Developments in industrial mineralogy: I. The …pygs.lyellcollection.org/content/pygs/49/2/95.full.pdf · naturalist and environmentalist they form the backbone of the landscape;

PROCEEDINGS OF THE YORKSHIRE GEOLOGICAL SOCIETY, VOL. 49, PART 2, PP. 95-104, 1992

Developments in industrial mineralogy: I. The mineralogy of brick-making

A . C . DUNHAM

(Presidential Address delivered at York, 9th December, 1989)

Department of Geology, University of Leicester, University Road, Leicester LEI 7RH

SUMMARY: Fuels, metals and non-metallic rocks and minerals have all contributed to the wealth of the United Kingdom, though in different proportions during different periods of history. Statistical returns show the very low level of metal production, and a waning production and value of all types of fuel at the present time. Only the exploitation of industrial rocks and minerals shows continuous growth. The transformation of clay to bricks is used to show how the application of "pure" mineralogical techniques can describe and explain the changes on firing, and lead to improved industrial firing procedures. Chemical and mineralogical analyses of brick-clays demonstrate a wide range of composition, with quartz and smectite decreasing with age and illite and chlorite increasing. Bricks reflect this variation in the mineral assemblages found after firing. The effect of the length of the firing cycle on the final mineral assemblage is discussed using pure kaolinite as an example.

Rocks satisfy different needs to different people. To the naturalist and environmentalist they form the backbone of the landscape; to the geologist they constitute the evidence from which to reconstruct the history and origin of the earth; to the applied geologist they form the milieu and raw materials to make the structures and artefacts of modern civilisation. All too often these three views seem to be in total opposition, but it is of increasing importance that all views play their part in geology more harmoniously in the future. An inescapable need, for present and future, is a good supply of raw materials with the right properties to enable the manufacture of almost everything we use. At the same time, these resources must not be developed in ways which destroy vital parts of the environment. The most efficient (in all senses) development will rely heavily on geological evidence, which can account for the origin and distribution of the family of rock types. This should enable improved exploration methods and better value judgements to be made concerning the future development of all types of natural geological resource.

In this address I review recent work on one type of raw material, Common Clay and Shale, and the mineralogy and chemistry underlying its use in the manufacture of bricks and tiles. The work presented here is based in part on the Minerals Industry Research Organisation research programme in the University of Leicester; this is abbreviated to LU-MIRO on later pages.

The change from clay to brick is very like the process of high-temperature thermal metamorphism, but with one important difference. In the geological situation, the length of the heating event is measured in tens to many thousands of years whereas in brick-making the entire heating cycle is complete within a day or two. Thus, a study of the mineralogical changes provides a useful insight into the early thermal changes rarely recorded by nature, although geologically stable assemblages of minerals may not be attained. In addition, it is useful for the brick-maker to know what minerals are present in his raw materials, and what they are transformed to in the final product. This should enable much better prediction about the behaviour of a new clay, and possibly a reduction in the firing time, with consequent savings in fuel and volume of noxious emissions.

© Yorkshire Geological Society, 1992

1. RAW MATERIALS

Bricks have been made from a variety of fine-grained, clay-rich rocks from almost every geological system. The properties required of a brick-clay include the ability to be moulded or pressed into a suitable shape that is retained without distortion or excessive shrinkage on drying and firing, and the appropriate mineralogy to change into new phases with a desirable overall colour, compressive strength, water absorption and durability after firing.

The ideal clay deposit is at or very close to the surface, so that it can be easily quarried. Reserves must be sufficient to justify the investment in the brick-making plant, fuel, transport and staff wages. The deposit must be as uniform as possible in its mineralogy to minimise problems in blending the raw materials and in control of the kiln temperatures. In addition, it is desirable that gypsum, pyrite, and siderite and calcite nodules are absent (Carp 1987; Prentice 1988).

The minerals in brick-clays belong to seven major groups. /. Quartz is a major constituent of all brick-clays. 2. Clay minerals form the second major constituent. Six assemblages are commonly found in United Kingdom brick-clays:

a. Illite — Kaolinite — Chlorite — Smectite b. Illite — Kaolinite — Chlorite c. Illite — Kaolinite— —Smectite d. Illite — Kaolinite e. Illite — — Chlorite — Smectite f. Illite — — Chlorite

Vermiculite and sepiolite occur sporadically. Mixed-layer clays are also commonly found; the bulk of the smectite occurs in this way, but it is never a major clay constituent. Illite is present in all assemblages and chlorite is present in almost all. Kaolinite is absent from some of the geologically older brick-clays. The main variation within, and between, many geological formations lies in the kaolinite/illite ratio, and the quartz/total clay mineral ratio. 3. Feldspars are usually present, though never in large amounts, as both plagioclase (generally sodic) and K-feldspar. 4. Carbonates are important in some clays (e.g. calcite in the Oxford and Gault clays and dolomite in the Mercia

