The Crystal Field Concept (CFC) in Geosciences: Does the Crystal Field 3 Stabilization Energy of Cr + Rule Its Intercrystalline Partitioning Behavior?

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Mineral Spectroscopy: A Tribute to Roger G. Bums © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D. Dyar, C. McCammon and M. W. Schaefer

The crystal field concept (CFC) in geosciences: Does the crystal field 3 stabilization energy of Cr + rule its intercrystalline partitioning behavior?

K. LANGER and M. ANDRUT* Institute of Mineralogy and Crystallography, Technical University, D-10623 Berlin, Germany

Abstract-According to crystal field concepts, CFC (BURNS,1970a and 1993), 3dN-ion partitioning between different phases in geochemical systems or between different structural sites in multisite phases is governed by the crystal field stabilization energies of the 3dN-ions within the coexisting phases or the different sites, respectively. Although CFC was qualitatively shown to be valid in many systems, other authors claim that ionic sizes and distances within the polyhedra incorporating the 3dN-ions playa more important role. A decision over CFC and this latter predominantly geometrical concept may be taken when quantitative relations of the type

N C3d = f(CFSE3dN) or K<;~~t~)T) = f(t::'cFSEi~'J - Ph2)) with positive f are found. Such quantitative relations may be seen when influences of different bulk

N chemistry and p,T-conditions during formation on C3d are ruled out. Therefore, we determine C N 3d and CFSE3dN, for the case of Cr'", on coexisting minerals in the following parageneses, which 3 contain Cr + above the trace level (> 1000 ppm): 1 and 2: GtJCpxJKy, grosspydite ZagadochnayaJ Yakutia; 3: GtJCpxJAmph, eclogite UralslRussia; 4: Cor/Paragl Phlog, Raj Iz/Antarctica; 5: Cor/Kyl Fuchs, MysorelIndia; 6: Cor/Amph, Longido/Tanzania; 7: OpxJCpx, Cpx exsolution in Opx from an eclogite UralslRussia. The results quantitatively confirm CFC for parageneses 1 through 6. In

case of paragenesis 7, a negative correlation between CCr3+ and CFSEcr3+ was observed. This deviation from CFC can be explained on the basis of crystallochemical arguments.

INTRODUCTION In the splitting schemes of the lower part of Fig. Outline of the Crystal Field Concept, CFC 1, the spectroscopic ground states of the free ions THE d-ELECTRONSof 3dN-ions in the gaseous state are represented by symbols derived from Russell- are subject to the spherical fields of their nuclei Saunders- or LS-coupling, (2S+1)L],of the d-elec- and, therefore, are degenerate with respect to both trons. The crystal field split states are characterized energy and symmetry. When incorporated into by the symbols of the irreducible representations nonspherical fields, e.g., ligand fields of regular of their symmetry (cf e.g. MCCLURE, 1958; DUNN or distorted octahedra in a mineral structure, the et al., 1965; SCHLAFERand GLIEMANN, 1980). degeneracies are at least partly lifted. The respec- One essential aspect of all the splitting schemes N tive spectroscopic states of the 3d -ions are split to shown in Fig. I is that the electronic ground states produce an electronic ground state and concomitant in the crystal field are energetically lower than the excited state or states. This effect is called crystal spectroscopic states of the free ions from which field splitting. It is characterized by the crystal field they originate. This energy gain of the 3dN-ions on parameter lODq, or in the case of octahedral fields, incorporation into a crystal field is called crystal 1ODqoct.. These relations are outlined in Fig. I, field stabilization energy, CFSE, and is indicated which presents the octahedral crystal field split- for the case of an octahedral field in Fig. 1 by tings of geochemically important 3dN-ions with to- multiples of Dq for all 3dN-ions 1 ~ N ~ 9. tal d-electron numbers N between I and 9. The crystal field stabilization energy in octahe- For the same 3dN-ion ligand-distances, the crys- dral fields, point group Oh, is obtained from tal field parameters behave as

-CFSE3dN,oct = [4n - 6(N - n)]' Dq (2) lODqoct:lODqcub:lODqtetr = 1:( - ~)( - ~) (1) or from in cubic and tetrahedral fields 1. -CFSE3dN.tetr,cub= [6n - 4(N - n)] . Dq (3) *Present adress: GeoForschungszentrum Potdsdam, Telegraphenberg, D-144407 Potsdam, Germany. for tetrahedal or cubic fields of point group T, IThe - sign is a consequence of the reversal of elec- or Oh, respectivly. N is the total number of d- tronic states compared to their sequence in octahedral electrons and n is the d-electrons in the crystal fields. field-derived ground state (t2g in octahedral fields 29 30 K. Langer and M. Andrut

1 4 5 6 de 9 d d2 d3 d d d « d n" v" Cr3+ Mo3+ Fe3• 0:]+ l 2 v2• C';- Mnz. Fe2• Co" Ni • Cu2+

20 3F 4F 50 6$ 50 4F 3F 20

7; C3i (1) (1)

~0 6$ A1g (2)

-CFSEoct 4Dq 8Dq 12Dq 6Dq o 4Dq 8Dq 12Dq 6Dq

FIG. L d-electron configurations, spectroscopic ground states, (2s+1)LJ,of the free ion case, crystal field-splitting of these free ion ground states as well as crystal field stabilization energies of the whole series of 3dN-ions in an octahedral environment, point group Oh (after SCHLAFER and GLIE- MANN, 1980; in case of d2 and d", 6Dq was corrected to 8Dq). The ordinate in the split level diagrams is energy in arbitrary units. For further explanations cf text.

