BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 64. PP. 869-878, 1 FIG.. 1 PL. AUGUST 1953

FISSILITY OF MUDROCKS

BY ROY L. INGRAM

ABSTRACT The breaking characteristics of a mudrock can be represented on a triangular diagram with massive, flaky-fissile, and flaggy-fissile as end members. Fissility in is usually associated with a parallel arrange- ment of the micaceous clay particles and nonfissility with a random arrangement. Experiments and obser- vations indicate that the clay minerals attain a parallel arrangement by gravity settling or by flocculation and , unless the particles are adsorbed on irregularly shaped sesquioxide or silica particles or grow randomly in a gel. The nature of the cementing agent determines whether a will be flaky or flaggy. If the cementing agent can hold the material in a large slab, the shale will be flaggy. If the amount or the tenacity of the cementing agent is small, the shale will be flaky. Cementing agents other than organic mat- ter tend to hinder parallel to the clay particles causing a decrease in fissility and an increase in massiveness. Moderate increases the fissility of a shale. In general the type of fissility does not correlate with the type of clay minerals present in a random collection of mudrocks.

CONTENTS TEXT Effect of organic matter 875 Page Effect of iron 875 Introduction 869 Effect of aluminum 876 Review of the literature 870 Effect of silica 876 Empirical classification of fissility 870 Post-depositional changes 876 Analyses and studies of mudrocks 871 Compaction 876 Color 871 Penecontemporaneous clay minerals 876 Mechanical analyses 871 Wave action 876 Carbonate Content 872 Weathering 876 Organic Content 872 Conclusions 877 Clay-mineral studies 872 References cited 877 Microscopic studies 873 Experimental reproduction of fissilities 874 ILLUSTRATIONS Gravity settling 874 Figure Page Flocculation 874 1. Triangular diagram showing a megascopic Discussion of results 874 classification of mudrock-breaking types 871 Gravity settling 874 Plate Facing page Flocculation 875 1. End members of mudrock-breaking types 872

INTRODUCTION necessary to define certain terms as used in this paper. The purpose of this study is to explain the Fissile—possessing the property of splitting origin of fissility and nonfissility in nonlam- along approximately parallel surfaces inated mudrocks. Fissility caused by separa- Massive—nonfissile tion along planes between beds or laminae of different lithologic characteristics is outside the Mudrock— of which at scope of this paper. least 50 per cent is silt and clay, with no The literature shows that the terminology connotation as to the relative percentage used in describing argillaceous sediments is not of silt and clay and as to the breaking standardized (Lewis, 1924; Bradley, 1931a; characteristics Twenhofel, 1937; Krynine, 1948; Dapples, Siltrock—mudrock with silt dominant over Krumbein, and Sloss, 1950). Therefore it is clay 869