by guest on September 5, 2018http://pygs.lyellcollection.org/Downloaded from

http://pygs.lyellcollection.org/

96 A. C. DUNHAM

S i o 2 A

3 02C a O + M g O F e

Mudstone). Carbonates may cause problems during firing if the evolved carbon dioxide cannot escape. Fine grinding may be required, so that C 0 2 can escape easily without decrepitation of the carbonate minerals. 5. Iron-bearing minerals may include oxides (hematite, goethite, amorphous oxide/hydroxide), carbonates (siderite) or sulphides (pyrite, marcasite). Siderite is the most common iron-rich phase in the Carboniferous mudstones, whereas oxides or hydroxides tend to dominate in the other formations. Pyrite can be a deleterious phase if sulphur is unable to escape on firing. 6. Minor phases include anatase, gypsum, and apatite. Gypsum causes problems with scumming; soluble alkali sulphate may form, which causes efflorescence. 7. Organic material is present in many brick-clays (e.g. Oxford Clay), and may provide an important fuel element.

In the recent LU-MIRO study of 243 brick-making clays representing almost all the currently used raw materials in the United Kingdom forty-six different mineral assemblages were recorded. Quartz, illite, apatite and anatase were common to almost all. Thirty seven of the assemblages were found in only one geological formation, eight in two and only one in three formations, indicating considerable scope for the development of a wide range of mineral assemblages following firing of the clays.

The variability of both the mineral assemblages and the mineral proportions gives rise to considerable variation in the chemical composition of the brick-clays. The analyses assembled by Ridgeway (1982) are plotted in Figure 1, with the compositions of the minerals found in British brick-clays. Figure 1A shows the variations of the A^CtySiC^ ratio as a result of varying proportions of quartz and clay minerals,and the tight control which the two K-bearing minerals, muscovite/ illite and K-feldspar, have over the spread of data points. The effect of the presence of carbonates is shown in Figure IB, where the alumina/silica variation is still visible, but there is a further spread of data points towards the carbonate corner. Figure 1C shows the variation of A1 2 0 3 , CaO+MgO and F e 2 0 3 . This plot is useful in spreading out the range of compositions found in brick-clays; these oxides are the most abundant in clays after silica, with the exception of carbon dioxide in some. This plot is used later to illustrate the development of different mineral assemblages from raw materials with different starting mineralogies. Note that all are saturated in silica, so that quartz is present in all compositions shown, in addition to the minerals shown in Figure 1C. The five oxides used in these plots generally account for about 90% of the total, though there is a variation from 69 to 97% in Ridgeway's (1982) compilation.

In spite of the variations in chemistry and mineralogy demonstrated above, a number of estimates have been made of the average composition of mudrocks, shales and slates (Clarke 1924; Pettijohn 1975; Ridgeway 1982). Rather surprisingly, they are very similar to averages computed from Ridgeway's data (1982) and from the analysis of 243 samples in

Fig. 1. Plots of the bulk chemistry of brick-clays in the United Kingdom. Data from Ridgeway (1982). Also plotted as solid squares are the positions of the minerals found in the clays: quartz (Qz), albite (Ab), anorthite (An), K-feldspar (Kf), kaolinite (Ka), chlorite (CI), smectite (Sm), muscovite-illite (M-l), calcite (Ca), dolomite (Do), siderite (Si), goethite (Go), apatite (Ap).



MINERALOGY OF BRICK-MAKING 97

Table 1 Average shale and slate compared with two estimates of average brick-

clay in the United Kingdom Average Average UK brick-clays

shale slate (Clarke (Pettijohn (Ridgeway LU-MII 1924) 1975) 1982)

2 Si0 58.1 58.5 57.0 57.5 2 T i 0 0.6 0.8 0.9 1.0

3 02A l 15.4 17.3 17.7 17.1 3 02F e 4.0 3.0 6.2 6.3

FeO 2.4 4.4 MnO tr 0.1 tr 0.1 MgO 2.4 2.6 2.2 2.0 CaO 3.1 1.3 3.5 3.4

0 2N a 1.3 1.2 0.6 0.6 0 2K 3.2 3.7 2.7 2.8

P2O5 0.2 0.1 tr 0.1

2 co 2.6 1.2 3 S 0 0.6 0.3 LOI 8.7 8.5

C 0.8 1.2 0 2H 5.0 3.9

LOI: loss on ignition

the LU-MIRO brick study. Two earlier estimates are compared with these two sets of analyses related solely to brick-clays in Table 1. The most significant differences lie in the N a 2 0 and K 2 0 contents; the brick-clays have less of both oxides. A number of authors have also commented on an apparently regular change in the mineralogy with age; kaolinite and smectite are more abundant in younger rocks whereas illite and chlorite are more abundant in older rocks (Potter et al. 1980; Blatt 1982). The change from smectite to illite observed in many oil reservoir sandstones and associated mud-rocks as depth, temperature, pressure and age all increase, has excited much interest {e.g. Jeans 1989; Muan et al. 1985; Freed & Peacor 1989). A change of mineralogy may imply a change in chemistry or it could be isochemical To examine the variation in composition with the time in brick-clays the averaged data from the LU-MIRO study for both oxides and minerals is shown in Figure 2. Each data point represents an average of between 2 and 54 analysed samples. Silica is the only major oxide to decrease with increasing sample age, though with great scatter in the data in more recent times. A1 2 0 3 , F e 2 0 3 , N a 2 0 , K 2 0 and MgO all show increases with time. The loss on ignition data is very scattered and the organic carbon very low.

