i B U L L E T I N of the International Association of ENGINEERING GEOLOGY No 28 P A R I S 1 9 8 3 j de I'Assocation Internationale de GI~OLOGIE DE L'INGENIEUR

C H E M I C A L A N D M E C H A N I C A L S T A B I L I T Y O F H E M A T I T E I R O N - O R E IN S E A W A T E R

LA S T A B I L I T E C H I M I Q U E ET M E C A N I Q U E DU M I N E R A I DE F E R H E M A T I T E D A N S L ' E A U DE M E R

VERHOEF P.N.W. (1) and VAN DALEN j,pj.(2)~ Delft University of Technology, Department of Mining Engineering (1) and Department of Chemical Technology (2), P.Ox.Box 5028, 2600 GA DELFT, The Netherlands

Abstract

Hematite ore was considered as a potential high-density rock-fill material for the constuction of a major coastal protec- tion work in The Netherlands (the Storm Surge Barrier in the Eastern Scheldt). Three groups of hematite ore from the Itabira District, Brazil, were investigated for their chemical and mechanical stability in a prefeasibility study. Chemical leaching tests showed only minor chemical activity in one of the three investigated groups of rock samples.The microstruc- ture of the ore was shown by slake durability testing and a sand blast test to be a significant factor influencing mechanical resistance against abrasive processes. The iron ore contains an important volume percentage of cryptocrystalline limonite. The mechanical characteristics of the ore are influenced by the microstructural distribution of the limonite. Redistribution of limonite within the ore and release of limonite from the ore takes place in flowing water, which might lead to visual pollu- tion of the environment if this material is used as rock-fill in coastal protection constructions.

R~sum~

On examine l'usage du mineral de fer h6matite pour r6aliser des enrochements pour la construction d'un ouvrage important pour la protection de la c6te des Pays-Bas (Digue protectrice contre les temp6tes, Schelde-Est). Trois groupes du mineral h6matite provenant du District d'Itabira, au Br6sil, ont 6t6 examin6s quant i la stabilit6 chimique et m6canique dans une 6rude de faisabilit6. Un essai de lessivage chimique montrait une activit6 chimique tr6s limit6e pour un des trois groupes de roches examin6es. La structure microscopique est un facteur important qui contr61e la r6sistance m6canique contre les processus abrasffs. Le mineral de fer contient un pourcentage important en volume de limonite cryptocrystalline. Les carac- teristiques m6caniques du minerai sont influenc6es par la distribution micostructurale de la limonite. II y a redistribution de la limonite darts le minerai et une perte de limonite se produit dans l'eau courante. Cette "perte" peut produire une pollu- tion visuelle de l 'environnement si ce mat6riel est utilis6 comme enrochements darts des constructions de protection des c6tes.

I n t r o d u c t i o n

The design of the Eastern Scheldt Storm Surge Barrier (Offringa, 1983) requires the use of lfigh-density rock-fill materials. Especially in the so-called "negative overlap"; in the area between the 200 x 42 m foundation mattresses, rock-fill with a grading of 40 - 250 mm and a density of about 5 t/m 3 was thought to be needed to cope with the very high water currents (a once a year event of 3 - 5 m/s was expected at some localities) during the building stages of the project. Once laid the rock filter material should stay intact for a period of 200 years, the expected life-time of the storm surge barrier. Since this barrier is a permeable structure seawater remains constantly in contact with the rock-fill. Water velocities in the permeable rock-fill could reach values of about 0.2 m/s.

After a search for possible rock materials with the required density, such as peridotite, eclogite and iron-ores, hematite iron-ore from the Itabira district, Brazil, was choosen as the most suitable. To investigate its suitability tests, such as grain-size analysis, density, aggregate impact value and grain shape analysis were performed by the Road Research Laboratory of the National Water Authority (Rijkswater- staat, The Netherlands) (Table 1.). It was decided to complement this work with a more detailed study on the stability of iron-ore in seawater.

