Feature and Particle Analysis

In particle analysis it is a frequent requirement to find a small and rare mineral grain containing a specific element inside a large sample. The challenge here is to find as many as possible (if not all) of these grains, and to do it in a reasonable time. Manual methods are tedious and time consuming, while an automated feature analysis program – like ESPRIT Feature – can be very fast.

Feature analysis is the study of shapes, the calculation of its morphological parameters and positions and the tabulation of these results. For further information depth, the chemical composition can also be measured and particles can be classified subsequently by applying user defined rules to the obtained data (e.g. must contain Zr). With respect to analysis speed, the compromise lays in the resolution and magnification of the recorded and analyzed images. The number of phases that are analyzed by the system, the counts per spectrum as well as the count rate affect the speed.

In this case the analyzed sample was a lunar basaltic meteorite Dhofar287-A thick section hosting small grains containing baddeleyite (zirconium oxide mineral ZrO₂ or zirconia with a stoichiometric ratio of Zr: 74.03 wt. % to O: 25.97 wt. %). The expected size of these grains can be as small as 3 μm or as big as more than 60 μm. All these grains needed to be found to measure them with radiometric dating methods such as a U-Pb using an ion probe. Grains under 3–6 μm would not be measurable with this method, therefore size filtering was a necessity, too.

The particles were initially detected by their morphological parameters and their brightness. As zirconium is a rather heavy element it can expected that baddeleyite particles will appear bright in the BSE images acquired for the search, binarization thresholds for image analysis were set accordingly. In the end this procedure found 997 possible baddeleyite particles in 90 fields of view and produced a 70 megapixel composite image of the 9 x 5 mm² sample for particle revisiting. EDS analysis of the initially found particles resulted in a list of only 11 particles that had the right size and composition for subsequent task of dating the sample.

The whole procedure took less than 1 hour 30 minutes, whilst a manual search would have not only taken many hours but would have included the risk of missing particles. This shows that using an particle analysis program like ESPRIT Feature is actually the only feasible approach for small particle search in large samples.

Bruker GSR Professional is an automatic gunshot residue analysis package that combines ease of use with full control over results. Set up in minutes, using a four-step wizard, it provides complete information in a short time. Analytical results can be efficiently checked using the review function to either revisit single particles or rerun whole analyses with different parameter sets.

4-step wizard for speed and convenience: The unique wizard of GSR Professional permits an easy setup and to perform the analysis of up to 20 samples using individual search conditions:

• Initial setup – load configurations and identify sample
• Define the search conditions – select search area, minimum particle size, particle classification, etc.
• Search setup summary – review settings and return to previous steps for changes, if necessary
• Run measurement – apart from allowing the user to pause or to stop a measurement, this dialog provides information on measurement progress and data

Other selected highlights:

• Flexible sample holder configuration
• EDS detector setup
• Automatic calibration of beam current, EBSD detector, EDS and microscope magnification
• Simple revisiting and validation, stage can be driven directly to a particle which can then be reanalyzed
• Manual or automatic reclassification
• Comprehensive reporting

Energy dispersive spectrometry (EDS) using scanning electron microscopes (SEM) and micro-X-ray fluorescence spectrometry (Micro-XRF) can be performed rapidly since the introduction of silicon drift detector (SDD) technology. The combination of SEM-EDS and benchtop Micro-XRF allows an advanced workflow for the characterization of ore. Applied to the examples discussed here, ore thick sections from an offset dike of the Sudbury Igneous Complex (SIC), this translates to:

1. Detecting the presence of high demand elements rapidly using Micro-XRF for determining the value of ores.
2. Locating mineral phases of economic interest with SEM-EDS using automated feature analysis.
3. Analyzing them at high spatial resolution using low accelerating voltages to gain insights in arsenide, telluride and sulfide deposit models.

Micro-XRF allows to determine the distribution of elements with Z > 10, also in traces (down to 20 μg/g) with a spatial resolution of >25 μm. Samples with sizes up to 20 x 16 cm² can be mapped with the Bruker M4 TORNADO Micro-XRF spectrometer within 4 h to locate regions of interest for subsequent high-resolution SEM studies.

Using the QUANTAX EDS system, minerals in large areas can be classified by automated feature analysis with SEM stage control, a combination of morphological classification with chemical analysis. Analyzing only features of interest by selecting the corresponding threshold in the backscattered electron (BSE) micrograph significantly reduces measurement and evaluation time. Spectra were acquired by point measurements in the center of each grain or by scanning the complete grain. Saving stage coordinates helps to relocate grains to carry out further analyses using spectrum imaging techniques (see Bruker Application Note # EDS-14).