Transverse Mount vs Normal Polished Section

You might already know that the samples for a mineralogy test by an automated SEM system, such as MLA or QEMSCAN, need to be prepared as a tubular round polished section. Most of the automated SEM systems use a 30 mm round polished section, while some other systems might use one-inch sections.

FEI QEMSCAN 650-F Chamber with 14 polished section Holder.

In either case, samples need to be crushed, split and stage pulverized to the desired size fraction and be embedded into epoxy resin to prepare the polished section.

30 mm round polished section

So far so good, but one problem with tubular polished section – especially when working on larger size fractions- is the segregation of the coarser and heavier particles over the fine and lighter particles. This simply skews the quantification of the modal mineralogy by an overestimation in heavy minerals over the lighter minerals. Schematically, below depicts what happens:

Schematic segregation of particles in a epoxy resin section.

There are a few ways to go around this problem. One of the popular methods is to use a transverse mount polished section. To prepare such sections, we have to cut through the normal section, and subsequently mount each half vertically and scan through the full profile to avoid segregation effect.

Steps to prepare a transverse mount polished section.

Below you can see a BSE overview of such sections, where the segregation is obvious. As you can see, the left side of the picture more bright minerals (heavier minerals) can be seen, while the right end accumulates more of the finer particles and clays.

BSE image from the vertical cut through a tubular section.

Just to demonstrate practically the segregation effect, the table below compares the results of a modal mineralogy test from both normal polished sections (NS) and transverse mount (TM), for two samples.

Minerals quantification: Normal Polished Section vs. Transverse mount. NS: Normal Section. TM: Transverse Mount Section.

Now you can have a better understanding of how the mineralogy quantification could be skewed during polished section preparation, despite a correct/right mineral identification and a good measurement.

It is noteworthy that segregation is more visible with coarser particles. Also for some minerals is more active, for example, pyrites tend to segregate more in comparison to other heavy minerals, vs. illite tend to float on top of the section more than other silicates.

So next time when you send samples to a mineralogy lab, make sure that you ask about their procedure to avoid segregation… If they do not have any suggestion, then ask for a transverse mount for modal mineralogy and QA/QC the mineralogy results with a reconciliation of major elements by chemical analysis vs. calculated assay by mineralogy.

Hope you found this post useful...