Samples of three groups of rocks were provided for a pre- feasibility investigation. It was decided to perform leaching

experiments to test chemical interaction with seawater and to make a petrographic examination of the ore. The latter study led on to experiments on mechanical stability.

H e m a t i t e i ron-ore f r o m Brazil ( I tab i ra dis t r ic t )

The geology of the Itabira district and the hematite ore is very well documented (Dorr and Miranda Barbosa (1963); Dorr (1964, 1978)) and it was possible to develop a clear picture of the natural occurrences of the ore. This enabled classification of the three groups of samples inves- tigated into the geological types distinguished by Dorr and Miranda Barbosa (1963).

The hematite ore studied here is developed from the so- called "itabirite" banded iron-ore deposits. These banded iron-ores consist of layers of alternating quartz and iron oxides, It is thought that these iron-ores resulted from chemical precipitation in seawater during rather unique circumstances in Precambrian times (3200-2000 x 106 years ago). The banded iron-ore deposits in Brazil also underwent deformation and metamorphism during subse- quent orogenic episodes. This led to the typical "itabirite" type banded iron ores. The deformation during the oro- genic episodes caused the formation of folds and related development of schistosity planes in the rock. Locally quartz was dissolved from the itabirite and rock bodies consisting mainly of hematite (Fe2 03) remained.

234

Tab. 1 : Engineering geological properties of hematite iron-ore

Sample group n bulk density

~q

A 10 4.95 B 10 5.15 B* 10 5.15 C 10 4.82

Mg/m 5 ,~ - - - - - - - - - 4 -

a n - 1

0.14 0.04

0.34

3.46 0.89

5.97

porosity %

i trn-- 1

2.22 0.65

6.3

A.I.V.% (B.S. 812)

12.7

Specimens stu died microscopically

ASWl ASW2 ASW3

B1 B2 C1 C2 C3

4.82 4.92 4.78 5.15 5.19 4.36 5.13 5.06

6.7 2.9 6.5 0.9 0.4

14.8 0.4 1.3

(*) Samples tested by the Road Research Laboratory (Rijkswaterstaat).

Since the climate in Brazil is tropical and humid, extreme weathering of the rocks is possible. Especially along channel ways for water, such as joints, faults or schistosity planes, weathering can take place. The development of limonite, extremely fine-grained (cryptocrystalline) material, consis- ting probably of iron hydroxides and hydrated oxides ( F e 2 0 3 . n H 2 0 ) is related to the weathering ptocess. There is a definite increase of limonite content of the rock with increasing weathering grade, although there is no clear understanding of the weathering process itself; it is not certain whether or not hematite (Fe203) can trans- form directly to limonite, or if any other material (such as quartz) is involved in the process (Dorr, 1964).

Dorr and Miranda Barbosa (1963) classified the hematite ore using structure and weathering grade. The deformed ore is generally schistose, displaying a pervasive axial- plane schistosity, but locally massive iron-ore occurs, which, because of the compact structure, comprises the highest grade ore. The durability of the ore can be descri- bed as hard, intermediate or soft, which relates to the limonite content of the rock and reters to crumbling under physical action.

It can be concluded from the literature (op. cit.) that the quality of the ore is a function of the geological history. The ore may or may not be strongly deformed (schistose or massive). It can be highly or only slightly weathered (soft or hard). It is important to note that the whole range of qualities can be present in one mine (Dorr and Miranda Barbosa, 1963).

