Spectroscopic study of chromium, iron, OH, fluid and mineral inclusions in uvarovite and fuchsite
aa,ba Antonio Sanchez Navas, B. J. Reddy and Fernando Nieto
a Departamento de Mineraloía y Petrología, Facultad de Ciencias, Universidad de Granada, Fuentenueva S/N, 18002, Granada, Spain
b Department of Physics, Sri Venkateswara University, Tirupat 517502, India
3+Octahedrally-coordinated Cr possesses peculiar spectral features which made easy to identify it in minerals, even in minor amounts. Chromium has been studied in uvarovite and fuchsite by optical and EPR spectra. Optical, EPR, FT-infrared and EPMA studies have also let to 3+3+determine the presence of Fe and Ti and fluid inclusions within uvarovite and fuchsite.
Absorption and scattering effects on the optical spectra obtained for Cr-bearing samples, resulting from the presence of inclusions, are also discussed in this work.
Author Keywords: Uvarovite; Fuchsite; UV-Vis–NIR absorption spectroscopy; EPMA; EPR; 3+3+FT-infrared; Cr; Fe; Solid and fluid inclusions
Natural uvarovites and fuchsites range widely in composition, reflecting growth under different conditions. They can contain besides chromium other transition elements such as titanium and iron. Oxidation states of iron and titanium in such minerals may reflect growth under different conditions. Relationship between composition and crystal-field effects in chromium and iron is of considerable interest . Uvarovite often exhibits weak birefringence. Explanations for the anomalous behavior of the optical properties in cubic garnets are given in literature [2, 3, 4 and 3+5]. Crystal structure and distortion of symmetry due to ordering of cations such as Al, Ca, Fe 2+and Fe in different garnets have been reported [2, 3, 4, 6, 7, 8 and 9]. There is no much information available on spectroscopic study of garnets with significant uvarovite component and the effect of iron on it. Optical absorption spectra of chromium rich garnets were studied by investigators but a few of them only gave complete chemical analyses, for better interpretation and comparison of the optical data [10 and 11]. Synthetic end member uvarovite crystallizes with cubic garnet symmetry Ia3d, contrary to the natural birefringent uvarovite-grossular solid solutions exhibiting triclinic and orthorhombic symmetry .
Fuchsite refers to muscovite family. Muscovite, KAl(AlSiO)(OH) is one variety of micas with 23102
low content of iron and possesses best insulating properties. In muscovite, Al has slightly distorted octahedral coordination with one longer Al---O bond than the other five Al---O/Al---OH bonds. In chromian muscovites (fuchsites) Cr replaces Al in appreciable amounts [13 and 14]. It is well established that the presence of transition metal ions either as principal constituents or, as frequently as minor ones profoundly influence the optical properties of many silicates, carbonates, sulphates, phosphates and several other minerals. Knowledge of chemical analysis and optical absorption study often makes possible to identify the cation, its valence state and its site symmetry in a crystal lattice. Explanation for the origins of color and pleochroism of many 3+2+silicate minerals lies in understanding the roles of Fe and Fe bound mainly to oxygens in the
crystal lattices [15, 16, 17 and 18]. To understand the crystal-field effects, a systematic investigation of EPR, UV-Vis–NIR and IR absorption spectroscopy have been taken up. As a
part of the program this paper presents the results of the investigations on chromium rich uvarovite and fuchsite. The results of the present work we study how the absorption by 3+3+octahedrally-coordinated Crand Fe influence the color of minerals, and the scattering and
absorption effect on this color by the mineral inclusions, which may also contain iron. For this
we have investigated the spectroscopic behavior of Cr in uvarovitic garnet of podiform chromitites from the Moa-Baracoa massif, Cuba; as well as in fuchsite from Bahia, Brazil. A detailed discussion of compositional data from EPMA analyses also allows us to establish the role of mineral inclusions on the color of the studied uvarovite and fuchsite. Hydrogen species, HO and OH have also been determined in the studied garnet and muscovite. 2
2.1. Sample description
The petrology and geochemical settings of Al- and Cr-rich chromitites from the Mayari-Baracoa Ophiolitic Belt (Eastern Cuba) are described along with their mineral chemistry . Studied uvarovite occurs in chromitite pods of the Moa-Baracoa massif, in the eastern ophiolitic belt of Cuba. Uvarovite is concentrically layered with chromite. Garnet compositions show a uvarovite-grossular solid solution series . These chromium rich samples, uvarovite and fuchsite studied in the present work come from the collections of the museum, Department of Mineralogy and Petrology, University of Granada, Granada, Spain. Uvarovite fragments which are emerald green color were carefully separated from brownish red chromite of the chromitite rock, with the help of an electrically operated, motor driven pen type driller. Polished thin sections were prepared from tiny rock fragments of uvarovite and fuchsite selected from the main matrix samples, and after coated with carbon for SEM and EPMA analyses. Since both the samples are fragile, they were made into fine powder for EPR and optical studies at room temperature. 2.2. Measurements
