THE MICACEOUS MINERAL OF THE YORKSHIRE FIRECLAYS

Transcription

1 THE MICACEOUS MINERAL OF THE YORKSHIRE FIRECLAYS By R. W. GRIMSHAW The Leeds Fireclay Company Limited, Farnley, Leeds, A. G. SADI~eR Department of Mines and Technical Surveys, Mines Branch, Ottawa, Canada, and A. L. ROBERTS Houldsworth School of Applied Science, The University of Leeds. [Received 5th April, 1962] ABSTRACT Pure samples of the micaceous component of Yorkshire fireclays have bee~ separated. Examination of these by chemical, optical, X-ray, and thermal methods indicates that the material is in all likelihood an intimate mixture (but not an interstratification) of mica and a kaolintype mineral--probably the b3 disordered kaolinite commonly found in fireclays. The evidence also suggests that this kaolin-type mineral is the end-prodact of weathering of the mica, and a possible mechanism is suggested. INTRODUCTION Cart, Grimshaw and Roberts (1952) showed that, although they vary considerably in chemical composition, most British fireclays contain three principal mineral components, quartz, a kaolin-type mineral*, and a micaceous mineral of variable composition. Many minerals other than these may be present in the fireclays, but usually in small amount and the combined proportions of the three main types often exceeds 90 per cent. of the total mineral content. The relative percentages of the minerals quartz, kaolin-type, and mica-type vary considerably with different clays, as also does the grain size range of each component. These facts explain the variation in composition and properties of fireclays from different seams and localities. In previous papers the nature of the clay mineral has been described (Grimshaw, Westerman and Roberts, 1948) and it has been established that the micaceous component could be likened to a *This mineral has been variously named livesite, fireclay-mineral, b3 dis-. ordered kaolinite and kaolinite pm. 110

2 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 1 l l hydrous mica in that it contained less alkalis and more combined water than normal muscovite (Carr, Grimshaw and Roberts, 1953). The purpose of the research presently described was to investigate the nature of the micaceous component in fireclays, and to assess its origin and the causes of variation in its chemical composition. MATERIALS Separation and Purification. In fireclays the kaolin-type mineral is always present as extremely fine particles, the quartz is invariably composed of rounded grains mostly relatively larger but covering a wide size range, whilst the micaceous material is in the form of plates also probably covering a wide particle-size range. Therefore, mineral separation should best be effected by the technique of dispersion followed by sedimentation. In the method adopted, the clay was blunged in hot water containing a dispersing agent and the suspension flushed through a nest of sieves and washed thoroughly. The kaolin-type mineral being of extremely small particle size, passed through the sieves, whilst the larger grains of quartz, coal, and mica were retained. These larger particles were thus separated into a series of narrow size ranges in which each grain had approximately the same crosssectional area. The quartz and coal particles were, however, roughly spherical in shape, whereas the mica consisted of flat, plate-like crystals with a thickness approximately one-thirtieth of the largest diameter. When suspended in water, the mica plates were observed to fall with their principal face horizontal. They also fell considerably more slowly than spherical quartz particles of the same maximum diameter and of approximately the same density. In mixtures containing grains of similar maximum diameter, separation of the two minerals could, therefore, be effected by fractional sedimentation. A sample collected on a sieve was stirred up with water and allowed to settle in a large beaker until no quartz remained in suspension. The supernatant liquid, containing only mica plates and coal fragments, was then poured off through the sieve on which the particular sample had been collected. The process was repeated frequently, until the sedimented sample of quartz was virtually free from mica. The mica collected on the sieve was also free from contamination with quartz, but still contained an appreciable amount of organic material. It was impracticable to separate this by fractional sedimentation and so a heavy-liquid method was adopted in which the mica and coal particles were dispersed in a mixture of bromoform and alcohol (specific gravity 2.6) and centrifuged. The density of the mica was about 2.7 and that of the coal approximately 1"5. The high specific gravity of the liquid ensured that the mica crystals impregnated with organic matter were also excluded from the final fraction. The mica was collected and washed thoroughly with