S t a b i f i ~ o f h e m a t i t e on the E a r t h ' s su r f ace

Hematite is known as one of the most stable minerals on the Earth's surface. It is the end-product of the oxida- tion series

Fe ~ FeO ~ Fe304 (magnetite) ~ Fe20 ~ Chematite)

However, it is rather obscure under what conditions hydra- tion of hematite to form limonite is possible. The exact nature of the cryptocrystalline limonite is a problem in this respect. Often limonite is thought to be FeO(OI-I) (goethite) (Krauskopf, 1979) and the relative stability of goethite with respect to hematite

2 FeO(OH) = Fe203 + H20

has been the subject of considerable argument. Langmuir (1971) showed that kinetic as well as particle size effects control the occurrence of goethite and hematite in sedi- ments and soils. He shows that, especially, f'me-grained (< 0.1 /am) goethite is not stable with respect to coarse- grained hematite under geological conditions. With ageing dehydration of goethite to form hematite occurs, but reaction kinetics is very slow and experimental attempts to perform this reaction have had very little success.

The geological observations in Brazil, as summarized above, suggest the formation of limonite from hematite by weathe- ring processes. If hydration reactions should occur, they are accompanied by volume increase, and this might lead to mechanical deterioration of the rock. X-ray diffraction analysis of limonite material trom the iron-ores failed to show the presence of goethite. This suggests that it is not possible to describe the behaviour of limonite by assuming a goethite (FeO(OH)) composition tbr this cryptocrystalline material.

Desc r ip t i on o f the s t u d i e d mate r i a l

Three groups of samples were available for study. Group A (unknown mine) consisted o f rather rounded lumps of ore with abundant foliation planes and locally voids. Some samples were clearly folded. The voids have been developed on the intersection of two sets of discontinuity planes, as a result of weathering (Fig. 1). This ore could be classi- fied as intermediate to hard, schistose ore (Dorr and Miranda Barbosa, 1963). The ore is covered by fine-grained red-brown limonite dust. Group B (Esperanza mine). Lumps of this ore are generally hard and massive. (No discontinuity plane developed). Some specimens however do contain a layering or discontinuity. The ore has a me- taUic lustre and has less red limonite dust covering as type A. Group C (Aguas Claras mine). Rounded lumps of ore with a red limonite dust covering as type A. Layered and schistose specimens occur and also specimens with voids. This type of ore can be classified as hard, schistose.

Reflection microscopy showed that the ore consists mainly of the mineral hematite (Fe203) in volume percentages of about 75-80 %. In the ores B and C magnetite (Fe304) also occurs ( < 5 %). The ore is composed of an aggregate of hematite (-+ magnetite) grains (grain size +- 100/zm). A layering may be developed, detined by grain size variation

235

/ /

J

/ / . / /- / /

i / /j

Fig. 3 :

Fig. 1 :

Photomicrograph of a polished surface of specimen ASW3. IAmonite (tim.), opaque, is present interstitially (1), along hematite grain boundaries (2) and along the rims of voids (4) (cf. fig. 2).

Fig. 2 :

Schematic drawing of folded iron-ore specimen, showing development of voids (i) along the intersection of layering (I) and axial-plane schistosity (II). Scale bar is 5 em.

�9 i i i

Sketch of the various microstructures observed in the iron-ore Limonite (black) may be present (1) intersti- tiaUy in between hematite (stippled) grains, (2) along hematite grain boundaries, (3) along microcracks (often aligned with grain boundaries), (4) along the rims of, or within, voids, (5) as a cloudy replacement structure within individual hematite grains.

or compositional (e.g. -+ magnetite) variation. In some samples the hematite crystals are elongated, tabular and parallel to foliation planes. 10-25 volume percent of the ores is occupied by cryptocrystalline rusty-brown material, referred to as limonite. X-ray diffraction of l imonite sho- wed the presence of hematite --- kaoline in the limonite. Dil: ferential thermal analysis (DTA) indicated hematite and gel- type bonded water. No goethite was proved by either techniques. It is suggested that the limonite consists of amorphous, gel-type Fe compounds and perhaps of struc- tures of the type Fe 2 0 3 . nt-I 2 O although further analytical efforts are required to substantiate this.

