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Luminescence in diamonds of the Sao Luiz placer (Brazil)
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ScienceDirect Russian Geology and Geophysics 56 (2015) 729–736 www.elsevier.com/locate/rgg
Luminescence in diamonds of the São Luiz placer (Brazil) V.P. Mironov a,*, A.L. Rakevich a, F.A. Stepanov a, A.S. Emel’yanova a, D.A. Zedgenizov b,f, V.S. Shatsky b,c,e, H. Kagi d, E.F. Martynovich a,e a
Irkutsk Division of the Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 130a, Irkutsk, 664033, Russia b V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia d Geochemical Research Center (GRC), Graduate School of Science, University of Tokyo, Tokyo, 113-0033, Japan e Irkutsk State University, ul. Karla Marksa 1, Irkutsk, 664003, Russia f Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 30 September 2013; accepted 21 April 2014
Abstract Plates made of diamonds from the São Luiz province (Brazil) were investigated by confocal scanning luminescence microscopy. The samples have many macroinhomogeneities (cracks and inclusions), but there is a quasi-uniform distribution of luminescence centers in the bulk. At all investigated points of the crystals, the same group of centers was observed: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by relatively small fluctuations in the ratio of the intensities of individual bands in the spectra. Nitrogen centers of different degrees of aggregation (H4, N3, and 575 nm, with four, three, and one nitrogen atom, respectively) coexist in these crystals. In the same zones of the samples, the distribution of blue luminescence (N3 centers) is diffuse (uniform), but the distribution of yellow-green luminescence is characterized by layering on (111). This might be a consequence of the tangential growth of octahedron faces or a result of plastic deformation of the crystals and dislocations along (111). © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: diamond; defect; luminescence; laser; spectrum; microscopy
Introduction Diamonds can preserve particles trapped during diamond growth as inclusions for a long time, which makes them a unique geologic object. The characteristics of diamonds from Yakutian deposits were studied relatively thoroughly for a long time (Beskrovanov, 1992; Garanin, 1990; Lang, 1974; Lang et al., 2007; Mironov, 1993; Mironov and Antonuk, 1994; Mironov and Mityukhin, 2001; Varshavskii, 1968; Yakubova and Genshaft, 1973): diamond luminescence, the composition of intrinsic and impurity defects, their distribution over crystals, internal morphology, and some others. Diamonds have an intricate internal structure, with growth zones and pyramids, stress zones, and traces of plastic and brittle deformations. Three (Beskrovanov, 1992) or four (Mironov, 1993) successive stages of crystallization are recognized. They
* Corresponding author. E-mail address: [email protected] (V.P. Mironov)
differ in the physicochemical conditions of growth, which is reflected in the composition of defects and their distribution over the crystals. The estimated depths at which the generation and growth of these samples took place are 100–150 km, which does not permit judging about processes in deeper zones of the Earth’s mantle or their composition by these diamonds. Studies of such diamonds were focused on inclusions and isotope composition of carbon, inclusions in diamond, and many other important aspects. In the review (Shirey et al., 2013), the state of knowledge of natural diamonds, including studies by modern analytical methods, was discussed. Diamond formation, the sources of carbon in the mantle, the role of carbon in mantle melting, and the geologic history of the mantle were considered. The main problems to be solved in the future studies were defined. Deep mantle layers are usually described by theoretical models based on the results of experimental petrological studies and seismological data. Not long ago mineral assemblages whose formation requires temperatures and pressures corresponding to the conditions at depths of >660 km were
1068-7971/$ - see front matter D 201 5, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2015.04.004
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detected as inclusions in diamonds from Brazilian placers (Harte et al., 1999; Kesson and FitzGerald, 1991). In most of the papers, not enough attention is paid to the description of diamonds as such. For example, zoning was revealed in Brazilian diamonds by optical microscopy, cathodoluminescence, and photoluminescence, but no detailed studies of the internal structure reflected in the zoning, including those with the use of the point-resolved luminescence spectroscopy of specific microzones of these diamonds, were carried out before. Therefore, we examined diamonds of the well-known São Luiz deposit (Brazil) which contained macro- and microinclusions, whose formation, according to (Kaminsky et al., 2001), took place at the depth of the seismically detectable transition zone and lower mantle.
Samples, equipment, and methods Most of the studied diamonds of the São Luiz deposit were subrounded crystals with traces of partial dissolution. Part of the diamonds were irregularly shaped intergrowths. To perform experiments, plates along the planes (001) or (011) were made of these diamonds. The plates were ~0.5 mm in thickness and 3–10 mm2 in area. They usually did not contain the genetic center of the crystal, so that it was hardly possible to restore their growth history with the succession of crystallization stages. A set of plates made of Yakutian diamonds was used as a reference for comparison. The application of confocal scanning luminescence microscopy in this work yields the pattern of distribution of luminescence over the sample or luminescence spectra for crystal zones few microns in size, including those at the temperature of liquid nitrogen (77 K) under excitation with nearby lasers. The main measurements were taken using a MicroTime 2000 complex. This complex is a multipurpose instrument yielding a luminescent image of the object under study with its coloring on the luminescence decay and permitting correlation luminescence spectroscopy studies with sensitivity, which makes it possible to detect the luminescence of molecules. Luminescence is recorded by time-correlated counting of photons. Modules of avalanche diodes are used as photoreceivers. Luminescence is excited using pulsed semiconductor lasers with wavelengths of 375, 405, 470, 532, and 640 nm. The duration of the laser pulses is ~70 ps, and the frequency of their recurrence is regulated within a range from 31.25 kHz to 80 MHz. The microscope objective is moved along three coordinates by piezopositioners, which ensure a minimum positioning interval of 10 nm. The software permits obtaining data by scanning the sample along two coordinates, along one coordinate, and from the given point. The complex has the mode of automatic layer-by-layer scanning of the volume with the preset interval of movement along the third coordinate. For the recording of spectra, the complex is equipped with a QE6500 (Ocean Optics) spectrometer. Optical signal is transmitted to the spectrometer through optical fiber and makes it possible to record lumines-
