i ch ch lo r i t e s . Th e

i ro n h y d ro x id es ap p aren t l y can b e f i x ed i n t h e i n t e r -

l ay er o f t h e a l t e r ed mica t o fo rm ch lo r i t e wi th h ig h er

i ro n co n t en t s t h an t h a t i n th e h o s t mica .

S p e c i m e n R - C o - 7 , f r o m t h e o x i d a t i o n z o n e o f t h e

C o - N i d e p o s i t d i s c u s s e d i n E x a m p l e I , w a s c o l l e c t e d

a t t h e c o n t a c t b e tw e e n a p a r a g n e i s s a n d a m i n e r a l i z e d

b as i c ro ck . In t h i s co n t ac t r o ck , t h e o r i g in a l o l i v in e

i s en t i r e l y r ep l aced b y se rp en t i n e an d v e ry f i n e -

g r a i n e d m a g n e t i t e ; s m a l l r e m n a n t s o f p y r o x e n e a r e

e n c l o s e d in h o r n b l e n d e , a n d c o a r s e - g r a i n e d p h l o g o -

p i t e wi th g reen ch lo r i t i zed ed g es i s i n p a r t r ep l aced

a n d s u r r o u n d e d b y a c i c u l a r c h l o r i t ~ s e r p e n t i n e - t a l c

ag g reg a t es . Th e ro ck i s b r ecc i a t ed an d co n t a in s d i s se -

m i n a t e d s u l p h id e s a n d a f ew p a t c h e s o f c o a r s e -

g ra in e d ca l c i t e an d t r emo l i t e . A f ew b r ig h t -g ree n A1 -

sp in e l s en c lo se p h lo g o p i t e f l ak es p a r t l y a l t e r ed t o

ch lo r i t e . 'Go e th i t e ' c ry s t a l l i zed a lo n g 0 01 p l an es o f

s o m e m i c a s . T h e p u r p o s e s o f t h e e l e c t r o n m i c r o p r o b e

a n a l y s i s w a s t o d e t e r m i n e c h e m i c a l t r e n d s d u r i n g

m e t a m o r p h i c a l t e r a t i o n o f p h l o g o p i t e 'A ' t o t h e f o l-

l o win g r es id u a l ch lo r i t e s :

' C ' = d e p l e a t e d , c h l o r i ti z e d p h l o g o p i t e u n d e r

'g o e th i t e ' co a t i n g s ;

' D ' = g r e e n c h l o r i t iz e d e d g es o f t h e p h l o g o p i t e

a d j a c e n t t o c h l o r i t e - s e r p e n t i n e - t a l e a g g re g a t e s ,

f o rm e d i n M g - F e - r i c h e n v i r o n m e n t ;

' E ' = c h l o r i t iz e d p h l o g o p i t e e n c l os e d i n A l - sp i n e l,

f o r m e d i n A l - r i c h e n v i r o n m e n t ;

'F ' = ac i cu l a r f l ak es o f ch lo r i t e - t a l e ad j acen t t o su l -

p h id es . S t ru c tu ra l f o rmu lae o f t h e o r i g in a l p h lo g o p i t e

a n d o f re s i d u a l a l t e r a t io n p r o d u c t s s h o w c h e m i c a l d i f -

f e r en ces d ep en d in g o n d iv e r se en v i ro n m en t s o f c ry s -

t a l l i za t i o n . Th e f r esh p h lo g o p i t e 'A ' : (S i5 .2 A1 2 .8 )

(Alo.5 Tio.1 Feo.8 Mg4.v M no. ol) (Ca,Nao.o8 KI.86 )

(OH)s .8 (O)2o .1 co n t a in s r e l a t i v e ly l o w S i /A1 r a t i o i n

th e t e t r ah ed ra l l ay e r t h e reb y r e f l ec t i n g l o w co n -

cen t r a t i o n o f s i l ica i n t h e h o s t u l t r ab as i c ro c k . Th e

c h l o r i t e c o m p o n e n t ' C ' u n d e r t h e ' g o e t h i te ' c r us t ,

'C ' = (S is .s A12.2) (A12.7 Tio . l Fe l M gs M no .0 (OH)16

( 0 ) 2 o c o n t a i n s a s m a l l e r i o n i c p r o p o r t i o n o f S i a n d

m o r e M g a n d M n t h a n t h e h o s t p h l o g o p i t e . W h e n

th e ' g o e th i t e ' co a t i n g i s i n c lu d ed i n t h e s t ru c tu ra l f o r -

m u l a , i r o n c o n t e n t s o f t h e c h l o r i t e c o m p o n e n t s ' C ' ,

' D ' a n d ' E ' a r e s i m i l a r a n d a b o u t t w i c e a s h ig h a s

th e i ro n co n t en t i n t h e h o s t p h lo g o p i t e 'A ' . I ro n h y d -

ro x id es p rec ip i t a t e e i t h e r a s g o e th it e ' co a t i n g s o n t h e

su r f ace o f t h e p h lo g o p i t e ( 'C ' ) , o r a r e f i x ed i n t h e i n -

t e r la y e r o f th e K - d e p l e t e d p h l o g o p i t e t o f o r m g r e e n

ch lo r i t i zed ed g e 'D ' . S t ru c tu ra l f o rm u la fo r ch lo r i t e

c o m p o n e n t ' D ' : ( S i 6 A12) (A lv l F% .4 Mg6.1 Mn o.2)

(OH)2 o (O)1s i n d i ca t es a m ark e d i n c rease o f ir o n an d

man g a n ese . Th e r es id u a l ch lo r i t e 'E ' c ry s t a l l i zes i n A1 -


5/10

Na tural alteratio n of mica 251

Cr en v i ro n m en t . I t h as a r e l a t i v e ly h ig h Al - co n t en t ,

s imi l a r t o t h a t i n ch l o r i t e 'C ' , an d 0 3 ~o Cr i n t h e

s t ru c tu re . Th e Al - co n t en t d ec reases i n g r een ch lo r i -

t i zed ed g es o f t h e p h l o g o p i t e , r each in g t h e l o w es t co n -

cen t r a t i o n i n t h e ac i cu l a r f l ak es 'F ' wh ich a r e i n t e r -

g ro wn wi th su lp h id es . S t ru c t u ra l f o rmu lae ca l cu l a t ed

o n t h e b as i s o f 5 6 an d 4 4 v a l en c i es i n d i ca t e u n u su a l

c o m p o s i t i o n o f t h e t e t r a h e d r a l l a y e r o f f la k e s ' F ' , i n -

f e r r i n g t h e p resen ce o f ch lo r i t e - t a l c i n t e rg ro wt h s i n

a b o u t e q u a l p r o p o r t i o n s o f t h e t w o c o m p o n e n t s .

