AN SEM STUDY OF PHYLLOSILICATE DIAGENESIS IN SANDSTONES AND MUDSTONES IN THE WESTPHALIAN COAL MEASURES USING BACK-SCATTERED ELECTRON MICROSCOPY

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1 Clay Minerals (1986) 21, AN SEM STUDY OF PHYLLOSILICATE DIAGENESIS IN SANDSTONES AND MUDSTONES IN THE WESTPHALIAN COAL MEASURES USING BACK-SCATTERED ELECTRON MICROSCOPY J. M. HUGGETT BP Petrleum Develpment Limited, Britannic Huse, Mr Lane, Lndn EC2 Y 9BU (Received 18 July 1985; revised 31 July 1985) A B S T R A C T : Back-scattered electrn imaging was used in cnjunctin with energy-dispersive spectral analysis t study phyllsilicate minerals in plished thin-sectins f Carbniferus (Westphalian) sandstnes and assciated mudstnes frm the East Midlands and frm the diageneticauy higher grade area f Abernant in the Suth Wales calfield. Chemical diagenesis prceeded mre rapidly in the sandstnes than in the mudstnes. In the East Midlands, greater flw f freshwater thrugh the sandstnes than the mudstnes prduced a higher degree f alteratin f muscvite t kalinite in the sandstnes. Chlrite is present in the Well 1 and Well 2 East Midlands sandstnes but nt the mudstnes nr any Well 3 samples. The chlrite may be an early diagenetic replacement f bitite. Much chlrite has been replaced by kalinite. In the Abernant samples chlrite is present in bth the sandstnes and mudstnes. In the frmer it appears t have replaced detrital bitite. In the latter the chlrite is t fine-grained t discern whether it is detrital r authigenic. The chlrite may have frmed during the Hercynian rgeny. Much f the mica and chlrite in the Abernant sandstnes has been replaced by illite, pssibly in the pst-rgenic perid. The scarcity f kalinite in the Aberr.ant samples reflects the lack f freshwater leaching. This paper cmpares phyllsilicate diagenesis in interbedded Westphalian mudstnes and sandstnes, frm a range f diagenetic grades, using back-scattered electrn imaging (BEI) f carbn-cated, plished thin-sectins in an SEM with a semi-quantitative EDS analysis facility. XRD was used fr phyllsilicate identificatins. Use f plished specimens vercmes the prblems f artefacts in the characteristic spectra encuntered with X-ray analyses btained by fracture-surface SEM studies. Analyses can be btained frm grains f 2/~m diameter r greater. In BEI mde, areas f different mean atmic number prduce the image cntrast. Thus minerals can be identified by calibrating the grey tnes in the image against knwn samples (e.g. quartz) and by the use f characteristic mrphlgies. The ptimum magnificatin f the Camscan Series 4 micrscpe in BEI mde is frm x4 t x5, which facilitates petrgraphic studies f mudstnes as well as sandstnes and therefre their direct cmparisn. The cmbinatin f the imaging reslutin f an SEM, the image cntrast f BEI and the semi-quantitative analytical facilities is cnsequently a very useful tl fr detailed studies f diagenetic transfrmatins f phyllsilicate minerals The Mineralgical Sciety

64 J. M. Huggett SAMPLING Samples were btained frm three BP wells in the East Midlands and frm an NCB brehle in the anthracite zne f the Suth Wales Calfield at Abernant, Mid-Glamrgan.

