Luminescence behaviour and Raman ...

Journal of Luminescence 131 (2011) 2317–2324

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Luminescence behaviour and Raman characterization of dendritic agate in the Dereyalak village (Eskis-ehir), Turkey L. Paralı a, J. Garcia Guinea b, R. Kibar c, A. Cetin c, N. Can c,n a

Celal Bayar University, Turgutlu Vocational High School, 45400 Turgutlu-Manisa, Turkey Museo Nacional Ciencias Naturales, Jose Gutierrez Abascal 2, Madrid 28006, Spain c Physics Department, Faculty of Arts and Sciences, Celal Bayar University, 45140 Muradiye-Manisa, Turkey b

a r t i c l e i n f o

abstract

Article history: Received 5 April 2011 Accepted 30 May 2011 Available online 22 June 2011

Results are presented for the cathodoluminescence (CL), X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and simultaneously two thermal analyses of (DTA/TGA) ¨ ukl ¨ u¨ spectroscopy of dendritic agate which occurs in the Pliocene polymictic conglomerates of the Hoy Formation, North West of the Dereyalak village (Eskisehir, Turkey). Micro-Raman measurements were performed on dendritic agate and then strong quartz and moganite peaks were identified at 465 and 501 cm  1, respectively. Thermal analysis shows the loss of water and hydroxyl units occurs in 2 distinct stages; at 796 and 808 1C. Spatially resolved CL results at room temperature were recorded for chosen 3 different areas. Grey area (100% SiO2) displays the lowest CL emission. Brown area (99.7% SiO2 and 0.3% Fe2O3) contains exsolved non-detected ironed phases such as goethite-lepidochrocite to explain the brown colour and the iron point substitutional defects attributed to the 643 nm CL emission. White outer (98.7% SiO2 and 1.3% Al2O3) would be strongly disordered as observed in the ‘‘amorphous’’ Raman spectrum containing as inferred from the spectrum CL on the outer areas, particularly non-bridging oxygen hole centres (NBOHC) (317 nm) and [AlO4]1/H þ (380 nm) centres produced by large amounts of aluminium in the lattice (1.33% Al2O3). When it comes to collect the data in the time resolved CL spectrum, at least three broad emission bands were detected in: a green band of low intensity at about 496 nm, intense orange band at about 600 nm, and a red band at 670 nm. The CL emission at 670 nm shows some relationships between the hydroxyl or alkali content and the abundance of O2 (super 3-) centres and E 10 centres. Another conspicuous observed feature in the CL spectra of agates is the existence of an orange emission band centred at around 600 nm. The predominance of the yellow CL emission band and the high concentration of E10 centres are typical for agates formed by acidic volcanism processes. & 2011 Elsevier B.V. All rights reserved.

Keywords: Agate Raman Cathodoluminescence DTA TGA Turkey

1. Introduction Agate has a high silica purity that is generally 497% with nonvolatile impurities o1% [1]. The total water (H2O and Si–OH groups) is the major impurity with a concentration up to 2% [2]. Remarkably, most agate is a mixture of two silica polymorphs: a-quartz and moganite. The crystal structure of moganite has been described as the alternate stacking of layers of left- and right-handed quartz on the unit-cell scale [3]. Moganite was eventually recognized as an independent polymorph of silica in 1999 (IMA No 99-035). Agate occurs on every continent but an understanding of the agate genesis has proved to be problematic. Likely origins are either the direct precipitation of chalcedony or the deposition of

n

Corresponding author. Tel.: þ90 236 241 21 56; fax: þ 90 236 241 21 58 E-mail address: [email protected] (N. Can).

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.05.057

amorphous silica (gel or powder) that matures into chalcedony. Agate has not been made in laboratory time and unsolved problems include the method of silica transportation and deposition, together with the mechanism of crystallization. The majority of recent workers would accept that agates form at temperatures o100 1C [4]. The geochemical study demonstrated that the formation of agate can be a complex, multi-steps process which takes place during the formation and alteration of the parental volcanic rock [5]. Although agates are composed almost entirely of SiO2, it is the trace quantities of various other elements that give agates their colour and lead to their characteristic banding. The trace element and the isotope data lead to the conclusion that the SiO2 necessary for the agate formation is mobilized during the alteration of the volcanic process by own hydrothermal solutions (auto metasomatism). We agree with the idea proposed by some researchers that hydrothermal solutions are the source of the silica in all cases of agate genesis in an igneous environment [6].