These variations in chemistry are the result of changes in the mineralogy. Quartz and smectite show an overall decrease with increasing age of sample, but illite and chlorite both increase with age. Kaolinite occurs in very variable amounts with no obvious trend. Calcite and dolomite are also rather irregular but decrease with increasing age. Siderite has a peak in the Carboniferous. Plagioclase appears to increase while K-feldspar shows no clear trend.

The statistical significance of these trends has been examined by calculating the correlation coefficients on standardised data between mean geological formation age and the mean percentage of a mineral or oxide in each formation. The results are shown in Table 2. This is divided into four sections, depending on the level of significance of the correlation coefficient, evaluated using Student's t test. Only chlorite and illite show correlation coefficients with formation age significant at the 0.1% level, though quartz lies between

the 0.1 and 1% levels. Illite is also strongly correlated with chlorite (r = 0.83), K 2 0 (r = 0.93), A1 2 0 3 (r = 0.64) and F e 2 0 3

(r = 0.63). Plagioclase, smectite and calcite lie between the 1 and 5% levels so may still be significant. All the other phases show rather poor correlation with age. Amongst the oxides, A1 2 0 3 , F e 2 0 3 , K 2 0 and MnO show highly significant correlations with age (significant at the 0.1% level). CaO and inorganic carbon lie in the 1 to 5% region with N a 2 0 , P 2 0 5 , total carbon and S only just below the 5% level.

These results confirm the trends indicated in previous work. How can these trends be explained? There are many factors at work controlling the mineralogy of a sediment. The minerals may be detrital (e.g. quartz, clay minerals, feldspars, calcite), biogenic (calcite, organic material), or the results of cementation and later diagenetic changes (calcite, smectite conversion to illite). For these clays, chosen because bricks are or have been made from them, there are both marine (e.g. Oxford Clay, Gault Clay) and non-marine or brackish clays (e.g. Weald Clay, glacial clay) and possibly wind blown material (loess). The source areas of many will have been different, and the sampling sites within the same geological formations are at different distances from possible shore lines. It seems rash, under these circumstances to try to derive any conclusions about the changes of clay compositions with time. However, a suite of trace elements is currently being determined on the 243 samples collected. These, with careful

Table 2 Correlation coefficients between geological age and the mineralogy

and chemistry of United Kingdom brick-clays Mine

Positive ralogy

Negative Cher

Positive nistry

Negative

(Group A) Chlorite 0.73 Illite 0.69

A 1 2 0 3 0.78 F e 2 0 3 0.78 K 2 0 0.64 MnO 0.61

(Group B) Quartz -0.50

(Group C) Plag. Feld 0.35 Smectite -0.39

Calcite -0.36 CaO -0.37 Inorg. C -0.34

(Group D) Fe-oxide 0.29 Anatase 0.28 Apatite 0.16 Pyrite 0.13

Dolomite -0.26 Kfelds -0.15 Siderite -0.10 Gypsum -0.09

N a 2 0 0.32 P 2 O s 0.32 T i 0 2 0.18 MgO 0.06

Total C -0.33 S -0.32 LOI -0.28 S i 0 2 -0.27 Organ.C -0.21

Group A: Less than 1:1000 probability of correlation by chance Group B: Between 1:100 and 1:1000 probability of correlation by

chance Group C: Between 1:20 and 1:100 probability of correlation by

chance Group D: Greater than 1:20 probabililty of correlation by chance

The critical values for the correlation coefficients, with 32 degrees of freedom are:

Probability Level 0.1% 1.0% 5.0%

Correlation Coefficient 0.54 0.44 0.34

Data from the LU-MIRO Brick Research Project



98 A. C. DUNHAM

1 5

10-1

Wt%

5 4

• K 2 0 ° CaO • Fe 2 0 3 o MgO

6 0 1 • Quartz • Smectite o illite • Chlorite

4 0 4

Wt%

2 0 4

0

8 0 - ,

6 0

Wt%

4 0 -

2 0 -

MgO^ o -<$>'

o & o - ^ -

•Q<* o

Chlorite

Quartz

-o • Illite

«? - OO —

Smectite - •"

1 0 n

• S i0 2

• AI 2Q 3

• S i0 2 _ _ - - 54

Wt%

T"-' i - - l _ ^ V - . „ 54

0

2 0

1 5 4

Wt%

1 0 -

5 -

• LOI A Organic C

Plagioclase

K-feldspar

V

o

4 0 !

0 4 • A . A mi A mi ^-J^Jk * r ^L—jfo£m

2 0

Wt%

0

1 0

1 4

• Na 2 0

•

Wt% •

104

5 0 0 3 0 0

Age (Ma) 1 0 0

Calcite

Dolomite

Siderite

• •

5 0 0 3 0 0

Age (Ma) 1 0 0




selection of samples, and integration of sedimentological and faunal information should allow more specific conclusions to be drawn about changes with time in composition of clays from specific environments. We should note the fact, vital to the brick-maker, that there are changes in chemical and mineralogical composition with time. Neither the rheological properties nor the firing behaviour of an Ordovician mudstone is likely to be the same as the Oxford Clay or a Recent glacial lake clay.