cence spectra and the kinetics of the luminescence decay simultaneously. The plates were preliminarily studied under an ordinary microscope (visually), with the definition of their features: the presence and localization of optical inhomogeneities, cracks, and inclusions. Afterward abnormal birefringence patterns were controlled through crossed polaroids and the pattern of distribution of birefringence over the area of the samples was determined. The distribution of luminescence over the area of the plates was photographed under excitation using lasers with different wavelengths. Zones differing in color or luminescence intensity were distinguished. In the distinguished zones of the plates, which the authors found the most interesting, luminescence spectra were measured at 77 K, and the kinetics of the luminescence decay was measured for some of them. To reduce surface effects, luminescence measurements were taken in zones ~100 m in size under the plate surface. Luminescence was excited using lasers with wavelengths of 375, 405, 470, and 532 nm successively, which ensured the higher selectivity of excitation of specific centers. Excitation using a laser with a wavelength of 375 nm activates the luminescence of centers in the blue, green, and red spectral regions; in general, the luminescence spectrum corresponds to those on excitation using the widely used pulsed nitrogen laser with a wavelength of 337 nm. A laser with a wavelength of 405 nm excites luminescence in the blue (partly), green, and red spectral regions; a laser with a wavelength of 470 nm, in the yellow-green and red spectral regions; and a laser with a wavelength of 532 nm, only in the centers in the red spectral region. Also, the selectivity of excitation is determined by the coincidence of the excitation wavelength with the excitation spectrum for the given defect. For example, the luminescence band for the centers at 575 nm is excited by lasers with wavelengths of 375, 405, and 532 nm rather than 470 nm. As it will be demonstrated below, this suggests that the diamond luminescence in the red spectral region consists of at least two superposed bands. Some data on the kinetics of diamond luminescence obtained by scanning confocal microscopy are presented in (Martynovich et al., 2013).
Results of experiments Observations in crossed polarizing filters (Fig. 1) show birefringence patterns in the plates, which are known in the literature as “tatami.” Such patterns are typical of diamonds of the type IIa, which have low contents of nitrogen impurity (1019 cm–3 or less). The recording of absorption spectra in the UV and IR ranges confirmed that the total nitrogen content of those samples is no more than 1019 cm–3, the B1 defect being predominant in content. Such crystals make up no more than 5% of Yakutian diamonds. Along with the “tatami”-type birefringence, the samples show birefringence which is due to stresses around inclusions. Apparently, the stresses are high enough, because cracks (more often, (111)) are observed in some zones with stresses. Such cracks might have been present
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Fig. 1. Characteristic birefringence patterns, observed through crossed polarizing filters.
Fig. 2. Nonuniformities in the distribution of luminescence (internal structure) over the studied samples on excitation using a laser with a wavelength of 405 nm. a, Sample D20; b, sample D46; c, sample D53.
in a whole crystal or have appeared owing to the partial relaxation of internal stresses on the cutting of the samples. In zones with high internal stresses, there is an elevated concentration of dislocations, whose development also causes partial relaxation of stresses. Besides, green and red luminescence is particularly intense in these zones. Examples of distribution (internal structure) of luminescence in diamond plates are shown below. Block–mosaical structure, which is well-known from the literature, and blurred inhomogeneities (Fig. 2a) coexist with distinct even layers (probably, growth zones) along (111) (Fig. 2b) and complicated structures (Fig. 2c), which might be due to twinning or the intergrowth of two or more crystals. The luminescence spectra on the change of the excitation source are different (Fig. 3), but they differ considerably less on the change of the point of measurement. The spectra at different crystal zones are similar, with the same group of spectral lines and bands (Fig. 4). Visible differences in the luminescence color are determined by fluctuations in the relationships between the intensities of blue, red, and green luminescence. For example, diamond D46 (Fig. 2b) shows a rectilinear zoning (111); note that excitation using different lasers causes changes in its luminescence, but the character of the zoning remains unchanged (Fig. 5). Measurement of luminescence spectra
showed that the zoning is determined by fluctuations in the intensity of the luminescence of several centers of the same kind, as in sample D20. The spectral composition of luminescence in Fig. 5 is similar to the spectra in Figs. 3 and 4. The spectra for almost all the studied samples and their zones are similar to those for sample D20; they differ from one sample to another or from one zone to another within a sample only in the relationship between the intensities of vibronic bands. The presence of characteristic lines and bands (Fig. 6) might be the distinguishing feature of the São Luiz diamonds. Zones with inclusions and internal cracks, including those filled with a colored substrate, are conspicuous in plate D74. The specific features of the crystal luminescence are observed exactly in the vicinity of these zones. Figure 7a shows a zone near a large inclusion which is surrounded by a crack not exposed to the plate surface. The crack might have developed owing to a difference in the thermal expansion coefficients of the inclusion and diamond. The resulting high pressure in this zone might have caused not only the development of an inner crack but also diffusion of part of the inclusion material along this crack. Microparticles of the inclusion material are observed in the photograph (Fig. 7a). Micromapping of the luminescence of a zone 80 × 80 μm in size at 77 K on excitation using a laser with a wavelength of 405 nm revealed
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Fig. 3. Luminescence spectra (77 K) for the same zone P2 of crystal D20 on excitation with lasers. 1–4, 375, 405, 470, and 532 nm, respectively.