S t ru c t u ra l f o rm u la fo r t a lc co m p o n en t 'F ' : (S i8 ) (F e l .2

Mg4.6) (OH)5 .3 (O)19 .3 ind icates the p resence of i ron

in t h e t a l c s t ru c tu re wh i ch c ry s t a l l i zed i n an i ro n - r i ch

e n v i r o n m e n t a d j a c e n t t o p y r r h o ti t e . T h e E x a m p l e I I c

i l lu s t r a te s t h a t c h e m i c a l t r e n d s d u r i n g a l t e r a t i o n o f

m i c a a t t h e c o n t a c t b e t w e e n b a s i c a n d g r a n i t i c r oc k s ,

a n d c o m p o s i t i o n s o f r e s id u a l c h l o r i te s d e p e n d o n

l o c a l l y p r e v a i li n g e n v i r o n m e n t a l c o n d i t io n s .

l id . Pa rtly altered bri.qht-green phlo qopites in miner-

alized ultrabasic rocks.J?om Ni-deposits .

Br ig h t -g reen ,

p a r t l y c h l o r i ti z e d p h l o g o p i t e s o c c u r i n a s s o c i a t i o n

w i t h p y r r h o t i t e - p e n t l a n d i t e o r e i n M a r b r i d g e ( Q u e -

b e c ) a n d i n W e r n e r L a k e ( O n t a r i o ) C u - N i d e p o s i t s ,

a n d i n s e r p e n t i n i z e d u l t r a b a s i c r o c k s o f t h e T h o m p -

so n Nick e l Be l t (M an i to b a) . Becau se o f t h e i r f r eq u en t

a s s o c i a t i o n w i t h F e - N i s u l p hi d e s, t h e g r e e n p a r t l y

a l t e r ed p h lo g o p i t es a r e b e l i ev ed t o b e i n d i ca t i v e o f

F e - N i m i n e r a l i z a ti o n . I n t h e s e r p e n t i n i z e d u l tr a b a s i c

r o c k s , w h i c h a r e c h a r a c t e r i z e d b y l o w c o n c e n t r a -

t i o n s o f A1 an d S i , p h lo g o p i t e i s t h e so l e p r i mary

r o c k - f o r m i n g m i n e r a l c o n t a i n i n g a l u m i n a a n d

a lk a l i s .

G r e e n p h l o g o p i t e s w i th K - d e f i c i e n t c h l o r i ti z e d

ed g es i n sp ec i men TH-8 7 5 a r e en c l o sed i n f i n e -

g r a i n e d s e r p e n t i n e -c h l o r i te - t a l c a g g r e g a t e s w h i c h c o n -

t a in l e s s AI t h an t h e ch lo r i t e - se rp en t i n e - t a l c g ro u n d -

m a s s i n a c o n t a c t r o c k o f E x a m p l e I I e . A l t h o u g h t h e

i ro n co n t en t i n c rea ses i n a l l g r een p a r t l y a l t e r ed p h lo -

g o p i te s , th e i n t e r m e d i a t e p h a s e o f ' g o e t h i t e ' - c o a t e d

f l ak e s w a s n o t o b s e r v e d i n p h l o g o p i t e s T H - 8 7 5 . F r e s h

p h lo g o p i t e 'A ' , i t s g r een , ch lo r i t i zed p o r t i o n s 'B ' , an d

c h l o r i te - t a l c p s e u d o m o r p h s ' C ' c o n t a i n s i m i l a r c o n -

cen t r a t i o n s o f si l ica , b u t A1, T i , Cr an d K co n t en t s

m a r k e d l y d e c r e a s e f r o m ' A ' t o ' C ' , w h e r e a s F e a n d

M g c o n t e n t s i n c r e a s e . T e n t a t i v e s t r u c t u r a l f o r m u l a e

f o r s p e ci m e n ' B ' w e r e c a l c u l a t e d a s f o r m i c a a n d a s

fo r ch lo r i t e , a f t e r su b t r ac t i n g 6 0 p e r cen t o f t h e f r esh

c o m p o n e n t ' A ' . T h e p r o p o r t i o n s o f fr e sh m i c a a n d

c h l o r i te w e r e e s t i m a t e d f r o m x - r a y d i f fr a c t io n p a t t e r n s

b y measu r in g t h e i n t en s i t y r a t i o s o f t h e i r b asa l r e f l ec -

t i o n s (R imsa i t e , 1 9 6 7 b) . Th e r es i d u a l a l t e r a t i o n p ro d -

uct 'C ' con ta ins ve ry l i t t le A1203 (1 '2~o) an d i t s s t ruc -

t u ra l f o rmu la was ca l cu l a t ed as fo r ' t a l c ' , a f t e r su b -

t r ac t i n g 1 5 p e r cen t o f ch lo r i t e co mp o n en t 'B ' . Th e

f r esh p h lo g o p i t e 'A ' i s ch a rac t e r i zed b y l o w S i /A1

ra t io ; (S is . 2 A12.8) (A10.29 Tio . 1 Feo .4 Mgs.7 Cro .1

Nio .0 0 KI .~ (OH)3 .3 (O)1 9 + T h e p h lo g o p i t e f ro m th e

u l t r a b a s i c r o c k a s s o c i a t e d w i t h N i d e p o s i t s c o n ta i n s

Cr an d Ni i n t h e s t ru c t u re an d t h i s i s r e l eased d u r in g

i t s a l t e r a t i o n . Th e b r i g h t -g reen ed g es 'B ' co n t a in l e s s

K , A I, T i a n d C r, a n d m o r e O H , F e a n d M g t h a n

t h e h o s t p h l o g o p i t e . B e ca u s e o f t h e m a r k e d d e c r e a s e s

in Al -con ten ts ( f rom 17 .8 to 1 2~o A1203 in 'C ' ) , the

g r e e n e d g e s ' B ' a n d t h e u l t i m a t e a l t e r a t i o n p r o d u c t

ch lor i te- - ta lc 'C ' c on t ain insuff icien t S i an d A1 to f i l l

the te t r ah edr al po si t ion s: (S i5 .5 A12.r Tio.o6 Feo .o4)TM

(Feo .7 Mg6q Cro .06 Nio .00 K1.1 (OH)5

O ) 1 9 . 2

in

g reen , d ep l e t ed p h lo g o p i t e ed g es ca l cu l a t ed as 'mica ' ,

a n d i n t he c h l o r i te c o m p o n e n t ' B ' o f th e s a m e g r e e n

edges, ca lcu la ted as 'ch lo r i te ' : (S iT Al l ) (A12.4 Tio.ov

Fe l Mg8. 2 Cro .07 Nio .00 (OH)15 (0)20 .2 . T he s t ruc-

t u r a l f o r m u l a c a l c u l a t e d a s c h l o r i t e f o r s p e c i m e n

' C ' h a s a l s o u n u s u a l c o m p o s i t i o n o f t h e t e t r a h e d r a l

l ay er : (S i7 .1 Alo .3 Feo .JV(F eo .3 Mg12.4) (0H)1 3 . 6

(O)21.2.

