Albitization of plagioclase crystals in the Stevens sandstone (Miocene), San Joaquin Basin, California, and the Frio Formation (Oligocene), Gulf Coast, Texas: A TEM/AEM study

W. G. HIRT Geology Department, Center for Higher Education, San Ramon, California 94583 H.-R. WENK Geology/Geophysics, University of California, Berkeley, California 94720 J. R. BOLES Geology Department, University of California, Santa Barbara, California 93106

ABSTRACT

Conventional Transmission Electron Microscopy (CTEM) and An-alytical Electron Microscopy (AEM) studies of partially albitized pla-gioclase crystals taken from drill cores from the Stevens sandstone (Mio-cene), San Joaquin, California, and the Frio Formation (Oligocene), Gulf Coast, Texas, reveal that replacement of Ca-rich plagioclase cores by nearly pure albite (Ab96-Ab100) occurs along submicroscopic (—15 nm wide) en echelon (001) and (110) cleavages. The cleavages are the result of changes in the localized stress regime created by dissolution of adjacent phases. Photomicrographs show albite-lined brittle cleavage crosscutting albitized semibrittle fractures. Such crosscutting relation-ships can be explained by a reduction in effective stress associated with the albitization process. On a macroscopic scale, this reduction in ef-fective stress implies that the transition from hydrostatic to lithostatic pressure is discontinuous.

INTRODUCTION

Diagenesis, in particular the dissolution/precipitation of auto-genic phases, can drastically affect the porosity and permeability of potential hydrocarbon reservoirs (Schmidt and McDonald, 1980; Franks and Forester, 1984; Land, 1984; Kaiser, 1984) as well as the geophysical properties of sedimentary rocks (Loucks and others, 1984). The recognition of the widespread development of secondary porosity (Hayes, 1979) and its control over the migration path and accumulation site of hydrocarbons has led to the investigation of factors controlling numerous dissolution and precipitation reactions. One such reaction is albitization, the replacement of plagioclase by nearly pure albite.

During subsidence of basins containing arkosic/subarkosic sand-stones, authigenic albite is generated by at least two different mech-anisms (Coombs, 1954; Helmhold and van de Kamp, 1984; Gold, 1987; Milliken, 1989; Burch, 1991). Nearly pure albite can be found as (1) interstitial fracture fills and overgrowths precipitated from saturated pore fluids or (2) products replacing the Ca-rich cores of plagioclase grains.

In this paper, the term albitization refers exclusively to authigenic albite that is the product of detrital plagioclase replacement. For our study, we selected partially albitized crystals from cores used in pre-vious studies (Boles, 1982; Boles and Ramseyer, 1988) of the North Coles Field, Stevens sandstone, San Joaquin Valley, California, and the Frio Formation, Gulf Coast, Texas. These sedimentary units ex-hibit a simple burial history indicating that the in situ temperatures

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(120-160 °C) and pressures (21-65 MPa) measured during drilling represent the maximum PT conditions to which the sandstones have been subjected (Boles and Ramseyer, 1987). Samples from different wells at different depths afforded the opportunity to investigate dis-parate temperature and pressure regimes within the zone of albitiza-tion. Samples from two distinct sedimentary units provided the op-portunity to investigate albitization of plagioclases with different prelithification histories. The Frio Formation samples exhibit a vol-canic provenance as evidenced by the presence of numerous rhyolitic and trachytic rock fragments (Loucks and others, 1984). The large, equant, subhedral plagioclases of Stevens sandstone reflect a plutonic origin (Boles, 1984).

The focus of this investigation will be to explain: 1. Why albitization occurs at a higher temperature (100-160 °C)

than the 50-85 °C temperature range predicted from strictly thermo-dynamic considerations (Boles, 1982; Frank and Forester, 1984; Kai-ser, 1984; Boles and Ramseyer, 1987; Ramseyer and others, 1992).

