X-RAY ABSORPTION SPECTROSCOPY OF GEOLOGICAL MATERIALS

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X-RAY ABSORPTION SPECTROSCOPY OF GEOLOGICAL MATERIALS

G. Calas, J. Petiau, A. Manceau

To cite this version:

G. Calas, J. Petiau, A. Manceau. X-RAY ABSORPTION SPECTROSCOPY OF GEO- LOGICAL MATERIALS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-813-C8-818.

�10.1051/jphyscol:19868156�. �jpa-00226059�

Colloque C8, supplement au n o 12, Tome 47, dgcembre 1986

X-RAY ABSORPTION SPECTROSCOPY OF GEOLOGICAL MATERIALS

G. CALAS, J. PETIAU and A. MANCEAU

Laboratoire de Mineralogie-Cristallographie, U A 09 CNRS, Universites d e Paris V I et V I I , F-75252 Paris Cedex 05, France

RESI7ME. Les minCraux contiennent de nombreux ClCments sous forme d'impuretCs, k faibles concentrations. Ceci complique l'appproche des solutions solides naturelles, notamment en ce qui concerne l'insertion de ces ClCments dans le rCseau du minkral. Les distributions intracristallines de ces dldments (distribution intersites, homog6n6it6 d e la topographie de leur distribution) peuvent Stre dtudides en Spectromgtrie d'absorption X. Les mingraux form& 2 basse tempgrature (hydroxydes , silicates , argiles) reprbentent des rnatkriaux particuliirernent intdressants 2 cause de leur dtat divisd, de leur rndlange intime avec d'autres phases e t de I'irnportance des phCnom&nes d'adsorption : la spectromdtrie d'absorption X a perrnis de faire des p r o g r b importants dans la comprdhension des processus d'altgration qui en sont $ I'origine.

ARSTRBCT: Naturally occuring minerals contain many elements at low concentration. This makes it difficult t o interpret t h e natural solid solutions, as i t must be ascertained whether or not these elements a r e contained in the mineral lattice. Intracrystalline element distribution (intersite distribution, homogeneity of their distribution) may be studied by X-ray

Absorption Spectroscopy (XAS). Low temperature minerals (hydroxides, silicates, clay minerals) represent interesting materials on account of their finely divided character, of their intimate mixing with other phases and of t h e importance of the adsorption phenomena. XAS allows t o understand better the alteration processes which a r e a t their origin.

INTRODUCTION

Geological materials encompass mainly the rock-forming minerals and the ore minerals. The minerals give information about the various geological processes, e.g. the transfer of elements in the earth (including ore deposit formation) or the estimate of physico-chemical parameters of such processes as magmatism, metamorphism or rock hydrothermal alteration and surficial weathering.

The geological materials are often similar to well known synthetic materials, but set specific problems mainly because of their high impurity content. They contain minor or trace elements which are used by t h e geochemists t o decipher t h e various formation processes and t h e origin of the minerals and t h e rocks which a r e formed from. Mineralogists and geochemists have used a wide variety of approaches t o get quantitative information on t h e actual structure and composition of t h e naturally occuring minerals. These methods include X-ray and neutron diffraction, nanoscale imaging and microanalysis (electron and ion rnicroprobes, HRTEM, STEM) and a wide range of spectroscopic techniques (Mossbauer, EPR, NMR, Raman and infrared, UV-Visible

...

