1. University of Technology, Sydney
The Materials Chemistry of Opal or The Colloidal Chemistry and Materials Properties of Opal
Paul Thomas
University of Technology, Sydney
Collaborators:
Abhi Ray, Tony Smallwood, Leslie Brown, Peter Simon, Jean-Pierre Guerbois, amongst others…
Surface Processes (67510) - Spring 2007

2. What is Opal? - Opal Composition
Opal is an hydrated silica - SiO2.nH2O  6 to 12% H2O (70 to 90% molecular) Based on the Anhydrous mass (after heating to 960oC)  98.5% SiO2 (anhydrous mass %)  1% Al2O % K2O, Na2O, CaO… Generally Amorphous – Sedimentary Opal Sometimes paracrystalline – Volcanic opal
Surface Processes (67510) - Spring 2007

3. Origins of Opal A number of Models:
Weathering – Sanders, Darragh, Gaskin
Microbial – Watkins, Behr
Syntechtonic Fluids - Peckover
Mound Springs - Devison
Formation through diagenesis of inorganic silica and homogeneous precipitation of a monodispersed silica colloid
Feldspar  Dissolution  Homogeneous Precipitation  Sedimentation
2KAlSi3O8 + 3H2O  Al2Si2O5(OH)4 + 2KOH + 4SiO2
Feldspar water kaolin alkali colloidal silica
Surface Processes (67510) - Spring 2007

4. Mintabie Opal (cp6p02)
Surface Processes (67510) - Spring 2007

5. Ordered array of monodispersed amorphous silica spheres
Surface Processes (67510) - Spring 2007

6. Play of Colour
Silica has no intrinsic colour (trace elements may provide fixed colour)
Precious opal has ordered arrays of monodispersed silica spheres
Play of colour is due to Bragg diffraction of visible light
Sphere diameter must be 150 to 350 nm
Surface Processes (67510) - Spring 2007

7. Surface Processes (67510) - Spring 2007

8. – growth does not always result in a monodispersed colloid
Idaho Opal
– growth does not always result in a monodispersed colloid
Surface Processes (67510) - Spring 2007

9. Idaho Opal
Surface Processes (67510) - Spring 2007

10. Growth and Precipitation
Colloidal particles form through polymerisation of the dissolved silica
Particles grow and system matures through an Ostwald ripening process to produce a monodispersed colloid (only 1st generation survives)
Monodispersed colloid can then concentrated (e.g. by sedimentation) into ordered arrays to produce precious opal
Arrays are cemented together with ‘silica cement’
Surface Processes (67510) - Spring 2007

11. Coober Pedy White Play of Colour Opal
Surface Processes (67510) - Spring 2007

12. No heat treatment 200oC 400oC 600oC 800oC 980oC
Surface Processes (67510) - Spring 2007

13. Volcanic v Sedimentary Opal
Volcanic Opal –
Opal-CT
2.0 g/cm3
Water content 9 to 18% (75% molecular)
Sedimentary Opal –
Opal-AG
2.1 g/cm3
Water content 4 to 9% (90% molecular)
Surface Processes (67510) - Spring 2007

14. Curve (a) Opal – AG Coober Pedy Curve (b) Opal – CT Tintenbar
X-Ray Diffraction
Curve (a) Opal – AG Coober Pedy
Curve (b) Opal – CT Tintenbar
Surface Processes (67510) - Spring 2007

15. Electron Microscopy
Electron micrograph of Opal-A Coober Pedy precious opal sedimentary
showing spheres approximately 240nm
100,000x
Electron micrograph of Opal-CT Tintenbar precious opal
volcanic
showing etched spheres leaving “voids” approximately 240nm
100,000x
Surface Processes (67510) - Spring 2007

16. Opaline Water – two types
Molecular Water
H2O – Bulk (A)
H2O – Surface (B)
Silanol Water
Si-OH – Bulk (A)
Si-OH – Surface (B)
85 to 95% of water is molecular
Surface Processes (67510) - Spring 2007

17. nmr of Opal 29Si nmr (reference - kaolin) 29Si-1H Cross polarised nmr
- 111 ppm  Q4 – 0 SiOH per Si (i.e. 0 linkage breaks)
- 103 ppm  Q3 – 1 SiOH per Si (i.e. 1 linkage break)
- 94 ppm  Q2 – 2 SiOH per Si (i.e. 2 linkage breaks)
29Si-1H Cross polarised nmr
Magnifies SiOH peaks
Surface Processes (67510) - Spring 2007

18. Q4 Q3 1H-Cross Polarised 29Si nmr
Surface Processes (67510) - Spring 2007

19. H-cross polarised Si nmr of Tintenbar and Lightning Ridge Opals showing relative proportions of Q2, Q3 and Q4.
High proportion of Q2 and Q3 suggest a more open structure.
Curve fitting suggests that:
Tintenbar opal has 26% silanol (SiOH) and 74% molecular water
Lightning Ridge Sedimentary opal has 10% silanol (SiOH) and 90% molecular water Q3 Q4 Q2
Surface Processes (67510) - Spring 2007

20. Thermogravimetric Analysis - TG
5.7 % Loss
9.8 % Loss
Surface Processes (67510) - Spring 2007

21. Differential Thermogravimetry - DTG
Surface Processes (67510) - Spring 2007

22. Formation and Environment
Properties of opal is geological environment specific
Environment has an important role in the formation
Tintenbar – basaltic rocks
CP/LR – kaolinitic claystones
Why is volcanic opal less dense, but more crystalline?
pH – volcanic > sedimentary
Electrolyte concentration – volcanic > sedimentary
Is temperature a factor?
Surface Processes (67510) - Spring 2007

23. Gas analysis under hot vacuous extraction – DEGAS
Water loss corresponds to >99.96% of total mass loss
Trace amount of methane detected
Hydrogen from micellar decomposition of Si-OH

24. What Next? Further characterisation of opal:
Thermal analysis and diffusion coefficients of water
DEGAS experiments
Molecular characterisation of water (FTIR, nmr, QENS (quasielastic neutron scattering)
Short range structure analysis by SANS (Si-O-Si bond angles)
Long range structure analysis USANS (packing of spheres)
Surface Processes (67510) - Spring 2007

25. Neutron Beam Experiments
In November we have 9 days of beam time BENSC (Berlin Neutron Scattering Centre) To do QENS, SANS and USANS But we need some samples!
Surface Processes (67510) - Spring 2007

26. Thanks
Surface Processes (67510) - Spring 2007