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8/20/2019 30-226-458 hgh hgh.pdf http://slidepdf.com/reader/full/30-226-458-hgh-hghpdf 1/9 458 Zoisite amphibolite with corundum from Tanganyika. By P. M. GAME B.Sc. Department of Mineralogy, British Museum Natural History). [Taken as read 10 June 1954.] OCCURI~ ENCE. HE most striking rock now described occurs in the Northern Province of Tanganyika Territory, just west of the Great North road, and south of the track from Longido to Engare Naibor. A small group of specimens was collected by Mr. A. G. Clough of Industrial Minerals Ltd., and presented to the British Musemn Natural History) in 1952 B.M. 1952,133). The outcrops occur in the highly metamorphosed Basement Complex which forms small hills The Matabatu Mountains) descending, at their margin, by step-faulting to a peneplaned surface cut out of much younger, unconformable volcanics ; the faulting in the marginal area of the basement rocks forms the eastern side of the Rift Valley. The b~se- ment rock adjacent to the zoisite-amphibolite is usually an amphibolite with conspicuous large, brown-red garnets. The occurrence of this garnet-amphibolite is usually an indication of the proximity of the zoisite-amphibolite. The outcrops of the zoisite-amphibolite are small. The largest is just over 100 yards long and about 50 yards wide. The foliation corresponds with that of the surrounding metamorphic types. It is, therefore, difficult to determine whether the amphibolite is a highly altered intrusive rock or whether it has been formed by metasomatism or is a migmatic pro- duct. Probably each of these processes has played a part in its pro- duction. The zoisite-amphibolite shows a continuous gradatioI~ from a very dark amphibolite containing very little zoisite to a much paler apple- green, zoisite-rich rock with little amphibole, but usually containing large, red corundum crystals. The schistosity of the zoisite-rich types is poorly defined. In thin sections under the microscope the rock shows the association of corundum, amphibole, and zoisite; a few sections show, in addition, a little plagioclase and colourless mica. There is also a secondary, partly fibrous product.
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458
Zoisite amphibolite with corundum from Tanganyika.
B y P . M . G A M E B.Sc .
D e p a r t m e n t o f M i n e ra lo g y , B r it is h M u s e u m
N a t u r a l H i s t o r y ) .
[ Ta k e n a s r e a d 10 J un e 1954. ]
OCCURI~ ENCE.
H E m o s t s t r ik i n g r o c k n o w d e s cr ib e d o c c ur s i n t h e N o r t h e r n P r o v i n c e
o f T a n g a n y i k a T e r r it o r y , j u s t w e s t o f t h e G r e a t N o r t h r o a d , a n d
s o u t h o f t h e t r a c k f r o m L o n g id o t o E n g a r e N a i b o r . A s m a l l g r o u p o f
sp e c i m e n s w a s c o l l e c te d b y M r . A . G . C l o u g h o f In d u s t r i a l M i n era ls L t d . ,
a n d p re se n t e d t o t h e B r i t ish M u se m n N a t u ra l H i s t o ry ) i n 1 95 2 B .M .
1 95 2,1 33 ). Th e o u t c ro p s o c c u r in t h e h i g h l y m e t a m o rp h o s e d B a se m e n t
C o m p l e x w h i c h fo rm s sm a l l h i ll s Th e M a t a b a t u M o u n t a in s ) d e sc e n d i n g ,
a t t h e i r m a rg i n , b y s t e p - f a u l t i n g t o a p e n e p l a n e d su r f a c e c u t o u t o f m u c h
y o u n g e r , u n c o n fo rm a b l e v o l c a n ic s ; t h e f a u l t i n g i n th e m a rg i n a l a r e a o f
t h e b a s e m e n t r o c k s f o r m s t h e e a s t e r n s id e o f t h e R i f t V a ll e y. T h e b ~ s e -
m e n t r o c k a d j a c e n t t o t h e z o i s it e - am p h i b o l i te i s u s u a l ly a n a m p h i b o l i t e
w i t h c o n sp i c u o u s l a rg e , b ro w n - re d g a rn e t s . Th e o c c u r r e n c e o f t h i s
g a r n e t - a m p h i b o l i te i s u s u a l ly a n i n d i c a ti o n o f t h e p r o x i m i t y o f t h e
zo i s i t e -amphibo l i t e .
Th e o u t c ro p s o f t h e z o i s i te - a m p h i b o l i te a r e sm a ll . Th e l a rg e s t is ju s t
o v e r 1 00 y a rd s l o n g a n d a b o u t 5 0 y a rd s w i d e . Th e fo l ia t i o n c o r r e sp o n d s
w i t h t h a t o f t h e su r ro u n d i n g m e t a m o rp h i c t y p e s . I t is , t h e r e fo re , di ff ic u lt
t o d e t e rm i n e w h e t h e r t h e a m p h i b o l i t e i s a h i g h l y a l t e r e d i n t ru s i v e ro c k
o r w h e t h e r i t h a s b e e n f o r m e d b y m e t a s o m a t i s m o r i s a m i g m a t i c p r o -
d u c t . P r o b a b l y e a c h o f t h e se p ro c es se s h a s p l a y e d a p a r t i n it s p ro -
duc t ion .