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868156

C8-814 JOURNAL DE PHYSIQUE

INTERSITE DISTRIBUTION

Rock-forming silicates (olivines, pyroxenes, amphiboles and micas) contain non-equivalent sites of similar coordination numbers (Ml/M2 sites in olivines and orthopyroxenes, Ml/M4 in amphiboles) in which transition elements usually substitute for six-fold coordinated magnesium. In the case of olivines (M2Si04, where M=Mg, Fe, Ni..), which are the main mineral component of rocks derived from the earth mantle, two octahedral sites may be occupied: MI and M2. On Figure 1 is reported the absorption K-edge of iron in a natural olivine (=Mgl.gFeo.3 SiO4) and in the end member term Fe2SiOq. The edge maximum is broader in the latter in which both MI and M2 sites are equally occupied by divalent iron. The significantly distinct and narrower edge maximum may be interpreted as arising from an inequal occupancy of the M1 and M2 sites in the more diluted ferrous olivine confirming the data previously obtained by other spectroscopic techniques (optical absorption and Mossbauer spectroscopy): the M i site is preferentially occupied in low-iron olivines (Fig. 1). In Mg-Fe clinopyroxenes, a strong inequal site occupancy is also known at low iron content from crystal structure refinements, but in this case the iron site preference is towards the strongly distorted M2 sites of C2, symmetry. The distortion can be seen by the splitting of the edge absorption maximum (1). Even in the case of the presence of only one site, such effects may also occur: the statistical disorder induced by the substitution of ~ ecations in MgO-FeO solid solutions leads to a ~ + broadening of the edge features (2). In both cases, iron K-edge spectroscopy confums previous data obtained by using other techniques, but it illustrates the possibility to get site occupancy of other elements which we have no data for, on account of their low concentration in natural minerals (other d-elements, rare-earth elements).

Another group of minerals which exhibit variable site occupancy is the spinel family. The inversion degree (proportion of 6-fold coordinated divalent atoms: 0 in normal and 1 in inverse structures) is a useful tracer of the formation conditions and edge spectroscopy may give information about the distribution of the elements among tetrahedral and octahedral sites. It must be however pointed out that the apparent resolution of the edge features (relative position, broadening due to core level lifetime and experimental resolution) strongly depends on the energy of the studied edge, which is important when comparing the edge of various transition elements occuring in the same mineral.

This is apparent on the 3d-element absorption K-edges (Fig.2) in chromites where the four resonance lines of the nickel K-edge are less resolved than those of manganese K-edge. The iron K-edge structure and position in natural chromites from Brazil has shown (3) that they have a more inverse character than other natural chromites. They are more easily altered than these latter, to form the so- called "femt-chromites" which clearly exhibit edges characteristic of magnetites. The partial inverse character of these femt-chromites indicates a strong reorganization of the crystal lattice, probably as a result of dissolution-recrystallization sequences, because the low alteration temperatures (300-400°C in hydrothermal conditions) do not favour diffusion processes in these compounds once they crystallized. It may also be concluded that the resistance of natural chromites to hydrothermal alteration processes is strongly dependent on their inversion character.

More quantitative is the use of the bound states which give rise to pre-edge features in the absorption K-edge of transition elements. The decomposition of the pre-edge into gaussian components after background-substraction has been proved to be useful in the determination of iron coordination in silicate glasses (4). More simply the intensity of this preedge can be used to give an upper limit to the 4 fold16 fold site occupancy of an element. This was used in the case of phyllosilicates, and particularly in poorly crystallized clay minerals. In the case of nontronite, the ferric end-member of the dioctahedral 2/1 phyllosilicates, Bonnin et al. (5) have shown that pre-edge intensity and structure confirm other spectroscopic data on the low tetrahedral iron content of this mineral, on the contrary to previous interpretation of Mossbauer data. Similar studies were made on nickel-containing phyllosilicates, where tetrahedral nickel was accounted for to explain the anomalous swelling behaviour of some samples: it has been recently shown by pre-edge analysis that nickel atoms are mainly 6-fold coordinated, as was confirmed by optical absorption spectroscopy (6). A recent study of edge structure of titanium in various silicate minerals has shown, together with EXAFS data, that this element occurs only in distorted sites in six-fold coordination (7). The distortion which occurs in some phases gives rise to a significant enhancement of the pre-edge feature, which contrasts with the very low absorbance of the pre-edge in of titanium in regular octahedra.

Finally it must be pointed out the potential importance of edge spectroscopy to study phase transitions. Of particular importance is the olivine group which is known to transform into spinel silicate at 120-160 kbar at 1000'C, these pressure conditions being typical of the upper earth mantle.

As we have shown above, the edge structure is strongly dependent upon the site geometry. As an illustration of the potentiality of this approach, the iron K-edges of Fe-end member olivine Fe2SiOq

8 3 2 0 8 3 4 0 8 3 6 0 E N E R G Y ( e V 1

+ in chromites M2Cr04 (M=Mn, Fe, Co and Ni from top to bottom).

