Application of Thermal Methods to the Characterisation of the States of Water in Precious Opal
- Publisher:
- Academica Greifswald
- Publication Type:
- Conference Proceeding
- Citation:
- CEEC-TAC5 & MEDICTA 2019, 2019, pp. 49 - 49 (1)
- Issue Date:
- 2019-08-15
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| Filename | Size | |||
|---|---|---|---|---|
| BoA CEEC-TAC5 Medicta2019 Opal Paper.pdf | 4.22 MB |
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Precious opal is a hydrous silica (SiO2.nH2O) that is formed through a dissolution-precipitation process forming hydrated silicas with variable water content [1]. The gemmological value of precious opal is defined by its microstructure where the prized play-of-colour (POC) observed is a result of the diffraction of visible light from an ordered arrays of silica spheres. The monodispersed colloid of silica spheres are formed through an Ostwald type ripening process. Once the colloidal particles have grown to a suitable size for Bragg diffraction of visible light (ca. 200 to 400 nm in diameter; i.e. = 2dnsin where n is the refractive index of opal), aggregation occurs most likely through a homogeneous colloidal crystallisation process which results in the ordered array. Subsequently the array is solidified through precipitation of a second generation of silica which cements the array onto a solid coherent mineral specimen. The microstructure of the array can then be observed through the hydrofluoric etching of fresh fracture surfaces which reveal the ordered arrays of the monodispersed spherical particles. As the process of opal formation occurs in solution, a hydrous silica is formed. The silica network itself can be of an amorphous nature (opal-A) or is paracrystalline (opal-CT; cristobalite containing tridymite stacking faults) and contains water in the form of molecular and silanol (bound) water. The molecular water is trapped in cages, capillary pores and interstitial voids while the silanol water is present at the surface, internal interfaces (e.g. at capillary or void surfaces) and in the silica network as isolated broken bridges. The type and amount of each of these species of water is dependent on the environment in which opal formation occurs (e.g. temperature and pH) and hence water may be used as a probe to characterise the morphology structure of opal. Both thermal and spectroscopic methods have been used to successfully characterise the types of water present in opal and given the range of types of water, multiple characterisation techniques are required. This paper will outline the application of both thermal and spectroscopy methods in the characterisation of opal. TG data are presented to identify the total water content (molecular and silanol), low temperature DSC data is used to characterise the proportion of crystallisable water present in opal and thermoporosity based on DSC is used to determine the characteristic pore size of water filled pores. The silanol water content is determined from the 1H-CP-29Si MAS NMR while the distribution of molecular and silanol water is also gleaned using FT-NIR. The total water content is not indicative of the origin and formation environment of opal, however, the distribution of the different types of water present in these hydrous silicas is strongly dependent on the formation environment. The distribution of species type coupled with a knowledge of the origin of the opal allows the development of a fundamental model for opal genesis. [1] “Water in Opal – What Can It Tell Us?”, Paul Thomas, Laurie Aldrich and Anthony Smallwood, InColor Magazine, 41, Winter, 62-69, 2019.
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