Clay Minerals (1984) 19, 471-481

A U T H I G E N I C C H L O R I T E S : P R O B L E M S W I T H C H E M I C A L A N A L Y S I S A N D S T R U C T U R A L

F O R M U L A C A L C U L A T I O N

C. D. C U R T I S , B. J. I R E L A N D * , J. A. W H I T E M A N , R. M U L V A N E Y t AND C. K. W H I T T L E

Departments of Geology and Metallurgy, The University of SheffieM. Mappin Street, Sheffield SIO 2TN

(Received 8 April 1983; revised 10 October 1983)

A B S T RA C T: Analytical transmission electron microscopy was applied to some authigenic chlorites occurring as grain coatings in sandstones. Compositional variation proved to be relatively slight: all were magnesian chamosites. The coating chlorites were often intimately mixed with extremely fine-grained (0.01-0.2 #m) hematite but analytical 'contamination' was avoided because of the very high resolution of both observation (spot location) and analysis. One example of a water sensitive ('swelling chlorite') coating was also studied. This proved to have a very much more variable composition even within a single section. The coating appeared to include both chloritic and vermiculitic components. The effect of this on structural formulae is discussed and a model proposed in which the 'talc' layer may be common to both components.

Chlorite is found as an authigenic mineral in many sandstones. It usually occurs as grain coatings, less commonly filling pores. It has been suggested that these coatings can be so continuous that they prevent overgrowth of framework grains from saturated porewaters, thereby preserving porosity.

The chlorite structure consists of a 2 :1 layer (talc-like) plus an interlayer hydroxide sheet (Bailey, 1980) which in sedimentary examples is iron-rich. This is readily attacked by acids such as those used in some borehole completions and the resulting solutions are unstable. Any trace of oxygen causes precipitation of gelatinous iron(III) hydroxides which reduce permeability. The same result is obtained when coatings are dislodged. Some 'chlorites' behave as if interstratified with smectites and are sensitive to changes in pore- water composition. Swelling may directly reduce permeability.

A subjective review of the literature suggests that much less time has been devoted to studying chlorites than to the other common clay minerals, smectite, illite and kaolinite. There seems to be general agreement that much authigenic chlorite forms at temperatures in excess of 150 o C during burial diagenesis, sometimes at the expense of kaolinite (Boles & Franks, 1979; Foscolos & Powell, 1979; Hutcheon et al., 1980). A similar transition from berthierine (7 A trioctahedral structure) to chamosite has been reported recently by Iijima & Matsumoto (1982). Few chemical data are available for different authigenic chlorite varieties in spite of the fact that detailed X-ray structural information has been available for some time (Hayes, 1970). Chlorites and 7 A trioctahedral structures would appear to

* Present address: Amoco Canada Petroleum Company Ltd., Calgary, Alberta, Canada. Present address: British Antarctic Survey, Madingley Road, Cambridge, UK.

�9 1984 The Mineralogical Society

472 C.D. Curtis et al.

contain dominantly divalent iron whereas in glauconite, the other important authigenic iron-rich clay, the iron is essentially in the ferric form.

The problem with most clay mineral studies is that it is virtually impossible to separate pure mineral concentrates for chemical analysis. Even electron microprobe analysis is somewhat unsatisfactory because the spot used is typically greater than the dimensions of authigenic clay particles (it is also very difficult to see what is being analysed). We present here new analytical data for a small selection of chlorites obtained by analytical scanning transmission electron microscopy (STEM). This technique overcomes these problems. Other 'microprobe' difficulties still remain. In particular, it is not possible to obtain direct estimates of water content or relative proportions of iron(II) and iron(III).

S A M P L E S S T U D I E D

Our initial aim was to characterize, chemically and mineralogically, an interesting chlorite present as grain coatings in the Cretaceous Tuscaloosa Sandstone of Louisiana, USA. Five samples were obtained from a borehole which encountered this formation at a depth of some 5.5 km (samples A01 to A05, interval 5553 to 5560 m). Two more were obtained from another hole which met the same formation at much shallower depth (A06 and A07, 2393 and 2394 m respectively). The deep samples are litharenites or sub-litharenites. All have relatively early chlorite rims and are variously iron-stained. Porosity has been reduced by limited kaolinite and carbonate precipitation. Nevertheless, porosity values >20% were recorded for some of these deeply buried rocks. The shallower Tuscaloosa samples are feldspathic litharenites with porosity >25%. Chlorite is again present as grain coatings and porosity is somewhat reduced by kaolinite.

