Mica and vermiculite in South Africa*

J. S. At,. Inst. Min. Metal/., vol. 89, no. 1.Jan. 1989. pp. 1-12.

Mica and vermiculite in South Africa*by J.J. SCHOEMANt

SYNOPSISIn South Africa and in the rest of the world, the two mica minerals that have the most important commercial

value are muscovite and vermiculite.Muscovite has been making a comparatively small but steady contribution to South Africa's mineral exports since

about 1960.Vermiculite mining and concentration were started by the late Or Hans Merensky at Phalaborwa during 1946.

Vermiculite has enjoyed good overseas sales since then, and during the past four years has become an importantearner of foreign currency for South Africa.

The various aspects of the micas that could be of general interest are briefly reviewed, such as the history oftheir exploitation, their mineralogy and chemistry, and their mining, concentration, production, sales, and future.

SAMEVATTINGIn Suid-Afrika, asook in die wereld as 'n geheel, is die twee mika (glimmer) minerale met die belangrikste

handelswaarde, muskowiet en vermikuliet.Muskowiet het 'n betreklike klein, maar bestendige bydrae gelewer tot Suid-Afrika se minerale uitvoere vanaf

ongeveer 1960.Verm ikuliet mynbou en konsentrering was begin te Phalaborwa deur wyle Or. Hans Merensky gedurende 1946.

Vermikuliet het goeie oorsese verkope geniet vanaf 1946 en gedurende die afgelope vier jaar 'n belangrike buitelandsevaluta verdiener vir Suid-Afrika geword.

Die verskeie aspekte van die mikas wat van algemene belangstelling kan wees word kortliks bespreek soos diegeskiedenis van hul ontginning, mineralogie en chemie, mynbou, konsentrering, produksie, verkope en toekoms.

INTRODUCTION

The only two species of mica that occur very commonlyare muscovite and biotite. These two micas are impor-tant constituents of many igneous, sedimentary, andmetamorphic rocks, and of all kinds of loose surfacedeposits derived from them such as river and beach sands,and soils. Muscovite is, in fact, one of the most in-destructable of minerals in nature and can withstand anydegree of weathering. Biotite is rather less-resistant in thisregard.

A third species of mica, phlogopite, is much moresparsely distributed. It is found in certain ultrabasic rockssuch as kimberlites and the ultrabasic members of alkalineand carbonatite complexes; for example, the PalaboraIgneous Complex. It is also found in certain metamor-phosed dolomites and serpentinites.

The only two micas of established commercial valuein South Africa are muscovite and phlogopite-muscovitefor its ,own properties, and phlogopite essentially becauseof its alteration product, hydrophlogopite alias vermicu-lite.

MICA-GROUP MINERALS

Physical PropertiesThe various micas have certain very characteristic

common physical properties. The most notable is theirflaky form due to highly perfect crystallographic basal

* This is the latest paper in our Mineral Review Series.t Formerly Head of Geological, Surveying, and Mine Technical

Services, Palabora Mining Company Limited, P.G. Box 65, Phala-borwa, 1390 Transvaal. At the time of writing, Consultant to thatCompany.

@ The South African Institute of Mining and Metallurgy, 1989. SAISSN 0038-223X/3.00 + 0.00. Paper received November 1987.

cleavage. Mica normally takes the form of numeroussingle flakes adhering naturally together to form books.The individual flakes, which are pliable, resilient, andtough, can be separated like the pages of a book and canbe rendered as thin as some 20 Ilm by delamination.

A further notable feature of all micas is that the flakeshave glass-smooth surfaces and have high reflectivity. Inthis regard, the Germanic name glimmer is undoubtedlymost descriptive and apt.

The colour varies according to the particular species.Muscovite is colourless or of a pale colour, biotite isbrown to black, phlogopite has a purple or bronze colour,and lepidolite (litha mica) is rose- or lilac-coloured.

The micas have very low electrical and heat conduc-tivity, and can withstand high temperatures. In thisrespect, the two pertinent micas are muscovite with anupper limit of 550°C and phlogopite with a much higherlimit of lOOO°C.

The size of the flakes varies, generally in relation tothe texture of the rocks in which they occur. The coarsestmicas, as is to be expected, are found in igneous rocksof pegmatitic structure. The granitic pegmatites are thesource of coarse muscovite, including the exceptionallylarge crystals of sheet mica. Coarse phlogopite is a con-stituent of pegmatoid ultra basic rocks such as those ofthe Palabora Complex. The occurrences of pyroxenepegmatoid and the serpentine pegmatoid shown on thegeological map of Fig. 2 contain a substantial propor-tion of coarse phlogopite and of its derivative, hydro-phlogopite.

Chemical CompositionThe chemical composition of various micas is as fol-

lows:

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1989

Constituent I 11 III IV V VI

SiO2 40,25 38,53 36,61 35,93 38,74 46,90A~O, 10,45 8,35 8,40 9,05 10,38 31,80Ft;O, 2,50 6,31 4,50 5,11 8,96

4,39FeO 2,18 0,64 0,09 0,16 1,98MgO 26,11 25,74 26,91 26,15 21,77 0,60CaO Nil Nil Nil 0,50 Nil 0,05NazO 0,25 Nil Nil Nil 0,27 0,70KzO 10,35 4,73 0,05 0,16 7,82 10,50H2O+ 6,48 14,58 22,10 11,32 7,50 4,47H2O 10,36TiO2 1,15 1,12 0,89 0,93 2,11 0,05P20S Nil 0,01 Nil 0,05 Nil -F 0,62 0,92 0,55 0,67 0,53 -Cr2O, 0,07 0,12 0,49 0,48 Nil -MnO 0,02 0,Q3 0,02 0,02 0,04 -

100,43 101,08 100,61 100,89 100,10 99,46

~KA~ (SiO4)3

H2K (MgFe)3(AI, Fe)(SiO4)3

Phlogopite (magnesium mica) ~KMg3 (SiO4)3'

Of the several other species in the mica group, only lepi-dolite (litha mica) is of more than academic interest.

