Abstract
Single pulse, solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T1) relaxometry. Opal T1 relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of relaxation) and shape (form of the curve) parameters. Relaxation is at least an order of magnitude faster than for silica glass and quartz, with Q3 (silanol) usually faster than Q4 (fully substituted silicates). 95% relaxation (Q4) is achieved for some Australian seam opals after 50 s though other samples of opal-AG may take 4000 s, while some figures for opal-AN are over 10,000 s. Enhancement is probably mostly due to the presence of water/silanol though the presence of paramagnetic metal ions and molecular motion may also contribute. Shape factors for opal-AG (0.5) and opal-AN (0.7) are significantly different, consistent with varying water and silanol environments, possibly reflecting differences in formation conditions. Opal-CT samples show a trend of shape factors from 0.45 to 0.75 correlated to relaxation rate. Peak position, scale and shape parameter, and Q3 to Q4 ratios offer further differentiating feature to separate opal-AG and opal-AN from other forms of opaline silica. T1 relaxation measurement may have a role for provenance verification. In addition, definitively determined Q3/Q4 ratios are in the range 0.1 to 0.4 for opal-AG but considerably lower for opal-AN and opal-CT.
Highlights
Opal [1,2] is a generic term applied to commonly-found and naturally-formed hydrated silicas which lack the regular crystalline structure of quartz, moganite, cristobalite or tridymite [3]
We find that the major peaks are about 5 ppm more negative than quartz, consistent with previous work though at different absolute values probably caused by the use of external standards, experimental variations and differences in natural materials
Dehydration may occur for proximal silanol groups. While both effects may be in operation, we find no convincing support through correlation of any of the nuclear magnetic resonance (NMR) parameters with the X-ray powder diffraction (XRD) pattern trends, such as high shape parameters (G32925 and GNEW09)
Summary
Opal [1,2] is a generic term applied to commonly-found and naturally-formed hydrated silicas which lack the regular crystalline structure of quartz, moganite, cristobalite or tridymite [3]. XRD peaks are broad in all cases, for opal-A [2,4–9], suggesting a disordered “paracrystalline” environment. Other variability, such as in the amount of molecular water [7,10–13], trace elements [14–22] and the mixture of silanols (Q3 site with one oxygen as a silanol [23–25] with the remainder bridged to other silicon atoms) with fully substituted (Q4 site all Si-O-Si bridged) species leads to a consensus that opal is not a true mineral. The structures of opal-A and opal-CT remain unresolved [3,12,26,27].
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