DSC of natural opal: insights into the incorporation of crystallisable water in the opal microstructure

  • Boris ChauviréEmail author
  • Paul Stephen Thomas


Low-temperature DSC on a wide range of opal-A and opal-CT samples was carried out to estimate the proportion of crystallisable water and to determine the size of water-filled cavities. A wide range of crystallisable water contents in the range 4.9 to 41.9% of the water contained in opals were observed, although the proportion of crystallisable water did not correlate with structure. Pore size and pore size distribution were estimated from the melt temperature depression and heat flow data, respectively. Opal-CT was observed to have smaller water-filled pores (radii < 2 nm) than opal-A (radii from 2.5 to 4.9 nm), suggesting that molecular water may be contained between nanograins in the microstructural units (spheres or lepispheres). A narrower pore size distribution was calculated for opal-CT, and no melting of the crystallisable water was observed where bulk water would be expected to melt, suggesting the absence of larger voids. The melting peaks for opal-A, on the other hand, transitioned into the melting of bulk water suggesting the presence of significantly larger water-filled pores, an observation consistent with the microstructure observed in SEM micrographs.


Opal-A Opal-CT Porosity DSC TG 



The Dr Eduard Gübelin Association for Research & Identification of Precious Stones supported these analyses with the Dr Eduard Gübelin Research Scholarship 2016. The authors would like to thank Jean-Pierre Guerbois for his precious assistance to the TG.


