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The 450 nm (2.8 eV) cathodoluminescence emission in quartz and its relation to structural defects and Ti contents

  • Jens Götze , Colin M. MacRae ORCID logo , Yuanming Pan , Nicholas C. Wilson , Aaron Torpy and Andreas Audédat
Published/Copyright: December 31, 2023
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American Mineralogist
From the journal American Mineralogist Volume 109 Issue 1

Abstract

The origin of the common blue 450 nm (2.8 eV) cathodoluminescence (CL) emission in natural and synthetic quartz has been investigated using a combination of CL microscopy and spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, and trace-element analysis by electron micro-probe analysis as well as inductively coupled plasma-mass spectrometry (ICP-MS). The study shows that the appearance of the ~450 nm emission band can be attributed to two different defects in quartz. First, a transient luminescence can be explained by structural defects in oxygen deficient quartz. The luminescence model implies self-trapped exciton (STE) emission related to oxygen vacancies. This type of CL emission is frequent in high-purity synthetic quartz and natural quartz of hydrothermal origin. Second, in Ti-rich quartz from natural samples (e.g., quartz phenocrysts in rhyolites) and synthetic quartz of Ti-difusion experiments, an additional 450 nm (2.8 eV) emission was detected, which is stable under the electron beam. The intensity of this ~450 nm emission band correlates with the concentration of trace Ti in quartz, and substitutional Ti4+ at the Si4+ position was proved by EPR spectroscopy. In quartz crystals with elevated Ti concentrations both intrinsic and extrinsic blue CL emissions at ~450 nm can coexist, hindering a thorough characterization and quantification of the CL signal. A reliable distinction of the two diferent CL emission bands is possible by fitting the peaks of the CL spectra, and the peak width of the 450 nm emission can be used to diferentiate the STE from the Ti4+ emission. However, the definitive technique is through the observation of CL peak shape change over time at a point by collecting a time series of CL spectra in conjunction with EPR spectroscopy and trace-element analysis of the Ti concentration.

Keywords: Cathodoluminescence (CL); hyperspectral CL; quartz; blue CL emission; electron paramagnetic resonance (EPR); Ti concentration

References cited

Alonso, P.J., Halliburton, L.E., Kohnke, E.E., and Bossoli, R.B. (1983) X-ray induced luminescence in crystalline SiO2. Journal of Applied Physics, 54, 5369–5375, https://doi.org/10.1063/1.332715Search in Google Scholar

Audétat, A., Miyajima, N., Wiesner, D., and Audinot, J.-N. (2021) Confirmation of slow Ti diffusion in quartz by diffusion couple experiments and evidence from natural samples. Geology, 49, 963–967, https://doi.org/10.1130/G48785.1Search in Google Scholar

Barfels, T. (2001) Kathodolumineszenz amorpher und kristalliner Modifikationen von SiO2 and GeO2, 168 pp. Ph.D. thesis, Rostock University, Germany.Search in Google Scholar

Cerin, D., Götze, J., and Pan, Y. (2017) Radiation induced damage in quartz at the Arrow uranium deposit, southwestern Athabasca Basin, Saskatchewan. Canadian Mineralogist, 55, 457–472, https://doi.org/10.3749/canmin.1700003Search in Google Scholar

Chamberlain, K.J., Morgan, D.J., and Wilson, C.J.N. (2014) Timescales of mixing and mobilisation in the Bishop Tuff magma body: Perspectives from diffusion chronometry. Contributions to Mineralogy and Petrology, 168, 1034, https://doi.org/10.1007/s00410-014-1034-2Search in Google Scholar

Cooper, G.F., Morgan, D.J., and Wilson, C. J.N. (2017) Rapid assembly and rejuvenation of a large silicic magmatic system: Insights from mineral diffusive profiles in the Kidnappers and Rocky Hill deposits, New Zealand. Earth and Planetary Science Letters, 473, 1–13, https://doi.org/10.1016/j.epsl.2017.05.036Search in Google Scholar

Drivenes, K., Larsen, R.B., Müller, A., and Sørensen, B.E. (2016) Crystallization and uplift path of late Variscan granites evidenced by quartz chemistry and fluid inclusions: Example from the Land’s End granite, SW England. Lithos, 252–253, 57–75, https://doi.org/10.1016/j.lithos.2016.02.011Search in Google Scholar

Fisher, A.J., Hayes, W., and Stoneham, A.M. (1990) Structure of the self-trapped exciton in quartz. Physical Review Letters, 64, 2667–2670, https://doi.org/10.1103/PhysRevLett. 64.2667Search in Google Scholar