Ile . Re actio ns indicating removal of silica at th e in-

itial s tages of alteration fo llow ed b y relative increases

at advanced stages o f alteration. S e r p e n t in i t e T H - 8 6 6

f r o m t h e T h o m p s o n N i c k e l B e lt , M a n i t o b a , w a s

c h o s e n t o s h o w v a r i a t i o n s in t h e c o n c e n t r a t i o n o f

s i li c a w i t h a d v a n c i n g s t a g e s o f a l t e r a ti o n . T h e b r i g h t-

g r e e n c h l o ri t i z e d p h l o g o p i t e i s a s s o c i a te d w i t h F e - N i

su lp h id es i n t h e se rp en t i n i zed u l t r ab as i c ro ck . Th i s

mica ex ib i t s t r en d s o f a l t e r a t i o n s imi l a r t o t h o se i n

Ex a mp le I I e . Th e M g in c reases f ro m 2 7 p e r cen t i n

f r esh mica 'A ' t o 4 4 -7 ~o i n r es id u a l a l t e r a t i o n p ro d u c t

9 o/

'D ' w i th d ec reas in g A1 , f ro m 1 4 '4 ~o i n 'A ' t o 0 2 / o

i n ' D ' . H o w e v e r , m i c a a n d i t s a l t e r a t i o n p r o d u c t s i n

T E - 8 6 6 d i f f e r i n t h e c o m p o s i t i o n o f t h e t e t r a h e d r a l

l a y e r a n d i n t h e m a r k e d d e c r e a se o f t h e s i l ic a c o n t e n t

i n t h e g reen ch lo r i t i ze d ed g e 'B ' ( f ro m 3 9 ~ i n 'A ' t o

3 0 ~ i n 'B ' ) . Th e f r esh p h lo g o p i t e 'A ' an d t h e r es id u a l

a l t e r a t i o n p ro d u c t s 'B ' , 'C ' an d 'D ' co n t a in i n su f fi c i en t

S i an d Al t o f i l l t h e t e t r ah ed ra l p o s i t i o n s . Th e g reen

co lo r o f t h e ch lo r i t i ze d ed g e 'B ' i s a t t r i b u t ed t o i t s

g r e a t e r F e a n d O H c o n t e n ts , d e c r e a s e o f T i , a n d t o

F e 2 0 3 / F e O b e i n g > 1. A l t h o u g h t h e S i c o n t e n t i n -

c r eases i n t a l c 'D ' , A1 i s a lmo s t ab sen t , t h u s p a r t o f

M g a n d a l l T i a n d F e w e r e p l a c e d i n th e t e t r a h e d r a l

l ay er t o f i l l t h e v acan t p o s i t i o n s . Th ree p o ss ib i l i t i e s

a r e s u g g e s t e d t o a c c o u n t f o r t h e u n u s u a l c o m p o s i -

t i o n s o f t h e t e t r ah ed ra l l ay e r o f t h e sh ee t s i l i ca t es i n

t h e u l t r a b a s i c r o c k s :

( 1 ) t h e t e t r a h e d r a l l a y e r h a s u n u s u a l c o m p o s i t i o n :

Si5.84 A10.o4 Fe0 .66 Mg v46 in R T-866 'D ' ;

( 2) t h e t e t r a h e d r a l l a y e r m a y h a v e u n o c c u p i e d p o s i -

t i o n s , an d t h e t o t a l ch a rg e (v a l enc i es ) mig h t h av e

c h a n g e d ;

(3) ad d i t i o n a l p h ases , su ch as b ru c i t e an d o th e r

o x i d e s a n d h y d r o x i d e s a r e p r e se n t .

T h e s u g g e s te d p o s s i b i l it i e s w a r r a n t s p e c i a l x - ra y

an d e l ec t ro n mic ro sco p e s t u d i es .

T h e c h l o r i t e ' C ' h a s u n u s u a l c o m p o s i t i o n o f b o t h

t e t r a h e d r a l a n d o c t a h e d r a l l ay e r s , w i t h a m a r k e d d e f i-

c i en cy o f A1 an d ex cess o f M g th a t i n f e r s th e p resen c e

o f b r u c it e i n s u b - m i c r o s c o p i c i n te r g r o w t h s . T h e u n -

c o m m o n A l - p o o r c h l o r i t e s a n d F e - r i c h t a l c v a r i e t i e s

o b s e r v e d i n t h e m i n e r a l i z e d u l t r a b a s i c r o c k s a l s o w a r -

r a n t m o r e d e t a i l e d a n a l y s es . T h e u l t r a b a s i c r o c k s

s t u d i e d h a v e n o s e c o n d a r y m i n e r a l s w h i c h c o u l d

a c c o m m o d a t e t h e K r e l e a s e d f r o m t h e a l t e r e d p h l o g o -

p i te .


6/10

252 J. RIMSAITE

E X A M P L E I l l

Transition o f biotite to lepidomelane b y replacement

o f octahedral A1, Ti and M g b y Fe, and secondary mica-

ceous alteration products in a nepheline deposit.

B rown b io t i t e , g reen l ep idome lane wi th va r i ab le

M g - c o n t e n t s a n d t h ei r o x i d a t i o n p r o d u c t s h a v e

a l ready been de s c r ibed by R ims a i t e (1967a ) and by

F a r m e r et al. (1971). La te r e l e c t ron m ic ropro be ana ly -

s e s o f the b io t i te a nd l ep idome lane ind ica ted loca l

va r i a t ions in M g-con ten t s , r e s u l t ing f rom remnan t s

o f b rown M g-bea r ing b io t i te . A new tep idome lane

concen t ra t e , con ta in ing l es s than 0 -1~o M g O, was p re -

pa red fo r chemica l ana lys i s . The l ep idome lane fo rms

f rom b io t i t e by ga ins o f i ron , a s do the g reen Fe - r i ch

ch to r i t i z ed edges and bands in ph logop i t e d i s cus s ed

in Example I I . In the l ep idome lane , how eve r , r ep lace -

me n t o f A1, T i an d M g t akes p lace in the oc tahedra l

l aye r w i thou t a l t e ra t ion to ch lo r i t e .

Origin of lepidomelane . The l ep idome lane occu rs a s

b lack to da rk g reen coa rs e c rys t a l s con ta in ing magne -

t it e inc lus ions s u r rounded by s econ da ry a lb i te in a

nephel ine syeni te (Fig . le ) . Th e difference in i ron con -

tent be tween the ear ly brow n biot i te A' and la te lepi-

dom e lane 'B ' r e s emble s tha t be tween g reen ch lo r i t i z ed

mica edges and the i r hos t ph logop i t e . The p re s ence

of i ron du r ing c rys t a l l iz a t ion o f the i ron - r i ch m ica s

i s in fe r red by the a s s oc ia t ion wi th m agne t i t e and wi th

Fe -N i s u lph ide s. S t ruc tu ra l fo rmulae o f b io t i te 'A ' :

3+ F e~.~ M gl .9 Mno.08)

S i s A 13) (A l l T i0 .23 Feo.4

(Ca, Nao.o7 Kl.82 ) (O Hz,88 Fo. 2 C10.2) 0 2 0 . 6 a n d o f

F o3+ Fe2.~lepido mela ne 'B ' (Si5 .2 A12.8) (Alo.7 Ti0 .o7 ,~0.v

Mg 0.ol M no .10 (Cao.ol K1.9) (OH3.3 Fo.oT) 020.3 in-

d ica te s ign i fi c an t r ep lacemen t s in the oc tah edra l

l aye r ; i ron and M n inc rea s e a t t he expense o f A1,

M g and Ti . C hanges in the OH,O-s hee t s a re a l s o im-

por t an t : t he newly - fo rmed l ep idome lane con ta ins

l i tt l e fluo r ine and no ch lo r ine , i n con t ra s t t o the ea r ly

b io t i t e wh ich con ta ins re l a t ive ly abundan t F and C 1 .