2. Why regions of albitization need not be located proximally to intergranular or intragranular pores.

3. How cations in the pore fluids access the interior of plagioclase grains.

4. Why adjacent plagioclase grains can exhibit different degrees of albitization (Boles, 1984, and Siebert, 1984).

TECHNIQUES

Thin sections of the samples were examined optically with the polarizing microscope and selected plagioclase crystals were trans-ferred to copper grids for further thinning by ion bombardment to achieve electron transparent foils for use in the transmission electron microscope (TEM). Thinned foils (34) were surveyed in a JEOL 100 C TEM for areas of suitable electron transparency and interest. Sub-sequently, 13 samples were examined at the National Center for Elec-tron Microscopy (NCEM) at Lawrence Berkeley Laboratories with a JEOL 200 CX Analytical Electron Microscope (AEM) with the Scan-ning Transmission Electron Microscope (STEM) probe focused to a cross-sectional diameter of 20-30 nm.

To minimize the effects of beam damage and contamination, all samples were chilled to —160 °C. Acquisition times were kept to under 2 min while maintaining dead times <20% and count rates of 2,000-3,000 counts/s. To eliminate possible electron channeling ef-fects (the orientation-dependent, preferential intensity enhancement of one element over another created by localizing Bloch wave am-plitudes on a given atomic site [for example, Tafto and Buseck, 1983;

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Figure 1. Illustration of the typical ori-entation relationship between authigenic al-bite (Ab) seen in the center and its andesine host (An). The area on the perimeter out-side the light region is the andesine host.

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Lorimer, 1987]), all spectra were collected while the foil was in a random orientation as defined by Lorimer (1987). Plagioclase com-positions were calculated using Al/Si ratios normalized to their albite and anorthite components. Potassium, if present at all, was below the minimum detectability of the instrument (~2 wt%). Sodium was pref-erentially lost during progressive irradiation and was an unreliable measure of plagioclase composition. This result agrees with earlier studies (Lorimer, 1987).

RESULTS

Observations on the Authigenic Albite/PIagiocIase Orientation Relationship

Sample NC9120A3 (Fig. 1) from the Stevens sandstone illustrates a typical example of the orientation relationship between the authi-

genic albite and its host plagioclase. X-ray spectra collected from the albitized region (labeled Ab) yielded a composition of An4, while the host andesine yielded a Si/Al ratio equivalent to An40. Inspection of selected area diffraction (SAD) patterns from the two areas reveals a near perfect match of reflection positions with a slight shift in relative intensities between the An-rich SAD (lower right) and the Ab-rich region (upper left).

Interpretation of Orientation Observations

Throughout our study, authigenic albite consistently exhibited an orientation relationship to the host plagioclase. The orientations are so close that the SAD patterns superimpose exactly. The slight shift in intensities indicates an orientation difference of not more than a few degrees. Such an orientation relationship implies that the albite grew from a template obtained from the dissolving plagioclase rather than

HIRT AND

Figure 2. Fine-scaled modulated oligoclase (O) with albite (Ab) decorating an (001) en echelon cleavage. Note the absence of the mod-ulations in the vicinity of the en echelon cleavage.

precipitating in a void. To better delineate the nature of the replace-ment process, a survey of samples was undertaken to examine the plagioclase/albite boundary in partially albitized feldspars.

Observations on the Authigenic Albite/Plagioclase Boundary

En echelon cleavages (most often [001], but also [110] cleavages) were the one microstructure common to occurrence of authigenic albite. The cleavages can be found forming the actual albite/plagio-clase boundary or can be located medially within the albite phase, as in sample NC9091-G (Fig. 2). Sample NC9091-G is an oligoclase (An16) containing fine-scale (40 nm wide) modulations and an authi-genic albite phase. STEM line scans across the modulations reveal no chemical variation. An en echelon cleavage (001) runs medially through the albite phase and is surrounded by an imperfection-free zone.