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Clayrock—mudrock with clay dominant over Milner, 1940, p. 386-387; Ailing, 1946, p. 26; silt Pettijohn, 1949, p. 288); (2) settling of flaky Mudstone—massive mudrock particles in water with the flat surfaces lining Silistone—massive siltrock up parallel to the surface of deposition (Mohr, Claystone—massive clayrock 1932, p. 130; Kerr, 1937, p. 545; Keller, 1946, Mud sliale—fissile mudrock p. 71; Krumbein, 1947, p. 102); (3) orientation Silt shale—fissile siltrock of clay particles by weak currents (Keller, Clay shale—fissile clayrock 1946, p. 71); (4) penecontemporaneous and Table 1 summarizes this scheme. epigenetic growth of clay minerals parallel to the bedding (Lewis, 1924, p. 578; Keller, TABLE 1.—NOMENCLATURE or SEDIMENTARY ROCKS 1946, p. 70; Twenhofel, 1939, p. 293-294; CONTAINING MORE THAN SO PER CENT SILT Payne, 1942, p. 1717; Pettijohn, 1949, p. 278); AND/OR CLAY (5) presence of illite (Grim and Cuthbert, No con- 1945, p. 94; Keller, 1946, p. 71); (6) parting notation as to breaking Massive Fissile caused by expansion after the removal of over- character- istics burden by erosion (Hedberg, 1936, p. 245); (7) low carbonate content (Campbell, 1946, p. No connotation Mudrock Mudstone Mud 832); (8) high sulfur and carbon content (Camp- as to relative shale bell, 1946, p. 832); (9) weathering phenomena amounts of (Bain, 1896, p. 293; McKelvey, 1946, p. 28-29); silt and clay (10) necessary small grain size of the nonclay Siltstone Silt Silt predomi- Siltrock minerals (Ailing, 1945, p. 739); (11) minute nant over clay shale Clay predomi- Clayrock Claystone Clay stratification (Grout, 1932, p. 269; Twenhofel, nant over silt shale 1939, p. 293-294). Bradley (1931b, p. 319) and Grout (1932, p. 269) were intentionally non- committal as to the origin of fissility in many Mudrocks of many varieties were collected. mudrocks. Analyses were made in hopes of finding para- Nonfissility has been explained by: (1) dep- meters that vary with fissility. Attempts were osition of a large quantity of colloidal clay made to reproduce experimentally fissile and gel, thereby preventing the reorientation of nonfissile mudrocks. Finally, post-depositional randomly oriented clay particles (Lewis, 1924, changes that might influence fissility were con- p. 580); (2) random growth of clay minerals sidered. in a clay gel (Keller, 1946, p. 69-70); (3) stirring REVIEW OP THE LITERATURE by wave action (Weller, 1930, p. 129); (4) consolidation of massive clay soils (Keller, A perusal of the literature pertaining to the 1946, p. 71); (5) wind deposition of argillaceous structure of mudrocks reveals: (1) the emphasis material (Keller, 1946, p. 71); (6) presence of on the explanation of fissility whereas non- iron oxides in clay (Keller, 1936, p. 55; Ailing, fissility apparently has been assumed to need 1945, p. 740); (7) deficiency of potassium (Lewis, no explanation, and (2) the large amount of 1924, p. 581); and (8) presence of excessive pertinent work by the soil scientists which has nonclay size, nonclay mineral materials (Ailing, not been assimilated into the geologic litera- 1946, p. 18, 25, 35). Pettijohn (1949, p. 280) ture. presented no explanation for the massiveness of Ideas expressed in geologic literature as to argillites even though they do not "differ in the cause or partial cause of fissility are: (1) any important respects from normal shales rotation of flaky clay particles into positions and slates." perpendicular to the force exerted by the over- lying material with concurrent flattening of clay aggregates (Grabau, 1913, p. 175; Lewis, EMPIRICAL CLASSIFICATION OP FISSILITY 1924, p. 520, 578; Harker, 1932, p. 157-158; Mudrocks were collected from the Eocene Hedberg, 1936, p. 269; Hatch and Rastall, of Colorado; the Cretaceous of Iowa and Colo- 1938, p. 122; Twenhofel, 1939, p. 293-294; rado; the Jurassic, Triassic, and Permian of