2. MINERALOGICAL CHANGES ON FIRING

2.1. The minerals in bricks The minerals found in a brick depend on a number of factors. These include the chemical composition and mineralogy of the raw materials, the firing temperature, the length of the firing cycle, and the atmosphere under which the firing is carried out. Table 3 shows the minerals found in bricks made from a variety of geological formations, and has been compiled from X-ray diffraction analyses of 60 bricks, representing the 12 ages. Other phases may be present, but in very small amounts (<3%). Moreover, assemblages can differ within a given formation.

A number of phases are present in all the bricks examined. Two types of amorphous material may exist together or singly. These are phases within which the crystal structure has been destroyed, so that no characteristic X-ray peaks can be observed, though a broad hump is usually present in the X-ray trace (e.g. the change from kaolinite to metakaolinite), and glass, representing melt formed during the firing process. Quartz also occurs in all samples; this is a residual phase and normally less abundant in the brick than in the raw material. Hematite is also present in all examples, and imparts the red colour to many bricks. Even in yellow bricks hematite is still commonly present (Drakapoulos 1988), though in smaller amounts. In bricks fired in a reducing atmosphere, however, magnetite may take the place of some or all of the hematite. The black cores to some bricks generally owe their colour to the reduction of iron, where sulphur, for example, was unable to escape.

A number of phases originally present in the raw material may survive firing, in addition to quartz. In bricks that are underfired illite is the most persistent of the clay minerals (Grim 1968; Onike 1985). Calcite is also found occasionally, but may result from the recarbonation of lime. Both types of feldspar may survive, but new feldspar, particularly anorthite in Ca-rich materials, may also be formed during firing. Anatase is a common survivor, like zircon, which is usually present in amounts too low to be detected by X-ray diffraction, but has been found on petrographic examination.

Mullite may form on the breakdown of clay minerals (e.g. kaolinite breaks down to mullite and cristobalite). Where present its needle form imparts strength to the bricks. It may also precipitate from the melt phase. From more calcium-rich raw materials a number of new minerals are formed: diopside, wollastonite and melilite. Magnesium-rich compositions may produce forsterite. Cordierite is a rare constituent of bricks; it

Table 3 Minerals found in bricks

Formation 1 2 3 4 5 6 7 8 9 10 11 12 Amorphous x x x x x x x x x x x x Quartz x x x x x x x x x x x x Cristobalite - - (x) x - - - x - (x) - -K-feldspar x x x (x) (x) x x - - x x x Plagioclase ( x ) - - - - x - - x - - -Pyroxene - - - - ( x ) x - - x - - -Wollastonite _ _ _ _ X - - - x - x -Melilite - - - - x ' x - - x - - ( x ) Mica - - - - - - - - x -Mullite x x x x — — — x — x — — Forsterite _ _ _ _ ( x ) - - - - - - -Cordierite - - (x) - - - - - - - - -Anatase x - ( x ) - - - - - - - - -Hematite x x x x x x x x x x x x Ilmenite (x) x (x) (x) - - - - - - - -Anhydrite _ _ _ _ _ x - - - - - -

7 Wadhurst Clay 8 Weald Sands 9 GaultClay

10 Weald Clay 11 Glacial lake clay 12 Loess

Key: 1 Ordovician 2 Silurian 3 Carboniferous shales 4 Etruria Marl 5 Mercia Mudstone 6 Oxford Clay

x present in all samples examined, (x) occurs in some samples examined,

absent.

generally forms at temperatures above that of normal brick-firing (1000-1100°C).

Cristobalite is the most commonly occurring high-temperature silica phase. Tridymite might be expected from examination of the "normal" phase diagram for the silica minerals (Tuttle & Bowen 1958). The kinetics of reactions presumably play an important part in the nucleation of cristobalite rather than tridymite, perhaps due to the similarity of the ring structures of silica tetrahedra in cristobalite with the rings in the silica layers of clay minerals.

Anhydrite may form by the breakdown of calcite to form lime, followed by the reaction of the lime with S 0 2 produced on the breakdown of pyrite or marcasite in the raw material. Gypsum is usually present in the raw materials for bricks with anhydrite, so a second source may be dehydration of the gypsum.

2.2. Stages in the transformation of clay to brick The heating process begins with the drying of the green brick to drive off the absorbed water. The dried brick then enters the main kiln, where it is gradually raised to an appropriate soaking temperature, and is held there for a period, after which it is allowed to cool gradually. The time taken for this cycle is very variable, but the total time within the kiln is generally 24-48 hours, reaching temperatures generally less than 1100QC. Six transformation stages can be recognised:

1. Dehydration of clay minerals, gypsum and iron hydroxides

Fig. 2. Plots of mean oxide and mineral values against time. The data is from the LU-MIRO research programme. Each data point represents a mean of from 2 to 54 analysed samples. The percentages of minerals were obtained by a combination of quantitative X-ray diffraction analysis and normative calculation, using the minerals known to be present in each rock from the X-ray diffraction analysis and by other means.