Fig. 4 : Photomicrograph of a polished surface of specimen B2. Interlocking aggregate of magnetite (mt) and hematite (hm). Limortite (lira) occurs interstitially and as a cloudy replacement of hematite.

The limonite occurs (Fig. 2) :

1. in between hematite grains (interstitial) 2. along grain boundaries of hemati te 3. along microcracks in the ore (mostly along grain-

boundaries) 4. along the rims of voids in the ore 5. locally intracrystalline as cloudy replacement structures

in hematite crystals (in type B and C)

In samples of group A limonite occurs interstitially, along grain boundaries and in voids. The voids are parallel to layering and foliation (Fig. 3). In samples of group B limo- nite occurs interstitially in between hematite grains and as local cloudy replacements, intracrystalline in hematite grains (Fig. 4). In samples of group C limonite occurs interstitially and locally along grain boundaries.

The microstructural distribution of limonite in the hematite aggregates suggested that some of the ore-types might not be very resistant against abrasive processes. Especially in samples from group A individual hematite grains are

236

completely surrounded by limonite. Progressive release of such limonite-surrounded grains (Fig. 2) by flowing water or another abrasive agent seemed feasible, and this sugges- ted that it would be useful to perform tests to measure the abrasive resistance of the ores. The microstructure of group A seemed unfavourable, group B favourabte and group C intermediate with respect to this property .

The microscopic observations on the studied samples are summarized in Table 2.

E x p e r i m e n t s o n the c h e m i c a l s t a b i l i t y o f i r o n - o r e i n s e a w a t e r

Chemical interaction between iron-ore and seawater was tested in leaching experiments on a laboratory scale. The material was ground to increase the contact area between seawater and ore. For the determination o f the iron concen- tration in seawater, flameiess atomic absorption spectro- metry was choosen. This technique allows concentrations of about 10 microgram per liter to be measured in very small volumes of solution with reasonable accuracy

(-+ 10 %), but does not discriminate between dissolved iron or dispersed colloidal l imonite.

Samples of the three groups of ore, A, B and C were ground and sieve fractions of, initially, 0.14).2 mm and finally, 0.5-1.0 mm were used for the leaching experiments. In the initial experiments a t tempts were made to wash the fines (limonite) off the powder before starting the tests. Sea- water and powder were gently shaken during these tests. Attr i t ion of the powder during shaking caused limonite to disperse in the fluid. The never ending release of limonite from the powdered iron-ore rendered this type of experi- ment useless. It was not possible in this way to distinguish between increase of iron concentrat ion in the solution due to mechanical abrasion or due to chemical dissolution. To circumvent the problems met in the initial experiments, column experiments were set-up. In this way individual powder particles could not move with respect to each other to avoid abrasion effects. Seawater could be added slowly to the powder in the column and the possible dis- solution could take place in a static environment. Seawater replenishment to avoid saturat ion of the solution, could take place.

Tab. 2: Summary of petrography of hematite iron-ore

sample hr. mineral

A SW 1

ASW2

ASW3

B 1

B2

C1

C2

C3

hematite (hm) limonite (lira) talc voids

hematite limonite voids

hematite limonite voids

hematite magnetite goethite limordte

hematite magnetite (mt ) goethite limonite voids

hematite magnetite limonite voids

hematite magnetite limonite

hematite magnetite goethite limordte voids

vol. %/1

40-60 15-20

+ 0-25

75 20

5

75 20

5

85 5

+

10

7O 15 +

15 +

70 20 !0 +

80 5

15

80 +

+

20 +

grainsize ~m) average

100 502

100

75 502 50

60 502

100

80 60 10 502

80 80 10 60

200 150 2002 100

75 50 502

r a n g e

50-300

100-mm

25-200

50-100 25-3002 50-mm

75-100 50-100

10-1002

75-150 75-150

50-1002 50-mm

100-400 100-200 100-5002

50-200 10-100 10-2002

40 20-50 40 10 202 10-1002

50-200

microstructure

schistose, pores lim : interstitial, parallel grain boundaries, parallel foliation, in pore spaces

i schistose, voids parallel to foliation and layering. tabular hm

I lira : interstitial, parallel grain boundaries

schistose, lira : interstitial and along g~ain boundaries

massive, layered lim : interstitial and as "'cloudy" replacement of hm.