FE K-EDGES

7 0 9 0 7110 7130

ENERGY (eV)

Figure I . Absorption K-edge of iron in a natural olivine (FelMg=O.l5) and in end-member fayalite (Fe2SiOq )

and spinel germanate Fe2GeOq (isomorphic of the high-pressure spinel silicate but stable under under ordinary P-T conditions) are shown on Figure 3. As on Figure 1, it is apparent that the presence of iron in the two sites of the olivine structure gives broader spectra than in the case of the spinel germanate which has an unique octahedral site.

FE K-EDGES

7090 7110 7130

ENERGY ( e V )

F i ~ u r e 3: Absorption K-edge of iron in FeZSi04 olivine and spinel g e m n a t e equivalent Fe2Ge04.

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DETERMINATION OF OXIDATION STATES

Edge spectroscopy has been used to determine the oxidation states of transition elements, in particular during hydrothermal alteration and weathering conditions where their geochemicab behaviour (e.g. solubility) is strongly dependent upon their actual oxidation state. Such a study has been made on chromium-containing phyllosilicates from the alteration zone of the brazilian chromite deposits already mentioned above (3). Previous wet chemical analysis have indicated that the chromium which is found in this deposit in alteration phases (chlorites) occurs in these minerals in the hexavalent state. However no chromate groups were found from the edge structure analysis, and the +3 oxidation state was determined with a good accuracy, on account of the high intensity of the chromate shape resonance (8). A copper-bearing phyllosilicate (vermiculite) has also been studied to determine the copper oxidation state (9).

Another application is presented in this issue (10) concerning the oxidation state of cobalt in Mn-oxides which represent one of the main cobalt ore minerals although these phases are still imprecisely charcaterized. The edge position has shown that this element occurs in the trivalent state.

Furthermore Co-0 distances measured by EXAFS indicate the possibility of a low-spin state which could explain the occurence of this unusual valence state because the exceptional crystal-field stabilization energy of this electronic configuration and its low solubility in natural waters. This is the first direct evidence of this oxidation state in natural oxide minerals formed by continental weathering.

INTRACRYSTALLINE ELEMENT DISTRIBUTION

EXAFS provides an unique way to verify the reality of random substitution in solid solutions. A particularly favourable case concerns the mixing of 3d elements with Mg or A1 as in silicates or mixed manganese-aluminium oxi-hydroxides because of the n phase shift difference between both groups of elements in the studied wavevector range.

EXAFS spectra of nickel-containing phyllosilicates formed at low temperature under lateritic weathering conditions show that the intracrystalline distribution of nickel within the Mg-containing octahedral sheets is never random (1 1-13). The nickel-atoms are segregated into Ni-rich domains the minimum size of which may be calculated. Such a clustering indicates non-equilibria processes in these low-temperature minerals. it is thus expected that an increasing formation temperature and the subsequent faster cation exchange kinetics will favour ideal mixing of mineral components. EXAFS could be an unique tool for giving information about the temperature during the crystallization of these minerals.

Heterogeneous distribution of atoms in phyllosilicates depends on the substitution mechanism. Heterovalent substitution implying local charge compensation seems to hinder cation clustering in these minerals. This has been shown in chromium containing phyllosilicates from the brazilian ore deposit mentioned above (3). Cation C I U S ~ ~ M ~ occurs differently in simple oxides (14).

Fe K-edge EXAFS of compounds belonging to the MgO-FeO solid solution indicates a random substitution of Fe and Mg atoms. On the contrary, a Li0.1Fe0.1Mg1.802 sample exhibited the same second shell composition as a-LiFe02, indicating a clustering of Li+ and ~ ecations in this rock- ~ + salt derived structure. This difference between simple oxides and multisite silicates must be taken into account when modelling the behaviour of geological materials.

Other applications of EXAFS for determining the location of atoms in complex materials concern the evidence of disordered iron oxides associated to natural kaolinites (15) and volcanic . ,

glasses (1).