T h e o c c u p a n c y o f t h e i nt e r la y e r d o e s n o t c h a n g e

m u c h , b u t l e p i d o m e l a n e l o s e s N a a n d g a i n s R b

(0 .04% in 'A ' and 0 '7~o R b2 0 in 'B ') . Un l ike b r igh t -

g reen ox id ized edges in Example I I , t he l ep idome lane

has a de f i c i en t hydrox) l g roup , r e s u l t ing p robab ly

f rom ox id a t ion o f f e r rous i ron a t t he expens e o f hy -

d roxy l . Ho weve r , i t i s pos s ib le tha t t he Fe 3 rep lace s

A I3+ in the oc tahedra l l aye r ; t he re fo re the s t ruc tu ra l

fo rmula o f the lep idome lane w as ba s ed on 44 va len -

c ie s, a s s uming tha t t he to t a l num ber o f (+ ) cha rge

d id no t change .

Oxidation o f lepidomelane. Oxid ized l ep idome lane

'C ' fo rmed on the s u r face and a long ba s a l c l e avage

p lanes o f the h os t mica . T he ox id ized fe rr ic l ep idome-

lane s hows s imi la r va r i a t ions in M g-con ten t s a s thos e

obs e rved in the pa ren t l ep idome lane , The concen t ra t e

p r e p a r e d f o r an a l y si s c o n t a i n e d o n l y 0 ' 1 ~ M g O , a n d

i t s x - ray d i f f rac tion pa t t e rns ind ica ted no ch lo r i t e o r

verm icul i te impurit ies . Ho wev er , broa d basa l re flec-

t ions on x - ray s ing le -c rys t a l We is s enbe rg pa t t e rns ,

pos s ib ly due to ca. 2 pe r c en t ' s t rong ly ads o rbed

wa te r ' , de t e rmined by TG and i . r . analys e s, s ugges t

tha t t he ox id ized l ep idome lane 'C ' migh t r ep re s en t

a t r ans i t ion phas e be tween mica and an expanded

minera l , such as chlori te or vermiculi te . Th e oxid ized

lep idome lane 'C ' ha s a lmos t a l l f e rrous ox ide ox id ized

to ferr ic oxide , which increases the pos i t ive charge

in the oc tahedra l layer , thereby reducing the resul t ing

nega t ive cha rge o f the t e t rahedra l a nd oc tahedra l

l aye rs . Pa r t i a l l o s s o f the K appa ren t ly re s u l t s f rom

the increased pos i t ive charge of the oc tahedra l layer .

The ox id ized l ep idome lane re t a ins the s ing le - l aye r

mon oc l in i c s truc tu re o f the hos t l ep idom e lane 'B ' .

C o m p a r e d w i t h t h e t h r e e - l a y e r r h o m b o h e d r a l o x i -

d ized b io t i te de s c r ibed a s an ' i n t e rmed ia te m ica by

R ims a i t e (1967a ), l ep idome lane 'B ' and i ts ox ida t ion

prod uc t 'C ' have a d i f fe ren t c rys t a l s t ruc tu re , and l es s

M gO. The s t ruc tu ra l fo rmula fo r the ox id ized l ep ido -

me lane 'C ' was based on '44 + z ' = 47 va lencies ,

whe re '+ z ' was de te rmined f rom the cha rge d if fe rence

be tween the o r ig ina l l ep idome lane 'B ' and ox id ized

por t io n 'C ' (R imsa it e, 1970) . A l tho ugh the l ep idome-

lane has loca l magnet i te and a lbi te inc lus ions (Fig .

l e) , c l e an a n d h o m o g e n e o u s c o n c e n t r a t e s ' B ' a n d ' C '

we re ob ta ined by ca re fu l e l e c t romagne t i c and heavy

l iqu id s epa ra t ions , fo l lowed by repea ted op t i c a l an d

x- ray exam ina t ion o f concen t ra t e s du r ing the i r p rep -

ara t ion. I t i s be l ieved tha t the increase in va lencies

'+ z ' i s r e a l r e s u lt ing ma in ly f rom ox ida t ion o f i ron ,

whe reby the s t ruc tu ra l fo rmula ba s ed on 47 va lenc ie s

for the oxidized lepidornelane represents th is ra re

mineral without distortion: (Si5.z A12.8)~V(A10.6Tio.05

Fe4.~3+ Fe2+o.o6Mg0 .1 Mno.l )vl(Cao.~ K I.4) (OH )3.5 0~1

( H 2 0 ) o +

Secondary micaceous minerals in a nepheline deposit

M os t o f the nephe l ine c rys t a l s a re pa r t ly to co m-

p le t e ly a l t e red to f ine -g ra ined s econda ry mine ra l s :

mus cov i t e - s e r i c i t e , ' hyd ronephe l ine ' , z eo l i t e s and a

f ine -g ra ined aggrega te compos ed o f ch lo r i t e - s e rpen -

t in e mica in t e rg rowths , com m only re fe rred to a s ' g i e-

s eck it e' . S ome o f thes e s eco nda ry m ine ra l s a re s ub -

mic ros cop ic in si ze ; o the rs a re 40 #m in d iame te r . Th e

s econda ry mine ra l s a l s o c rys t a l l i z e in c av i t i e s and

frac tures . Biot i te f lakes part ly a l te red to chlori te have

been discussed by Rimsai te (1967a) . Textura l re la t ion-

ships sugges t a t leas t three perio ds o f c rys ta l l iza t ion

of the secondary sheet s i l ica tes .

Sericite. Seric i te f lakes , about 40#m in diameter ,

r ep lace hos t nephe l inc a long the f rac tu re s . The f ine -

g ra ined s e r ic it e 'E ' , concen t ra t ed f rom a nephe l ine

hos t , d i f fe r s f rom the coex i s t ing p r imary coa rs e -

g ra ined mus cov i t e D ' by le ss er M g , F and C I con-

tents , and b y sl ight ly hig her conc entr a t ion o f ferrous

i ron , a s c an be s een f rom the i r s t ruc tu ra l fo rmulae :

' D = S i 6 A12) (A13.v Fe 3+ F ~

2 4 w 0 2 6 Mgo.o3) (Nao.a5

K 1 . 7 5 ) ( O H 3 . 9 F o . 0 6 C 1 0 . 0 5 ) 0 2 0 , a n d ' E ' = ( 8i 5. 9

A12 .0 (A13.7 3+ 2+

eo .23 F eo .13 M go .01) (C ao .03 Nao .19

Kl.vS) (OH+7 Fo.o ,0 O19.6.