nant degeneracies of states are gradually removed, or eg in tetrahedral and cubal fields for the one electron case.) leading to further splitting of the E- and T -states, In addition to the type and symmetry of the coor- shown in Fig. 1. When ground states are subject dination polyhedra, two peculiarities of the crystal to such low symmetry splitting, as is the case for 3 5 B field determine the crystal field splitting parameter all 3dN-ions except for 3d , 3d , and 3d (cf Fig. 10Dq of a given 3dN-ion, 1), then the resulting ground state is lowered again (i) The nature of the ligands: In the case of rock compared to the high symmetry case, Con- N forming-minerals and related geochemical sys- sequently, CFSE3dN is higher at such sites for 3d - tems, these are predominantly 02-, OH-, H20, F-, ions with degenerate crystal field ground state. and S2-, Generally, lODq increases within the The energy gaps between the ground and excited "spectrochemical series': (TSUCHIDA, 1938a, b, crystal field states of Fig, 1, eventually split by low and c; SHIMURA and TSUCHIDA, 1956) which for symmetry, are in the. range of 25000 to 4000 cm .', the just mentioned ligands is F- < OH- < 02- corresponding to 3.1 to 0.5 eV. This means that

< H20 < S2- (BURNS, 1993). they occur in the UV to NIR spectral region with (ii) The mean 3dN-ion ligand-distance: In the case wavelengths of 400 to 2500 nm. Electronic reso- of octahedral fields of ligands with effective charge nant absorption spectroscopy within the aforemen- ZL, crystal field theory yields tioned region is used to measure such energies and

3dN. 2 to extract 10Dq and, from this CFSE Polarized 10D = ~ (ZL X e ) '4 (4) radiation and crystallographically oriented crystal qoct 3 R5 X r slabs are necessary in the case of anisotropic crys- 4 te.g., DUNN et al., 1965; LEVER, 1968), wherein r tals where symmetry related selection rules for the = mean fourth power radius of the d-electrons of electronic transitions are active. the central ion, a quantity that is thought to be The importance of these crystal field considera- approximately constant for 3dN-ions of .the same tions and their experimental evaluation is obvious valence, and R = the mean cation-ligand distance, from the following: in the early fifties, it was shown N On decendent symmetry from Oh or Td, the rem- that the heat of hydration of 3d -ions as well as Crystal field stabilization energy 31

N the lattice energy of 3d -metal sulfides can only ion] depending on the type of 3dN-ion and of its be correctly calculated when the crystal field stabi- structural site. N lization energy of the 3d -ions was included into The problem is, thus, to decide between CFC the calculations that were based on the electrostatic and the just mentioned geometrical approach in the approach, valid for spherical ions with 3do- case of 3dN-ion partioning in a concentration range configuration. above the trace level. This problem can only be This result stimulated many geoscientists to sug- solved by checking for quantitative relations be- gest that the crystal field stabilization energy, N tween 3d -ion concentrations and CFSE3dN CFSE3dN, should also affect or even rule the ther- N modynamic properties of 3d -ion bearing minerals (5) and the behavior of 3dN-ions in homogeneous or inhomogeneous geochemical systems, e.g., their or between distribution coefficients and differences intra- and intercrystalline distribution behvior and in the crystal field stabilization energies their partitioning between crystals and melt (WIL- K~~/!$~~,T)= f(f1CFSE~~l+- Ph2) (6) LIAMS, 1959; BURNS and FYFE, 1964, 1966, 1967a and b; BURNS et al., 1964; CURTIS, 1964; MERLINO, Such quantitative relations may be found only 1965; SCHWARCZ, 1967; STRENS, 1968; BURNS, when any effects of the bulk chemistry and of tem- 1968, 1970a, and 1976; BURNS and SUNG, 1978), perature and/or pressure changes on the parti- This approach was further stimulated by the pio- tioning of 3dN-ions in the respective geochemical neering book of BURNS (1970b, 1993), system, e.g., between the minerals of a rock, can N Considering 3d -ion distributions, different be ruled out. Therefore, our approach is to obtain CFSE3dN in phases with different cation-ligand dis- CFSE3dN and C3dN for coexisting, homogeneous tances and different site symmetries may force the minerals. Results of such work are presented here N 3d -ions predominantly into one of the phases in for the case of Cr3+ in paragenetic minerals (cf multiphase assemblages. This concept to interpret ANDRUT, 1995). The behavior of this 3d3-ion was interphase distribution is called crystal field con- studied because its CFSEcr3+ is highest among the cept, CFC, throughout the paper. It predicts that geochemically significant 3dN-ions and because its 3dN-ions fractionate into those sites and/or phases ground state in octahedral crystal fields, strongly wherein CFSE3dN is highest (BURNS, 1970b, 1993), preferred by this ion (BURNS, 1970b and 1993), is THE PROBLEM AND AN APPROACH nondegenerate (cf Fig. 1). Furthermore, the energy TO SOLVE IT of the spin-allowed transition 4A2g - 4T2g of C~+[6l (Fig. 1) is equal to lODqoct (TANABE and SUGANO, CFC proved capable of qualitatively interpreting 1954 and b). geochemical observations on 3dN-ion partioning between melt and crystals, between coexisting minerals in magmatic and metamorphic rocks from the SAMPLES AND METHODS