Sample NC9120-C (Fig. 3) further illustrates the details of the relationship between albitization and en echelon cleavage formation. STEM analyses of the area of tensional shear where the two cleavages overlap show that the moire fringe patterns are the result of chemical

OTHERS

variation (albite formation) and not due to rotation of the host plagj-oclase. Probe position 1 sampled the host plagioclase and yielded a composition of An35. Probe positions 2 and 3 sampled the areas con-taining the moire fringes and yielded compositions of An18 and An29, respectively. Inspection of the photomicrograph reveals that probe positions 2 and 3, though focused to a cross-sectional diameter <20 nm, sampled volumes from both the authigenic albite and the host plagioclase phase. The analyses therefore represent minimum albite compositions.

Sample NC9120-C (Fig. 4) provides important evidence as to the timing of the en echelon cleavage formation. Two distinct types of partings are displayed. The first runs northwest-southeast and consists of an open fracture in the upper left corner, a dislocation-free zone adjacent to the fracture tip, and a dislocation pileup in front of the dislocation-free zone. Such a series of microstructures typifies a semibrittle deformation (Park and Ohr, 1986; Clarke and Chiao, 1989). Running north-south and crosscutting the dislocation pileup is an en echelon (001) cleavage system. The en echelon cleavage exhibits sharp tips and no dislocations, typical of brittle fractures as described by Argon (1986). Results of a STEM line profile running southwest to northeast are illustrated in Figure 5. Enrichment in the Ab component of the plagioclase is observed both in the dislocation pileup preceding the semibrittle fracture and in the area of light contrast decorating the en echelon cleavage.

Interpretation of Albite/Plagioclase Boundary Observations

En echelon cleavage was the only microstructure consistently associated with albitization. The temporal relationship between for-mation of en echelon cleavage and initialization of albitization can be ascertained from Figure 2.

The fine-scale modulations show no chemical variation and dis-appear in the imperfection-free zone surrounding the en echelon cleav-age. This relationship implies that cleavage formation occurred after the lamellae structure formation. The balance of surface energy and internal energy terms in fracture theory predicts that an increase in surface energy generated by crack formation should be compensated by a reduction in internal strain energy (Griffith, 1920; Lawn and Wilshaw, 1975). Such a reduction in internal strain would be mani-fested as a reduction in microstructure (for example, the disappear-ance of the lamellae structures near the cleavage).

The medial location of the en echelon cleavage within the albite shows that the cleavage predates the albitization. If cleavage forma-tion postdated the albitization, for example, if cleavage formed due to unloading during drilling or as an artifact of sample preparation, the parting should follow the path of highest lattice strain, which would be along the plagioclase/albite boundary.

Analysis of sample NC9120-C (Fig. 3) is consistent with the interpretation that cations use pre-existing en echelon cleavage to access the interior of the plagioclase grains. One would predict that the replacement of the host plagioclase would occur first in areas of great-est strain within the plagioclase. Figure 3 is interpreted to have cap-tured the albitization in its initial stage. The strained plagioclase struc-ture in the region of tensional shear between the overlapping cleavages would act as a nucleation sight for replacement by authigenic albite.

Studies of how strain is accommodated in a variety of rocks under triaxial compression (Friedman, 1975; Wawersik and Fairhurst, 1970) have yielded a sequence of stages for cracking. Both authors define a stage I, from 20 to about 60 MPa, as a pressure range where pre-existing pores and fractures are closed due to increased confining

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Figure 3. Albite nucleation in areas of tensional shear where en echelon cleavages overlap. Upper left insert shows probe positions.

pressure. This pressure range agrees with the pressure where albiti-zation occurs. The 60 MPa upper limit is especially intriguing as it agrees closely with the upper pressure limit (21-65 MPa; 2.44- to 3.1-km depth) for the albitization process reported in the literature (Boles, 1982; Kaiser, 1984). If diagenetic albitization is the result of pore fluid migration along open fractures, then when the confining pressures achieve —60 MPa, these partings close and the mechanism for albitization ends. The implication is that brittle fracture formation requires two conditions of the surrounding stress field, both of which must be met before fracture can occur. The first condition is that some critical value of the differential stress, crc - <r„ (where erc is the resolved stress lying within the cleavage plane and parallel to the cleavage propagation direction and cr„ is the resolved stress normal to the cleavage plane) must be attained before fracturing can proceed. The second and equally important condition is that the confining pressure (crn) must be below some critical value, —60 MPa, or the parting will heal.