3 112 R.W. GRIMSHAW, A. G. SADLER AND A. L. ROBERTS alcohol and then water. Examination under the microscope confirmed the purity of the samples. Thirteen samples of micaceous material were collected from fireclays from six localities in Yorkshire. The fractions and their reference numbers are listed in Table 1. TABLE 1--Fractions of the micaceous material collected from the raw clays (sample reference numbers in brackets). Clay Sieve size(b.s.s.) New Tong Outcrop Lepton... Halifax... Hazelhead... Gildersome... Cockersdale Old Tong Outcropill... (1)... (6)... (9)... (1o)... (11)... (12)... 03) (2) (3) (4) (7) (8) -- (5) Lepton and Tong Outcrop are clays associated with the Better Bed in the Lower Coal Measures and these yielded several size ranges. Mica was also separated from a sample of Tong Outcrop clay which, although from the same seam, had been mined at an earlier date and from a different site: this has been termed Old Tong Outcrop. Halifax and Hazelhead clays are also from the Lower Coal Measures but occur below the Hard Bed Coal. These samples yielded mica only in the smallest size range. The two remaining clays, Gildersome and Cockersdale, are from the Brown Metal Bed in the Middle Coal Measures. These are also clays of comparatively small grain size and only the smallest grain-size fraction could be obtained from them. Also included in the results are the data for a sample collected by Carr (1952), separated from Old Tong Outcrop clay. This is referred to as sample 14. RESULTS Chemical Analysis. Chemical analyses of the various samples, together with those of a muscovite (M) and a hydromuscovite (HM), are given in Table 2 and the atomic proportions of the elements are shown in Table 3. The first significant fact apparent from the results is the large variation in the composition of the samples. Even the different fractions of the material collected from the same raw clay sample are far from constant in composition. It is important to note, however, that the total alkalis are always lower and the water content always higher than in ideal muscovite, and that there is a close relationship between the proportions of these alkalis and the water of constitution.

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6 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 115 In ideal micas the alkali ions form bridging bonds between successive layers, and all the water of constitution is present as OHwhich forms an integral part of the structure. The maximum theoretical amount of water which can be present in the ideal muscovite structure, KAl2(AlSi3)O10(OH)2, is 4-5 per cent.; therefore, if a similar structure is presumed for hydrous mica, the excess water must be present in extra sites, probably interlayer. The total water contents and the total alkali contents are compared in Fig. 1. Calcium has been included with the alkalis because this element is considered by most authorities to occupy interlayer positions in a micaceous structure and not to be part of the lattice. The relationship between the water and alkalis is approximately linear and it is of significance that if the best straight line is drawn through the points shown in Fig. 1, it cuts the percentage water ordinate at about 14 per cent. H20. The combined water in the minerals of the kaolin group, A12Si2Os(OH)4, amounts to 13.9 per cent. At the opposite end, the projected line passes close to the point representing ideal muscovite (11.8 per cent. K20, 4.5 per cent. H20) i~ KAOLINITE! I I 1 I I '1 I 1 Io 0 O'q~. Q o~ 6 "- MUSCOVITE "0 & 2 I l l I I I I I i ~6 i~) 20 PERCENT K.20 * NaTO * C~O FIG. 1--Comparison of total water content and total alkalis (including calcium) in the micaceous material. The atomic ratio between silicon and the total amount of elements which can occupy layer-lattice sites (i.e., A1, Fe, Ti, and Mg) is in the range 1.0:0.9 to 1-0:1.3 for all samples. Although the ideal ratio for muscovite is 1:1, the range for the micaceous material is within the limits found in substituted naturally-occurring micas. No simple relationship exists between the SIO2:A1203 ratio and the proportions of alkali ions. In this respect, the theory that hydrous micas form part of a continuous series between muscovite and montmorillonite is not continued in the micaceous material from the Yorkshire fireclays. If this were so, the SIO2:A1203 ratio would show an increase with decreasing alkali content.