earth's crust and mantle between different struc- Crystal fragments of coexisting minerals with suitable tural sites in multi site mineral structures, or solid grain size were drilled out of a series of chromium con- solution thermodynamics and phase transitions of taining rocks, compiled in Table 1. 3dN-ion bearing minerals. This was comprehen- For measurement of polarized single crystal spectra, anisotropic mineral crystals were oriented by means of sively presented 1970 and 1993 by Roger BURNS. spindle stage techniques (BLOSS, 1981) such that one of Despite the abundant qualitative evidence for the the main directions of their indicatrix, either X, Y, or Z validity of CFC, concepts based on ionic radii and (notation of TROGER, 1952), became the rotation axis. site geometry in mineral structures are often Two crystal fragments of each of the coexisting minerals thought to be superior in the interpretation of parti- were prepared to facilitate measurements with E parallel to X, Y and Z. The oriented crystals were embedded and tioning patterns of not only the spherical 3do-ions ground to obtain the suitable thickness, then polished from N N but also of nonspherical3d - or 4f -ions (e.g., JEN- both sides. Quartz crystals oriented parallel to c, placed SEN, 1973; BEATTIE, personal comm.). The argu- aside the crystals to be studied, served to determine the ment for this is that the Madelung part of the lattice thickness of the mounts by evaluating the birefringence of quartz. In this way, single crystal slabs with thicknesses energy of crystals is much higher than any crystal between 50 and 500 µm and containing the main optical field effects therein. However, this may be true directions (Y,Z), (X,Z), or (X,Y), respectively, were ob- only for trace amounts of 3dN-ions but not for solid tained. The procedures of preparation and thickness mea- solutions with concentrations of 3dN-ion end mem- surements are described elsewhere (LANGER, 1988). The slabs had lateral dimensions between about 200 and 900 bers higher than 1 to 2 mole%, because CFSE N 3d µm. Their orientation was checked by conoscopic obser- typically has values between 50 and 260 [KJ/g- vations, and is thought to be correct to within 3° to 5° for 32 K. Langer and M. Andrut

Table I. Parageneses studied and their sources. Abbreviations: Gt = garnet, Kpx = clinopyroxene, Opx = orthopyroxene, Ky = kyanite, Amph = clinoamphibole, Ru = corundum (ruby), Parag = paragonite, Phlo = phlogopite, Fu = fuchsite.

Pargeneses Symbols used for Source Obtained from studied minerals studied N.V. Sobolev, Gamet Zag CI-Gt, Zag cioe Grosspydite xenolith Z13, Clinopyroxene Zag CI-Cpx, Zag GI-Cpx Zagadochnaya, Yakutia Novosibirsk, Russia Kyanite Zag CI-Ky, Zag GI-Ky

Gamet MCI-AGt Eclogite MCI, A.N. Platonov, Clinopyroxene MC-IA Cpx Urals, Russia Kiev, Ukraine Clinoamphibole MCI-AAmph Corundum RIRu Raj Iz, Antarctica A.N.Platonov, Paragonite RI Parag Kiev, Ukraine Phlogopite RI PWo Corundum MyRu Mysore, India S. Herting -Agthe, Kyanite MyKy Techn. Univ. Berlin Fuchsite MyFu Corundum LonRu Amphibolite S. Herting-Agthe, Clinoamphibole Lon Amph Longido, Tanzania Techn. Univ. Berlin Clinopyroxene OPXCPX-Cpx Exsolution of clinopyroxene in A.N. Platonov, Orthopyroxene OPXCPX-Opx orthopyroxene from an eclogite, Kiev, Ukraine Urals

'Two sets of the minerals garnet, clinopyroxene, and kyanite were extracted from the grosspydite xenolith, CI and GJ.

observation in the green part of the spectrum. In the case ses on M various points on a grain were performed. From of OPXCPX-Kpx (Table 1), i.e., the clinopyroxene lamel- these, standard deviations were calculated for the means lae exsolved from orthopyroxene at an orientation inde- of N; analyses of one individual point i as well as of all pendent from that of orthopyroxene could not be N analyses on M points. When the standard deviation of achieved. The orthopyroxene slab was oriented parallel the mean of all N measurements on M points on a grain to Y, hence EIIX and EIIZ spectra could be measured on does not exceed 2s of N; measurements on one individual Opx. The indicatrix of the clinopyroxene lamellae is tilted point i, there is no significant compositional inhomogene- against that of Opx. Therefore, the clinopyroxene spectra ity with respect to the elements measured in the crystal were, in this case, polarized Ell ca.X and Ell ca.Y. How- grain. ever, this will not greatly influence 10Dq, as the baricenter X·ray diffraction. For data evaluation, the molar vol- 3 umes, VM in [dm ] , of the minerals under study were of the 4A2g - 4T2g transition does not strongly depend on the polarization. needed (see below). These data were calculated from the 3 Polarized single crystal spectra, 10g(IolI) = f(I/), in the volumes of the unit cells, Ve in [A ], by spectral range 40000 to 4000 em -1 were scanned in 10-27 x V x N a single-beam microscope spectrometer (Zeiss UMSP 80) V _ e L (7) M- Z using UV-transparent lOx objectives (Zeiss ULTRA- FLUAR) as condenser and objective. Details of the proce- 23 dure are given elsewhere (LANGER and FRENTRUP, 1979; (NL = 6.022*10 mole ", Z = number of formula units LANGER, 1988). Luminous field and measuring dia- per unit cell). Cell volumes were obtained from the lattice phragms were 80 and 32 µm, respectively, in the UVI constants determined from powders prepared from a num- VIS or 100 and 60 µm, respectively, in the NIR. Spectral ber of additional crystals extracted from the rock samples band widths and step widths were 1 nm each in the UV I in addition to the coexisting ones, which had been pre- VIS or 8 and 10 nm, respectively, in the NIR. Spectra pared for spectroscopy and subsequent microprobe. The were obtained as averages of up to 200 scans, the refer- powder diffractograms were scanned on a Huber Guinier ence I, = f(l/) being obtained in the quartz plate (see diffractometer G 645 (courtesy W. DEPMEIER, University above). Special care was used to select optically clear of Kiel), using silicon as external standard, measuring spots on the crystal slabs studied. Curve decon- volution and band positions were achieved by means of EXPERIMENTAL RESULTS AND THEIR a peak fit program, assuming Gaussian functions to repre- EVALUATION sent the component bands in complex spectra. Microprobe analyses on a variety of different points Composition of the coexisting minerals studied on each crystal were performed on a Cameca Camebax in the ZELMI laboratory of the Technical University Berlin The crystallochemical formulae of the coexisting subsequent to the spectral measurements. To check for the homogeneity of the crystals studied, N repeated analy- minerals studied as well as their lattice constants Crystal field stabilization energy 33