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The host-rock of agates used in this study crops out on over 1.5 km, as a layer of 5–25 m thickness. It consists of well-rounded pebbles, mainly ultramafic rocks, sandstones, limestones and volcanic materials. The conglomerates unconformable overlay the Cretaceous ophiolitic complex. The tectonic contact, with an E–W direction, is accompanied by another NE–SW trending major fault. Most of agate nodules occur along these fault zones. The mainly white and black agate nodules range from 5 to 30 cm in diameter and show a zoned macrostructure. The transparent core is surrounded by a translucent white zone and followed by a zone rich in black dendrites. The field observations and the mineralogical data, including the internal texture of agates suggest that the Dereyalak agates could have formed by sepiolite replacement with low-T silica-rich hydrothermal solutions which came most likely along the fracture systems [7]. Luminescence is light emission when energy is deposited into a material. The means of delivering energy can be divided into two broad types: stimulation, where the magnitude of the incident energy is less than that of the emitted light, and excitation, when the incident energy is greater. Forms of stimulation include heat (thermoluminescence, TL); excitation includes electron beam irradiation (cathodoluminescence, CL) or X-irradiation (radioluminescence, RL). Many minerals are luminescent, and applying these signals to solve geological problems is an important and expanding field. CL petrography is a wide-spread mineral-prospecting tool. Cathodoluminescence of quartz has various causes: CL spectra may include lines, usually associated with particular elements, and bands related to various types of crystal defects, but only the latter are observed in the case of agate. Sharp emission lines are not generally shown by quartz and other silica polymorphs as the ions that produce these lines (e.g. REE, Cr3 þ ) are not incorporated into the quartz structure. Quartz CL been investigated by many workers [5] and references therein. Studies of agate using CL are quite limited [8–10]. Three

emission bands were identified: a blue band (assigned to the [AlO4/ M þ ] to [AlO4]0 transition); a yellow band (related to intrinsic defects), and a red band attributed to non-bridging oxygen hole centres (NBOHC). Recently it was published a report on agate and chalcedony from igneous and sedimentary hosts [11]. All CL scans of the agates examined have produced red and violet emissions together with either a blue, yellow, or orange mid-range emission band. Raman spectroscopy is used as a powerful tool for mineral phase identification in geological samples, and for characterising the crystal chemistry of heterogeneous materials. A special case is represented by mineral polymorphs, such as the various SiO2 phases. Previous micro-Raman investigations on a wide range of types of microcrystalline silica samples have evidenced the presence of a tetrahedrally coordinated silica polymorph—moganite, in virtually every sample [12]. Luminescence study of dendritic agate from North West of the Dereyalak village—Eskis- ehir (Fig.1) and its physicochemical characterization were performed by X-ray diffraction (XRD), thermal analysis (DTA and TG), time resolved CL and CL coupled to an ESEM (ESEM-CL) techniques and micro-Raman.

2. Materials and methods The chemical content of the dendritic agate samples were performed by the accredited ALS Chemex Laboratory, Canada, using the XRF for major oxides, ICP-AES for trace elements and WST-SIM for loss of igneous analysis techniques (Table 1). The typical dendritic sample has been certificated with ‘‘IZ11005163’’ code number. Table 1 gives the bulk composition and trace element analysis of dendritic agate from Turkey using inductively coupled plasmaatomic emission spectrometry (ICP-AES) and XRF. The ICP-AES

Fig. 1. Dendritic agates used in this study have been located within volcanogenic conglomerates. The thickness of volcanogenic conglomerates at north-west of Dereyalak village is approximately 5–25 m. The size of dendritic agate nodules located in conglomerate units is about from 5 to 30 cm [7].