Fitting, H.-J., Barfels, T., Trukhin, A.N., and Schmidt, B. (2001) Cathodoluminescence of crystalline and amorphous SiO2 and GeO2. Journal of Non-Crystalline Solids, 279, 51–59, https://doi.org/10.1016/S0022-3093(00)00348-3Search in Google Scholar

Gorton, N.T., Walker, G., and Burley, S.D. (1997) Experimental analysis of the composite blue cathodoluminescence emission in quartz. Journal of Luminescence, 72-74, 669–671, https://doi.org/10.1016/S0022-2313(96)00242-6Search in Google Scholar

Götte, T., Pettke, T., Ramseyer, K., Koch-Müller, M., and Mullis, J. (2011) Cathodoluminescence properties and trace element signature of hydrothermal quartz: A fingerprint of growth dynamics. American Mineralogist, 96, 802–813, https://doi.org/10.2138/am.2011.3639Search in Google Scholar

Götze, J. (2009) Chemistry, textures and physical properties of quartz—geological interpretation and technical application. Mineralogical Magazine, 73, 645–671, https://doi.org/10.1180/minmag.2009.073.4.645Search in Google Scholar

Götze, J. (2012) Application of cathodoluminescence microscopy and spectroscopy in geosciences. Microscopy and Microanalysis, 18, 1270–1284, https://doi.org/10.1017/S1431927612001122Search in Google Scholar

Götze, J. and Hanchar, J. (2018) Atlas of cathodoluminescence (CL) microtextures. GAC Miscellaneous Publication No. 10. Geological Association of Canada, St. John’s, Newfoundland and Labrador, Canada, 248 pp.Search in Google Scholar

Götze, J., Plötze, M., and Habermann, D. (2001) Origin, spectral characteristics and practical applications of the cathodoluminescence (CL) of quartz—A review. Mineralogy and Petrology, 71, 225–250, https://doi.org/10.1007/s007100170040Search in Google Scholar

Götze, J., Plötze, M., Graupner, T., Hallbauer, D.K., and Bray, C. (2004) Trace element incorporation into quartz: A combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis and gas chromatography. Geochimica et Cosmochimica Acta, 68, 3741–3759, https://doi.org/10.1016/j.gca.2004.01.003Search in Google Scholar

Götze, J., Plötze, M., and Trautmann, T. (2005) Structure and luminescence characteristics of quartz from pegmatites. American Mineralogist, 90, 13–21, https://doi.org/10.2138/am.2005.1582Search in Google Scholar

Götze, J., Schertl, H.-P., Neuser, D.K., Kempe, U., and Hanchar, J. (2013) Optical microscope-cathodoluminescence (OM–CL) imaging as a powerful tool to reveal internal textures of minerals. Mineralogy and Petrology, 107, 373–392, https://doi.org/10.1007/s00710-012-0256-0Search in Google Scholar

Götze, J., Pan, Y., Stevens-Kalceff, M., Kempe, U., and Müller, A. (2015) Origin and significance of the yellow cathodoluminescence (CL) of quartz. American Mineralogist, 100, 1469–1482, https://doi.org/10.2138/am-2015-5072Search in Google Scholar

Götze, J., Lessig, F., Möckel, R., and Georgi, U. (2017a) Zur Mineralogie von Vulkaniten im Bereich des Kemmlitzer Porphyrs (Oschatz Formation, Nordwestsächsisches Becken). Veröffentlichung Museum für Naturkunde Chemnitz, 40, 77–94.Search in Google Scholar

Götze, J., Pan, Y., Müller, A., Kotova, E.L., and Cerin, D. (2017b) Trace element compositions and defect structures of high-purity quartz from the Southern Ural region, Russia. Minerals (Basel), 7, 189, https://doi.org/10.3390/min7100189Search in Google Scholar

Götze, J., Möckel, R., Breitkreuz, C., Georgi, U., and Klein, A. (2020) Zur Mineralogie von Vulkaniten und Lithophysen im Bereich des unterpermischen Leisniger Porphyrs (Nordwestsächsisches Becken). Veröffentlichungen Museum für Naturkunde Chemnitz, 43, 5–44.Search in Google Scholar

Götze, J., Pan, Y., and Müller, A. (2021) Mineralogy and mineral chemistry of quartz—A review. Mineralogical Magazine, 85, 639–664, https://doi.org/10.1180/mgm.2021.72Search in Google Scholar

