Clays and Clay Minerals, Vol. 30, No. 3,232-236, 1982.

NOTES

H I G H V O L T A G E E L E C T R O N M I C R O S C O P Y O F A U T H I G E N I C

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

Key Words--Authigenesis, Chlorite, Electron microscopy, Energy dispersive X-ray analysis, Illite, Kao- linite, Sandstone.

Previously published transmission electron microscopic (TEM) studies of clay minerals from sandstones have em- ployed conventional 100 kV instruments, and most have been made with clay extracts (e.g., Stoch and Sikora, 1976). There are two major disadvantages of this procedure. First, the tex-

tural or spatial relationships between the clays themselves and between the clays and other minerals cannot be ascertained, and second, no correlation can be made with optical thin sec- tions. Scanning electron microscopy (SEM) has also been ex- tensively used to study the morphology (e.g., Keller, 1976) and

Figure 1. Transmission electron micrographs of authigenic clay minerals in pore spaces in the Eagle Sandstone, typical mi- crostructure. The minerals can be identified either by SAD patterns and/or by "finger-print" analyses by STEM. The inserts show grains tilted to excite the basal reflections shown in the diffraction patterns. The grain in insert (i) is illite (basal spacing 9.8 ]k), in (ii) is kaolinite (basal spacing 7.2 ]k), and in (iii) is chlorite (basal spacing 13.8 A). Grains marked q are quartz.

Copyright �9 1982, The Clay Minerals Society 232

Vol. 30, No. 3, 1982 Electron microscopy of clay minerals in sandstones 233

Figure 2. The same area as Figure 1 but taken at 100 kV. Note the much smaller area transparent to the beam.

textures (e.g., Bennet et al., 1981) of clay minerals in sand- stones, but the very nature of the specimens for SEM pre- cludes thin section correlation. Furthermore, the fractograph- ic methods of SEM specimen preparation prevents unequivocal determination of the complete sequence of diagenetic clay mineral formation in any one sample because fractures in sandstones are normally intergranular and thus bias the ex- posed surfaces to those exhibiting minerals formed in the later stages of diagenesis.

In this note we show that by using a 1000 kV high-voltage transmission electron microscope it is possible to study the microtextures and microstructures of the authigenic clay min- erals in the pore spaces of a sandstone, the structure of quartz and other mineral overgrowths, and the textures and struc- tures of any minerals that may be included within the quartz overgrowths. Thus, high-voltage electron microscopy (HVEM) can be used as 'an ultra high-resolution petrographical micro- scope.'

METHODS

Materials

The sandstone used in this study was the Eagle Sandstone from the Coal Measures (Westphalian B) of the East Midlands Basin, U.K.

Preparation of electron-transparent samples The sandstone specimens for TEM were prepared from

standard uncovered, petrologic thin sections using an alcohol- soluble adhesive (Lakeside 70-C cement). Copper grids (3 mm in diameter) were glued with a nonalcohol-soluble adhesive (e.g., araldite AY 18 with hardener HZ 18 or s ecotine) over se- lected areas of the thin section. The entire thin section was then floated off the glass slide in alcohol, and the selected areas were cut out of the section. The selected areas were further

thinned in an argon ion-milling machine (Barber, 1970). For conventional 100-kV TEM the foil thickness had to be less than 0.5 /zm, whereas for HVEM the optimal results were achieved with thicknesses between 1 and 2/zm. To ensure that

AI

CuK~

CuKB

ENERGY

5i CuKa

z AI

Mg

CuK~

ENERGY

Figure 3. XRA traces or "finger-print" analyses by STEM of the grains in insert (i) and (iii) of Figure 1.

234 Huggett and White Clays and Clay Minerals

Figure 4. Electron micrographs showing the effect of increasing voltage on mineral penetration. Little detail can be seen in (a) taken at 100 kV; some detail appears in (b) (200 kV); whereas in (c) (1000 kV~ the fine structure in the kaolinite grain can be seen, together with the small grains at the grain edge.

thinning was as even as possible, with minimal loss of entire grains, it was necessary to treat the porous sandstone samples with an impregnating agent, such as araldite, before preparing the thin sections. When thinned, the specimens were given a thin, conducting coat of carbon before insertion into the mi- croscope.