are presented in Table 2. The crystallochemical of this paper to present the spectra measured on all formulae were calculated from the wt% values of the crystals studied (Table 1 and 2) and all parame- the oxide components. These values were calcu- ters extracted. This will be the aim of a forthcoming lated as the averages of repeated measurements on paper (ANDRUTand LANGER,in preparation). Here, several spots on the respective crystal slab, includ- we shall concentrate on the problems of the extrac- ing the spot on which the spectral measurements tion of lODq, using Figs, 2 to 5 as examples. had been performed. From a comparison of the All the minerals studied contain Cr3+ in coordi- standard deviations of mean values of repeated nation octahedra with site symmetries lower than analyses on one spot with those obtained by aver- Oh - m3m. For example, in garnet, the octahedra aging the data measured on several points on one have site symmetry 3, in corundum 3, in clinopy- slab, there was no indication of significant zoning roxene M(1) 2 and in kyanite 1. As pointed out in or other chemical inhomogeneities of the minerals the introduction, fields of lower symmetry than Oh studied. Typical examples of such ;standard devia- - m3m give in principle rise to two effects, im- tions are those of wt% Cr203 in kyanite from the portant in the extraction of correct 10Dqet3+-values: Zagadochnaya parageneses Cl and Gl (cf Table 1): (i) splitting of the excited, triply degenerate ZagCI-Ky: T(F) states of c2+ and (ii) symmetry-related selection rules for the re- n = 3 measurements on the same spot : spective transitions between the nondegenerate 2,17%, s=0.06% crystal-field ground state of Cr3+ and the now non- measurements on M = 7 different spots : degenerate excited states, originating from the 2.20%, s=0.09% aforementioned T-states by low-symmetry splitting. Such symmetry-related selection rules make ZagGI-Ky: allowance for a transition only, when the electric n = 3 measurements on the same spot : field vector of the exciting radiation has a special 1.78% s=0.05% orientation with respect to the electric dipole mo- ment vector of the transition considered (cf e.g, measurements on M = 7 different spots : WILSON et al., 1955; COTTON, 1971). 1.83%, s=0.09% The latter effect causes the "pleochroism" of crystals of low-symmetry minerals and other crys- From this, it may be estimated that the uncer- talline material. Other effects may contribute in the tainty in ne,3+, i.e. the number of Cr3+ -ions per case of chemically complex natural minerals. For formula unit, of the formulae in Table 2 occurs in example, charge-transfer phenomena, may also the third decimal point. This is important to note, give rise to strongly polarized absorption bands as ne,3+ will be used to calculate the volume- (SMITH and STRENS, 1976). normalized chromium concentrations held by the coexisting phases (see below). A third effect that eventually makes the extraction of 10Dqer3+ from the spectra difficult is: (iii) band-overlap of the (4A2g - 4T )-derived The spectra and the extraction of lODqcr3+ 2g from them Cr'" -transition, or transitions respectively, by other excitations occurring in chemically complex natu- As shown in the introduction, the crystal field ral minerals. parameter lODqer3+ is equal to the lowest-energy An example of (i) is the splitting of the two spin- 3 spin-allowed electronic transition of c2+, Azg allowed Cr + -bands in the clinopyroxene spectra - 4T2g(F). This transition occurs in the absorption of Fig. 4. This splitting, as well as the band polar- spectra of Cr3+-bearing, oxygen-based structures as izations (ii) that are obvious in Fig, 4, was interpre- strong, broad band, centered between about 18500 ted to be caused by the nonregular symmetry of 3 and 15500 cm ", depending on the properties of the Cr + -containing M(l)-octahedra of the clinopy- the crystal field in the respective structural sites as roxene structure, the effects (i) and (ii) being most described by eqn. (4). The half widths of this band consistent with a D2d. - 42m pseudosymmetry range up to about 2500 cm ". (ABS-WURMBACH et al. 1985). Another example Examples of such spectra, measured during this of effect (ii) is the polarization of band intensities study on coexisting c2+-bearing garnet, clinopy- in the kyanite spectra of Fig. 3. An example of roxene and kyanite, extracted as paragenesis G 1 effect (iii) is the overlap between the (4Azg - 4T2g)_ 3 from the Zagadochnaya grosspydite xenolith, are derived Cr + -band in the kyanite spectra of Fig, 3 shown in Figs. 2 to 5. It would go beyond the scope with a band originating from Fe2+ - Ti4+ charge- 34 K. Langer and M. Andrut

E 00 o ci o ci 0\ 0\ 0\

__,-., N ,-.. 0\ ~~ vv ...... ,,-.,'-'_ '_"'lfi M ~ t-- ~~ ~ 00 ~-~~- M '-' -M r- 0 M N 0 .,.., - 00 t--\0 _r> M- \0 '"..". -t-- t-- -- 00 -.iVl""'; -.:ir-tr'l ...j. 0\