Th e z o i s i t e - a m p h i b o l i t e sh o w s a c o n t i n u o u s g rad a ti oI~ f ro m a v e ry
d a rk a m p h i b o l i t e c o n t a i n i n g v e ry l i t t l e z o i s i t e t o a m u c h p a l e r a p p l e -
g re e n , z o i s i t e - r i c h ro c k w i t h l i t t l e a m p h i b o l e , b u t u su a l l y c o n t a i n i n g
l a rg e , re d c o ru n d u m c ry s t a l s. Th e s c h i s t o s i ty o f t h e z o i si te - r ic h t y p e s
is p o o r l y d e fi n ed . I n t h i n s e c ti o n s u n d e r t h e m i c ro sc o p e t h e ro c k sh o w s
t h e a s so c i a t io n o f c o ru n d u m , a m p h i b o l e , a n d z o i s i te ; a f e w se c ti o n s
show, in add i t ion , a l i t t l e p l ag ioc lase and co lour l ess mica . There i s a l so
a s e c o n d a ry , p a r t l y f ib ro u s p ro d u c t .
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ZOISITE-AMPHIBOLITEFROM TANGANYIKA 45 9
I n a d d i t i o n t o t h e s e a m p h i b o l i t e s a n d z o i s i t e - a m p h i b o l i t e s t h e c o l l e c -
t i o n p r e s e n t e d t o t h e m u s e u m i n c l u d e d o n e s p e c i m e n o f a w h i t e z o is it e-
r o c k ; t h i s i s m o n o m i n e r a l i c a n d s l i g h t l y c o a r s e r - g r a i n e d t h a n t h e g r e e n
v a r i e t y , w i t h t h e s a m e g r a n u l a r t e x t u r e .
Cvrundum. The c o r u n d u m f o r m s c o n s p i c u o u s r u b y - r e d c r y s t a l s u p
t o 3 c m . i n d i a m e t e r , w h i c h f r e q u e n t l y s h o w h e x a g o n a l , i d i o b l a s t i c o u t -
lin e s. A l l t h e c r y s t a l s s h o w i n t e n s e c r a c k i n g a n d g r a n u l a t i o n , o b v i o u s l y
t h e r e s u l t o f s tr e ss . T h i s c r a c k i n g h a s d e s t r o y e d t h e o t h e r w i s e g e m
q u a l i t y o f t h e m i n e r al . S o m e c r y s t a l s a re r i d d l e d w i t h s m a l l z o is it e i n -
c l us io n s a n d c o n t a i n , i n a d d i t i o n , a f e w o p a q u e g r a in s , p r o b a b l y o f
i ro n - o r e. A n i n t e r e s t in g f e a t u r e i s t h e f r e q u e n t i n t e r g r o w t h w i t h a m p h i -
b o l e a t b o u n d a r i e s o f t h e t w o . T h e c o r u n d u m g r a i n s t h e n s h o w n o w e ll -
d e fi ne d b o u n d a r i e s b u t r a t h e r a ' f r e t t e d ' a n d c o m p l e x p e r i m e t e r w i th
n a r r o w t o n g u e s o f a m p h i b o l e r a d i a l ly a r ra n g e d . T h e c o r u n d u m s h o w s
a d i s t i n c t p i n k i s h t i n g e i n m i c r o - s e c t i o n s o f t h e u s u a l t h i c k n e s s .
Amphibole. The a m p h i b o l e u s u a l l y f o r m s i r r e g u l a r g r a i n s o f v a r i a b l e
siz e, t h e m a x i m u m l e n g t h b e i n g a b o u t 0 .4 r a m . W h e n a s s o c ia t e d w i t h
p l a g i o c l a s e , h o w e v e r , i t s o m e t i m e s s h o w s s m a l l , s h a r p l y i d i o b l a s t i c
d i a m o n d - s h a p e d s e c t io n s w i t h th e p r is m a s t h e p r o m i n e n t f o rm . O t h e r
g r a i n s s h o w p i n a e o i d a l c l e a v a g e s i n a d d i t i o n t o t h e p r i s m c l e a v a g e , t h u s
p r e s e n t i n g a h i g h l y ' s c a r r e d ' a p p e a r a n c e a n d c o n s p i c u o u s r el ie f. T h e
m i n e r a l is p a l e i n c o l o u r a n d i s o n l y f a i n t l y p l eo e h r o ic .
T h e a b s o r p t i o n i s V f l > a , w i t h ~ a n d f i b l u e - g r e e n , a p a l e y e l l o w -
g r e e n t o a l m o s t c o l o ur le s s. T h e v a l u e s o f 2 V a n d o f y : c w e r e d e t e r m i n e d
o n a L e i t z 4 - a x i s s t a g e ; a n d f i b y t h e i m m e r s i o n m e t h o d , i n s o d i u m -
l ig h t , o n t h r e e s e le c t e d g r a in s s h o w i n g t h e c e n t r a l e m e r g e n c e o f a n o p t i c
a x is . T h e v a l u e s o f V a n d a w e r e d e t e r m i n e d b y c a l c u l a t io n f r o m t h e
v a l u e s o f fl a n d 2 V c o m b i n e d w i t h t h e v a l u e o f y - - s , w h i c h w a s
m e a s u r e d w i t h a B e r e k c o m p e n s a t o r .
2 V y : 8 2 ~ ( m e a n o f 5 v a l u e s : r a n g e 7 8 8 489176
7 : c = 2 1 ~ ( m e a n o f 5 v a l u e s : r a n g e 1 9 2 289176
fl = 1"648 (m ea n o f 3 va lu es , 1"647, 1"648, 1"650).
y - - a - 0"021 ; a 1 .639 (calc . ) , ~ 1-660 (calc . ) ; sp . gr . d~~ ~ 3"13.
