Pauline ANDRIEUX Sabine PETIT Alain DECARREAU FRE3114 CNRS, HydrASA Université de Poitiers 40, ave. du Recteur Pineau 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

BeidelliteNontronite Tetrahedral charge Montmorillonite Octahedral charge Fe 3+ -Montmorillonite (Theoretical)

(Si (4-x) Al, Fe 3+ x ) Fe 3+ 2 O 10 (OH) 2 M + x IV VI Fe 3+ - nontronite (Si (4-x) Fe 3+ x ) Fe 3+ 2 O 10 (OH) 2 M + x nontronite (Si (4-x) Al x ) (Al, Fe 3+ ) 2 O 10 (OH) 2 M + x Beidellite

Ditrigonal cavity Tetrahedral sheet Octahedral sheet R(VI), OH R(IV) O Tetrahedral sheet dioctahedral (tv) dioctahedral (cv) Ditrigonal cavity Tetrahedral sheet Octahedral sheet R(VI), OH R(IV) O Tetrahedral sheet Schematic representation of the octahedral sheet

dioctahedral (tv)dioctahedral (cv) O H \/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\ h OH R = Al, Fe 3+

dioctahedral (tv)dioctahedral (cv) OH R = Al, Fe 3+ Fe 3+ H+H+ Al O vacancy

Ferruginous smectite SWa-1 NIRMIR  OH region +  OH (Si 3.70 Al 0.30 ) (Al 0.57 Fe Mg 0.10 ) O 10 (OH) 2 Na

Ferruginous smectite SWa-1 NIRMIR  Fe 3+ 2 OH  Fe 3+ 2 OH Wavelength (µm)

Ferruginous smectite SWa-1 NIRMIR AlAl  AlFe 3+ OH  AlFe 3+ OH Wavelength (µm)

Ferruginous smectite SWa-1 AlAl 4563 AlAl  Al 2 OH  Al 2 OH NIRMIR Wavelength (µm)

OH Fundamental vibrations OH combination  OH OH overtones (µm) (cm -1 ) _ MIR NIRUV Vis Far IR  OH  OH  OH SiO  SiO H2OH2OH2OH2O H2OH2O H2OH2OH2OH2O 2W OH >> W 2 OH X = 1/2W 2 OH - W OH = cm -1 Petit et al. (2004) Phys. Chem. Minerals, 31, X : anharmonicity constant

Wavenumber (cm -1 ) Wavelength (µm) SWa Reflectance Reflects different conditions of formation NIR is most often not self – sufficient but it could help AlFeOH Fe 2 OH (Al 2 OH)

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: SiO 2 Na 2 O metal source: salt (chloride, nitrate…) equilibrated agent: HCl, NaOH, ….. Ex: nontronite 4 SiO 2 Na 2 O + 2 FeCl HCl  8 NaCl + H 2 O + Si 4 Fe 2 O 11 beidellite 4 SiO 2 Na 2 O + 2 AlCl 3 +2 HCl  8 NaCl + H 2 O + Si 4 Al 2 O 11 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 ( °C), Rayner [1962] calculated a half-reaction time of 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 ( mg) + distilled water (30 cc) (+ HCl or NaOH) Teflon Metal

(Theoretical) Fe 3+ - nontronite (Si (4-x) Fe 3+ x ) Fe 3+ 2 O 10 (OH) 2 M + x nontronite

Starting from Fe 2+ : 2 SiO 2 Na 2 O + FeCl 2  Si 2 FeNa 2 O 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 Fe 3+ Si 2 O 6 ) is obtained at higher temperatures (Decarreau et al. Eur. J. Mineral. 16, 85-90, 2004) (Decarreau et al. Clays&Clay Min , 2008)

T (°C) gel Si 3.25 Fe Fe 3+ 2 O 10 (OH) 2 Na (Decarreau et al. Clays&Clay Min , 2008)

%Réflectance (Offset) Wavenumber (cm-1) Wavelength (µm) Fe 3+ 2 OH H2OH2O  Fe 3+ 2 OH H2OH2O fresh gel starting gel nontronite ?

100°C 110°C 125°C 150°C 75°C 90°C Wavenumber (cm-1) +  Fe 3+ 2 OH2 Fe 3+ 2 OH H2OH2O H2OH2O ? %Reflectance (offset) Wavelength (µm)

- 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) Fe 3+ x Fe 3+ 2 O 10 (OH) 2 Na + x after Decarreau et al. (2004) Eur. J. Mineral.,

(Theoretical) (Si (4-x) Al, Fe 3+ x ) Fe 3+ 2 O 10 (OH) 2 M + x IV VI Fe 3+ - nontronite (Si (4-x) Fe 3+ x ) Fe 3+ 2 O 10 (OH) 2 M + x nontronite (Si (4-x) Al x ) (Al, Fe 3+ ) 2 O 10 (OH) 2 M + x Beidellite

Starting gel Al/FeFe 3+ /Fe 2+ T (°C)pH f result 0.2/1.8Fe °C10.82nontronite 170°C10.81nontronite 200°C10.68nontronite 220°C10.28nontronite + zeolite Fe °C11nontronite + zeolite 170°C10.78nontronite + zeolite 200°C10.66nontronite + zeolite 0.4/1.6Fe °C10.12hisingerite 200°C10.23hisingerite 220°C10.23hisingerite Fe °C11.59nontronite + zeolite 200°C11.27nontronite + zeolite 10.71nontronite 220°C10.81zeolite +nontronite

Position (°2Theta) Cuk  Counts 15.9 Å 4.52 Å 3.64 Å 2.58 Å Å XRD powder pattern Å % Reflectance Wavenumber (cm-1) Wavelength (µm) Fe 3+ 2 OH H2OH2O H2OH2O +  Fe 3+ 2 OH

Starting gel Al/FeFe 3+ /Fe 2+ T (°C)pH f result 1/1Fe °C11.51nontronite + zeolite 10.39Al-nontronite 6.73Fe 3+ -beidellite 1.8/0.2Fe °C10.38beidellite + zeolite Fe °C9.34beidellite 7.30beidellite + zeolite 4.99kaolinite 7.67beidellite + kaolinite

1.8Al0.2Fe 0.2Al1.8Fe 0.4Al1.6Fe 1Al1Fe pHi=6.3, pHf=6.7 2Fe 1Al1Fe pHi=8.4, pHf= Wavenumber (cm-1) % Reflectance (Offset) 4462 Wavelength (µm)  Fe 3+ 2 OH +  Al 2 OH +  AlFe 3+ 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