_p,;>, +-' 0.. >. y';l¥'" y';l¥'" -uuu-- -ddd-- .,tIlbObO., ., tIlbOOil NNN ~~~ Crystal field stabilization energy 35

c- dd-transitions The other possible interpretation is to determine 30 ~ the baricenters of the two polarized components, 'A" -'TI, (F) derived from (4A2g - 4T2g), in the Eline and E]», spectra and to calculate the mean value, This then 20 yields lODqcub for the regular approach. In the case 'A" - 'T" (F) of corundum, the latter procedure yields IODqcub- values which are only 1.3% lower than those obtained in the first mentioned and, with respect to 10 the symmetry, correct procedure. Because a strict symmetry treatment is not possible in the case of the other minerals studied and as the deviation between the crystal field parameter 30 25 20 15 10 as obtained in the strict vs. the cubic approach is

l wavenumber [lO'cm· ] small, the 10Dqcub-approximation is adopted for the evaluation of all the spectra measured in this study. 3 FIG. 2. Single crystal spectrum of the Cr + -bearing If so, why are polarized and not unpolarized spectra (Pyr26Gross49Alm20)-garnet ZagGl-Gt (cf. Tables land 2) and assignment of the spin-allowed dd-transitions of used, which are more easily obtained? Such a pro- Cr3+[6J. Other bands in the UVNIS and NIR ranges are cedure might produce erroneous lODqcub-values 2 dd-transitions of Fe +[8] The crystal slab was 247 µm thick.

Cr' dd-transitions transfer in edge-connected octahedra in the struc- 'A" - 'Tlg (F) ture of this mineral (SMITH and STRENS, 1976). ~ o As a consequence, the extraction of 10Dqcr3+- 50 ~ " values necessary for our purpose of quantitatively 'A" - 'T" (F) 40 checking CFC is a nontrivial problem and, hence,

we ought to shortly comment on it for the Cr3+_ 30 containing, coexisting minerals studied here. '6 20 corundum- The trigonal field of the Cr3+_ ~

centered octahedra yields two bands in the lODq- ~ 10 'u 50 region at around 17900 and 18300 cm ", the first !.::: occurring in Eline and the second in Ellno (cf e.g, ""

can recalculate 1ODqtrig, obtained from the exact 35 30 25 20 15 10

I treatment for the point symmetry 3 of the corun- wavenumber [lO'cm· ) dum octahedra to the cubic field parameter by FIG. 3. Polarized single crystal spectra of the Cr3+_ Dqcub = Dqtr;g + 7/[8 X DT (8) bearing kyanite ZagGI-Ky icf. Tables 1 and 2) and assignment of dd-transitions of Cr'" as well as Fe2+ _Ti4+ (KONIG and KREMER, 1977), wherein DT is a pa- charge-transfer. The two narrow low-energy bands of rameter taking into account the trigonal distortion Cr3+ are due to spin forbidden transitions of this ion. Wavenumber positions of component bands are obtained of the octahedra along C3. DT can be determined by the curve fitting process (cf. text). Spectra EIIX and from the energies of all the transitions of Cr3+ in EIIZ are measured on a slab.lY (-Ky-3), 237 µm thick, the corundum-spectra (KONIG and KREMER, 1977). the spectrum EIIY on a slab 12 (- Ky-l), 227 µm thick. 36 K. Langer and M. Andrut

1 Cr • dd-transitions hibit in the spectral region around 16000 cm ", where the (4A2g - 4T2g)-derived bands occur, a 'A" -'T" (F) complex band system. This contains at least two strong and broad components, a high energy com- 50 'A" - 'T1, (F) ponent at around 17200 cm ", polarized ax < ay

40 < az, and a low energy component at around 15500 cm ", which is not observed in the EIIX- 30 spectrum and, in EIIY and EIIZ, with ay < az polar-

q " ElfX ization (Fig. 3), In addition, there occur two narrow 20 '" " 50 '" bands between 14000 14500 cm ", caused by spin- 3 10 40 forbidden transitions of Cr +, which show up with remarkably high intensity in kyanite (Fig. 3). These 30 bands do not playa role in the extraction of 10Dqcub

50 and, therefore, will not be further commented upon E/~r here. The assignment of the strong component 40 10 bands is based on the following results published

30 so far: (a) Pure, synthetic Cr'" -bearing kyanites (LANGER 20 and SEIFERT, 1971), with chromium-contents com- EIIZ 10 parable to those of the natural kyanites studied here, show the (4A2g - 4T2g)-derived transition of Cr3+ at 16850 cm " in unpolarized powder remis- 25 20 15 10 30 sion spectra (LANGER, 1976). Single crystal spectra,