Hydronepheline . Hydronephe l ine i s a f ine -g ra ined

aggr egate of ser ic i te -analc i te in tergro wths , iden t i f ied

in x - ray powd e r d i f f rac t ion pa tt e rns . Th e pa r t i c l e s iz e

i s l es s than 2 #m, v i sibl e unde r an e l ec t ron m ic ros cope


7/10

N a t u r a l a l t e r a t io n o f m i c a 2 5 3

us ing magn i f i c a t ion x 60 ,000 . The 'hydrone phe l ine ' is

an a l t e ra t ion p r odu c t o f nephe l ine , bu t loca l ly it r e -

places a l l ear l ie r-formed minera ls , inc luding fe ldspar

and s econda ry s e r i c i t e . Area s rep laced by 'hydrone -

phe l ine ' have a p ink , pa tchy appea rance . The hyd-

ronephel ine 'F ' d i ffers f rom seric i te 'E ' and muscovi te

'D ' by l es se r Fe , A1 and K con ten t s an d h ighe r M g ,

N a a n d H 2 0 c o n t en t s . H i g h e r c o n c e n t r a t i o n o f N a

i s a ccou n ted fo r by the p re s ence o f ana lc it e , and C a O

is con ta ine d in ca lc i te impurit ies . Th e excess H 2 0 in

the f ine -g ra ined micaceous mine ra l s i s a ' s t rong ly

ads o rb ed wa te r ' wh ich rema ins a f t e r hea t ing a t 110~

a n d w a s s t u d i e d i n s o m e d e t a i l b y D T A , T G A a n d

i .r . analyse s. The ana lc i te (15% ) and C a O were s ub -

t rac ted f ro m chemica l ana lys i s befo re ca lcu la t ing the

s t r u c t u r a l f o r m u l a f o r t h e m i c a c o m p o n e n t o f t h e

'hydronephe l ine ' :

(Si6 A12)

AI3.9 3+ 2+

e0.03 Feo.o3 Mgo.2)

N a o . 1 9 K 1 . 7 ) 0 H 3 . 9 7 F0 .o l ) 020

(ads o rbed HzO = 0 .5 ) .

R ep lacem en t o f nephe l ine 'H ' by s e ri ci te 'E ' and by

hydronep he l ine 'F ' r e s u l t s i n the re leas e o f s od ium

wh ich has crys ta l l ized in secon dary minerals , ana lc i te ,

z eo l i te s and in s econda ry a lb i te T tha t s u r round s

magne t i t e c rys t a l s in l ep idome lane 'B ' .

Fine-grained chlorite-serpentine-mica aggregates

( gieseckite ) G .

Thes e ve ry f ine -g ra ined ( s ub -mic ro -

s cop ic ) agg rega te s p roduce ve ry poo r x - ray pow de r

d i f f ract ion pa t t e rns o f ch lo r i t e- s e rpen t ine an d mica .

A f ib rous va r i e ty o f the aggrega te s c rys t a l l i z e s a long

f rac tu res and jo in t s in the nephe l ine s yen i te , an d mas -

sive agg reg ates replace nepheline, feldspars, b iotite,

mus cov i t e and o the r mine ra l s p roduc ing g reen i s h -

g rey a l t e ra t ions in the hos t rock . Th e g reen i s h , a l t e red

patch es va ry in size from a few mil l imeters to a few

cen t ime te r s and loca l ly m ay cons t i tu t e m ore than 50

pe r c en t o f the rock . I t i s in t e re s ting to po in t ou t

tha t t he f ine -g ra ined micaceous a l t e ra t ion p roduc t s ,

and the me ta s oma t i c l ep idome lane , do no t f ix the

re l eas ed an ions , F and C 1, and con ta in ma in ly hy -

d roxy l in the OH,O-s hee t s . S t ruc tu ra l fo rm ula was

ca lcu la t ed fo r the ch lo r i t e componen t 'G ' a f t e r s ub -

t rac t ing e s t ima ted 5 pe r c en t nephe l ine and 15 pe r

cen t mus cov i t e :

Si6 .4 A l l .6 ) A13 .75 3+ 2+

e o . 3 F e l . 7 M g 6 )

O H l s F o . o 2 C10.o2) 020 5.

The fo l lowing s equence o f c rys t a l l iz a t ion o f mica -

ceous a l t e ra t ion p roduc t s was obs e rved in the nephe -

l ine depos i t :

(1) bro wn biot i te 'A ' re leases Mg , A1, Ti , F an d

C ! and a l t e r s to l ep idome lane wh ich ga ins Fe a nd

O H ;

(2) nephel ine ' H ' a l te rs to ser ic ite 'E ' and subse-

quen t ly to hydronephe l ine 'F ' and re l ea s e s s od ium;

(3) the re l ea sed s o d ium can be a ccou n ted fo r in

p a r t b y t h e p r e s e n c e o f s e c o n d a r y N a - b e a r i n g

minera ls , ana lc i te , zeol i tes and secondary a lbi te T

which rep lace p r imary mine ra l s and fo rm in in t e r -

s ti ce s s u r ro und ing magn e t i t e in l ep idom e lane ;

(4) ch lo r i t e - se rpen t ine -mica aggrega te s 'G ' c rys t a l -

l ize in rock frac tures and replace nephel ine syeni te

in i r r egu la r pa tches the reby p roduc ing a pecu l i a r

s po t t ed g reen i s h -g rey a l te ra t ion o f the rock .

E X A M P L E I V

Alteration of biotite in granitic rock s containing quartz

and sulphides

Altera t ion of b iot i te in Si-Al-r ich rock s is d ifferent

from tha t observ ed in Si-defic ient , u l t raba s ic rock s .

In the A1-S i - r i ch rocks a f fec ted by low t empe ra tu re

m e t a m o r p h i s m o r h y d r o t h e r m a l a l t e r a t i o n , b i o t i t e

a l t e rs to A l - r i ch ch lo r i t e by lo s ing K , S i and Ti a nd

9gaining A1. At the in i t ia l s tage of a l te ra t ion, the t i tania

exso lves as futile needles (Rimsaite, 1973b, Fig. 12)

a n d t h e b i o t i t e c h a n g e s i n c o l o r f r o m b r o w n t o

g reen . C hem ica l changes du r ing th ree s t ages o f a lt e r -

a t ion we re s tud ied on f re s h and ch lo r i t i z ed b io t i t e s

in a l t e red m e ta -g reywacke f rom M arbr idg e n icke l

depos i t , Que bec (Example IVa) . The K re l ea s ed f rom

the b io t i t e s t ruc tu re i s e i the r r emoved f rom the s i t e

o r i t c an be f ixed in the s econda ry mus cov i t e wh ich

replaces biot i te (Fig . Id) . The muscovi te which is

f o r m e d b y r e p l a c e m e n t o f b i o t it e c o n t a i n s b r o w n

pa tches o f r em nan t a l t e red b io t i t e and i s u s ua l ly

phengi t ic .

Example 1Va. Fres h and a l t e red b io t i t e s in va r ious

degree s o f ch lo r i t i z a t ion we re concen t ra t ed f r om

m e t a m o r p h o s e d g r e y w a c k e R M - 1 0 c o l l e c t e d i n M a r -

b r idge No . 2 n icke l depos i t , Quebec . Gene ra l geo logy

and d i s t r ibu t ion o f the g reywacke a re d i s cus sed by

Da ws o n (1966 , M ap 1179A) , and mine g eo logy i s de -

s c r ibed by B uch en and B lowes (1968) . The me ta -

g reywack e cons i s ts o f a l t e rna t ing b io t it e - -ch lo r it e an d

qua r t z -p lag ioc la s e bands , and con ta ins d i s s emina ted

pyr i t e. The ro ck i s a l t e red a long f rac tu re s . C renu la t ed

zig-zag veinle ts of chlori te in biot i te can b e seen in

F ig . l ( c ) . Wi th advanced a l t e ra t ion , ch lo r i t i z a t ion

s p reads in to the b io t i t e bands .

