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

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% Al2O3 + 0.5% K2O, Na2O, CaO… Generally Amorphous – Sedimentary Opal Sometimes paracrystalline – Volcanic opal Surface Processes (67510) - Spring 2007

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

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

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

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

Surface Processes (67510) - Spring 2007

– 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

Idaho Opal Surface Processes (67510) - Spring 2007

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Thanks Surface Processes (67510) - Spring 2007