of crystals with nCr3+ = 0.3 pfu, measured with Ellc

3 and E.ic which is close to the EIIZ and EIIX mea- FIo. 4. Polarized single crystal spectra of the Cr +_ surements of the present study, show the baricenter bearing (Jd4sDi47 )-clinopyroxene ZagG 1-Kpx (cf Tables 1 1 and 2). Baricenters of Cr3+ dd-transitions are indicated. of this band at 16480 cm- with al.c < aile polariza- Wavenumber positions of component bands are obtained tion (LANGER, 1984 and 1988). Hence, the strong, by the curve fitting process. Spectra EI!X and EIIY were high-energy component observed here is identified measured on a slab ..lZ (-Cpx-3), 110 µm thick, the EIIZ spectrum on a slab X (-Cpx-5), 68 µm thick. as the 10Dq-band of cil+. We determined 10Dqcub by averaging the baricenters of this component band as obtained by curve resolution of the three polarized spectra (Fig, 3). because the position of the baricenters depend on (b) Most kyanites from metamorphic rocks, the polarization. Therefore, all three polarizations containing traces of chromium only, exhibit a typi- are needed, whereas an unpolarized spectrum of a cal blue color, the intensity of which is often undu- crystal plate is a mixture of only two. Also, effect lous or zoned within the crystals. The polarized (iii) can only be identified, if present, on the basis spectra of such kyanites were first obtained by of polarized spectra, WHITE and WHITE (1967) and are dominated by a Garnet- The trigonal site distortion of the gar- strongly polarized, intense band system with maxi- net octahedra incorporating Cr3+ is so small (No- mum at 16000 cm ", absent in EIIX and with ay VAK and GIBBS, 1971; ARNI et al., 1985) that any > az. After an intense debate about the origin of band splitting that might result from effect (i) is these spectral features, it became obvious (SMITH indicated only by a slight asymmetry of the strong and STRENS, 1976; PARKIN et al., 1977) that the spin-allowed dd-bands of Cr3+, even at low temper- band system originates from Fe2+ - Ti4+ charge- atures (TARAN et al., 1994). Symmetry-related selection rules for the site symmetry '3 of the garnet transfer transitions in the c-parallel chains of edge- octahedra are destroyed by the Ia3d symmetry of connected M(l)-, M(2)-octahedra of the structure 3 the overall structure. Hence, an evaluation based (BURNHAM, 1963). Contributions of Fe2+_Fe + on the baricenters and yielding lODqcub is in this charge-transfer in the low-energy wing of the band case the only possible way to obtain the required system do also contribute when chemistry and con- data from the spectra, an example of which is ditions of the kyanite formation allows for it. Be- shown in Fig. 2. cause all features of the low-energy component Kyanite- The spectra obtained (e.g. Fig. 3) ex- band under discussion in our kyanite spectra corre- Crystal field stabilization energy 37

2 4 spond to those of the Fe + - Ti + charge-transfer geometry. This is the case for the M(1,2,3) sites of band identified in blue kyanites (I.e.), its assign- the structure (e.g., PAPIKE et al., 1969), which are, ment to charge-transfer of this type is obvious. therefore, assumed to incorporate chromium, clinopyroxene- The identification of the 10Dq- There is no information in the literature about band in the spectra (cf Fig. 4) and the extraction the distribution of Cr"' in trioctahedral biotite or 3 of 10Dqeub of Cr + in the M(1)-octahedra of the dioctahedral muscovite and paragonite that can clinopyroxene structure (CLARK et al., 1969) pose help in the interpretation of the spectra of our sam- no problems thanks to the spectral studies of ABS- ples RI Phlo, My Fu and RI Parag (Tables 1 and WUMBACH et al. (1985) and of KHOMENKO and 2). Effects (i) and (ii) are not observed in the case PLATONOV (1985), The baricenters of the (4A2g of RI Phlog; low band intensity may be a conse- - 4T2g)-derived band system in EIIX, EIIY and EIIZ quence of the low Cr3+-concentration in this sample polarization (Fig. 4) were determined and averaged (Table 2). My Fu and RI Parag, with pleochroic to obtain lODqcub in all clinopyroxenes of Table 1. -schernes ay ~ az < ax and ay < az ~ ax, respec- The only difficulty arose with the clinopyroxene tively, of the lODq-band system, do not show any exsolution from orthopyroxene, i.e. the spectra of low symmetry splitting although band intensities the OPXCPX-Cpx sample in Table 1. Due to the are strong enough to detect it. In any case, Cr3+ (100) intergrowth of the clinopyroxene ex solution will preferentially enter the octahedral sites with lamellae in the orthopyroxene matrix, the indicatrix lowest distortion from regular geometry, i.e. in of clinopyroxene is tilted against the face of the phlogopite the M(1)-sites with trans-configuration slab IIY of orthopyroxene, corresponding to (100) of the two OH-groups. FAYE (1968), in his interpre- within the error of the orientation. Therefore, the tation of the spectra of a fuchsite from Madagaskar, E-vector of the measuring radiation could only be considers the M(2)-position with cis-configuration approximately oriented parallel to X or Y of clino- of OH to be "relatively distorted," whereas the pyroxene in this case. M(1)-position, normally vacant in dioctahedral mi- Orthopyroxene-Considering the radius of Cr3+ cas, is close to regular geometry (e.g. BAILEY, and the mean M-O distances within the octahedral 1984). M(I)- and M(2)-sites of the structure (CAMERON Due to very weak band intensity and a possible and PAPIKE, 1981), as well as the fairly weak distor- polarization ay ~ az ~ ax> the 10Dq-band in RI tion from regular geometry of M(I) as compared Phlo could only be obtained from the EIIX spec- to M(2), it is to be expected that Cr3+ enters the trum, which bears on the accuracy of CFSEcrH for former site, charge balance being achieved e.g. by this coexisting mineral. In case of the dioctahedral 3 4 AI +[4] for Si +[4] substitution, Entrance of chro- micas, lODqcub was calculated from the baricenters mium into the M(1)-sites of the orthopyroxene for all three polarizations. structure is confirmed by the evaluation of the po- In summary- The spectral results on Cr3+ in the 3 larized spectra of a synthetic Cr + -bearing enstatite various coexisting minerals studied here, as well (ROSSMAN, 1980). Thus, our evaluation of the spec- as the considerations about their evaluation, lead tra of OPXCPX-Opx (Table 1) is based on these to values of 10Dqcub and these to values of results, 10Dqeub being obtained as the average of CFSEcrH as listed in columns 2 to 4 of Table 3. the baricenters of the lODq-band in EIIX and EIIZ lODqcub values are estimated to be correct to within polarization. EIIZ could not be measured on this about ::':30 cm-I, corresponding to ::':0.43 KJ/g- sample (see above), and, in this polarization, the atom for the values of CFSEcrH, except for baricenter of the lODq-band system occurs at OPXCPX and RI Phlo as discussed before, slightly higher wavelengths, i.e. lower energies The choice of concentration unit compared to the two former polarizations (Ross- When Cr3+ partitions between different phases MAN, 1980). Therefore, the lODqcub-value obtained of a system, a common concentration unit, valid for here may be too high by about 100 cm ", which all the phases, must be chosen. Thus, wt% Cr203 would correspond, in CFSEcrH, to + 1.4 kcal/g- or mol% Cr3+ -end member in the respective solid atom-,». solution are inappropriate as they relate the Cr3+_ Clinoamphiboles, dioctahedral and trioctahe- concentration to the very phase just under consider- dral micas- The 10Dq-band system in the spectra ation, Therefore, the concentrations must be recal- obtained on the clinoamphiboles of Tables 1 and culated to a unit volume, e.g. l drrr' or I, of all the 2 exhibits, if any, only a very weak low-symmetry phases studied. This is achieved by splitting and also weak polarization with ay > ax ~ az, Hence, Cr"' is incorporated in octahedral ccr'+[g - atomlQ = nCr3+/VM (9) sites that are only slightly distorted from regular wherein nCrH are Cr3+ -ions per formula unit of the 38 K. Langer and M. Andrut respective crystallochemical formulae of the min- ene (see above). Crystallochemical peculiarities erals and VM their molar volumes in drrr' as ob- involved in the Cr3+ -substitutions based on tained from eqn. (7). The values of Velo as obtained CaM2MgMl ~ NaM2CrMl in clinopyroxene and on from the X-ray diffraction data, of VM, nCrH and, MMlSiT~ CrMIAh in orthopyroxene, whereby the 2 finally, of CCt3+ in g-atom/l are compiled in columns additional presence of significant amounts of Fe + 5 to 8 of Table 3. The error in CCr3+,resulting from in the latter phase may also be of influence for the the uncertainties in the determination of Vel and exceptional sample. of Cr3+ -analyses by microprobe, is given in Calculating KD,cr+3-values and plotting them as parentheses. a function of the difference of the crystal field stabi- 1ization energies of the respective two phases Ph 1 CRYSTAL FIELD STABILIZATION AND and Ph2 (Fig. 6), also shows positive correlations INTERCRYSTALLINE PARTITIONING OF CR3+- THE CONCLUSIONS for the various parageneses, except for OPXCPX with a slightly negative KD· Figure 5 shows a plot of the uniformly normal- From this it is obvious that the data obtained ized Cr3+ -concentrations, Ccc'+[g-atom/l], as a show a quantitative, positive correlation between function of the crystal field stabilization energy, Cr3+ [g-atom/l] and CFSEcr3+, as predicted by CFSE 3+, that the chromium ions gain in their re- cr CFC, except for a clinopyroxene exsolution in or- spective structural matrix, From this figure, it is thopyroxene, a deviation that may be explained by obvious that there exists a positive, quantitative possible errors of the 10Dqcub-determination and correlation between the two quantities, as postu- crystallochemical peculiarities of the substitutions lated by eqn. (5), the function f being of the same type for all the parageneses studied. The only ex- in this case. ception, OPXCPX, may be explained by a value This proves the CFC concept to be valid, at least 3 of CFSE that is too high in the case of orthopyrox- for Cr + -concentrations above the trace level.

