NON-CRYSTALLINE HYDROUS FELDSPATHOIDS IN LATE PERMIAN CARBONATE ROCK

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1 Clay Minerals (1991) 26, NON-CRYSTALLINE HYDROUS FELDSPATHOIDS IN LATE PERMIAN CARBONATE ROCK C. BENDER KOCH Laboratory of Applied Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark (Received 19 December 1990) ABSTRACT: The EDTA-insoluble residues from five samples (two from the oolitic shoal facies and three from the lagoonal facies) of the Late Permian Ca-2 unit (Zechstein) have been investigated by X-ray diffraction, infrared spectroscopy, and scanning and transmission electron microscopy with energy dispersive X-ray analysis (EDXA). The results show that spheres of non-crystalline hydrous feldspathoids (with AI/Si molar ratio between 1.5 and 2.2) dominate the residues of samples from the oolitic shoal facies. Samples from the lagoonal facies are dominated by crystalline material (muscovite and quartz), but two of the samples contain a small number of spheres. Analyses of these samples by selective area diffraction and EDXA revealed the presence of small amounts of noncrystalline hydrous feldspathoids with AI/Si molar ratio between 0.1 and 1.9. Carbonate rocks commonly contain small amounts of non-carbonate minerals. Few investigations of carbonates have considered these non-carbonate minerals, although knowledge of their mineralogy may be helpful in characterizing the sedimentology and diagenesis of the carbonate rock. As only few of the non-carbonate minerals can be studied directly by optical microscopy methods, it is generally necessary to dissolve selectively the carbonate minerals to obtain a suitable amount of the non-carbonate minerals for studies by other methods. In a recent investigation of the Late Permian Ca-2 unit (Zechstein) from the Lcgumkloster-1 well (Denmark), non-crystalline material was found in the EDTAinsoluble residue (Stentoft et al., 1991). This investigation also indicated that abundant noncrystalline material occurred in the oolitic shoal facies, whereas only small amounts were indicated in the lagoonal facies (for details on the position of the well and carbonate diagenesis of the Ca-2 unit see Stentoft, 1990). Previous investigations on the noncarbonate minerals of the Zechstein units have studied the acid-insoluble residues (Ftichtbauer, 1958; Ftichtbauer & Goldschmidt, 1959; Smykatz-Kloos, 1966; Braitsch, 1971). These investigations reported the presence of well-crystallized diagenetic quartz, feldspars, chlorite and detrital muscovite in the residues, but made no mention of the presence of non-crystalline material. The purpose of this study is to give a more detailed description of the non-crystalline material in the EDTA-insoluble residues. Samples EXPERIMENTAL Three plugs from the lagoonal facies and two plugs from the oolitic shoal facies were investigated. Following crushing of the clean core plugs to mm size, the materials were The Mineralogical Society

2 528 C. Bender Koch powdered in an agate mortar. Dolomite and anhydrite were dissolved by boiling for <16 h in an EDTA solution adjusted to ph 11 with NaOH (Bodine & Fernalld, 1973) using an initial solid/solution ratio of 1 : 100 and analytical grade reagents. The residues were washed with water and weighed after drying at 60~ and 40% r.h. for 24 h. The amounts of sample treated and residue obtained are given in Table 1. Due to the small amounts of residue for most samples, no size fractionations were carried out. Methods The residues were investigated by X-ray diffraction (XRD) as unoriented and oriented samples using a Philips PW1050 diffractometer and Co-Ko~ radiation. Unoriented samples were prepared by backloading of the Na-saturated samples into a brass container with sample dimensions 30 x 17 x 1.5 mm. Oriented samples (100 mg of sample) were prepared using the glass slide method following Mg-saturation, and ethylene glycol vapour used to test for swelling minerals. Infrared (IR) spectra were obtained using a Perkin Elmer 580 spectrometer and the pressed pellet technique (1-3 mg in 300 mg KBr). Scanning electron micrographs (SEM) were obtained using a JEOL JSM-U3 instrument operated at 25 kv. Transmission electron micrographs (TEM) and selected area electron diffraction patterns (SAED) were obtained using a Philips EM 430 microscope operated with an acceleration voltage of 300 kv and the chemical composition of the particles determined using an EDAX energy-dispersive X-ray (EDX) attachment. Samples for SEM and TEM were dispersed in water using ultrasonic treatment, pipetted on to carbon-coated copper grids, and dried. In addition, samples for SEM were coated with Pt. RESULTS AND DISCUSSION The random powder X-ray diffractograms of samples 72/76 and 30 are shown in Fig. 1. The diffractogram of sample 72/76 is characterized by a number of fairly sharp peaks due to the presence of well-crystallized minerals such as muscovite, quartz, K-feldspar, and trace amounts of pyrite. In contrast, the diffractogram of sample 30 exhibits only very few sharp peaks (due to quartz, a small amount of muscovite, and one unidentified mineral, having a d-spacing of nm), and is dominated by a very broad diffraction peak having a TABLE 1. Sample codes, depth, facies, weight of sample treated and wt% of insoluble residue. Sample Depth Sample wt Residue no. m(b.kb.) Facies g wt% lagoonal * oolitic shoal oolitic shoal lagoonal / / lagoonal * includes a trace of dolomite.