For purposes of comparison two additional samples were investigated. A16 is a sub- litharenite from the Cretaceous Falher Formation of Alberta, Canada, sampled at a present burial depth of 1876 m. In addition to chlorite rims, this sandstone has an extensive (later) carbonate cement. Sample E79-1 contains 'swelling chlorite' as grain coatings and also partially replacing trachytic volcanic fragments. The 'chlorite' is pale olive-green in thin section; birefringence is low, although somewhat higher than that of the pale-green Tuscaloosa chlorite.

X-ray diffraction traces were taken from grain-coating 'concentrates' obtained by gently crushing and 'rolling' samples under distilled water in an agate pestle and mortar. Specific size fractions were then obtained by sedimentation and, in certain cases, subsequently saturated with various cations. Traces were taken from highly orientated smear-mounts on glass slides.

The Tuscaloosa sandstones all proved to be rich in chlorites, some concentrates being virtually monomineralic (Fig. 1). It can be seen from Fig. 1 that the 14- 15 A reflection is unaffected by glycollation but contracts to 13.6 A on heating. The 002 reflection virtually disappears. This behaviour is characteristic of chlorite.

The olive-green 'swelling chlorite' in sample E79-1 exhibits much more complex behaviour. The traces in Fig. 2 are from <0.5 /lm separates, the first three being K-saturated (air-dry, glycollated and heated at 550~ respectively) and the remainder equivalent traces from an Mg-saturated sub-sample. The K-saturated sample behaves much like the natural, untreated clay. The 14 A reflection is little affected by glycollation (slight expansion) and contracts marginally on heating. These features are indicative of chlorite.

Chemical compositions of authigenic chlorites 473

)

] I I o 7 10 14 A

FIG. 1. X-ray diffraction traces of Tuscaloosa chlorite. (a) 0.5-2-0 ~m fraction; air dry. (b) <0.5 /2m fraction; air-dry. (c) <0-5 Bm fraction; glycol vapour. (d) <0.5 /lm fraction;

550~ 1 h.

Conversely, however, the 001 reflection is more intense than the 002 reflection and part of the sample collapses to 10 tk on heating. This is indicative of a vermiculitic component.

The Mg-saturated sample expands (completely) to 16.6 A on glycollation and contracts on heating to give both 10 and 13 A reflections. This behaviour is intermediate between that of chlorite and vermiculite. It is possible that it is due to 'islands' of brucite interlayering within a talc-like structure, as envisaged for swelling chlorites by Brindley (1961, p. 286). The same general model was preferred by Bain & Russell (1981) to explain the properties of a swelling chlorite found in an altered Tertiary basalt. Within a heterogeneous structure of this type, 'islands' would tend to behave as chlorite whereas parts of the structure without interlayer brucite would behave much more like smectite or vermiculite, depending on interlayer charge.

Iron(II) and iron(III) determinations by rapid dissolution wet chemical methods were made on the coating concentrates for samples A01 and E79-1. The former contained almost exclusively ferrous iron whereas the latter had significant (~ 30%) ferric iron. These values were assumed in individual mineral calculations discussed below.

A N A L Y T I C A L S C A N N I N G T R A N S M I S S I O N E L E C T R O N M I C R O S C O P Y

Ultrathin sections were prepared from optical thin sections by ion-beam milling following the procedure of Phakey et al. (1972). These were then examined in the Phillips 400T

474 C. D. Curtis et al.

7 10 14 A

FIG. 2. X-ray diffraction traces of 'swelling chlorite' E79-1. <0.5 pm fraction, K + saturated: (a) air-dry; (b) glycol vapour; (c) 550~ 1 h. <0.5 pm fraction, Mg+-saturated: (d) air-dry;

(e) glycol vapour; (f) 550~ 1 h.

Analytical TEM/STEM system with EDAX energy-dispersive X-ray analysis. This system has many advantages for clay mineral work. Thin-section study allows textural information, and single-crystal electron diffraction patterns can be obtained from the analysis 'spot' . This, in turn, can be very much smaller than is possible with the electron microprobe (Doig et al., 1980). The more sharply focussed the beam, of course, the greater its intensity, and volatilization of elements such as Na, Ca and K can be a problem. The smallest acceptable area for a given mineral is determined by temporal analysis of line- intensity ratios. A more detailed account of analytical STEM methods, as used in this laboratory, is given by Ireland et al. (1983). Veblen (1983) described a range of complex phyllosilicate minerals, specifically those intermediate between mica and chlorite, using a similar approach.

