BEHAVIOR OF BxOy COMPLEXES IN A L K A L I N E H Y D R O T H E R M A L SOLUTIONS
(EXPERIMENTAL AND SYNTHETIC FLUID INCLUSION STUDY)
S M I R N O V , S . Z . ' . D E M I N , S. P. 1 , T H O M A S , V . G . '
1
I M P S B R A S , pr. ac. K o p t y g a , 3 , N o v o s i b i r s k , Russia.
E-mail: [email protected]
Introduction
T h e f i n d i n g of sassolite d a u g h t e r crystals in fluid inclusions in p e g m a t i t e minerals f r o m a r o u n d the w o r l d ( S m i r n o v et al.,
2 0 0 0 ; P e r e t y a z h k o et al., 2 0 0 0 ; T h o m a s et al., 2 0 0 0 ) entails an important c o n c l u s i o n that b o r o n c o u l d b e m a j o r c o m p o n e n t o f
h y d r o t h e r m a l fluids. T h i s f u r t h e r raises a host of questions regarding b o r o n b e h a v i o r and s p e c i a t i o n u n d e r natural c o n d i t i o n s
associated with crystallization of h y d r o t h e r m a l minerals. T h i s work r e p r e s e n t s the results of the fluid inclusion s t u d y o f q u a r t z
g r o w n f r o m s o d i u m - b o r a t e h y d r o t h e r m a l solutions.
Experimental setup and results
F o r investigation of b o r i c acid behavior under h y d r o t h e r m a l c o n d i t i o n s w e h a v e c h o s e n c o m p o s i t i o n s within wellcharacterized system N a 2 0 - B 2 0 3 - H 2 0 . Quartz crystals w e r e g r o w n o n seed at 5 2 0 ° C and 1.5 kbars. T h e c o m p o s i t i o n s o f
starting solutions are s h o w n in the T a b l e . T h e starting c o m p o s i t i o n w a s c h o s e n with the a i m of o b t a i n i n g f l u i d i n c l u s i o n s
c o n t a i n i n g borate d a u g h t e r crystals in quartz. I n c l u s i o n s o b t a i n e d in quartz o v e r g r o w t h h a v e b e e n s t u d i e d m i c r o thermometrically and by R a m a n s p e c t r o s c o p y within 2 0 - 3 5 0 ° C t e m p e r a t u r e range.
T h e growth of q u a r t z and fluid inclusion e n t r a p m e n t h a v e been o b s e r v e d in s o l u t i o n s c o n t a i n i n g 0 . 1 - 6.5 wt. % of N a O H .
S e e d s and charge c o m p l e t e l y dissolved in experimental solution with 12 wt. % N a O H , and, a f t e r the run, t h e s y s t e m c o n s i s t e d
of water solution and glass-like phase. T h e f o r m a t i o n of glass-like p h a s e ( G L P ) w a s also o b s e r v e d in e x p e r i m e n t s with 6.5 wt.
% of N a O H along with quartz g r o w t h .
Quartz o v e r g r o w t h s contain a b u n d a n t primary t w o - p h a s e fluid inclusions, r a n g i n g in size f r o m 10 to > 1 0 0 |xm. T h e
inclusions consist of water solution and a gas bubble. T h e glass-like p h a s e was not o b s e r v e d in fluid inclusions. D a u g h t e r
crystals of sassolite were o b s e r v e d o n l y in inclusions c o r r e s p o n d i n g to 0.1 and 0.8 wt. % N a O H in t h e starting solution. T h e
last melting crystals in inclusions o b t a i n e d in other runs appear to be ice. E u t e c t i c t e m p e r a t u r e s of i n c l u s i o n s d e c r e a s e
progressively with increasing N a O H content in the starting solution.
Table. Starting solution compositions and results of the study
wt. %
NaOH
H3BO3
Glass-like phase
Fluid inclusions
Starting solution
Raman lines within
borate region, cm 1
Te
Tm
C
°C
20C
350C
871
0.1
10.8
-3
39-45*
876
0.8
10.9
-5.1 - - 6 . 3
20*
876
867
1.8
10.8
-8.4--9.0
-2.1 --2.7*
879
870
3.1
10.8
-12.7
-1.8*
876
874
6.5
21.3
-35
-17.5*
874
867
-2.7®
766, 788,
969
759, 781,
959, 867
6.5
10.8
12.9
10.7
12.9
21.3
-11
N o quartz growth
wt. %
Si02
Na20
B203
H20
Total
N0 G L P formation
G L P was not analysed
57.84
15.43
8.37
7.64
89.28
56.99
16.76
8.70
6.50
88.95
46.61
16.38
16.71
7.48
87.18
T e - eutectic t e m p e r a t u r e ; T m - temperature of final melting: * - dissolution of sassolite; * - ice m e l t i n g
G L P is typically located at the b o t t o m of container and substitute f o r charge particles. T h e results of e l e c t r o n m i c r o p r o b e
and S I M S m i c r o a n a l y s e s are presented in Table. T h e T a b l e s h o w s that G L P consists p r i m a r i l y of S i 0 2 , N a 2 0 a n d B 2 0 3 .