Fig. 4. Luminescence spectra for some points of crystal D20 on excitation using a laser with a wavelength of 375 nm.
considerable inhomogeneities (Fig. 7b). The observed block– mosaical pattern was consistent with the distribution of the intensity of luminescence and its time constant of decay in this zone. Such inhomogeneities might be due to an accumulation of microinclusions. The recording of luminescence spectra in the colored microzones in Fig. 7b showed that the spectra for all the blocks had intense bands of N3 centers and a weak band of centers at 575 nm (Fig. 7c); the distribution of these centers was close to uniform. The block–mosaical structure is due to variations in the intensity of the broad red band (maximum at ~740 nm), which might have been caused by microinclusions. Micromapping of the same crystal zone on excitation using a laser with a wavelength of 470 nm revealed only traces of block–mosaical inhomogeneity, which appear as weakly perceptible features in rectilinear microlayering. The microlayering is evidenced by variations in the spectral composition and intensity of luminescence (mainly red). For a detailed study of the detected microlayering, we selected a crystal zone with no visible inhomogeneities but with distinct layering on excitation using a laser with a wavelength of 470 nm. Studies of the vicinity of this point
yielded the following results. On excitation using a laser with a wavelength of 375 nm, the distribution of luminescence over the zone under study (80 × 80 μm) is close to uniform (Fig. 8a), but well-defined layering is observed on excitation of the same crystal zone using lasers with wavelengths of 470 and 532 nm (Fig. 8b, c). The characteristic thickness of the layers is 10–20 μm. The differences in the intensity of luminescence between neighboring layers with more or less bright luminesence reach a factor of 1.5, but the spectrum shape experiences only a minor change. When the sample is rotated by 45° and 90° from the initial orientation, the distribution patterns and spectra of luminescence remain almost unchanged. Such a microlayering was also detected in the reference set of plates made of Yakutian diamonds (Fig. 8d). In the core of diamond D73, a large inclusion is surrounded by numerous smaller ones (Fig. 9a). On excitation using a laser with a wavelength of 405 nm, scattered radiation reveals a figure which might be a phantom of an early growth form of crystallization (Beskrovanov, 1992) (Fig. 9b). Luminescence spectra recorded on excitation using different lasers (Fig. 9c) showed that this zone contains the same lumines-
Fig. 5. Luminescence of the same zone of crystal D46 on excitation with lasers: a, 375 nm; b, 470 nm; c, 532 nm.
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Fig. 6. Characteristic lines and bands in the luminescence spectra for diamonds of placers in the São Luiz province.
Fig. 7. Microinhomogeneities around an inclusion in sample D74. a, General view of the vicinity of the inclusion (10×); b, micromapping of the zone 80 60 μm (77 K, laser with a wavelength of 405 nm) near a semitransparent zone around the inclusion (50×); c, spectra for some microscopic areas of panel b, which show the most dramatic differences in the luminescence color and intensity.
cence centers as other zones of this crystal, and they are similar to the spectra for other diamonds from this source. The phantom outlines are explained by the distribution of the microinclusions, which cause the elevated scattering and lower intensity of luminescence, rather than by differences in luminescence (the content of lattice defects). Note that the above-mentioned features are well-pronounced not in all the diamonds. For example, sample D69 shows a zoning close to rectilinear; on excitation using lasers with wavelengths of 375, 405, and 470 nm, the spectra show only minor differences from those for Yakutian diamonds, and only on excitation using a laser with a wavelength of 532 nm do the spectra have characteristic features: centers at 575 nm and a red band with a maximum at ~700 nm. Discussion The studied diamond samples from the Brazilian placer have similar optical characteristics. The distribution of luminescence over the area of the plates is characterized either by a blurred block–mosaical structure or by a zoning (probably, growth zoning).
The position of characteristic lines and bands in luminescence spectra (77 K) is shown in Table 1. Well-known lines are identified at 415, 496, and 575 nm; worse studied lines, at 491, 523, and 536 nm; and a broad structureless band with a maximum at ~700 nm, against whose background the center at 640 nm is hardly seen. This is somewhat unexpected, because the centers at 575 and 640 nm usually coexist owing to similar structures. It is presumed that both centers contain a single nitrogen atom and a vacancy. However, the band with a maximum at 700 nm is not the oscillatory branch of the system at 640 nm, as was shown by measurements of the kinetic characteristics of the luminescence decay in the same diamonds (Martynovich et al., 2013). The time constant of decay of the red band with a maximum at ~700 nm is ~70 ns, which is considerably more than the time constant of decay of the luminescence of the centers at 640 nm (~13 ns), which is known from the literature. Such a band of red luminescence was previously detected in diamonds from deep xenoliths of kimberlites of Yakutia (Beskrovanov, 1992). As it follows from Table 1, no new, previously unknown, lines or bands are found in the spectra. The N3 center (zero-phonon line at 415 nm) is observed in all the samples.
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Fig. 8. Inhomogeneities detected in sample D74 on excitation with lasers: a, 375 nm; b, 470 nm; c, 532 nm; d, microlayering in a Yakutian diamond (reference set) on excitation using a laser with a wavelength of 470 nm.
Based on green luminescence, the diamonds are divided into two groups: with predominant H4 centers (zero-phonon line at 496 nm) and with predominant H3 centers (zero-phonon line at 503.2 nm). The following nitrogen–vacancy structural models are known for these centers from the literature: H3 defect–A defect plus vacancy and H4 defect–B1 defect plus vacancy. The data of the structural models suggest that the studied samples are divided into two groups based on the composition of the impurity defects contained in them: The B1 defect is the main impurity in samples D20–D53, and the A defect, in samples D69–D74. This is consistent with the results of IR spectroscopy, because the limit of detection of the content of the impurities from IR absorption is no more
than 1017 cm–3 and part of the samples are identified as “nitrogen-free.” Luminescence spectroscopy is more sensitive, at least by two orders of magnitude, which permits detecting nitrogen defects in “nitrogen-free” diamonds. The spectra for the diamonds with the B1 defect (H4 center, zero-phonon line at 496 nm) contain a center with a zero-phonon line at 536 nm, which is poorly studied as yet. A center with a zero-phonon line at 491 nm is observed in part of the diamonds with both B1 and A defects. According to literature data, this center is observed in diamonds with plastic deformations; it might contain dislocations. On excitation using a laser with a wavelength of 375 nm, luminescence of the highest intensity is luminescence with a
Fig. 9. Internal structure of diamond D73. Luminescence in the same zone of crystal D46 on excitation with lasers: a, 375 nm; b, 470 nm; c, luminescence spectra for this zone, recorded on excitation with different lasers.