2 64 J. M. Huggett SAMPLING Samples were btained frm three BP wells in the East Midlands and frm an NCB brehle in the anthracite zne f the Suth Wales Calfield at Abernant, Mid-Glamrgan. The East Midlands samples were frm depths f ~7 m in Well 1,9 msn Well 2 and 15 m in Well 3. Whilst nt maximum burial depths, the amunt f uplift is unlikely t have varied significantly between wells. Well 3 samples may therefre be assumed t have been the mst deeply buried, and thse frm Well 1 the least buried. The Abernant samples were btained frm a present-day depth f ~75 m, thugh maximum burial was prbably much greater. The burial histry is cmplicated in Suth Wales by flding and faulting during the Hercynian rgeny. Clearly burial depth is an inapprpriate measure f maximum diagenetic grade. Vitrinite reflectance was therefre used as this is a guide t thermal maturity. Vitrinite reflectance values fr the East Midlands are lw--r max% = (persnal cmmunicatin, P. M. Curran, BP Research Centre, Sunburyn-Thames) in the vicinity f the three wells. This range f values crrespnds t a lw diagenetic grade. Vitrinite reflectance values frm the Abernant Clliery indicate R max% = 2.32 (Gill et al, 1979), which crrespnds t a high diagenetic grade. The apprximate thicknesses f the sampled sandstnes and mudstnes, and the sample depths, are shwn in Table 1. Petrgraphy RESULTS The detrital mineralgies f the sandstnes d nt vary significantly between the fur lcatins. The sandstnes are all sub-lithic quartz-arenites. The detrital mineralgy cnsists predminantly f quartz with subrdinate feldspar, minr muscvite, chlrite, iuite and rare bitite. Authigenic minerals are quartz vergrwths, kalinite, chlrite, siderite, calcite, ankerite, ferran dlmite and minr pyrite. Authigenic kalinite is nly abundant in the East Midlhnds samples and authigenic chlrite nly in the Abernant samples. Tw carbnates nly (usually siderite and ferran dlmite) are generally present. Sme pressure slutin has ccurred, particularly in the Abernant sandstnes. Detailed descriptins f the petrgraphy and diagenesis f ther similar Westphalian sandstnes frm the East Midlands are given by Huggett (1982, 1984). The mudstnes frm the East Midlands differ slightly frm thse frm Abernant (Table 2), the frmer cntaining abundant silt-grade quartz, feldspar and muscvite whilst the latter cnsist almst entirely f clay with minr silt-grade muscvite and chlrite. Minr kalinite was identified by XRD in all the mudstnes, thugh it was nt bserved with SEM in the Abernant mudstnes. In Well 3 bth the sandstnes and mudstnes cntain detrital muscvite, illite and authigenic kalinite. In Wells 1 and 2 the sandstnes cntain chlrite, muscvite, illite and authigenic kalinite, but the mudstnes cntain n chlrite. In the Abernant brehle muscvite, illite and chlrite are present in bth the sandstnes and mudstnes. Phyllsilicate diagenesis Phyllsilicates identified by EDS with the SEM were chlrite, muscvite, bitite, illite and kalinite (Table 2). Over 15 analyses were btained. The analyses quted in the text

3 SEM phyllsilicate diagenesis in sandstnes and mudstnes 65 ~ rl,~, ~ ~ 66&6 >, [.., r,~ C m AA&6.1

4 66 J. M. Huggett TABLE 2. Phyllsilicates present in the sandstnes and mudstnes f Wells I-3 and the Abernant brehle. (d) = detrital; (a) = authigenic. Sandstnes Mudstnes Well 1 muscvite (d) muscvite (d) chlrite (d) rare kalinite (a?) kalinite (a) illite (d) minr iuite (d) Well 2 muscvite (d) muscvite (d) chlrite (d) rare kalinite (a?) rare bitite (d) illite (d) kalinite minr iuite (d) Well 3 muscvite (d) muscvite (d) kalinite (a) rare kalinite (a?) minr illite (d) illite (d) Abernant brehle muscvite (d) muscvite (d) chlrite (d) chlrite (d) illite (a + d) illite (d) have been calculated as xides nrmalized t 1%. The analyses are listed in Tables 3-6 and the pints at which they were taken are shwn in Figs 1-1. Because these are nly semi-quantitative analyses, the data have been used t demnstrate the general nature f the diagenetic reactins rather than t calculate specific reactins fr individual phyllsilicate grains as in a previus study (White et al., 1985). Slightly high SiO 2 values are prbably artefacts caused by adjacent quartz grains. The analyses are presented as AFK plts (Fig. 12) and have als been pltted by the methd f Velde (1977) with the c-rdinates MR3-2R3-3R 2 (Fig. 11). The calculatins invlved in prducing these diagrams are: MR 3 = Na + + K + + (2Ca 2+) 2R 3 = (A13+ + Fe 5+ _ MR3)/2 3R 2 = (Mg 2+ + FEZ+)/3 The MR a ple represents the bulk cmpsitin f feldspars, these being the mst alkali-rich minerals assciated with clay minerals; kalinite and ther pure alkali-free dictahedral phases plt at the 2R 3 ple. Talc and ther pure alkali-free trictahedral phases plt at the 3R 2 ple. Illite as a detrital clay is present in all samples. As it des nt appear t have participated in the diagenetic reactins it is nt discussed. East Midlands samples Well 1. The mudstne samples cntain mstly illite with lesser amunts f silt and clay-grade muscvite, quartz and authigenic pyrite (Fig. 1). The muscvite appears fresh,

