Phys Chem Minerals

DOI 10.1007/s00269-011-0434-3

ORIGINAL PAPER

Origin of the color in cobalt-doped quartz

Luiz Carlos B. de Miranda Pinto · Ariete Righi · Fernando Soares Lameiras · Fernando Gabriel da Silva Araujo · Klaus Krambrock

Received: 8 February 2011 / Accepted: 24 April 2011© Springer-Verlag 2011

Abstract Synthetic Co-doped quartz was grown hydro-thermally in steel autoclaves at the Technological Center ofMinas Gerais (CETEC), Brazil. The quartz samples, origi-nally yellow in the as-grown state acquired blue colorationafter prolonged heat treatment times at 500°C near thealpha–beta transition temperature. UV–VIS–NIR absorp-tion spectroscopy shows the characteristic spectra of Co3+

before heat treatment. After heat treatment, the opticalabsorption spectrum is dominated by two split-triplet bandsthe Wrst in the near infrared region centered at about6,700 cm¡1 (1,490 nm) and the second in the visible spec-tral range at about 16,900 cm¡1 (590 nm). Both split-tripletbands are typical for Co2+ ions in tetrahedral coordinationenvironments. From the absence of electron paramagneticresonance (EPR) spectra, we conclude that the Co2+ foundin the optical absorption spectra of the blue quartz is notdue to an isolated structural site in the quartz lattice.Instead, the blue color is associated with electronic transi-tions of Co2+ in small inclusions in which the Co site has

tetrahedral symmetry. The non-observation of polarization-depend optical absorption spectra is also in agreement withthis model. The results for Co2+ in quartz are diVerent fromCo-bearing spinel and staurolite and other silicates likeorthopyroxene, olivine, and beryls. The formation processof the color center is discussed.

Keywords Synthetic quartz · Color · Cobalt · Electron paramagnetic resonance · Optical absorption

Introduction

The most abundant quartz in nature, known as rock crystal,is colorless and transparent. There are many varieties ofcolored quartz, such as citrine, rose quartz, amethyst,smoky quartz, and blue quartz, found in natural and syn-thetic forms (Nassau 1980, 1983; Ballman 1961) withapplications in the jewelry market. Although colored trans-parent quartz is very abundant on earth, synthetic coloredanalogs are widely used in jewelry, mainly because ofdeeper homogeneous colors compared with their naturalcounterparts. Colored varieties especially amethyst, citrine,ametrine, prasiolite, and blue quartz are actually grown inRussia, Japan, and China.

The color mechanism of quartz has been associated withthe incorporation of impurity ions in structural sites of thequartz lattice and color center formation based on ioniza-tion irradiation and heating. Although studied since a longtime, the origin of color in many quartz specimens is stilluncertain. For example, quartz with iron content in therange of 20–80 ppm may be colorless, violet (amethyst),yellow to brown (citrine) or green (prasiolite). The origin ofthe amethyst color is still controversially discussed.According to Lehmann (Lehmann and Bambauer 1973;

L. C. B. de Miranda PintoTechnological Center of Minas Gerais (CETEC) and REDEMAT, Av. José Cândido da Silveira, 2000, Belo Horizonte 31170-000, Brazil

A. Righi · K. Krambrock (&)Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte 31270-901, Brazile-mail: klaus@fisica.ufmg.br

F. S. LameirasNuclear Technology Development Center (CDTN) and REDEMAT, Belo Horizonte, Brazil

F. G. da Silva AraujoDepartment of Physics and REDEMAT, Federal University of Ouro Preto, Ouro Preto, Brazil

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Lehmann 1975), the amethyst color is associated with ironincorporated in two diVerent structural sites in the quartzlattice. In both sites, the original ferric iron, Fe3+, does notproduce color and is transformed by ionization radiation tosubstitutional Fe4+ and interstitial Fe2+. The violet amethystcolor is only observed if both sites are nearby, and the elec-tronic transitions are therefore due to charge transferbetween both charge states. Recent publications found evi-dence for the unusual charge state of Fe4+ in structural sitesof quartz (Di Benedetto et al. 2010), however, still are notconclusive in relation with the cause of color in amethyst.

