Download an x-ray investigation of some argillaceous rocks from the skipton

Clay Minerals (1969) 8, 29
A N X - R A Y I N V E S T I G A T I O N OF SOME
A R G I L L A C E O U S R O C K S F R O M THE
SKIPTON ANTICLINE, YORKSHIRE
M. J. P U R T O N * AND R. F. Y O U E L L
The University of Leeds
(Read at the April 1960 Meeting of the Clay Minerals Group at Aberdeen)
ABSTRACT: X-ray analyses of argillaceous rocks from the Skipton anticline show that
their average composition is quartz 30 %, calcite 27 %, feldspar 5 .%0,illite 15 %, kaolin
17 ~ . Pyrite and other constituents account for the remaining 6 %. The quartz, calcite
and feldspar are considered to be detrital, the variations in their quantity and size
reflecting changes in the source of the sediments. The evidence indicates the general
predominance of kaolin over illite, which is contrary to other published results on
Carboniferous marine shales. A kaolin-rich source is necessary to account for the
composition of the sediments. An occurrence of unusually pure and well-crystalline
melanterite is recorded from the Draughton Limestone.
INTRODUCTION
This paper presents the results of X-ray powder analysis of rocks of the Skipton
anticline, particular attention being paid to the clay mineral content and the use
of this information in interpreting the history and deposition of these rocks. The
geological field work was carried out by the first author, and the X-ray analysis
jointly.
The area from which these samples came is one of the classic areas of the basin
facies of the Lower Carboniferous in the Craven Lowlands. It was described in
detail by Hudson & Mitchell (1937). The rocks in the succession are mudstones,
shales, and argillaceous limestones, with limestone predominating at three levels.
They were laid down in four sedimentation periods, each initiated by an earth
movement, in which the sedimentation succeeded from deep to shallow water. The
succession is up to 5000 ft in thickness, as follows:
Lower Namurian
Visean
Skipton Moor Grit
Upper Bowland Shales (El)
Berwick Limestone (El)
Middle Bowland Shales (P2)
Nettleber Sandstone (P1)
Lower Bowland Shales (P1)
* Present address: Welwyn Hall Research Association, Church Street, Welwyn, Herts.
M. J. Purton and R. F. Youell
30
Visean
Tournaisian
r
l
{
Draughton Shales
Draughton Limestone (So_D1)
Skibeden Shales with Limestones (S)
Embsay Limestone (C2S1)
Halton Shales with Limestones (C)
Haw Bank Limestone (C1)
The large quarries in the area give excellent exposures in the Limestone series.
TECHNIQUE
A Brindley and Spiers 20-cm diameter powder camera was used for this investigation. Exposures were made with the plane surface of the specimens at 3~ or 9 ~ to
the incident beam, giving satisfactory information over the ranges of 30 kX to 3kX
and 7 kX to 1 kX respectively. Copper radiation was used for most constituents
of the shales. Iron compounds caused secondary emission and a heavy background
to the film. In practice this background was useful in drawing attention to certain
iron compounds with poor diffraction patterns which were not easily identifiable.
Cobalt radiation was also used on the iron rich specimens. In view of the large
amount of material investigated, it was necessary to combine fairly rapid examination with reasonable accuracy. The films were examined against a scale calibrated
to read interplanar spacings directly in kX units. In general mica, kaolin, quartz
and calcite could be identified by inspection and other components by reference
to the tables of Brindley (1951), the A.S.T.M. index (1949), and in doubtful cases
to photographs of standard mineral samples taken on the same camera.
Pyrite was difficult to distinguish in the powder patterns because its principal
lines were sometimes obscured by lines from other strongly diffracting minerals.
Chemical tests were carried out on such samples if there were any doubts, and
separation by sedimentation into coarse and fine fractions was followed by another
diffraction pattern to give reliable X-ray data. An assessment of the composition
was made by comparison of the intensities of the powder lines, proper allowance
being made for the very strong lines given by well-crystallized clay minerals. The
proportionality between powder line intensities and the amount present of constituent minerals can be affected if one of these components has a relatively heavy
absorption of X-rays. Experiments with mixtures of known composition showed
that if iron compounds were present in quantity they could have a significant effect,
otherwise the error was not serious for the general picture required for this work.