SEM phyllsilicate diagenesis in sandstnes and mudstnes 67 TABLE 3. Semi-quantitative EDS analyses f phyllsilicates frm Well 1. 693.29/1 693.29/2 7.1/1 7.1/2 7.1/3 7.1/4 7.1/5 Analysis (Fig. 1) (Fig.

5 SEM phyllsilicate diagenesis in sandstnes and mudstnes 67 TABLE 3. Semi-quantitative EDS analyses f phyllsilicates frm Well / /2 7.1/1 7.1/2 7.1/3 7.1/4 7.1/5 Analysis (Fig. 1) (Fig. 2) si TiO A Fe23/FeO MnO MgO CaO Na KzO i.e. the layers are nt splayed apart, but analyses indicate that it is very slightly K-depleted (see Table 3, analyses /1 and /2) cmpared with values f 1-11% fr fresh muscvite (Deer et al., 1966). N chlrite is present in the mudstne. Minr kalinite was detected by XRD but it was nt identified with the SEM. PhyUsilicate minerals present in the sandstne samples are detrital muscvite and ferran chlrite, authigenic kalinite and minr detrital illite. The muscvite and chlrite grains are up t 5 gm lng. N analyses f fresh muscvite r chlrite were btained, and nne f the grains were f fresh appearance. All muscvite analyses indicate K-depletin (e.g. Table 3, 7.1/1 and 7.1/3). Muscvite is cmmnly intergrwn with kalinite and assciated with pre-filling bks f kalinite. Analyses f the kalinite intergrwths in Fig. 2 (Table 3,7.1/2 and 7.1/5) indicate sme cntaminatin, prbably due t fine intergrwths f muscvite which are t narrw t be reslved with the SEM, ccurring within bands f kalinite. Analysis f the kalinite bks in Fig. 2 (Table 3, 7.1/4) indicates that they are almst 'pure' kalinite. The tw mrphlgical frms f kalinite may be due t tw distinct mechanisms f frmatin. The kalinite intergrwths may be in situ replacements and the kalinite bks may have precipitated frm slutin. Chlrite grains als have kalinite intergrwths. Sme grains shw patchy grey and white cntrast prbably due t variatin in irn cntent. Well 2. The muscvite in the mudstne samples appears fresh but is very depleted in K (K~O mstly 3-5%). Variatin in irn cntent is reflected in the range f grey tnes shwn by the muscvite (Fig. 3). Sme muscvite grains shw faint tnal banding, which with their very lw K2 values (Table 4, analysis 92.6/2) suggests fine-scale kalinite intergrwths; these may crrespnd t traces f kalinite detected by XRD. N chlrite was detected in the mudstne samples. In the sandstnes, grains f detrital muscvite and ferran chlrite ccur cmmnly with kalinite intergrwths (Figs 4 and 5). Nne f the analyses f the phyllsilicates in Fig. 4 is f a pure phase. The light grey areas have a cmpsitin which is prbably leached bitite (Table 4, analyses 924.4/3 and 924.4/4), and the intermediate grey areas have a cmpsitin which is prbably leached muscvite, perhaps cntaminated with kalinite. The bitite has less K than the muscvite suggesting preferential leaching f the bitite interlayer catins. The bitite als has very high SiO 2 values, prbably due t 'cntaminatin' by adjacent quartz grains. The dark grey areas infilling the areas between

6 68 J. M. Huggett z~;~~ ~ i ~!~i ~ i,~ ~i...~ ~i~, ~... ~i~:~,!:~i~ ~, i~, ' ~ ~ ~!~i~ ~ ~,~ i~,~ ~ ~, '~ -~~ 9 i~, ;;~,i~ ~ %!i 475. ~; ~!i~ ~ 9~ ~, ~,i!~ii~iii~ii~ ~ ~ ~ i ; ~ '~i~, ~ 7! i ~ i ~ 7, 3 ~ 9, 9

