In a previous post we discussed the analysis modes that are fundamental to the data collection on Zeiss Mineralogic. In this post we’ll explore some of the basics of Mineral lists and some of the pitfalls to watch out for. As with any automated mineralogy software there are some foundational aspects that need to be established for the sample before any Mineral List can be set-up. These things include telling the software how many samples there are in the SEM, where those samples are, and in the case of non-standard sections, how big the samples are. The beam then needs to be calibrated and brightness and contrast set at a value that is suitable for the sample being analysed. With these set-up and the analysis mode decided it is then possible to move on to defining the Mineral List. A well developed Mineral List is a necessary pre-requisite to any data collection and interpretation.
Mineral Lists
Mineral lists define the minerals you are searching for and the priority order that you assign to each. They can include as many or as few minerals as required for the job. Each mineral is defined by the weight percent value of each element present with a certain tolerance placed on those values (e.g. quartz has a weight percent Si content of ~47% and a minimum and maximum value for EDX analysis could be given as 42% and 52%). Qualifiers can also be included, such as element ratios for minerals with a solid solution (i.e. Mg>Fe for clinochlore). In theory it is pretty straightforward but there are a number of items and subtleties to bare in mind.
- Mineralogic Mining lists operate a “first-match” criteria. This means that the mineral list should start with the most specific and critical phases at the top, followed by generic definitions (i.e. “silicate gangue”) at the bottom. The first mineral definition to match the EDX spectra from a given pixel / area will be assigned to that pixel / area.
- It is sometimes necessary to include elements that are absent from the mineral phase within the definition (“excluders”). A good example is with calcite where the failure to specify that phosphorous is not present (i.e. set a max of P = 0.01%), might result in the mis-identification of fluorapatite as calcite. Equally, if the same isn’t done for ‘S’ then gypsum may be misidentified. “Excluders” are a common requirement in jobs with complex mineralogy.
- Mineralogic allows the use of morphochemical and greyscale data as additional interpretation tools. This can be particularly useful in resolving edge effects, or mixed pixel spectra.
It will be a fruitless task trying to define 27 different types of Na-Amphibole in a mineral list. Other tools such as a microprobe are required for complex overlapping minerals such as these.- Without a carbon correction beware that carbonate minerals can’t include information on C content. Therefore siderite, for example, will have a very similar EDX analysis to goethite and may be confused as such.
- Overlapping spectra can throw a spanner into mineral identification. Common ones to look out for are Na with Zn, Pb with Mo, As & S and As with Mg.
- An ancillary technique such as optical microscopy and / or XRD can be an excellent and necessary sanity check for your mineral list development.
Mineral lists are fully transferable and portable between jobs so for repeat work on similar project types it is possible to use previously developed lists. Indeed, for routine monitoring work it is imperative that the mineral list is correct first time and then is kept constant to track mineralogical change with time, for example in operational mineralogy. However, for more general usage the porting of a mineral list should always be done with caution. The mineral list isn’t a black box, and no two geological projects are exactly identical. Failure to take this into consideration will result in bad data and incorrect interpretations. On the other hand a well developed mineral list underpins good quality modal mineralogy and particle liberation studies.
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