V.P. Mironov et al. / Russian Geology and Geophysics 56 (2015) 729–736 Table 1. Lines and bands in samples from the São Luiz placer Sample no. Luminescence lines and bands, nm D20
415
491
496
–
536
575
700
D43
415
491
496
–
536
575
700
D53
415
491
496
–
536
575
700
D69
415
–
–
503.2
–
575
700
D73
415
–
–
503.2
–
575
700
D74
415
491
–
503.2
–
575
700
735
Both in the studied diamonds from the São Luiz placer and in the reference samples of Yakutian diamonds, it was found that the distribution of blue luminescence (N3 centers) is close to uniform, but the distribution of yellow-green luminescence in the same zones of the samples is characterized by microlayering. Apparently, this is explained by the possibilities of the applied method rather than by the genetic features of the crystals, and this calls for a separate study.
Conclusions zero-phonon line at 415 nm (N3 centers) and that of centers at 575 nm. Red luminescence is sometimes more intense than blue. Also, there are less intense luminescence bands in the green spectral region. As it is known, N3 centers contain three nitrogen atoms and a vacancy (N3V), whereas the center at 575 nm contains one nitrogen atom and a vacancy (NV). The coexistence of the N3 center and the center at 575 nm in the same crystal zones calls for an additional study, because it contradicts the hypothesis that “small” nitrogen defects aggregate into “large” at high temperatures and pressures. The center at 575 nm is almost not excited using a laser with a wavelength of 470 nm (Fig. 3). A line at 536 nm and a red band with a maximum at 700 nm, which are distinguishing features of these diamonds, are well excited with this laser along with a line at 523 nm, which is sometimes also recorded in diamonds from other deposits. The line at 523 nm is assigned to the system of the lines of the S2 center. Measurement of the kinetics of decay of this line showed that the characteristic time is ~14.3 ns, which is close to the rate of decay of the luminescence of H4 centers. We consider the line at 523 nm to be independent of the S2 system. A laser with a wavelength of 532 nm excites red luminescence with an intricately shaped spectrum. The spectrum contains a band of centers at 575 nm with oscillatory repetitions and a band with a maximum at 700 nm. Also, a line at 572 nm is observed in the spectra, which is due to the Raman scattering of the laser with a wavelength of 532 nm in diamond. In the kinetics of the luminescence decay, there are also two components of decay: with a time constant of 23.2 ns for the center at 575 nm and the kinetic component, with a characteristic time of ~70 ns, of a band with a maximum at 700 nm. Thus, the samples from the São Luiz placer contain the following centers: N3, S2, H3, H4, and a center at 575 nm, as well as lines at 523 and 536 nm and a band with a maximum at 700 nm, which might be independent. The same group of centers is recorded in all the crystal zones, differing only in the intensity of lines and bands. According to V.V. Beskrovanov (1992), such crystals are quasi-homogeneous. Note that red luminescence is also often observed in diamonds from Yakutian kimberlites, but it is here determined by different centers. The spectra for Yakutian diamonds from deep xenoliths, including peridotite xeloliths (Beskrovanov, 1992), are similar to those for the São Luiz diamonds, but they were obtained by different techniques, so that direct comparison is not quite correct.
The studied samples from the São Luiz placer have many macroinhomogeneities (cracks and inclusions) but show the quasi-uniform distribution of luminescence centers in the bulk. At all the studied crystal points, the same group of centers is detected: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by the narrow variations in the relationship between the intensities of luminescence of specific bands. The quasi-uniformity in the distribution of defects over the samples does not permit defining the succession (stages) of crystallization. The samples crystallized in one stage or homogenized at high temperatures and pressures during the postgrowth period. At each point of the crystals, nitrogen centers showing different degrees of aggregation are observed: H4, N3, and 575 nm, which contain several, three, and one nitrogen atom, respectively. In the same places of the samples, the distribution of the luminescence of N3 centers has a diffuse (uniform) pattern, whereas the distribution of yellow-green luminescence is characterized by microlayering along (111), which might be a consequence of tangential growth or plastic deformation of the crystals and dislocations along (111). The fact that some centers (N3) show a uniform distribution and others show layering is not discussed in the literature and requires an additional study. The limited number of the studied samples does not permit defining any statistical regularities. Note that the application of scanning confocal luminescence microspectroscopy with selective and spatially selective excitation of centers using several lasers opens up new opportunities for the study of minerals, including their defect and impurity compositions.
References Beskrovanov, V.V., 1992. The Ontogeny of Diamond [in Russian]. Nauka, Moscow. Garanin, V.K., 1990. To the problem of the discreteness of natural diamond formation. Mineralogicheskii Zh. 12 (5), 28–36. Harte, B., Harris, J.W., Hutchison, M.T., Watt, G.R., Wilding, M.C., 1999. Lower mantle mineral associations in diamonds from São Luiz, Brazil, in: Mantle Petrology: Field Observations and High Pressure Experimentation: A Tribute to Francis R. (Joe) Boyd. The Geochem. Soc., Houston, Tex., Spec. Publ. 6, pp. 125–153. Kaminsky, F.V., Zakharchenko, O.D., Davies, R.M., Griffin, W.L., Khachatryan-Blinova, G.K., Shiryaev, A.A., 2001. Superdeep diamonds from the
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Juina area, Mato Grosso State, Brazil. Contrib. Mineral. Petrol. 140 (6), 734–753. Kesson, S.E., FitzGerald, J.D., 1991. Partitioning of MgO, FeO, NiO, MnO and Cr2O3 between magnesian silicate perovskite and magnesiowüstite: implications for the origin of inclusions in diamond and the composition of the lower mantle. Earth Planet. Sci. Lett. 111 (2–4), 229–240. Lang, A.R., 1974. On the growth-sectorial dependence of defects in natural diamonds. Proc. R. Soc. London A, 340 (1621), 233–248. Lang, A.R., Bulanova, G.P., Fisher, D., Furkert, S., Sarua, A., 2007. Defects in a mixed-habit Yakutian diamond: Studies by optical and cathodoluminescence microscopy, infrared absorption, Raman scattering and photoluminescence spectroscopy. J. Cryst. Growth 309 (2), 170–180. Martynovich, E.F., Mironov, V.P., Rakevich, A.L., Stepanov, F.A., Emel’yanova, A.S., Shatskii, V.S., Zedgenizov, D.A., 2013. Spectral–temporal luminescence microscopy of superdeep diamonds of the Juína province. Izv. Vuzov. Fizika, 56 (2/2), 227–232.