T h e s e v a l u e s, i n c o n j u n c t i o n w i t h t h e r e l a t iv e l a c k o f c o l o u r a n d f e e b le
p l e oc h r o is m , s h o w t h a t t h e a m p h i b o l e is a n i r o n - p o o r h o r n b l e n d e b e -
l o n g i n g t o t h e t r e m o l i t e - e d e n i t e s e ri es ( a b o u t 4 5 m o l e c u l a r % e d e n it e ). 1
1 A. N. W inchell , Elements of optical mineralogy, 4th edit ., 1951, pt. II , fig. 324,
p. 433 . [M.A. 11 463.]
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460 1 . M. GAME ON
P l a g i o c l a s e . - - T h i s is o n l y s p a r s e ly p r e s e n t , b e i n g o b s e r v e d i n o n l y o n e
o f t h e h a l f - d o z e n s e c ti o n s e x a m i n e d . I t f o r m s h i g h l y i rr e g u l a r g r a i n s
u p t o 3 m m . i n d i a m e t e r , f r e s h a n d u n z o n e d , e n c lo s in g m a n y c r y s t a l s o f
a m p h i b o l e a n d z o is it e. T h e g r a i n s s h o w o n e s e t o f f in e t w i n - l a m e l l a e
i n t e r s e c t e d a t a s m a l l a n g l e b y a p r o m i n e n t c l e a v a g e , A s e c o n d c l e a v a g e ,
n e a r l y p e r p e n d i c u l a r t o t h e f ir s t, is o c c a s i o n a l l y v i s ib l e a s il l- d e f in e d
i n t e r m i t t e n t c r ac k s . M e a s u r e m e n t o n t h e u n i v e r sa l s ta g e s h o w e d t h e
t w i n n i n g t o b e o n t h e p e r i c li n e la w , t h e p r o m i n e n t c l e a v a g e t o b e ( 0 01 ),
a n d t h e c o m p o s i t i o n t o a p p r o a c h A n 10 0.
T h e a n g u l a r v al u e s o b t a i n e d w e r e : c a 102 ~ t i f f 5689~ ~ , 1 0 4 ~ 2 V ~
7789~ T h e s e a n g le s , i n t e r p r e t e d o n T e r t s c h ' s c u r v e s 1 f o r c a lc i c p l a g i o -
c la se s , c o n f i r m t h e c o m p o s i t i o n to b e t h a t o f a n a n o r t h i t e ( a b o u t A n gv ).
T h e v a l u e s a c c o r d r e a s o n a b l y w e l l w i t h t h o s e d e r i v e d f r o m t h e l o w -
t e m p e r a t u r e c u r v e s f o r a p la g i o c la s e o f t h i s c o m p o s i t io n . I n t e r p o l a t i o n
f r o m T e r t s c h ' s h i g h - t e m p e r a t u r e c u r v e s g iv e s m u c h le ss c o n s i st e n t A n
v a l u e s. ( T h e c a , a n d [ J f l v a l u es a r e b e y o n d t h e r a n g e s o f t h e h i g h -
t e m p e r a t u r e c u r v e s .) I n v ie w , h o w e v e r , o f t h e n e c e s sa r il y a p p r o x i m a t e
d e t e r m i n a t i o n s ( o ne s e t o f t w i n l a m e l l a e b e i n g to o n a r r o w f o r m e a s u r e -
m e n t s a v e i n o n e o r t w o g ra in s ) a n d t h e p o ss ib le d o u b t f u l a c c u r a c y o f
t h e h i g h - t e m p e r a t u r e d e t e r m i n a t i v e c u r v e s , 2 t h e d i a g n o s is o f th e l o w -
t e m p e r a t u r e m o d i f i c a ti o n c a n o n l y b e r e g a r d e d a s t e n t a ti v e .
ZOISITE.
T h e a b u n d a n c e o f t h e z o i si te a n d i ts u n u s u a l o p t i c a l p r o p e r t i e s w e r e
c o n s i d e r e d t o j u s t i f y d e t a i l e d c h e m i c a l a n d o p t i c a l e x a m i n a t i o n . T h e
m i n e r a l o c c u r s i n tw o f o r m s - - a c o m m o n g r e en v a r i e t y a n d a c o l o ur le s s
m o d i f i c a t io n , t h e l a t t e r r e p r e s e n t e d i n o n l y o n e r o c k - s p e c i m e n . A c h e m -
i c a l a n a l y s i s o f a g r a v i t y - s e p a r a t e d , h a n d - p i c k e d s a m p l e o f t h e g r e e n
z o i s i t e g a v e t h e r e s u l t s s h o w n i n t a b l e I .
T h e f e a t u r e o f t h e a n a l y s i s is t h e l o w v a l u e f o r H e O a n d t h e h i g h v a l u e s
f o r A 1 a n d C a. E v e n a f te r r e c a lc u l a ti o n t o th e b a s is o f 5 2 ( 0 , 0 H ) t h e
c a t i o n t o t a l is a b o u t 3 % h i g h e r t h a n t h a t r e q u i r e d t o s a t is f y th e f o r m u l a
C a 2M a Sia O 12 0H . O n t h e a c t u a l ( e m p i r ic a l ) O , O H c o n t e n t t h e c a t i o n
t o t a l i s 5 % h i g h .