Finally, EXAFS of 3d-elements in natural manganese oxides (10) shows that cobalt and nickel are incorporated in a distinct location in the mineral structure. Cobalt atoms occur in highly ordered area which contrast with the disorder observed on the first and second shells around Mn atoms, although the Co-0 and Mn-0 distances are found to be similar. Together with the determination of C O ~ + from edge position and structure, these data explain h e exceptional role these Mn-oxides play in trapping selectively the cobalt. Ni-0 distances arehlgher than Mn-0 ones and correspond either to the building of incomplete Ni(OH)2 layers or to a wbstitution in Al(OH)3 layers, depending on the nature of the studied phases. The distinct crystal chemistry of these two rather similar elements explains the contrasted location of their ore deposits, the nickel being more often associated with secondary phyllosilicates.

EVOLUTION OF IRON HYDROXIDE GELS

The formation processes of minerals are of primary importance to interpret their occurences.

On account of their low solubility, ferric oxides and oxi-hydroxides play an important role in all the

hydroxides which progressively polymerize with a subsequent dehydration and dehydroxylation the relative importance of which determine the nature of the resulting mineral: a-Fe2Og (hematite), a- FeOOH (goethite) or y-FeOOH (lepidocrocite). The poorly crystallized precursors are known by X- ray diffraction and Mossbauer spectroscopy, wich show some common structural characteristics, whatever the final mineral which will derive from. EXAFS experiments have been made on gels obtained either by direct precipitation of solutions containing ferric ions or by neutralization and oxidation of solutions containing ferrous ions. These gels are in fact clearly distinct: the iron local surrounding on the second shell is similar to either one of the two fenic hydroxides, although with a higher structural disorder (16). The physicochemical parameters which govern the formation of these hydroxide gels have thus a direct influence on the resulting mineralogy through the control of the local structures.

EDGE STRUCTURE OF SULFIDE AND ARSENIDE MINERALS

Sulfide and arsenide minerals are the most important ore minerals. The knowledge of their crystal chemistry helps in understanding the mechanisms of metallic element concentrations in the earth crust. Because of their semiconducting character, some spectroscopic methods are of limited use, as optical absorption or electron paramagnetic resonance. Copper has a strong geochemical affinity for sulfur (chalcophilic character) and is one of the elements which are essentially found in sulfides and sulfosalts (sulfo-arsenides and sulfo-antimonides). This element is usually described as occuring only in the monovalent state inbese minerals. The edge spectra are however distinct if one compares three minerals which have the same structure of zincblende type and in which copper i s similarly coordinated by sulfur (Fig. 4). Chalcopyrite, CuFeS2, exhibits a pre-edge, indicating that the d-orbitals are not fully occupied, on the contrary to the two other, enargite, CugAsSq, and tennantite, Cu12As4S13. This pre-edge arises from a non pure d-10 character and it has been correlated with iron and sulfur K-edge features (17). The cation-anion partial density-of-states correlation gives also constraints on conduction mechanisms in these semi-conducting minerals.

COPPER K-EDGE

8970 8990 9010 9030 ENERGY ( e V )

CONCLUSIONS

Fiaure 4: Copper absorption K-edges in various sulfides.

The future development of X A S in Earth Sciences concerns as well the minerals as their formation environments (silicate melts and glasses (18), hydrothermal solutions) t o understand better the structural controls of the partition of minor and trace elements during the crystallization of the minerals. Crystalline nucleation has been studied by XAS in some silicate glasses of cordierite composition around titanium and zinc (19) or zirconium (20) as well as in glasses of spodumene composition (21). Another type of

C8-818 JOURNAL DE PHYSIQUE

materials which has received some attention are the metamict materials where a severe irradiation by radioactive substitutional elements during geological periods causes strong radiation damages which destroys the periodicity of the lattice. The concerned minerals contain strongly charged cations (e.g. Ti, Zr..) in sites of high coordination number and are mainly complex niobium- tantalum-titanium oxides (pyrochlores..) (22) apd silicates (zircon) (23). Order-disorder transition corresponds to a change in the sharing between the polyhedra as well as significant coordination changes around elements such as titanium.

