MLA Measurement Modes
Depending on the area of application and the project task, various measurement modes can be selected for a MLA analysis. It is differentiated between modes that analyze the entire sample area and modes that capture only specific phases. For the latter, trigger values are defined which start the analytical detection of certain phases. These can be related to a threshold of gray scale values as well as to a specific element. The two measurement modes XBSE and XMOD can be used to automatically perform an acquisition of reference X-ray spectra and to create a reference mineral spectrum library.
XBSE - Extended BSE liberation analysis
The XBSE mode is the quasi-default measurement mode of MLA for granular materials. XBSE analyses each phase (area of consistent BSE grey value) with a single centroid X-ray point. This results in a very fast measurement as it requires only a relatively small number of X-ray analyses. If a sample contains minerals of rather similar BSE grey values, which is the case quite frequently, the XBSE mode should not be used but the GXMAP measurement mode. Using XBSE the full range of results provided by MLA software can be extracted.
XMOD - X-ray modal analysis
The XMOD measurement mode is a point counting method similar to point counting with an optical light microscope. XMOD uses the BSE image to discriminate particles from the background and collects the X-ray spectra from particles only, where it collects one X-ray spectrum at each counting point. As no image processing procedure has to be conducted hundreds of thousands of measuring points can be analyzed within a few minutes. However, the XMOD measurement mode solely results in modal mineralogy information of the sample. Neither particle shape and size information nor mineral association, locking and liberation data can be computed.
GXMAP - Grain-based X-ray mapping
The GXMAP measurement mode combines X-ray mapping with the XBSE image processing steps of BSE image acquisition, particulation, and segmentation prior X-ray spectra collection. Custom BSE grey value triggers or specific X-ray spectrum triggers can be defined for detailed X-ray mapping of phases/grains of interest. Phases outside of these ranges are analyzed by a single centroid X-ray point similar to XBSE mode. GXMAP has an advantage over XBSE as soon the sample material contains minerals of similar BSE grey values, which is the case quite frequently. Using GXMAP the full range of results provided by MLA software can be extracted.
BSE - Standard BSE liberation analysis
The BSE measurement mode does not acquire X-ray spectra, but collects a series of BSE images only. A mineral discrimination is feasible only if the sample material contains minerals having a sufficient BSE grey level contrast and the grey values can be assigned to specific minerals.
SPL - Sparse phase liberation analysis
The SPL measurement mode is a search mode using triggers to find minerals of interest which are present in low amounts (typically < 1 wt.%) in the sample material. Typical examples for areas of application are the search for precious metals, the investigation of sulfides in tailings or penalty element-bearing minerals in concentrates. Custom BSE grey value thresholds can be used to search for the minerals of interest. Only particles containing matching mineral grains will be analyzed but not the full sample area. The measurement of the particles can be conducted by either single X-rays (SPL_XBSE) or X-ray mapping (SPL_GXMAP). Often, the SPL measurement can utilize a higher BSE image magnification than a measurement of the full sample area, such as GXMAP, since only a small area of the sample is actively analyzed. As only certain types of particles in the sample are analyzed, SPL does not provide bulk mineral information. However, the list of results is similar to XBSE and GXMAP, with respect to the mineral selective acquisition of data.
SXBSE - Super extended BSE liberation analysis
The SXBSE mode is a special measurement mode, which is used only in particular cases. SXBSE is an adjusted XBSE mode and enables to analyze specific minerals of interest by long count spectra. For minerals of interest an X-ray spectrum trigger can be applied. The X-ray acquisition time of a long count spectrum (over 1,000,000 counts) of one measuring point can be 20 seconds and more. The long count spectra can be used to obtain accurate elemental quantification for the minerals of interest. This measurement is suitable for minerals with variable stoichiometry, e.g., sphalerite. It should be noted that this measurement mode is not a standard mode, due to the lengthy X-ray spectra acquisition times and thus long measurement times. However, the full range of results provided by MLA software can be extracted from SXBSE.
RPS/XSPL - Rare phase search / Extended sparse phase liberation analysis
The RPS measurement mode uses elemental triggers to find minerals of interest. In contrast to the newer XSPL measurement mode, which replaces the RPS mode, the RPS mode requires manual interaction and characterization after particles of interest were found. In contrast to the standard RPS measurement mode, XSPL uses an automated RPS technique and combines SPL and GXMAP features with the RPS elemental classification. For setting the RPS triggers pure element standards are needed. XSPL is a fast and accurate measurement mode for samples having very low concentrations of the mineral of interest. As this is a selective measurement mode it is the same as for SPL, XSPL does not provide bulk mineral information.