Re: Standards for EPMA-Fluorine

[AndrewLocock]

Is end member topaz easily synthesized (or available commercially)?   

The topaz from the Thomas Range, Utah, USA, is very widely available in mm-scale gem-quality brown crystals, and is very close to the end-member Al2(SiO4)F2. F-quantities for this material are reported by Penfield & Minor (1894) - 20.37 wt%; Ribbe & Rosenberg (1971) - 20.4(6) wt%, specimen 2; and Barton et al. (1982) - 20.3 wt%. The theoretical maximum F content of Al2(SiO4)F2 is 20.646 wt%.

Unit cell refinement from powder diffraction data acquired for topaz from the Thomas Range gives b: 8.789 angstroms,
consistent with F = 20.1(5) wt% based on a quadratic regression of the b-cell dimension data of Alberico et al. (2003) and Diego Gatta et al. (2006). (One-sigma standard deviation of last decimal place in parentheses). This independent determination is in good agreement with the previously published data.

I would suggest that this topaz may be useful as a reference material.

References:

Alberico, A., Ferrando, S., Ivaldi, G. and Ferraris, G., 2003. X-ray single-crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China)–Structural foundation of the correlation between cell parameters and fluorine content. European Journal of Mineralogy, 15(5), pp.875-881.

Barton, M.D., Haselton, H.T., Hemingway, B.S., Kleppa, O.J. and Robie, R.A., 1982. The thermodynamic properties of fluor-topaz. American Mineralogist, 67(3-4), pp.350-355.

Gatta, G.D., Nestola, F., Bromiley, G.D. and Loose, A., 2006. New insight into crystal chemistry of topaz: A multi-methodological study. American Mineralogist, 91(11-12), pp.1839-1846.

Penfield, S.L. and Minor, J.C., 1894. On the chemical composition and related physical properties of topaz. American Journal of Science, (281), pp.387-396.

Ribbe, P.H. and Rosenberg, P.E., 1971. Optical and X-ray determinative methods for fluorine in topaz. American Mineralogist, 56(9-10), pp.1812-1821.

[BenjaminWade]

Hi Andrew
Very interesting. If I purchased a gemmy topaz from Thomas Range, in your opinion what would be the best way to quantify the amount of F in it to use it as a EPMA standard? Powder diffraction? I am assuming that electron diffraction on TEM wouldn't have the precision?

Cheers

[AndrewLocock]

Hi Andrew
Very interesting. If I purchased a gemmy topaz from Thomas Range, in your opinion what would be the best way to quantify the amount of F in it to use it as a EPMA standard? Powder diffraction? I am assuming that electron diffraction on TEM wouldn't have the precision?
Cheers

Hello,
I can only suggest a procedure similar to what we did (I don't think that cell dimensions from TEM are very precise):

We purchased a gemmy topaz from the Thomas Range, and cut off a small piece with a thin diamond saw. This piece was then finely powdered (polycrystalline sapphire mortar and pestle with ethanol). The dried fine powder was run on a very well-calibrated powder X-ray diffractometer (from NIST standards), and the unit cell dimensions were refined using Rietveld methods (Co K-alpha radiation, 5 to 140 degrees 2-theta, refinement in space group Pbnm starting with PDF card #97-015-5352).

The unit cell and compositional data from Alberico et al. (2003) and Diego Gatta et al. (2006) cited above were used to derive a quadratic relationship in Excel between composition and the b unit cell parameter. The following values of the b parameter were adopted for the end-members: Al2(SiO4)F2 = 8.788 angstroms; Al2(SiO4)(OH)2 = 8.930 angstroms.  The quadratic equation relating F in wt% (y) in topaz to the b parameter (x) is: y = -530.386x^2 + 9,258.119x - 40,379.022, with R² = 0.970.

From comparison of the calculated and tabulated wt% F data, the standard deviation of the absolute differences is 0.5 wt%. This is taken as the one-sigma uncertainty in composition. The measurement uncertainty of Rietveld cell refinement should be in the 4th decimal place (or less).

Thus, we consider that our specimen with b = 8.789 angstroms has F = 20.1 wt% with a one-sigma uncertainty of 0.5 wt%.

Cheers,
Andrew

[BenjaminWade]

Hi Andrew
Fantastic, thanks for the great explanation. I have seen a number of gemmy topaz on ebay and dealers, so I guess another job to add to the list..

Cheers