massive interlocking aggregate hm - m t - l i m

I mt locally replaces to goethite. Lira and voids present around a vein with coarse h m

layered (compositional) locally much rot. intergrowths of mt-hm. one layer has much lim along grainboundaries

elongated hm grains l im: interstitial. Very locally along grainboundaries. Locally as "cloudy" inclusions in hm.

interlocking aggregate l im-hm locally pores and locally very rich in ~m. (25%)

1) volume percentages estimated with density diagrams. 2) grain size of interstitial aggregates of cryptocrystalline limonite.

237

About 40 g (e 15 ml) of material (sieve fraction 0.5- 1.0 mm) was rinsed with water in a 250 ml flask, to remove the adhering "fines" formed during the grinding, until fur- ther washing had no visible effect and the solution was more or less clear. Each of the sample materials was poured in a glass column (�9 1 cm) up to a height of 15 cm. The bottom side of the column was provided with a porous stopper and could be closed with a tap. Waterflow through the columns was regulated with a 4 channel peristaltic pump (range: 150 ml/min - 1 ml/min). After pouring the powder the columns were flushed with seawater for 10 mi- nutes at 100 ml/min. Then the experiments and sampling for atomic absorption analysis commenced. Samples of eluate (0.5 ml) were taken at the outlet of the peristaltic pump. They were mL,~ed with 0.5 ml diluted nitric acid (10 ml concentrated acid per litre) for conservation pur- poses and analysed for the iron content. To prevent growth of algae the columns and seawater supply were protected against light. The seawater used was decanted Eastern Scheldt water (pH = 8.25).

Initially the seawater was eluted constantly with a velocity of 1 ml/min. After three days the iron release in the solu- tion decreased and no significant increase of Fe with res- pect to seawater-proper could be measured. Then the flow of seawater was stopped. Only after longer periods of sta- tic contact of seawater and ore-powder did the iron content in the seawater increase in sample B. No significant increase was noted in samples A and C. Sample B apparently released 9 /2g/I/hour of iron into the seawater (Fig. 5).

13 5

Fig. 5 :

0

0

1 I 5 I0

"-IME [OARS}

15 70

Results of leaching experiment of hematite-ore powder (500 urn-1 /~m) in Eastern Scheldt seawater. Only sample B showed Fe dissolution.

If dissolution is considered as a surface process, the thick- ness of the dissolved layer after 200 years can be calculated to be about 1/am. Even if in practice the dissolution would be a factor 1 000 higher, this still is quite negligible. These laboratory experiments, of course, did not exactly simulate the natural physical-chemical conditions in the Eastern Scheldt and no attention has been paid to possible bacterio- logical activity.

Examination of the sieve fractions (0.5-1 mm) before and after the leaching experiments with the reflection micro- scope showed no obvious effects on sample material A. Only some limonite was deposited on the outer boundary of the powder grains (Fig. 6). Examination of material B showed a marked difference. During the contact with sea- water new, rusty brown, grains developed, identified as aggregates of limonite. In contrast with B, the quantities of limonite in the grains of group C appeared to have decreased. In samples A and C rusty limonite aggregates

Fig. 6 : Photomicrographs of iron-ore powder (A) before (a) and after (b) the leaching experiment. The precipitation of limonite along the powder-grains after the experiment can be clearly seen (b).

were already present before the chemical leaching. In sample C the number of limonite aggregates appears to have increased. Of course, the same grains could not be studied under the microscope before and after the experiment and only limited value should be placed on these observa- tions (Table 3).