100 A. C. DUNHAM

Raw Material

Quartz

l l l i le/Mica

Chlori te

Kaolinite

K-Feldspar

Plagioclase

Calci te

Siderite

Gypsum

Pyrite

Fe-oxide/hyd.

Organic material

200

Approximate Temperature (°C)

400 600 B00 1050 800 600 400 200 Brick

°ZL,^ Y/////////Z^

//////A ////'//////////

(CO,) ,

-«=£zZZZZZ2

.Z' Burn-ofl ////S/////S///

Time (hours) 0

Stages

24 ^ 36 Soak

5 —•+*

Quartz

Crislobal i te

Mullite

K-Feldspar

Glass

Plagioclase Wollastonite

Pyroxene Meli l i te Anhydri te

48

Fig. 3. The changes in mineralogy on firing. This cartoon follows the firing cycle from the raw minerals, on the left of the diagram up a 24 hour ramp to the soak temperature (here represented by a 12 hour soak at 1050°C), followed by a 24 hour cooling period to the brick, represented by the minerals on the right hand side of diagram. (OH, C 0 2 , S) in brackets represent the points where these species are evolved from a mineral. Element symbols with dashed lines indicate where elements are liberated from one mineral to form another. The numbered stages are discussed in the text.

2. Loss of C 0 2 , S and hydrocarbons 3. The alpha/beta-quartz transition 4. Solid-state mineral reactions 5. Melt production 6. Reactions on cooling These stages can be followed in Figure 3, which shows the

mineralogical and chemical changes taking place during firing of clay.

2.2.1. The dehydration of clay minerals, gypsum and iron hydroxides

The dehydration of relatively pure clay mineral samples takes place over different ranges of temperatures depending on the clay minerals present (Grim 1968). Kaolinite loses its structural water around 500°C, changing to metakaolinite, which X-ray diffraction shows to be almost structureless (Redfern 1987). Chlorite loses its water at a higher temperature, generally beginning around 750°C, but the temperature depends on the mineral chemistry. Illite loses its water at even higher temperatures, retaining recognisable X-ray diffraction peaks up to around 1000°C (Onike 1985). The evolved gases are lost to the atmosphere. These dehydration reactions may be important to subsequent reactions in that the collapsed structures may allow reorganisation within themselves to form other phases (e.g. metakaolinite changing to mullite and cristobalite), or they may react with other phases to form new ones (e.g. metakaolinite plus lime forming pyroxene). However, differential thermal analysis (DTA) combined with thermo-gravimetric analysis (TG) of brick-clays suggests that the dehydroxylation for mixtures takes place in the temperature range from around 450°C to 650°C. Two contrasting DTA and TG traces, for Oxford Clay and Etruria Marl, are shown in Figure 4. The loss in weight and endothermic peak is clearly

Fig. 4. Differential Thermal Analysis / Thermogravimetric traces for typical Oxford Clay and Etruria Marl samples. +ve and -ve indicate exo- and endothermic reactions respectively. The dashed line is the base line in an inert sample. The percentage weight loss indicated by the TG curve is shown on the vertical

OXFORD CLAY

1000 900 800 700 600 500 400 300 200 100

Temperature (°C)

ETRURIA MARL

1000 900 800 700 600 500 400 300 200

Temperature C'C)




seen in the Oxford Clay sample. Goethite, or other indeterminate iron hydroxide phases, lose water at around 325°C; the peak is well seen in the Etruria Marl DTA and TG traces. Gypsum, if present, loses its water below 200°C, as shown in the Oxford Clay trace.

2.2.2. The loss of C02, SO3 and hydrocarbons The carbonate minerals lose carbon dioxide on heating, calcite beginning around 650°C and dolomite a little higher. It is not possible to be precise about the temperatures because this depends on a number of factors such as the mineral grain size, the rate of heating, and how easily the evolved carbon dioxide is lost to the atmosphere. A calcite endothermic peak and loss in weight is seen on both examples in Figure 4.

Pyrite loses sulphur on heating, by oxidation, at around 400-450°C (Brownell 1976, fig. 74). This reaction may contribute to the higher-temperature organic material peak in the Oxford Clay DTA/TG trace. Unless reducing conditions are required (to make a very dark coloured brick) it is essential that S 0 3 can escape. In addition, some may react with calcite to form anhydrite. The remaining iron may form hematite or magnetite, depending on the activity of oxygen in the kiln atmosphere.

Hydrocarbons burn off by about 450°C forming an important source of thermal energy in those clays, like the Oxford Clay, which contain significant quantities. It is most important that the gases evolved should be able to escape, otherwise unwanted reducing conditions may pertain.

2.2.3. The alpha/beta-quartz transition At around 573°C alpha(low) - quartz undergoes a structural transformation into beta(high) - quartz. As this transition involves rotation rather than the breaking of bonds the reaction is reversible, so any high quartz present after the high

temperature soak will invert to low quartz during the cooling part of the firing cycle. This is more than just a mineralogical curiosity because of the considerable change in the thermal expansion rates around 573°C. In Figure 5 the change in volume is plotted against temperature for a number of the minerals found in brick-clays. The expansions of quartz and cristobalite are much greater than for most other minerals, but the most significant factor is the very large change during the alpha/beta-quartz transformation. During the rise of temperature this causes the brick to expand, but if cooling is too fast then highly deleterious microcracking can occur (Brownell 1976).