1 H . T ertsch, Zur H cchtemperaturoptik bas ische r Plagioklase. Tschermaks
M in. Petr. M itt ., 1942, vol. 54, pp. 193 ~1 7. [M .A. 9-270.]
D. L. Reynolds, T he difference in optics between volcanic and plutonic plagio-
c la se s. . . Ge ol . Mag., 1952, vol. 89, pp. 2334 50. [M.A. 12-137.]
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ZOISITE AMFHIBOLITE FROM TAN GANYIKA
TABLE I . Chem ica l ana lys is o f g reen zo is i te f rom n or the rn Tang any ika .
(B.M. 1952,133.) An alys t , D. I . Bo thw el l .
4 6 1
Em pi r i ca l un i t -ce l l Ce ll con ten t s on bas is
A t o m i c r a ti o s , c o n t e n t s ) o f 5 2 ( 0 ,O H ) .
SiOe .. . 39 .]6 Si . . . 0.652 12-33 12.08
A120a .. . 33.50 A1 0.657
242~
12 17~
F%O3. .. 1 7 5 F e . .. 0-022 0 42~ 12 92 0 41~ 12 66
Fe O . .. n i l
Cr203. . . 0 33 Cr . .. 0-004 0 .08} 0-0 8]
CaO ... 25'02 Ca .. . 0.446 8.43 I 8'26 t
M gO . . . ni l 8.43 8 '26
MnO . .. 0 .014 Mn . .. 0 - 0 0 0 2 0 .004J 0 0041
N a 2 0 . . . n i l
K2 0 . .. n i l
I-IeO~- 0 57 0 ... 2 742 5 53 07 52 00
ILI2O 0-06 O H . . . 0-063 1 18]
TiO 2 . .. 0 001
100.40
Sp. gr. (d~~ 3 .364•
Orienta t ion . Crys ta ls
o f b o t h g r e e n a n d c o l o u r le s s z o is i te s s h o w a
s in g l e c l e a v ag e . T h e c l e a v a g e c r ac k s a r e t r u l y p l a n e , t h o u g h r a t h e r
w i d e l y s p a c ed . C o n o s c o p i c o b s e r v a t i o n s o n i s o l a t e d c r y s t a l s s h o w e d t h a t
a ( = B x o ) i s i n a l l i n s t a n c e s p e r p e n d i c u l a r t o t h e c l e a v a g e p la n e , w h i c h
t h e r e f o r e c o r r e s p o n d s t o t h e y fi p l a n e . T h e o r i e n t a t i o n i s , t h e r e f o r e , t h a t
o f f e r r i f e r o u s z o i s i te w i t h 5 % o r m o r e F e 2 0 a ( t h e fi - z o is i te o f T e r m i e r) . ~
I r o n - f r e e z o i s i te h a s t h e o p t i c p l a n e p a r a l l e l t o t h e c l e a v a g e ; y e t t h e
a n a l y s is s h o w s l es s t h a n 2 % F e 2 0 a. P r e l i m i n a r y m e a s u r e m e n t s o f 2 V y
s h o w e d v a l u e s o f a b o u t 2 5 ~ f o r t h e g r e e n a n d 3 5 ~ f o r t h e c o l o u r l e s s
v a r i e t i e s . T h e s e v a l u e s c o r r e s p o n d w i t h t h o s e u s u a l l y q u o t e d f o r i r o n -
f r e e z o i s i t e , i . e . 3 0 ~ 1 7 7B o t h t h e c o l ou r le s s a n d g r e e n z o i si te s w e r e f o u n d
t o h a v e e x t r e m e l y s t ro n g d i s p e rs i o n w i t h r ~ v . T h i s d i sp e r si o n , o n t h e
o t h e r h a n d , a g r e e s w i t h t h a t g i v e n f o r a f e r r i f e r o u s z o i s i te . I t i s t h u s
e v i d e n t t h a t t h e o p t i c s d o n o t c o n f o r m ( in r e l a t i o n to t h e c h e m i c a l c o m -
p o s i t i o n ) w i t h th o s e p r e v i o u s l y p u b l i s h e d f o r z o is i t e . T h e o r i e n t a t i o n a n d
t h e d i s p e r s i o n i n d i c a t e a f e r r i fe r o u s z o i s i te , w h i l e t h e o p t i c a x i a l a n g l e
s u g g e s ts t h a t t h e m i n e r a l h as l i t t l e i ro n , a s c o n f i r m e d b y a n a ly s is .
I n o r d e r t o e s t a b l is h w i t h c e r t a i n t y t h e r e l a t i o n b e t w e e n t h e o p t i c a l
a n d c r y s t a l l o g r a p h i c d i r e c t i o n s t h r e e s i n g l e - c r y s t a l X - r a y o s c i l l a t io n
p h o t o g r a p h s w e r e t a k e n b y G . F . C l a r i n g b u l l a b o u t a , f i , a n d y . T h e l a y e r -
a Ca lcu la ted f rom the de te rm ined va lue o f the sp. g r . t oge the r w i th the un i t -ce f f
d imen s ions a 16-30, b 5 .60 , c 10 .21 ~ . (L. W aldb auc r and D. C. McCann, Am er.
Min. 1935, vo l. 20, p. 107 .) [M.A. 6-179 .]
2 p. Te rm ier, Bu ll . Soc. Fra ng . Min. 1898, vo l. 21, pp. 148-170.