Identification of clay minerals by TEM Two methods of identification are available on transmission

electron microscopes; these are, selected area electron dif- fraction (SAD) and X-ray analysis. The clay minerals (i.e., kaolin-group minerals, smecti tes, micas, and chlorites) can be readily identified by SAD techniques by their basal spacings. The tilting stages on most TEM instruments allow particles to be rotated into a position for the basal planes to diffract. This

can be detected m micrographs as the grains appear at their darkest. Examples are shown in Figure 1.

The bulk chemistry of a mineral can be determined by anal- ysis of X-rays (XRA) emitted as a result of electron irradiation of the sample. This technique is less precise for mineral iden- tification than SAD; however , it can assist in the identification of clay minerals which have similar d-spacings, e.g., different types of chlorites, smectites, etc. Mineral phases can also be differentiated by comparing "finger print" elemental analy- ses. Figure 3 shows the XRA traces for grain (i) and (iii) of Figure 1. Full analyses of the individual grains can also be ob- tained.

The analyses of micaceous clays from areas I and 2 in Figure 1 are given in Table 1. These analyses are only semi-quanti- tative and do not include water or elements lighter than atomic

Figure 5. Relationship between optical and electron micrographs. (All scale units are/xm.) (a) An optical micrograph of the prepared specimen foil. The area to be studied in the HVEM is arrowed. (b) Low magnification electron micrographs of the area selected for study (arrowed). Note a piece of the foil has dropped off during loading into the HVEM. (c) and (d) high magnification micrographs of the selected area. The holes arrowed are the same as those arrowed in Figure 1.

Vol. 30, No. 3, 1982 Electron microscopy of clay minerals in sandstones 235

Figure 6. (a) Dislocation network in a quartz overgrowth. The dislocations are approximately parallel to the growth direction. (b) Contact between the overgrowth O and host grain h. Note the difference in dislocation network style across the grain boundary. (c) An illite (basal spacing = 9.9 ]k) inclusion in a quartz overgrowth.

number 11. The data have been normalized to 100% and re- calculated as percentages of various oxides. The HVEM ap- paratus used in the present study lacked XRA facilities for rea- sons of lens geometry. Analyses were therefore performed on a JEOL 120CX scanning transmission electron microscope (STEM) with a LINK 860 energy dispersive spectrometer.

RESULTS AND CONCLUSIONS

Effect of increasing voltage on transparent area

Conventional TEM instruments operate at 100 kV, or oc- casionafly at 200 kV, and the area transparent to the electron beam at this voltage is substantially less than that at 1000 kV. The effect of increasing the accelerating voltage on the area of foil transparent to the beam can be seen in Figure 4. At 100 and 200 kV little detail of the kaolinite grain can be seen, whereas at 1000 kV the fine structure and the smaller particles at the grain edge become apparent. At 1000 kV it is also pos- sible to relate features seen in the electron microscope to those seen in the petrographic microscope (Figure 5). With 100-kV instruments such a comparison is impossible, and the spatial relationships between clays and other minerals are difficult to

Table 1. Semi-qualitative energy dispersive X-ray analyses of two illites (1 and 2) shown in Figure 3. The data have been normalized to 100% and recalculated as percentages of var- ious oxides.

Wt. % oxide

Grain 1 Grain 2

SiO2 61.0 50.3 TiO2 0.3 0.5 A1203 28.8 33.3 FeO 1.6 2.8 MnO 0.0 0.0 MgO 1.5 1.3 CaO 0.3 0.3 Na20 0.1 2.4 1<20 6.3 8.8

resolve even at 200 kV. The problem is mainly due to the fact that the clays etch much more slowly than does the quartz and araldite cement. Consequently, in very thin areas, such as those needed for 100 and 200 kV TEM studies, the quartz etch- es out, and the quartz-rich areas of the foils tend to fall apart.

microstructures and microtextures

Examples of the microtextures which can be observed by HVEM are shown in Figures 1 and 6. The montage of Figure 1 shows typical microtextures of a group of authigenic, inter- grown clay minerals. Most of these particles are morpholog- ically similar and are difficult to distinguish by SEM. How- ever, SAD and XRA in the STEM shows that kaolinite, micaceous clay and chlorite are intergrown.