Chemically considered, the micas are silicates, and inmost cases orthosilicates, of aluminium with potassiumand hydrogen, and also often magnesium and ferrousiron. Ferric iron, lithium, and other elements also occurin certain species.

The mica species all yield water on ignition, mostlyfrom 4 to 6 per cent. This can be regarded as water ofconstitution, and hence the micas are not proper hydroussilicates. As will become evident later in this review, thisis in contrast to the vermiculites, which are hydroussilicates.

It can be seen from Table I, that the conversion ofphlogopite to hydrophlogopite is a matter of loss ofalkalies and a gain in water. The same holds for the con-version of biotite to hydrobiotite. Muscovite apparentlyis resistant to such conversion because of its resistanceto weathering. Therefore there are, for practical purposes,only two minerals in the vermiculite group, if one canuse the term, namely hydrophlogopite and hydrobiotite.

A theoretical chemical formula for hydrophlogopite isgiven later in this review under the heading 'Vermiculitiza-tion'. Hydrobiotite would have a similar formula but withless or no magnesium content.

Muscovite (potassium mica)

Biotite (magnesium-iron mica)

TABLE ICHEMICAL ANALYSES OF MICAS AND VERMICULITES

I Coarse unaltered phlogopite, VOD orebody (GeversI, p. 154,analysis I)

11 Golden-yellow vermiculite, VOD orebody (GeversI, p. 154,analysis V)

III Brownish-yellow, VOD ore body (GeversI, p. 154, analysis VI)IV Surface-weathered, leathery, golden-yellow vermiculite, VOD

orebody (GeversI, p. 154, analysis VIII)V Fine-grained vermiculite (hydrobiotite), southwest of Loolekop

(GeversI, p. 154, analysis IX)VI Muscovite, Marble Bath Mine, Mica (McLachlan & Lazar (Pty)

Ltd, 1984)

MuscoviteHistory

Muscovite owes its name to the Latin vitrum musco-viticum or muscovy glass, formerly a popular name forthe mineral. In the following text mica is synonomouswith muscovite.

In South Africa, the areas around Mica in the easternTransvaal and the Kenhardt and Namaqualand districtsof the northwestern Cape Province have been the onlylocalities in which the production of muscovite of anyimportance has taken place (Fig. 1).

In the Mica area, muscovite production on a mention-able scale first started in 1909, but was hampered by thefact that the nearest railway station was at Pietersburg,a distance of 190 km over a primitive earth road. After1912, when the Selati railway line came into operation,the conditions for transportation and marketing improvedconsiderably. The railway line happened to pass over thismica field, hence the name Mica Siding-later, MicaStation.

The pegmatite belt in the northern Cape Province hashad a longer and more interesting and colourful historythan the Mica field, but not because of its muscovite pro-duction, which has never been important. Since well intothe past century, its attraction has always been the greatvariety of rare and valuable minerals that have beenfound and recovered from this vast pegmatite belt. Sub-stantial quantities of beryl have been produced over a longperiod, and also quantities of tourmaline, gadolinite,cassiterite, spodumene, columbite-tantalite, garnet,lepidolite, scheelite, wolframite, etc., from thousands ofsmall to comparatively large workings by individualdiggers, syndicates, and mining companies.

UsesInitially, the demand was for muscovite in compara-

tively large laminae, known as sheet mica, with dimen-sions of 5 to over 20 cm, but production declined andsheet mica has not been marketed in South Africa since1970, except of the order of 50 to 200 kg per year in cer-tain years.

Sheet mica has always been a scarce commodity becauseit makes up only a small portion of the total mica inpegmatites, and the perfect quality, ruby mica, is notavailable from South African pegmatites. Effective sub-stitutes for sheet mica have been evolved, which, coupledwith its scarcity, has virtually phased it out of worldmarkets.

However, the much more abundant mica of the pegma-tites, the smaller size flake mica, is put to many uses. Itis concentrated out of the host pegmatite and is almostentirely finely ground, mainly in wet form, water-groundmica, but it is also ground dry, dry-ground mica.

Water-ground and dry-ground micas have a variety ofapplications as filler and brightener for paints, filler inplaster boards, rolled roofing, asphalt shingles, specialcements, moulded electrical insulation, fire-resistantboards, rigidized plastics, a component in wallpapercoating, concrete and tile surfacing, welding-rod coatings,absorbents, special greases, lubricant in the manufactureof moulded rubber products such as tyre moulds, heatinsulation of boilers and steam pipes, and drilling muds.A small quantity is even used in cosmetics. A smaller

2 JANUARY 1989 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

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Fig. 1-0ccurrences of muscovite in South Africa

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amount of flake mica is not processed to ground mica,but goes mainly into the production of mica paper, asa substitute for sheet mica.

An extremely fine mica product, less than 40 ~m insize, has become a competitor to the conventionallyground mica. It is micronized mica, and is produced bythe acceleration of fine particles of flake mica to very highspeeds in a confined space so that they collide and dis-integrate. Micronized mica was first introduced in Europeto retain the paint market previously held by water-ground mica.

OccurrencesThe two muscovite-producing regions of South Africa

are shown in Fig. 1. In order of current economic im-portance they are the Mica region of the north-eastTransvaal and the northwestern Cape region.

The occurrences of muscovite in the eastern Transvaalare along a belt 4 to 8 km wide, extending from west ofMica, eastwards towards the Lebombo Mountains in theKruger National Park, a distance of about 100 km.

The deposits of commercial muscovite from this areaare restricted to coarse-grained pegmatites intrusive intoArchaean granites and gneisses, The pegmatites are asmuch as several hundred metres in length and 20 m ormore in width; they are more or less parallel to oneanother, and their strike direction is approximately east-northeast. They are composed essentially of quartz andfelspar, with scattered muscovite as flakes and sometimes

as books, the latter forming irregular nests and pocketsup to 3 m in size.

The easily recoverable near-surface muscovite from alarge number of quarries in pegmatite outcrops has beenmined out, and very little is now obtained from openworkings. The once-numerous mines have now beenreduced to only a few with a significant output.