2.2.4. Solid-state mineral reactions, and formation of melt At low temperatures reactions between two solid phases generally take place very slowly. However, around the soak temperatures reactions may occur much more readily. In addition single minerals, pairs or even larger numbers of mineral phases may begin to melt at eutectic points or cotectic phase boundaries. The nature of reaction products is reviewed by Grim (1962,1968) and by Onike (1985).

The clay minerals, particularly illite and kaolinite, begin to form new minerals at around 900°C. Exothermic peaks can be seen on both DTA traces in Figure 4 corresponding to this change. Following dehydroxylation, the kaolinite structure degrades so that there are very few, weak, characteristic X-ray diffraction peaks on a broad hump; at around 900°C mullite, and spinel or gamma-alumina are formed. Between 1000° and 1100°C cristobalite joins the mullite (Onike et al. 1986). The melting temperature of the reaction products formed from kaolinite (c. 1590°C) is well above any normal brick-firing temperature. Illite keeps a recognisable structure to around 1000°C, when a number of new phases may be formed, depending on the composition and structure of the original mineral and the length of heating of the sample. These phases

Fig. 5. Plots of temperature versus percent volume expansion for a number of minerals found in brick. Data from Clark

Temperature (°C) ( 1966 ) .



102 A. C. DUNHAM

A 3O2A I

3 02CaO+MgO F e

Fig. 6. (A) Plot of the range of bulk chemistry for Coal Measures shales from Ridgeway's 1982 compilation, with the minerals present in the raw material (in brackets) and the minerals found in the bricks (indicated by symbols inside the main triangle). A? marks the possible composition of glass. Data from LU-MIRO project.

(B) Similar plot for two different types of clay used at Broomfleet Brickworks (Dunham & Mwakarukwa 1984). Solid symbols inside the main triangle represent the buff brick raw materials, open symbols the red brick minerals. Bb and Br are the bulk compositions of the buff and red brick clays; Ar and Ab are the compositions of the amorphous/ glass phase in the red and buff bricks respectively.

include spinel, leucite, sanidine, corundum and mullite. Illite has the lowest melting temperature amongst the clay minerals, beginning to melt around 1050°C. Chlorite breaks down around 900°C, the new phases including olivine, spinel and enstatite. Finally, smectites begin to form some or all of the following at about 900°C: quartz, cristobalite, cordierite, mullite, enstatite, spinel and anorthite. The above description refers to the behaviour of pure minerals, but in a brick clays reactions between phases and the intervention of a melt may

alter considerably the resulting mineralogy. Those phases found in commercial bricks are listed in Table 3. The kinetics of the reactions are of paramount importance, a topic to be discussed later in this address.

Raw materials that contain calcite produce lime, a most reactive compound. This may contribute to the formation of a variety of new minerals (diopside, wollastonite, gehlenite and anorthite) by reaction with other phases such as the dehydrated clays. Lime may also contribute to the mixture which forms a melt. K-feldspar remains stable until it forms part of the melt possibly below 1000CC. Na-rich plagioclase will also melt at a similar temperature, though Ca-rich plagioclase may persist through the firing process. The hematite/ magnetite formed at lower temperatures may persist through firing, may be taken into solid solution by mullite, or may contribute to the melt. Anatase, apatite and zircon, if present in the raw material, probably also persist through the firing.

2.2.5. Reactions on cooling The melt produced is a most important constituent of the brick. It acts as the binder, imparting considerable strength to the brick on cooling. New phases may crystallise from the melt; a liquid remaining on cooling will form glass. New phases formed from the melt may include mullite, hematite, pyroxene, melilite, anorthite and K-feldspar. The other important mineralogical event during cooling is the beta/ alpha- quartz inversion (see above).

2.3. The dependence of brick mineralogy on bulk composition The change in mineralogy between the raw clay and the fired brick, and the different mineral assemblages produced by firing different bulk compositions can be illustrated using a plot of CaO+MgO - Al 26 3 - F e 2 0 3 . Figures 6A and 6B show the clay bulk compositions, minerals in the unfired clay and the minerals in the bricks for three different starting materials. Figure 6A shows the starting and finishing mineralogy for typical Coal Measures shales. Figure 6B shows a similar plot for two different clay mixes used in the past at Broomfleet, Humberside (Dunham & Mwakarukwa 1984). In the Carboniferous shale the small amount of calcium in the raw material is contained in the glass; there is no plagioclase present in the brick. The mixes of Recent estuarine clays have varying but greater percentages of calcite in the raw material, compared with the Coal Measures shales. The minerals in each of the Broomfleet mixes are the same (kaolinite, illite, vermiculite, plagioclase, K-feldspar, calcite and an iron oxide/ hydroxide), but the proportions are different. The raw material for the buff brick is the richest in calcite, and develops gehlenite, wollastonite, plagioclase (anorthite) and pyroxene on firing. The red brick carries much more mullite, but plagioclase (anorthite) is the only calcium-bearing mineral. Note also the difference in composition of the glassy phases in the two Broomfleet bricks (Dunham & Mwakarukwa 1984).