0.01 ppm

4 ppm 5 ppm 0.04 ppm o 0.5 ppm 1 ppm o 5 ppm 2.35 ppm o 0.01 ppm o0.5 ppm o 0.01 ppm o0.1 ppm

0.13 ppm

Zr Zn Yb Y W V U Tm Tl Tb Ta

Th

o 2 ppm o 0.01 ppm 0.6 ppm

0.3 ppm

0.3 ppm

117 ppm

o 5 ppm

0.09 ppm

2.1 ppm

0.05 ppm

o 1 ppm

34.0 ppm

o 0.2 ppm Sr

o 0.05 ppm 0.3 ppm

Sn Sm

o0.03 ppm o 0.03 ppm

Rb Pr

0.07 ppm o 5 ppm

Pb Ni

6.85 ppm 50 ppm

Nd Nb

7.7 ppm

Mo Lu

1.2 ppm 25.7 ppm o1 ppm

La

Ce Ba Ag

Co

Cr

Cs

Cu

Dy

Er

Eu

Ga

Gd

Hf

Ho

98 0.01% 4.39 0.01% o 0.01 0.01% o 0.01 0.01% 0.02 0.01% 0.67 0.01% 0.13 0.01% 91.2 0.01%

0.67 0.01%

0.88 0.01%

0.03 0.01%

0.03 0.01%

0.01 0.01%

o 0.01 0.01%

0.02 0.01%

Total LOI BaO SrO P2O5 MnO TiO2 Cr2O3 K2O Na2O MgO CaO Fe2O3 Al2O3 SiO2

Table 1 Average chemical bulk (by XRF) and trace element (by ICP–AES) analysis of gem-quality dendritic agate from the Dereyalak Village—Eskis- ehir region. Some trace elements in extraordinary ratios, such as Ni, Cr, Sr, Ba are characteristic.

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and XRF analyses reveal that dendritic agate is mainly composed of SiO2 (91.2%) and also abundance of Al2O3, Fe2O3, CaO and Na2O are seen in the range of 0.13%, 0.67%, 0.67%, and 0.03%, respectively. Thirty-five elements (Ag, Ba, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Hf, Ho, La, Lu, Mo, Nb, Nd, Ni, Pb, Pr, Rb, Sm, Sn, Sr, Ta, Tb, Th, Tl, Tm, U, V W,Y) were measured by inductively coupled plasma mass spectrometry (ICP-AES) and XRF, of which 10 are rare-earth elements. As can be seen from Fig. 2, SEM images obtained from opaque parts of agates are usually composed of a granular texture images. In the SEM analyses made in the samples taken from the opaque parts of agates it has been observed that quartz crystals are much larger than fibres. Quartz crystals can be easily seen when this tissue is enlarged 1.500 times. X-ray diffraction analyses were performed to powder crushed and milled from the dendritic agate material. Outer zone (rich in terms of pyrolusit) and transparent zone were firstly separated from the samples. XRD measurements were carried out using a Philips X’Pert Pro X-ray diffractometer with Bragg Brentano geo˚ for the identification of metry using Cu Ka line (l ¼ 1.5418 A) ˚ diffraction matching crystallographic phases. The d-spacing (A) using the comparative matching technique is based on the positions of peaks with relative intensities [%(I/Io)Z2] greater than 2% using with 2-theta values below 701, and tolerance range of 70.01. The main mineral component of microcrystalline SiO2, such as chalcedony, jasper, chert, or flint, is low or a-quartz. Based on X-ray ¨ diffraction, Florke et al. [13] have evidenced a new silica polymorph, ‘‘moganite’’ (type locality: Mogan, Gran Canaria, Spain) that often forms an intimately intergrowth with a-quartz in many microcrystalline SiO2 varieties. Moganite was approved as a new mineral by IMA in 1999 [14]. Conventional X-ray diffraction (XRD) (long-range order) is constrained in identifying moganite, due to the nano-range size of moganite crystallites and its close structural relationship – thus strong similarities in the XRD pattern – to aquartz, to which it is usually associated. Numerical data obtained from the experimental XRD analyses of the investigated of dendritic agate samples were tried to match to those ideal microcrystalline quartz building phases, such as cryptocrystalline a-quartz and cryptocrystalline quartzine (called moganite [Mo]). They were compiled from the PDF Cards [15] and some crystal structure databases of the material using a comparative matching technique [16–18]. Therefore, Fig. 3 clearly demonstrates that moganite (Mo) silica phase are present in the overlapped diffraction bands as additional to main agate silica phase. The thermal properties of the materials were evaluated on a Perkin Elmer Diamond TG/DTA instrument. A weighed amount of the sample to be analysed was placed in a cylindrical alumina crucible mounted on one of the 2 mm diameter alumina rods. The sample was heated from room temperature up to 1400 1C in a dynamic air atmosphere at a rate of 283 K/min. All time-resolved cathodoluminescence (CL) spectra were taken using 14 keV electrons at current densities of around 0.4 mA cm  2. The primary electron beam was normally pulsed using a Thandor TG501 5 MHz function generator as a sine function with a frequency of 90 Hz, except for the lifetime measurement. The CL response was gated using an Egand Ortholoc-SC 9505 two phase lock-in amplifier. It is worth noting that the broad diameter of the beam significantly reduces any instability due to secondary electron emission and surface charging. The light coming from the sample was focused via a quartz lens onto the entrance slit of a grating monochromator with f/4 light collection. Optical spectra were obtained using an f/4 scanning monochromator; a cooled redsensitive photomultiplier tube and lock-in amplifier measured the photomultiplier output. Two different measurements can be performed namely alternating current (AC) and direct current (DC) in the system. DC measurements can potentially record optical