3 Hydrous feldspathoids in carbonates 529 Q Q Q M A IV1 M ~f~/v #'" k.,.fil "\'~'~r d,nm I I 5'0 4'0 3'0 2'0 ~ 0 FIG. 1. X-ray diffractograms of unoriented, Na-saturated samples. Sample 72/76 (upper trace) and sample 30 (lower trace). Major peaks are labelled: Q = quartz, M = muscovite, F = K-feldspar, HF = hydrous feldspathoid. Intensity scale is 100 cps. Co-K0~ radiation. maximum at ~32~ (d = 0.32 nm) and a width at half height of ~10~ The intensity of the broad peak indicates that non-crystalline material dominates this sample. No changes in the diffractograms of the oriented samples could be detected following Mg-saturation and glycolation indicating the absence of swelling minerals. The diffractograms of samples 30 and 26 are similar except for the presence of two unidentified peaks in the latter sample (with d-values of and nm) and the diffractograms of samples 5 and 61 are similar to that obtained from sample 72/76 except for the absence of pyrite in both samples, the absence of K-feldspar in sample 5, and the presence of a trace of dolomite in sample 5. These results show that non-crystalline material is dominant in samples from the oolitic shoal facies. Fig. 2 shows the IR spectra of samples 72/76 and 26. Both samples exhibit absorption bands at 3400 and 1640 cm -1 due to water, and at 2830, 2920, 2930, 1465 and 1380 cm -1 due to aliphatic groups in crude oil that has migrated into the formation. These absorption bands diminish and disappear with progressive heating of the KBr discs. The spectrum of sample 72/76 also exhibits absorption bands at 1025, 915,535,472 and 425 cm -1 assigned to muscovite (Russell, 1987). The specturm of sample 26 is characterized by broad absorption,bands at 1010, 700 and 448 cm -1 with shoulders at 1150, 865 and 560 cm 1. None of these absorption bands can be assigned to the small amounts of well-crystallized minerals

4 530 C. Bender Koch j/jj i l+~o 30~00 i zo~o ' 16~o ' lz~o ' 8~o ' ~Go WAVENUMBER (CM -1) FIG. 2. IR spectra of samples 72/76 (upper trace) and 26 (1 mg Na-saturated sample in 300 mg KBr; heated at 110~ for 16 h). identified by XRD in this sample. However, the spectrum is very similar to that obtained from synthetic hydrous feldspathoids with a Si/A1 molar ratio of -2 (Farmer et al., 1979), suggesting that a similar mineral is the major component of the sample. As for synthetic feldspathoids (Farmer et al., 1979), the absorption band at 865 cm -1 (assigned to SiOH groups) disappears on heating the KBr discs. The IR spectra of sample 5 indicate the presence of trace amounts of dolomite, but otherwise the spectra of samples 5 and 61 are similar to that of sample 72/76. The IR spectra of samples 26 and 30 are similar. The quartz in the samples is only present as crystals too large to be transparent to IR and consequently no absorption bands due to this mineral could be detected. SEM micrographs of sample 72/76 are dominated by flaky particles <2 /~m in size (Fig. 3a) interpreted as detrital muscovite. In addition, a few very large sharp-edged particles interpreted as authigenic quartz, are present. Samples 26 and 30 are dominated by particles of variable size consisting of intergrown /~m spheres (Figs. 4a and 4b) which appear to be hollow (Fig. 4c). The few particles observed with a platy outline (muscovite) are all covered with fibrous overgrowths (Fig. 4d). The particles in samples 5 and 61 resemble those of sample 72/76, but also contain small numbers of spheres (Fig. 3b). Several particles in each sample were investigated by SAED and EDX. SAED of the spheres revealed three diffuse rings at , 0.21 and 0-12 nm, demonstrating the noncrystalline structure of these particles. Farmer et al. (1979) reported similar values for synthetic hydrous feldspathoids. Table 2 lists the elemental composition of spherical particles exhibiting only diffuse SAED patterns. As SAED of the spherical particles in sample 61 all revealed the presence of a small crystalline component, the analyses are not included. (From the presence of 4-8% K the spheres probably cover the centre of muscovite). EDX analyses of non-crystalline spherical particles in sample 26, following Mgsaturation at room temperature, indicate that approximately half of the Na can be exchanged by Mg, demonstrating cation exchange properties of the feldspathoids. No spheres were found in sample 72/76. The Si/A1 molar ratios of the non-crystalline particles in samples 26 and 30 are not