Fe-related citrine with yellow to brown colors is due toferric iron and or Fe3+–Fe2+ charge transfer, respectively.Most citrine is produced by heating certain amethyst speci-mens to about 400–500°C. This process is irreversible.Although the cause of color in citrine is not proofed, it isbelieved that Fe3+ inclusions in form of oxides or hydrox-ides are responsible for their color (Lehmann 1977). Thecolor of Fe-related green quartz, called prasiolite, is attrib-uted to Fe2+ in distorted octahedral symmetry. Recent EPRinvestigations of amethyst gave some indications to theexistence and formation of diVerent Fe3+ clusters uponheating; however, no correlation experiments with the colorof these specimens were done (Sivaramaiah et al. 2011). Agood example that inclusions are responsible for the coloris that of massive rose quartz for which pink dumortieritenanoinclusions were identiWed by high-resolution transmis-sion electron microscopy (Ma et al. 2002).

In case of blue synthetic quartz, the color mechanism isalso not yet clariWed. There are three diVerent origins ofcolor proposed. For natural blue quartz, the color has beenassociated with light scattering by microinclusions ofilmenite or rutile (Zolensky et al. 1998), whereas the causeof color in synthetic blue quartz is of completely diVerentorigin. Wood and Ballman (Wood and Ballman 1966) ana-lyzed samples of cobalt-doped quartz from SawyerResearch Products and from Western Electrics by UV–VISabsorption. They observed the typical Co2+ triplet withabsorption bands centered at 540, 600 and 640 nm and pro-posed that Co2+ replaces for Si4+ with Na+ ions in thenearby interstitial sites as charge compensators. Lehmann(Lehmann 1969) proposed that Co3+ ions are initially incor-porated during growth in the quartz structure in octahedralstructural interstitial sites yielding yellow coloration. Aftera heat treatment in air at about 500°C, the cobalt ions arereduced to Co2+ and change their position to a tetrahedralinterstitial site, turning the color to blue. Lee and Chung(Lee and Chung 1999) concluded that none of these mecha-nisms occurs and attributed the blue color of syntheticquartz due to nanoclusters of colloidal particles Co-Si–OHwith diameters of about 20 nm.

In this work, cobalt-doped quartz crystals were grown attwo diVerent temperatures in order to elucidate the color

mechanism of synthetic blue quartz. The samples wereinvestigated by inductively coupled plasma emission(ICPE), UV–VIS–NIR absorption spectroscopy, Fouriertransform infrared absorption (FTIR), and electron para-magnetic resonance (EPR).

Experimental

Two cobalt-doped synthetic quartz crystals, labeled as sam-ples A and B, were grown in a 1,150 cm3 steel autoclavewithout internal coating at the Pilot-Plant of HydrothermalSynthesis of Crystals at CETEC, Brasil. In both cases, theutilized mineralizer was 1 M K2CO3 and the dopant wascobalt oxide (Co2O3) with 1.16 g/l concentration. The dis-solution temperature was 350°C (§1°C) with temperaturegradient of 20°C (§1°C) for sample A and 15°C (§ 1°C)for sample B between the dissolution and the crystallizationzones of the autoclave. The samples A and B were obtainedduring 12 and 20 days, respectively, with an averagegrowth rate of 0.39 mm/day for both. The as-grown sam-ples were submitted to heat treatment at 500°C in air in aLindberg tubular furnace for 24, 48, 72, 96, and 120 h.Sample plates with dimensions of 3 £ 3 £ 3 mm3 and3 £ 3 £ 1 mm3 were cut perpendicular to the optical c-axisand polished for UV–VIS and infrared absorptions from theas-grown and heat-treated quartz.

The samples were analyzed with a Perkin-Elmer ICPEOptima 3000 plasma emission spectrometer in order todetermine the impurity concentrations in the crystals. Theresults are summarized in Table 1. EPR spectra wererecorded with a homemade spectrometer which includes a500mW klystron (Varian), a commercial resonance cavity(Bruker) working at X-band (9.39 GHz), an electromagnet(Varian) with maximum Weld amplitudes of 800 mT and aHe Xux cryosystem (Oxford) for low temperature measure-ments. The UV–VIS–NIR spectra of the samples wereobtained with UV-3600 Shimadzu UV–VIS–NIR spectro-photometer with 50 nm/s scan speed and 0.1 s integrationtime at room temperature. For polarized absorption spectra,a Glan–Thompson Polarizer was used. The infrared spectrawere obtained in transmission geometry with an ABBBomem spectrometer and a Specac cryostat at low tempera-ture. The infrared spectra of the samples were measured at87 K with 128 scans and 4 cm¡1 resolution.

