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Minerals Engineering 20 (2007) 1344–1350

Quantitative measurement of copper mineralogy usingmagnetic resonance

Daniel Bennett a,*, David Miljak a, Joe Khachan b

a Commonwealth Science and Industrial Research Organisation (CSIRO), Division of Minerals, PMB 5, Menai, New South Wales 2234, Australiab Department of Physics, University of Sydney, New South Wales 2006, Australia

Received 10 May 2007; accepted 14 August 2007Available online 1 October 2007

Abstract

A prototype instrument using magnetic resonance has been constructed for quantitative measurement of selected copper minerals, inbulk samples obtained from a number of commercial ore deposits. The instrument has been developed to provide rapid on-line deter-mination of phase concentration. The on-line measurement of copper mineral phase would be of considerable assistance to the miner-alogist or process engineer for process optimisation. Ultimately, the underlying method may be applied in ore streams on conveyors orbatch samples. This paper describes the development of the instrument and measurement methods, with particular emphasis on chalco-pyrite, focusing on the design route for instrument deployment. Quantitative results regarding detection limits and accuracy for dilute oreconcentrations are presented. In addition, effects due to crushing and grinding are discussed. Owing to the very high selectivity of thetechnique, it has been possible to demonstrate mineral phase resolution of 0.1 wt% in bulk samples.Crown Copyright � 2007 Published by Elsevier Ltd. All rights reserved.

Keywords: Sulphide ores; Ore mineralogy; Sorting methods; Process optimisation; On-line analysis

1. Introduction

The significant driver for the research described here isto enable optimisation of mining and mineral recovery pro-cesses by use of an on-line measurement of mineralogy.This would provide the opportunity for improved oreblending, grade control and gangue rejection at both themining and process stage. For example, on-line measure-ment would enable the input–output balance in a processto be determined accurately. Selected key mineral phasescould be measured continuously, perhaps with processalarms implemented above a predefined detection level.Other possible applications include the measurement ofoxides and sulphides in flotation circuits, since their ratiocan affect reagent use, flotation control and mineral recov-ery. It would be of great benefit if feedstock mineralogycould be measured accurately and controlled for process

0892-6875/$ - see front matter Crown Copyright � 2007 Published by Elsevie

doi:10.1016/j.mineng.2007.08.009

* Corresponding author.E-mail address: daniel.bennett@csiro.au (D. Bennett).

optimisation since costly periods of process underperfor-mance could be avoided.

There exists a variety of methods used to determine min-eral phase. For example, X-Ray Diffraction (XRD) andelectron microprobe techniques are widely used for mineralcharacterisation and are capable of quantifying mineralogyfor process optimisation. Other techniques such as Ramanand Mossbauer spectroscopy, fluorescence and reflectancecan also provide mineral specific information. Opticalmethods in particular have found use in rapid on-line oresorting and characterisation (Bennett et al., 2004). For avariety of reasons, many of the methods have not beendeployed widely for on-line measurement of large bulkore samples (several hundred grams or more). On-linemethods ideally should have simple instrument hardware,high levels of mineralogical discrimination, high sensitivityand accuracy, adequate measurement speed, as well as thecapability to measure large bulk samples with minimal orno sample preparation. All these required attributes willgenerally not be associated with any one method. However,

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D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350 1345

specific processes could significantly benefit from deploy-ment of a suitable method for on-line analysis and control.

In this paper we present the particular advantages ofusing magnetic resonance methods, which are especiallyamenable to copper and other transition metal minerals.The very high selectivity of the technique and the bulk mea-surement of kilogram-size samples are demonstrated in lab-oratory tests on a variety of natural ore samples. Wedescribe the development of a spectrometer for the mea-surement of specific minerals, and as an example, estimatethe direct bulk content of chalcopyrite in a copper concen-trate diluted as low as 0.1 wt%. The method is suited to anumber of applications, such as non-contact, direct bulkmineralogical analysis on conveyor streams, slurries,bore-core or routine laboratory sample analysis.