Infrared absorption spectra of the samples were recorded with KBr slices on Nicolet-20SXB ?1FTIR spectrometer (4000–400 cm). Diffuse reflectance spectra in the UV-Vis–near-infrared
were recorded on Varian Cary 5E UV-Vis–NIR spectrophotometer (200–2000 nm) for the
powder samples. Although this technique is based on scattering, its effect on extinction is subtracted by the use of a non-absorbing standard. Specular reflection is also eliminated by this technique. All this allows that measured diffuse reflectance can be directly interpreted as transmittance. EPR spectra were measured using Bruker ESP 300E ESR spectrometer operating at X-band frequencies. Secondary electron (SE) and backscattered (BSE) micrographs were performed with a field emission scanning electron microscope (FESEM) LEO 1525 equipped with an energy-dispersive X-ray spectroscopy system (EDX) to provide qualitative elemental analyses. Quantitative chemical analysis was carried out with CAMEBAX SX-50 automated electron microscope in the wavelength dispersive mode under the conditions: acceleration voltage 20 kV; probe current 5 nA; electron beam diameter 0.5 μm. Natural and
synthetic samples were employed as standards.
3d-ions such as chromium and iron have unfilled d shells. The crystal-field determines the most important aspects of their spectra. When an octahedral crystal-field becomes more intense, the spectroscopic states are split into several crystal-field states, and their relative energies change. 33+A d (Cr) ion in an octahedral field (O) will have electronic transitions from the ground state h4444A(F) to the excited states, T(F), T(F) and T(P), called spin allowed transitions. In additions 22112223+5to this, some spin forbidden transitions arise from E, T, T states. In O field, Fe ion (d) 12h644444gives rise to a number of multiplets A, A, A, E, T, T and some other states. The 112126transitions are represented from the ground state, A to other excited states. The energies of 1transitions are expressed in terms of crystal-field (Dq) and interelectronic repulsion parameters n(B & C), are presented in the form of matrices for different d configurations .
4.1. Scanning electron microscopy and electron microprobe
Optical microscopy observations showed that uvarovite garnet possesses emerald green color, with a vitreous luster. The crystals are rhombic dodecahedra. Fuchsite appears as up to one centimeter plates in basal sections with whitish green in color, containing large amounts of rutile inclusions (Fig. 1). Brownish red rutile crystals develop prismatic morphologies when size is up to 0.2 mm in length, and acicular ones for the smallest crystals (less than 5 μm in length). A set
of compositional analyses of uvarovite and fuchsite obtained from EPMA are represented in Table 1 and Table 2. Compositions cover a wide range of uvarovite-grossular solid solution for the studied garnets. Totals are significantly lower than 100%. Ti concentration is very low and Fe is negligibly small compared to Cr. Thus optical absorption features can expect to be dominated by chromium. Interestingly, the concentrations of Ti and Fe are significant when compared to Cr in fuchsite (Table 2), and compositional trends are observed for these elements ( Fig. 2), probably due to the included rutile. Hence its optical spectrum must differ from that of uvarovite.
4.2. UV-Vis–NIR absorption spectra
Absorption spectra for the two samples were measured at room temperature in the region 200–
2000 nm. For uvarovite sample, Cr is the only transition metal in sufficient high concentration to give rise absorption bands due to electronic d–d transitions. Possible spectral features caused
by Fe and Ti cannot be over ruled completely since they are present in very low amount as determined by EPMA. The spectrum of uvarovite displayed in Fig. 3 is characterized mainly by ?1two broad and intense absorption bands at 610 and 430 nm (16,390 and 23,260 cm) which 3+are typical for Cr octahedrally-coordinated by O atoms [20 and 21]. The bands show a slight 3+asymmetric shape, due to tails of UV-centered bands, as well as energy splittings. Cr in an
octahedral symmetry (O) shows three spin-allowed (broad and intense bands) transitions. h?1Accordingly, the observed two broad bands at 16,390 and 23,260 cm are assigned to the spin-4444allowed d–d transitions, A(F)?T(F) and A(F)?T(F) [21, 22 and 23]. Both these bands, 2g2g2g1g?1at their lower energy tails, exhibits shoulders (13,985, 14,285, 14,705, 14,925 and 22,220 cm).