Results and discussion

The results of inductively coupled plasma emissionspectroscopy (ICPE) are shown in Table 1. The cobalt con-centration in the two crystals was about 82.5 and129.1 ppm for sample A and B, respectively. It can be seen

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that B sample, which was grown with temperature gradientof 15°C, incorporated about 56% more cobalt than sampleA which was grown with temperature gradient of 20°C inagreement with the observations of Lee and Chung (1999).The incorporated cobalt concentration into quartz isstrongly dependent on the growth temperature and nearlyindependent of the concentration of cobalt in the nutrient.

Some other observations should be drawn from theTable 1. The iron and potassium contents in our samplesare relatively high. This fact is explained by the reaction ofthe potassium carbonate (K2CO3) mineralizer used in ourgrowth process which is known to attack the steel walls ofthe autoclave. Normally, such mineralizer is used for thegrowth of amethyst, citrine, or ametrine. The iron content ismuch higher for sample A compared with sample B and forK on the contrary. The concentration of Al is low andshould come from the nutrient. However, as shown later,the iron content measured by ICPE has no or minor inXu-ence on the color of the quartz specimen.

Our as-grown samples are originally yellow in contrastto samples grown by Lee and Chung, which appeared blue(Lee and Chung 1999). Our samples acquire a blue colorafter heat treatment at 500°C which is more intense forlonger heat treatment times (24, 48, 72, 96 and 120 h). Forsample B deeper color intensity was observed due to morecobalt concentration in the crystal. After each heating step,UV–VIS–NIR and infrared absorption measurements werecarried out in the spectral regions 220–2,000 nm and3,000–3,800 cm¡1, respectively.

Figure 1 shows the UV–VIS–NIR spectra of the as-grown and the heat-treated (500°C for 120 h) sample A.Sample B has similar UV–VIS–NIR spectra but the absor-bance intensities are higher than for sample A, which isconsistent with the higher Co concentrations as found fromthe ICPE measurements (see Table 1). The absorption spec-trum of the as-grown sample with yellow color shows onedominant and broad absorption band at about 25,500 cm¡1

and a much less intense at about 15,500 cm¡1, apart from astrong UV absorption centered at about 42,000 cm¡1. Afterheat treatment in air at 500°C for prolonged times, justbelow the �–� phase transition of low-temperature quartz,two new split-triplet absorption bands appear. The Wrst withless intensity in the near infrared spectral region centered atabout 6,700 cm¡1 and the second with higher intensity inthe visible spectral region centered at about 16,900 cm¡1,as shown in insert of Fig. 1. The latter absorption band

together with strong UV absorption leads to the blue colorof the heated Co-doped quartz.

The absorption bands in as-grown Co-doped syntheticquartz with yellow color have been attributed earlier toCo3+ ions in octahedrally coordinated sites (Lehmann 1969)with a crystal Weld splitting parameter 10 Dq of about18,000 cm¡1, which is similar to octahedrally Co3+ coordi-nation in corundum (18,300 cm¡1) (McLure 1962). In thelatter, the dominant absorption band of Co3+ is found at22,800 cm¡1 and a less intense band at about 15,600 cm¡1

which are similar to the absorption band positions 25,500and 15,550 cm¡1 found in our yellow cobalt-doped quartz.Other valence states of Co can be excluded, also that ofCo3+ in tetrahedrally coordination. The two optical transi-tions at 15,500 and 25,500 cm¡1 belong to transitions fromthe 1A1 ground state to the 1T1 and 1T2 excited states ofCo3+ in octahedral coordination environment. For tetrahe-dral symmetry, the 10 Dq value should be about 4/9 of theobserved one (Burns 1970), which is not consistent with theoptical spectrum. The strong UV absorption centered atabout 42,000 cm¡1 is probably due to intense charge trans-fer transitions related with metal–ligand transitions, i.e., Corelated or Fe related.