SEM phyllsilicate diagenesis in sandstnes and mudstnes 69 and beynd the muscvite fragments are prbably kalinite with muscvite cntaminatin, either frm the visible fragments r frm fine-scale

7 SEM phyllsilicate diagenesis in sandstnes and mudstnes 69 and beynd the muscvite fragments are prbably kalinite with muscvite cntaminatin, either frm the visible fragments r frm fine-scale intergrwths. (This example indicates sme f the uncertainties which arise when attempting t identify fine-grained mixtures f phases and t recgnize incmplete reactins.) Fig. 5 shws an irn-rich chlrite (Table 4, analyses /2 and /3) intergrwn with kalinite (Table 4, analyses /1 and /4). The mttled appearance f the chlrite may reflect variable irn cntent. The minr K cntent f bth chlrite and kalinite may be due t a few mica layers r t cntaminatin frm adjacent illite. In this example very thin remnants f chlrite are visible intercalated with the replacive kalinite. Well 3. In the mudstne samples there is abundant silt-grade muscvite, but n chlrite was identified. In a few grains, intergrwths f mre than ne phase, e.g. analyses /1 and /2 (Table 5 and Fig. 6), indicate muscvite and kalinite. Analyses f grains withut visible intergrwths frm the same sample (1475.7/ /6) indicate very similar cmpsitins. These grains may als be intergrwths f kalinite and muscvite but t fine t be reslved with the SEM used. In the sandstne samples fresh muscvite is very rare and n chlrite was detected. The muscvite is generally depleted in K and has cmmn brad intergrwths f kalinite up t 2 #m wide. Hwever, sme grains have been prtected frm leaching pre-fluids by detrital illite (Fig. 7), have very few kalinite intergrwths, very little lss f K (Table 5, analysis / 1), r physical separatin f the phyllsilicate 1 ayers. Abernant brehle The mudstnes cntain less silt-grade material than the East Midlands mudstnes and als differ in having sme silt-grade ferran chlrite in additin t muscvite and illite. N intergrwths were visible in the phyllsilicates (Fig. 8), but may be beynd the limit f detectin. The analyses indicate a mderate Fe and Ti cntent, and lw K (Table 6, 746/1-746/5). In the sandstne samples, bth muscvite and chlrite are present. The chlrite spradically appears fresh (Fig. 9; Table 6, 74/5). Hwever, bth chlrite and muscvite grains are extensively replaced by illite (Fig. 1). Frm the analyses it is generally uncertain whether bitite r chlrite has been replaced by illite. Relatively large Ti cntents suggest FIG. 1. Well 1 mudstne. Detrital muscvite (analyses 1 and 2) and quartz in a matrix f illite. Bright white 'grains' are authigenic pyrite frambids. FIG. 2. Well 1 sandstne. Muscvite (analyses 1 and 3) with intergrwths f kalinite (analyses 2 and 5). Kalinite bks (analysis 4) have partially filled the pre space. The bright white rhmbs with the muscvite/kalinite intergrwth are siderite. FIG. 3. Well 2 mudstne. The range f grey tnes shwn by the muscvite grains (analyses 1-6) reflects the variatin in irn cntent. The ther grains are quartz and the fine-grained white mineral is ankerite. FIG. 4. Well 2 sandstne. Muscvite intergrwn with kalinite (analyses 1, 2 and 5). Pssible leached bitite (analyses 3 and 4). The bright white fine-grained mineral is siderite. FIG. 5. Well 2 sandstne. Irn-rich chlrite (analyses 2 and 3) intergrwn with kalinite (analyses 1 and 4). FIG. 6. Well 3 mudstne. Silt grade 'muscvite' and quartz in an illite matrix. The muscvite appears t have fine-scale kalinite intergrwths which are nt always reslved (analyses 1-6). The bright white mineral is pyrite.