In the northwestern Cape Province, the pegmatites areassociated with late- to post-tectonic granites and gneissesof Mokolian age, in a belt 15 to 20 km wide and some450 km long, between Kenhardt in the east and Steinkopfin the west.

ProductionPegmatites are notoriously difficult to mine because

of the sporadic and unpredictable distribution of all theeconomic minerals, including muscovite, recovered fromthem. In South Africa, the mining of mica can be payableonly if felspar and other minerals are recovered at thesame time. Muscovite has been mined for its own sakeonly at Noumas No, 1 Mine in the Steinkopf ColouredReserve, halfway between Steinkopf and Vioolsdrif.

In 1986, ground mica was produced mainly by twocompanies. The Mica area (Gelletich Mining Industries)contributed 92,3 per cent of the total mass, with thebalance from the Steinkopf Reserve (Garieb Minerals).Of the total mass of mica produced in South Africaduring 1986, water-ground mica contributed 60 per cent,the rest being dry-ground mica and usable flake mica,

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1989 3

Production", t

Sheet Flake Ground Sales ValueYear mica mica micat Total t R

1957 1198 2507 484141958 1930 1823 31 6121959 1706 1753 32 4081960 0,8 2514 530 3045 1842 73 0901961 2468 2134 102 2421962 2224 1909 1030251963 2123 1971 1409791964 3068 2871 189 6231965 0,9 493 1775 2269 2351 1735071966 2235 2196 1620081967 4618 7441 338 6091968 7918 9989 432 1831969 6449 10 131 471 8741970 10,6 5419 2121 7551 7589 4999261971 7160 6707 4203621972 4247 5393 3633941973 6009 7678 5167641974 2767 2719 272 6011975 984 3066 40501976 1413 2500 1709 297 1551977 1989 2935 3333 4890911978 2428 2616 3075 585 7611979 1897 1784 1974 5403061980 1736 17361981 53 1479 15321982 1533 15331983 45 1654 16991984 1787 17871985 2153 21531986 2509 2509

World M.E."Production

Country t 070 Rank 070 Rank

USA 125 000 57,1 1 74,2 1Canada 12000 5,5 3 7,1 2France 10 100 4,6 4 6,0 3India 8000 3,8 5 4,7 4Norway 4000 1,8 6 2,4 5RSA 2200 1,0 7 1,3 6Other M.E. t 7200 3,3 4,3

Total M.E. 168 500 77,1 100,0USSR 50 000 22,9 2

Total world 218000 100,0

The production and sales of muscovite from 1957 tothe end of 1986 are given in Table U, which is a com-bination of two tables derivetl from publications by DeVilliers and Hugd and by Gossling3. The annual salesrevenues after 1979, at the request of the producers, arenot available for publication.

TABLE 11PRODUCTION AND SALES OF SOUTH AFRICAN MUSCOVITE2,3

. Production figures are based on total salest Including usable flake mica

In respect of the world production of mica in 1985,Table III shows that South Africa's contribution is small.

Future MarketsAlternative materials such as polyamides, polyesters,

polystyrene, alumina, steatite ceramics, fused quartz, andsilicon rubber have lately made inroads as substitutes formica, notably sheet mica.

The total annual world tonnages of mica are small com-pared with those for other base minerals, and these rela-tively small tonnages of mica are made into a largenumber of technical products requiring a variety of dif-ferent raw materials and supplied in relatively small quan-tities to widely dispersed markets around the world.

Therefore, despite the threat of substitutes, the tradi-tional outlets of mica products are considered relativelystable with limited growth within the construction, auto-motive, and oil industries. The production and marketing


TABLE IIIWORLD PRODUCTION OF MUSCOVITE

Source: BGS World Mineral Statistics, 1981-85

" M.E. denotes mica-exporting countriest Spain, Yugoslavia, Madagascar, Morocco, Mozambique, Sudan, Zim-

babwe, Mexico, Argentina, Brazil, Sri Lanka, Portugal

of mica should progress slowly yet with stability and,despite pronouncements to the contrary, there appear tobe no major disturbing influences on the horizon.

MiningThe pegmatite mines of the eastern Transvaal and

northwestern Cape Province are essentially felspar-muscovite mines with accessory pure quartz. The pegma-tites of the Mica area are of the barren type, with littleor no valuable minerals such as common and ruby corun-dum, common and emerald beryl, cassiterite, and gemtourmaline. The pegmatites of the northwestern CapeProvince are more often of the complex unhomogeneoustype, and valuable accessory minerals have been recoveredfrom them.

The main constituent of most pegmatites in SouthAfrica is rock composed of intergrowths of felspar andquartz in variable proportions. The quartz has no valuebut has to be mined together with the accompanyingfelspar and mica. In earlier times, the miners tried to mineout the felspar and muscovite selectively, and for thisreason the older quarries are often unbelievably irregularexcavations resembling dolomite sink holes and caverns.The later quarries, where such selectivity was not applied,are more conventional excavations.

In the Mica area, opencasting was followed by under-ground mining. One of the first mines was the now-abandoned Lady Godiva Mine. Some years ago, theMarble Bath Mine operated by Gelletich Mining Indus-tries also progressed from open to underground mining.At the latter operation, access to the underground work-ing is by a 3D-degree inclined haulage shaft fitted witha 450 mm-gauge track. Ore and waste are hand-loadedinto cocopans in the stopes, and are hauled via the in-clined shaft to the surface by mechanical rope hoisting.

The mine has reached a depth of about 120 m belowsurface and has natural ventilation. The rock is drilledwith compressed-air jackhammers in a stoping operation.It is blasted with Dynagel cartridges used in conjunctionwith igniter and safety fuse.

The article4 used for reference does not give the ton-

nage output for this mine, but it is not a large-scaleoperation.