2. Radio frequency techniques

The response of materials to Radio Frequency (RF)electromagnetic fields covers a broad range of phenomena.Materials can often be characterised by the predominantinteraction with either the electric or magnetic radio fieldcomponents. The electric response of rocks and minerals,when considered over a broad range of radio frequencies,is often dominated by moisture content and density (Ulabyet al., 1990). Likewise, most minerals show only weakresponse to applied magnetic fields, and even then theresponse is usually dominated by small amounts of com-mon ferrimagnetic minerals such as magnetite. It is there-fore uncommon for magnetic or electric measurements toprovide highly quantitative discrimination of mineralphase, especially for phase mixtures. However, one majorexception to this generalisation is Nuclear Magnetic Reso-nance (NMR), which encapsulates several different classesof radio spectroscopy. NMR is commonly used to deter-mine chemical structure, bonding and the dynamics ofatomic motion in solids, yet it rarely used for on-line min-eral process measurement due to difficulties in obtaininghighly uniform static magnetic fields over large volumes,together with the low sensitivity found for some transitionmetal elements.

However, there are other classes of magnetic resonancethat can be exploited for mineralogy measurement, wherethe application of a static magnetic field is not required.This is an important advantage, since the significant hard-ware associated with magnetic field generation and controlis avoided. For example, Nuclear Quadrupole Resonance(NQR) (Das and Hahn, 1958; Abragam, 1961) is stronglydependent on bonding and crystal structure. The resonantfrequency is highly specific to the associated mineral, and astatic magnetic field is not required to observe NQR reso-nances. Another class of radio spectroscopy, antiferromag-netic nuclear magnetic resonance, arises due to internalmagnetic fields related to magnetic ordering imposed bythe crystal structure. The internal field removes the require-ment for external application of a static magnetic field.Only selected minerals can be measured using antiferro-

magnetic NMR or NQR, as the techniques are physicallylimited to specific combinations of nuclei and coordination.Some copper ores are particularly suited to detection(Abdullin et al., 1987). Chalcopyrite, a commerciallyimportant copper mineral, has a room-temperature antifer-romagnetic resonance, 63Cu at 18.45 MHz. The results ofthis paper focus on this resonance, which is being studiedfor the purpose of obtaining on-line quantitative informa-tion on chalcopyrite concentration in conveyor streams.

Most resonances of interest occur below 100 MHz,where radio waves have penetration depths typically tensof centimetres in most ores. Therefore, the radio methoddescribed below is a true bulk technique, potentially capa-ble of analysing material volumes of many litres. Com-pared to other methods of analysis, these measurementshave the benefit of avoiding sub sampling and sample prep-aration, and consequently any sampling bias related tothese processes. Additionally, surface oxidation does notaffect the measurement due to the small ratio of surfacenuclei to those in the bulk. Significant moisture levels,either existing on the surface or within the bulk normallydo not pose a problem at the RF frequencies.

Another significant advantage is that the resonances arerarely subject to interferences from other minerals. This isbecause resonances are relatively narrow and at the sametime widely distributed throughout the spectrum. Sulphidesand oxides like chalcopyrite, cuprite and tenorite are veryeasily distinguishable, allowing combinations of copperminerals in real ore samples to be measured. For example,we have successfully measured small levels of cubanite inchalcopyrite ores by exciting specific cubanite resonances.Typical gangue materials like aluminosilicates have reso-nances at much lower frequencies, and do not interfere.Even if there is overlap in the spectra, discrimination canbe obtained by exploiting differences in dynamic responsethat are particular to each mineral species. These factsmean that NQR and antiferromagnetic NMR methodscan provide very high levels of copper mineral phasediscrimination.

Another potential capability resulting from the absenceof interferences is the detection of relatively low mineralconcentration, compared to other methods. In principle,spectrometer noise may be reduced by sufficient signalaveraging, allowing the resolution of very low concentra-tion. It is anticipated that for most applications the meth-ods described below will typically require several minutesof signal averaging time, acceptable in most on-line appli-cations, such as a moving conveyer.

3. CSIRO spectrometer

CSIRO has developed a bench spectrometer for lab-scale bulk mineralogical investigation. Significant elementsof the spectrometer shown in Fig. 1 include; a radio fre-quency transmitter that can be modulated to provide exci-tation pulses, a tuned sample coil, switching circuits and anRF receiver for signal detection. Control electronics are

Tuned Resonant Circuit Sample Solenoid

1 kW Amplifier

Transmit Electronics Reciever Electronics

Radio Frequency Pulse

Logger and Computer Analysis

IsolationSwitch

Preamplifier

Fig. 1. Schematic of the spectrometer operation.

ig. 3. A typical resonant echo generated in a laboratory prepared.0 wt% concentration chalcopyrite sample (1000 s for 10,000 acquisi-ons). The amplitude of the echo provides an estimate of the concentra-on of chalcopyrite.