Table 1 Growth parameters and concentration of impurities in cobalt-doped quartz crystals as determined by ICPE

Sample Dissolution temperature (°C)

Growth temperature (°C)

Temperature gradient (°C)

Co (�g/g) Fe (�g/g) Al (�g/g) K (�g/g)

A 350 330 20 82.5 343.7 25.0 91.0

B 350 335 15 129.1 71.9 23.9 288.1

Fig. 1 Optical absorption spectra of sample A measured at room tem-perature in the NIR–VIS–UV region for the as-grown (dashed line)and for the sample submitted to thermal treatment at 500°C heat for120 h (solid line). The dotted lines represent three Wtted Gaussiancurves for octahedrally coordinated Co3+. Insert Detailed spectrum ofspin-triplet absorptions of tetrahedrally coordinated Co2+ in the NIRand VIS spectral range

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The two triplet absorption bands in blue synthetic quartzare centered at about 6,700 cm¡1 and 16,900 cm¡1. Onlythe latter split-triplet absorption band was measured byLehmann and was attributed to the split 4A2—

4T1 transitionof Co2+ ions in distorted tetrahedral coordination (Lehmann1969) with a 10 Dq value of about 4,000 cm¡1. Octahe-drally coordinated Co2+ sites are not consistent with theoptical spectrum. Similar optical absorption spectra of Co2+

in tetrahedral coordination were observed in natural Co-bearing spinel and staurolite (Taran et al. 2009) and syn-thetic Co-doped Mg—spinel (Deren et al. 1994). For thenatural Co-bearing staurolite and spinel, two spin-tripletbands associated with Co2+ in tetrahedral coordination wereobserved: one in the near infrared and the second in the vis-ible spectral range. For the synthetic Mg-spinel, three spin-triplet bands were detected.

Table 2 compares the split-triplet bands of the men-tioned crystal systems which are similar. According toTaran (Taran et al. 2009 and references therein), the twosplit-triplet absorption bands arise from spin-allowed tran-sitions of the 4A2 ground state to the excited 4T1(

4F) and4T2(

4P) excited states of Co2+ in tetrahedral coordination.The splitting of the T level by spin–orbit and vibrationalcoupling results in three absorption lines. Some additionalless intense, superposed lines originate from diVerent spin-forbidden transitions. It is important to note that the inten-sity ratios for the two absorption bands in staurolite andspinel are similar also to the one observed in our Co-dopedblue quartz.

Figure 2 shows the evolution of UV–VIS absorbancepeak intensity at 16,900 cm¡1 as a function of the heattreatment time at 500°C for samples A and B. For bothsamples, the absorbance and also the color increase contin-uously with the heat treatment time up to 72 h. No visiblevariation in coloration was observed for longer duration ofmore 72 h, indicating that all trivalent cobalt was reducedto divalent cobalt. The inset in this Wgure shows a compari-

son between UV–VIS spectra of samples A and B after heattreatment at 500°C for 120 h. The absorbance of sample Bis about 60% more intense than that of sample A consistentwith the 56% higher cobalt concentration found by ICPE.

Figure 3 shows the infrared absorption spectra in the3,000–3,800 cm¡1 region for the as-grown and heat-treated(500°C) sample A for diVerent times. Three weak sharpbands can be observed superimposed upon a broad bandcorresponding to the H2O molecular impurities (Katz1962). The three sharp bands localized at 3,390, 3,433, and3,580 cm¡1, called s2, s3, and s4, respectively, were attrib-uted earlier to OH stretching vibration due to alkali-OHdefects observed in cultured quartz (Katz 1962; Lipson andKahan 1985). Katz (1962) attributed the bands at3,400 cm¡1 and at 3,580 cm¡1 as due to OH¡ perturbed byLi+ and K+ ions, respectively. It should be noted that thebroad absorption band is not strongly aVected by the heattreatment (500°C). The two sharp bands increase slightly inintensity with the heat treatment time, whereas the thirdband at 3,580 cm¡1 is not aVected. A new absorption peakappears at 3,226 cm¡1 and its intensity increases continu-ously with the thermal treatment. This band is not related tothe cobalt ion, which may produce IR band at much lowerwavenumbers, but can be related to the lattice organizationafter the heat treatment which creates Si–OH bonds result-ing from the combination of SiO2 with the water molecule.Natural and synthetic quartz crystals presented the samebroad absorption band at 3,230 cm¡1 which was associatedwith OH-H+ vibration, denoted “ice” bands (Wood 1960;Dood and Fraser 1965). A similar IR band is also observedin heat-treated amethyst, high temperature boric oxide glassand also in high temperature sol–gel-derived mullite pre-cursors (Parsons and Milberg 1960; Voll et al. 1998). Thequartz lattice is probably distorted because of the growth,