Table 3. Crystal field splitting, 10DqcrH , crystal field stabilization enrgy, CFSEcr3+ , and concentration of

chromium, C + in [g-atom/l], in the parageneticminerals studied. Vel' VM and nCt3+ are the unit cell volume, Ct3 molar volume and atoms per formula unit, respectively. a) the extraction of 10Dq is discussed in the text. b} The M(I,2,3) positions are taken into account (cf. text),

I) Cr3+ CCr3+ Crystals 10Dqcr3+ CFSECr3+ CFSECt3+ Vel VM n 3 (cf. Tab 1) [em"] [ern"] [Kl/g-atom] [A ] [dmvmole] [g-atorn/l] Zag CI-Gt 17030 20435 244.5 1600.5(1.6) 01190(1) 0.093 0.77 (3) Zag CI-Cpx 15 340 18408 220.2 428 (4) 0.0644(6) 0.027 0418(18) Zag-C1.Ky 17 100 20520 245.5 293.6 (4) 0.0442(1 ) 0.045 1.09 (9) Zag Gl-Gt 17090 20 510 245.3 1600.5(1.6) 0.1190(1) 0.076 0.63 (3) Zag GI-Cpx 15 265 18 318 219.1 428 (4) 0.0644(6) 0.023 0.36 (2) Zag GI-Ky 17 145 20579 246.1 293.6 (4) 0.0442(1 ) 0.042 0.94 (7) MCI-A Gt 1544.1 (5) 0.1162(1) 0.001 MCI-ACpx 15435 18522 221.6 424.2(1.3) 0.0639(2) 0.019 0.14 (2) MCI-A Amph 15 200 18240 218.2 8994(1.5) 0.2708(5) 0.024 0.089(15)b RI Ru 18 130 21 756 260.3 255.4 0.0256(1 ) 0.029 112 (6) RI Parag 16605 19926 238.4 957 (5) 0.1441(8) 0.088 0.61 (4) RI Phlo 16695 20034 239.3 1002 (4) 0.1509(6) 0.113 0.75 (4) My Ru 18 140 21 768 260.4 254.0(2) 0.0255(1 ) 0.01 I 0.44 (2) MyKy 17 150 20580 246.6 293.0(4) 00441(3) 0.009 0.204(17) My Fu 16255 19 506 233.3 928.3(9) 0.1398(1) 0.014 0.100(4) Lon Ru 18 100 21 720 259.8 255.1(1) 00265(1) 0.025 0.98 (6) Lon Amph 15 550 18636 222.9 904.2(1.2) 0.2723(3) 0.217 0.80 (3) OPXCPX-Cpx 15 640 18 768 224.5 440 (10) 0.062(15) 0.046 0.70 (4) OPXCPX-Opx 15 900 19085 228.3 830 (10) 00625(7) 0.023 037 (5) Crystal field stabilization energy 39