A very important matter was the behaviour of clay minerals or powders with plate
like particles producing preferential orientation which substantially affects the
resulting intensities for component minerals. When the extent of this behaviour
is known, it can be used to improve the value of the analysis of mixtures by
strengthening specific powder lines of substances present in small quantities. This
controlled preferential orientation is a development of the specimen preparation
technique described by one of the authors (Youell, 1960) during work on the
X-ray investigation of argillaceous rocks
31
chlorite group of minerals. Other techniques for improving the accuracy of the
Brindley and Spiers camera for qualitative analysis were developed for the present
work. In quantitative analysis of clay minerals accuracy is sometimes controlled
not by correction of the X-ray data for the effects of orientation, alignment, absorption, matrix, and other mathematically or mechanically manipulable factors, but by
the variation of diffraction patterns p e r se of specimens which may differ inappreciably in chemical composition and particle size. In the structures of the kaolin group
of minerals, the variation between true kaolinites and chamosites is noteworthy.
Fireclay-type kaolin, illite and iron oxides, in that order, were those principally
affected in this way in the present work. With well-crystalline components like
quartz, and calcite, the accuracy of the analysis depends largely on the time available for the measurement of the data. Accurate determinations were made on a
selection of specimens, using every possible improvement of the Brindley & Spiers
technique, and in some cases data were checked against standard mixtures. For the
present work, a moderately accurate general picture for a very large range of
specimens was clearly of considerable value. The weakness of improving techniques
to the limit of accuracy on a few selected specimens is that such samples may not
necessarily be representative of the whole layer being investigated. If the authors
appear to be labouring this topic unnecessarily, it may be pointed out that perhaps
too little attention has been paid in the past to a proper compromise between
experimental accuracy, representative selection of samples, and (equally important)
a correlation of the geological, physical and chemical aspects of a problem. In the
present work, all the data, both of high and moderate accuracy, are quoted together
to 10% for major components and 5% for minor components. The particle size of
the components was estimated from the sharpness of the powder diffraction lines
and by comparison with standard photographs, the high resolution and clarity
obtained with the Brindley and Spiers camera making this straightforward.
Only mild pretreatment was used when preparing the specimens for analysis,
so that the particle size measured from the diffraction patterns was not substantially
influenced by vigorous or prolonged grinding. The term 'kaolin' is used for all
components of the group of minerals including the fireclays, reserving 'kaolinite'
for the pure well crystallized mineral.
RESULTS
The results are shown in Table 1. Where different specimens from the same bed in
a horizon gave similar results, the tabulated information is an average. Where
substantial variation of composition, particle size or other features occurred, each
specimen is quoted separately. The results are in the order of the outcrops within
the horizons. Apart from the quarry exposures, natural outcrops were poor and
often drift-covered. There is, therefore, a predominance of results from the limestone series, although sufficient samples were collected to ensure representation of
all the principal series. In general it may be concluded that:
M. J. Purton and R. F. Youell
32
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34
M. J. Purton and R. F. Youell
1. The specimens are predominantly fine-grained quartz and calcite, these two
averaging 57%.
2. Where quartz, calcite and feldspar occur together they are, with very few
exceptions, all of similar grain size.
3. The clay mineral content of the shales averages 32%. Those normally present
are a muscovite-type clay mica (illite) and a fireclay-type kaolin, the latter usually
predominating as indicated below:
Haw Bank Limestone
Halton Shales
Embsay Limestone
Skibeden Shales
Draughton Limestone
Draughton Shales
Bowland Shales
Millstone Grit
Ratios of numbers of specimens
Kaolin predominant Proportions equal Illite predominant
9
3
1
1
1
3
3
4
1
1
3
2
5
3
1
2
1
4
-
4. Chlorite is rare; it was found only as a trace in three specimens. No montmorillonite was recorded.
5. Other components were pyrite, gypsum, hematite, goethite, limonite, gibbsite,
siderite, dolomite and melanterite.
6. The clay mineral could sometimes be correlated with field observations.
The specimens which were clayey or showed well-marked strainslip cleavage had
a high clay mineral content. The average clay content in this category was 63 %
as compared with the average of 32% for all the specimens.
Only two specimens which did not appear clayey had a clay content of over
50%. One of these was a paper shale and the other a friable mudstone. One other
example of a paper shale contained 70 % quartz.