Mironov, V.P., 1993. Regularities in the internal structure of diamonds of the Malaya Botuobiya kimberlite field. Fiziko-Tekhnicheskie Problemy Razrabotki Mestorozhdenii Poleznykh Iskopaemykh, No. 3, 110–118. Mironov, V., Antonuk, B., 1994. Distribution of the luminescent centers in Yakutian diamonds. Arch. Mineral. 50 (2), 3–11. Mironov, V.P., Mityukhin, S.I., 2001. Absorption, luminescence, and internal morphology of diamonds from placers of the Tungusskaya area (Lower Tunguska basin). Geologiya i Geofizika (Russian Geology and Geophysics) 42 (5), 831–840 (830–839). Shirey, S.B., Cartigny, P., Frost, D.J., Keshav, S., Nestola, F., Nimis, P., Pearson, D.G., Sobolev, N.V., Walter, M.J., 2013. Diamonds and the geology of mantle carbon. Rev. Mineral. Geochem. 75 (1), 355–421. Varshavskii, A.V., 1968. Abnormal Birefringence and Internal Morphology of Diamond [in Russian]. Nauka, Moscow. Yakubova, S.A., Genshaft, Yu.S., 1973. Some features of the internal structure of natural diamonds. Almazy, No. 7, 2–6.
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Luminescence in diamonds of the São Luiz placer (Brazil) V.P. Mironov a,*, A.L. Rakevich a, F.A. Stepanov a, A.S. Emel’yanova a, D.A. Zedgenizov b,f, V.S. Shatsky b,c,e, H. Kagi d, E.F. Martynovich a,e a
Irkutsk Division of the Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 130a, Irkutsk, 664033, Russia b V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia c A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia d Geochemical Research Center (GRC), Graduate School of Science, University of Tokyo, Tokyo, 113-0033, Japan e Irkutsk State University, ul. Karla Marksa 1, Irkutsk, 664003, Russia f Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 30 September 2013; accepted 21 April 2014
Abstract Plates made of diamonds from the São Luiz province (Brazil) were investigated by confocal scanning luminescence microscopy. The samples have many macroinhomogeneities (cracks and inclusions), but there is a quasi-uniform distribution of luminescence centers in the bulk. At all investigated points of the crystals, the same group of centers was observed: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by relatively small fluctuations in the ratio of the intensities of individual bands in the spectra. Nitrogen centers of different degrees of aggregation (H4, N3, and 575 nm, with four, three, and one nitrogen atom, respectively) coexist in these crystals. In the same zones of the samples, the distribution of blue luminescence (N3 centers) is diffuse (uniform), but the distribution of yellow-green luminescence is characterized by layering on (111). This might be a consequence of the tangential growth of octahedron faces or a result of plastic deformation of the crystals and dislocations along (111). © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: diamond; defect; luminescence; laser; spectrum; microscopy
Introduction Diamonds can preserve particles trapped during diamond growth as inclusions for a long time, which makes them a unique geologic object. The characteristics of diamonds from Yakutian deposits were studied relatively thoroughly for a long time (Beskrovanov, 1992; Garanin, 1990; Lang, 1974; Lang et al., 2007; Mironov, 1993; Mironov and Antonuk, 1994; Mironov and Mityukhin, 2001; Varshavskii, 1968; Yakubova and Genshaft, 1973): diamond luminescence, the composition of intrinsic and impurity defects, their distribution over crystals, internal morphology, and some others. Diamonds have an intricate internal structure, with growth zones and pyramids, stress zones, and traces of plastic and brittle deformations. Three (Beskrovanov, 1992) or four (Mironov, 1993) successive stages of crystallization are recognized. They
* Corresponding author. E-mail address: [email protected] (V.P. Mironov)
differ in the physicochemical conditions of growth, which is reflected in the composition of defects and their distribution over the crystals. The estimated depths at which the generation and growth of these samples took place are 100–150 km, which does not permit judging about processes in deeper zones of the Earth’s mantle or their composition by these diamonds. Studies of such diamonds were focused on inclusions and isotope composition of carbon, inclusions in diamond, and many other important aspects. In the review (Shirey et al., 2013), the state of knowledge of natural diamonds, including studies by modern analytical methods, was discussed. Diamond formation, the sources of carbon in the mantle, the role of carbon in mantle melting, and the geologic history of the mantle were considered. The main problems to be solved in the future studies were defined. Deep mantle layers are usually described by theoretical models based on the results of experimental petrological studies and seismological data. Not long ago mineral assemblages whose formation requires temperatures and pressures corresponding to the conditions at depths of >660 km were
1068-7971/$ - see front matter D 201 5, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2015.04.004
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detected as inclusions in diamonds from Brazilian placers (Harte et al., 1999; Kesson and FitzGerald, 1991). In most of the papers, not enough attention is paid to the description of diamonds as such. For example, zoning was revealed in Brazilian diamonds by optical microscopy, cathodoluminescence, and photoluminescence, but no detailed studies of the internal structure reflected in the zoning, including those with the use of the point-resolved luminescence spectroscopy of specific microzones of these diamonds, were carried out before. Therefore, we examined diamonds of the well-known São Luiz deposit (Brazil) which contained macro- and microinclusions, whose formation, according to (Kaminsky et al., 2001), took place at the depth of the seismically detectable transition zone and lower mantle.