Prof. W. Depmeier, Kiel, made the Guinier-diffractometer available for the lattice constants determinations. Dr. M. Wildner, Vienna, helped by commenting on the energy level calculations and Profs. G. Franz and D. Lat- tard helped through discussions of geochemical and petro- logical problems. The Deutsche Forschungsgemeinschaft, Bonn-Bad-Godesberg, supported this study within the major research program' 'Elementverteilung" under grant no. La 324/31. To all of them, the authors' thanks are due. '\.. u <... o REFERENCES o .~" 0.8 ABS-WURMBACH 1., LANGER K. and OBERHANSLI R. 1l f Cpx (1985) Polarized absorption spectra of single crystals c "o of the chromium bearing c]inopyroxenes kosmochlore c \ \ and Cr-aegirine-augite. N. Jb. Miner. Abh. 152, 293- 0.4 f Opx 319. \ ANDRUT M. (1995) Kristallfeldstabilisierungsenergie und interkristalline Verteilung von Ubergangsmctallionen in silikatischen Paragenesen. Dr.-Disscrtation, Tech- nische Universitat Berlin, 123 pp. 210 220 230 240 250 260 ARNI R, LANGER K. and TILLMANNSE. (1985) Mn3+ in crystal field stabilization energy of Cr3+ [KJ/g-atom +1 garnets. III. Absence of Jahn- Teller distortion in syn- cc3 3 thetic Mn + -bearing garnet. Phys. Chern. Minerals 12, FIG. 5, Plot of the Cr'<-concentration, normalized to 279-282. the volume unit for all minerals studied (cf text), Ccc'+ BAILEY S. W. (1984) Crystal chemistry of the true micas. versus the CFSEcr3+ in their structure. The different sym- In Micas (ed. S.W. BAILEY). Rev. in Mineral. 13, pp. bols represent the various parageneses studied: Cl , Gl, 13-60, Mineralogical Society of America. MCI-A, RI, My, Lon, OPXCPX (cf. Table 1); abbrevia- BLOSS F. D. (1981) The spindle stage: Principles and tions as in Table 1. practice. Cambridge University Press, Cambridge. BURNHAM C. W. (1963) Refinement of the crystal structure of kyanite. Z. Kristallogr. 118,337-360. BURNS R G. (1968) Enrichments of transition metal ions ~ in silicate structures. In Symposion on the origin and ~ distribution of elements, Section 5: Terrestrial abun- if • ; 4.0 RulFu dances (ed. T.H. AHRENS). Pergamon Press, New York, ,z 1151-1164. BURNS R G. (1970a) Crystal field spectra and evidence 3.0 ~... for cation ordering in olivine minerals. Amer. Mineral. U Ru/Ky Ky/Cpx Ky/Cpx <... 55, 1608-1632. 0 E.., 2.0 Ky/Gt •• ° • BURNS R G. (1970b) Mineralogical applications of '0 .1..Cpx/Am h KylFu R'iFu crystal field theory. Cambridge University Press, 15 ~ p [, 0"". Cambridge. '"0 Phlo/Parag RulPhl0 GtlCpx RulAmph c 1.0 BURNS R G. (1976) Partioning of transition metals in • 0 .52" 0.5 mineral structures in the mantle. In The physics and ¤ Opx/Cpx chemistry of minerals and rocks (ed. R G. J. STRENS). '"0.. 0.0 Wiley, New York, 555-572. o 10 20 30 40 BURNS R G. (1993) Mineralogical applications of crystal PbI Ph2 field theory. Cambridge University Press, Cambridge, (CFSE - CFSE )Cd+ [KJ/g-atofficd+l 2nd ed. BURNS R G., CLARK J. Rand FYFE W. S. (1964) Crystal FIG. 6. Partition coefficient KD,crH,(p,T{,,"Ph2 as a function of the difference of the crystal field stabilization field theory and applications to problems in geochemis- energy of chromium in the respective two phases try. In Chemistry of the earths crust (ed. V. VINOGRA. CFSE(Phl-Ph2).Symbols and abbreviations as in Fig. 5. DOV). Proc. Vernadsky Centeno Symp. 2,88-106. BURNS R G. and FYFE W. S. (1964) Site preference energy and selective uptake of transition metal ions during magmatic recrystallization. Science 144, 1001-1003. BURNS R G. and FYFE W. S. (1966) Distribution of ele- Acknowledgments-Prof. A. N. Platonov, Kiev, kindly ments in geological processes. Chern. Geol. 1, 49-56. provided samples MC-l, RI, and OPXCPX; Prof. N. V. BURNS R G. and FYFE W. S. (l967a) Trace element Sobolev, Novosibirsk, sample Z13 from Zagadochnaya; distribution rules and their significance. Chern. Geol. and S. Herting-Agthe, Berlin, samples My and Lo. Drs. 2,89-104. Helfmeier and Galbert, ZELMI-Iaboratory of the Tech- BURNS R G. and FYFE W. S. (1967b) Crystal field theory nische Universitat Berlin helped in the microprobe analy- and the geochemistry of transition elements. In Re- ses; H. Reuff, Berlin, carefully prepared slabs from ori- searches in Geochemistry (ed. P.H. ABELSON), Wiley, ented crystal grains extracted from the rock samples; and New York. 40 K. Langer and M. Andrut

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