Concentration ProcessesConcentration involves the excavation of the musco-

vite, and also the felspar and quartz, out of the brokenaggregate of pegmatite and wall-rock, and includesvarious arrangements of crushing, handsorting, trommel-ling, screening, washing, and blowing according to thepreference and ingenuity of the individual operator. Inthe fields of both Mica and the northwestern CapeProvince, this can vary from primitive procedures to afairly high degree of sophistication. The Marble BathMine at Mica can serve as an example of the latter.

After blasting at the Marble Bath Mine, the felspar andquartz are sorted out underground from the mica andwaste rock. On the surface, the former two minerals areseparated on a sorting belt and stored. The waste rock,made up of felspar-quartz intergrowths (Quaggaklip) andwall-rock schists and granites, is mainly sorted out under-ground and, on reaching the surface, is put onto wastedumps. The remaining constituent, the muscovite, is keptseparate underground as much as possible, and is con-veyed to the beneficiation plant admixed with some fel-spar, quartz, and waste rock.

This muscovite-rich material is passed over a 25 mmvibrating screen. The oversize is handsorted on a conveyorbelt, where the felspar and quartz are picked out. Theremaining rock goes to waste. The muscovite is sent eitherto a dry-grinding plant or to a mica stockpile. The minus25 mm undersize is passed through an air separator, andthe concentrate is deposited on the mica stockpile. Themiddlings fraction is sun-dried and then recleaned twicethrough an air separator, and the residual tailings go towaste.

The mica stockpile, which contains material that is tobe water-ground, is ground in series through three Chileanmills. Each of these consists of two steel milling wheelsof 1,83 m diameter, rolling round the floor of a cylin-drical pan. The discharge from the third Chilean mill ispassed over a 4 mm screen. The oversize, consisting ofmica flakes and rock grit, is sent to the mill middlingsplant, where it is trommelled to remove the grit. Thetrommel oversize is returned to the Chilean mills togetherwith the stockpiled mixture of mica flake and grit.

The finely ground screen undersize pulp is size gradedover three screens in series to produce three size grades,namely 60, 180, and 325 mesh. These products are allow-ed to settle out in tanks of various sizes over a 48-hourperiod. After the water has been decanted, the dewateredpulps from the tanks are pumped onto large drying sur-faces known as Roman tables. On these drying tables,the pulp is heated from below by means of hot air sup-plied by burning coal. The dry pulp is broken up in asmall hammer mill and is then bagged according to theabove-mentioned size grades and sold as water-groundmica.

In respect of dry-ground mica, Gelletich Mining In-dustries started to produce this material in 1982. SouthAfrica's needs for dry-ground mica were formerly metby imports, but the plant at the Marble Bath Mine cannow supply the demands of the domestic market. Unlikewater-ground mica, it is not a payable export because oflow prices in the consuming countries.

This material is produced by putting mica from theabovementioned mica stockpile through a series of ham-mer mills and then over screens to produce two commer-cial grades, minus 20 and minus 60 mesh. By the use ofother screens, other size grades can also be produced.

BiotiteBiotite is used on a limited scale in other countries in

place of muscovite, but in South Africa it has no applica-tions. The Palabora Complex holds considerable quan-tities of biotite in two places-on the western side ofLoolekop, the zone of predominantly mica rock shownin Fig. 2, and also in a large patch (not denoted in Fig.2) about 1,5 km southwest of Loolekop.

PhlogopiteThe word phlogopite comes from the Greek word

meaning 'fire-like' in allusion to its colour.

Geological SettingAs mentioned previously, phlogopite is a characteristic

component of ultrabasic rocks in alkaline and carbonatitecomplexes, of which there are three well-known examplesin the Transvaal. The Klein Letaba Complex to the eastof Bandelierkop and the Glenover Complex in the farnorthwestern Transvaal contain considerable reserves ofphlogopite, but the most important reserves are in thePalabora Complex.

Fig. 2 gives a picture of the lithological assemblage con-stituting the Palabora Igneous Complex. The ultrabasicrocks form its central core. This is a deeply eroded vol-canic pipe-probably the deep root zone of a very ancientvolcano that erupted some 2000 million years ago. In Fig.2, it is the area of irregular outline measuring some 6 kmnorth-south and some 2,5 km east-west.

The early intrusion of the ultrabasic rocks was followedby a final alkaline phase of intrusion that resulted in manybodies of syenitic rock scattered over a wide regionaround the central ultra basic pipe of Fig. 2. The syeniticrocks are resistant to erosion, and have formed the manypicturesque bare rock tors and rocky hills of the Phala-borwa lowveld region.

The areas of pyroxene pegmatoid and serpentine peg-matoid of Fig. 2 are underlain by phlogopite rock inwhich much of this mineral is of coarse flake structure.

The mining at Phalaborwa has shown that the phlogo-pite in the abovementioned areas is almost completelyaltered to vermiculite to a maximum of some 50 m belowsurface. This top zone is followed downwards by a tran-sition zone of vermiculite intermingled laterally with moreor less vermiculitized phlogopite. This zone is probablyfairly thick everywhere. It is, in turn, underlain by un-altered phlogopite to great depth.

UsesIn the past, phlogopite was evidently a more costly

commodity than muscovite. To quote from a 1955 pub-lication5: 'The higher cost of phlogopite prevents it fromcompeting successfully against muscovite, except for elec-trical insulation, for which it is preferable'. Therefore,in the past, this consideration may have prevented its usein industry.

Vermiculite mining at Phalaborwa has exposed great


Fig. 2- The Palabora Igneous ComplexPMC = Palabora Mining CompanyPP&V = Palabora Phosphate & Vermiculite Company-now part of Palabora Mining CompanyVOD = Vermiculite Operations Department of Palabora Mining Company


reserves of phlogopite that can be recovered relativelycheaply. The properties of muscovite and phlogopite aremuch the same, and it seems feasible that phlogopitecould be a less costly substitute for muscovite. Theprimary criterion, of course, is that the phlogopite mustbe fresh and unaltered.