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Fig. 2. Electron microscope images showing quartz grains that is causing occurrence white zone in dendritic agate samples [7].

signals from the electron gun source, but this is rejected in the AC operating mode. During the experiments we used AC measurements. In AC measurement, an Ortholoc-SC9505 two phase lock-in amplifier was employed. This instrument was used in normal mode where it performs in exactly the same way as an ordinary lock-in amplifier, e.g. only one channel is needed. Output from the PM tube is first fed into a Brookdeal 5002 current pre-amplifier mounted near to the tube. This pre-amplifier provides a more suitable input to lock-in, being able to sink a considerable DC current. Its output can be varied to provide the optimum amount of AC gain against DC reduction. The calibration of the wavelength and intensity was achieved with reference lamps whose emission spectra are known. Bausch & Lomb grating monochromator is calibrated with a low-pressure mercury vapour lamp with welldefined emission lines at 546.07, 576.96 and 579.06 nm. The monochromator can be driven mechanically to appropriate position, which enables precise acquisition data. The calibration of the apparatus can thus be easily controlled. To cover the entire range of the spectrum, two lamps whose spectra are broad bands with no line are needed. For the UV range (200–400 nm), a deuterium lamp whose spectrum corresponds to a broad band between 160 and 380 nm was used. Correction of a CL spectrum requires multiplication of the raw spectrum by the system response. All spectra shown here have been corrected for the system response. The electron beam was chopped at frequencies from 90 to 900 Hz and the photomultiplier output was measured on a lock-in amplifier. Modulation of the exciting electron beam has been accomplished by means of a pair of parallel plates inserted into the beam column. One advantage of a square wave may be that if the excitation beam is non-uniform, it still goes cleanly on and off the sample, rather than increase erratically as it sweeps into place. It may also influence the lower frequency limit as some of the harmonics will operate in the lock-in; similarly, there may be a disadvantage at high frequency with a square wave where the harmonics are above the working range. The spectral coverage is 200–800 nm and the

spectral dispersion is 2.8 nm mm  1 of slit width. The width of the entrance and exit slits is usually 1 mm, and therefore the spectral dispersion is 2.7 nm for the majority of the spectra presented in this work. Reducing the slits allows a better resolution, but produces an important decrease in the measured intensity. The samples were examined using an ESEM XL30 microscope of FEI Company. The microscope has a chemical EDS probe and a MONOCL3 Gatan probe to record CL spectra and panchromatic and monochromatic plots. The PMT covers a spectral range of 185–850 nm, and is more sensitive in the blue parts of the spectrum. A retractable parabolic diamond mirror and a photomultiplier tube are used to collect and amplify the CL emission. The sample was positioned 16.2 mm beneath the bottom of the CL mirror assembly. The excitation for CL measurements was provided at 25 kV electron beam. The spectral system was calibrated with a standard mercury lamp. The used 300 mm Czerny-Turner mono-chromator is a standard part of the MonoCL3 system having a focal length of 300 mm, aperture f/4.2, grating size of 69 mm  69 mm, dispersion of 1.8 nm mm  1 for 18001/mm grating and resolution grating dependent with a maximum of circa 0.5 nm and a wavelength accuracy of 70.2 nm, automated backlash removal with the position determined by software. Selected polished agate sections were prepared for both ESEMEDS and micro-Raman plane measurements. The micro-Raman spectroscopy study was performed by single spectra and hyperspectral line-scans using a new Thermo Fischer DXR Raman Microscope (West Palm Beach, FL 33407, USA), which has pointand-shoot Raman capability of one micron spatial resolution. We select the 20  objective of the confocal microscope together with a laser source 532 nm at 6 mW in mode laser power at 100%. The average spectral resolution in the Raman shift ranging from 50 to 800 cm  1 was 4 cm  1, i.e., grating 900 lines/mm and spot size 2 mm. The system was operated under OMNIC 1.0 software fitting working conditions such as pinhole aperture of 25 mm, bleaching time 30 s; four exposures average timed 10 s each.