7 116 R.W. GRIMSHAW, A. G. SADLER AND A. L. ROBERTS Microscopic Examination. In ordinary light under low-power magnification, the purified mineral was transparent and characteristically platy. The flakes had no definite shape and had obviously been eroded. At higher magnifications, many cracks and fissures could be observed, occurring mostly at the edges of the plates, but in some samples persisting through the whole flake. There was no regularity in the direction of the cracks, so that they could not be attributed to cleavage planes. Saffranine and malachite green stained the flakes along the cracks in the crystals suggesting the presence of a fine-grained or amorphous material. Although the majority of the flakes were colourless or faintly green, some were partly or entirely of a brownish colour. Inclusions of a red or brownish material were numerous. A welldefined cleavage was observed parallel to the basal plane. The perimeters of many flakes had fractured unevenly giving a characteristic stepwise or contour appearance. Examination under polarized light showed that the flakes exhibited varying pleochroism. The clearer crystals showed very weak, the greener ones medium, and the brown ones quite strong pleochroism. Under crossed nicols, the extinction was straight and the birefringence was weakly negative. With convergent light and crossed nicols, good interference figures were obtained on the larger crystals, although they were somewhat diffuse. 2V could be readily calculated and although the value varied for each sample, all were between 30 ~ and 40 ~ as compared with muscovite angles of ~ (Heinrich et al., 1953). From the optical evidence, the micaceous materials from the fireclays are all of a very similar character. In most of their optical properties (Table 4), they are indistinguishable from muscovite mica. TABLE 4---Properties of the micaceous material. Property Specific Gravity Colour Form Cleavage Refractive Indices Birefringence Inclusions Pleochroism Extinction Interference Figure Results 2"6-2-7 (variable). Mainly colourless-green-brown. Generally auotriomorphic but occasionally showing hexagonal Outlines. One direction of cleavage parallel to the basal plane. ct = (maximum) r= (minimum) Negative, (average). Red and brown on edges and cleavages. Colourless crystals--faint. Green--weak. Brown--distinct. Straight. Biaxial, 2V = 30-40".

8 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 117 Differential Thermal Analysis. The differential thermal curves for the mica samples are shown in Figs. 2 and 3; all gave similar curves, differing only in the overall magnitude of the peaks. Four endothermic effects can be distinguished at ~ 630~ 680~ and 830~ Only one exothermic peak at 975~ is evident. Apart from the endothermic peak at 830~ the heights of the remaining peaks shows little relative variation, although the size of the peaks for different samples varies considerably. I I! I m... ~ _ ~ n g Outcrop. ~-150 BSS 7.0~ Water ~ Ou~crop ~4-~B5S 8.9~ Water " 6. Old Tong OUtcrop ~er Te~erature in *C I f! i., 2O0 EO0 6oo 8o0 t OO0 FIG. 2--Differential thermal curves for the micaceous material. In Fig. 4 the endothermic peak area measurements are compared with the total water content of each mica sample. On the same graph, the appropriate values for muscovite mica (4.5 per cent. HzO) and kaolinite (13.9 per cent. HzO) are included. When a straight line is drawn through the points representing these two minerals, the points representing the mica samples are sufficiently close to suggest that there is a relationship. Unground muscovite mica gave no detectable endothermic reactions within the temperature range over which the mica peak areas were measured. Differential thermal curves of muscovite illustrated in the literature

9 ]18 R. W. GRIMSHAW, A. G. SADLER AND A. L, ROBERTS 1 I I "l Water 9 -. r- 9 I I 1 i 200 ~ Temperature in oc Fio. 3--Differential thermal curves for the micaceous material~ 10 9 I I I I I I [ I i, v ] j. KAOLINITE 7 y 6 o- Og" O 0 9 O 3 2 MUSCOVITE " I J J I ~i v i i i I i i i ~ I i~ 15 D~R~ENT Fio. 4--Relationship of total water content to endothermic peak area. WATI~R

10 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 119 show a broad endothermic peak of variable size at about 900~ However, it is probable that these samples have been ground to facilitate packing in the sample holder. Mackenzie and Milne (1953) filed a muscovite, to avoid the more drastic treatment of grinding, but failed to detect any endothermic peaks below 1000~ Although the best straight line relating water content to the endothermic peak area in Fig. 4 lies close to the value for kaolinite, the temperatures at which the peaks occur do not correspond with that of kaolinite. However, other kaolin minerals exhibit en ~00 t~o0 ~OO 1000 T~P~RATURE IN ~ FIG. 5JDifferential thermal curves tbr the micacoous material ground for varying periods of time. dothermic peaks (Davis, Rochow and Rowe, 1950) which correspond to those of the micaceous material. Grimshaw (1947) reported that the kaolin minerals halloysite, b3 disordered kaolinite, and kaolinite, although showing different thermal characteristics, gave approximately equal endothermic peak areas on differential thermal curves for the pure minerals, and suggested that the size of these peaks was directly related to the amount of structural water in the minerals. The endothermic effects in both the kaolin minerals and the micaceous material are caused by loss of structural water. Despite the fact