Habermann, D. (2002) Quantitative cathodoluminescence (CL) spectroscopy of minerals: Possibilities and limitations. Mineralogy and Petrology, 76, 247–259, https://doi.org/10.1007/s007100200044Search in Google Scholar

Harrowfield, I.R., MacRae, C., and Wilson, N.C. (1993) Chemical imaging in electron microprobes. Proceedings of the 27th Annual MAS Meeting, Microbeam Analysis Society, 547–548.Search in Google Scholar

Jourdan, A.-L., Vennemann, T.W., Mullis, J., Ramseyer, K., and Spiers, C.J. (2009) Evidence of growth and sector zoning in hydrothermal quartz from Alpine veins. European Journal of Mineralogy, 21, 219–231, https://doi.org/10.1127/0935-1221/2009/0021-1881Search in Google Scholar

Krickl, R., Nasdala, L., Götze, J., Grambole, D., and Wirth, R. (2008) Alpha-irradiation effects in SiO2. European Journal of Mineralogy, 20, 517–522, https://doi.org/10.1127/0935-1221/2008/0020-1842Search in Google Scholar

Landtwing, M. and Pettke, T. (2005) Relationships between SEM-cathodoluminescence response and trace-element composition of hydrothermal vein quartz. American Mineralogist, 90, 122–131, https://doi.org/10.2138/am.2005.1548Search in Google Scholar

Leeman, W.P., MacRae, C.M., Wilson, N.C., Torpy, A., Lee, C.-T.A., Student, J.J., Thomas, J.B., and Vicenzi, E.P. (2012) A study of cathodoluminescence and trace element compositional zoning in natural quartz from volcanic rocks: Mapping titanium content in quartz. Microscopy and Microanalysis, 18, 1322–1341, https://doi.org/10.1017/S1431927612013426Search in Google Scholar

Lehmann, K., Pettke, T., and Ramseyer, K. (2011) Significance of trace elements in syntaxial quartz cement, Haushi Group sandstones, Sultanate of Oman. Chemical Geology, 280, 47–57, https://doi.org/10.1016/j.chemgeo.2010.10.013Search in Google Scholar

Luff, B.J. and Townsend, P.D. (1990) Cathodoluminescence of synthetic quartz. Journal of Physics Condensed Matter, 2, 8089–8097, https://doi.org/10.1088/0953-8984/2/40/009Search in Google Scholar

MacRae, C.M., Wilson, N.C., and Brugger, J. (2009) Quantitative cathodolu-minescence mapping with application to a Kalgoorlie scheelite. Microscopy and Microanalysis, 15, 222–230, https://doi.org/10.1017/S1431927609090308Search in Google Scholar

MacRae, C.M., Wilson, N.C., and Torpy, A. (2013) Hyperspectral cathodoluminescence. Mineralogy and Petrology, 107, 429–440, https://doi.org/10.1007/s00710-013-0272-8Search in Google Scholar

Mashkovtsev, R.I., Li, Z., Mao, M., and Pan, Y. (2013) 73Ge, 17O and 29Si hyperfine interactions of the Ge E1′ center in crystalline SiO2. Journal of Magnetic Resonance, 233, 7–16, https://doi.org/10.1016/j.jmr.2013.04.016Search in Google Scholar

Matthews, N.E., Huber, C., Pyle, D.M., and Smith, V.C. (2012) Timescales of magma recharge and reactivation of large silicic systems from Ti diffusion in quartz. Journal of Petrology, 53, 1385–1416, https://doi.org/10.1093/petrology/egs020Search in Google Scholar

Monecke, T., Bombach, G., Klemm, W., Kempe, U., Götze, J., and Wolf, D. (2000) Determination of trace elements in the quartz reference material UNS-SpS and in natural quartz samples by ICP-MS. Geostandards and Geoanalytical Research, 24, 73–81, https://doi.org/10.1111/j.1751-908X.2000.tb00588.xSearch in Google Scholar

Müller, A., Kronz, A., and Breiter, K. (2002) Trace elements and growth patterns in quartz: A fingerprint of the evolution of the subvolcanic Podlesi granite system (Krušné Hory, Czech Republic). Bulletin of the Czech Geological Survey, 77, 135–145.Search in Google Scholar

Müller, A., Wiedenbeck, M., Van den Kerkhof, A.M., Kronz, A., and Simon, K. (2003) Trace elements in quartz—a combined electron microprobe, secondary ion mass spectrometry, laser-ablation ICP-MS, and cathodoluminescence study. European Journal of Mineralogy, 15, 747–763, https://doi.org/10.1127/0935-1221/2003/0015-0747Search in Google Scholar

