|QEMSCAN® Sample Blocks|
The quality of information provided by automated mineralogy solutions, as with any other analytical technique, depends first of all on the quality of the measured sample. It depends on proper sample collection, subsampling procedures, sample preparation and presentation. It is therefore most appropriate to start this review of the Top 10 Automated Mineralogy papers with this classic, although difficult to access, technical note by Jackson, Reid and Wittenberg on "Rapid production of high quality polished sections for automated image analysis of minerals". The 1984 seminal paper on sample preparation describes in great detail a method developed to mount particles for automated mineralogy analysis which remains, with minor adjustments, the standard protocol applied in SEM-EDS laboratories across the world to this day.
The paper starts with laying out the requirements for the production of a representative section mount, so that the mineralogy in the cross-section is representative of the overall parent sample mineralogy. The fundamental requirements are to provide a random, even 3-D distribution of particles, without segregation of particles by mass, density or grain size, or introducing preferential orientation. In addition, a high degree of surface integrity in the sectioned plain needs to be achieved in order to minimise bias introduced by pitting, plucking, grain shattering, or the preferential removal of less competent minerals.
Jackson et al. describe seven steps of sample preparation, including screen sieving, rotary riffling, mechanical dilution, epoxy mounting, grinding, polishing and conductive coating.
1) Screen sieving and/or cyclosizing in an ultrasonic ethanol bath to remove loose aggregation, oversized foreign matter, and provide optimal narrow size ranges for measurement.
2) Random subsampling of the parent material to a few grams is achieved by a rotary riffler.
3) Mechanical dilution of the particle samples is discussed to prevent segregation, minimizes particle-to-particle contact, and to assists random particle orientation. Jackson et al. suggest mixing the sample with crushed graphite of similar size range and surface angularity as an inert filler.
4) Even and random 3-D distribution is achieved by mechanical shaking the mixture in cylindrical plastic moulds. Subsequently, the dry mixture is cast by covering it in resin and hardener and stirring the sample. The epoxy-sample slurry can be evacuated to remove air bubbles.
5) Grinding the hardened block consists of two stages; the first to cut away surface epoxy and particle layers of preferred orientation well into the mixture of particles and filler, the second to remove damaged damage areas.
6) Final polishing with a sequence of diamond paste cloth laps is applied to improve the surface finish. Between grinding and polishing stages, the sample block is cleaned in an ultrasonic bath using a detergent solution or alcohol.
7) Finally, the sectioned sample surface is sputter-coated with a 20-30 nm carbon film making it electro-conductive.
The fundamental sample preparation protocol for automated SEM-EDS analysis laid out by Jackson et al. remains largely unchanged over the past 25 years and is widely applied by the leading service providers in the mining industry.