R e s u lt s o f c h em i c a l a n d e l e c tr o n m i c r o p r o b e a n a l y -

s e s a re compared in Tab le 1 , R M -10 , inc lude thos e

o f f re sh b io t i te 'A ' . Pa r t ly a l t e red b io t i te w i th exs o lved

ru t il e 'B ' was ana lys ed b y an e l ec t ron mic rop ro be ,

and a lmos t en t i r e ly ch lo r i t i z ed b io t i t e 'C ' w i th a f ew

rut i le needles , and green chlori te without rut i le by

we t chemica l ana lys i s . C ons ide r ing tha t e l e c t ron mic -

rop robe ana lys e s we re made in po l i s hed th in s ec t ions

on s ma ll a rea s , r ep re s en t ing o n ly a few s qua re m ic -

rons o f a f l ake and chemica l ana lys e s we re mad e o n

c o n c e n t r a t e s c o n t a i n i n g m a n y t h o u s a n d s o f m i c a

flakes represent ing different s tages of a l te ra t ion, the

ana lys e s compare remarkab ly we l l and s how de f in i t e

chemica l t r ends w i th p rog re s s ive a l t e ra t ion . The

ch lo r i t e and b io t i t e p ropor t ions in pa r t ly a l t e red

micas we re ca lcu la t ed f rom the i r x - ray pow de r d i ff rac -

t ion cha r t s . The re s u l t s i n Tab le 1 s how g rad ua l de -

crease s for K, Ti, Si, F a nd C1, differential losses of


8/10

254 J. R1MSAITE

Rb a n d S r, an d i n c reases o f AI an d M g . I t i s i n t e r es t -

i n g t o p o in t o u t t h a t d u r i n g h y d ra t i o n , t h e f l u o r in e

i s ap p aren t l y l e s s s t ab l e i n t h e m ica s t ru c tu re t h an

th e ch lo r i n e .

Exa mp le IVb. Al terat ion of b ioti te in a kaol ini tized

gneiss . Replacem ent o f b iot ite by

M g - F e - A 1

chlorites

and muscovite. In t h e a l t e r ed g n e i s s R-Kp -5 , b io t i t e

i s a lm o s t en t i r e l y a l t e r ed t o ch l o r i t e . A f ew r em n an t s

o f f resh b io t i t e a r e p r ese rv ed i n q u ar t z an d a f ew

p a t c h e s o f a l t e r e d b i o t i te r e m a i n i n m u s c o v i t e w h i c h

rep l aces t h e b io t i t e (F i g . l d ) . E l ec t ro n m ic ro p r o b e

an a ly se s o f f resh b io t i t e 'A ' , en c l o sed i n q u ar t z ; o f

a l t e r e d b i o ti t e r e m n a n t ' B ' i n m u s c o v i t e ; o f M g - r i c h

' C ' , F e - r i c h ' D ' a n d A l - r ic h ' E ' c h l o r i te s ; a n d o f m u s -

co v i t e 'M ' i n d i ca t e t h e fo l l o wi n g ch em i ca l tr en d s : d u r -

i n g a l t e r a t i o n o f b io t i t e t o M g - r i ch ch lo r i t e ( 'A ' t o

'C ' ) , t h e K , S i , T i , M n an d F e d ec rease , wh i l e A1 an d

M g a p p a r e n t l y i n c r e a s e . D e e p - g r e e n c h l o r i t e ' D ' i s

m o s t a b u n d a n t a n d s h o w s s i m i l a r c h e m i c a l t re n d s ,

b u t c o n t a i n s l e ss M g a n d m o r e F e t h a n c h l o r i t iz e d

b i o t i t e s ' B ' a n d ' C ' , w h e r e a s c h l o r i t e ' E ' c o n t a i n s

m u ch m o re A1 2 0 3 (2 7 % ) an d ab o u t t en t im es l e s s

i ro n (1 .8 % F eO) t h an t h e F e- r i ch ch l o r i t e 'D ' ( 1 8 @ ' /o

A1 2 0 3 an d 2 6 .1 % F eO ) . Th e m u sco v i t e 'M ' r ep l aces

b i o t i t e w h e r e b y t h e T i , F e , M g a n d M n a r e r e m o v e d

f r o m t h e b i o t i t e a n d s u b s t i t u t e d b y S i , A L N a a n d

K in t h e m u sco v i t e s t ru c t u re . Du r in g t h e r ep l acem en t

o f b io t i t e b y m u sco v i t e , t h e b io t i t e i s a h o s t an d i n

p ar t a su p p l i e r o f t h e A1, S i an d K, b u t i o n i c p ro p ro -

t i o n s ch an g e i n t h e i n t e r l ay e r (h ig h er p ro p o r t i o n o f

Na) , i n t h e t e t r ah ed ra l l ay e r (h ig h er S i / A1 r a t i o ) an d

in t h e o c t ah ed ra l l ay e r o f t h e m u sco v i t e , a s can b e

seen f ro m th e fo l l o win g s t ru c t u ra l f o rm u l ae :

b io t i te 'A ' : (S is.v A12.3) (Alo .1 Tio .4 Fe2 .9 Mg2.2

Mno.03)

KI.v2) OH)4,7 O)19.6;

M g- ric h ch lor ite 'C ': (Si5.6 A12.4) (A12.1 Tio.04 Fe3.3

Mg6.7 Mno.02 Cro .01)(OH)16 0)20 ;

Fe -r ic h ch lor i te 'D ' : (S is .s A12.5) (A12.3 Tio .04 Fe4 ., ;

Mg4.8 Mno.04 Cro.02) 0H)22 O)17;

Al -r ich ch lor i te 'E ' : (S i6 .4 Al l .6 ) (A14.8 Feo .3 Mgs.3)

(OH)27 (O)15;

M u sco v i t e 'M ' : (S i 6 .5 Al v s ) (A13 .6 T io .o l F eo . l z

Mgo.43) (Nao.18 K1.6 6) 0H )3.5 0)20.3.

T h e m u s c o v i t e ' M ' a n d A l - r i c h c h l o r it e ' E ' a r e t h e

l e a s t a b u n d a n t a l t e r a t i o n p r o d u c t s o f b i o t i t e i n t h e

g n e i s s R-Kp -5 . Ch em i ca l d i f f e r en ces b e t ween t h e r es i -

d u a l a l t e r a t i o n p r o d u c t s r e p l a c i n g t h e b i o t i t e a r e

a t t r i b u t e d t o t h e l o c a l d i s t r i b u t i o n a n d a v a i l a b i l i t y

o f t h e p a r t ic u l a r i o n s d u r i n g t h e a l t e r a t io n . A l t e r a t i o n

o f b io t i t e t o A1 -M g - r i ch ch lo r i t e i s r e l a t ed t o t h e

a l t e r a t i o n o f f e l d s p a rs w h i c h a r e r e p l a c e d b y f in e -

g r a i n e d q u a r t z , m i n o r c h l o r i t e a n d k a o l i n i t e t h e r e b y

r e l e a s in g a l k a l i s a n d a l u m i n a .