11 120 R.W. GRIMSHAW, A. G. SADLER AND A. L. ROBERTS that the reactions in these two minerals take place at different temperatures, the results confirm that the overall peak area is proportional to the amount of water lost. Dry grinding of the micaceous material resulted in a change of shape and size of the endothermic peaks (Fig. 5). The lowtemperature endothermic peak ( ~ gradually increases with grinding time, the higher temperature peaks progressively disappear, and a broad endothermic effect appears between 50~ and 450~ The latter can be removed by treating the ground sample with dilute hydrochloric acid followed by sodium hydroxide solution, and is in all probability due to the presence of hydrated amorphous silica and alumina. The peaks produced by grinding the micaceous material are much more pronounced than those given by muscovite when similarly treated: even light crushing with a pestile and mortar is sufficient to enhance the peak at 575~ X-Ray Examination. X-ray diffraction powder photographs were taken with a 19 cm Debye-Scherrer camera, using Cu Ka radiation. The photographs were all similar to that reported previously (Carr, Grimshaw and Roberts, 1953) and are indistinguishable from that obtained for a mixture of muscovite and kaolinite. The principal feature of the photographs was the presence of diffraction lines corresponding to basal spacings of 10)~ and 7,~. The results do not correspond to previously reported illites and hydromuscovites (Grim, Bray and Bradley, 1937; Nagelschmidt and Hicks, 1943) which show only a 10,~ basal spacing. The X-ray evidence indicates that the micaceous material from the Yorkshire fireclays is a mixture of muscovite and kaolinite. The possibility of it being a mixed-layer mineral, of the type discussed by Brown and MacEwan (1951), is precluded because in such circumstances the basal spacing would be intermediate between those of the pure mineral types involved and depend upon the relative amounts of the two layers. Dilute hydrochloric acid treatment of the unground micaceous material did not affect the X-ray pattern. On heating to 500~ the 10A basal spacing line remained unchanged, whilst the line corresponding to 7~ disappeared. These two facts suggest that the 7 A line is not indicative of a chlorite-type mineral with a weak first order reflection at 14 A and a strong second order at 7 A, since chlorites are generally attacked by acid, and heat treatment at 500~ does not usually destroy a true chlorite structure. The X-ray diffraction photographs of the micaceous samples differ in the relative intensities of the 10 A and 7 A lines, and, furthermore, there is a decided correlation between the total amount of water in the micaceous material and the relative intensities of the 10 A and 7 A lines (Table 5).

12 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 121 TABLE 5--Relationship of total water content of the micaceous samples to ratio of the intensities of the 10.~ and 7 A lines. Sample BSS No. Relative Intertsity 10A7A Total Water Content Lepton... Old Tong Outcrop'"... Hazelhead... Lepton... Halifax... Cockersdale... Lepton... New Tong Outcrop... Gildersome New Tong Outcrop '40 1 "30 1' " Single crystal rotation photographs gave the parameters a~-5.2~, b a, typical of the silicate layer minerals. The pattern obtained with the X-ray beam perpendicular to the c axis was identical with that of muscovite. Weissenburg photographs obtained on these micas independently by H. S. Yoder of the Carnegie Institute, Washington, and A. A. Levinson of the Ohio State University were both interpreted as representing a normal 2-layer monoclinic muscovite. The most likely explanation of the X-ray evidence alone, is that the micaceous material in the Yorkshire fireclays is a mixture of muscovite and a kaolin-type mineral or minerals. Cation-Exchange Capacity. Cation-exchange capacities (Worrall, Grimshaw and Roberts, 1958) obtained using the ammonium acetate method are shown in Table 6. TABLE 6---Cation-exchange capacity and total water content of the micaceous material and associated minerals. Mineral Muscovite Kaolinite b3 Disordered kaolinite Gildersome mica BSS Lepton mica BSS Halifax mica BSS Cation-exchange capacity (m-eq100g) 2"55 5"37 39 "30 4"50 11 "36 12 "68 Total water (%) 4"50 13"90 13 "90 6"70 10"78 9"36 There appears to be a direct relationship between the cationexchange capacity and the total water content, suggesting that the higher values of the former may be due to an alteration mineral in