1346 D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350

also used to define the timing, generation and acquisitionof RF signals.

The sequence to obtain resonant signals from the spec-trometer consists of several steps. Firstly, an RF pulse froma gated power amplifier is fed to the tuned sample coil,which produces a strong RF magnetic field inside the sam-ple. The target nuclei inside the sample couple to the RFfield by precessing, and after each pulse, re-radiate a weakRF field which is detected by the same coil. This resultingsignal is transmitted via the transmit-receive switch into thepreamplifier, and then into the detection system. These sig-nals are averaged over many acquisitions and the magni-tude is then calculated and processed for optimal noiserejection.

To excite the resonances, pulse schemes typical of NMRare used. In minerals such as chalcopyrite with relativelyshort relaxation times, the Hahn echo sequence, illustratedin Fig. 2, may be used to excite the resonances. The partic-ular pulse separation employed is dependent on the mineralspecies. For a given pulse sequence and system gain, theamplitude of the echo signal is proportional to the numberof nuclei within the target mineral. A typical chalcopyrite

Fig. 2. The Hahn echo sequence used to excite signal echoes inchalcopyrite. Two radio frequency pulses are applied, the first 4 ls wideand the second 8 ls wide. Pulse separation s is 240 ls. Echoes appear at atime of 2s = 480 ls after the first excitation pulse. This sequence isrepeated once every 100 ms. Echoes from each sequence are recorded andthen averaged to increase the signal to noise ratio.

F1titi

echo signal is shown in Fig. 3. This echo was obtained on asample of total volume 70 cm3, with the chalcopyrite pres-ent as a powder (particle size less than 100 lm), diluted at1.0 wt% concentration in silica powder. Throughout theexperiments described below, the value of the echo peakwas used to quantify the concentration of chalcopyrite.All measurements were performed at room temperatureon samples placed in cylindrical plastic containers, andlocated within a coil volume of 70 cm3. This volumeenabled measurement of various samples, such as mineralconcentrate powders, agglomerations of larger particlesas well as large single fragments of fined grained ore orlarge crystals.

In most magnetic resonance experiments, the absolutevalue of received signal may not be of particular signifi-cance, and control of the absolute response is not para-mount. However, for quantitative determination ofmineralogy, the absolute response must be carefully con-trolled across different sample loading or sample presenta-tions (Shultz and Karr, 1969). For the results obtainedbelow, several techniques have been applied to obtain spec-trometer stability, including the monitoring of amplifier andcoil characteristics. Another differentiator of these experi-ments, compared to most other magnetic resonance experi-ments, is the relatively large sample volume. Larger volumesgenerally require higher working voltages during the excita-tion phase. This places technical constraints on the RF engi-neering and transmit-receive switch, which must adequatelyprovide isolation against the larger terminal voltages.

4. Results and discussion

4.1. Chalcopyrite dilution measurements

To demonstrate the quantitative capabilities of the tech-nique, chalcopyrite powder was diluted with commercial

D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350 1347

grade powdered silica, across a range of concentrations,and the spectrometer tested for predictive capability. Nat-ural chalcopyrite ore concentrate (greater than 90% purity,less than 100 lm particle size) was used as the test material.Nine samples were prepared with dilutions rangingbetween 0.6 wt% and 11.0 wt% chalcopyrite. Samples wereplaced wholly inside the sample coil and were subjected toHahn pulse sequences to excite the resonance echo. Theecho signal to noise ratio was improved by averaging eachpulse sequence 1000 times (with a period of 0.1 s, represent-ing a signal integration time of 100 s) before further dataanalysis. A correction for noise was applied to the echomagnitude (Bernstein et al., 1989).

