Spectroscopic study of chromium, iron, OH, fluid and mineral inclusions in uvarovite and fuchsite

Spectrochimica Acta Part A 60 (2004) 2261–2268

Spectroscopic study of chromium, iron, OH, fluid and mineral inclusions in uvarovite and fuchsite Antonio Sanchez Navas a,∗ , B.J. Reddy a,b , Fernando Nieto a a

Departamento de Mineralo´ıa y Petrolog´ıa, Facultad de Ciencias, Universidad de Granada, Fuentenueva S/N, 18002 Granada, Spain b Department of Physics, Sri Venkateswara University, Tirupat 517502, India Received 30 October 2003; accepted 26 November 2003

Abstract Octahedrally-coordinated Cr3+ 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 determine the presence of Fe3+ and Ti3+ 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. © 2003 Elsevier B.V. All rights reserved. Keywords: Uvarovite; Fuchsite; UV-Vis–NIR absorption spectroscopy; EPMA; EPR; FT-infrared; Cr3+ ; Fe3+ ; Solid and fluid inclusions

1. Introduction 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 [1]. Uvarovite often exhibits weak birefringence. Explanations for the anomalous behavior of the optical properties in cubic garnets are given in literature [2–5]. Crystal structure and distortion of symmetry due to ordering of cations such as Al, Ca, Fe3+ and Fe2+ in different garnets have been reported [2–4,6–9]. 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 [10,11]. Synthetic end member uvarovite crystallizes with cubic garnet symmetry Ia3d, contrary to the natural birefrin-

∗

Corresponding author. Fax: +34-958-243368. E-mail address: [email protected] (A. Sanchez Navas).

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.11.024

gent uvarovite-grossular solid solutions exhibiting triclinic and orthorhombic symmetry [12]. Fuchsite refers to muscovite family. Muscovite, KAl2 (AlSi3 O10 )(OH)2 is one variety of micas with 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 [13,14]. 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 silicate minerals lies in understanding the roles of Fe3+ and Fe2+ bound mainly to oxygens in the crystal lattices [15–18]. 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 octahedrally-coordinated Cr3+ and Fe3+ influence the color of minerals, and the scat-

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tering 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, H2 O and OH have also been determined in the studied garnet and muscovite.

2. Experimental 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 [19]. 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 [9]. 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 FTIR spectrometer (4000–400 cm−1 ). 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 ␮m. Natural and synthetic samples were employed as standards.

3. Theory 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. A d3 (Cr3+ ) ion in an octahedral field (Oh ) will have electronic transitions from the ground state 4 A2 (F) to the excited states, 4 T (F), 4 T (F) and 4 T (P), called spin allowed transitions. 2 1 1 In additions to this, some spin forbidden transitions arise from 2 E, 2 T1 , 2 T2 states. In Oh field, Fe3+ ion (d5 ) gives rise to a number of multiplets 6 A1 , 4 A1 , 4 A2 , 4 E, 4 T1 , 4 T2 and some other states. The transitions are represented from the ground state, 6 A1 to other excited states. The energies of transitions are expressed in terms of crystal-field (Dq) and interelectronic repulsion parameters (B & C), are presented in the form of matrices for different dn configurations [20].

4. Results 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 ␮m in length). A set of compositional analyses of uvarovite and fuchsite obtained from EPMA are represented in Tables 1 and 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

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Table 2 Analyses of fuchsites No. 1

2

3

4

5

6

SiO2 TiO2 Al2 O3 Cr2 O3 Fe2 O3 ∗ MnO MgO CaO Na2 O K2 O

46.307 1.533 34.870 0.205 2.303 0.012 0.519 0.003 0.475 10.676

45.907 1.503 34.972 0.325 2.255 0.026 0.483 0.007 0.457 10.599

45.869 1.433 34.925 0.158 2.271 0.012 0.539 0.007 0.426 10.618

45.965 1.858 34.145 0.412 2.315 0.000 0.509 0.000 0.431 10.505

46.188 1.576 34.897 0.425 2.295 0.000 0.489 0.001 0.442 10.483

45.862 1.668 34.517 0.412 2.306 0.005 0.494 0.001 0.439 10.560

Total

96.903

96.533

96.257

96.141

96.797

96.266

6.083 1.917 3.483 0.151 0.021 0.228 0.001 0.102 3.813 0.000 0.121 1.789 1.910

6.055 1.945 3.493 0.149 0.034 0.224 0.002 0.095 3.814 0.001 0.117 1.783 1.901

6.064 1.936 3.507 0.142 0.017 0.226 0.001 0.106 3.840 0.001 0.109 1.791 1.901

6.089 1.911 3.421 0.185 0.043 0.231 0.000 0.101 3.752 0.000 0.111 1.775 1.886

6.070 1.930 3.477 0.156 0.044 0.227 0.000 0.096 3.800 0.000 0.113 1.758 1.871

6.068 1.932 3.453 0.166 0.043 0.230 0.000 0.097 3.780 0.000 0.113 1.783 1.896

Si [4] Al

Fig. 1. Optical image of rutile inclusions within fuchsite. Rutile crystals show prismatic and acicular morphologies depending on the size.