EXAMPLE V

Na tura l alteration o f phlogopite to vermiculite and talc

S p ec im en s o f co ar se -g ra i n ed p h l o g o p i t e , s l i g h t l y t o

m a r k e d l y a l t e r e d to v e r m i c u l i t e a n d t o t a l c o n s l ic k -

en s i d e su r f aces were co l l ec t ed i n t h e Oly m p u s v e rm i -

c u l it e m in e , O n t a r i o . T h e p h l o g o p i t e - v e r m i c u l i t e

v e in s , co n t a in in g co m m erc i a l v e rm icu l i t e , t r an sec t

a l t e r e d s y e n it e r o c k s . T h e p h l o g o p i t e w a s s e p a r a t e d

f ro m th e v e rm icu l i t e h o s t wh ere i t o ccu r s a s d i sc r e t e

r e m n a n t s . I t c o n t a i n s a b o u t 1 0 p e r c e n t o f v e r m i c u li t e

' V ' w h i c h w a s e s t i m a t e d f r o m x - r a y d i f f r a c t i o n p a t -

t e r n s u s i n g t h e p r o c e d u r e d e s c r i b e d b y R i m s a i t e

(19 73 , F ig . 1 0 ) . Th e v e rm icu l i t e co n cen t r a t e 'V ' co n -

t a i n s a b o u t 3 0 p e r c e n t o f p h l o g o p i t e c o m p o n e n t ' P h '

wh ich was su b t r ac t ed b e fo re ca l cu l a t i n g t h e s t ru c tu ra l

fo rm u la . Ta l c 'T ' r e t a in s t h e g en era l h ab i t o f p h lo g o -

p i t e -v e rm icu l i t e b u t u n d er a m ic ro sco p e i t i s s een t o

b e c o m p o s e d o f fi n e p a r ti c l e s, a r r a n g e d i n p a r a l l e l

r o w s a n d c o n t a i n s s p e c k s o f m a g n e t i t e a n d h e m a t i t e

w h i c h c a u s e i t s b l a c k a p p e a r a n c e i n h a n d s p e c i m e n .

T h e n a t u r a l a l t e r a t i o n o f p h l o g o p i t e t o v e r m i c u l it e

a n d t a l c t a k e s p l a c e in a l k a l i n e . N a - r i c h r o c k s b y

h y d r a t i o n a n d l o s se s o f a n i o n s a n d c a t i o n s f r o m t h e

i n t e r la y e r a n d f r o m t h e t e t r a h e d r a l a n d o c t a h e d r a l

l ay er s , an d b y r e l a t i v e i n c rease o f S i an d Mg .

Na tural ahe ration o f phlooopite to vermiculite.

In

a d d i t i o n t o h y d r a t i o n w h i c h a c c o m p a n i e s l o s s e s o f

t h e i n t e r l ay er K an d Na , t h e p h lo g o p i t e l o ses A1 , T i ,

Rb , F e , Mn , F an d CI (Tab l e l ) . Th e a b o v e l o sses

o f c a t i o n s a n d a n i o n s a r e c o m p e n s a t e d b y r e l a ti v e

i n c re a s e s o f Si , M g a n d H 2 0 . H o w e v e r , t h e i n c r e a s e d

s i l ica co n t en t i s i n su ff i c ien t t o c o m p en s a t e fo r t h e l o s s

o f a l u m in a wh ich r esu l t s i n a S i -Al -d e f i c i en t t e t r a -

h e d r a l l a y e r a n d i ts u n u s u a l c o m p o s i t i o n , o r i n v a c a n t

t e t r a h e d r a l p o s i t io n s . T h e p r o p o r t i o n s o f f l u or i n e l o s t

ex ceed s t h a t o f ch lo r i n e , a s a l r ead y o b se rv ed i n

E x a m p l e I V a . G e n e r a l c h e m i c a l t r e n d s d u r i n g n a t u r a l

a l t e r a t i o n o f p h l o g o p i t e ' P h ' t o v e r m i c u l i t e 'V ' a r e

s im i l a r t o t h o se o b se rv ed i n ex am p le I I e (n o t sh o wn )

w h e n p h l o g o p i t e a l t e rs t o b r i g h t -g r e e n c h l o r i te b y l o s -

i n g K, AI , T i , an d g a in in g Mg . H o wev e r , t h e b r i g h t -

g reen ch lo r i t e g a in s a l so F e an d d u r in g t h e i n i t i a l

s t ag e o f a l t e r a t i o n , l o ses so m e o f i t s Si . Th e m ain

d i f f e r en ce b e tween r eac t i o n s i n ex am p le I I c an d V

i s i n t h e b eh av io u r o f i r o n : i n t h e g reen , p a r t l y a l t e r ed

p h lo g o p i t e RT-8 6 6 'B ' i r o n i n c reases an d i s f i x ed i n

th e s t ru c tu re , wh ereas i n v e rm icu l i t e W ' , i r o n d e -

c reases an d i s r em o v ed .

Na tura l ahe ration Of phlogopite vermiculite inter

growths to ta lc .

D u r i n g t h e n a t u r a l a l t e r a t i o n t o t a l c

o n ' s l i ck en s id e ' su r f aces a lo n g ro ck - f r ac tu res , t h e v e r -

m icu l i t i zed p h lo g o p i t e l o ses t h e i n t e r l ay e r ca t i o n s ,

t e t r a h e d r a l a n d o c t a h e d r a l A 1, T i a n d p a r t o f F e , a s

w e l l a s F a n d C I f ro m t h e O H , O - s h e e t s . P a r t o f t he

in t e r l ay er wa t e r i s a l so ex p e l l ed f ro m th e v e rm icu l i t e ,

b u t t a l c r e t a in s ex cess wa t e r o r h y d ro x y l a s a su b s t i -

t u t e fo r F an d CI , an d p o ss ib ly i n h y d ro x id e im p u r i -

t i e s wh ich m ig h t b e i n t e rg ro w n wi th t a l c . Th e p res -

en ce o f p o ss ib l e im p u r i t i e s i n v e rm icu l i t e 'V ' an d i n

t a l c 'T ' i s i n f e r red b y t h e S i -d e f i c ien t t e t r ah e d ra l l ay e r

a n d b y a s l i g h t e x c es s o f t h e o c t a h e d r a l M g . A l t h o u g h

S i - co n t en t co n s id e rab ly i n c reases i n ta l c T , i t s q u an -

t i t y i s i n su f f ic i en t to f i l l th e t e t r ah ed ra l p o s i t i o n s . Th e

t e t r ah ed ra l lay e r i n b o th v e rm icu l i t e 'V ' an d t a l c q

h as a p ecu l i a r co m p o s i t i o n , r e su l t i n g f ro m co n s id e r -

ab l e l o s ses o f AI an d i n su f fi c i en t i n c reases o f S i. I n

o rd er t o f i l l t h e ap p aren t t e t r ah ed ra l v acan c i es , p a r t


9/10

Fig. l(a). Sulphide bands (Po=pyrrhot ite; Pn:pe ntl an dit e) in a crenulated mica layer of fresh

biotite schist RT-18.

l(b). Biotite (B) partly replaced by and coated with iron and uranium oxides in gneiss R-Kp-5.

l(c). Biotite flakes replaced by zig-zag veinlets of chlorite (CH) and serpentine (SP) and feldspar

altered to sericite aggregates (S) along a fracture in greywacke RM-10.

l(d). Dark remnants of biotite in muscovite (white band) and residual chlorite (grey). The titani a

removed from the altered biotite crystallized as anatase (A) in gneiss R-Kp-5.

l(e). Magnetite (black) surrounded by secondary albite (white) in lepidomelane NEB-1.

l(f). Biotite bands (B) partly replaced by Ni-goethite (Fe) and jarosite (Y) in oxidised portion of

biotite schist RT-18.

