Determination of Fluorine Concentration in Topaz Using Raman Spectroscopy

Abstract

Fluorine concentration in topaz cannot be measured precisely and accurately with most methods regularly applied in mineralogy, especially not electron microprobe analysis, without major caveats. We present a new, easy to use protocol for determining F in topaz using Raman band positions. The method is based on the observation that several Raman band positions are near linear functions of F concentration in wt% over a range covering most naturally occurring topaz. A related correlation between F concentration and lattice constants is the basis fo the well established method to determine F in topaz, which is based on powder X-ray diffraction (pXRD) data. Unlike this method, our approach has the advantage of high spatial resolution and being non-destructive. We present correlation equations that allow to measure F in topaz between 16 and 20.5 wt% with an accuracy of 0.5 wt% and a precision of 0.1 wt%. We recommend the use of the equation F [wt%]=−113.6+0.329×(v˜562−v˜155), where v˜562 and v˜155 are the fitted positions of two Raman bands in cm−1. Alternatively, the equations F [wt%]=181.8−1.043×v˜155 or F [wt%]=308.0−1.074×v˜268 can be used. The method was tested by obtaining a F distribution with Raman mapping on a natural greisen sample from Zinnwald/Cínovec and was capable of resolving micrometer sized F-heterogeneities in topaz.