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2321

Angle [°2θ]

d - value

d - value α2 [ Å ]

Peak widht [°2θ]

Peak int. [Count]

Back.int. [Count]

Rel.int [%]

Signif.

α1 [ Å ]

5.585 19.970 20.830 21.640 26.620 28.680 30.990 36.545 39.450 40.260 42.425 44.605 45.775 50.110 54.845 55.275 59.920 64.040 65.740 67.680 68.140 68.325

15.8107 4.4425 4.2609 4.1033 3.3458 3.1100 2.8833 2.4567 2.2823 2.2382 2.1289 2.0297 1.9805 1.8189 1.6725 1.6605 1.5424 1.4528 1.4193 1.3832 1.3750 1.3717

15.8500 4.4535 4.2715 4.1135 3.3542 3.1178 2.8904 2.4629 2.2879 2.2438 2.1341 2.0348 1.9855 1.8234 1.6767 1.6647 1.5463 1.4564 1.4228 1.3867 1.3784 1.3751

0.180 0.300 0.180 0.180 0.210 0.240 0.480 0.270 0.210 0.210 0.270 0.300 0.270 0.300 0.210 0.120 0.240 0.120 0.240 0.120 0.240 0.090

174 193 1884 324 12144 86 31 515 724 216 420 92 256 1011 246 151 497 110 22 412 645 543

635 159 164 169 135 114 96 77 71 69 64 59 58 61 55 56 55 50 52 53 53 53

1.4 1.6 15.5 2.7 100.0 0.7 0.3 4.2 6.0 1.8 3.5 0.8 2.1 8.3 2.0 1.2 4.1 0.9 0.2 3.4 5.3 4.5

0.84 0.83 11.90 1.38 59.53 0.84 1.47 15.42 13.44 5.59 15.49 0.95 11.34 30.11 4.68 1.04 9.43 0.91 0.97 1.26 5.66 1.26

Fig. 3. XRD pattern and initial numerical data of dendritic agate.

3. Results and discussion 3.1. Thermal analyses It can be assumed that the dendritic agate from Turkey contains water either in the form of H2O and/or HO. According to the empirical relationship [19]; as defined in Brand et al. [20], the H2O can be obtained from

which involve the loss of water. These are as follows: Step 1 ¼0.34% at 796 1C Step 2 ¼37.78% at 808 1C The mass loss step at 808 1C is attributed to the condensation of hydroxyl units and the formation of H2O. 3.2. Cathodoluminescence

H2 O wt% ¼ ðNa2 O wt%þ 1:4829Þ=1:1771: ¨ ukl ¨ u¨ Formation, North Thus the average water content in the Hoy West of the Dereyalak village (Eskisehir, Turkey) the dendritic agate samples could be calculated as 1.29 wt%. The high resolution thermogravimetric analysis combined with the differential thermogravimetric curve is shown in Fig. 4. There are two mass loss steps

In this study, measurements were made using two different CL systems as explained below. 3.2.1. Spatially resolved cathodoluminescence Dendritic agate exhibits CL in blue, green and red wavelengths of visible light, and usually shows strong zoning—a feature that

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evidences sequences of crystallization during rock formation. However, there are limited literatures on the actual chemical components that determine the luminescence of this material. Such information is imperative to quantitative analysis of CL. Under the chemical EDS three different regions of the dendritic agate have been investigated. Therefore we recorded data in order to see influences on CL spectrum of those regions. The most

Fig. 4. Simultaneous differential thermal analysis and thermogravimetric analysis (DTA/TGA) pattern of dendritic agate from Turkey. The TGA curve has been corrected for buoyancy of the atmosphere. The drift observed in the dotted red lines is typical of the effect of buoyancy. Mass loss measurement is, therefore, being corrected in the straight red line, so the mass gain is an experimental artefact. The measurements were carried out from room temperature up to 1400 1C at a constant heating rate of 10 1C per minute.