8 61 J. M. Huggelt ~6~66666~ ~ 6 ~ ~ I "~ ~B ~6~66666& ~t &6&&6&66&,q m ~ _ &66~666& e~ 6&&~ ~&6&666 6&4&~666&. '.L r~.d m &6A~6&66@ m < [.. ~ A & ~ &6~A6~66~ ~aias~a,a ~ 6 ~ ~ &6&&6&66~ ~ 6 ~ ~ ~6~A6a664 ffff~ffoo Nz~ lm < gg~~ X~ ~

S E M phyllsilicate diagenesis in sandstnes and mudstnes 611 FIG. 7. Well 3 sandstne. Detrital muscvite (analysis 1) prtected frm leaching pre-fluids by detrital illite (analysis 3).

9 S E M phyllsilicate diagenesis in sandstnes and mudstnes 611 FIG. 7. Well 3 sandstne. Detrital muscvite (analysis 1) prtected frm leaching pre-fluids by detrital illite (analysis 3). Only minr alteratin t kalinite has ccurred at the grain terminatins and in the smaller muscvite fragment (analysis 2). FIG. 8. Abernant brehle mudstne. Less silt-grade material is present than in the mudstnes frm Wells 1-3. The pale elngate grains (e.g. analysis 3) are mstly chlrite. FIG. 9. Abernant brehle sandstne. Apparently fresh chlrite. Analysis 5 implies that it is either partially altered t illite, r the analysis is 'cntaminated' by the mass f illite t the left f the chlrite grain. FIG. 1. Abernant brehle sandstne. Chlrite r bitite (analyses 2 and 4) being replaced by illite (analyses 1 and 3). The relatively high Ti values suggest that bitite is mre likely t be the parent mineral. that the riginal phyllsilicate in Fig. 1 was bitite (Table 6, analyses 74/1-74/4). X R D results indicate that m i n r kalinite is present, althugh it was nt bserved by SEM. Discussin f the MR 3-2R 3-3R 2plts (Fig. 11) The m u d s t n e analyses in Well 1 (Fig. 1 la) cluster a r u n d illite, in Well 2 (Fig. 1 l b ) the range trends slightly t w a r d s kalinite, and in Well 3 (Fig. 1 lc) there is a b r a d spread f analyses between illite and kalinite. This reflects the increase in authigenic kalinite with depth. In the A b e r n a n t brehle (Fig. 1 ld) the range lies between the areas where 'pure' illite and ' p u r e ' chlrite wuld plt, suggesting that the grains a n a l y s e d are intergrwths f illite and chlrite, b e y n d the reslutin f the S E M used.

10 612 J. M. Huggett ~~-..~.< A~6&66~6~ -q cm ~&&A66&64 u~ &6&A66&~4 ~6 6 ~ A <i ~ < ~ z ~

11 SEM phyllsilicate diagenesis in sandstnes and mudstnes 613 "6 I ~-'.-- / ~ r.l: I g I a I ~..,;;;,' ij,#j ~., 9 ~..=_ ( 9 ii ~ i I 8 ~_ II II II I! I

