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e-mail: [email protected]
Pauline ANDRIEUX
Sabine PETIT
Alain DECARREAU
FRE3114 CNRS, HydrASAUniversité de Poitiers
40, ave. du Recteur Pineau86022 POITIERS Cedex
FRANCE
- determine the experimental conditions which led to mineral crystallization (in very simplified systems)
constrain possible conditions of formation for those minerals
- obtain good reference minerals with monitored crystal chemistry
determine their spectroscopic fingerprint
Beidellite Nontronite
Tetrahedral charge
Montmorillonite
Octahedral charge
Fe3+-Montmorillonite
(Theoretical)
(Theoretical)
(Si (4-x) Al, Fe3+x) Fe3+
2 O10 (OH)2 M+x
IV VI
Fe3+- nontronite(Si (4-x) Fe3+
x) Fe3+2 O10 (OH)2 M+
x
nontronite(Si (4-x) Alx) (Al, Fe3+ ) 2 O10 (OH)2 M+
xBeidellite
Ditrigonal cavity
Ditrigonal cavity
Tetrahedral sheet
Octahedral sheet
R(VI), �
OHR(IV)
OTetrahedral sheet
dioctahedral (tv) dioctahedral (cv)
Ditrigonal cavity
Ditrigonal cavity
Tetrahedral sheet
Octahedral sheet
R(VI),
OH
R(IV)
OTetrahedral sheet
Schematic representation of the octahedral sheet
dioctahedral (tv) dioctahedral (cv)
O H\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
h
OH
R = Al, Fe3+
dioctahedral (tv) dioctahedral (cv)
OH
R = Al, Fe3+
Fe3+
H+
Al
O
vacancy
Ferruginous smectite SWa-1
NIR MIROH region+ OH
(Si3.70Al0.30 ) (Al0.57 Fe3+1.33 Mg0.10) O10(OH)2 Na+
0.40
Ferruginous smectite SWa-1
NIR MIR
2.29
4361
+ Fe3+2OH
12.2
820
Fe3+2OH
Wavelength (µm)
Ferruginous smectite SWa-1
NIR MIR
AlAl
4563
2.24
4464
+ AlFe3+OH
AlFe3+OH
11.4
875
Wavelength (µm)
Ferruginous smectite SWa-1
AlAl
4563
AlA
l92
0
2.19
4563
+ Al2OH
456310.87
920
Al2OH
NIR MIR
Wavelength (µm)
OH
Fundamental vibrationsOH combination
OH
OH overtones (µm)
(cm-1)_
MIRNIRUV VisFar IR
OHOH OHSiO SiO11000 4000 400
0.9 2.5 25
H2OH2OH2OH2OH2O
2WOH >> W2OH
X = 1/2W2OH - WOH = -85.6 cm-1
Petit et al. (2004) Phys. Chem. Minerals, 31, 585-592.
X : anharmonicity constant
Wavenumber (cm-1)
4600 4350 4100
2.17 2.30 2.44Wavelength (µm)
SWa-1
2.24
4464
2.29
4361
Ref
lect
ance
Reflects different conditions of formation
NIR is most often not self – sufficient but it could help
AlFeOHFe2OH
(Al2OH)
Requirements:
- minimize the variables of the system
- reproducibility
- homogeneous and definite chemical composition
Use an amorphous gel with the clay stoechiometry
Basic reaction :
silica source: SiO2Na2O
metal source: salt (chloride, nitrate…)
equilibrated agent: HCl, NaOH, …..
Ex: nontronite
4 SiO2Na2O + 2 FeCl3 + 2 HCl 8 NaCl + H2O + Si4 Fe2 O11
beidellite
4 SiO2Na2O + 2 AlCl3 +2 HCl 8 NaCl + H2O + Si4 Al2 O11
Coprecipitation of gel with the clay stoechiometry
Decarreau (1983) ’s protocol
The coprecipitate is:
- centrifuged and washed
- dried and crushed (or frieze dried)
The starting material is ready to use.
Aim:
Reproduce in laboratory clay formation
Clays similar to clays formed at the earth surface are synthesized by hydrothermal treatment at T <= 250°C (at equilibrium water P)
From kaolinite syntheses performed at several temperatures (180-300°C), Rayner [1962] calculated a half-reaction time of 16.104 years at 20°C.
Problem: we cannot use geological times !
The rate constant of an heterogeneous chemical reaction in aqueous phase is given by :
k = A.exp - (E/RT)
Ageing time of clay synthesis can be minimized by an increase of T.
< 100°C : PFA reactors (copolymer of ethylene tetrafluor)
From 100 to 240°C : reactors with metal bodies and removable Teflon liners
Starting material (300-500 mg)
+distilled water
(30 cc)
(+ HCl or NaOH)
Teflon
Metal
(Theoretical)
Fe3+- nontronite(Si (4-x) Fe3+
x) Fe3+2 O10 (OH)2 M+
x
nontronite
Starting from Fe2+:
2 SiO2Na2O + FeCl2 Si2FeNa2O6 + 2 NaCl
after drying the gel, iron is oxidized
pH is adjusted to 12.5 with NaOH
ageing time 4 weeks
T = 75, 90, 100, 125, 150 °C *
*aegirine (Na Fe3+Si2O6 ) is obtained at higher temperatures (Decarreau et al. Eur. J. Mineral. 16, 85-90, 2004)
(Decarreau et al. Clays&Clay Min. 322-337, 2008)
0 10 20 30 40 50 60 70 80 90
T (°C)
150
125
110
100
90
75gel
001 02-11004 13-20 06-33
15-24-31
Si3.25Fe3+0.75 Fe3+
2 O10(OH)2 Na+0.75
(Decarreau et al. Clays&Clay Min. 322-337, 2008)
51986975
20
25
30
35
40
45
50
55
60
65
%R
éfle
cta
nce
(O
ffse
t)
4500 5000 5500 6000 6500 7000 7500
Wavenumber (cm-1)
Wavelength (µm)
2.29
1.43
1.92
43732Fe3+2OH
H2O
Fe3+2OH
H2O
fresh gel
starting gel
nontronite
2.414144
?