Samples, equipment, and methods Most of the studied diamonds of the São Luiz deposit were subrounded crystals with traces of partial dissolution. Part of the diamonds were irregularly shaped intergrowths. To perform experiments, plates along the planes (001) or (011) were made of these diamonds. The plates were ~0.5 mm in thickness and 3–10 mm2 in area. They usually did not contain the genetic center of the crystal, so that it was hardly possible to restore their growth history with the succession of crystallization stages. A set of plates made of Yakutian diamonds was used as a reference for comparison. The application of confocal scanning luminescence microscopy in this work yields the pattern of distribution of luminescence over the sample or luminescence spectra for crystal zones few microns in size, including those at the temperature of liquid nitrogen (77 K) under excitation with nearby lasers. The main measurements were taken using a MicroTime 2000 complex. This complex is a multipurpose instrument yielding a luminescent image of the object under study with its coloring on the luminescence decay and permitting correlation luminescence spectroscopy studies with sensitivity, which makes it possible to detect the luminescence of molecules. Luminescence is recorded by time-correlated counting of photons. Modules of avalanche diodes are used as photoreceivers. Luminescence is excited using pulsed semiconductor lasers with wavelengths of 375, 405, 470, 532, and 640 nm. The duration of the laser pulses is ~70 ps, and the frequency of their recurrence is regulated within a range from 31.25 kHz to 80 MHz. The microscope objective is moved along three coordinates by piezopositioners, which ensure a minimum positioning interval of 10 nm. The software permits obtaining data by scanning the sample along two coordinates, along one coordinate, and from the given point. The complex has the mode of automatic layer-by-layer scanning of the volume with the preset interval of movement along the third coordinate. For the recording of spectra, the complex is equipped with a QE6500 (Ocean Optics) spectrometer. Optical signal is transmitted to the spectrometer through optical fiber and makes it possible to record lumines-
cence spectra and the kinetics of the luminescence decay simultaneously. The plates were preliminarily studied under an ordinary microscope (visually), with the definition of their features: the presence and localization of optical inhomogeneities, cracks, and inclusions. Afterward abnormal birefringence patterns were controlled through crossed polaroids and the pattern of distribution of birefringence over the area of the samples was determined. The distribution of luminescence over the area of the plates was photographed under excitation using lasers with different wavelengths. Zones differing in color or luminescence intensity were distinguished. In the distinguished zones of the plates, which the authors found the most interesting, luminescence spectra were measured at 77 K, and the kinetics of the luminescence decay was measured for some of them. To reduce surface effects, luminescence measurements were taken in zones ~100 m in size under the plate surface. Luminescence was excited using lasers with wavelengths of 375, 405, 470, and 532 nm successively, which ensured the higher selectivity of excitation of specific centers. Excitation using a laser with a wavelength of 375 nm activates the luminescence of centers in the blue, green, and red spectral regions; in general, the luminescence spectrum corresponds to those on excitation using the widely used pulsed nitrogen laser with a wavelength of 337 nm. A laser with a wavelength of 405 nm excites luminescence in the blue (partly), green, and red spectral regions; a laser with a wavelength of 470 nm, in the yellow-green and red spectral regions; and a laser with a wavelength of 532 nm, only in the centers in the red spectral region. Also, the selectivity of excitation is determined by the coincidence of the excitation wavelength with the excitation spectrum for the given defect. For example, the luminescence band for the centers at 575 nm is excited by lasers with wavelengths of 375, 405, and 532 nm rather than 470 nm. As it will be demonstrated below, this suggests that the diamond luminescence in the red spectral region consists of at least two superposed bands. Some data on the kinetics of diamond luminescence obtained by scanning confocal microscopy are presented in (Martynovich et al., 2013).
Results of experiments Observations in crossed polarizing filters (Fig. 1) show birefringence patterns in the plates, which are known in the literature as “tatami.” Such patterns are typical of diamonds of the type IIa, which have low contents of nitrogen impurity (1019 cm–3 or less). The recording of absorption spectra in the UV and IR ranges confirmed that the total nitrogen content of those samples is no more than 1019 cm–3, the B1 defect being predominant in content. Such crystals make up no more than 5% of Yakutian diamonds. Along with the “tatami”-type birefringence, the samples show birefringence which is due to stresses around inclusions. Apparently, the stresses are high enough, because cracks (more often, (111)) are observed in some zones with stresses. Such cracks might have been present
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Fig. 1. Characteristic birefringence patterns, observed through crossed polarizing filters.
Fig. 2. Nonuniformities in the distribution of luminescence (internal structure) over the studied samples on excitation using a laser with a wavelength of 405 nm. a, Sample D20; b, sample D46; c, sample D53.
in a whole crystal or have appeared owing to the partial relaxation of internal stresses on the cutting of the samples. In zones with high internal stresses, there is an elevated concentration of dislocations, whose development also causes partial relaxation of stresses. Besides, green and red luminescence is particularly intense in these zones. Examples of distribution (internal structure) of luminescence in diamond plates are shown below. Block–mosaical structure, which is well-known from the literature, and blurred inhomogeneities (Fig. 2a) coexist with distinct even layers (probably, growth zones) along (111) (Fig. 2b) and complicated structures (Fig. 2c), which might be due to twinning or the intergrowth of two or more crystals. The luminescence spectra on the change of the excitation source are different (Fig. 3), but they differ considerably less on the change of the point of measurement. The spectra at different crystal zones are similar, with the same group of spectral lines and bands (Fig. 4). Visible differences in the luminescence color are determined by fluctuations in the relationships between the intensities of blue, red, and green luminescence. For example, diamond D46 (Fig. 2b) shows a rectilinear zoning (111); note that excitation using different lasers causes changes in its luminescence, but the character of the zoning remains unchanged (Fig. 5). Measurement of luminescence spectra
showed that the zoning is determined by fluctuations in the intensity of the luminescence of several centers of the same kind, as in sample D20. The spectral composition of luminescence in Fig. 5 is similar to the spectra in Figs. 3 and 4. The spectra for almost all the studied samples and their zones are similar to those for sample D20; they differ from one sample to another or from one zone to another within a sample only in the relationship between the intensities of vibronic bands. The presence of characteristic lines and bands (Fig. 6) might be the distinguishing feature of the São Luiz diamonds. Zones with inclusions and internal cracks, including those filled with a colored substrate, are conspicuous in plate D74. The specific features of the crystal luminescence are observed exactly in the vicinity of these zones. Figure 7a shows a zone near a large inclusion which is surrounded by a crack not exposed to the plate surface. The crack might have developed owing to a difference in the thermal expansion coefficients of the inclusion and diamond. The resulting high pressure in this zone might have caused not only the development of an inner crack but also diffusion of part of the inclusion material along this crack. Microparticles of the inclusion material are observed in the photograph (Fig. 7a). Micromapping of the luminescence of a zone 80 × 80 μm in size at 77 K on excitation using a laser with a wavelength of 405 nm revealed
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Fig. 3. Luminescence spectra (77 K) for the same zone P2 of crystal D20 on excitation with lasers. 1–4, 375, 405, 470, and 532 nm, respectively.