In a grain-supported sandstone, achieving the condition of ob-taining a critical value of the stress differential would not be difficult near point contacts with surrounding grains. By decreasing the area over which two grains lie in contact, the stress at the contact could, in theory, become infinite (though in reality it would be limited to the strength of the weaker material). Contact with adjacent grains could then create a stress differential, <rc - crn, of sufficient magnitude to propagate fracture.

It may prove that the second condition of maintaining confining pressure (<xn) below a critical value of about 60 MPa is the constraining factor on cleavage formation and therefore on albitization. Terzaghi (1945) conducted uniaxial compression tests showing how the effec-tive confining pressure is lowered by an increase in pore pressure (^effect ive = «^confining ~ < W p r e s s u r e ) - 1 " the Stevens sandstone and the Frio Formation, the dissolution of adjacent matrix material partially surrounding a plagioclase crystal at depth and the subsequent influx of pore fluid could reduce the effective confining pressure on the

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Figure 4. Andesine exhibiting (001) en echelon cleavage (running north-south) crosscutting semibrittle fractures (north-west-southeast). The crosscutting relation-ship illustrates a transition from semibrittle to brittle strain mechanisms and implies a reduction in effective stress. Phyllosilicate (Ps) in lower center of micrograph.

crystal, thrusting it from a regime of semibrittle to brittle behavior. The brittle behavior would be expressed in the crystal by the formation of en echelon cleavages, and the crosscutting relationship observed in Figure 4 would be established. Such a mechanism for cleavage for-mation and albitization could explain the following thin section observations:

(1) The lack of a proximal spatial relationship between inter-granular/intragranular pores and regions of albitization. This lack of a spatial relationship can be explained by the fact that, although pore formation and fluid influx reduce the effective stress, the actual albi-tization takes place in the areas of en echelon cleavage overlap and along the surfaces of the submicroscopic cleavages, which need not occur at the pore/crystal interface. Because such a mechanism de-pends on cleavage formation, it also explains why stress-induced albitization at grain-grain contacts is more common in the hydrostat-ically pressured Stevens reservoirs than in the lithostatically pressured Frio reservoirs (Boles, 1984).

(2) Unaltered plagioclases found next to completely albitized grains. This apparent contradiction can be explained by the orientation dependence between the direction of stress reduction and the cleavage plane. To be effective in generating cleavages, secondary pore for-mation would have to be situated so as to reduce the confining pressure (crn) and not trc. Additionally, compositional variation may also play a role. Increased Si activity along stress contacts could drive the albitization reaction accounting for differences in the reactivity of plutonic and volcanic detrital plagioclases within the zone of albiti-zation (Ramseyer and others, 1992).

(3) The distinct lack of en echelon cleavages seen in unalbitized samples and in the unaltered regions of partially albitized grains (Fig. 6). Unless a reduction in the resolved confining stress, crn, occurs, formation of cleavages and albitization is prohibited.

Implication of Matrix Dissolution on the Hydrostatic/Lithostatic Transition

The normal sequence of deformation in homogeneous material with increasing pressure is brittle to semibrittle to ductile deformation. Yet, the crosscutting fractures seen in sample NC9120-C (Fig. 4) appear to contradict this sequence. Neither fracture demonstrates the asymmetric tips typical of fractures induced by stress corrosion (Mc-Donald, 1986). Nor do the fractures display dislocation densities or blunted tips typical of ductile deformation (Neuman, 1986; McDonald, 1986). Assuming that the dislocation pileup is associated with the crack tip and not the result of strain surrounding some crystal imper-fection above or below the plane of the foil, this photomicrograph shows that the semibrittle deformation (running northwest-southeast in Fig. 4) was active during albitization and does not represent some earlier deformation event. The fact that both cracks have been albi-tized implies that they were not created by unloading during drilling or core collection. Pore fluids must have accessed the crystal's interior by migration along open partings. Cation diffusion occurred through areas of high tensional strain associated with the crack tip and pro-gressed along dislocation cores in the pileup preceding the crack. The brittle deformation, marked by the en echelon cleavages running north-south in Figure 4, crosscuts and postdates the semibrittle de-formation. The creation of brittle fracture during albitization after semibrittle fracture during albitization could be explained by a reduc-tion in effective stress caused by the dissolution of proximal phases in the matrix. On a macroscopic level, the stress reduction illustrated in the microstructures implies that the transition from hydrostatic to lithostatic pressure is a discontinuous function, diagrammed sche-matically in Figure 7. The discontinuities in the transition could be associated with dissolution of different phases in the matrix. Such a