100°C
110°C
125°C
150°C
75°C
90°C
60
64
68
72
76
80
5000 6000 7000
Wavenumber (cm-1)
+ Fe3+2OH2 Fe3+
2OH
H2O
H2O
?%
Ref
lect
anc
e (
offs
et)
6982 5204 4373 4144
1.43Wavelength (µm)
1.92 2.29
2.41
1.47
6800
- Well crystallized nontronite can be synthesized under either oxidizing or partially reducing conditions.
- the range of synthesis pH is narrow (12 to 12.5)- at lower pH hematite or hisingerite are
formed- at higher pH and for temperatures >180°C
aegirine is formed
Thermodynamic equilibrium diagram of aegirine with 2:1 phyllosilicates with the following structural formula: Si (4-x) Fe3+
x Fe3+2 O10 (OH)2 Na+
x
after Decarreau et al. (2004) Eur. J. Mineral., 85-90.
(Theoretical)
(Si (4-x) Al, Fe3+x) Fe3+
2 O10 (OH)2 M+x
IV VI
Fe3+- nontronite(Si (4-x) Fe3+
x) Fe3+2 O10 (OH)2 M+
x
nontronite(Si (4-x) Alx) (Al, Fe3+ ) 2 O10 (OH)2 M+
xBeidellite
Starting gel Al/Fe Fe3+/Fe2+ T (°C) pHf result
0.2/1.8 Fe3+ 150°C 10.82 nontronite
170°C 10.81 nontronite
200°C 10.68 nontronite
220°C 10.28 nontronite + zeolite
Fe2+ 150°C 11 nontronite + zeolite
170°C 10.78 nontronite + zeolite
200°C 10.66 nontronite + zeolite
0.4/1.6 Fe3+ 170°C 10.12 hisingerite
200°C 10.23 hisingerite
220°C 10.23 hisingerite
Fe2+ 170°C 11.59 nontronite + zeolite
200°C 11.27 nontronite + zeolite
10.71 nontronite
220°C 10.81 zeolite +nontronite
1000
2000
3000
4000
5000
6000
3 13 23 33 43 53 63
Position (°2Theta)
Co
un
ts
3 13 23 33 43 53 63
6000
5000
4000
3000
2000
1000Position (°2Theta) Cuk
Cou
nts
15.9 Å4.52 Å
3.64 Å
2.58 Å 1.528 Å
XRD powder pattern
1.919 Å
2.294373
2.224510
1.915224
1.436980
40
45
50
55
60
65
70
75
80
85
90
95
100
% R
efle
cta
nce
4500 5000 5500 6000 6500 7000 7500
Wavenumber (cm-1)
Wavelength (µm)
1.466833
2 Fe3+2OH
H2O
H2O
+ Fe3+2OH
Starting gel Al/Fe Fe3+/Fe2+ T (°C) pHf result
1/1 Fe2+ 200°C 11.51 nontronite + zeolite
10.39 Al-nontronite
6.73 Fe3+-beidellite
1.8/0.2 Fe3+ 220°C 10.38 beidellite + zeolite
Fe2+ 220°C 9.34 beidellite
7.30 beidellite + zeolite
4.99 kaolinite
7.67 beidellite + kaolinite
%R
efle
cta
nce
(O
ffse
t)
4500 5000 5500 6000 6500 7000 7500 8000
Wavenumber (cm-1)
Wavelength (µm)
1.8Al0.2Fe
0.2Al1.8Fe
0.4Al1.6Fe
1Al1Fe pHi=6.3, pHf=6.7
2Fe
1Al1Fe pHi=8.4, pHf=10.4
6982
1.43
7100
1.41H2O
5204
1.92
6800
1.47H2O
2 Fe3+2OH
2 Al3+2OH
+ R3+2OH
1.8Al0.2Fe
0.2Al1.8Fe
0.4Al1.6Fe
1Al1Fe pHi=6.3, pHf=6.7
2Fe
1Al1Fe pHi=8.4, pHf=10.4
4200 4400 4600 4800
Wavenumber (cm-1)
% R
efle
ctan
ce
(O
ffse
t)
4462
Wavelength (µm)
4566
4373
2.29
2.24
2.19 + Fe3+2OH
+ Al2OH
+ AlFe3+OH
- nontronite can crystallize under partially reducing or oxidizing conditions if available water, Si, Fe and alkaline pH without biology and organic acids
- poorly crystalline nontronite can be obtained for days at low temperature
- however, the pH conditions range is narrow
- the range increases when Al increases (same with Mg)
- pH conditions hardly control crystal-chemistry of synthesized clays
- high T are not convenient for nontronite
- nontronite/zeolite paragenesis may correspond to the same geochemical conditions
What is the «stability » of nontronite (or hingerite) under rather acidic atmosphere ? (no H+ activity ?)
YESTERDAY topic
(Identification of phyllosilicates)
- poorly crystalline nontronite give the same NIR signal than well crystallized one (width of the OH combination band does not decrease significantly)
- doublet (or triplet) in the 2.2 µm region does not necessarily reflect the presence of several minerals (… and is the mystery of the doublet at 2.2 and 2.28 µm solved? )
- NIR alone is most often not enough to characterize muti-component samples unambiguously