C o m m o n feature of all s a m p l e s is a p r o m i n e n t deficit of analytical totals. T h e G L P is h i g h l y u n s t a b l e u n d e r e l e c t r o n b e a m ,
indicating that s o m e constituents m a y b e lost in the c o u r s e of analysis. T y p i c a l l y a l k a l i n e m e t a l s and water a r e very m o b i l e in
h y d r o u s glass structure. In order to eliminate the loss of N a d u r i n g the analysis, w e used d e f o c u s e d ( 2 0 p.m) e l e c t r o n b e a m at
l o w b e a m current ( 1 0 nA). T h u s , the N a loss would not b e greater than 10 rel.%. It is a p p a r e n t f r o m the T a b l e , that a f t e r
c o m b i n a t i o n of S I M S data with electron m i c r o p r o b e analysis analytical totals still r e m a i n b e l o w 1 0 0 % . W e b e l i e v e that t h e
m o s t possible reason f o r this is the failure to d e t e r m i n e water b y S I M S , which w a s p r e v i o u s l y r e p o r t e d b y I h i n g e r et al. ( 1 9 9 4 )
f o r h y d r o u s glasses with water content >5 wt. %. A s s u m i n g that the total deficit is d u e to w a t e r loss, t h e water c o n t e n t c a n b e
assessed as 16 to 2 0 wt. % .
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Raman and IR-spectroscopy of inclusions and GLP
On the basis of p r e s e n c e of 876-867 cm" 1 band in R a m a n spectra of fluid inclusions within the e x p l o r e d t e m p e r a t u r e range,
w e c o n c l u d e d that the o n l y detectable species in solution is H3BO3. Integral intensity of this band d e c r e a s e s as N a O H content
increases in the starting solution. H e a t i n g of inclusions up to 3 5 0 ° C d o e s not result in a p p e a r a n c e of n e w b a n d s within the 5 0 0
- 1200 cm" 1 range, typical of the R a m a n spectra of alkali-borate and p o l y b o r a t e species. T h e only e x c e p t i o n s to this rule are
inclusions in quartz g r o w n in solution with 6.5 wt. % N a O H and 10 wt. % H 3 B 0 3 (see Table). T h e b a n d s at 7 6 6 - 7 5 9 and 9 6 9 9 5 9 cm" 1 in these inclusions indicate the p r e s e n c e of b o r o n in f o u r - and three-fold c o o r d i n a t i o n respectively, w h e r e a s H3BO3
a p p e a r s o n l y on heating in small a m o u n t s . T h e d e c r e a s e of integral intensity of the 8 7 6 cm" 1 band is o b s e r v e d f r o m 100 to
3 5 0 ° C f o r e x p e r i m e n t s with 0 . 1 - 3 . 1 w t . % N a O H and 10 wt. % H 3 B 0 3 ; f o r e x p e r i m e n t with 6.5 wt. % N a O H and 21 wt. %
H3BO3, the integral intensity of the 8 7 6 cm" 1 b a n d increases in this t e m p e r a t u r e range.
IR spectra of G L P s a m p l e s with highest B 2 0 3 content consist of b r o a d b a n d s centered at 4 5 0 and 1050 cm" 1 that could be
assigned to vibration of S i - O - S i bonds, b a n d s at 1650, 3 4 0 0 and 3 6 0 0 cm" 1 that could be assigned to H 2 0 and O H vibrations,
and less intensive at 6 8 0 , 1340 and 1400 cm" 1 . T h e bands at 6 7 0 cm" 1 of borosilicate glasses are typically a s s i g n e d to S i - O - B
vibrations (Sigoli et al.. 2 0 0 1 ) . B a n d at 1340 cm" 1 is p r o b a b l y d u e to B ( 3 ) - 0 - B ( 4 ) and 1400 cm" 1 c o u l d b e a s s i g n e d to B ( 3 ) - 0
vibrations ( V a l y a s h k o , V l a s o v a . 1966). B ( 4 ) - 0 vibrations can not be detected by IR spectroscopy, b e c a u s e they are usually
m a s k e d by stronger S i - O - S i vibrations.
Discussion
A c c o r d i n g to the p u b l i s h e d experimental data, the system N a 2 0 - B 2 0 3 - H 2 0 c o n t a i n s three non-variant eutectic p o i n t s within
the r a n g e of e x p e c t e d fluid-inclusion c o m p o s i t i o n s . For inclusions in quartz g r o w n in solutions with N a 0 H / H 3 B 0 3 of < 0 . 3 the
e x p e c t e d eutectic p o i n t s a r e close to each other at -1 and -1.5°C. F o r c o m p o s i t i o n s with N a 0 H / H 3 B 0 3 of 0 . 3 to 0.6, the
e x p e c t e d eutectic point is -5.8°C. D a u g h t e r p h a s e s of sassolite and h y d r o u s s o d i u m tetraborate should exist u p to ~ 5 0 ° C or
a p p e a r o n cooling. O u r t h e r m o m e t r i c data given in the T a b l e s h o w that the b e h a v i o r of fluid inclusions d o e s not c o r r e s p o n d to
w h a t w o u l d be e x p e c t e d o n the basis of p h a s e d i a g r a m for the system N a 2 0 - B 2 0 3 - H 2 0 . W e interpret this p h e n o m e n o n as an
indication of f o r m a t i o n of a new s o l u b l e substance, lowering eutectic and ice-melting temperatures and d e c r e a s i n g H 3 B 0 3
content in the solution. T h e data suggest that this s u b s t a n c e most p r o b a b l y is c o m p r i s e d of sodium, b o r o n and silica.