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P r e s s u r e Figure 7. Schematic diagram of the transition from hydrostatic to

lithostatic pressure as a function of depth. Heavy line represents possible path during diagenesis.

discontinuous pressure curve as a function of depth agrees with Sibson (1990) and implies similar discontinuities in compaction and porosity formation curves.

P r o b e Pos i t ion Figure 5. Line profile across area in Figure 4. Top photomicro-

graph shows probe positions (position 1 in lower left to position 20 in upper right).

Figure 6. Bright field image of a fracture in a typical unalbitized plagioclase. Note lack of en echelon cleavages.

CONCLUSIONS

(1) Authigenic albite (Ab96-Ab100) replacing Ca-rich plagioclase exhibits an orientation relationship to its host and occurs along sub-microscopic, en echelon (001) and (110) cleavages. The cleavages result from localized stress differentials created by dissolution of prox-imal matrix phases.

(2) Albitized brittle cleavage seen crosscutting albitized semibrittle microstructures indicate that a reduction in effective stress is associated with the albitization process. On a macroscopic scale, the effective stress reduction implies a discontinuous transition from hy-drostatic to lithostatic pressure.

ACKNOWLEDGMENTS

We thank K. Krishnan, R. Gronsky, C. Echer, and J. Turner of the National Center for Electron Microscopy, Lawrence Berkeley Laboratory, for their patience and willingness to provide access and instruction on the facility's equipment. We also wish to acknowledge D. Barber, L. Goodwin, and A. Meike for their helpful comments in reviewing this research. This research was supported by National Science Foundation Grants EAR-8816577 and EAR-9104605.

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REFERENCES CITED

Argon, A. S., 1986, Chemistry and physics of fracture, in Latanison, R. M., and Jones, R. H., eds., Chemistry and physics of fracture: Boston, Massachusetts, NATO ASI Series, p. 3-11.

Boles, J. R., 1982, Active albitization of plagioclase, Gulf Coast Tertiary: American Journal of Science, v. 282, p. 165-180.

Boles, J. R., 1984, Secondary porosity reactions in the Stevens sandstone, San Joaquin Valley, California, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 217-224.

Boles, J. R., and Ramseyer, K., 1987, Diagenetic carbonate in Miocene sandstone reservoir, San Joaquin Basin, California: American Association of Petroleum Geologists Bulletin, v . 71, p. 1475-1487.

Boles, J. R., and Ramseyer, K., 1988, Albitization of plagioclase and vitrinite reflectance as paleothermal indicators, San Joaquin Basin: Society of Economic Paleontologists and Mineralogists Pacific Section, v. 60, p. 129-139.

Burch, T. E., Nagy, K. L., and Lasaga, A. C., 1991, Dependence of albite dissolution and precipitation kinetics on solution saturation state at 80 °C: Geological Society of America Abstracts, v. 23, no. 5, p. 260.

Clarke, D. R., and Chiao, Y., 1989, Application of transmission electron microscopy to the observation of dislocation emission from crack tips at elevated temperatures: Ultramicroscopy, v. 29, p. 203-211.

Coombs, D, S., 1954, The nature and alteration of some Triassic sediments from Southland, New Zealand: Royal Society of New Zealand Transactions, v. 82, p. 65-109.

Franks, S. G., and Forester, R. W., 1984, Relationships among secondary porosity, pore-fluid chemistry and CO2, Texas Gulf Coast, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 63-79.