Müller, A., Breiter, K., Seltmann, R., and Pécskay, Z. (2005) Quartz and feldspar zoning in the eastern Erzgebirge volcano-plutonic complex (Germany, Czech Republic): Evidence of multiple magma mixing. Lithos, 80, 201–227, https://doi.org/10.1016/j.lithos.2004.05.011Search in Google Scholar

Neuser, R.D., Bruhn, F., Götze, J., Habermann, D., and Richter, D.K. (1995) Kathodolumineszenz: Methodik und Anwendung. Zentralblatt Geologie Paläontologie Teil I, H 1, 287–306.Search in Google Scholar

Perny, B., Eberhardt, P., Ramseyer, K., and Mullis, J. (1992) Microdistribution of aluminium, lithium and sodium in quartz: Possible causes and correlation with short-lived cathodoluminescence. American Mineralogist, 77, 534–544.Search in Google Scholar

Ramseyer, K. and Mullis, J. (1990) Factors influencing short-lived blue cathodoluminescence of alpha quartz. American Mineralogist, 75, 791–800.Search in Google Scholar

Ramseyer, K., Baumann, J., Matter, A., and Mullis, J. (1988) Cathodoluminescence colours of alpha-quartz. Mineralogical Magazine, 52, 669–677, https://doi.org/10.1180/minmag.1988.052.368.11Search in Google Scholar

Rinneberg, H. and Weil, J.A. (1972) EPR studies of Ti3+-H+ centers in X-irradiated alpha-quartz. The Journal of Chemical Physics, 56, 2019–2028, https://doi.org/10.1063/1.1677493Search in Google Scholar

Rusk, B.G., Reed, M.H., Dilles, J.H., and Kent, A.J.R. (2006) Intensity of quartz cathodoluminescence and trace-element content in quartz from the porphyry copper deposit at Butte, Montana. American Mineralogist, 91, 1300–1312, https://doi.org/10.2138/am.2006.1984Search in Google Scholar

Rusk, B., Lowers, H.A., and Reed, M.H. (2008) Trace elements in hydrothermal quartz: Relationships to cathodoluminescent textures and insights into vein formation. Geology, 36, 547–550, https://doi.org/10.1130/G24580A.1Search in Google Scholar

Salh, R., Fitting Kourkoutis, L., Schmidt, B., and Fitting, H.-J. (2007) Luminescence of isoelectronically ion-implanted SiO2 layers. Physica status solidi (a), Applications and Materials Science, 204, 3132–3144, https://doi.org/10.1002/pssa.200622571Search in Google Scholar

Sigel, G.H. and Marrone, M.J. (1981) Photoluminescence in as-drawn and irradiated silica optical fibers: An assessment of the role of non-bridging oxygen defect centres. Journal of Non-Crystalline Solids, 45, 235–247, https://doi.org/10.1016/0022-3093(81)90190-3Search in Google Scholar

Skuja, L. (1994) Direct singlet-to-triplet optical absorption and luminescence excitation band of the twofold-coordinated silicon center in oxygen-deficient glassy SiO2. Journal of Non-Crystalline Solids, 167, 229–238, https://doi.org/10.1016/0022-3093(94)90245-3Search in Google Scholar

Skuja, L. (1998) Optically active oxygen-deficiency-related centers in amorphous silicon dioxid. Journal of Non-Crystalline Solids, 239, 16–48, https://doi.org/10.1016/S0022-3093(98)00720-0Search in Google Scholar

Stevens-Kalceff, M.A. (2009) Cathodoluminescence microcharacterization of point defects in α-quartz. Mineralogical Magazine, 73, 585–605, https://doi.org/10.1180/minmag.2009.073.4.585Search in Google Scholar

Stevens-Kalceff, M.A. and Phillips, M.R. (1995) Cathodoluminescence microcharacterization of the defect structure of quartz. Physical Review B: Condensed Matter, 52, 3122–3134, https://doi.org/10.1103/PhysRevB.52.3122Search in Google Scholar

Van den Kerkhof, A.M., Kronz, A., Simon, K., and Scherer, T. (2004) Fluid-controlled quartz recovery in granulite as revealed by cathodoluminescence and trace element analysis (Bamble sector, Norway). Contributions to Mineralogy and Petrology, 146, 637–652, https://doi.org/10.1007/s00410-003-0523-5Search in Google Scholar