13 122 R.W. GRIMSHAW, A. O. SADLER AND A. L. ROBERTS the micaceous material, and that this may be disordered kaolinite or some related mineral. Weight-Loss Experiments. The samples were dried at 200~ and then heated to a constant weight at temperature increments of 10~ The results for three samples are shown in Fig. 6. The lattice water was lost in three distinct stages at 280~ ~ and 470~ confirming the belief in the variable chemical nature of the material. lz, t~ I I i i [ KAOLINITE t o F LEPTON 2~0-300 BS5 "~ALIFAX. 2~0-300 ~SS 9 8 ] LEPTON." BSS 7' I )oo t, oo TEblPE~IATURE IN ~ Fro. 6--Dehydration curves of the micaceous materials. DISCUSSION The evidence from the various methods of analysis overwhelmingly suggests that the micaceous material from the Yorkshire fireclays is not a single mineral entity. X-ray, optical, and thermal methods indicate that in each apparently single crystal, there are two components, one of which is probably a true mica of the muscovite type, and the other a mineral having many of the characteristics of kaolinite. If the assumption is made that the micaceous material consists of true micas (either muscovite, paragonite, or margarite) which are thermally inert to differential thermal analysis, plus some kaolin-

14 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 123 l I l 0 [ ~ 9 I ~66~I ~ ~ ~ I-i I~l ~ ~ ~ c~ 0 oo.. 9 ~ ~ - ~ ~g ~z 0

15 124 R.W. GRIMSHAW, A. G. SADLER AND A. L. ROBERTS type mineral which causes the endothermic effects on differential thermal curves, a mineral balance sheet which fits the chemical results can be drawn up as follows. In each sample, the total amount of residual mica is calculated from the sodium, potassium, and calcium contents, and the required amounts of alumina, silica, and water subtracted from the corresponding chemical analysis. The percentages of the constituents remaining then represent the composition of the decomposition products (Table 7). If the contents of each oxide in the remainder percentages are added, and the molecular proportions of each element calculated, the averaged empirical formula is: Si2.00(Al, Fe3+, Ti, Mg) 1. s (OH)4 as compared with kaolinite: Si2AI20 5(OH)4 g l l l l 90 8O 7O O. # # ~ ~o N 39 ~ 20 lo. i 1 I I I I t~ 5 6 ' '~'' 8 9 ;o' ii 12 ' i '~' i~ 15 PERCENT WATE~ FiG. 7--Graph of percentage remainder against total water content. The percentage remainder in each sample is plotted against the total water content in Fig. 7. When the resulting curve is extrapolated to I00 per cent. remainder, the water content of such a material is a little less than 14 per cent. as compared with 13"9 per cent. for a pure kaolin-type mineral. Figs. 8 and 9 compare percentage of the remainder with its water content and the peak area on differential thermal curves of the micaceous samples. The peak areas for a 100 per cent. remainder and a material containing 13.9 per cent. water, obtained by extrapolation of the curves, are close to the measured value for kaolinite which is also shown. With the exception of the peak temperatures of the endothermic reactions, the total effect of the micaceous materials in the experiments carried out is similar to that of a simple mixture of true mica

16 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 125 i00 9O J I 0 K Aol, I ~IJ'~ ao 7o 6o 5o ~.o O O" o." o Q G " I 10 I I I I 1 I I i I to PEAK AREA [1OGO's SQ. ~'s) Fig 8--Graph of percentage remainder against endothernfic peak area I0 9 i L I i r i I I t KAOLINITE, I i r J i i Q, B < 7 6 (D, C~9;,,@ i i 1 i ~ ~ PEAK AREA (l~o's SQ ~'S) Fro. 9---Graph of remainder water content against endothermic peak area.