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Colorado; the Pennsylvanian of Iowa and (1) those with a flaggy structure are predom- Colorado; the Mississippian of Iowa; and the inantly black or dark gray, (2) those with a Ordovician of Wisconsin. Three dominant flaky structure are predominantly gray or types of breaking characteristics were observed: gray black and, in general, are not so dark as massive, flaggy-fissile, and flaky-fissile (PI. 1). the flaggy ones, (3) those with a massive struc- Massive mudrocks have no preferred direc- tion of cleaving or breaking. Most of the frag- Massive Flaky ments are blocky. A few of the fragments may be platy, but the orientation of the platy frag- ments in the outcrop is random. Flaggy shales split into fragments of varying thicknesses but with the width and the length many times greater than the thickness and with the two essentially flat sides approximately par- allel. Most of the fragments in the flaggy shales have the length and width at least 50 times greater than the thickness. Some of the frag- ments may break so as to fit the description of the flaky shales, but these are not so dominant as the flaggy fragments. Flaky shales split along irregular surfaces Flaggy parallel to the bedding into uneven flakes, thin FIGURE 1. TRIANGULAR DIAGRAM SHOWING A chips, and wedgelike fragments, whose length MEGASCOPIC CLASSIFICATION OF MUDROCK- seldom exceeds 3 inches. BREAKING TYPES Some samples combine varying amounts of massiveness, flagginess, and flakiness, producing ture have a predominance of colors other than a continuous coverage of a triangular diagram gray black or black—i.e., gray, white, yellow, if the three dominant types are placed at the and red, (4) some exceptions exist to the above apices of the triangle (Fig. 1). This classifica- trends, the most notable being the occurrence tion is only qualitative, but a given mudrock of predominantly flaggy red mudrocks. can be located easily if the end members are kept in mind. This was verified by the close Mechanical Analyses agreement in the location of a number of sam- The dispersing and centrifuging procedure ples on the triangular diagram by several per- used for mechanical analyses was evolved by sons. Jackson, Truog, and their students in the All attempted verbal descriptions for the Soils Department of the University of Wisconsin breaking characteristics of all possible com- (White and Jackson, 1946, p. 150-154). This binations of massiveness, flagginess, and flaki- procedure would have been unnecessary if only ness were too unwieldy. Mudrocks with inter- mechanical analyses were desired; but, since mediate characteristics can be described as separations were necessary to obtain samples dominantly massive, dominantly flaggy, or for x-raying, this procedure was most expedient. dominantly flaky. Fifty samples, representative of the varia- Mudrocks with splintery breaking character- tions in fissility were chosen for more detailed istics do exist. The splintery nature is most studies. Each sample was split into four size probably caused by closely spaced fractures groups: sand and gravel (greater than 20 and is not considered in this paper. microns), silt (2-20 microns), coarse clay (0.2- 2 microns), and fine clay (less than 0.2 microns). ANALYSES AND STUDIES OF MUDROCKS No correlation exists between the results of Color the mechanical analyses and the type of fissility. This is expected since only a small amount of Attempts to correlate color with type of clay is necessary to control the behavior of a fissility yielded the following observations: sediment (Nutting, 1927, p. 185; Sided, 1938,

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p. 130; Ailing, 1946, p. 26), and the addition decrease in fissility with a given type of shale. beyond the minimum required should have Because so many other factors influence fissility, little effect. it is easy to see why the carbonate content of a The results of a mechanical analysis of a random collection of mudrocks does not vary mudrock have little meaning since they depend with fissility. on so many variables. Clay minerals vary widely in their ability to be dispersed (Hauser and Organic Content LeBeau, 1946, p. 202). Different dispersing ions cause different degrees of disaggregation The organic content of the mudrocks was (Gripenberg, 1939, p. 532; Grim, 1939, p. 488; estimated by the Schollenberger chromate Hauser and LeBeau, 1946, p. 203). Sideri titration method (Schollenberger, 1931, p. 483- (1936, p. 461, 465) has shown that clay, or- 486). The organic content is high for flaggy ganic matter, and fine sand form a system that shales, moderate for most flaky shales, and low is impossible to disperse completely. The small- for most massive mudrocks. These results are est grains are always aggregates. In experiments discussed later in the paper. performed by the writer even 15 to 20 treat- ments with 30 per cent H2O2 had little effect in Clay-Mineral Studies removing organic matter from some black shales. Microscopic studies show that the The techniques used for the preparation, mudrocks contain less sand and silt than the x-raying, and interpretation of the x-ray diffrac- analyses indicated. Even if one assumes that tion data are those developed by Jackson and complete dispersal could be attained, it does his students (White and Jackson, 1946, p. not follow that this duplicates the conditions 150-154). Each clay was saturated with cal- of deposition of the sediments. cium ions and glycerol solvated. Unfiltered Ailing (1945, p. 739, 751) has shown that an iron radiation was used on wedge-shaped sam- increase in silt and sand causes a decrease in ples in a camera with a diameter of 143.2 mm. fissility. This relationship was not investigated Montmorillonoid, chlorite, illite, kaolinite, in this work. But this is not the only parameter and mica intermediates were identified. No of fissility since some clayrocks are fissile and correlation exists between the presence or some are massive. amount of any clay mineral and the type of fissility in the 50 samples studied. The clay Carbonate Content minerals identified are distributed among all the types of fissility. An estimate of the carbonate content was Several workers have expressed the idea obtained by adding a standardized acid in ex- that the type of clay mineral determines cess to a powdered sample and back titrating whether or not a mudrock will be fissile. Payne with a standardized base using a phenol- (1942, p. 1717) and Grim and Cuthbert (1945, phthalein indicator (Willard and Furman, p. 94) believe that sediments composed of 1940, p. 136-152). The acid consumed was illite will have a greater tendency toward assumed to have been used in reacting with fissility than those containing other clay min- CaCOs. For these 50 samples no correlation erals because illite will not break into individual exists between the amount of CaCO3 and the layers in the presence of water as will montmo- type of fissility. rillonoid and kaolinite. Marshall (1936, p. 29) Peterson (1944, p. 44) observed a decrease expects a stronger tendency toward oriented in the platiness of a dried puddled kaolinitic coagulation with those clay minerals, illite soil upon the addition of Ca(OH)2. This increase and beidillite, that possess the strongest sur- in blockiness or granularity is attributed to face charge as a stronger pull will be exerted the cementing effect of Ca(OH)2. Campbell on the adsorbed cations. (1946, p. 832) observed a similar trend in the Some observations and experiments show the New Albany shale, where the fissility varied kinds of clay minerals in fissile and massive indirectly with the lime content. Undoubtedly mudrocks. Kerr (1937, p. 545) described the the cementing action of carbonates causes a fissile Attapulgus, Georgia, clay as containing