Fig. 4 shows a linear regression between echo magnitudeand chalcopyrite sample concentration. The error bar inpredicted concentration for each sample is the standarddeviation of ten repeated measurements of the echo magni-tude. The error in these measurements is predominantlydue to incoherent thermal noise in the sample coil andspectrometer preamplifier. For the time interval used, thenoise contributes to the uncertainty at each of the sampleconcentrations, thus setting the lower limit on concentra-tion resolution. As can be seen the results of chalcopyritediluted in silica demonstrate good linearity. The standarderror of the regression is 0.3 wt%. It is anticipated that inreal applications most common gangue materials will notaffect the quantitative determination of the target species.Tests have also been performed using alumina and magne-tite as the dilutant. Alumina yielded results very similar tothose for silica. However for magnetite the standard errorincreased to 0.6 wt%, which is attributed to strong electri-cal loading and consequent reduction of magnetic flux gen-erated in the sample coil. It should be noted that puremagnetite dilutant probably represents an extreme situa-tion with regard to electrical loading, not common to mostapplications or representative of real ores.

Fig. 4. Relation between spectrometer-predicted concentrations versusprepared chalcopyrite concentration in silica, for the range 0.6–11.0 wt%.Integration time is 100 s.

In another set of measurements the detection limitsand resolution were investigated. A second set of chalco-pyrite samples diluted with silica were prepared, withchalcopyrite concentrations ranging from 0.1 wt% to1.0 wt%. Each sample was subjected to 10,000 excitationpulses (10 Hz repetition rate or 1000 s acquisition time)and the signal amplitude measured. These results areshown in Fig. 5, where the size of the error bars isdefined by the standard deviation of five independentmeasurements of the magnitude of the sample signal.The standard error of the regression is 0.08 wt%. Forthese particular experimental conditions, the lowestdetectable level of chalcopyrite is 0.2 wt%. The smallerrors observed demonstrate spectrometer stability overthe short-to-medium term, where multiple measurementshave occurred over a period of a number of days, andalso show the feasibility of controlling spurious coherentspectrometer noise to levels suitable for low concentra-tion detection.

The investigation of different mixtures of gangue andsulphide mineralogy on the resolution and detection limitis continuing. Initial experiments described here areencouraging, and there is reason to believe that similar res-olution would be obtainable in other mixtures. For all mea-surements reported in this paper, the resolution remainsdominated by spectrometer thermal noise. Also, as detailedin subsequent sections there has been no observation of anysystematic interference from other sulphide minerals, evenwhere these phases collectively have higher concentrationthan the chalcopyrite.

While the demonstrated resolution and detection limitswould be useful in mineral applications, there would be abenefit in reducing the acquisition time. It is anticipatedthat better than 0.1 wt% chalcopyrite resolution will beachievable with a several minute integration time, by acombination of increasing the sample volume, reducing

ig. 5. High resolution measurements of the spectrometer-predictedoncentration versus prepared chalcopyrite diluted in silica for the range.1–1.0 wt%. Integration time is 1000 s.

Fc0

Fig. 6. Line shapes of selected chalcopyrite concentrates.

1348 D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350

first-stage preamplifier noise and further optimising theexcitation pulses. Straightforward scale-up of this natureis possible since the resolution is constrained by thermalnoise in the spectrometer rather than mineralogical inter-ferences. This has been confirmed in preliminary measure-ments with approximately 250 cm3 coil volume, wheresignal to noise ratio for chalcopyrite has been improvedin accordance with expectations. The signal to noise ratiovaries for different minerals and is principally controlledby the dynamics of the response (Abragam, 1961). Basedon preliminary measurements, it is expected that similarresolution and detection limits will be obtainable for theminerals cuprite, tenorite, covellite and cubanite so fartested.

4.2. Consistency of response across chalcopyrite sample

As magnetic resonance is highly sensitive to local fieldsaround nuclei, the question arises as to whether the tech-nique will be overly sensitive to small changes in crystallineparameters. These changes may include structural defects,impurities or crystalline stress. Very strong changes inresponse may hamper instrument calibration. In order todetermine the natural variation of magnetic resonanceresponse, multiple chalcopyrite samples were obtainedfrom eight different deposits. The samples consisted of sixconcentrates, plus a number of samples with large particlesize (>10 mm). The resonant frequency for six of the sam-ples was found to be identical, within experimental error.The two remaining samples showed small frequency shiftsless than 0.5%, which is not expected to present serious cal-ibration problems.