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462
P M GAME ON
l i ne s p a c i n gs o f t h e t h r e e p h o t o g r a p h s i n d i c a t e d t h e a p p r o x i m a t e v a l u e s
1 6, fl 5 , ~ 1 0 ~_. C o m p a r i n g t h e s e v a l u e s w i t h t h o s e o f W a l d b a u e r a n d
M c C a n n i t is e v i d e n t t h a t a c o i n c i d e s w i t h a , fl w i t h b , a n d ~, w i t h c .
T h e c l e a v a g e i s t h u s s h o w n t o b e ( 1 0 0 ) a n d t h e o p t i c a x i a l p l a n e i s
p a r a l l e l t o ( 01 0). A t fi r st s ig h t t h e o r i e n t a t i o n o f b o t h t h e s e p l a n e s
a p p e a r s t o d i ff e r f r o m t h a t o f t h e t e x t b o o k f ig u re s ( fo r e i t h e r i ro n - f r e e o r
f e r r i f e ro u s z o i s it e ), w h i c h s h o w t h e c l e a v a g e a s (0 10 ), a n d t h e o p t i c a x i a l
p l a n e a s ( 01 0) f o r i r o n - f r e e a n d ( 00 1) f o r f e r r i f e r o u s zo i s it e . H o w e v e r ,
t h e t e x t b o o k o r i e n t a t i o n s a r e b a s e d o n a x i a l r a t i o s : a : b : c = 0 " 6 2 : 1 :
0 -3 4 , d e r i v e d f r o m t h e e a r li e r m o r p h o l o g i c a l m e a s u r e m e n t s .
C o m p a r i n g t h e se r a t io s w i t h t h e X - r a y p a r a m e t e r s i t is e v i d e n t t h a t :
a ( f ro m m o r p h o l o g i ca l m e a s u r e m e n t ) - - c ( fr o m X - r a y m e a s u r e m e n t )
b . . . . . . ) a . . . . . . )
c . . . . . . ) = b . . . . . . )
M akin g t hes e t r an s fo rm at i on s (g iv ing a : b : c ~ 0" 625 : 1 : 0 "344) i t is
a p p a r e n t t h a t t h e c l e a v a g e d o es , i n f a c t , c o i n ci d e w i t h t h e (0 10 ) p l a n e
o f t h e t e x t b o o k f i g u re s a n d t h a t t h e o p t i c a x i a l p l a n e i s t h e (0 01 ) p l a n e
o f t h e s e a u t h o r s . I n o t h e r w o r d s , t h e o r i e n t a t i o n a g r e e s w i t h t h a t g i v e n
i n t e x t b o o k s f o r f e r ri f e r o u s z o i si te .
Size and arrangement of crysta ls. Th e
g r e e n z o i s i t e u s u a l l y s h o w s
d i s t in c t e l o n g a t i o n p a r a l l e l t o fi, w i t h p a r t i a l a l i g n m e n t o f t h e c r y s t a l s.
T h e l e n g t h : b r e a d t h r a t i o i s o f t h e o r d e r o f 3 : 1, b u t r e a c h e s 6 : 1 i n s o m e
g r a i n s . T h e c r y s t a l le n g t h v a r i e s f r o m a b o u t 8 9 o 189 r a m . , b u t i s s o m e -
t i m e s r e d u c e d i n p r o x i m i t y to t h e c o r u n d u m c r y st a ls . T h e g r e e n z o is it e
s h o w s f e w c r y s t a l f a c e s ; m a n y g r a in s s h o w s u b - r o u n d e d t e r m i n a t i o n s .
T r a n s v e r s e c r a c k s a r e e x t e n s i v e ly d e v e l o p e d a n d a r e a p r o m i n e n t f e a t u r e .
T h e c o l ou r le s s z o i s it e , f o r m i n g v i r t u a l l y a m o n o m i n e r a l i c r o c k , s h o w s
n o e l o n g a t i o n o r s c h i s to s i t y . T h e g r a i n s t e n d t o b e e q u i d i m e n s i o n a l ;
t h e a v e r a g e d i a m e t e r is 2 r a m . ( m a x i m u m 4 r a m . ). T h e g r a i n s f o r m a
p o l y g o n a l m o s a i c a n d m a n y s h o w p a r t ia l c r y s t a l f o r m , t h o u g h c r y s t a l
f a ce s o f te n t e n d t o b e m o d i fi e d d u e t o m u t u a l i n t e rf e re n c e d u r i n g g r o w t h .