Extraction of PotassiumSouth Africa is well endowed with many minerals but

it has poor reserves of potash and alumina, and to dateno economic deposits of these two commodities have beenfound. For this reason, Palabora Mining Company hascarried out full-scale investigations over recent years toextract the 10 per cent K2O contained in phlogopite,with the recovery of alumina as a byproduct. Two pro-cesses were evolved: the Gypsum Process and the AcidProcess. The latter gave the more promising results, but,to quote from Basson6: 'it is safe to say that a vastreserve of potassium exists and that a process or processeshave been developed for the selective recovery of this.It may not be an economic viability under current condi-tions, but it can be exploited whenever the need arises'.

In respect of the extraction of potash from phlogopite,the prime criterion of a fresh unaltered state also applies.With the process of weathering and consequent vermicu-litization, the K2O content of fresh phIogopite is reducedto a trace in vermiculite, as demonstrated by the chemicalanalyses of Table I.

To obtain sufficient fresh phlogopite for uses such asthose that apply to muscovite need not be a problembecause the tonnages required are not likely to be large.For use as a source of potash, however, large tonnagesof phlogopite would have to be processed, and the avail-ability of sufficient fresh material could pose a costproblem.

VERMICULITE-GROUP MINERALS

HydrophlogopiteHistory

The history of vermiculite mining in South Africa isessentially the history of vermiculite (hydrophlogopite)mining at Phalaborwa. Comparatively small quantitieswere produced for limited periods in the past from otherdeposits such as the occurrence near Bandelierkop in thenorthern Transvaal, but production from them has longsince ceased.

The discovery of the remarkable degree to which ver-miculite expands on heating was probably made a longtime ago. A small book of vermiculite, when heated, canexpand to thirty times its original volume, stretching outslowly into a long worm-like form. This characteristic hasgiven the name to this mineral-from the Latin ver-miculari, which means to breed worms.

The excellent fire-resistant and heat- and sound-insula-tion properties of expanded vermiculite eventually result-ed, some fifty years ago, in its being used for thesepurposes.

In a review of the Phalaborwa vermiculite, Gevers1wrote: 'Although reddish brown mica (phlogopite) hasbeen mentioned by Dr Melior as early as 1906 and in spiteof the fact that considerable quantities of "rotten mica"had been found to be associated, particularly with thecoarse and lumpy apatite in the pyroxenite to the South

East of Loo1ekop, no notice seems to have been takenuntil 1936.'

In that year, samples of Russian vermiculite were sentto South Africa from London. One sample eventuallyreached Phalaborwa through Government agencies, andwas shown to Carter Cleveland, a wandering Americanpioneer prospector from the State of Alabama who hadmade his home in these lowveld regions. He confirmedthat he had seen large books of rotten mica that had ex-perienced exfoliation during a grass fire.

This started prospecting for vermicuIite in the PaIaboraComplex. The hydrophlogopite area to the north ofLoolekop (now the VOD area shown in Fig. 2) soon cameunder the ownership of Dr Hans Merensky, the legendarydiscoverer of platinum and diamonds, millionaire, andvisionary. By 1946, the open-pit mine and concentratingplant were in operation. All Merensky's vermiculite in-terests were purchased by Palabora Mining Company in1963, which simply continued the operation as the Ver-micuIite Operations Department (VOD). Thus vermiculitehas been mined and concentrated continuously at Phala-borwa since 1946.

Another concern, the Palabora Phosphate & Vermicu-lite Company, started a similar operation in 1942, a fewyears before Merensky had initiated the abovemention-ed venture. A plant was erected to concentrate apatite(calcium phosphate) and vermiculite (hydrophlogopite)out of the orebody, denoted as the PP&V Area on Fig. 2.The operation, however, was shortlived and survived foronly six years. The vermiculite rights to this orebody wereacquired by Dr Merensky, and thus in 1963 also becamethe property of Palabora Mining Company.

Around 1936, the deposits of hydrobiotite (black ver-miculite) in the PaIabora Complex were also prospectedbut abandoned in favour of the hydrophlogopite deposits.

VermiculitizationIn the PaIabora Complex, vermiculite has resulted from

the hydration of phlogopite (and biotite) by the loss ofalkali and the addition of water. Vermiculite is thereforeessentially a complex hydrous silicate of magnesium andaluminium with varying amounts of iron, possibly ofisomorphous replacement. The ideal formula for hydro-phlogopite is given7 as

22 MgO. 5 A~O3' FeO. 22 SiO2. 40 H2O.

At Phalaborwa all evidence indicates that surfaceweathering, under the influence of percolating meteoricwater, was the main cause of the conversion of phlogopiteto vermiculite. According to Gevers1,

The main effect in the process of vermiculitisation is the pro-gressive leaching of the alkalies, mainly K2O, from 9,9 percent inthe phlogopite through 4,73 percent in the transitional material topractically nothing (0,05 percent) in the golden yellow vermiculite.At the same time the total water content increases from 6,76 per-cent in the phlogopite through 14,58 percent in the intermediatematerial to 22,10 percent in the high grade vermiculite.

These changes are clearly evident from a comparisonof the chemical analyses given in Table I.

As would be expected, there is a definite relationshipbetween the expansion coefficient, or as more common-ly termed the exfoliation ratio, and the water content.This ranges from nil for fresh phlogopite, through inter-mediate stages of hydration and expandabiIity, to the last-


named high-grade vermiculite containing about 22 percent water and having an average exfoliation ratio ofaround 26 times the original volume. The vermiculite isexpanded by commercial users in gas or dieseline burnersat temperatures of 850 to 1O00°C.

Hanekom et al.7 describe the process of vermiculitiza-tion as follows:

Generally the process of vermiculitisation begins from the edges

of books and flakes of phlogopite and works progressively inwards.

In such a way books of vermiculite resulted, showing yellow ver-

miculite in the marginal portion and reddish brown phlogopite in

the core. The alteration is accompanied by a change in colour and

a concomitant loss in elasticity and transparency on the part of thephlogopite.

The gradations are seen clearly in the open-pit mine. Thenear-surface vermiculite is generally a pale yellow, andthe colour grades downwards through golden-yellow toamber and brownish varieties.