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 two broad and intense absorption bands at 610 and 430 nm (16,390 and 23,260 cm−1 ) which are typical for Cr3+ octahedrally-coordinated by O atoms [20,21]. The bands show a slight asymmetric shape, due to tails of UV-centered bands, as well as energy splittings. Cr3+ in an octahedral symmetry (Oh ) shows three spin-allowed (broad and intense bands) transitions. Accordingly, the observed Table 1 Analyses of uvarovites No. 1

2

3

4

5

SiO2 TiO2 Al2 O3 Cr2 O3 FeO∗ MnO MgO CaO

36.968 1.877 12.655 10.965 0.167 0.015 0.005 35.317

37.730 0.921 13.815 10.483 0.257 0.057 0.022 35.396

37.963 1.123 14.909 9.243 0.279 0.021 0.027 35.797

36.776 1.820 12.283 11.629 0.298 0.037 0.091 35.103

36.697 1.948 12.217 11.796 0.329 0.040 0.100 35.030

Total

97.970

98.680

99.364

98.049

98.167

5.909 0.226 2.385 1.386 0.022 0.002 0.001 6.049

5.960 0.109 2.573 1.309 0.034 0.006 0.005 5.991

5.932 0.132 2.746 1.142 0.036 0.002 0.006 5.994

5.891 0.219 2.319 1.473 0.040 0.004 0.022 6.025

5.875 0.235 2.306 1.493 0.044 0.004 0.024 6.009

Si Ti Al Cr Fe Mn Mg Ca ∗

Total Fe calculated as FeO. Normalization to 24 O.

[6] Al

Ti Cr Fe Mn Mg
[6] Ca Na K [12]




∗

Total Fe calculated as Fe2 O3 . Normalization to 20 O and 4 OH.

two broad bands at 16,390 and 23,260 cm−1 are assigned to the spin-allowed d–d transitions, 4 A2g (F) → 4 T2g (F) and 4 A2g (F) → 4 T1g (F) [21–23]. Both these bands, at their lower energy tails, exhibits shoulders (13,985, 14,285, 14,705, 14,925 and 22,220 cm−1 ). 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 t2 electrons is same both in excited and ground states [22]. The first two lines at 13,985 and 14,285 cm−1 are designated as N and R lines, 4 A2g (F) → 2 Eg (G). The other sharp lines at 14,705 and 14,925 cm−1 are attributed to the components of 4 A2g (F) → 2 T1g (G) transition known as R lines. The B line observed 22,220 cm−1 is assigned to the spin-forbidden transition, 4 A2g (F) → 2 T2g (G). The assignments are made with the help of Tanabe and Sugano diagram drawn for d3 configuration with C = 4.5B [20]. The first spin allowed transition, 4 A2g (F) → 4 T2g (F) is a direct measure of crystal-field strength, 10Dq and B is the degree of interelectronic d–d repulsion parameter evaluated from the expression: B=

(2m1 − m2 )(m2 − m1 ) 27m1 − 15m2

where m1 and m2 are the energies of the first and second spin allowed transitions, respectively. The value of B is found to be 700 cm−1 . The crystal-field stabilization energy (CFSE)

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Fig. 2. Bivariate diagrams of diverse chemical elements in fuchsite, using the analyses reported in Table 2.

for Cr3+ Oh symmetry is calculated by the formula: CFSECr3+ = 65 10Dq Although the chromium content in fuchsite is clearly minor than that in uvarovite (Table 2) poor defined bands centered at 620 and 440 nm (16,130 and 22,730 cm−1 ) of the optical spectrum of Fig. 5, can be assigned to the Cr3+ . The colors of green micas, pyroxenes and amphiboles often result from traces of Cr3+ as much as from the primary iron component [24]. Therefore, transmission window that occurs between the two main bands of fuchsite centered at