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Fi(iURK 1. MASSIVU FICJURK 2. FLAKY

FKIUKK 3. FLAGGY The fla^'My specimens \vc,rc broken from larger slabs. UNO MICMBI-:HS OF MnnMocK-BUKAKiNG TYPES, x y±.

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parallel montmorillonite flakes. Keller (1946» tation can be determined since extinction is p. 69-70) observed that the massive Chelten- parallel or approximately parallel to the cleav- ham fireclay of Missouri contained randomly age (Grim, 1939, p. 470; Ross, 1945, p. 181). oriented montmorillonite and illite. Peterson Mudrocks with the clay-mineral flakes parallel (1944, p. 37, 41, 44) puddled clay and fine give extinction under crossed nicols when the sand, subjected the mixture to ten cycles of direction of light vibration parallels the long alternate wetting and drying, and studied the direction of the clay-mineral flakes. With a resulting product megascopically and micro- random orientation of the clay particles, the scopically. With the clay being either pure birefringence of the clay matrix does not change kaolinite or montmorillonite a massive struc- with rotation of the stage. The degree of pre- ture was obtained, but with the addition of ferred orientation can be estimated from the 3 per cent of montmorillonite to kaolinite a change of birefringence with rotation of the platy structure resulted. These observations stage. and experiments show that montmorillonitic The mudrocks studied are composed pri- mudrocks may be either fissile or massive, yet marily of a clay or organic matter matrix with Peterson's work implies that the type of clay embedded grains of detrital minerals, shells, mineral affects the breaking characteristics. and shell fragments and some pigments of iron None of the samples studied by the writer oxides and organic matter. Clay is present as: contained halloysite, but mudrocks composed (1) flaky particles or aggregates with extinction of halloysite are probably massive. Brindley, parallel to the long direction of the flakes, Robinson, and Goodyear (1948, p. 427) found (2) flaky particles with admixed calcite, the natural halloysites to have a random arrange- admixed calcite making it impossible to de- ment of the halloysite particles. They obtained termine the extinction direction of the clay, partial orientation by pressures of 12,000 to (3) radial aggregates, and (4) a matrix of clay 16,000 pounds per square inch and state, "this, particles too small to be seen individually. The we believe, is the first time such an orientation clay particles in the matrix can have preferred effect has been detected with halloysite." Their or random orientation. Organic matter occurs electron-microscope photographs of sedimented as: (1) opaque fragments that are either equi- halloysite revealed no orientation of the lath- dimensional or linear in cross section and (2) shaped crystals. brownish or blackish organic stains. Quartz Thus, in a random collection of mudrocks and other detrital minerals are present as there is no apparent correlation between fissility angular grains of fine sand, silt, and clay. The and the type of clay mineral. The writer's quartz grains are evenly disseminated through- success in producing fissile and massive mud- out the clay matrix except for a few specimens rocks experimentally using the same clay mix- which contain laminae of fine sand and silt. ture is more evidence against a dominating Calcite is present as: (1) scattered grains and influence of different clay minerals. This does (2) concentrated patches. Many of the sections not eliminate the possibility that clay minerals cut across fossils. Iron oxides occur as fine influence fissility, for other parameters of dust, covering or staining the other materials. fissility may mask clay-mineral effects, and The fissility of a mudrock depends primarily further research in clay mineralogy may sub- on the of the clay minerals. All the fissile divide the known clay minerals into more mudrocks exhibit some degree of parallelism species. of the flaky constituents; the degree of fissility in general, is proportional to the degree of pre- Microscopic Studies ferred orientation. All the flaggy shales studied possess a lenticular type of parallelism; clear Thin sections were studied to determine the lenses of clay or clay and calcite are embedded microstructural features of fissile and non- in an opaque organic matrix. Especially notable fissile mudrocks. The clay minerals provide the is the tendency for the organic stain to be as- main structural control for mudrocks. Even sociated with clays that show preferred orienta- though most of the clay particles are too small tion. One sample is laminated by the alterna- to be seen with the microscope, the mass orien- tion of layers with and without an organic stain.