5 Hydrous feldspathoids in carbonates 531 E E a).-&.q

6 532 C. Bender Koch FIG. 4. SEM micrographs of samples from oolitic shoal facies; (a) spherules (sample 26); (b) spherules (sample30); (c) hollowspheres (sample30); (d) platyparticle (muscovite)with fibrous overgrowth (sample30). constant, but vary between 1.6 and 2.2. This range is in fairly good agreement with a mean Si/Al ratio of 2 for these samples as estimated by comparison with IR spectra obtained from synthetic feldspathoids (Farmer et al., 1979). The few analyses of amorphous particles from sample 5 indicate an even more variable chemical composition, but as the IR spectrum of this sample is dominated by muscovite, no compositional information on the non-crystalline phase can be obtained by this method. As Farmer et al. (1977) demonstrated that transformation of allophane and imogolite into feldspathoids could be achieved by heating in simple alkaline solutions, it must be considered whether the feldspathoids in the samples studied could be artefacts. However, the fact that not all the samples contain feldspathoids, and that dissolution features are absent on the detrital muscovite, indicates that the E D T A treatment itself does not induce formation of feldspathoids from the detrital minerals. Furthermore, the presence of elements in the non-crystalline particles which are strongly complexed by E D T A (e.g. Fe), would not be expected if the particles formed from E D T A solution.

7 Hydrous feldspathoids in carbonates 533 TABLE 2. Elemental composition (mole%) and SilAt mole ratio of non-crystalline particles in the samples (Na-saturated). Only elements with Z -> 11 (Na) are detected. Sample no. Si AI Na K Mg Ca Fe S CI Si/AI n.a. n.a. n.a. n.a. n.a. n.a n.a. n.a. n.a. n.a. n.a. n.a n.a. n.a. n.a. n.a. n.a. n.a n.a. n.a. n.a. n.a. n.a. n.a n.a. = not analysed. The feldspathoids show that at one stage of diagenesis the pore fluid contained A1, Si, Na, K and Fe ions and thus had a more complex composition as compared to the pore fluid evidenced by diagenetic quartz only. The study has provided no indication of the origin of the components of the feldspathoids oa- the timing of the precipitation, and further studies on the distribution of feldspathoids in the rock and their elemental composition are needed before a possible mechanism for their formation in these environments can be elucidated and their geological implications can be understood. The well used in the present study is located close to the paleocoast (Baartman, 1978) and it is possible that at this position diagenesis may have differed from that at wells situated in the centre of the basin. This difference may explain the absence of non-crystalline materials in previous investigations. ACKNOWLEDGMENTS Thanks are due to F. Kragh, A. Henschel and M.D. Bentzon for help in obtaining the micrographs. REFERENCES BAARTMANN J.C. (1978) in RASMUSSEN L.B. (1978) Geological aspects of the Danish North Sea Sector. Danmarks Geolgbke Underscgelse. Series , 85 pp. BOOtNE M.W. JR. & FERNALLD T.H. (1973) EDTA dissolution of gypsum, anhydrite, and Ca-Mg carbonates. J. Sed. Pet. 43, BRAt'rSCH O. (1971) Salt Deposits'. Their Origin and Composition. Springer-Verlag, Berlin, 297 pp. FARMER V.C., SMITH B.F.L. & TAIT J.M. (1977) Alteration of allophane and imogolite by alkaline digestion. Clay Miner. 12, FARMER V.C., FRASER A.R. & TAIT J.M. (1979) Characterization of the chemical structures of natural and synthetic aluminosilicate gels and sols by infrared spectroscopy. Geochim. Cosmochim. Acta 43, F0CHTBA~ER H. (1958) Die petrographische Unterscheidung der Zecbsteindolomite im Emsland dutch ihren Saurertickstand. Erddl und Kohle 11, Fi3EHTBAUER H. & GOLDSCHMIDT H. (1959) Die Tonminerale der Zechsteinformation. Belt. Miner. Petr. 6, RUSSELL J.D. (1987) Infrared methods. Pp in: A Handbook of Determinative Methods in Clay Mineralogy (M.J. Wilson, editor). Blackie, Glasgow. SMVKATZ-KLoos W. (1966) Sedimentpetrographische und geochemische Unter suchungen an Karbonatgesteinen des Zechsteins Teil II. Spezieller Tell Contr. Mineral. Petrol. 13,

534 C. Bender Koch STENTOFT N. (1990) Diagenesis of the Zechstein Ca-2 carbonate from the Lcgumkloster-1 well Denmark. DGU SERIE B 12, 42 pp. STENTOFF N., FRYKMAN P.

8 534 C. Bender Koch STENTOFT N. (1990) Diagenesis of the Zechstein Ca-2 carbonate from the Lcgumkloster-1 well Denmark. DGU SERIE B 12, 42 pp. STENTOFF N., FRYKMAN P., RASMUSSEN K.L. & KOCH C.J.W. (1991) EDTA-insoluble residues from the Zechstein Ca-2 unit (Late Permian), the LCgumkloster-1 Well, Denmark. Geologie en Mijnbouw 70,

NOTE FIBROUS CLAY MINERAL COLLAPSE PRODUCED BY BEAM DAMAGE OF CARBON-COATED SAMPLES DURING SCANNING ELECTRON MICROSCOPY

Clay Minerals (1991) 26, 141-145 NOTE FIBROUS CLAY MINERAL COLLAPSE PRODUCED BY BEAM DAMAGE OF CARBON-COATED SAMPLES DURING SCANNING ELECTRON MICROSCOPY Authigenic fibrous clays often occur in the pore