Fig. 4. Luminescence spectra for some points of crystal D20 on excitation using a laser with a wavelength of 375 nm.
considerable inhomogeneities (Fig. 7b). The observed block– mosaical pattern was consistent with the distribution of the intensity of luminescence and its time constant of decay in this zone. Such inhomogeneities might be due to an accumulation of microinclusions. The recording of luminescence spectra in the colored microzones in Fig. 7b showed that the spectra for all the blocks had intense bands of N3 centers and a weak band of centers at 575 nm (Fig. 7c); the distribution of these centers was close to uniform. The block–mosaical structure is due to variations in the intensity of the broad red band (maximum at ~740 nm), which might have been caused by microinclusions. Micromapping of the same crystal zone on excitation using a laser with a wavelength of 470 nm revealed only traces of block–mosaical inhomogeneity, which appear as weakly perceptible features in rectilinear microlayering. The microlayering is evidenced by variations in the spectral composition and intensity of luminescence (mainly red). For a detailed study of the detected microlayering, we selected a crystal zone with no visible inhomogeneities but with distinct layering on excitation using a laser with a wavelength of 470 nm. Studies of the vicinity of this point
yielded the following results. On excitation using a laser with a wavelength of 375 nm, the distribution of luminescence over the zone under study (80 × 80 μm) is close to uniform (Fig. 8a), but well-defined layering is observed on excitation of the same crystal zone using lasers with wavelengths of 470 and 532 nm (Fig. 8b, c). The characteristic thickness of the layers is 10–20 μm. The differences in the intensity of luminescence between neighboring layers with more or less bright luminesence reach a factor of 1.5, but the spectrum shape experiences only a minor change. When the sample is rotated by 45° and 90° from the initial orientation, the distribution patterns and spectra of luminescence remain almost unchanged. Such a microlayering was also detected in the reference set of plates made of Yakutian diamonds (Fig. 8d). In the core of diamond D73, a large inclusion is surrounded by numerous smaller ones (Fig. 9a). On excitation using a laser with a wavelength of 405 nm, scattered radiation reveals a figure which might be a phantom of an early growth form of crystallization (Beskrovanov, 1992) (Fig. 9b). Luminescence spectra recorded on excitation using different lasers (Fig. 9c) showed that this zone contains the same lumines-
Fig. 5. Luminescence of the same zone of crystal D46 on excitation with lasers: a, 375 nm; b, 470 nm; c, 532 nm.
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Fig. 6. Characteristic lines and bands in the luminescence spectra for diamonds of placers in the São Luiz province.
Fig. 7. Microinhomogeneities around an inclusion in sample D74. a, General view of the vicinity of the inclusion (10×); b, micromapping of the zone 80 60 μm (77 K, laser with a wavelength of 405 nm) near a semitransparent zone around the inclusion (50×); c, spectra for some microscopic areas of panel b, which show the most dramatic differences in the luminescence color and intensity.
cence centers as other zones of this crystal, and they are similar to the spectra for other diamonds from this source. The phantom outlines are explained by the distribution of the microinclusions, which cause the elevated scattering and lower intensity of luminescence, rather than by differences in luminescence (the content of lattice defects). Note that the above-mentioned features are well-pronounced not in all the diamonds. For example, sample D69 shows a zoning close to rectilinear; on excitation using lasers with wavelengths of 375, 405, and 470 nm, the spectra show only minor differences from those for Yakutian diamonds, and only on excitation using a laser with a wavelength of 532 nm do the spectra have characteristic features: centers at 575 nm and a red band with a maximum at ~700 nm. Discussion The studied diamond samples from the Brazilian placer have similar optical characteristics. The distribution of luminescence over the area of the plates is characterized either by a blurred block–mosaical structure or by a zoning (probably, growth zoning).
The position of characteristic lines and bands in luminescence spectra (77 K) is shown in Table 1. Well-known lines are identified at 415, 496, and 575 nm; worse studied lines, at 491, 523, and 536 nm; and a broad structureless band with a maximum at ~700 nm, against whose background the center at 640 nm is hardly seen. This is somewhat unexpected, because the centers at 575 and 640 nm usually coexist owing to similar structures. It is presumed that both centers contain a single nitrogen atom and a vacancy. However, the band with a maximum at 700 nm is not the oscillatory branch of the system at 640 nm, as was shown by measurements of the kinetic characteristics of the luminescence decay in the same diamonds (Martynovich et al., 2013). The time constant of decay of the red band with a maximum at ~700 nm is ~70 ns, which is considerably more than the time constant of decay of the luminescence of the centers at 640 nm (~13 ns), which is known from the literature. Such a band of red luminescence was previously detected in diamonds from deep xenoliths of kimberlites of Yakutia (Beskrovanov, 1992). As it follows from Table 1, no new, previously unknown, lines or bands are found in the spectra. The N3 center (zero-phonon line at 415 nm) is observed in all the samples.
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Fig. 8. Inhomogeneities detected in sample D74 on excitation with lasers: a, 375 nm; b, 470 nm; c, 532 nm; d, microlayering in a Yakutian diamond (reference set) on excitation using a laser with a wavelength of 470 nm.
