1. Silicate Sediments Dissolution during interaction with Seawater
Ben Gurion University, Israel
Geological and Environmental Studies Department
Silicate Sediments Dissolution during interaction with Seawater
Daniel Winkler
Under supervision of Prof. Jiwchar Ganor (Ben Gurion University) and Yehudit Harlavan (Geological Survey of Israel)

2. Background
1. Silicate minerals contain Si. In addition they contain: Al, Mg, Ca, Fe, etc. They may also contain Sr in trace amount.
2. During contact with seawater, these minerals may dissolve, given that the solution is under-saturated with respect to these minerals.
3. With dissolution, the ions are released to the solution.
4. We may track the changes the solution is undergoing in order to compute the dissolution-rate.

3. Sr
Strontium (atomic number 38) is an alkaline-earth metal which has four naturally occurring stable isotopes: 88Sr which is the most abundant (82.58%), 87Sr (7.0%), 86Sr (9.86%), and 84Sr (0.56%). Among these isotopes, only 87Sr is radiogenic (a product of first-order 87Rb beta-decay).

4. SrCO3 SrSO4 Strontianite Celestite Other Minerals
Strontium appears in the following minerals:
Strontianite
SrCO3
in hydrothermal veins
in evaporitic rocks
Celestite
SrSO4
Other Minerals
Sr2+
1.13Å 
Ca2+
0.99Å 
Cationic exchange
Accessory Cation
+14%
Common minerals
Anortite CaAl2Si2O8
Apatite Ca5(PO4)3(OH,F,Cl)
Calcite CaCO3
K-feldspar KAlSi3O8 (Sr2+ ↔K+)

5. 87Sr/86Sr Rb-Sr decay: spontaneous process
Decay parameter
Father
Daughter Sr n +p
[beta particle = electron]
spontaneous process
is independent of environmental conditions.
Decay constant is known to us:
λ=1.42*10-11years-1
Isotopic composition
87Sr/86Sr

6. Sr budget of the Ocean: ? Interaction with sea-floor sediments
Basic assumptions
Sr is homogenized in the oceans (long residence time in comparison with mixing rate)
There are differences in the isotopic composition between the various donors.
Interaction with sea-floor sediments
(Unknown) ?

7. 87Sr/86Sr variations through time:
0.7092
Today
Jurassic
today
ערך נוכחי
0.7082
triassic
0.7070
perm
0.7076
carbon
0.7077
devon
Jurassic
0.7087
silur
0.7077
ordovican
Via Marine carbonates
Wickmann (1947) ↓
Burke (1982)
0.7091
Cambrium

8. 87Sr/86Sr in the oceans is affected by the following:
Faure 1986
Old crust rocks
young volcanics
carbonates
fractions
Typical Values

9. carbonates young volcanics Old crust volcanics carbonates today today
Jurassic

10. One homogenous river-bed Dry sieving to fractions of
Roded Quartz-Diorite
Eilat Granite
Yehoshafat Granite
Amram Rhyolite
Sampling
One homogenous river-bed
Dry sieving to fractions of
60-90µm
Ultra-sonic shaking in Ethanol and drying in 60c for 24hr (removal of Clay particles)
Leaching in acetic acid 0.5M for 1hr and in DDW washing x3 (removal of dust)
Dissolution experiments

11. Batch experiments
Closed system (Batch). Each tube represents one single point.
All experiment tubes have the same conditions (pH, Temp, Solution/Sediment ratio)
After some time the solution is separated from the sediment for chemical analyses. t1 t2 t3 t4 t5 t6 t7 t8
Solution 40gr
Sediment 0.4gr

12. Filtering via 0.22μm mesh
t0+t
Temp=25C
Analyses of Si,Al:
Spectrophotometer
Perkin Elmer Lambda 2S
precision 5%±
Analyses of Sr, Mg, Ca, Fe :
ICP-MS [at G.S.I]
Perkin Elmer SCIEX ELAN
precision 10%±
Sr separation
87Sr/86Sr Analysis
Nu Plasma MC-ICP-MS
precision 0.002% ±

13. Comparison of Seawater and borax solutions:
Ionic strength
Conc.
Elec.
charge
Synthetic Seawater Solution:
NaCl, KCl, MgCl2x6H2O, KBr, MgSO4x7H2O ,CaCl2x2H2O
Borax Solution:
Na2B4O7x10H2O+ HCl