MiningBecause the vermiculite has resulted from surface

weathering, the vermiculite ore forms a surface layer thatgrades downwards into rock containing too much phlogo-pite, which is an undesirable contaminant of the ver-miculite concentrate. Thus, the vermiculite orebody, thatis the VOD orebody, does not extend to more than some50 m below the surface, but it has a lateral spread of morethan 2 km2.

The vermiculite ore has been mined from the start byopen-cast methods, the bench height being maintainedat 5 m. The vertical blastholes are spaced at 3 m, with a2,5 m burden. The holes are a standard 65 mm in dia-meter, drilled with a track-mounted ptleumatic drill.

The explosive is a blend of granular ammonium nitrateand fuel oil. In the dry holes, the detonation is by elec-trical detonators connected in series. In the wet holes,blasting is effected with high-explosive primer cartridgesset off by detonating fuse. The powder factor is satisfac-tory at 6,5 t per kilogram of explosive.

The method of mining has a severe restraint placed onit by the fact that it must be extraordinarily selective. Thisis necessary because of the many size gradings of ver-miculite required to satisfy market requirements. The con-centrating plants are also designed and geared to meetthese same demands. This means that mining is strictlycontrolled on an hour-to-hour basis so that a balancedore feed in respect of flake sizing is continuously sup-plied to the concentrator.

The ore grade and quality are controlled by a combina-tion of visual evaluation of all the blasthole chippings andlaboratory assays. In the laboratory, flake sizing of thesamples of blasthole chippings is determined by screen-ing and exfoliation ratio through heating of the samplesin high-temperature ovens to 1O00°c.

Based on the visual and laboratory assays, the piles ofbroken ore after blasting are subdivided into loading'blocks'. These are regarded as individual entities, andare loaded separately one from another. Loading fromthese blocks is so arranged that a consistently blendedfeed results.

The bulk exfoliation ratios pertaining to the six gradesof final concentrate must be controlled as well as pos-sible, and is done by the blending of ores from shallower


and deeper levels in the open pit. This adds a further con-straint on the style of mining.

The VOD open pit puts out about 8500 t per day ofore and waste. So it is a fairly large-scale operation but,for the reasons given above, the mining style is rathersmall-scale. There is no question of going to a more bulkystyle of mining through, say, an increased bench heightor the use of large loaders and trucks.

The loading and hauling equipment consists of threerubber-tyred front-end loaders of 3,6 m capacity and sixtrucks of about 30 t payload. The open pit operates ona six-day week, with two 8-hour shifts per day.

The additional earth-moving operation, that is thereclamation of the concentrator tailings dump, requiresthe loading and transportation of about 1300 t per day.This is done with one 3,6 m rubber-tyred front-end loaderand one 25-ton tractor-loader.

ConcentrationAlthough chemically a well-altered product, vermiculite

is still a distinct mineral and retains the flaky structureof phlogopite, its parent mineral. This flaky structuremakes it possible, by winnowing, to separate the ver-miculite from the gangue minerals that, by contrast,assume a granular form after comminution. This win-nowing does not basically differ from the age-old pro-verbial process of 'separating the grain from the chaff',but here the 'chaff' is the desired commodity.

Like phlogopite, the vermiculite also occurs mainly inbook form, but these books have to be delaminated; thatis, the books have to be rendered into thin flakes in theconcentrating process, or at least into thinner books forefficient winnowing. As the coarser grades of vermiculiteare higher-priced than the finer grades, a further requisitein milling is that the vermiculite should not be subjectedto unnecessary comminution.

The concentrating processes are therefore essentiallyexercises in size gradings by screenings, with concomi-tant winnowings.

Markets and SupplyThe market for vermiculite has shown an interesting

progression over the years. Initially, only the coarsergrades were in demand, but this has changed in that con-sumers have found increasing applications for fine tovery-fine flake material. This has had a beneficial finan-cial effect by increasing the total tonnage demand, andthe finer material that was formerly valueless now hasvalue.

This has made the retreatment of the old tailings dumpand the recent erection of an additional concentratingplant to treat these tailings an economic viability. Some7,7 Mt of tailings have been brought back onto treatablereserves, with an average grade of about 27 per cent forvermiculite larger than 35 mesh, amounting to a total ver-miculite content of 2,1 Mt. If the smaller vermiculite isalso taken into account, a further 2,7 Mt of tailings atan average grade of 46 per cent vermiculite can be addedto the tailings-dump reserves.

As shown in Table IV, there are no fewer than six com-mercial grades of vermiculite for which there is a market.

Ore-concentration PlantThe various sections making up the vermiculite-ore

ISO screen sizes, mm

RangeProduct

grade Middle size From To

Premium 5,66 16,00 2,80Large 2,80 8,00 1,40Medium 1,40 4,00 0,71Fine 0,71 2,00 0,355Superfine 0,355 1,00 0,180Micronized 0,180 0,50 0,090

DR"I'Pl..,

"'. i

i.IIY'"',

"'"''" "'.

r_I

PLAJlT IiSrI..SECO.DARY - ,..",... uC(

S" TOSE<O..., "U,

..UI

"

J ~ W'N"""'R.,;g SCALE

Q CRUIHER 0 I,N

t? OR'ZZLVICREEN 1 lUCK" CONVEVGK

cr--o"ELT CONVEYOR= ICREW CONVOVOR

~

Fig. 3- The vermlcullte-concentratlng plant at Palabora Mining Company

TABLE IVMARKETABLE GRADES OF VERMICULITE

concentrator at Palabora Mining Company are shown inFig. 3.

The raw ore from the open pit is tipped by the minehaulage trucks into a stationary sloping-bar grizzly toseparate the boulders of diopside and serpentine largerthan 300 mm, which are the two gangue constituents. Theundersize material is passed over a second stationarygrizzly at a gap 'Of 150 mm. The oversize rocks are alsodiscarded as a coarse low-grade waste product.