12 614 J. M. Huggett The sandstnes shw a greater spread f analyses than the mudstnes because the grains are larger and analyses culd mre ften be btained frm a single phase in the frmer, rather than frm a fine-scale multiphase intergrwth. In the East Midlands wells the spread is als greater because chlrite ccurs in the sandstnes but nt the mudstnes. The 'pure' chlrite analyses are generally frm relatively large unaltered grains. 'Pure' kalinite analyses in Fig. 1 l a are frm kalinite bks (Fig. 2). There are n pure kalinites in Fig. 1 lc because all the analyses were f intergrwths. The chlrite analyses pltting midway between 2R 3 and 3R 2 in Fig. 1 ld are frm the fresh chlrite shwn in Fig. 9. The gradual lss f chlrite with increasing well-depth is shwn by the narrwing spread f sandstne phyusilicate analyses frm Wells 1 t 3. This lss f chlrite with depth is due t replacement by kalinite (Fig. 5). Discussin f the AFK plts (Fig. 12) Of the three elements, A1 is mst abundant in all analyses except thse f a few irn-rich chlrites (Figs 12a,b). In the East Midlands wells, sandstne and mudstne analyses shw a strng mve away frm Fe in the phyusilicates twards A1, with increase in depth (Figs 12a-c). The prprtin f K decreases slightly with depth, reflecting the leaching f micas and their replacement by kalinite. The sharp decrease in Fe reflects the replacement f bitite and chlrite by kalinite. In the Abernant brehle there is generally higher Fe and less K than in Wells 1 t 3 (Fig. 12d), reflecting the greater prprtin f chlrite and scarcity f muscvite in the Abernant samples. As in Fig. 11, the sandstnes shw a greater cmpsitinal spread than the mudstnes because the grains are larger and analyses were btained mre frm a single phase than fine-scale plyphase intergrwths in the mudstnes. CONCLUSIONS This brief study shws that it is pssible t make direct petrgraphic bservatins f mudstnes and thus directly cmpare mudstne and sandstne diagenesis. Diagenesis has further diversified initially similar ranges f phyllsilicate types in interbedded sandstnes and mudstnes. In the sandstnes, chemical diagenesis has prceeded mre rapidly than in the mudstnes due t higher prsities and permeabilities and cnsequent greater pre-fluid flw. In the East Midlands wells, greater flw f freshwater thrugh the sandstnes than the mudstnes had prduced a higher degree f alteratin f muscvite and chlrite t kalinite in the sandstnes. The tw kalinite mrphlgies may indicate tw mechanisms f frmatin: the pre-tilling bks having precipitated frm slutin, and the intergrwths with ther phyllsilicates being an in situ replacement. The degree f kalinizatin increases with well depth, thugh whether the increased burial is a genuine cause f the reactin is uncertain. The kalinizatin may be very early, have ccurred at the Permian uncnfrmity, r may be recent. The latter tw are unlikely t be the nly causes because this wuld have resulted in a reduced degree f leaching with depth. The fact that chlrite ccurs in the tw shallwer wells but is absent frm the deepest ne further suggests that uncnfrmity leaching is nt the prime cause f kalinizatin. Hence early kafinite frmatin seems the mst prbable mechanism.

13 SEM phyllsilicate diagenesis in sandstnes and mudstnes 615 v r c t~ "6 = ~6 tn -[3 ~_ t~ t~ " E ~6 v E v < e~ ~L >, >. = t~ "6 "6 = "6.< t~ t'~

14 616 J. M. Huggett In the Abernant samples chlrite is present in bth the mudstnes and sandstnes. In the latter it appears t have replaced bitite; in the frmer it is t fine-grained t discern whether it is detrital r authigenic. It wuld nt be unreasnable t expect chlrite frmatin during the Hercynian rgeny, when a high diagenetic grade was reached in the Abernant area. Hwever, the pssibility f a detrital surce cannt be discunted. The illitizatin f much f the mica and chlrite prbably ccurred during the pst-rgenic phase f uplift and ersin. The scarcity f kalinite reflects an absence f significant freshwater leaching. ACKNOWLEDGMENTS l am grateful t BP Petrleum Develpment Limited and the NCB (Suth Wales) fr permissin t publish this paper. I particularly wish t thank Mr Steve Dumpletn (NCB) fr supplying cre material. REFERENCES DEER W.R., HOWIE R.A. & ZtrSSMAN J. (1966) An Intrductin t the Rck-Frming Minerals. Lngmans, 528 pp. GILL W.D., KnALAF F.I. & MASSOUD M.S. (1979) Organic matter as indicatr f the degree f metamrphism f the Carbniferus rcks in the Suth Wales Calfields. J. Pet. GeL L HUGGETT J.M. (1982) The grwth and rigin f authigenic clay minerals in sandstnes. PhD Thesis, University f Lndn. 1-IUGGETT J.M. (1984) An SEM study f phyllsilicates in a Westphalian Cal Measures sandstne using back-scattered electrn imaging and wavelength dispersive spectral analysis. Sediment. Gel. 4, VELDE B. (1977) Clays and Clay Minerals in Natural and Synthetic Systems. Elsevier, Amsterdam, 218 pp. WHITE S.H., HUGGETr J.M. & SHAW H.F. (1985) Electrn ptical studies f phyllsilicate intergrwths in sedimentary and metamrphic rcks. Mineral. Mag. 49,

Interference is when two (or more) sets of waves meet and combine to produce a new pattern.

Interference is when two (or more) sets of waves meet and combine to produce a new pattern. Interference Interference is when tw (r mre) sets f waves meet and cmbine t prduce a new pattern. This pattern can vary depending n the riginal wave directin, wavelength, amplitude, etc. The tw mst extreme