SEM-MLA

Teemu Karlsson, Geological Survey of Finland, P.O. BOX 1237, FI-70211 Kuopio, FINLAND, e-mail: teemu.karlsson(at)gtk.fi

Introduction

The Mineral Liberation Analyzer (MLA) is an automatic mineral analysis system that identifies minerals in polished sections of drill core or ground materials, and quantifies a wide range of mineral characteristics, such as mineral abundance, grain size and liberation. The MLA combines a large specimen chamber automated Scanning Electron Microscope (SEM), multiple Energy Dispersive X-ray detectors with state-of-the-art automated quantitative mineralogy software. The software controls SEM hardware to quantitatively analyze mineral and material samples. (Sylvester 2012)

Figure 1. MLA-instrument in the Mineral Processing laboratory of the Geological Survey of Finland (GTK Mintec). The scanning electron microscope is FEI Quanta 600 and the energy dispersive X-ray analyzer is EDAX Genesis with two detectors. Photo: Nina Dodd, GTK.

Description of the method

Polished sections of waste rock and tailings material are prepared to determine the mineralogical composition by MLA method. To prepare polished sections the rock samples should be cut or ground. MLA measurements are based on backscattered electron (BSE) image of the polished section and analysis for determining grain boundaries and locations for X-ray spectral acquisition, and classification of the characteristic X-ray spectra of mineral species by comparison to a library of reference spectra (Sylvester 2012). In practice, to determine mineral abundances, after measuring thousands of points, the hits per mineral will be counted and based on the results the surface area and weight percentages of different minerals are calculated. (Heikkinen & Laukkanen 2007, Sylvester 2012)

The recommended minimum particle size is 5 μm for a routine automated inspection. Special arrangements allow smaller particle sizes of 0.5 μm to be investigated. The automated routine analytical protocol gains ED spectra from 200 000 particles with average P50 particle size of 30 μm within 6 hours of measurement. (Sylvester 2012)

Appropriate applications

The method is applicable e.g. for the determination of:

Mineral abundances
Grain size
Degree of liberation of specific minerals from gangue
Microstructures in mine tailings

Advantages of the MLA for mineralogical investigation over traditional optical microscopy include (Sylvester 2012):

High resolution; fine-grained or complexly intergrown minerals at the scale of micrometers can be distinguished
Speed; the replacement of tedious manual analysis by systematic, computer-automated analysis
Accuracy; increased through-put and the number of mineral grains examined provides a more statistically representative analysis of a sample, reducing also the potential for operator bias and human error

Disadvantages of the MLA (Sylvester 2012, Heikkinen & Laukkanen 2007):

The difficulty of distinguishing minerals with very similar compositions (e.g., gypsum and anhydrite) and polymorphs (e.g., calcite‒aragonite; rutile‒anatase‒brookite; kyanite‒sillimanite‒andalusite)
The lack of widely available standard reference materials to demonstrate accuracy and precision of results
The amounts of mineral coal (e.g. anthracite, bituminous coal, lignite) cannot be analysed by MLA, it has to be calculated manually for example based on the amount of non-carbonatic carbon

Method maturity

MLA method is well documented and widely used, e.g. at the Mineral Processing Laboratory of Geological Survey of Finland (GTK Mintec).

References

Heikkinen, P. M. & Laukkanen, J. 2007. Rikastushiekkojen ja sivukivien mineraloginen ja kemiallinen koostumus sekä neutraloimispotentiaalisuhteet – tuloksia kahdeksalta eurooppalaiselta kaivokselta. Geologian tutkimuskeskus, arkistoraportti S/49/0000/2007/93. 22 p.

Sylvester, P. 2012. Chapter 1: Use of the mineral liberation analyzer (MLA) for mineralogical studies of sediments and sedimentary rocks. Mineralogical Association of Canada Short Course 42, St. John’s NL, May 2012, p. 1-16.