Fig. 4 shows the minor bands of uvarovite in expanded scale. These bands are also termed as lines. The line or band is sharp if the number of t electrons is same both in excited and ground 2?1states . The first two lines at 13,985 and 14,285 cm are designated as N and R lines, 42?1A(F)?E(G). The other sharp lines at 14,705 and 14,925 cm are attributed to the 2gg42?1components of A(F)?T(G) transition known as R′ lines. The B line observed 22,220 cm is 2g1g42assigned to the spin-forbidden transition, A(F)?T(G). The assignments are made with the 2g2g3help of Tanabe and Sugano diagram drawn for d configuration with C=4.5B . The first spin 44allowed transition, A(F)?T(F) is a direct measure of crystal-field strength, 10Dq and B is 2g2g
the degree of interelectronic d–d repulsion parameter evaluated from the expression:
where m and m are the energies of the first and second spin 12?1allowed transitions, respectively. The value of B is found to be 700 cm. The crystal-field 3+stabilization energy (CFSE) for Cr O symmetry is calculated by the formula: h
Although the chromium content in fuchsite is clearly minor than that in uvarovite (Table 2) poor ?1defined bands centered at 620 and 440 nm (16,130 and 22,730 cm) of the optical spectrum of 3+Fig. 5, can be assigned to the Cr. The colors of green micas, pyroxenes and amphiboles often 3+result from traces of Cr as much as from the primary iron component . Therefore,
transmission window that occurs between the two main bands of fuchsite centered at 620 and ?13+440 nm (16,130 and 22,730 cm) is responsible for the green color of the sample. These Cr 4444bands are assigned to A(F)?T(F) and A(F)?T(F) spin allowed transitions, being the 2g2g2g1gformer one a 10Dq band. From observed energies of these two bands B is evaluated to be ?1665 cm. On the examination of fuchsite spectrum recorded in the region 300–850 nm, in
expanded scale (Fig. 6), there are two weak shoulders appear at 680 and 550 nm (14,705 and ?1?14218,180 cm). The one on red side located at 14,705 cm is identified as A(F)?T(G) band 2g1g3+?13+64due to Cr ion. The other small band at 18,180 cm may be due to Fe ion of A(S)?T(G) 1g2g
?13+transition. Similar features around 19,000 cm are ascribed to octahedral Fe ion in a number
of iron bearing samples [16, 25, 26, 27 and 28]. In oxide mineral like corundum Al sites ?1substitution by trivalent iron cause broad absorption bands around 1400 and 1800 cm .
However the observed peak seem to be narrow, so it cannot be the result of Al substitution by 3+?1Fe in the structure of the muscovite. In any case the small peak at 18,180 cm is not well 3+resolved. The third spin allowed band expected for Cr in both, uvarovite and fuchsite is also
hidden under the absorption edge which spreads into UV region (Fig. 3 and Fig. 5) represented by low energy tail of an intense absorption caused by metal-oxygen charge transfer [1, 11 and 30]. For the purpose of comparison Table 3 provides the energies of the band positions with 3+their assignments, crystal-field parameters and crystal-field stabilization energies of Cr in both 3+the samples of the present investigations, together with that of Cr in alexandrite ,
uvarovites from Russia  and fuchsite from Madagascar . By looking the data presented 3+in the Table 3, it is clear that both the samples of the present study contain Cr in octahedral
sites and this ion is largely responsible for the green color of the minerals. The magnitude of the 10Dq band is a direct measure of the crystal-field strength and it is of the same order (Table 3). 3+So, the Cr---O bond distances might be similar in uvarovite, fuchsite and alexandrite. Water molecules, hydroxide ions and fluid inclusions are important components of many natural and synthetic minerals and also related technological materials. Prominent OH and HO bands 2
appear in the infrared and near-infrared and their energies are host-dependent. The NIR spectra of the two samples under study show (Fig. 3 and Fig. 5) overtone and combination ?1stretching + bending modes of HO around 7000 and 5200 cm, respectively [21 and 32]. The 2?15200 cm overtone bands have low intensity both in uvarovite and fuchsite, correspond to vibration bending of the HO molecule, and indicate the existence of low amounts of molecular 2?1water as fluid inclusions in both minerals . The other band near 7000 cm correspond to O--
-H stretch is typical in hydrogen-bearing minerals, such as fuchsite; and its presence in uvarovite indicate the existence of hydrogarnet substitution (OH)?SiO  which is coherent 44
with the low totals found in EPMA.