R a m a n s p e c t r o s c o p y d a t a support the p r e s e n c e of n e w borate-containing s u b s t a n c e , expressing it through the d e c r e a s e of
integral intensity of H3BO3 b a n d that f o l l o w s the increase of N a O H content. Further d e c r e a s e of integral intensity o b s e r v e d on
heating probably indicates slight increase of the content of n e w species as quartz gets dissolved f r o m inclusion walls. T h e
a b s e n c e of n e w b a n d s (apart f r o m 8 7 6 cm" 1 ) in borate vibration region s h o w s clearly that in contrast to N a 2 0 - B 2 0 3 - H 2 0 s y s t e m
alkali-borate c o m p l e x e s d o not appear in significant a m o u n t s within the explored t e m p e r a t u r e range.
T h e a p p e a r a n c e of the G L P in final e x p e r i m e n t product indicates that liquid immiscibility o c c u r s in the c o u r s e of q u a r t z
g r o w t h at N a O H content > 6 . 5 wt. %. T h i s is the e v i d e n c e f o r liquid immiscibility, which is a c o m m o n f e a t u r e f o r N a 2 0 - S i 0 2 H 2 0 and N a 2 0 - B 2 0 3 - S i 0 2 systems ( V a l y a s h k o , 1990, Haller et al., 1970). Our analyses s h o w that G L P represents h y d r o u s
s o d i u m borosilicate liquid. A c c o r d i n g to S I M S and IR analyses, b o r o n partitions b e t w e e n G L P and water solution. B o r o n in
G L P exists in three- and, probably, f o u r - c o o r d i n a t e d f o r m s , and could be partially incorporated into silicon-oxygen network.
T h e a b s e n c e of G L P in p r i m a r y fluid inclusions indicates that it f o r m s along with quartz growth rather than at c o o l i n g after
e x p e r i m e n t . H e a v i e r s o d i u m borosilicate liquid a c c u m u l a t e s at the b o t t o m of the growth container. Q u a r t z seeds, s u s p e n d e d in
the container, g r o w f r o m water solution a n d d o not trap this liquid.
Conclusions
Our e x p e r i m e n t s h a v e s h o w n that in the c o u r s e of the quartz g r o w t h , s o d i u m - b o r a t e c o m p o n e n t s of h y d r o t h e r m a l fluid
reacts with dissolved silica to f o r m highly s o l u b l e substance. T h e e f f e c t is p r o m i n e n t even at very low N a O H c o n t e n t s and
m a n i f e s t s itself through d e c r e a s e of eutectic ice melting and sassolite dissolution temperatures. A p p a r e n t l y , the c h e m i s t r y of
the fluid inclusions c o r r e s p o n d s to the s y s t e m N a 2 0 - B 2 0 3 - S i 0 2 - H 2 0 , rather than N a 2 0 - B 2 0 3 - H 2 0 . H3BO3 r e m a i n s o n e of the
dissolved b o r o n f o r m s a l o n g with other f o r m s , which d i f f e r f r o m s o d i u m meta- and tetraborate c o m p l e x e s . T h e f r a c t i o n of
H3BO3 c o m p l e x e s d e c r e a s e s with increasing N a O H content and the temperature, and increase with increasing H3BO3 content.
At high N a O H content b o r o n partitions b e t w e e n water solution and h e a v y s o d i u m borosilicate liquid, w h e r e it p r o b a b l y exists
in three- and f o u r - c o o r d i n a t e d f o r m s , a n d partially is b o u n d e d to silicon-oxygen n e t w o r k , c o r r o b o r a t i n g the p r e s e n c e of
s o d i u m - b o r o n - s i l i c a c o m p l e x e s inferred f r o m m i c r o - t h e r m o m e t r i c and s p e c t r o s c o p y data.
Our data s h o w an i m p o r t a n t role of heavy i m m i s c i b l e liquids f o r b o r o n partitioning under h y d r o t h e r m a l conditions. D u r i n g
the g r o w t h of crystals in natural alkaline e n v i r o n m e n t , crystals g r o w i n g o n the ceilings of cavities w o u l d not trap h e a v y liquid.
Studies of such crystals c o u l d lead to underestimation of the b o r o n contents in the system. A n o t h e r s o u r c e of u n d e r e s t i m a t i o n
is the f o r m a t i o n of s o d i u m - b o r o n - s i l i c a species in e v e n slightly alkaline sodium solutions.
T h e study is s u p p o r t e d by R F B R grant 0 3 - 0 5 - 6 4 4 3 6 .
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