Vasyukova, O.V., Goemann, K., Kamenetsky, V.S., MacRae, C.M., and Wilson, N.C. (2013) Cathodoluminescence properties of quartz eyes from porphyry-type deposits: Implications for the origin of quartz. American Mineralogist, 98, 98–109, https://doi.org/10.2138/am.2013.4018Search in Google Scholar

Walker, G. (1985) Mineralogical applications of luminescence techniques. In F.J. Berry and D.J. Vaughan, Eds., Chemical Bonding and Spectroscopy in Mineral Chemistry, p. 103–140. Chapman and Hall.Search in Google Scholar

Wark, D.A., Hildreth, W., Spear, F.S., Cherniak, D.J., and Watson, E.B. (2007) Pre-eruption recharge of the Bishop magma system. Geology, 35, 235–238, https://doi.org/10.1130/G23316A.1Search in Google Scholar

Watt, G.R., Wright, P., Galloway, S., and McLean, C. (1997) Cathodoluminescence and trace element zoning in quartz phenocrysts and xenocrysts. Geochimica et Cosmochimica Acta, 61, 4337–4348, https://doi.org/10.1016/S0016-7037(97)00248-2Search in Google Scholar

Zhou, S., Čižmár, E., Potzger, K., Krause, M., Talut, G., Helm, M., Fassbender, J., Zvyagin, S.A., Wosnitza, J., and Schmidt, H. (2009) Origin of magnetic moments in defective TiO2 single crystals. Physical Review B, 79, 113201, https://doi.org/10.1103/PhysRevB.79.113201Search in Google Scholar

Zinkernagel, U. (1978) Cathodoluminescence of quartz and its application to sandstone petrology. Contributions to Sedimentology, 8, 1–69.Search in Google Scholar
Received: 2022-11-25
Accepted: 2023-02-21
Published Online: 2023-12-31
Published in Print: 2024-01-29

© 2024 by Mineralogical Society of America

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https://doi.org/10.2138/am-2022-8884
Keywords for this article
Cathodoluminescence (CL); hyperspectral CL; quartz; blue CL emission; electron paramagnetic resonance (EPR); Ti concentration

Articles in the same Issue

  1. The search for a universal law of crystal growth: The law of proportionate effect?
  2. Crystal growth according to the law of proportionate effect
  3. Melt-mediated re-equilibration of zircon produced during meltdown of the Chernobyl reactor
  4. High-pressure behavior and structural transition of beryl-type johnkoivulaite, Cs(Be2B)Mg2Si6O18
  5. Subsolidus breakdown of armalcolite: Constraints on thermal effects during shock lithification of lunar regolith
  6. Melting and melt segregation processes controlling granitic melt composition
  7. Magmatic degassing controlled the metal budget of the Axi epithermal gold deposit, China
  8. Formation of mixed-layer sulfide-hydroxide minerals from the Tochilinite-Valleriite group during experimental serpentinization of olivine
  9. Two discrete gold mineralization events recorded by hydrothermal xenotime and monazite, Xiaoqinling gold district, central China
  10. Formation of amphibole lamellae in mantle pyroxene by fluid-mediated metasomatism: A focal plane array FTIR study from the Carpathian-Pannonian region
  11. Origin of gem-quality turquoise associated with quartz-barite veins in western Hubei Province, China: Constraints from mineralogical, fluid inclusion, and C-O-H isotopic data
  12. The 450 nm (2.8 eV) cathodoluminescence emission in quartz and its relation to structural defects and Ti contents
  13. Correlation between Hinckley index and stacking order-disorder in kaolinite
  14. Structure and titanium distribution of feiite characterized using synchrotron single-crystal X-ray diffraction techniques
  15. Enrichment of precious metals associated with chalcopyrite inclusions in sphalerite and pyrite
  16. An UV/Vis/NIR optical absorption spectroscopic and color investigation of transition-metal-doped gahnite (ZnAl2O4 spinel) crystals grown by the flux method
  17. Understanding the unique geochemical behavior of Sc in the interaction with clay minerals
  18. Scandian actinolite from Jordanów Śląski, Lower Silesia, Poland: Compositional evolution, crystal structure, and genetic Implications
  19. Characterizing a new type of nelsonite recognized in the Damiao anorthosite complex, North China Craton, with implications for the genesis of giant magmatic Fe-Ti oxide deposits
  20. Genesis of Mesozoic high-Mg dioritic rocks from the eastern North China Craton: Implications for the evolution of continental lithosphere
  21. SEM and FIB-TEM analyses on nanoparticulate arsenian pyrite: Implications for Au enrichment in the Carlin-type giant Lannigou gold deposit, SW China
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