4.3. EPR spectra
Room temperature EPR spectrum of uvarovite in polycrystalline form is shown in Fig. 7. The 3+3+3EPR spectrum of uvarovite is characteristic of Cr with S=3/2. Cr ion belong to d system,
being a Kramer’s ion and each of the levels |?1/2> and |?3/2> will degenerate in the absence of external magnetic field and the separation due to spin-orbit interaction between them is 2D,
where D is zero-field splitting parameter. The degeneracy is lifted in the presence of external magnetic field and gives rise to three resonances correspond to |?3/2>?|?1/2>, |?1/2>?|1/2> and |1/2>?|3/2> transitions. For powder samples mainly perpendicular component is observed. In the present case the broad unsolved spectrum is due to high concentration of chromium. The value g=1.938 measured from the broad resonance can be assigned to |?1/2>?|1/2> transition 3+of Cr ion and agrees well with studies made on other fuchsite samples [35 and 36]. The *sample contains Ti and Fe as impurities. The weak absorption (marked with ) was observed at 3+3+low field giving rise to a g value of 3.567 is assigned to Fe impurity. No Ti signal could be
observed due to broad resonance.
The EPR spectra of fuchsite recorded in low and high field regions are presented in Fig. 8. For 3+Fe ion a number of resonances are expected ranging from g=0.8 to 6.0 depending on the
distortion and the population of Kramer’s doublets [37, 38 and 39]. In low field there are two *signals (marked with in Fig. 8A), one broad at g=6.318 and a sharp line at g=3.921. This 3+indicates a distinct distribution of site symmetry around substituted Fe ion. A major resonance
at g=4.3 denotes a strong rhombic distortion without any indication of cation coordination . There are three signals noticed in the high field region (marked with * in Fig. 8B). The weak 3+absorption at g=2.223 is due to Fe ion. However, the main resonance at g=1.965 is a strong 3+3+indication of Cr signal. The unsymmetrical nature of the spectrum perhaps due to both Cr 3+3+and Ti, since Ti impurity is also present in the sample. The weak signal at 3500 G with 5+g=1.889 may be due to Cr. It is not clear to assign this feature to any impurity.
4.4. FTIR spectroscopy and vibrational analysis
Representative infrared absorption spectra of the two samples are given in Fig. 9 and Fig. 10. ?1Both the spectra show main bands ranging from 3600 to 3400 cm, which are typical for
structurally incorporated hydroxyl groups with weak hydrogen bonds . The OH vibrational ?1spectra shown in Fig. 9 and Fig. 10 display one sharp absorption band at 3600 cm ?1overlapped by a broad band centered around 3400 cm. This last broad band has been
assigned to the valence vibrations of (HO) clusters in submicroscopic fluid inclusions [42 and 2n?143]. Two other broad, poorly resolved bands at 2900 and around 2500 cm are also ?1observed. The spectral region between 1200 and 400 cm hosts bands (not shown) that may
be essentially assigned to the vibrational modes of Si---O and SiO. 4
5. Discussion and conclusions
3+3+3+The results of the present work indicate that the transition metal ions Cr, Fe and Ti occur
mainly in octahedral sites of the muscovite structure. Although all these ions may influence significantly the color of the host mineral, green color observed in the two samples is related to the position and or intensity of the two main absorption bands that occur in the visible region of the spectrum at 600 and 450 nm. The properties of the first band seem to be reasonably 3+influenced by the degree of Al substitution for Cr and its site symmetry around substituted Cr 3+3+ion. The presence of Fe and Ti as impurities in fuchsite is readily reflected in both optical and 5+EPR spectra. In fuchsite, the assignment of the weak signal at g=1.889 for Cr casts serious 3+doubt which might be related to the presence of Ti impurity. Rutile inclusions, very abundant 3+4+within fuchsite, may explain for Ti impurities. However it is well known that Ti occurs as Ti,