17 126 R.W. GRIMSHAW, A. G. SADLER AND A. L. ROBERTS and a kaolin-type mineral. This explains why the earlier mineralogical analyses of the Yorkshire fireclays could be balanced with the chemical analyses (Grimshaw, 1947). In these, the clay mineral and quartz contents were obtained from differential thermal measurements, and the alkalis were regarded as components of the true micas, muscovite and paragonite. The measurements of the area of the clay mineral peak on the differential thermal curves would include the effect of the mineral admixed with the micaceous material and this would give a comparable thermal effect and have the same chemical composition as a kaolin-type mineral. It seems that with the present techniques there is no way of distinguishing between 'free' clay mineral and the material in the mica. Although a mineral balance can be compiled, there may be a great difference in the properties of clays which apparently contain the same minerals, but really differ greatly in 'free' clay mineral content. Only differential thermal analysis gives some idea of the amount of combined material, in the size of the inflection at about 680~ which follows the clay mineral peak. Even this probably only indicates the fraction of mineral admixed with the larger mica crystals. CONCLUSIONS All the results suggest that the micaceous material in the Yorkshire fireclays is composed of primary muscovite which has been broken down to a certain extent by weathering processes. It is most likely that the original mica has been altered along cleavages and other fissures and the secondary product remains in close association. The composition of the combination depends on the relative extent of the alteration reaction. That the change cannot be simply represented by postulating the replacement of alkali ions by hydronium ions, is borne out by X-ray evidence which indicates that two mineral entities are present and not a single type with variable lattice characteristics. The final product of weathering is a kaolin-type mineral (probably b]3 disordered kaolinite) formed in cracks and fissures in the outermost parts of the mica plates. The process of weathering: commences with the leaching of the alkali ions from interlamellar positions and their replacement by water. Subsequently, a complex structural change is involved, and its true nature can only be guessed. Intermediate products are most certainly present as indicated by differential thermal curves, which show at 630~ and 680~ endothermic peaks that are not present in the curves of either of the end products. It may be that a chlorite- or vermiculite-type complex is first formed which has a meta-stable existence and when it is freed from the restraining influence of the mica lattice it reverts to the more stable kaolin-type structure. The important conclusion to be derived from the evidence is that a mineral of the kaolin group is the final product of the normal

18 MICACEOUS MINERAL OF YORKSHIRE FIRECLAYS 127 breakdown of muscovite mica. This change is the result of epigene agencies in which water under slightly acid conditions is the principal activator. REFERENCES Bgow~, G., and MACEWAN, D. M. C., X-ray Identification and Crystal Structures of Clay Minerals (G. W. Brindley, editor). Mineralogical Society, London, Chapter XI, p CARR, K., Ph.D. Thesis, University of Leeds. CARR, K., GRIMSHAW, R. W., and ROBERTS, A. L., Trans, Brit. Ceram. Soc., 51,334. CARR, K., GRIMSHAW, R. W., and ROBERTS, A. L., Trans. Brit. Ceram. Soc., 52, 139. DAVIS, D. W., RocHow, T. G., and ROWE, F. G., Preliminary Reports: Reference Clay Minerals. American Petroleum Institute, Research Project 49. GRIM, R. E., BRAY, R. M., and BRADLEY, W. F., Amer. Min., 33,813. G~MSHAW, R. W., Ph.D. Thesis, University of Leeds. GRIMSHAW, R. W., WESTERMAN, A., and ROBERTS, A. L., Tratts. I. Int. Ceram. Congr., p Hut, WdCH, E. W., LEVINSON, A. A., LEVANDOWSKI, D. W., and HEWlTT, C. H., Studies in the Natural History of Micas. Engineering Research Institute, University of Michigan, Project M 978, Final Report, p. 57. MACKENZIE, R. C., and MILNE, A. A., Miner. Mag., 30, 178. NAGELSCHMIDT, G., and HICKS, D., Miner. Mag., 26, 297. WORRALL, W. E., GRIMSHAW, R. W., and ROaERTS, A. L., Trans. Brit. Ceram. Soc., 57, 363.