620 and 440 nm (16,130 and 22,730 cm−1 ) is responsible for the green color of the sample. These Cr3+ bands are assigned to 4 A2g (F) → 4 T2g (F) and 4 A2g (F) → 4 T1g (F) spin allowed transitions, being the former one a 10Dq band. From observed energies of these two bands B is evaluated to be 665 cm−1 . 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 18,180 cm−1 ). The one on red side located at 14,705 cm−1 is identified as 4 A2g (F) → 2 T1g (G) 23260

22220

0.35

Abs

0.30

0.6

Abs

14925 14705

0.8

0.25

14285 13985 cm -1

16390

23260 16390

0.4

0.20

7040

5180cm-1

0.2

0.15 400 500

1000

1500

2000

500

600

700

Wavelength (nm)

Wavelength (nm)

Fig. 3. UV-Vis–NIR spectrum of uvarovite shows typical absorption bands at 16,390 and 23,260 cm−1 for Cr3+ octahedrally-coordinated.

Fig. 4. Expanded spectral range between 350 and 750 nm of uvarovite spectrum of Fig. 3, displaying chromium minor bands at 13,985, 14,285, 14,705, 14,925 and 22,220 cm−1 .

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Table 3 Energies of the band positions with their assignments, crystal-field stabilization energies (CFSE) and crystal-field parameters (cm−1 ) for Cr3+ ion in different samples Assignment

Alexandrite (Ural Mts., USA)

4A

2g (F)

→ 2 Eg (G)

4A

2g (F)

→ 2 T1g (G)

Uvarovite (Moa-Baracoa, Cuba) (present work)

Uvarovite (Saratov, Russia) 14260 14370

→ 4 T2g (F) 2 2g (F) → T2g (G) 4 A (F) → 4 T (F) 2g 1g

16650∗ 21000 25000

13985 14285 14705 14925 16390∗ 22220 23260

CFSECr3+ Dq B

19980 1665 930

19670 1640 700

4A

2g (F)

4A

∗

14700

Fuchsite (Bahia, Brazil) (present work)

Fuchsite (Tamative region, Madagascar)

14705

14800

16250∗

16130∗

22600

22730

19500 1625 650

19360 1615 665

16700∗ 21200 24100 24600 20040 1670 770

Indicates magnitude of ∆oct (10Dq).

band due to Cr3+ ion. The other small band at 18,180 cm−1 may be due to Fe3+ ion of 6 A1g (S) → 4 T2g (G) transition. Similar features around 19,000 cm−1 are ascribed to octahedral Fe3+ ion in a number of iron bearing samples [16,25–28]. In oxide mineral like corundum Al sites substitution by trivalent iron cause broad absorption bands around 1400 and 1800 cm−1 [29]. However the observed peak seem to be narrow, so it cannot be the result of Al substitution by Fe3+ in the structure of the muscovite. In any case the small peak at 18,180 cm−1 is not well resolved. The third spin allowed band expected for Cr3+ in both, uvarovite and fuchsite is also hidden under the absorption edge which spreads into UV region (Figs. 3 and 5) represented by low energy tail of an intense absorption caused by metal-oxygen charge transfer [1,11,30]. For the purpose of comparison Table 3 provides the energies of the band positions with their assignments, crystal-field parameters and crystal-field stabilization energies of Cr3+ in both the samples of the present investigations, together with that of Cr3+ in alexandrite [31], uvarovites from Russia [11] and fuchsite from Madagascar

[16]. By looking the data presented in the Table 3, it is clear that both the samples of the present study contain Cr3+ 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). So, the Cr3+ –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 H2 O bands appear in the infrared and near-infrared and their energies are host-dependent. The NIR spectra of the two samples under study show (Figs. 3 and 5) overtone and combination stretching + bending modes of H2 O around 7000 and 5200 cm−1 , respectively [21,32]. The 5200 cm−1 overtone bands have low intensity both

0.7

1.0

0.6

0.5

Abs

Abs

0.8

0.6

0.4 22730

0.4

22730

7090

16130

16130 5210cm-1

18180

0.3

14705cm-1

0.2

0.2 500

1000

1500

Wavelength (nm)

Fig. 5. Optical absorption spectrum of fuchsite. Beside chromium spectral features at 16,130 and 22,730 cm−1 , an intense band near 7000 cm−1 due to O–H stretch and a minor one at 5200 cm−1 , corresponding to vibration bending of the H2 O molecule, are also observed.

400

600

800

Wavelength (nm) Fig. 6. Expanded spectrum of fuchsite recorded in the region 300–850 nm. Two weak shoulders appear at 14,705 and 18,180 cm−1 .

