Supplementary Information - Crystalline silica quantification
Until recently, quantification of single mineral phases, such as cristobalite, in
heterogeneous powders was difficult without prior knowledge of the complete
mineralogical and structural composition of the sample so that quantitative methods such as
Rietveld refinement could be applied (Rietveld 1969) following X-ray diffraction (XRD).
Here, rapid quantification of silica polymorphs (cristobalite, quartz and tridymite) was
carried out using a new technique enabled by, and unique to, an XRD with a curved 120°
2position-sensitive detector (XRD-PSD) in static geometry relative to the beam and
sample (Le Blond et al. 2009). The fixed geometry PSD records highly reproducible
diffraction data by measuring the diffracted intensity at all angles simultaneously
throughout the angular range 0° to 120° 2. The simultaneous collection allows extremely
short collection times for samples.
The Internal Attenuation Standard (IAS) method of quantification allows the user to attain
the wt. % of a single phase of interest within the sample without needing to know the full
mineralogy of the bulk sample. A pseudobinary is produced by mixing an IAS with the
sample; this allows the X-ray attenuation coefficient of the total sample mixture to be
calculated, thereby removing the need to match every phase in the mixture with appropriate
standards. Only the single phase of interest needs to be matched with an appropriate
standard to quantify its proportion in the sample mixture (see Le Blond et al. (2009) for a
full theoretical and methodological overview).
1
Ash samples were ground in an agate pestle and mortar, to reduce grain size to between ~
5-20 µm diameter. 0.35 g of the sample was weighed out on a microbalance, into which ~
30 wt. % pure phase powdered IAS (zinc oxide, a compound not found in the ash) was
added. The powders were then thoroughly mixed in a pestle and mortar, packed into the Al
circular mount and analyzed with an Enraf-Nonius X-ray diffractometer with INEL curved
PSD at the NHM, London, using monochromated Cu K1 radiation. XRD patterns for the
pristine ash samples (i.e., without the IAS added), a sample of pure phase IAS (ZnO in
these experiments), the pseudobinary mixture (sample plus IAS), and phase-pure standards
cristobalite, quartz and tridymite (i.e. the phases required to be quantified) were collected.
Labradorite (plagioclase feldspar) standard was also included to aid differentiation between
cristobalite and plagioclase feldspar peaks, some of which overlap.
References
Le Blond JS, Cressey G, Horwell CJ and Williamson BJ (2009) A rapid method for
quantifying single mineral phases in heterogeneous natural dust using X-ray
diffraction. Powder Diffraction 24: 17-23
Rietveld HM (1969) A profile refinement method for nuclear and magnetic structures. J
App Crystal 2: 65-71
2