These two diagrams suggest that the mineralogy of a fired clay should be predictable if the bulk composition and the firing temperature are known. However, firing time can also play an important part in the production of particular mineral assemblages.

2.4. Time-dependent reactions It has already been shown that both time and temperature are important constraints on the production of a specific mineral assemblage from a particular starting composition. The effects




are illustrated using the Time-Temperature-Transformation diagram (TTT-diagram) for a pure kaolinite (Onike etal. 1986; Redfern 1987). The diagram is shown in Figure 7. Each TTT-diagram applies to just one composition, in this case SPS-grade kaolinite from Cornwall, supplied by English China Clays. The diagram is also relevant only for a pressure of one atmosphere. A field on the diagram shows the window of time and temperature within which a particular mineral assemblage is found and a boundary line on the diagram shows the limits of occurrence of a particular phase.

1 5 0 0 - 1

Mullite + Melt

1 0 0 0 -

O o CD i_

»

CO CD Q. E

.CD

5 0 0

M E T A K A O L I N I T E

0.9 N

0.8 „

0.6 >. ^ >• ^ ^ ^ 0.3 " ^ x . 0.2 o.i ^ - " - ~" -

K A O L I N I T E

i i 111 11 i 5 1015

Minutes 30 1 2 5

Hours

Time

12 1 2 4 Days

8 14

Fig. 7. Time - Temperature - Transformation diagram for kaolinite (Onike et al. 1986; Redfern 1987). The solid lines indicate boundaries between mineral stability fields. The mineral assemblage within each field is listed. The dashed lines show the weight fraction of OH lost during the change from kaolinite to metakaolinite.

At low temperatures kaolinite is stable; above about 400°C kaolinite begins to dehydrate. The dashed lines show the weight fraction of combined water that has been driven off (Redfern 1987). Note that these curves slope downwards from left to right; kaolinite can be dehydroxylated at lower temperatures if enough time is available. Above the dehydroxylation zone metakaolinite is the only phase found. This probably consists of small domains of kaolinite within a degraded structure. The dehydroxylation is not complete until much higher temperatures are reached, since the OH-groups diffuse out slowly. The first new phases to appear are mullite and Al-Si spinel (boundary 1), though both are present in rather small amounts until much higher temperatures are reached. The exothermic event on the DTA trace for kaolinite (Figure 8) represents the nucleation of these phases. Boundary 2 represents the upper limit of the presence of kaolinite X-ray diffraction peaks. Boundary 3 is the upper limit to the apearance of spinel. Above this line mullite increases rapidly in both amount and crystal size. The final new phase to appear is cristobalite, which is found in the C-shaped field (boundary 4), typical of homogeneous nucleation close to the melting point of the mineral. The disappearance of cristobalite above boundary 4 may indicate that the eutectic composition has been reached, though the temperature indicated by this work is well below that predicted from phase diagrams for the system S i 0 2 - A1 2 0 3 (Chesters 1973). Tridymite has not been detected, in spite of its presence in phase diagrams for S i0 2 at temperatures between 867° and 1470°C at atmospheric pressure. The absence of tridymite may be due to problems of nucleation; the cristobalite may form from the rather similar rings of silica tetrahedra in the metakaolinite. It is also possible that tridymite forms at high temperatures, but is not quenchable in our experiments. Other heating experiments on brick-clays, however, have produced tridymite at higher temperatures than cristobalite (A. S. McKnight, pers. comm. 1987). When time is short in geological terms the silica minerals do not behave as might be predicted by the phase diagram, which is based on the attainment of equilibrium.

A most important feature of the TTT-diagram for kaolinite, and indeed for other TTT-diagrams, is that at a given

K A O L I N I T E

1000 900 800 700 600 500 400 300 200 100

C ) UT e m p e r a t u r e (

Fig. 8. Differential Thermal Analysis/Thermogravimetric traces for kaolinite. SPS-grade kaolin from Cornwall, kindly supplied by ECC.



104 A. C. DUNHAM

temperature different mineral assemblages are possible, depending on the length of time the sample has been held at the elevated temperature. For example, at 1100°C the assemblages found with increasing time are: metakaolinite, metakaolinite + spinel + mullite, mullite + spinel, cristobalite + mullite + spinel, and finally cristobalite + mullite. Many of the mineral assemblage fields are elongated so that they slope from higher temperatures to lower temperatures with increasing firing times. This may be of great importance to commercial firing cycles because by firing at slightly higher temperatures the same mineral assemblages (i.e. a very similar product) can be produced with a much reduced firing time. A significant reduction in firing costs, a major part of the costs of brick-making, may thus be achieved. Work in progress in the LU-MIRO brick project has confirmed that the mineral assemblage fields in TTT-diagrams for individual brick-clays also show the same general orientations.