which does not show magnetic moment, in TiO (a white pigment). Therefore we propose that 23+Ti substitutes Al in the fuchsite structure. In fuchsite the intensities and position of OH vibrational bands in NIR and IR spectra are typical of a hydrogen-bearing mineral where hydrogen is weakly bonded. Fluid inclusions exist both in uvarovite and fuchsite samples as indicated by the presence of molecular water. OH ions also enter in the structure of the uvarovite through hydrogarnet substitution (OH)?SiO. 44
The role of mineral inclusions in the color of fuchsite may be relevant in the case of fuchsite due to the existence of numerous yellow and brownish-red rutile inclusions within. Large deviations from stoichiometry are observed in the crystalchemical formulae of fuchsite. This is the case of the low octahedral content which is below four atoms per formula unit (Fig. 2) and negatively correlated with Ti, due to the effect of the rutile inclusions. The positive correlation between Ti and Fe lead us to interpret the color of rutile inclusions as due to the existence of nanometer size inclusions of hematite. Development of platelets of nanometer size, which are coherently intergrown, within rutile crystals of micrometer size (less than 1 μm diameter) is frequent . 3+Effect of Ti and Fe gives rise to suppression of Cr features on the optical spectrum of fuchsite. 3+The presence of oxygen to ligand (Ti and Fe) absorption bands near UV as well as hematite
absorption bands at 500 and 800 nm may be inferred from the fuchsite spectrum. Particularly important for fuchsite color is the reduction of the transmission window in the area about 500 nm by the inclusions. Scattering due to the size of the inclusions (below micron) probably 4produce addition of a background (of the type: extinction=1/λ or 1/λ) to our fuchsite spectra. So
the presence of inclusions let explain the suppression effect of Cr spectral features in fuchsite spectrum of Fig. 5 and Fig. 6, when compared it with those of others fuchsites with similar (between 0.21 and 0.56%) Cr contents .
The minerals used in the present study were gratefully supplied by Manuel Rodríguez Gallego & Miguel Ortega Huertas (Museum in charge and Head of the Department). One of the authors (B.J. Reddy) is thankful to the University of Granada and the Ministry of Education, Culture & Sports, Government of Spain for the award of Visiting Professorship. Ms. Isabel Nieto is noted for her help in sample preparations. The authors thank Prof. Miguel Quiros (Department of Inorganic Chemistry, University of Granada) for his helpful comments on EPR analyses. We thank Miguel Angel Salas, Bendición Funes, Elena Villafranca, Alicia González and Miguel Angel Hidalgo from the Scientific Instrumentation Center of the University of Granada for their
help with UV-Vis–NIR & FTIR spectrophotometer recordings, EPR recordings, SEM studies and
EPMA analyses. Our thanks are also due to Agustin Rueda who helped in the preparation of the
samples for the present work. This work was financed by Research Project BT2000-0582
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Fig. 1. Optical image of rutile inclusions within fuchsite. Rutile crystals show prismatic morphologies depending on the size.
Fig. 2. Bivariate diagrams of diverse chemical elements in fuchsite, using the analyses reported in Table 2.
Fig. 3. UV-Vis-NIR spectrum of uvarovite shows typical absorption bands at 16,390 and 23,260 -13+ cm for Croctahedrally-coordinated.
Fig. 4. Expanded spectral range between 350 and 750 nm of uvarovite spectrum of Fig. 3, -1displaying chromium minor bands at 13,985, 14,705, 14,925 and 22,220 cm.
Fig. 5. Optical absorption spectrum of fuchsite. Beside chromium spectral features at 16,130 -1-1and 22,730 cm, an intense band near 7000 cmdue to O---H stretch and a minor one at 5200 -1cm, corresponding to vibration bending of the HO molecule, are also observed. 2
Fig. 6. Expanded spectrum of fuchsite recorded in the region 300-850 nm. Two weak shoulders -1appear at 14,705 and 18,180 cm.
Fig. 7. Room temperature EPR spectrum of uvarovite. Broad unsolved spectrum is due to high *3+concentration of chromium. Weak absorption (marked with) is assigned to Fe impurity.