The undenH'zematerial, now upgraded in respect of ver-miculite, is passed over a screenJitted with a screen clothcontaining apertures of 20 mm square mesh. The over-size is fed through a primary impact crusher with a 38 mm

setting between low bars on rotor and stationary beaterplates. The undersize joins the crushed oversize and isput on a stockpile before the dryers.

It is an essential condition for efficient winnowing thatthe vermiculite should be dried to a moisture content of4 per cent and be free-flowing, even down to sizes smallerthan 180 #tm. Two coal-fired dryers are used for the pitore, and a similar dryer for the reclaimed tailings.

After being dried, the ore is crushed in a closed crush-ing-screening circuit to minus 16 mm. In the process, thescreen oversize is passed through two winnowers todiscard a further low-grade waste-rock product.

As can be seen from Fig. 3, the remainder of the ore-concentrating plant consists essentially of winnowing andscreening circuits. The complicated arrangements arenecessary to produce the six different grades of concen-trate.

A similar but separate plant, the tailings concentrator,has recently been commissioned to treat the old tailingsdump for the extraction of vermiculite, but without acrushing plant, which, of course, is not necessary.

Fig. 4 gives a cross-sectional view of the standard typeof winnower employed .at Phalaborwa. It has beenevolved for the specific purpose of winnowing the localvermiculite.

The two concentrators give approximately the follow-ing performances:


Ore ConcentratorOpen-pit ore, t/d

Vermiculite grade, 070Dryer feed, t/d

Vermiculite grade, 070Recovery from pit ore, 070

460021,6

370028,0 + 425 J.tm38,0 + 180 J.tm60,0 + 425 J.tm65,0 +425 J.tm

650200000

Recovery from dryer feed, 070Vermiculite concentrate, t/dVermiculite concentrate, t/ a

Tailings ConcentratorTailings from dump, tf dVermiculite grade,Recovery, 070Vermiculite concentrate, t/ dVermiculite concentrate, t/ a

130031,5 + 180 J.tm42,5 + 180 J.tm

19060000

UsesVermiculite has a considerable range of uses in the

countries to which Palabora exports. It is invariably usedin the expanded form but, because of its high volume inthis condition, it is expanded as near as possible to theplace where it is applied. Mandoval Vermiculite (Pty) Ltd,Alberton, concerned with the exportation, give the fol-


Fig. 4-Wlnnowing at PalaboraMining Company

1 Feed (screw conveyor)2 Double-deck screen3 Winnower-feed chute4 Cataracts5 Winnower6 'Magic' rotating screen7 Grade0 concentrate conveyor8 Grade1 concentrate conveyor9 Middllngs conveyor

10 Tailings conveyor11 Dust hood12 Dust exhaust13 Screen-undersize chute14 Top-deck winnower-feed

chute15 Air-exhaust ducting16 Ductlng to dust baghouse17 Air Intake (belied)

lowing approximate proportions of usage: fire protection30 per cent, high-temperature insulation 20 per cent,ambient-temperature refrigeration and sound insulation10 per cent, agricultural uses 30 per cent, animal feed 5per cent, and minor uses 5 per cent.

Vermiculite can withstand temperatures of up to11O0°c. For fire protection, it is sprayed or moulded ontoany surface to the desired thickness. It can prevent, toan important degree, the warping, bending, or collapseof the reinforced-concrete or steel-beam frameworks ofhigh-rise buildings, bridges, etc.

Because of its inertness in respect to heat and fire, ithas a great advantage over light-weight plastic foams thatare all flammable under certain conditions and give offdeadly fumes on burning. This seems to have been thecause of the Kinross Mine tragedy that claimed so manylives. In respect of structures with a high fire risk likeoil rigs and petrochemical plants, it is an invaluablesafeguard.

In addition to its fire-proofing use, vermiculite is animportant high-temperature insulator. For ambient-temperature refrigeration and sound insulation, it isequally useful. For these two classes of insulation, thevermiculite is applied in much the same way as for fire-

II I

200 OOOTons/"" t-. .. .

I \ I1 1/\ /,'\. J ,...1-/

1 1\ J tV \ I1

f-150 OOOTons ~V \ V1/ , j

--I \/7' 'Y

~-/' """~1-100 OOOTons

~./Lt') 0 Lt') 0 Lt')

l/ to I"-- I"-- ex) ex)

10-""'"0") 0") 0") 0") 0")

". - - - - -50 OOOTons

.

proofing.Another major application is in agricultural fields since

it retains water and air better than natural soil does. Ithas become a much-favoured ingredient in composts,together with peat moss, pine bark, etc. It is also anabsorbent of fluid plant nutrients and also a filler forgranular or powdered fertilizer.

Vermiculite is also finding increasing application inanimal feeds, chiefly as an absorbent and filler fornutrients. Other uses are as a filler or extender ofpigments, paints, rubber, enamel, plastic, and ink.

In South Africa, it has some minor and unusual uses,for instance as a substitute for river sand to nest crocodileeggs. At a crocodile farm on the South Coast of Natal,there was great consternation recently. The female croco-diles became very upset because there was no vermiculitein which to lay their eggs owing to the late arrival of anexpected consignment of vermiculite!

Production and SalesThe main producer of vermiculite on the world scene

is the USA, followed by South Africa. Lesser producersare the Soviet Union, Argentina, Brazil, and India.

The US vermiculite is almost all produced at Libby,Montana, from a huge dyk~like basic igneous intrusion.The vermiculite is hydrobiotite, and not hydrophlogopiteas at Phalaborwa. This hydrobiotite is mainly consumedwithin North America.

Fig. 5 depicts the sales of South African vermiculite,being exclusively hydrophlogopite. Since Palabora hasbeen the sole producer, the graph refers only to thatsupplier.

The tonnages of vermiculite produced have always cor-responded closely to the tonnages sold, and there hasnever been an accumulation of unsold stocks.

The 1960 to 1974 figures are those provided by

Schoeman and Brabers8. The later yearly figures werekindly supplied by the marketing division of Rio TintoManagement Services Ltd, Sandton, Johannesburg.