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Fig. 7. Room temperature EPR spectrum of uvarovite. Broad unsolved spectrum is due to high concentration of chromium. Weak absorption (marked with ∗ ) is assigned to Fe3+ impurity.

in uvarovite and fuchsite, correspond to vibration bending of the H2 O molecule, and indicate the existence of low amounts of molecular water as fluid inclusions in both minerals [33]. The other band near 7000 cm−1 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)4 ↔ SiO4 [34] which is coherent 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 EPR spectrum of uvarovite is characteristic of Cr3+ with S = 3/2. Cr3+ ion belong to d3 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 of Cr3+ ion and agrees well with studies made on other fuchsite samples [35,36]. The sample contains Ti and Fe as impurities. The weak absorption (marked with ∗ ) was observed at low field giving rise to a g value of 3.567 is assigned to Fe3+ impurity. No Ti3+ 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 Fe3+ 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 [37–39]. In low field there are two signals (marked

Fig. 8. EPR spectra of fuchsite recorded in low (A) and high field (B) regions. In the low field region (A) two signals, marked with ∗ , are assigned to Fe3+ . In the high field region (B) three signals are also marked by star: a weak absorption at g = 2.223 is due to Fe3+ ion, the main resonance at g = 1.965 for Cr3+ and another weak signal with g = 1.889 may be due to Ti3+ .

with ∗ in Fig. 8A), one broad at g = 6.318 and a sharp line at g = 3.921. This indicates a distinct distribution of site symmetry around substituted Fe3+ ion. A major resonance at g = 4.3 denotes a strong rhombic distortion without any indication of cation coordination [40]. There are three signals noticed in the high field region (marked with * in Fig. 8B). The weak absorption at g = 2.223 is due to Fe3+ ion. However, the main resonance at g = 1.965 is a strong indication of Cr3+ signal. The unsymmetrical nature of the spectrum perhaps due to both Cr3+ and Ti3+ , since Ti3+ impurity is also present in the sample. The weak signal at 3500 G with g = 1.889 may be due to Cr5+ . 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 Figs. 9 and 10. Both the spectra show main

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bands ranging from ∼3600 to 3400 cm−1 , which are typical for structurally incorporated hydroxyl groups with weak hydrogen bonds [41]. The OH vibrational spectra shown in Figs. 9 and 10 display one sharp absorption band at ∼3600 cm−1 overlapped by a broad band centered around ∼3400 cm−1 . This last broad band has been assigned to the valence vibrations of (H2 O)n clusters in submicroscopic fluid inclusions [42,43]. Two other broad, poorly resolved bands at ∼2900 and around 2500 cm−1 are also observed. The spectral region between 1200 and 400 cm−1 hosts bands (not shown) that may be essentially assigned to the vibrational modes of Si–O and SiO4 .

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 influenced by the degree of Al substitution for Cr and its site symmetry around substituted Cr3+ ion. The presence of Fe3+ and Ti3+ as impurities in fuchsite is readily reflected in both optical and EPR spectra. In fuchsite, the assignment of the weak signal at g = 1.889 for Cr5+ casts serious doubt which might be related to the presence of Ti3+ impurity. Rutile inclusions, very abundant within fuchsite, may explain for Ti3+ impurities. However it is well known that Ti occurs as Ti4+ , which does not show magnetic moment, in TiO2 (a white pigment). Therefore we propose that Ti3+ 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)4 ↔ SiO4 . 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 ␮m diameter) is frequent [44]. Effect of Ti and Fe3+ gives rise to suppression of Cr features on the optical spectrum of fuchsite. The presence of oxygen to ligand (Ti and Fe3+ ) 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 produce addition of a background (of the type: extinction = 1/λ4 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 Figs. 5 and 6, when compared it with those of others fuchsites with similar (between 0.21 and 0.56%) Cr contents [16].

5. Discussion and conclusions

Acknowledgements

The results of the present work indicate that the transition metal ions Cr3+ , Fe3+ and Ti3+ 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

The minerals used in the present study were gratefully supplied by Manuel Rodr´ıguez 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,

Absorbance

0.20

3450

0.10

3550 2530

2920

0.00 4000

3500

3000

2000

2500 -1

Wavenumbers(cm )

Fig. 9. Infrared absorption spectrum of the hydroxyl-stretching region for uvarovite sample.

Absorbance

0.40

3630 0.20 3435 2890

2160

0.00 4000

3500

3000

2500

2000

-1

Wavenumbers(cm ) Fig. 10. IR absorption spectrum of the hydroxyl-stretching region for fuchsite sample.

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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 (S.E.U.I.D.-M.C.T, Spain). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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