Mechanical stabili ty o f the iron ore

Problems met during the washing procedures to remove the "fines" showed the continuous release of limonite from the iron-ore grains due to mechanical attrition. The microscopic observations pointed to a weak microstructure for samples of group A, with large interstitial limonite aggregates, voids and limonite along grain boundaries. Therefore, two types of experiment have been performed which give an impres- sion of relative durability of the rock.

First sand-blast tests have been performed on specimens of group A and B. The flat surface (q) 60 mm) of a test sample is abraded by sand blast under three atmospheres air pressure, using 3.5 kg quartz sand. The resulting weight loss was measured (ASTM (418-68)). The results (Table 4) show a marked difference between samples A and B. In

238

Tab. 3 : Observations on sieve fractions (500/am-1 mm) before and after leaching experiment with seawater

l. Observations with the unaided eye

before after

A. particles of ore, grey colour, metallic lustre. A few rusty-brown idem. metallic lustre higher. A few rusty-brown particles. particles

B. particles of ore, grey with reddish shade idem. somewhat higher dull metallic lustre, less reddish, some rusty- brown particles

C. particles of ore, dull grey. A few rusty-brown particles idem. a higher number of rusty-brown particles, some are clot.gated

2. Microscopic observations on polished sections

before 1 after

A. Very irregular ragged particles. Coarse voids Filled with limonite. Limonite has precipitated on the outside of the particles. No further Along the rims of the particles hematite grains apparently are / difference observed. detached along their grain boundaries

B. Irregular particles, much magnetite present. Particles are equidi- Many particles appear internally richer in limonite. Limonite occurs mensional and elongated. Besides particles with high timomte con- I along rims of some particles. In one polished section a rusty-brown tent also particles with a very low limonite content particle is present. This consists entirely of limortite

C. Irregular, ragged particles some with a high limonite content (grain Irregular, ragged grains. Limonite has precipitated on the outside of size of interstitial limonite aggregates is high). Magnetite is present, some particles. Percentage limonite within the particles appears lower. Limonite is commonly in contact with the surface of the particles.

Tab. 4 : Results of sand blast tests

For comparison the values for good quality concrete are given.

material weight loss average weight loss (g) (g)

A

B a

b C

concrete

75.3 77.3

100.7

8.1 7.5 8.0

10-20

84.4

7.9

this respect group A ores are markedly weaker than most rocks and concrete.

The slake durability test (Franklin and Chandra, 1972) is specially designed for clay-bearing rocks, but was also used to test the ores. It attempts to test the combined effect of attrition wear and slaking (cycles of wetting and drying). The test is performed in a 2 mm standard mesh drum, which rotates (20 r.p.m.) about a horizontal axis in a trough f'dled with water to a level 2 cm below the rotation axis. The drum is f'dled with about 10 lumps of ore (O 2.5 cm). The lumps of ore tumble against each other during rotation and lose material. Fragments smaller than 2 mm fall through the mesh. After each cycle the drum is dried (24 hours at 105~ and weighed. The test was not perfor- med in the way suggested by the ISRM (Brown, 1981), namely 2 cycles of rotation of 10 minutes with subsequent drying. Instead 9 cycles of 30 minutes were performed, to establish an equilibrium desintegration rate.

All groups (A, B, C) classify as "very highly durable" in the slake durability classification (Brown, 1981). Fig. 7 shows, however, the relative weakness of group A with respect to B and C material. The f'me material released during the tests consisted of hematite grains and limonite.

(2?

T

It;

z w

cE w o_

2 . 0

1.5

1.0

0 . 5

o

~ o ~

\ ~ o - o o--

! I I I I I I I

0

Fig. 7 :

0.5 1 1.5 2 2.5 3 3.5 t. z..5 SLAKING TIME (HOURSI

Percentages weight loss of the three groups of ore during the "slake durability test".