7. Variations occur in the grain size and distribution of the minerals from
different horizons. For example:
(i) The quartz in the samples from the Haw Bank Limestone and the Millstone
Grit is predominantly coarser grained than the quartz from other horizons.
(ii) Quartz is more abundant in Skibeden Shales than in the horizons below
this level.
(iii) No feldspar is recorded in the specimens from the Haw Bank Limestone,
but it is common as a minor constituent in samples from other horizons.
(iv) Illite tends to be dominant over kaolin in the samples from the Embsay
and Draughton Limestones.
(v) There is little iron in the specimens from the Skibeden Shales.
(vi) Dolomite is a relatively common constituent in the Haw Bank Limestone,
but it rarely occurs in other horizons. It was only recorded when there was
over 25 % of calcite in the rock. Dolomite was absent from only one specimen
out of nine with abundant calcite in the Haw Bank Limestone.
X-ray investigation of argillaceous rocks
35
DISCUSSION
Quartz, calcite and feldspar
After studying the proportions o f silica in the dissolved solids of fluviatile and
oceanic waters, Murray & Gravenor (1953) have suggested that colloidal silica
in sediments is authigenic. They considered that this silica combined with colloidal
alumina to form clay minerals, excess silica being deposited as quartz during
diagenesis, quoting Buckley (1951) in support of their theory. The evidence does
not appear sufficiently strong for a categorical conclusion. In the present work the
equivalence of particle size of quartz, calcite and feldspar over an extremely wide
range of specimens and levels is so remarkable that it appears to indicate the
possibility of a common origin. Notwithstanding Murray & Gravenor's conclusions,
it appears quite possible that these could be of detrital origin and not authigenic.
The coarse-grained quartz from the Haw Bank Limestone probably came from
a relatively near source compared with the finer quartz from later rocks and it may
have been derived from the Askrigg Block which was undergoing erosion at the
time. The influx of feldspar after this suggests that at least some of the sediments
came from another source. The influx of coarser quartz and greater quantities of
kaolin in the Millstone Grit can be correlated with the change of environment
during this period.
Clay minerals
Hudson & Mitchell (1937) considered that the beds of the Skipton Anticline were
laid down in sea water which varied in depth during each sedimentary cycle. They
stated that palaeontological evidence indicated only shallow water conditions. The
actual depth of water probably varied from a few fathoms or less for the widespread
erosion of hundreds of feet of sediment (Kuenen, 1950) to about 100 fathoms for
the deposition of the Bowland Shales off Stebden Knoll (Black, 1957).
The proportions of illite to kaolin reported in work on marine shales from the
Carboniferous show some discrepancies. Murray (1954) found only a small percentage of kaolin in his marine shales. Millot (1952) considered that marine
sediments normally contain up to 40% of kaolin, and 50% in certain cases. He also
stated that the amount of kaolin decreases with the amount of carbonate present.
The present work shows kaolin to predominate over illite in most cases. Results
on the Embsay and Draughton limestones, however, correspond more nearly to
the examples described by other authors.
If kaolin in marine sediments is regarded as detrital, there must have been a
plentiful supply to the area from a source rock such as granite, poor in bases, or
from a landmass where intensive leaching took place. The first possibility is the
more likely as there is no indication of intensive leaching of the pre-Carboniferous
surfaces in the Northern Province, and the feldspars found in the sediments would not
be expected to survive such treatment.
It is unlikely that the illite in the sediments was transported as separate
colloidal silica and alumina as suggested by Murray (1954). This hypothesis would
36
M. J. Purton and R. F. Youell
require that authigenic colloidal quartz be associated with the illite whereas the
fine grained quartz appears to be detrital. The illite was probably transported as
a degraded form which, on reaching a marine environment, absorbed cations to
form a typical clay mica structure.
The absence of montmorillonite from the sediments is expected since none has
been recorded from Palaeozoic rocks. The general lack of chlorite may have some
genetic significance since it is recorded in Carboniferous rocks in America (Murray,
1954) and from the Coal Measures in this country (Brindley & Robinson, 1948).
Other components
The only other non-detrital mineral in the shales was pyrite which was possibly
formed by bacterial action on organic matter below the ocean floor. No direct
determination of the amount of organic matter in the shales was made, but in some
cases during chemical tests a black, and often bituminous, scum was seen on the
surface of the partial solution. This organic scum did not occur in shales from
which pyrite was absent, but sometimes pyrite was present without any evidence
of such organic matter.