Some interesting applications have thus been already made, but important results are expected, in particular for the materials representative of the earth interior. This needs in-situ hinh tekperature-kigh pressure experiments (24), to have a direct access to the site evolution (sge com~ressibilitv and thermal expansion) of the minor components for the main minerals. The maior phasks have &ready been studied using XRD methodsLwith diamond anvil cells up to sev&al hundreds of kbars (25). Another major application concerns the study of mineral trace components using fluorescence detection systems. The location of these components inside the crystal lattice (substitutional or interstitial position) or as separated nano-inclusidns is of great importance to interpret the mineralhiquid partition coefficients of these elements. Finally dispersive XAS will give valuable information on site modifications during phase transition, mineral dissolution or phase crystallization, with the possibility of observing transient species. It is to be pointed out that all these studies are a complement of XRD and trace element imaging, two techniques which are increasingly used by earth scientists on synchrotron radiation (26).

REFERENCES

(1)- G. Calas and J. Petiau (1983). Bull. MinQal., 106, 33-55.

(2)- G.A. Waychunas, M.J. Apted and G.E. Brown (1983). Phys. Chem. Minerals, 10, 1-9.

(3)- G. Calas, A. Manceau, A. Novikoff and H. Boukili (1984). Bull. MinCral., 107,755-766.

(4)- G. Calas and J. Petiau (1983). Solid State Comm., 48, 625-629.

(5)- D. Bonnin, G. Calas, H. Suquet and H. PCzerat (1985). Phys. Chem. Minerals, 12,55-64.

(6)- A.Manceau and G.Calas (1986). J. Solid State Chem. (submitted).

(7)- G. A. Waychunas (1986). Amer. Mineral. (in press).

(8)- F.W. Kutzler, C.R. Natoli, D.K. Misemer. S. Doniach and K.O. Hodeson (1980). J. Chem. u \ ,

phys., 73, 3274-3288.

(9)- P. Ildefonse, A. Manceau and D. Prost. submitted to Clay Minerals.

(10)- A. Manceau, S. Llorca and G. Calas (1986). J. Physique Colloq., 4_7, (28-703 - C8-707.

(1 1)- A. Manceau, G. Calas and J. Petiau (1984). In "EXAFS and Near edge structure III", eds.

K.O. Hodgson, B. Hedman and J.E. Penner-Hahn, Springer Proc. Phys 2,358-361.

(12)- A. Manceau and G. Cala s (1985). Amer. Mineral., 70,549-558.

(13)- A. Manceau and G. Calas (1986). Clay Minerals, 21 (in press).

(14)- G.A. Waychunas, G.E. Brown and M.J. Apted (1986). Phys. Chem. Minerals, 13, 31-47.

(15)- D. Bonnin, S. Muller and G.Calas (1982). Bull. Mineral. 105,467-475.

(16)- J.M. Combes, A. Manceau and G. Calas (1986). J. Physique Colloq., g, C8-697 - ~ 8 - 7 0 1 (17)- P. Sainctavit, G. Calas, J. Petiau, R. Kamatak, J.M. Esteva and G. E. Brown (1986).

J. Physique Colloq., 47, C8-411 - C8-414.

(18)- G.E. Brown (1986). 3. Physique Colloq., 42, C8-661 - C8-668.

(19)- T. Dumas and 3. Petiau (1986). J. Non-Cryst. Solids, 81,201-220.

(20)- T. Dumas, A. Ramos, M. Gandais and J. Petiau (1985). J. Mater. Sci. Lett., 4, 129-132.

(21)- A. Ramos, M. Gandais and J. Petiau (1985). J. Physique, C8,491-494.

(22)- R.B. Greegor, R.W. Lyttle, B.C. Chakopumakos, G.R. Lumpkin and R.C. Ewing (1986).

14th I.M.A. Meeting Abstracts, 114.

(23)- M. Imafuku, I. Nakai, J. Akimoto, R. Miyawaki and Y. Sugitani and K. Koto (1986). Ibid.,

1 2 1 I 2 I .

(24)- R. Ingalls, G.A. Garcia and F.A. Stem (1978). Phys. Rev. Lett., 40, 334-336.

(25)- R.M. Hazen and L.W. Finger (1982). Comparative crystal chemistry. Wiley.

(26)- G. Calas, W.A. Bassett, J. Petiau, M. Steinberg, D. Tchoubar and A. Zarka (1984). Phys.

Chem. Minerals, 11, 17-36.