Saturday, 16 September 2017

Li Mineralogy by Automated SEMs: A case study

  Similar to other analytical methods, Automated SEMs have their own restrictions; one of the limitation of SEMs -more specific EDS Spectra analysis-  is that light elements such as Li or Be cannot be detected with the current X-Ray detectors (e.g. Reed 2005). This can effect the reliability of the mineral list or SIP file when dealing with one of these elements, such as Li exploration projects; however an integration with XRD and a  supplementary method such as Laser Ablation ICPMS, enables QEMSCAN or MLA to overcome this technical restriction.

In this post, we are going to review a case study on a Li mineralogy, from a prelim exploration project on Pegmatites with the main objective to identify and characterize main carrier of Li. The mineralogical study was done on 97 core samples with Li abundance of 0.07-0.15 % and 0.05-0.1% of W with the host rock being identified as Alkaline Feldspar Granite; here we will review the results for one sample.

The mineralogy test was initiated using QEMSCAN on one size fraction; the samples were stage pulverised to 125 µ for preparing 30 mm round polished sections; two sections per sample was prepared (one transverse and one normal). BMA measurement mode was used for Modal Mineralogy analysis and PMA mode for particle data. In order to validate the reliability of the SIP file, 25 samples with highest amounts of W and Li  were selected for Q-XRD analysis.

In granites and other similar rocks, the main carrier of Li minerals are silicates, like Pyroxenes ( e.g. Spodumene LiAl(Si2O6)), Micas.( e.g. Lepidolite  KLi2AlSi4O10F(OH)) or Feldspathoid  ( Petalite LiAlSi4O10); now regarding the inability of detecting Li by EDS spectra technique, the identification of these minerals can be tricky with probability of misclassification with other minerals or entries; Although elemental ratio (e.g. Si/Al) in the SIP entries can help -to some extend- to avoid such, however an ideal condition - of the sample, measurement settings, sample prep etc- is prerequisite for such discrimination.

After QEMSCAN test and comparison with XRD, it was concluded that the Li minerals in this sample are from Mica group, i.e. Zinnwaldite  (KLiFeAl2Si3O10(F,OH)2) and Lepidolite KLi2AlSi4O10F(OH), with approximate abundance of 2.8% and 1.13%, respectively. Without the XRD check prior to generate the SIP file, the chances of misclassification of K-Feldspar and Lepidolite could occur caused by  overlapping X-Ray counts for Si, Al and K (see the synthesized spectra).

                       Modal Mineralogy wt% by QEMSCAN
Using QEMSCAN SIP editor it is easier to overcome such misclassifications comparing to MLA, where mainly peak shapes match criteria is used for minerals identifications. In addition to elemental counts, as well as Si/Al and K/Al ratio, we also used “May Have” criteria in the SIP entries for both Orthoclase and Lepidolite.

Synthesized EDS spectra by SIP Editor

Li by chemical assay in this sample had resulted 1385 ppm , while calculated Li by QEMSCAN using empirical formula of Lepidolite and Zinnwaldite is ~ 0.11 %. This deficiency for Li is opposite of what was seen for other elements such as K, Si and Al, where they showed a tight reconciliation. So the possibility of underestimation of minerals’ quantification was ruled out. This can be explained by the presence of Li as a minor element in other minerals; looking back at the mineral list, the best candidates are Muscovite and/or Biotites. In order to confirm this, yet another method should be used. For this purpose Laser Ablation ICP-MS is suitable and we can perform the test directly on the same polished sections that were used for mineralogy test by QEMSCAN.

As a brief summary, the L.A. ICP-MS test confirmed the elevated abundance of Li in muscovite with range of 1000-4300 ppm in the analyzed samples; with the +10 wt% presence of Muscovite, this can definitely be the main reason for the difference of Li between mineralogy test and chemical assay. 

Despite the technical restrictions of MLA and QEMSCAN, they still play an important role in mineralogy projects,  using the right supplementary methods can help to overcome these restrictions.

Hope it was interesting for you!

Matt (Mahdi) Ghobadi

2 comments:

  1. Just a minor suggestion for the introduction: The element Rb is not reported by QemScan since I lacked a suitable mineral standard from which to extract the element signal. So it should not be classed as a 'light' element, but perhaps something like "Rb isn't reported due to a feature limitation within the QemScan software package" - if you still want to include a specific reference to that element.
    - Michael

    ReplyDelete