3. CONCLUSION

In this address I have examined recent work on one important non-metallic raw material and it products; clays and shales and their transformation into fired bricks, tiles and pipes.

Bricks and tiles have been manufactured for around 4000 years, but we are only just beginning to understand the relationship between the raw materials and the final product. The key lies in the mineralogy. To understand the nature of the firing reactions it is firstly essential to know which minerals and how much of each species are present, and to have an assessment of the texture. To this must be added systematic experimental investigations of the mineral reactions on firing. Of particular importance is the effect of time on the progress of these reactions. A research programme, supported by MIRO, the Energy Efficiency Office and a consortium of brick-makers, is currently underway at the University of Leicester to determine TTT-diagrams for twenty five brick-clays, covering the range of materials currently being used in the United Kingdom. This will provide both the basic scientific understanding of what is happening during firing, with some indications of mechanisms, and provide data which should enable the more efficient firing of raw materials.

Industrial mineralogy, the application of mineralogical science to industrially important raw materials and processes is an exciting branch of geology. It takes us back to the roots of geology when geologists were practical mining men and engineers, who needed geological information and ideas to solve practical problems. This combination of pure science and its applications seem to me to be the raison d'etre of geology.

Acknowledgements. It is a pleasure to be able to thank colleagues past in Hull and present in Leicester Universities. In particular, I thank Dr P. W. Scott, Dr A. S. McKnight and Mr. I. Warren for their help and advice, and Sue Button and Clive Cartwright who drafted the diagrams. Finally, I am grateful to the Minerals Industry Research Organisation, the Energy Efficiency Office, and a consortium of brick-makers for their permission to publish data from the LU-MIRO brick research project.

References BLATT, H . 1982. Sedimentary petrology. W . H . Freeman and

Company, San Francisco. 564pp. BROWNELL. W. E. 1976. Structural clay products. Applied Mineralogy

9 , Springer-Verlag, Vienna. 231pp. CARP, A. 1987. The exploration and evaluation of raw materials for the

heavy clay industry. British Geologist 13,160-163. CHESTERS, J. H. 1973. Refractories, production and properties. The

Iron and Steel Institute, London. 553pp. CLARK. JR. S. P. 1966. Handbook of physical constants. Geological

Society of America Memoir97, 587pp. CLARKE, F. W . 1924. Data of geochemistry. United States Geological

Survey Bulletin 770,841pp. DRAKAPOULOS, Y. D . 1988. A chemical and mineralogical

investigation of yellow building bricks. Unpublished M.Sc. thesis, University of Hull.

DUNHAM, A. C. & MWAKARUKWA, G. M . 1984. The mineralogy of brickmaking at Broomfleet, North Humberside. Proceedings of the Yorkshire Geological Society 44,501-517.

FREED, R. L. & PEACOR, D . R . 1989. Variability in temperature of the smectite/illite reaction in Gulf Coast sediments. Clay Minerals 24,171-180.

GRIM, R . E. 1962. Applied clay mineralogy. McGraw-Hill Book Company, New York. 422pp.

GRIM, R . E. 1968. Clay mineralogy. 2nd Edition. McGraw-Hill Book Company, New York. 596pp.

JEANS, C. V. 1989. Clay diagenesis in sandstones and shales: an introduction. Clay Minerals 24,127-136.

MUAN, A., NADEAU,P. H . , WILSON,M. J . , MCHARDY, W . J. &TAIT, J. M. 1985. The conversion of smectite to illite during diagenesis: evidence from some illitic clays from bentonites and sandstones. Mineralogical Magazine 49, 393-400.

ONIKE.F. N . 1985. Time - temperature - transformation (TTT) curves for and the thermal decomposition reactions of kaolinite, montmorillonite and two muscovite samples. Unpublished Ph.D. thesis, University of Hull.

ONIKE, F., MARTIN, G. D. & DUNHAM, A. C. 1986. Time -temperature - transformation curves for kaolinite. Materials Science Forum 7 , 73-82.

PETTIJOHN. F. J. 1975, Sedimentary rocks. 3rd Edition. Harpur and Row, New York. 628pp.

POTTER,P. £ . , MAYNARD.J. B . & PRYOR, W. A. 1980. Sedimentology of shale. Springer-Verlag, New York. 306pp.

PRENTICE, J. E. 1988. Evaluation of brick-clay reserves. Transactions of the Institution of Mining and Metallurgy 9 7 , B9-14.

REDFERN, S. A. T . 1987. The kinetics of dehydroxylation of kaolinite. Clay Minerals 22, 447-456.

RIDOEWAY, J. M. 1982. Common clay and shale. Mineral Resources Consultative Committee, Mineral Dossier 22, H.M.S.O., London.164pp.

TUTTLE, O . F. & BOWEN, N. L. 1958. The origin of granite in the light of experimental studies in the system NaAlSijOg- KAlSijOg-S i0 2 . Geological Society of America Memoir 74,153pp.

Manuscript received: 13th March, 1992.



Documents

Developments in industrial mineralogy: I. The …pygs.lyellcollection.org/content/pygs/49/2/95.full.pdf · naturalist and environmentalist they form the backbone of the landscape;