striking features of these analyses are seen in Fig. 5, where CL is compared among grey area-agate, brown area-agate and white outer-agate. Fig. 5 reveals that the peak-wavelengths and intensities of CL are dependent on SiO2, Fe2O3 and Al2O3 concentrations. As seen from Fig. 5(c), no clear emission bands have been observed in the grey area which consists of 100% SiO2. Five emission bands in the white outer area which includes 1.3% Al2O3 were detected in the CL spectra: UV bands at about 317 and 380 nm, sharp blue bands at about 437 and 458 nm and an intense red band at 643 nm. However when the brown area including 0.3% Fe2O3 compared to the other two areas one distinctive band at 643 has been observed. Moreover the intensity for brown area-agate at 643 nm is higher than that of white outer-agate. Different CL colours observed are caused by varying intensity ratios of these CL emission bands and indicate variations of lattice defects in the various zones [21,22]. The 450 nm emission is due to recombinations of self-trapped excitons, which involve irradiation-induced oxygen Frenkel pairs consisting of an oxygen vacancy and a peroxy linkage (  Si–O–O–Si ) [23]. The red emission band at about 643 nm is a conspicuous feature of CL spectra taken from moganite-rich zones. It is attributed to the recombination of electrons in the non-bridging oxygen band-gap state with holes in the valence-band edge. A number of different precursors of this non-bridging oxygen hole centre have been proposed, such as hydrogen or sodium impurities (Si–O–H,Si– O–Na groups), peroxy linkages (oxygen-rich samples), or strained silicon–oxygen bonds [23]. A likely precursor for the decreasing mean-relative intensity of the red emission and the thermogravimetrically determined defect-site water loss is a condensation reaction between neighbouring silanol groups: Si–O–HþSi–O–HSi–O–SiþH2O [6]. In SiO2 samples with hydroxyl concentrations higher than 200 ppm, radiolysis of hydroxyl groups can lead to the formation of non-bridging oxygen hole centres and atomic hydrogen [23]. Therefore, we conclude that high intensities of the 643 nm emission as particularly observed in moganite-rich zones can

Fig. 5. (a) Backscattered image of dendritic agate under the Environmental Scanning Electron, (b) major-element compositions of different phases of dendritic agate sample by EDS Microscope (ESEM), (c) 3 different phases selected for recording CL spectra in the dendritic agate sample and (d) micro-Raman spectra of 3 different dendritic agate morphologies represented in (a).

L. Paralı et al. / Journal of Luminescence 131 (2011) 2317–2324

probably be assigned to high concentrations of silanol groups. This conclusion supports results of Graetsch et al. [24] who found a higher H2O(SiOH)/H2O(molecular) ratio in moganite, compared with ¨ chalcedony. Florke et al. [25] concluded that H2O(SiOH) in moganite is preferentially bound in the twin-boundary structural elements reducing stress and thus is a constituent of the crystal structure. 3.2.2. Time-resolved cathodoluminescence CL emission spectra have been recorded between 300 and 800 nm for dendritic agate at 293 K with different excitation modulation frequencies. Note that although the intensity scales on the graphs are presented as ‘‘arbitrary units’’ they nevertheless are consistent throughout. Fig. 6 depicts CL emission spectra of dendritic agate at 293 K. Changes in the relative intensities of the CL bands can be observed when different excitation modulation frequencies are used. It is easy to see that all peaks decrease sharply as the excitation modulation frequency increase. The fact that such changes are apparent at the very low frequencies of modulation immediately indicates that there are some very long lifetime processes occurring. Various band and line emission features have similar but differing lifetimes, hence their relative intensities change with modulation frequency when using a lockin amplifier. As seen from Fig. 6 emission starts from near 300 nm and extends to 800 nm. The data at room temperature suggest that three bands occur at about 500, 600 and 680 nm. The origin of the emission peak observed at about 500 nm was in discussion during the last decade. Nassau and Prescott [26] first associated an [AlO4]0 centre from a substitutional Al3 þ with the emission band at 485 nm (2.55 eV). Itoh et al. [27] related the 2.5 eV emission to an extrinsic process due to the substitutional incorporation of impurity ions. Rink et al. [28] reported a narrow, intense 470 nm TL emission in quartz from Li-rich pegmatites which they related to Al and Ge defects. Hatipoglu et al. [10] reported a 530 nm CL peak in the first group nodular-shaped agate sample occurred in cavity-spaces of the acidic-characterized rhyolite host rock. In general, the 500 nm emission is the dominant emission band in pegmatitic quartz of different origin. A broad luminescence emission band centred at 520–590 nm was observed by Stevens Kalceff and Phillips [23] and Itoh et al. [27] by CL in irradiated samples of amorphous SiO2 and alphaquartz which they related to the self-trapped exciton. It was detected that the yellow CL emission is predominant in agates and hydrothermal vein quartz of acidic volcanic [9]. Since those quartz samples also have the highest content of lattice defects (especially high concentration of oxygen vacancies, i.e. E01 centres) these defects may be responsible for the yellow CL emission band 20000