A l l g r a i n s s h o w a n e x t e n s i v e s y s t e m o f c ra c k s , s o m e t i m e s r o u g h l y p e r -
p e n d i c u l a r t o t h e c l e a v a g e , b u t o f t e n f o r m i n g a n ir r e g u l a r n e t w o r k . F i v e
o f t h e b e s t - d e v e l o p e d c r y s t a l s i n t w o s e c t io n s o f t h i s c o l o u r l e s s z o i s i te
r o c k w e r e m e a s u r e d o n t h e u n i v e r s a l s t a g e i n o r d e r to d e t e r m i n e t h e
f a c e s r e p r e s e n t e d . E a c h o f t h e f i v e r e s u l t in g s t e r e o g r a m s w a s f ir s t t r a n s -
p o s e d , b r i n g i n g 7+ t o t h e c e n t r e . T h e f i v e t r a n s p o s e d s t e r c o g r a m s w e r e
t h e n s u p e r i m p o s e d t o b r i n g a , f i, a n d 7+ i n t o c o i n c i d e n c e . I n d e x i n g o f
f a c e s o n t h is f in a l s t e r e o g r a m w a s p e r f o r m e d b y p l o t t i n g t h e p o l es o n a
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ZOISITE AMPHIBOLITE FROM TANGANYIKA 4 6 3
g n o m o n o g r a m , b a s e d o n t h e X - r a y p a r a m e t e r s , w i th ~ (c) a t t h e c e n t re
( i. e. h a v i n g t h e s a m e o r i e n t a t io n a s th e s t e r e o g r a m ) . T h e f o r m s t h u s
i d e n ti f ie d a f t e r t r a n s f o r m a t i o n f r o m t h e X - r a y o r i e n t a t i o n (7+ = c) t o t h e
m o r p h o l o g i c a l o r i e n t a t i o n (7+ = a ) a r e a s f o ll o w s :
( 1 0 0) ( v i c i n a l f a c e 8 ~ f r o m 1 0 0 ), ( 0 1 0) , ( 0 21 ) ( v i c i n a l f a c e 4 ~ f r o m 0 2 1 ) ,
( 0 3 4 ) ( v i c i n a l f a c e 4 ~ f r o m 0 3 4 ) , ( 0 4 1 ) , ( 0 9 1 ) , ( 1 4 0 ) , ( 1 4 2 ) , ( 1 4 4 ) , ( 1 6 4 ) ,
(311) , (313) , (431) , (562) , (971) , (991) , (20 .5 .2 ) .
O f t h e s e o b s e r v e d f o r m s o n l y f iv e (1 0 0 ), ( 0 1 0 ), ( 0 2 1 ), ( 0 4 1 ), a n d ( 1 4 0) ,
a r e l is te d as c r y s t a l - f o r m s b y D a n a a n d G o l d s c h m i d t . W i t h t h e e x c e p -
t i o n o f (1 0 0) a n d (0 1 0) , s im p l e f o r m s a r e a b s e n t a n d t h r e e f a c e s a re v i c i n a l
t o f a c e s h a v i n g t h e q u o t e d i n d ic e s. ( T h e a n g u l a r d i ff e re n c e s i n t h e s e
i n s t a n c e s a r e o u t s i d e t h e r a n g e o f e x p e r i m e n t a l e r r0 r l) T h e h ig h i n d ic e s
m a y p e r h a p s b e d u e t o m u t u a l i n te r fe r en c e .
OPTICAL PROPERTIES.
Refract ive ind ices . Th e ~ a n d f i v a l u e s o f b o t h t h e g r e e n a n d w h i t e
z o i s i t e w e r e d e t e r m i n e d o n s e l e c t e d o r i e n t e d c r y s t a l s , u s i n g t h e i m -
m e r s i o n m e t h o d , w i th m i x t u re s o f m e t h y l e n e io d id e a n d m o n o b r o m o -
n a p h t h a l e n e . A B e l li n g h a m a n d S t a n l e y r e f r a c t o m e t e r w a s u s e d t o
m e a s u r e t h e r e f r a c t iv e i n d e x o f t h e m a t c h e d l iq u i d in s o d iu m - li g h t . T h e
v a l u e s o f b o t h ~ a n d fl w e r e o b t a i n e d f r o m c l e a v a g e f la k e s , s in c e th e
c l e a v a g e c o i n c i d e s w i t h t h e ~ fi p l a n e . C l e a v a g e fl a k e s t h u s g i v e a c e n t r e d
o b t u s e b i s e e t r i x i n t e r f e re n c e f i g u re a n d a r e e a s il y r e c o g n i z ab l e . T h e
v a l u e o f f l w a s d e t e r m i n e d , i n a d d i ti o n , f r o m g r a i n s s h o w i n g t h e c e n t r a l
e m e r g e n c e o f a n o p t i c a x i s ; g r a i n s h a v i n g t h i s o r i e n t a t i o n a r e a l so e a s i ly
d e t e c t e d b y t h e i r a b n o r m a l p u r p l e - y e l lo w i n t e r f e re n c e c o l o u r s r e s u l t in g
f r o m t h e e x t r e m e a x i a l d i s p er s io n o f t h e m i n e r a l .
F o r m o s t o f t h e r e f r ac t iv e in d e x m e a s u r e m e n t s t h e c o n t r o l d r o p
m e t h o d w a s u se d . T h e r e f r a c t i v e in d e x o f t h e l i q u id m i x t u r e w a s a d -
j u s t e d t o a v a l u e s o m e w h a t a b o v e t h e in d e x t o b e m e a s u r e d in t h e
m i n e r a l. T h e d r o p w a s s p l i t i n t o t w o p o r t i o n s ; t h e f ir s t p o r t i o n r e m a i n e d
o n t h e g l a s s s li de ( w i t h t h e i m m e r s e d g r a i n s ) w h i c h w a s p l a c e d o n t h e
m i c ro s c o p e s ta g e . T h e s e co n d p a r t o f t h e d r o p w a s p l a c e d u p o n t h e g l as s
p r i s m o f t h e r e f r a c t o m e t e r w h i c h w a s i m m e d i a t e l y a d j a c e n t t o t h e
m i c r o s c o p e , so t h a t t h e t w o g l as s s u r f a c e s ( c a r r y i n g t h e t w o p o r t i o n s o f
t h e l iq u i d d r o p ) w e r e s e p a r a t e d b y o n l y a f e w i n ch e s . T h e o p e r a t i o n w a s
c a r r i e d o u t in a r o o m f r e e f r o m d r a u g h t s . O w i ng to t h e m o r e r a p i d r a t e
o f e v a p o r a t i o n o f t h e m e t h y l e n e io d id e c o m p a r e d w i th t h a t o f t h e m o n o -
b r o m o n a p h t h a l e n e , t h e r e f r a c t i v e i n d e x o f t h e t w o d ro p s b e c o m e s
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4 6 4 ~ M G A M E O N
g r a d u a l l y lo w e r . T h e l i q u i d o n t h e s l i d e, t h e r e f o r e , s l o w l y p a s s e d t h r o u g h
t h e r e f r a c t iv e i n d e x v a l u e a t w h i c h it m a t c h e d t h e d e s i re d i n d e x o f t h e
g r a in , a n d a t t h is i n s t a n t t h e r e f r a c t o m e t e r r e a d i n g w a s n o t e d . T h i s
t e c h n i q u e w a s f o u n d t o g i v e r e a s o n a b l y c o n c o r d a n t v a l ue s . F o r
e x a m p l e , in o n e s e t o f 8 m e a s u r e m e n t s t h e d i ff e re n c e b e t w e e n t h e m a x i -
m u m a n d m i n i m u m v a l u e s w a s 0- 00 19 , a n d w a s r e d u c e d t o 0.0 01 1 b y
o m i t t i n g o n e r e a d i n g .