X-ray emission spectra were processed as suggested by Cliff & Lorimer (1975). Absorption and fluorescence effects are minimized when X-rays are generated within very thin foils. The concentration ratio of any two elements in a sample thus approximates

Chemical compositions of authigenie chlorites 475

closely to the ratio of emitted X-rays. Proportionality constants ('k values') relative to silicon are obtained from thinned sections of standard minerals (muscovite, biotite, jadeite, sodalite and sphene). The analysis procedure apportions the total counts collected (after background and overlap corrections) amongst the elements sought according to 'k' values.

The system is inferior to microprobe analysis in that counts are normalized and not evaluated on an absolute basis. Some interference problems can arise as a result of contamination by grid support materials. These are usually obvious at the spectrum- generation stage.

S T E M D A T A F O R C H L O R I T E S

Figs 3a, c and d are electron micrographs of typical chlorite fabrics exhibiting differing densities. Crystals are commonly curved although not greatly so. Selected-area diffraction patterns (Figs 3e, f) from crystals in appropriate orientation show well developed 001 rows with a basal spacing close to 14.1 ~. Occasionally, as in Fig. 3d, a single crystal is sufficiently separated from its neighbours to give an almost perfect diffraction pattern (Fig. 3f). A high degree of ordering is indicated by these patterns and this is supported by lattice images such as that in Fig. 3g. Sometimes more complicated stacking sequences are suggested (Fig. 3h) and interstratifications can be seen (single broad, pale stripe in Fig. 3h). Great care must be exercised in these interpretations, however, since similar effects can arise for quite different (e.g. instrumental) reasons. Our TEM observations suggest precipitation of authigenic chlorite from solution; this is consistent with the numerous, and generally more visually attractive, scanning electron micrographs which have graced the recent literature. The degree of crystal perfection is perhaps slightly surprising since many low-temperature phyllosilicates are supposedly prone to stacking disorder.

Fig. 3b is a rather poor micrograph of a rather common phenomenon. Extremely fine hematite crystals (or pure, crystalline iron oxide) are found in intimate association with the chlorite flakes. They are commonly 0-01-0.2 /~m in diameter. Such 'contamination' would be impossible to avoid during preparation for classical methods of chemical analysis and would probably pass undetected at the time of electron microprobe microanalysis (spot diameter 1-5 r

We found that the chlorite coatings in the shallow-depth Tuscaloosa sample A06 were heavily contaminated by hematite and that it was also present, although to a much lesser extent, in some of the deep samples. The excellent optical system of the transmission microscope was of great value here as the analysis spot could be located precisely. TiO 2 (rutile?) is a less common, but not infrequent, fine-grained 'contaminant'.

The effects of admixed hematite on chlorite analyses is clearly illustrated in Fig. 4 which is a simple triangular diagram for silicon, total aluminium and total iron (as molar oxide proportions). The analyses plotted are of chlorite coatings from Tuscaloosa samples A01, 2, 5, 6, and Falher sample A16. Most cluster tightly near the centre of the diagram but there is a significant 'tail' to much more iron-rich compositions. This is simply a 'mixing' curve due to hematite contamination and has nothing whatsoever to do with compositional variation within chlorite. The few 'Fe-poor' analyses might also represent 'mixing', but this time due to overlap with illite which is more difficult to detect.

The majority of analyses in Fig. 4 are from samples A16 (14) and A01 (16). Average analyses are presented in Table 1. Variation within both is slight, suggesting equilibrium

4 7 6

O

C. D. Curtis et al.

b

C d

e

g h

FIG. 3. STEM data for chlorites. (a) A16 chlorite; 1 /~m = 6.6 cm. (b) A06 chlorite plus hematite; 1 /~m = 8-4 crn, (c) A16 chlorite; 1 #m = 6.6 cm. (d) A02 chlorite; 1 /~m 10.8 cm. (e) Diffraction pattern from (c) (part of). (f) Diffraction pattern from single crystal in (d).

(g) Lattice image, A16: regular 14.1 ,~ spacing. (h) Lattice image, A06: less regular stacking?

Chemical compositions of authigenic chlorites

si

AI Fe

FIG. 4. Compositional variations in Tuscaloosa authigenic chlorites. The 'mixing curve' represents contamination by extremely fine-grained hematite (molar proportions).