D i s c u s s i o n

The resistance against abrasion (Table 4, Fig. 7) of the ore is a function of limonite content (Table 2), but also of the microstructural distribution of limonite in the ore. The results of the abrasive durability tests show a quali- tative correlation between microstructure of the ore and release of fine-grained material. If hematite grains are completely surrounded by limonite, these grains can be easily released from the structure. If limortite is in contact

239

with the surface of the ore lumps it easily can be washed out, as shown by the chemical leaching experiments. Redis- tribution of limonite during the chemical tests was pro- babily mainly by the action of flowing water. If a zone of weakness in the rock (e.g. schistosity planes coated with limonite, zones with limonite along hematite grains etc.) is in contact with the surface, progressive release of limo- nite may occur, which may eventually lead to disintegration of the ore lumps. The schistose ore of group A suffers parti- cularly from this microstructural drawback. In the massive ore of group B the limonite occurs at distinct interstitial places in the structure or within hematite grains and thus has no deteriorating effect on the mechanical stability.

Of course the chemical and abrasion tests performed on the iron-ores give only an indication on the expected behaviour of the rock-fill, ff used in the Eastern Scheldt Storm Surge Barrier. In situ tests, to monitor release of limonite in the actual environment, however, have not been performed. It was decided not to use the ore, since new data on the expected water current velocities during the construction phase of the barrier indicated even higher peak velocities. Another engineering solution, involving anchored gabions, is prefered at this stage to fill the gap between adjacent foundation mattresses.

Conclusions

Petrographic investigation proved a major aid in studying the suitability of hematite iron-ore for use as a high-density rock-fill material in the Eastern Scheldt Storm Surge Barrier. The laboratory research has not indicated major chemical activity in seawater, although one of the groups showed some. A major drawback of the ore is the release of f'me-grained material, limonite with hematite grains, during abrasion. It is possible that the fine limonite par- ticles could cause visual pollution of the environment of the type visible along railway tracks (rusty dust on pave- ment and rock-f'dl). The massive type of iron-ore suffers

least from these drawbacks and might perform satisfactorily if used as a high density rock-fill in seawater.

Acknowledgements

The permission of the "Deltadienst Rijkswaterstaat" to publish the results of this work and the kind cooperation of Mr. G.J. Laan (Rijkswaterstaat, Wegbouwkundige dienst), is gratefully acknowledged. Discussions with Dr. R.A. Ktihnel on the behaviour of iron hydroxides are appreciated. The assistance of the personnel of the Depart- ments of Mining Engineering and Chemical Technology, with special mention of Mr. W. Verwaal, was invaluable. Mrs. S. van Adrichem-Hagen cheerfully typed several ver- sions of the manuscript,

References

BROWN E.T. (1981): "Rock characterization, testing and moni- toring. ISRM suggested methods". Pergamon Press, Oxford, UK, pp. 92-94.

DORR John Van N. and MIRANDA BARBOSA A.L. de (1963): "'Geology and ore deposits of the Itabixa District, Minas Gerais, Brazil". Geological Survey Prof. Paper 341-C, 109 pp.

DORR John Van N. (1964): "Supergene iron ores of Minas Gerais, Brazil" Kconomic Geology, 59, pp. 1203-1239.

DORR John Van N. (1973): "Iron-Formation in South America". Economic Geology, 68, pp. 1005-1022.

FRANKLIN J.A. and CHANDRA R. (1972) : "The slake-durability test". Int. J. Rock Mech. Min. Sci., 9, pp. 325-341.

KRAUSKOPF K.B. (1979): "'Introduction to geochemistry", 2nd edition, McGraw-Hill, New York, pp. 211-214.

LANGMUIR Donald (1971): "Particle size effect on the reaction Goethite = hematite + water". A. J. Sci., 271, pp. 147-156.

OFFRINGA G. (1983): "The Storm Surge Barrier in the Eastern Scheldt". PubL by: Dosbouw v.o.f., Burgh-Haamstede (the Netherlands), pp. 39.