CCM--f.p. 254


10/10

Natural alteration of mica 255

of Mg ions were placed into the tetrahedral layer,

in addition to all Fe and Ti. Although they may be

present, no Mg-rich minerals, such as serpentine and

brucite, were observed on x-ray diffraction pattern.

SUMMARY AND CONCLUSIONS

The study was concerned mainly with intermediate

alteration products containin g various proportions of

fresh mica remnants in intergrowths with the residual

alter ation phases, identified by the x-ray analysis.

Results of this study provide data on diverse stages

of alteration of mica in solid rocks; o n remobilization

of anions and cations during the alteration (Table 1);

and on chemical compositions of the original micas

and their residual altera tion products, chlorites, ser-

pentine, vermiculite and talc which form by pseudo-

morphous replacement of the micas. Quantities of

released and gained constituents during various stages

of alteration can be calculated. Thus, during alter-

ation of 20% biotite to chlorite, 1 kg of mica schist

RM-10 releases 20g K; 25g Si; 0 5g Ti; 0.1g F;

50 ppm Rb which are ac companied by apparen t gains

of 50g OH ; 1'5g A1 and 2'5 g Mg (Example IVa).

Some of the released cations are removed from the

host rock, while the others may be fixed in newly-

formed secondary minerals.

Chemical composition of the residual chlorites

depends on the chemical composition of the original

mica, on the type of the host rock and on physico-

chemical environ ment during alteration. In ultrabasic

rocks the phlogopite alters to chlorite which has pre-

dominantly antigorite-ferroantigorite composition at

the initial stage, and to talc at the advanced stage,

by apparent gains of Si and Mg. In Si-Al-rich rocks,

the residual chlorites usually contain more A1 than

the host micas and variable proportions of increased

Mg and Fe which are fixed in the chlorite depending

on the local distribution and availability of these ions

due to decomposition of associated feldspars and Fe-

rich sulphides. In the oxidation zone of a Fe-Ni-sul-

phide deposit, the goethite-, hematite- and jarosite-

coated biotite alters to silica-rich amorphous flakes

by losing different proportions of all the other consti-

tuents.

Results of this study contribute to scarce data on

chemically-determined natura l alteratio n reactions in

solid rocks. The analyses obtained on fresh and

altered portions of micas (Table 1 and structural for-

mulae) indicate that alteration and hydration of micas

involve losses and substitutions in the interlayer, in

the octahedral and tetrahedral layers, and in the

OH,O-sheets, in additions to the replacement of

alkalis by hydroxides in the interlayer. Chemically

complex biotites commonly alter to less complex

oxides and hydrous silicates.

A c k n o w l e d g e m e n t s T h e

author wishes to thank the Staff

of the following mining companies for valuable suggestions

and assistance with selection of sampling sites: Cominco

Limited; Consolidated Canadian Faraday Limited; Fal-

conbridge Nickel Mines Limited; and the International

Nickel Company of Canada Limited. Electron microprobe

analyses by G. R. Lachance and chemical analyses by J.

L. Bouvier, both of the Central Laboratories and Technical

Services Division, Geological Survey of Canada, and criti-

cal reading by J. L. Jambor of the Regional and Economic

Geology Division, Geological Survey of Canada, are grate-

fully acknowledged.

R E F E R E N E S

Buchan, R. and Blowes, J. H. (1968) Geology and minera-

logy of a millerite nickel ore deposit, Marbridge No.

2 mine, Malartic, Quebec: Can. Min . Met . Bu l l . 1-6.

Carlson, H. D. (1958) Geology of the Werner Lake-Rex

Lake area: Ontario Department Mines Report 66, pt.

4.

Dawson, K. R. (1966) A comprehensive study of the Preis-

sac-Lacorne batholith, Abitiby county, Quebec:

Geol.

Surv. Can. Bull.

142.

Hoda, S. N. and Hood, W. C. (1972) Laboratory alteration

of trioctahedral micas: C l a y s a n d C l a y M i n e r a l s 20, 243-

258.

Farmer, V. C., Russell, J. D., McHardy, W. J, Newman,

A. C. D., Ahlrichs, J. L. and Rimsaite, J. Y. H. (1971)

Evidence for loss of protons and octahedral iron from

oxidized biotites and vermiculites: M i n e r a l . M a g . 38,

121-137.

Lachance, G. R. and Plant, A. G. (1973) Quantitative elec-

tron microprobe analysis using an energy dispersive

spectrometer: Geol. Surv. Can. Paper 73-1, part B, 8-9.

Maxwell, J. A. (1968) R o c k a n d M i n e r a l A n a l y s i s . Inter-

science-Wiley, New York.

Mehmel, M. (1938) Ab- und Umbau am Biotit:

Chem. d.

E r d e

ll, 30~332.

Rimsaite, J. (1967a) Bioti te intermediate between diocta-

hedral and trioctahedral micas:

C l a y s a n d C l a y M i n e r a l s

15, 375-393.

Rimsaite, J. (1967b) Studies of rock-forming micas:

Geol.

Surv. Can. Bull. 149.

Rimsaite, J. (1970) Structural formulae of oxidized and hy-

droxyl-deficient micas and decomposition of the hy-

droxyl group:

Contr . Miner . Pe tro l .

25, 225-240.

Rimsaite, J. (1973a) Mica group minerals and related sili-

cates in Canadian mineral deposits: Geol. Surv. Can.

Paper 73 l. part B~ 205-209.

R msaite, J. (1973b) Genesis of chlorite, vermiculite, serpen-

tine and secondary oxides in ultrabasic rocks:

Proe. Int .

Clay Conf. 1972, Madrid, Spain, 291-302.

Robert, M. and Pedro, G. (1973)Etablissement d'un

schema de l'evolution experimentale des micas trioctaed-

riques en fonction des conditions du milieu (pH con-

centration): P r o c . I n t . C l a y C o n f 1972, Madrid, Spain,

433-447.

Zurbrigg, H. F. (1963) Thompson mine geology:

Can. Inst .

M i n i n 9 Met . Bu l l .

56, 451 460.

Natural Alteration of Mica and Reactions Between Released Ions in Mineral Deposits

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