Based on green luminescence, the diamonds are divided into two groups: with predominant H4 centers (zero-phonon line at 496 nm) and with predominant H3 centers (zero-phonon line at 503.2 nm). The following nitrogen–vacancy structural models are known for these centers from the literature: H3 defect–A defect plus vacancy and H4 defect–B1 defect plus vacancy. The data of the structural models suggest that the studied samples are divided into two groups based on the composition of the impurity defects contained in them: The B1 defect is the main impurity in samples D20–D53, and the A defect, in samples D69–D74. This is consistent with the results of IR spectroscopy, because the limit of detection of the content of the impurities from IR absorption is no more
than 1017 cm–3 and part of the samples are identified as “nitrogen-free.” Luminescence spectroscopy is more sensitive, at least by two orders of magnitude, which permits detecting nitrogen defects in “nitrogen-free” diamonds. The spectra for the diamonds with the B1 defect (H4 center, zero-phonon line at 496 nm) contain a center with a zero-phonon line at 536 nm, which is poorly studied as yet. A center with a zero-phonon line at 491 nm is observed in part of the diamonds with both B1 and A defects. According to literature data, this center is observed in diamonds with plastic deformations; it might contain dislocations. On excitation using a laser with a wavelength of 375 nm, luminescence of the highest intensity is luminescence with a
Fig. 9. Internal structure of diamond D73. Luminescence in the same zone of crystal D46 on excitation with lasers: a, 375 nm; b, 470 nm; c, luminescence spectra for this zone, recorded on excitation with different lasers.
V.P. Mironov et al. / Russian Geology and Geophysics 56 (2015) 729–736 Table 1. Lines and bands in samples from the São Luiz placer Sample no. Luminescence lines and bands, nm D20
415
491
496
–
536
575
700
D43
415
491
496
–
536
575
700
D53
415
491
496
–
536
575
700
D69
415
–
–
503.2
–
575
700
D73
415
–
–
503.2
–
575
700
D74
415
491
–
503.2
–
575
700
735
Both in the studied diamonds from the São Luiz placer and in the reference samples of Yakutian diamonds, it was found that the distribution of blue luminescence (N3 centers) is close to uniform, but the distribution of yellow-green luminescence in the same zones of the samples is characterized by microlayering. Apparently, this is explained by the possibilities of the applied method rather than by the genetic features of the crystals, and this calls for a separate study.
Conclusions zero-phonon line at 415 nm (N3 centers) and that of centers at 575 nm. Red luminescence is sometimes more intense than blue. Also, there are less intense luminescence bands in the green spectral region. As it is known, N3 centers contain three nitrogen atoms and a vacancy (N3V), whereas the center at 575 nm contains one nitrogen atom and a vacancy (NV). The coexistence of the N3 center and the center at 575 nm in the same crystal zones calls for an additional study, because it contradicts the hypothesis that “small” nitrogen defects aggregate into “large” at high temperatures and pressures. The center at 575 nm is almost not excited using a laser with a wavelength of 470 nm (Fig. 3). A line at 536 nm and a red band with a maximum at 700 nm, which are distinguishing features of these diamonds, are well excited with this laser along with a line at 523 nm, which is sometimes also recorded in diamonds from other deposits. The line at 523 nm is assigned to the system of the lines of the S2 center. Measurement of the kinetics of decay of this line showed that the characteristic time is ~14.3 ns, which is close to the rate of decay of the luminescence of H4 centers. We consider the line at 523 nm to be independent of the S2 system. A laser with a wavelength of 532 nm excites red luminescence with an intricately shaped spectrum. The spectrum contains a band of centers at 575 nm with oscillatory repetitions and a band with a maximum at 700 nm. Also, a line at 572 nm is observed in the spectra, which is due to the Raman scattering of the laser with a wavelength of 532 nm in diamond. In the kinetics of the luminescence decay, there are also two components of decay: with a time constant of 23.2 ns for the center at 575 nm and the kinetic component, with a characteristic time of ~70 ns, of a band with a maximum at 700 nm. Thus, the samples from the São Luiz placer contain the following centers: N3, S2, H3, H4, and a center at 575 nm, as well as lines at 523 and 536 nm and a band with a maximum at 700 nm, which might be independent. The same group of centers is recorded in all the crystal zones, differing only in the intensity of lines and bands. According to V.V. Beskrovanov (1992), such crystals are quasi-homogeneous. Note that red luminescence is also often observed in diamonds from Yakutian kimberlites, but it is here determined by different centers. The spectra for Yakutian diamonds from deep xenoliths, including peridotite xeloliths (Beskrovanov, 1992), are similar to those for the São Luiz diamonds, but they were obtained by different techniques, so that direct comparison is not quite correct.
The studied samples from the São Luiz placer have many macroinhomogeneities (cracks and inclusions) but show the quasi-uniform distribution of luminescence centers in the bulk. At all the studied crystal points, the same group of centers is detected: N3, H4, 575, and a red band with a maximum at 690–700 nm. The visible nonuniformities in the distribution of luminescence over the area of the plates are determined by the narrow variations in the relationship between the intensities of luminescence of specific bands. The quasi-uniformity in the distribution of defects over the samples does not permit defining the succession (stages) of crystallization. The samples crystallized in one stage or homogenized at high temperatures and pressures during the postgrowth period. At each point of the crystals, nitrogen centers showing different degrees of aggregation are observed: H4, N3, and 575 nm, which contain several, three, and one nitrogen atom, respectively. In the same places of the samples, the distribution of the luminescence of N3 centers has a diffuse (uniform) pattern, whereas the distribution of yellow-green luminescence is characterized by microlayering along (111), which might be a consequence of tangential growth or plastic deformation of the crystals and dislocations along (111). The fact that some centers (N3) show a uniform distribution and others show layering is not discussed in the literature and requires an additional study. The limited number of the studied samples does not permit defining any statistical regularities. Note that the application of scanning confocal luminescence microspectroscopy with selective and spatially selective excitation of centers using several lasers opens up new opportunities for the study of minerals, including their defect and impurity compositions.
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Editorial responsibility: N.V. Sobolev