An experiment was performed to assess line shape acrossselected samples. The spectrometer excitation frequencywas scanned across the resonance frequency for each sam-ple in 50 kHz steps. This ‘‘piecewise’’ measurement of theresponse provides finer detail of line shape than does indi-vidual echo measurement. Fig. 6 shows a comparison ofline shape for four of the concentrate samples, each norma-lised to individual peak signal. Three of the samples showconsistent response, as line widths and central transitionfrequency overlap. However, one sample clearly showsbroadening, and a small upwards shift in frequency. Thesource of this broadening and shifting for this single sampleis currently being investigated.

XRD analysis on most of the available concentratesamples has indicated the presence of other significantphases such as cubanite, bornite, pyrite, chalcanthiteand quartz. Comparison against quantitative mineralphase and elemental analysis is currently being pro-gressed to compare absolute response for chalcopyriteacross the different samples. Given the relatively smallvariations in response of samples from widely differingore deposits, it would appear from these early measure-ments that the calibration of magnetic resonance tech-niques for chalcopyrite will be straightforward in mostapplications.

4.3. Size effects: small particle size limit

It would be advantageous to avoid significant processingor sample preparation procedures, especially for on-line orby-line measurements. It is therefore important to quantifythe effects of particle size on signal response. As describedin the previous section, it has been observed that multipleconcentrate samples sourced from different deposits weresuccessfully measured with a consistent response. Themean particle size for the samples, measured using laser dif-fraction methods, ranged between 17 and 24 lm. As eachconcentrate was derived from current industrial grindingmethods, it is likely that the magnetic resonance techniqueis compatible with existing comminution processes.

Other studies have reported significant effects associatedwith high intensity grinding (Shultz and Karr, 1969;Tkacova and Balaz, 1996). In one of these studies (Shultzand Karr, 1969), the cuprite resonant response diminishedto background under intensive grinding using a tungstencarbide vial, yet with hand grinding, the signal amplitudewas roughly unchanged. Other studies on chalcopyritespecimens (Huhn, 1985; Tkacova and Balaz, 1996; Balazet al., 1998) have identified significant changes to the struc-ture and magnetic susceptibility associated with high inten-sity grinding. Results indicated a range of both chemicaland physical interactions occurring during breakage, fromdefect production, stress/strain changes to the structure,and in extreme cases, chemical reactions where new miner-als formed. Such gross effects, imparted to the sample on alarge scale, are likely to strongly interfere with resonancemeasurements.

To investigate these effects on the echo signal, smallsamples of chalcopyrite concentrate were ground in a lab-oratory ring mill for several minutes. Microscopic inspec-tion of the product revealed severe changes to particlemorphology, with very irregular and darkened particles,and a large fraction of smaller fragments. The resonancein the ground sample was not observable due to excessivebroadening. These results are in line with the other studies

ig. 7. Chalcopyrite signal versus particle size fraction, for a mineralample obtained from an Australian sulphide deposit. The chalcopyriteignal is recovered at smaller size fractions.

D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350 1349

on the effect of high intensity grinding on magnetic reso-nance. However, it should be emphasised that strong echosignals are obtainable from all concentrate samples wehave obtained from processing operations. Clearly, typicalindustrial grinding processes do not significantly affect theresonant response. This is very encouraging for wide appli-cation of this technique.

4.4. Size effects: large particle size limit

It is well known that sample electrical conductivity canaffect magnetic resonance measurements. For example,metals subjected to NMR measurement are typically firstlypowdered to allow sufficient radio frequency field strengthto be generated inside individual particles. It is the ‘‘skindepth’’ (Jackson, 1998) of highly conducting materials thatlimits radio field penetration. To a lesser extent, semicon-ducting minerals will produce similar effects. Thereforethere is likely to be an upper limit to the size of individualhighly conducting particles that can be accurately charac-terised with radio techniques. The radio signal is affectedonly to the extent that nuclei deep inside large highly con-ducting particles cannot be readily excited. If the conduc-tivity is too high, ore samples would have to be crushedto smaller size before measurement.

