Babel or Esperanto? The need of finding a common language in heavy mineral analyses

Similar to the standardization efforts performed regularly in technical and chemical research, some branches of analytical geosciences have conducted interlaboratory comparisons by circulating either well certified or well homogenized standard samples. Without these certified natural reference materials, routine analyses using methods such as U?Pb, K?Ar, fission track and helium geochronology, coal petrography, illite “crystallinity” and many other geochemical analytical techniques would be impossible. In the field of sediment provenance studies, heavy mineral analysis plays a prominent role. Perspectives have been recently widened with development of advanced spectroscopic techniques such as Raman and automated procedures, which generate much higher numbers of observations than the classical technique based on polarizing microscopy. In this way the statistical robustness of the HM proportions and HM ratios can achieve significant improvements. However, automated procedures may suffer from distinct shortcomings and a systematic comparison of different laboratories and different techniques have not been performed so far.

We organized a round robin test and distributed two different heavy mineral concentrates to over forty laboratories. The goals of the interlaboratory test are (i) to outline the reproducibility and comparability of heavy mineral analyses, and (ii) compare the different techniques. We did not use natural HM samples in order to avoid the influence of polyphase (composite) grains, which may be treated differently by the users and thus introduce some subjectivity to the measurements. Instead, the mixtures were made from high purity monomineralic components, mostly from crushed monocrystals. In order to mimic the usual appearance of detrital grains, air abrasion was applied on most of the monomineralic samples to produce more?or?less rounded grains. The heavy mineral concentrates were sieved to 63?125 ?m size fraction. Precisely weighted masses of the different monomineralic components were mixed.

The participants were asked to apply their usual techniques to the test samples, report their procedure, the degree of experience of the observer, the heavy mineral counts and, separately, opaque, lithic fragments and unidentified grains. The results will be presented anonymously, and for the first time at the Dublin WGSG meeting.