Gypsum occurred frequently in the shales but it was only seen as small crystals
on the lamination surfaces. It was therefore interpreted as a secondary deposit
from meteoric waters obtaining their sulphur from the oxidation of pyrite and
their calcium from the abundant calcium present in the rocks.
One occurrence of melanterite, FeSO~. 7H20 was recorded from the Draughton
Shales in Hambleton Quarry near Bolton Abbey railway station. It occurred as
blue, green and white prismatic crystals up to a quarter of an inch in length between
the lamination planes of a fairly thin bedded shale. The long axes of the crystals
were perpendicular to the laminations. It is probably significant that there was no
calcite in the shale which might have provided the calcium necessary for the formation of gypsum. Palache, Berman & Frondel (1951) state that melanterite is usually
associated with pyrite ores as a secondary mineral but may also be in coal and
lignite deposits and in sheltered crevices in sedimentary rocks. Ferrous sulphate
is not exceptionally stable and it is surprising that it has not been oxidized to a
ferric form such as goethite or jarosite, KFe:~(SO~)2(OH)~. Hartley (1957) has
described jarosite from the Bowland Shales near Bolton Abbey and it would seem
that this is the normal form of secondary iron sulphate Jn shales. This melanterite,
however, occurs in a predominantly quartz-kaolin shale which is unlikely to contain
much potassium, thus precluding the formation of jarosite.
In many of the specimens brown and yellow encrustations were seen on the
lamination and joint surfaces in the shales. These were interpreted as secondary
iron oxides and identifiable as hematite, goethite and Iimonite.
In the massive mudstone from the Haw Bank Limestone, a well crystallized
kaolinite was found on the lamination surfaces. This would appear to be secondary
and derived from the poorly crystallized fireclay in the shale. The dolomite and
siderite are probably secondary. The dolomite in the Haw Bank Limestone occurs
X-ray investigation of argillaceous rocks
37
commonly, but only in specimens with considerable calcite. The significance of this
is uncertain.
It seems that the many causes of uncertainty in the interpretation make the
conclusions to some extent tentative. The correlation of the composition and
properties of the argillaceous rocks with their geological history is a difficult matter
as compared with the fairly straightforward analysis of the samples. There is a
clear need for data on the clay mineral components of rocks as a routine aid to
interpretation. Even with such data, however, the interpretation is at times far from
precise. There have been other cases where X-ray and older methods have been in
direct opposition as regards their conclusions; the status of chamosite related to
the chlorite and kaolin groups, and the composition of certain components of the
oolitic ironstones can be quoted as examples.
The importance of this study lies not only in the details of the results but a!so
in the possible uses of X-ray techniques in the study of problems relating to
Carboniferous facies and other related topics.
REFERENCES
A.S.T.M. INDEX(1949) Am. Soc. Testing iVIaterials, Alphabetical and Grouped Numerical Index of
X-Ray Diffraction data, and Supplements.
BLACKW.W. (1957) Trans. Leeds geol. Ass. 7, 24-33.
BRINDLrVG.W. (1951) X-ray Identification of Crystal Structures of Clay Minerals, Mineralogical
Society, London.
BRINDLEYG.W. & ROBINSONK. (1948) Trans. Leeds geoL Ass. 6, 75-94.
BUCKLEYH.E. (1951) Crystal Growth, Wiley, New York.
HAR'n-EYJ. (1957) Trans. Leeds geol. Ass. 7, 19-23.
HUDSONR.G.S. & MITCHELLG.H. (1937) Mere. geoL Surv. Summ. Prog. Pt. II, I--45.
KUENENP.H. (1950) Marine Geology Wiley, New York.
MILLOTG. (1952) Problems of Clay and Laterite Genesis Symposium of the American Institute of
Mineral, Meteorological and Petrological Engineers, New York, 107-114.
MURRAYH.H. (1954) Clays Clay Miner. 2, 46457.
MURRAYH.H. & GRAVENORC.P. (1953) Science, N. Y. 118, 25-28.
PALACHEC., BERMANH. & FRONDELC. (1951) Dana's System ofAIineralogy 7th edn, Wiley, New
York.
YOUELL R.F. (1960)An electrolytic method for producing chlorite-like substances from montmorillonite, Clay Miner. Bull. 4, 191.