477

TABLE 1. Authigenic chlorite analyses.

Chlorites ' Swelling Chamosite Chlorite' 'Low' 'High'

AM011 AM161 Boles 2 Iijima ~ E79-11 E01J094 E01JI04

Si 5.88 5.93 5.84 5.10 6.93 6.19 7.86 AI 2.12 2.07 2.16 2.90 1.07 1.81 0.14 AI 3.33 3.39 3.88 2.81 2.53 2.13 4.02 Ti 0.00 0.01 0.00 0.00 0.03 0.04 0.04 Fe 3+ 0.21 0.20 0.00 0.00 1.35 1.50 1.03 Fe 2+ 4-99 4.93 3.66 7.76 3.00 3.33 2.30 Mn 0.00 0.02 0.00 0.00 0.02 0.08 0.01 Mg 2.71 2.56 3.47 1.55 3.26 3.70 1.46 Ca 0.03 0.10 0.06 0.00 0.06 0.12 0.06 Na 0.01 0.00 0.10 0.00 0.19 0.17 0.35 K 0.02 0.02 0.05 0-00 0.47 0-16 0-85 Oct. 11.30 11.23 11.22 12.12 10-19 10.77 8.86 Int. 0-06 0.12 0.21 0.00 0.72 0.45 1.25

Analytical STEM data, this work. AM01 value is mean of 18 analyses, AMI6 mean E79-1 mean of 10 analyses. Previously published analyses are electron microprobe data.

From Boles & Franks (1979). 3 From lijima & Matsumoto (1982). 4 High- and low-silica individual analyses of'swelling chlorite' sample E79-1.

of 14 analyses and

assemblages . The two average analyses are r e m a r k a b l y similar and no t too different

(Mg2+/Fe 2§ rat io only) f rom recent e lectron m i c r o p r o b e da t a for authigenic chlori tes f rom

the Wi lcox F o r m a t i o n (Boles & Franks , 1979). It is clear that diagenet ic condi t ions

prevai l ing when these three chlori tes precipi ta ted were essential ly similar. All are relat ively

rich in a lumin ium and have oc tahedra l cat ion tota ls significantly less than the twelve

expec ted for t r ioc tahedra l chlorites. They are m u c h less r ich in i ron than e lec t ron micro-

probe analyses o f chamos i t e s recent ly repor ted by I imima & M a t s u m o t a (1982), poss ibly

478 C.D. Curtis et al.

reflecting the influence of Mg 2§ in marine porewaters. According to recent recom- mendations of the AIPEA Nomenclature Committee (Bailey, 1980), however, all should be termed chamosites. Also included in Table 1 is the average of 10 much more variable analyses from the 'swelling chlorite' sample E79-1. The two individual analyses with highest and lowest silica are also included. This mineral apparently has a much smaller degree of tetrahedral A1 substitution and a somewhat lower octahedral total. Although ferrous iron still exceeds ferric, the latter is much more important than in 'true' chlorites.

Sample E79-1 contains significant potassium. This has to be present in interlayer sites, presumably as a K-smectite or vermiculite component. Such high values could not possibly be incorporated in a simple chlorite structure.

These deliberations are made without proper acknowledgement of all the assumptions needed to convert X-ray emission count rates to mineral formulae (as in both electron microprobe and STEM analysis). No method of analysis actually measures anion content: classical methods assume oxygen equivalent to determined metals. Modern instrumental methods also assume hydroxyl content and rely on separate determinations (different sample) for iron(II)/iron(IlI) ratio. Analyses such as all those in Table 1 assume mineral identification and then calculation from metals on the basis of an assumed anion (oxygen and hydroxyl) content. For chlorites this is 20 oxygens plus 16 hydroxyls, for micas 20 oxygens and 4 hydroxyls-- the difference being hydroxyls associated with the interlayer trioctahedral sheet.

An attractive model for swelling chlorites (to the thermodynamicist, that is) is one in which the 2:1 layer is fixed and interstratification amounts to substitution of simple cations for charged hydroxide sheets. Table 2 column 1 represents a chlorite analysis based on A01 and A016 (Table 1). Column 2 is the same analysis with one half of the octahedral cation content replaced by a 'charge-equivalent' amount of interlayer potassium. The assumption (for want of a better one) is that the octahedral component of the 2: I layer has the same cation composition as that of the interlayer hydroxide sheet. The latter may then be replaced by an equivalent (in charge terms) number of interlayer cations.