Friedman, M., 1975, Fracture in rock: Reviews in Geophysics and Space Physics, v . 13, p. 352-358. Gold, P. B., 1987, Texture and geochemistry of authigenic albite from Miocene sandstone, Louisiana Gulf

Coast: Journal of Sedimentaiy Petrology, v . 57, p. 353-362. Griffith, A. A., 1920, The phenomena of rupture and flow in solids: Royal Society of London Philosophical

Transactions, v . A221, p. 163-198. Hayes, J., 1979, Sandstone diagenesis—The hole truth: Society of Economic Paleontologists and Min-

eralogists Special Paper 26, p . 127-140. Helmhold, K. P., and van de Kamp, P. C., 1984, Diagenetic mineralogy and controls on albitization and

laumonite formation in Paleogene arkoses, Santa Ynez Mountains, California, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 239-276.

Kaiser, W. R., 1984, Predicting reservoir quality and diagenetic history in the Frio Formation (Oligocene) of Texas, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 198-215.

Land, L. S., 1984, Frio Sandstone diagenesis, Texas Gulf Coast: A regional isotopic study, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 47-62.

Lawn, B. R., and Wilshaw, T. R., 1975, Fracture of brittle solids: London, England, Cambridge University Press, 204 p.

Lorimer, G. W., 1987, Quantitative X-ray microanalysis of thin specimens in the transmission electron microscope: A review: Mineralogical Magazine, v. 51, p. 49-60.

Loucks, R. G., Dodge, M. M., and Galloway, W. E., 1984, Regional controls on diagenesis and reservoir quality in lower Tertiaiy sandstones along the Texas Gulf Coast, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Association of Petroleum Geologists Memoir 37, p. 15-45.

McDonald, D. D., 1986, Surface chemistry in aqueous solutions, in Latanison, R. M., and Jones, R. H., eds., Chemistiy and physics of fracture: Boston, Massachusetts, NATO ASI Series, p. 255-286.

Milliken, K. L., 1989, Petrography and composition of authigenic feldspars, Oligocene Frio Formation, South Texas: Journal of Sedimentary Petrology, v. 59, p. 361-374.

Neuman, P., 1986, Plastic processes at crack tips, ¿«Latanison, R. M., and Jones, R. H., eds., Chemistiy and physics of fracture: Boston, Massachusetts, NATO ASI Series, p. 63-84.

Park, C. G., and Ohr, S. M., 1986, In sini TEM studies of crack tip deformation in Mo, in Latanison, R. M., and Jones, R. H.,eds. , Chemistry and physics of fracture: Boston, Massachusetts, NATO ASI Series, p. 437-442.

Ramseyer, K., Boles, J. R., and Lichtner, P. C., 1992, Mechanism of plagioclase albitization: Journal of Sedimentary Petrology, v. 62, p. 349-356.

Schmidt, V., and McDonald, D. A., 1980, Secondary reservoir porosity in the course of sandstone diagenesis: Educational Course Notes, Series 12,124 p.

Sibson, R. H., 1990, Conditions of fault-valve behavior, in Knipe, R. J., and Rutter, E. H., eds., Deformation mechanics, rheology and tectonics: Geological Society of America Special Publica-tion 54, p. 15-28.

Siebert, R. M., Moncure, G. K., and Lahann, R. W., 1984, A theory of framework grain dissolution in sandstone, in McDonald, D. A., and Surdam, R. C., eds., Clastic diagenesis: American Associ-ation of Petroleum Geologists Memoir 37, p. 163-175.

Tafto, J., andBuseck, P., 1983, Quantitative study of Al-Si ordering in an orthoclase using an analytical electron microscope: American Mineralogist, v. 68, p. 949-950.

Terzaghi, K., 1945, Stress conditions for failure of saturated concrete and rocks: Proceedings of the American Society for Test Materials, v . 45, p. 777-801.

Wawersik, W. R., and Fairhurst, C., 1970, A study in brittle rock fracture in laboratory compression experiments: International Journal of Rock Mechanics and Mineral Science, v. 7, p. 561-571.

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