A relatively minor part of the Palabora vermiculite isexpanded and consumed in South Africa, but most ofthe production is exported to a number of countries. Itis exported in raw unexpanded form. Transportation ismainly in the loose bulk state by rail to ports on theeastern seaboard.

It can be seen from Fig. 5 that there has been a steadyincrease in sales over the years, even if on a somewhatfluctuating basis. The sales respond principally to thestate of the building and construction industries in theconsumer countries, and therefore fluctuate accordinglyyear by year.

ReservesThe reserves of hydrophlogopite rock in the Phalabor-

wa deposits that are regarded as economic ore amountto roughly 22 Mt at an average grade of about 22 per centvermiculite. In addition, there is a further 34 Mt of rock,at an average grade of 21 per cent hydrophlogopite that,for certain reasons such as accessibility to the concen-trator plant in the VOD area (Fig. 2), are now classifiedas sub-economic ore. This material, however, is morethan likely to be exploited in the next century.

Thus, there is no reason for concern regarding thefuture of vermiculite mining at Phalaborwa.

Mention was made earlier of rock containing a mix-ture of vermiculite and phlogopite. A great amount hasalready been stockpiled, carrying more than 20 per centvermiculite and, furthermore, large reserves have becomereadily accessible at the bottom of the VOD open pit. Ifit does prove possible to utilize this material-even onlya portion of it-then there is no end in sight to the ver-miculite operations at Phalaborwa.

0 Tons

Fig. 5-Annual tonnages of vermiculite sold

JANUARY 1989 11JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

A practical and economical means for the separationof phlogopite from vermiculite would be of incalculablebenefit. Unfortunately, this has not been achieved to datebut, if it were possible, it can be stated with full con-fidence that vermiculite mining at Phalaborwa could lastindefinitely.

In contrast to the Palabora alkali complex, other alkalicomplexes such as Klein Letaba and Glenover are unlikelyto ever be exploited for their hydrophlogopite contentbecause of adverse factors such as remoteness and lackof infrastructure.

HydrobiotiteThe biotite in the two occurrences in the Palabora

Complex-to the west and southwest of Loolekop-hasbeen altered to vermiculite in the weathered zone. Thishydrobiotite (black vermiculite) , compared with thehydrophlogopite, is of inferior quality in that it is fine-grained and has a much lower expandability on beingheated.

It is improbable that these deposits will be worked,although the reserves of hydrobiotite are fairly consider-able.

ACKNOWLEDGEMENTS

The author thanks the Management of PalaboraMining Company Limited for the satisfaction of havingbeen able to write this review.

Special thanks go to Mr P. Retief of the VermiculiteOperations Department and to Mr N.S.L. Steenkamp of

Obituary: J.J. SchoemanJan Jacobus Schoeman was born on 28th May, 1919,

at Lydenburg and passed away at Warmbaths on 23rdJuly, 1988.

.He was educated at King Edward VII High School in

Johannesburg and the University of the Witwatersrand,where he obtained the following degrees:

B.Sc. Mining and Engineering 1940B.Sc. Mining Geology 1942D.Sc. Geology 1953.

In addition, he obtained the Certificate of Competencyfor Mine Managers.

During the earlier part of his career, Dr Schoemanworked for the Geological Survey of Kenya, and then asSenior Geologist with Messina (Transvaal) DevelopmentCompany Limited for the period 1950 to 1957. Duringthose early years of his career, he was primarily engagedin exploration activities, but he also worked for mines


the Technical Department, and to their respective staffmembers for the assistance given; also to Mr C. Lessingand staff of the Engineering Drawing Office for the fourfigures and to Mr B. de Wet for the drawing of the win-nower assembly.

The manager of the Marble Bath Mine at Mica, MrLR. Alexander, made possible a most informative andpleasant visit to that mica-felspar operation.

REFERENCES

I. GEVERS,T.W. Vermiculite at Loolekop, Palabora, north easternTransvaal. Trans. Geol. Soc. S. Afr., vol. 51. 1948.

2. DEVILLIERS,S.B., and HuGO, J.P. Mica. Mineral resources of theRepublic of South Africa. Pretoria Geological Survey, 1976.

3. GbsSLING, H.H. Mica. Johannesburg, Minerals Bureau of SouthAfrica, 1987.

4. ANoN. Bushveld mica for world markets. S. Afr. Min. Wld, Apr.1984.

5. JONES,W.R. Minerals in industry. London, Pelican Books, 1955.6. BASSON,C.F. Proceedings Potassium Symposium. 1985.7. HANEKOM,H.J., VANSTADEN,C.M., SMIT, P.J., and PIKE, D.R.

The geology of the Palabora Igneous Complex. Pretoria, GeologicalSurvey, Memoir 54. 1965.

8. SCHOEMAN,J.J., and BRABERS,A.J. Vermiculite. Mineral resourcesof the Republic of South Africa. Pretoria, Geological Survey, 1976.

BmUOGRAPHY

CHAPMAN,G.P. The world mica grinding industry and its markets.Transactions 4th Industrial Minerals International Congress,Atlanta. 1980.

DANA, E.S., and FoRD, W.E. Dana's text book on mineralogy. NewYork, WHey & Sons, 1932.

in Zimbabwe and South Africa, and for short spells inthe Sudan, Tanzania, and Zambia. From 1957 to 1962,he was Head of the Economic Geology Division of theGeological Survey of South Africa.

In 1962, Dr Schoeman joined Palabora Mining Com-pany Limited as Chief Geologist, to be promoted in 1963to Chief Mine Engineer and Geologist, the post he helduntil his retirement in February 1976. From the date ofhis retirement until his death, he rendered consulting ser-vices to Palabora.

He was the author or co-author of a variety of technicalpapers.

Despite his full mining career, Dr Schoeman still foundtime to venture into tobacco farming in Zimbabwe, andlucerne farming and cattle ranching in the Transvaal. Hewas a keen tennis player, and also painted a few can-vasses, particularly of the bushveld that he loved so much.

Documents

Mica and vermiculite in South Africa*