14. Dissolution experiments
Sample
size
[μm]
Experiment
Duration [days]
Albite
67-75
Seawater[without washing]
Borax solution [with washing]
172
152
K-feldspar
60-90
Seawater[with washing]
Amram Rhyolite
0-63
Yehoshfat Granite
140
Eilat Granite
162
Roded Quartz-Diorite
Minerals
Silicate sediments (Aluvium)
Washing = replacement of solution after one week of dissolution

15. Albite sample under the SEM:
After ultra-sonic cleansing
Before treatment

16. Seawater
Albite [NaAlSi3O8 ] dissolution experiment in Seawater solution
Mixing-line
Isotopic-composition variations
Al, Si release
Mixing between
2 components
What are the 2 components?

17. Albite sample under the SEM:
Apatite
Ca5(PO4)3(F,Cl,OH)
Biotite
K(Mg,Fe)3AlSi3O10(F,OH)2
Albite sample under the SEM:

18. Dissolution experiment:
הידרוניום
Dissolution experiment:
אלביט
Solutes → Mineral
Debye–Hückel term
Degree of saturation
Albite is under-saturated

19. PHREEQC - A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations
Under-saturation
Over-saturation

20. K-feldspar dissolution in seawater

21. Dissolution Rate Eilat Granite : SW and Bx
Both had the same washing x2 process

22. Eilat granite:
Seawater
Borax solution

23. Roded Quartz-Diorite in seawater: effect of washing

24. Various silicates dissolution in Seawater:
(without washing)
1.Plagioclase dissolves faster than K-feldspar.
2.Plag/KSP in Roded quartz diorite=50 (Bogoch et al.2002)
in Yehoshafat granite only 0.3 (Steinitz et al.2009).
3. Amram rhyolite size fraction :0-63μm (opposed to μm in the others)
Among the 3 silicates in sea-water experiments; the Roded quartz diorite has the highest Si concentration towards the end of the experiment, followed by Amram rhyolite and Yehoshafat granite (116, 82 and 46μM, respectively. See Figure 21a, page 60). This may be explained both by the mineralogy and by the size fraction of the samples. The ratio between plagioclase and K-feldspar in the samples is important since plagioclase dissolution rate is faster than that of K-feldspar (Langmuir, 1997). The ratio of plagioclase to K-feldspar is highest in Roded quartz-diorite (~50; Bogoch et al.2002) and indeed it has the highest Si concentration at steady-state (Figure 21a). The plagioclase to K-feldspar ratio in Yehoshafat granite is much lower (~0.3; Steinitz et al.2009) which may explain the lower Si release rate (compared to Roded quartz-diorite). As for Amram Rhyolite, this ratio is probably very low (<~0.5;Mushkin et al.2002), but not as in the other experiments, Amram rhyolite consists of the most fine particles (0-63μm, compared to the others, 60-90μm), and therefore probably dissolves faster than Yehoshafat granite.

25. Various silicate sediments in seawater:
(without washing)

26. Albite in seawater and borax solution: 87Sr/86Sr
Similar pattern in Roded-quartz-diorite experiment
אפטיט/ביוטיט.

27. Conclusions:
The albite sample dissolves incongruently (non-stoichometric) in seawater due to precipitation of secondary phases. The isotopic composition of Sr decreases in a hyperbolic manner. This means that there are two components in the sample with two different distinct isotopic composition of 87Sr/86Sr: albite with trace amounts of apatite/biotite.
2. The K-feldspar sample dissolves congruently in seawater (Si:Al ratio of 3:1). The sample does not contain Sr and therefore does not contribute to the 87Sr/86Sr ratio in the solution.
3. The dissolution rate in the Borax-HCl solution is faster than in the seawater solution, even though the pH is the same (pH~8.2).
4. In the beginning of the experiment (until day 20) the dissolution rate is fast and is decreasing with time. The reason is, the presence of fine reactive particles (large surface area in comparison with their volume).
5. In the albite experiements (and also in Roded-quartz-diorite) a shift in the 87Sr/86Sr isotopic composition was observed. This may result from the fact that both solutions (seawater and borax-HCl) does not have the same distance from equilibrium.

28. Thank you for listening..

29. Albite in borax solution
appendix
Albite in borax solution

30. K-feldspar in borax solution
appendix
K-feldspar in borax solution

31. Eilat Granite in borax solution
appendix
Eilat Granite in borax solution