The analysis in column 2 amounts to a potassium-deficient biotite or K-vermiculite, In

TABLE 2. Hypothetical interstratified chlorite-mica with identical 'talc' sheets; inter- stratification due only to substitution of interlayer cations for 'brucite' sheet.

Chlorite 'Mica' Mica recalculated 2 octahedral sheets 1 octahedral sheet charge 1.05 + Basis 20 (O) + total charge 2.1 + interlayer cations 1.05 K § 16 (OH)

Si 5.90' A1 2.10 AI 3.36" Ti 0.00 Fe 3+ 0.21 Fe 2+ 4.96 Mn 0.00 Mg 2.68 Ca 0.06 Na 0.00 K 0.02

8.00

-11.28

5.90"~ 8.00 7.51" 2.103 0.49 1.68"] 4.32" 0.00~ 0.00 0-10~5.60 0.13 2.481 3.16 0 .00 | 0.00 1.34~ 1.66 0.03) 0.04 0.00~ 1.09 0.00 1.06J 1.35

8.00

,10.66

Chemical compositions of authigenic chlorites 479

order to evaluate the possible contribution of such a component to swelling chlorite analyses, however, the analysis must be re-expressed on the same basis as a chlorite (36 anions) because this is the way in which the swelling chlorite analyses were treated. This is done in column 3. The much lower tetrahedral aluminium content and octahedral total (for a chlorite) erroneously suggested by this calculation are particularly noteworthy.

Mixing of interlayer cations with hydroxide sheets (not necessarily by interstratification which would modify X-ray diffraction patterns) in a hypothetical vermiculite/chlorite is indicated by the formulae in Table 3. Substitution of interlayer cations for hydroxide sheets results most noticeably in higher silicon values and (obviously) higher Ca 2+, Na § or K § levels. Rather surprisingly, the interlayer total is rather insensitive to this substitution.

The average analysis of swelling chlorite E79-1 in Table 1 is not too far removed from the vermiculite mix suggested in Table 3. This is encouraging, but many more samples need to be analysed before more definitive pronouncements are made on the applicability

of this model. One notable discrepancy is the oxidation state of iron. The presence of significant ferric

iron suggests the possibility of interstratification with glauconite (dioctahedral, ferric mica). Table 4 lists average compositional data for a range of glauconites and, in column 5, a recalculated formula equivalent to chlorite. Various mixtures of this with chlorite are listed in Table 5. Comparison with the average analysis for E79-1 in Table 1 shows that a rather smaller proportion of glauconite, especially of the more aluminian variety, would approximate fairly closely to the composition of the 'swelling chlorite'.

The two sets of calculations above show that the presence of micaceous or vermiculitic layers or ' islands' within chlorites will radically alter chemical compositions when expressed as mineral formulae. The alteration, however, is predictable in nature and thus offers promise for identification of specific intergrades by chemical analysis: provided, of course, that a sufficiently small and carefully chosen (to avoid overlap between mineral grains) area is selected for analysis.

TABLE 3. Expected chemical composition from mica-chlorite mixtures with identical 'brucite' sheets.

Chlorites with cation sheets. AM01/16 10% 'mica'* 20% 'mica'* 50% 'mica'*

Si 5.90 6.06 6-22 6.71 A1 2.10 1-94 1.78 1.29 AI 3.36 3.46 3.55 3.85 Ti 0.00 0.00 0.00 0.00 Fe 3+ 0.20 0-19 0.19 0.16 Fe 2+ 4.96 4- 78 4- 60 4.06 Mn 0.00 0.00 0.00 0.00 Mg 2.68 2.58 2.48 2.17 Ca 0.06 0-06 0.06 0.05 Na 0.00 0.00 0-00 0.00 K 0.02 0.15 0-29 0.69 Oct. 11-28 11.22 11.17 10.98 Int. 0.08 0-21 0.35 0.74

* 'Mica' component is that in Table 2.

480 C. D. Curtis et al.

TABLE 4. Some representative glauconite analyses.