Table 2 Split-triplet optical transitions (in cm¡1) of IVCo2+ in diVer-ent minerals: quartz (this work), spinel and staurolite (Taran et al.2009), and Mg-spinel from Deren et al. (1994)

Optical transitions of IVCo2+ Quartz Spinel Staurolite

4A2 ! 4T1 (4F) 5,350

5,740 6,640 5,750

6,700 7,070 6,230

7,530 7,730 7,380

8,2504A2 ! 4T1 (

4P) 15,650 16,080 15,650

16,900 16,840 17,240

18,420 18,260 18,830

20,940 20,920

Fig. 2 Absorption coeYcient � at 16,900 cm¡1 as a function of heattreatment time at 500°C for samples A (full circles) and B (opensquares). The inset shows VIS absorption spectra of samples A and Bheat treated at 500°C for 120 h

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the water content, and the presence of cobalt nanoinclu-sions and is reorganized by the heat treatment. Sample Bpresents similar infrared spectra.

Electron paramagnetic resonance (EPR) is a technique toreveal and identify isolated paramagnetic ions in diamag-netic host crystals. Before entering in the discussion aboutthe Co incorporation, we should explain the high iron con-tent observed by ICPE in our samples. The detected iron isnot due to structural iron impurities, because their EPRspectra were not detected. Reported syntheses (Nassau1980; Lee and Chung 1999) of cobalt-doped quartz in gen-eral use mineralizers as used for the production of colorlessquartz, i.e., sodium-based mineralizers such as Na2CO3

which does not react with the walls of the autoclave.Instead, our quartz crystals were grown with K2CO3 miner-alizer, which is used normally for the growth of amethyst,citrine, and ametrine (Nassau 1980; Lee and Chung 1999;Balitsky et al. 2000). This fact may explain the high ironcontent found in our samples. However, it is puzzling thatthe iron does not form color centers even after high-dosegamma irradiation. Probably, the growth temperature andthe small temperature gradient between the dissolution andthe growth zone do not favor the incorporation of iron instructural sites of the quartz lattice and the iron is accumu-lated in inclusions.

Our Co-doped quartz samples did not show any para-magnetic cobalt centers neither in the as-grown sample norin the heat-treated samples at 500°C measured in the tem-perature range from 5 to 300 K (spectra not shown). This isan important result because it excludes possibility of tetra-hedrally coordinated Co2+ being in structural sites.

For Co3+ ion (3d6) in octahedral Weld, the 3d orbitalstates are split. The two eg orbital states are raised in energyrelative to the three t2g orbital states. The energy diVerence

is denoted by � (Abragam and Bleaney 1986). The Co3+

ion with 6 electrons may show high- (S = 2) or low-spin(S = 0) conWguration. For the Wrst conWguration four elec-trons are in the t2g and two in the eg orbital states, whereasfor the latter all six electrons are in the t2g orbital states.EPR measurements did not reveal any paramagnetic state inthe investigated temperature range (4–300 K). Therefore,we conclude that Co3+ is in the low-spin conWguration as itwas observed in many other oxides (Burns 1970).