Intensity (a.u)

15000

AC Measurement Room Temp. 14 kV 1x1 mV

90 Hz 180 Hz 360 Hz 900 Hz 9000 Hz

10000

5000

0 300

2323

at 580 nm. Hatipoglu et al. [10] reported an emission line occurring in the middle-visible wavelength region (yellow region) at circa 590 nm. The common occurrence of the 580 nm emission band in cryptocrystalline quartz can possibly be related to rapid growth probably from a non-crystalline precursor. It was concluded that high intensities of the 650 nm emission in agate can probably be assigned to high concentration of O3 2 centres which are formed by irradiation [9]. Silanol groups, which are a constituent of the agate structure [1], are the favourable precursors of these non-bridging oxygen centres. A correlation of the abundance of OH related defects in quartz and the intensity of the 650 nm emission was also reported from igneous quartz [29]. 3.3. Raman spectroscopy Based on the features obtained in the Raman spectra the studied samples from Turkey were grouped into (chemically-) ‘‘homogeneous’’ (or ‘‘pure’’) and, respectively, ‘‘heterogeneous’’, the terms being related to the absence/presence of other mineral phases than SiO2 in the corresponding materials. Also, the spectra were grouped according to the main SiO2 phase present in the sample into microcrystalline and quasi-amorphous. In the case of homogeneous microcrystalline SiO2 samples (Fig. 5(b)), one representative measured data set was selected and plotted in each case. For heterogeneous materials, one measurement was plotted for each type of phase/mixture of phases. Fig. 5(d) shows the micro-Raman spectrum of sample of the dendritic agate sample between 50 and 750 cm  1. It was demonstrated that the ratio of quartz to moganite can be estimated qualitatively from the relative intensities of the strong quartz (465 cm  1) and moganite (501 cm  1) peaks in the Raman spectrum [12]. In this study, moganite and quartz were identified in dendritic agates from Turkey. Micro-Raman measurements in the range of 50–750 cm  1 reveal sharp bands with frequencies and relative intensities that are reproducible throughout the entire sample. The most intense peaks are centred at 125, 200, 465 and 501 cm  1, whereas groups of less intense bands are found in the ranges 250–300 and 350–400 cm  1. The peaks below about 400 cm  1 originate from torsional vibrations and O–Si–O bending modes. The stronger bands in these spectra, which occur in the range of 465 and 530 cm  1 (Fig. 5(d)), involve motions of O in Si– O–Si symmetric stretching-bending modes. The Raman frequencies of these modes are found to correlate with the number of tetrahedra that form rings within framework silicate structure [30]. It has been demonstrated that structures with four-member SiO4 rings, such as coesite and feldspars, have (T–O–T) modes above 500 cm  1, whereas structures with six-member rings, such as quartz, cristobalite, tridymite, and nepheline have (T–O–T) vibrations bands between 380 and 480 cm  1 [31]. These observations have been important for assigning spectral features associated with the distribution of rings in silicate glasses [32–34]. The position of the band at 501 cm  1 in the moganite spectrum is consistent with this proposed model, since the structure of moganite consists of four-member rings of corner-sharing SiO4 tetrahedra. The strong peaks at 465 and 501 cm  1 may be loosely correlated to the additional six- and eight-member rings also present in the moganite structure.

4. Conclusions 400

500

600

700

800

Wavelength (nm) Fig. 6. Cathodoluminescence (CL) spectra for dendritic agate recorded 90, 180, 360, 900 and 9000 Hz at room temperature.

CL measurements revealed gradual changes in luminescence and signal response across the individual bands; these probably reflect decreases in trace element (i.e. iron) concentration during band growth. It is proposed that CL supports the hypothesis that

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