However, it should be noted that bulk powder com-posed of small conducting particles allow much betterRF field penetration than single large pure ore masses ofthe same volume (Euler, 1978). The reduction in conductiv-ity of packaged powder samples is a complex function ofindividual particle morphology, particle contact area, sur-face oxidation and packing compression. At the relativelylow RF frequencies used in the experiments, the sameeffects are expected to apply, resulting in much lower RFshielding currents in the sample and improved RF fieldpenetration. This explains the fact that large volumes ofloosely compressed chalcopyrite powder can be success-fully measured, as described in the previous section.

To assess large individual particle size limits, measure-ments were firstly performed on relatively pure samplesof chalcopyrite having bright gold lustre and large visiblecrystals, with pieces up to 20 mm diameter. Strong signals,consistent with the particle mass were observed in eachcase. This is in accordance with the lower range of typicalconductivity reported for chalcopyrite (Pridmore andShuey, 1976) of about 10 S m�1, which corresponds to askin depth of 6.5 mm at 18.46 MHz.

However, in many sulphide deposits, chalcopyrite coex-ists with other minerals which may have higher conductiv-ity. To determine an indicative upper particle size limit,samples with relatively high conductivity (compared tothe pure chalcopyrite samples) were specially selected froman Australian massive sulphide deposit, with known dom-inant minerals being chalcopyrite and pyrite. Several dark,fine grained homogeneous mineral lumps (size 10 by 15 by25 mm) were crushed using a laboratory jaw crusher, sizedusing sieves and fractionated. Crushing was performed so

as to obtain roughly equal mass of approximately 40 gacross each size fraction. Given the fine grained nature ofthe samples, it is likely that the constituent minerals weredispersed evenly across the fractions. The fractions wereeach measured separately in the spectrometer. The signal,normalised with fraction mass, is plotted as a function offraction size in Fig. 7. At large particle size, only a veryweak signal is recovered (much weaker than signalsobtained from pure chalcopyrite samples of similar size).An increase in the resonant response was observed at smal-ler size, stabilising for fractions with maximum particle sizeless than 2.8 mm. Based on these measurements it may beconcluded that direct measurement of sulphide ore depositswith disseminated mineral grains of size less than a few mil-limetres to centimetres could be performed, depending onthe particle conductivity.

To summarise expected particle size effects on the mag-netic resonance technique, several different ore measure-ment scenarios may be considered. Firstly, it wouldappear that size effects will be minimal for ore streams withmineral particle size below a few millimetres, disseminatedthrough larger mineral bearing rocks. Typical mineralbearing rock has much larger skin depth than the sulphideparticles themselves, and so does not affect the magneticresonance measurement. Secondly, the measurement ofhighly conductive copper sulphide mineral with particlesize up to at least a few millimetres demonstrates thatloosely packed sulphide minerals are amenable to the tech-nique. Finally, there is the case of ore streams with mineralparticles much larger than a few millimetres. Depending onthe particle conductivity, this case could pose a difficultyfor the magnetic resonance method. Smaller mineral parti-cles would contribute preferentially to the measured signal,and the estimation of mineral concentration would becomea function of both the mineral particle size distributionand segregation of particles presented to the magneticresonance sensor. However, overall it is believed that a

Fss

1350 D. Bennett et al. / Minerals Engineering 20 (2007) 1344–1350

significant number of measurement applications would besuited to the magnetic resonance technique.

5. Conclusion

RF detection of bulk mineralogy offers several potentialadvantages, such as true bulk material analysis on largevolume samples, relatively simple hardware for signal gen-eration and detection, and the avoidance of mechanicalsampling and sample presentation procedures. The methodcan be very highly discriminating of mineral phase. Thedilution experiments reported here demonstrate that reso-lution of chalcopyrite concentrations better than 0.1 wt%is achievable. Measurements performed on a number ofsamples sourced from different deposits demonstraterobust response from both concentrate and lump mineralsamples. The demonstrated lower and upper particle sizelimits are likely to be compatible with many applicationsand deposits. Work is continuing to widen the applicationof the technique to other transition metal mineralogy withreduced integration times.

Acknowledgements

The authors gratefully acknowledge Dr. Barry Whit-tington and Ms. Nicki Agron-Olshina, CSIRO Minerals,for provision of some of the samples and associatedXRD analysis results.

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