The microstructures of a quartz overgrowth is shown in Figure 6a. Figure 6b shows the interface between a quartz overgrowth and its host grain. The dislocation networks in the overgrowth differ greatly from those in the host grain. The for- mer are growth dislocations which take up local strain during growth and which can aid in the growth process itself (Grant and White, 1978). These dislocations tend to be perpendicular to the growth interface. Dislocations in the old grain are typical of those in naturally deformed quartz (White, 1973) and reflect the metamorphic provenance of the quartz grain. Clay mineral inclusions within quartz overgrowth are shown in Figure 6c. Presumably, the quartz nearest the contact with the host grain was deposited before that at the present clay-quartz interface. The clays incorporated in the overgrowth therefore offer an opportunity to study changes in clay mineralogy during pro- gressive diagenesis as such clays are, by their position, pro- tected from the effect of changes of composition of pore fluids during diagenesis.

Advantages over other techniques

Scanning electron microscopy (SEM) has now become a widely used technique in the field of sandstone diagenesis, but the technique has serious petrographic shortcomings: (1) the observable surface is biased by the mechanical properties of the sandstone; (2) mineral identification is usually achieved by crystal habit augmented (or not) by XRA; (3) energy dispersive analyses of X-rays produced by rough surfaces suffer insu- perable problems of artefactual additions to the characteristic

236 Huggett and White Clays and Clay Minerals

spectra. The techniques discussed above for combined HVEM and STEM with X-ray analysis largely overcome the limitations of SEM. X-ray analysis by STEM has the advan- tage over that by SEM in that in thin foils there is minimal interference from surrounding grains. However, as can be seen from Figure 3, there is interference from the copper grid on which the specimen is mounted. The advantage over a con- ventional electron microprobe is the improved spatial reso- lution in TEM, and it is possible to analyze grains smaller than 0.1/~m in size.

SUMMARY

High voltage electron microscopy can be used both as an 'ultra-high resolution petrographic microscope' and, by the combined use of selected area diffraction and energy disper- sive X-ray analysis, as a precise analytical instrument. Al- though the preparation technique can equally well be used for 100 kV TEM, HVEM is better suited to the study of sand- stones because it enables thicker areas of foils to be examined, and mineral microstructures and microtextures canbe imaged over larger areas.

ACKNOWLEDGMENTS

We thank BP Petroleum Development Limited for the core material used in this study and H. F. Shaw and P. R. Grant for helpful criticism of the draft manuscript. We gratefully ac- knowledge financial support of the Natural Environmental

Research Council, J. Huggett through a research-studentship and S. H. White through grant GR/3/3848.

Department of Geology J .M. HUGGETT Royal School of Mines S.H. WroTE Imperial College of Science and Technology London, SW7 2BP, United Kingdom

REFERENCES

Barber, D. J. (1970) Thin foils of non-metals made for electron microscopy by sputter etching: J. Mater. Sci. 5, 1-8.

Bennet, R. M., Bryant, W. R., and Keller, G. M. (1981) Clay fabric of selected submarine sediments: Fundamental prop- erties and models: J. Sed. Petrography 51, 217-231.

Grant, P. R. and White, S. H. (1978) Cathodoluminescence and micro-structure of quartz overgrowths on quartz: Scan- ning Electron Microsc. 1978, 789-793.

Keller, W. D. (1976) Scan electron rnicrographs of kaolins collected from diverse environments of origin--l: Clays & Clay Minerals 24, 107-113.

Stoch, L. and Sikora, W. (1976) Transformations of micas in the process of kaolinitization of granites and gneisses: Clays & Clay Minerals 24, 156-162.

White, S. (1973) The dislocation structures responsible for the optical effects in some natura//y deformed quartzites: J. Mater. Sci. 9, 490--499.

(Received 15 December 1980; accepted 15 August 1981)