T h e v a l u e o f a w a s n o t d e t e r m i n e d d i r e c t ly , b u t w a s o b t a i n e d b y
c a l c u l a t i o n f r o m t h e m e a n v a l u e s m e a s u r e d f o r y , f l, a n d 2 V . O w i n g t o
t h e d i f f i c u lt y o f f i n d i n g s u i t a b l e c r y s t a l s h a v i n g t h e r e q u i r e d o r i e n t a t i o n ,
t h e t h e o r e t i c a l l y d e r iv e d i n d e x w a s c o n s i d e re d to b e m o r e a c c u r a t e .
Op tic axial ang le. This w a s m e a s u r e d i n so d i u m - l i g h t o n t h e u n i v e r s a l
s t a g e . S e c t i o n s o f a b o u t t w i c e t h e n o r m a l t h i c k n e s s w e r e u s e d . O w i n g
t o t h e s m a l ln e s s o f 2 V t h e u s u a l o r t h o s c o p ic m e t h o d w a s f o u n d t o g i v e
w i d e f l u c t u a t i o n s in t h e m e a s u r e m e n t s a n d m u c h m o r e p r e c i se v a l u e s
w e r e f o u n d t o r e s u l t f r o m c o n o s c o p i c o b s e r v a t i o n ( f u ll a p e r t u r e s o n s u b -
s t a g e d i a p h r a g m s , a n d i n s e r ti o n o f B e r t r a n d l en s) . F o u r t o f iv e r e a d i n g s
w e r e m a d e o f t h e p o s i t i o n o f e a c h o p t i c a x i s. T h e v a l u e s o f 2 V g i v e n i n
t a b l e I I a r e t h e r e s u l t o f 12 d e t e r m i n a t i o n s o f t h e g r e e n z o is i t e a n d 1 2
o f t h e c o l o u r l e s s v a r i e t y .
TABL~ II . Op tical da ta (mean values),
a. ft. y. 2Vv.
range
G re en z oisite . . . . . . 1 . 7 0 0 1 . 7 0 0 1 . 7 0 5 2 5 ~ (1 7~ -3 3 ~
W h i te zo isite . . . . . . 1 . 6 9 5 1 . 6 9 8 1 . 7 0 4 3389~ (28-37 ~
Dispers ion . I t
w a s a p p a r e n t d u r in g th e p r e l i m i n a r y o p t ic a l e x a m i n a -
t i o n t h a t t h e z o i s i t e p o s s e s s e d e x t r e m e l y s t r o n g a x i a l d is p e r s i o n , s i n ce
t h e i s o g y r e s o f w e l l - c e n t r e d i n te r f e r e n c e f ig u r es s h o w e d c o n s p i c u o u s
c o l o u r f r in g e s . I t w a s , t h e r e f o r e , d e c i d e d t o a t t e m p t t o m e a s u r e t h e
v a r i a t i o n i n t h e o p t i c a x i a l a n g le c a u s e d b y c h a n g e s in t h e w a v e - l e n g t h
o f t h e l i g h t . O w i n g t o t h e w e a k b i r e f r i n g e n c e , t h e s m a l l n e s s o f t h e c r y s -
t a le , a n d t h e i r e x te n s i v e c r a c k in g , t h e a c c u r a t e d e t e r m i n a t i o n o f 2 V
f o r l i g h t o f v a r y i n g w a v e - l e n g t h s p r e s e n t e d s o m e d i f fi c u l ty . T h e f i r st
m e a s u r e m e n t s , m a d e o n a n o p t i c a x i a l g o n i o m e t e r a n d o n a s t a g e g o n i o -
m e t e r , f i t t e d t o a S w i f t m i c r o s c o p e , w e r e n o t e n t i r e l y s a t i s f a c t o r y , a l -
t h o u g h t h e y c o n f i rm e d t h e o r d e r o f t h e d i s p er s io n ( r ed to v i o l e t) t o b e
a b o u t 1 0 ~
I t w a s f i n a ll y f o u n d t h a t m u c h m o r e r e l i a b l e r e s u l ts w e r e o b t a i n a b l e
u s i n g a u n iv e r s a l s ta g e m e t h o d . A s u i t a b l y o r i e n t e d c r y s ta l , g iv i n g o n e
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Z O I S I T E - A M P H I B O L I T E ~ R O M T A N G A : N Y I K A
4 6 5
a l m o s t c e n t r e d o p t i c a x is , w a s i s o l a te d ; b o t h u p p e r a n d l o w e r s u r f a c e s
w e r e c a r e f u l l y g r o u n d , t h u s p r o d u c i n g a p a r a l l e l - s i d e d p l a t e a b o u t
0 7 r a m . i n t h i ck n e s s . T h i s w a s m o u n t e d i n a c i rc u l a r d e p r e ss i o n o n a
g l a ss s lid e . A L e i t z 4 -a x i s s t a g e w a s u s e d f o r t h e m e a s u r e m e n t s ; t h e
s u b s t a g e d i a p h r a g m s w e r e o p e n e d t o g i v e c o n o s c o p i c d e f in i t i o n . A H i l g e r
32
>
3d
i I I t S I O O r
65 8 55 S O 43
WA V E LENGTH I N A N GS TR OM UNITS
F I c . 1 . Z o i s it e c o l o u rl e s s v a r i e t y ) f r o m T a n g a n y i k a . V a r i a t i o n i n o p t i c a x i a
a n g l e 2 V w i t h w a v e - l e n g t h o f l i gh t .