For Co2+ ion (3d7) with seven electrons in tetrahedralWeld, the two eg orbital states are lowest in energy and sepa-rated from the three t2g orbital states. For this conWguration,a high-spin state with S = 3/2 should be expected with threeelectrons occupying the three t2g orbital states in parallelconWguration. EPR spectra of Co2+ ions are generallyobserved at low temperatures (<30 K) due to short-spin lat-tice relaxation time and EPR spectra show strong aniso-tropic g tensors (>ge) and strong hyperWne interactions dueto 59Co isotope (I = 7/2, 100% natural abundance) exactlywhat is observed for MgO: Co2+ (Abragam and Bleaney1986). In our Co-doped blue quartz, no EPR spectrum dueto Co2+ has been detected in the investigated temperaturerange (4–300 K). From these measurements, we can con-clude that the tetrahedrally coordinated Co2+ ions found inthe optical absorption spectra are not due to isolated para-magnetic Co2+ ions in structural sites in the quartz lattice.Therefore, we can assume that Co2+ forms nanoparticles,such as oxide/hydroxide particles probably with antiferro-magnetic coupling, or Si-Co–OH colloid, as suggested byLee and Chung (1999).

These results for Co2+ in quartz are diVerent from othersilicates like orthopyroxene, olivine, and beryls (Taran andRossman 2001). In contrast to quartz, these silicates havesixfold coordinated octahedral sites available for the incor-poration of cobalt. We suggest that the coloration mecha-nism of cobalt-doped cultivated quartz is due to electronictransitions of Co2+ in these nanoinclusions, i.e., the parti-cles should be much smaller than the optical wavelengthused for the absorption measurements. The UV–VIS–NIRabsorption spectra are not consistent with scattering cen-ters. Therefore, the blue color is attributed to tetrahedralCo2+ in nanoinclusions. Polarized optical absorption spec-tra conWrm this conclusion. Figure 4 shows the opticalabsorption of the Co-doped blue quartz for light withthe electric Weld polarized parallel and perpendicular to thec-axis. No change in the optical absorption spectra isobserved for the two Co2+-related absorptions centered at6,700 and 16,900 cm¡1; therefore, the color is isotropicwithout any pleochroism. DiVerent results were obtainedfor Co-bearing staurolite and spinel (Taran et al. 2009) forwhich the optical absorption spectra of Co2+ are stronglypolarization dependent. In case of blue quartz, nanoinclu-sions containing Co are randomly oriented, and therefore,

Fig. 3 Infrared absorbance spectra of sample A with 82.5 ppm Coconcentration for the as-grown and for the same sample submitted todiVerent heat treatment times at 500°C

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no polarization dependence in the optical spectra isobserved.

For citrine produced from heated amethyst, the colororiginates probably from charge transfer Fe3+–O band cen-tered in the near UV spectral region. Also, this type of Fe3+

should be in nanoinclusions because it is silent in EPR mea-surements. It seems that the quartz lattice does not accom-modate easily impurity ions, and thermal treatments mayexpulse these ions at intermediate temperatures.

Conclusions

Synthetic Co-doped quartz was grown hydrothermally at330 and 335°C growth temperature and small temperaturegradient between the dissolution and growth zone usingpotassium carbonate as mineralizer. The as-grown quartzwas yellow and became blue after heat treatment at 500°Cfor prolonged times. The UV–VIS absorption spectra ofyellow (as-grown) and blue (heat-treated) quartz are consis-tent with electronic transitions of octahedral and tetrahedralcoordinated Co3+ and Co2+, respectively. The appearance ofthe 3,226 cm¡1 infrared absorption band after the heat treat-ment is probably due to formation of Si–OH bonds and theorganization of the quartz lattice which was distorted by thewater, the fast growth rate, and the presence of cobalt nano-inclusions. The non-observation of paramagnetic Co2+ ionsby EPR and the absence of polarization-dependent opticalabsorption let us conclude that Co2+ forms nanoparticles orcolloids in form of oxides or hydroxides with antiferromag-netic coupling, similar to Fe3+ oxide or hydroxide particlesin Fe-bearing citrine. In both cases, the particles should bemuch smaller than the wavelengths in the visible spectral

range because scattering eVects on particles were notobserved.

Acknowledgments The authors acknowledge Wnancial support fromFAPEMIG, CNPq, VALE, CAPES, Gorceix Foundation, and Materi-als Engineering Graduate Program of Federal University of Ouro Pre-to, REDEMAT/CETEC-UFOP-UEMG and acknowledge kindly thesuggestions of the referees, in special Michail Taran.

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