  1. 1.
    Iler RK. The occurrence, dissolution, and deposition of silica. In: The chemistry of silica: solubility, polymerization, colloid and surface properties and biochemistry of silica. Wiley, Wilmington; 1979, pp. 3–93Google Scholar
  2. 2.
    Liesegang M, Milke R, Kranz C, Neusser G. Silica nanoparticle aggregation in calcite replacement reactions. Sci Rep. 2017;7:14550.CrossRefGoogle Scholar
  3. 3.
    Xia Y, Gates B, Yin Y, Lu Y. Monodispersed colloidal spheres: old materials with new applications. Adv Mater. 2000;4095:693–713.CrossRefGoogle Scholar
  4. 4.
    Gardner LR. Mechanics and kinetics of incongruent feldspar dissolution. Geology. 1983;11:418–21.CrossRefGoogle Scholar
  5. 5.
    Jones JB, Biddle J, Segnit ER. Opal Genesis. Nature. 1966;210:1353–4.CrossRefGoogle Scholar
  6. 6.
    Dehouck E, Gaudin A, Mangold N, Lajaunie L, Dauzères A, Grauby O, et al. Weathering of olivine under CO2 atmosphere: a martian perspective. Geochim Cosmochim Acta. 2014;135:170–89. Scholar
  7. 7.
    Landmesser M. Mobility by metastability: silica transport and accumulation at low temperatures. Chem Der Erde Geochem. 1995;55:149–76.Google Scholar
  8. 8.
    Landmesser M. “Mobility by metastability” in sedimentary and agate petrology: applications. Chem Der Erde Geochem. 1998;58:1–22.Google Scholar
  9. 9.
    Jones JB, Segnit ER. The nature of opal I. Nomenclature and constituent phases. J Geol Soc Aust. 1971;18:57–68.CrossRefGoogle Scholar
  10. 10.
    Elzea JM, Rice SB. Tem and X-ray diffraction evidence for cristobalite and tridymite stacking sequences in opal. Clays Clay Miner. 1996;44:492–500.CrossRefGoogle Scholar
  11. 11.
    Langer K, Flörke OW. Near infrared absorption spectra (4000–9000 cm−1) of opals and the role of “water” in these SiO2–nH2O minerals. Fortschr der Mineral. 1974;52:17–51.Google Scholar
  12. 12.
    Darragh BPJ, Gaskin AJ. The nature and origin of opal. Aust Gemol. 1966;8:5–9.Google Scholar
  13. 13.
    Gaillou E, Fritsch E, Aguilar-Reyes B, Rondeau B, Post J, Barreau A, et al. Common gem opal: an investigation of micro- to nano-structure. Am Mineral. 2008;93:1865–73.CrossRefGoogle Scholar
  14. 14.
    Fritsch E, Ostrooumov M, Rondeau B, Barreau A, Albertini D, Marie A-M, et al. Mexican gem opals: nano- and micro-structure, origin of colour, comparison with other common opals of gemmological significance. Aust Gemol. 2002;21:230–3.Google Scholar
  15. 15.
    Fritsch E, Gaillou E, Rondeau B, Barreau A, Albertini D, Ostroumov M. The nanostructure of fire opal. J Non Cryst Solids. 2006;352:3957–60.CrossRefGoogle Scholar
  16. 16.
    Jones JB, Sanders JV, Segnit ER. Structure of opal. Nature. 1964;204:990–1.CrossRefGoogle Scholar
  17. 17.
    Darragh PJ, Sanders JV. The origin of colour in opal. Aust Gemol. 1965;7:9–12.Google Scholar
  18. 18.
    Day R, Jones B. Variations in water content in opal-A and opal-CT from geyser discharge aprons. J Sediment Res. 2008;78:301–15.CrossRefGoogle Scholar
  19. 19.
    Jones JB, Segnit ER. Water in sphere-type opal. Mineral Mag. 1969;37:357–61.CrossRefGoogle Scholar
  20. 20.
    Thomas PS, Šesták J, Heide K, Fueglein E, Šimon P. Thermal properties of Australian sedimentary opals and Czech moldavites. J Therm Anal Calorim. 2010;99:861–7.CrossRefGoogle Scholar
  21. 21.
    Brown LD, Ray AS, Thomas PS, Guerbois JP. Thermal characteristics of Australian sedimentary opals. J Therm Anal Calorim. 2002;68:31–6.CrossRefGoogle Scholar
  22. 22.
    Chauviré B, Rondeau B, Mangold N. Near infrared signature of opal and chalcedony as a proxy for their structure and formation conditions. Eur J Mineral. 2017;29:409–21.CrossRefGoogle Scholar
  23. 23.
    Graetsch H, Flörke OW, Miehe G. The nature of water in chalcedony and opal-C from Brazilian agate geodes. Phys Chem Miner. 1985;12:300–6.CrossRefGoogle Scholar
  24. 24.
    Boboň M, Christy AA, Kluvanec D, Illášová L. State of water molecules and silanol groups in opal minerals: a near infrared spectroscopic study of opals from Slovakia. Phys Chem Miner. 2011;38:809–18.CrossRefGoogle Scholar
  25. 25.
    Smallwood AG, Thomas PS, Ray AS. Characterisation of the dehydration of Australian sedimentary and volcanic precious opal by thermal methods. J Therm Anal Calorim. 2008;92:91–5.Google Scholar
  26. 26.
    Smallwood AG, Thomas PS, Ray AS. The thermophysical properties of Australian opal. In: Proceedings of the 9th international congress application minerals. Bisbane, Queensland; 2008, pp. 557–65.Google Scholar
  27. 27.
    Thomas PS, Guerbois J-P, Smallwood AG. Low temperature DSC characterisation of water in opal. J Therm Anal Calorim. 2013;113:1255–60.CrossRefGoogle Scholar
  28. 28.
    Rondeau B, Fritsch E, Mazzero F, Gauthier J.-P. Opal—the Craze for stability. In Color. 2011;4:2–5.Google Scholar
  29. 29.
    Pearson G. Role of water in cracking of opal. Aust Gemol. 1985;15:435–45.Google Scholar
  30. 30.
    Landry MR. Thermoporometry by differential scanning calorimetry: experimental considerations and applications. Thermochim Acta. 2005;433:27–50.CrossRefGoogle Scholar
  31. 31.
    Ishikiriyama K, Todoki M. Pore size distribution measurements of silica gels by means of differential scanning calorimetry. J Colloid Interface Sci. 1995;171:103–11.CrossRefGoogle Scholar
  32. 32.
    Brun M, Lallemand A, Quinson J-F, Eyraud C. A new method for the simultaneous determination of the size and shape of pores: the thermoporometry. Thermochim Acta. 1977;21:59–88.CrossRefGoogle Scholar
  33. 33.
    Chauviré B, Rondeau B, Mazzero F, Ayalew D. The precious opal deposit at Wegel Tena, Ethiopia: formation successive pedogenesis events. Canadian Miner. 2017;55(4):701–23.CrossRefGoogle Scholar
  34. 34.
    Jähnert S, Vaca Chávez F, Schaumann GE, Schreiber A, Schönhoff M, Findenegg GH. Melting and freezing of water in cylindrical silica nanopores. Phys Chem Chem Phys. 2008;10:6039–51.CrossRefGoogle Scholar
  35. 35.
    Endo A, Yamamoto T, Inagi Y, Iwakabe K, Ohmori T. Characterization of nonfreezable pore water in mesoporous silica by thermoporometry. J Phys Chem C. 2008;112:9034–9.CrossRefGoogle Scholar
  36. 36.
    Fukusako S. Thermophysical properties of ice, snow, and sea ice. Int J Thermophys. 1990;11:353–72.CrossRefGoogle Scholar
  37. 37.
    Segnit ER, Stevens TJ, Jones JB. The role of water in opal. J Geol Soc Aust. 1965;12:211–26.CrossRefGoogle Scholar

Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2019

Authors and Affiliations

  1. 1.School of Mathematical and Physical SciencesUniversity of Technology SydneyUtlimoAustralia
  2. 2.Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerreUniversité Grenoble AlpesGrenobleFrance

Personalised recommendations