m o n o c h r o m a t o r w a s e m p l o y e d in c o n j u n c t io n w i t h a h i g h - p r e ss u r e
m e r c u r y - v a p o u r 5 0 w a t t l a m p , th e e m i s s io n f r o m w h i ch w a s i n te n s e f o r
t h e ' m e r c u r y v i o l e t ' {4358 X . ) a n d ' m e r c u r y g r e e n ' (5 46 1 X . ) w a v e -
l e n g t h s a n d s a t i s f a c t o r i ly s t r o n g f o r w a v e - l e n g t h s i n t h e b l u e , y e l l o w , a n d
r e d o r d e rs o f t h e s p e c t r u m . T h e v a l u e o f 2 V w a s d e t e rm i n e d f o r s ix
d i f fe r e n t w a v e - l e n g t h s r a n g i n g f r o m 4 3 58 to 6 5 5 0 -~ . U s i n g e a c h e y e
a l t e r n a t e l y , 1 0 r e a d i n g s w e r e m a d e f o r th e v i o l e t a n d b l u e w a v e - I e n g t h s
a n d 6 r e a d in g s f o r th e r e m a i n i n g w a v e - l e n g t h s o f t h e p o s i t io n o f t h a t
o p t i c a x is w h i c h e m e r g e d a l m o s t v e r t i c a l ly (A 1). F o u r r e a d i n g s w e r e
m a d e o f t h e o t h e r m o r e i n c l i n e d a x i s ( A 2) w h i c h g a v e l es s p r e c i s e d ef in i -
t i o n o f t h e i s o g y r e. F r o m t h e s e t w o s e ts o f r e a d i n g s ( f o r e a c h o p t i c a x is )
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4 6 6
1 . M. G AM E ON ZO ISITE-AM PHIBO LITE
t h e m e a n p o s i t i o n o f t h e a c u t e b i s e c t r i x w a s d e t e r m i n e d . T h i s a n g u l a r
v a l u e (B x a) w a s s u b t r a c t e d f r o m t h e m e a n v a l u e o b t a i n e d f o r A 1 f o r e a c h
w a v e - l e n g t h , a n d t h e r e s u l t i n g a n g l e w a s d o u b l e d , t h u s g i v i n g 2 V . T h e
c o r r e c t i o n s r e q u i r e d f o r t h e d i f fe r e n c e i n r e f r a c t i v e i n d e x o f h e m i s p h e r e
{ 1 .6 4 9) a n d m i n e r a l ( 1 6 99 ), w e r e v e r y s l i g h t o w i n g t o t h e s m a l l i n c l i n a -
t i o n s . T h e r e s u l t a n t v a l u e s o f 2 V f o r t h e s i x d i f f e re n t w a v e - l e n g t h s a r e
g i v e n i n T a b l e I I I . I f a l o g a r i t h m i c h o r i z o n t a l sc a le ( H a r t m a n n d i s-
p e r s i o n p a p e r ) b e u s e d t o p l o t t h e s e v a l u e s , a n a l m o s t s t r a i g h t l i n e
r e l a t i o n s h i p r e s u l t s ( fig . 1) .
TABLE II I. Zoisite (colourless va rie ty) dispersion.
W ave -length . Optic axia l angle.
4358 A . 27.2 ~
4916 31.3
5461 33.9
5790 34.9
6250 36.2
6550 36.7
Acknowledgen~ents. I w i s h t o t h a n k D r . G . F . C l a r in g b u l l f o r t h e X - r a y
d a t a , M r . D . I . B o t h w e l l f o r t h e a n a l y s i s , D r . M . H . H e y f o r h e l p w i t h t h e
i n d e x i n g o f t h e z o i s it e , a n d M r . A . H . W e i r f o r h i s a s s i s ta n c e i n s o m e o f
t h e d i s p e r s i o n d e t e r m i n a t i o n s . D e t a i l s o f t h e f ie l d o c c u r r e n c e w e r e
k i n d l y s u p p l i e d b y M r . N . J . G u e s t o f t h e T a n g a n y i k a G e o l o g ic a l
S u r v e y .