Glauconite analyses Odin ~ Ireland 2 Deer 3 Chlorite*

Si 7.31 7.28 7.30 9.29 A1 0.69 0.72 0.70 - - AI 0-56 1-79 0.70 1.78 Ti 0.02 0.02 0.01 0.01 Fe 3+ 2.41 1.35 2.35 2.99 Fe 2+ 0.20 0.34 0.20 0-25 Mn 0-00 0.00 0.00 0.00 Mg 0.80 0-46 0.80 1-02 Ca 0.16 0.05 0.08 0.10 Na 0.07 0.01 0.04 0-05 K 1.32 1.49 1.30 1.66 Oct. 3.99 3.97 4.06 - - Int. 1.55 1.55 1.42 - -

* Analysis 3 recalculated on the basis of 56 (20 (O) + 16 OH).

Odin & Matter (1981). 2 Ireland (1982). 3 Deer et al. (1962).

charge equivalents

TABLE 5. Expected chemical composition from glauconite/chlorite mixtures.

Chlorite/glauconite mixtures AM01/16 10% glauconite 20% glauconite 50% glauconite

Si 5.90 6-24 6-58 7.60 A1 2.10 1-76 1.42 0.40 AI 3.36 3.33 3.30 3.22 Ti 0-00 0.00 0.00 0-01 Fe 3+ 0.20 0.48 0.76 1-50 Fe 2+ 4.96 4.49 4.02 2.61 Mn 0.00 0- 00 0.00 0.00 Mg 2.68 2.51 2.35 1.85 Ca 0.06 0.06 0,07 0.08 Na 0-00 0.01 0-01 0.03 K 0-08 0.25 0.40 0.87 Oct. 11.28 11.21 10.91 10.17 Int. 0.14 0.32 0.48 0.98

C O N C L U S I O N S

Compositional variation within a small number of authigenic chlorite samples has been documented by analytical STEM. Amongst true chlorites, compositional variation is relatively slight and all would be termed chamosites according to recent AIPEA recom- mendations (presumably the qualifier 'magnesian' would be appropriate).

The excellent observational facilities offered by STEM reveal two kinds of analytical problem likely to be encountered in studies of authigenic chlorite mineralogy. The first is contamination by extremely fine-grained hematite. This can be avoided in STEM but extreme caution should be exercised when classical analytical methods are used. Even

Chemical compositions o f authigen& chlorites 481

electron mic roprobe methods m a y fail to resolve chlor i te /hemat i te mixtures. The second arises when minera l formulae are calculated: on wha t basis should they be when inter-

strat if ication is a possibi l i ty? A n example of 'swell ing chlorite ' proved to differ significantly in compos i t ion f rom the

m agnes i an chamosi tes . The presence of a vermiculi t ic c o m p o n e n t is suggested and the

effects of this on calculated minera l formulae have been explored. One possibi l i ty is that

simple cat ions subst i tu te for charged hydroxide sheets alongside 2: 1 layers of cons t an t composi t ion . Whi ls t this simple model is theoret ical ly attractive, other features of the

'swell ing chlori te ' could be explained by a glauconi t ic c o m p o n e n t - - i n which case two

quite different 2 :1 layers are involved. Latt ice images demons t ra t e inters trat i f icat ion directly bu t insufficient da ta have been accumula t ed to differentiate be tween the two

possibilities out l ined here.

A C K N O W L E D G M E N T S

The Amoco Production Company, Tulsa Research Center, is acknowledged both for financial assistance and provision of the Tuscaloosa samples. The remaining samples were provided by the Amoco Canada Petroleum Company and the Exxon Production Research Company, Houston. The Natural Environment Research Council has supported this work through research grants to C.D.C. and provision of research studentships to B.J.I. and C.K.W. The Science and Engineering Research Council is acknowledged for capital equipment provision. Alan Saxby provided wet chemical data and Susan Forster prepared the manuscript. Dr M. J. Wilson is thanked for helpful comments relating to structural interpretation.

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IRELAND B.J. (1982) Transmission electron microscopy of authigenic clay minerals. PhD thesis, University of Sheffield.

IRELAND B.J., CURTtS C.D. & WHITEMAN J.A. (1983) Compositional variation within some glauconites and illites and implications for their stability and origins. Sedimentology 30, 769-786.

ODIN G.S. & MATTER A. (1981) De glauconarium origine. Sedimentology 28, 611-641. PHAKEV P.P., CURT1S C.D. & OERTEL G. (1972) Transmission electron microscopy of fine-grained phyllo-

silicates in ultra thin rock sections. Clays Clay Miner. 20, 193-197. VEBLEN D.R. (1983) Microstructure and mixed-layering in intergrown wonesite, chlorite, talc, biotite and

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