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The clay particles in the organic-stained laminae termine if the chemical composition of the clay have an orientation parallel to the lamination; system influences fissility. The procedure in those in the unstained organic layers have a these experiments was essentially the same as random orientation. The flaggy shales have for gravity settling except that the clays were appreciably more organic matter than the flocculated with various electrolytes, allowed flaky shales. Apparently the cementing action to settle, and then dried and impregnated. of the organic matter aids in causing the rocks Sodium chloride was added to gray, black, to break into large slabs. Massive mudrocks and red clay suspensions until they flocculated. have a random orientation of the platy clay The gray and black clays had a pronounced particles. parallel orientation of the clay minerals and a tendency toward parting parallel to the bottom EXPERIMENTAL REPRODUCTION OF FISSILITIES of the test tubes. The red clay showed a pre- ferred orientation in a few spots, but most of Gravity Settling the section exhibited a random orientation. Flocculation of a gray-clay suspension with Different types of clay systems were dried FeCls gave a red mudrock with a parallel to determine if different types of fissilities could orientation. This can explain why some red be reproduced experimentally. mudrocks may be fissile and have a parallel The clay suspensions obtained by centri- structure. A similar flocculation with A1C13 fuging during mechanical analysis were com- also gave a parallel orientation. bined according to color to get large volumes To determine the influence of a gelatinous of gray, black, and red suspensions. These sus- precipitate of Fe(OH)3 on the structure of a pensions were used in all the succeeding ex- mudrock, NKiOH was added to a gray-clay periments. suspension containing FeCls in solution until To duplicate these experiments, the follow- both the clay and Fe(OH)3 flocculated. The ing procedure should be used: (1) fill with clay resulting red mudrock had a random orienta- suspension a 2 by 15 cm. soft test tube that tion of the clay particles. A similar experiment has been flattened on the bottom, (2) evaporate in which A1(OH)3 gel was produced gave a to dryness in a 40°C. oven, (3) pour a mixture mudrock with a parallel orientation of the clay of Canada balsam and xylol on the dried clay, particles. (4) put in vacuum for 2 to 10 hours, (5) cure When a gray-clay suspension containing in a 55°C. oven, (6) make a thin section and Na2Si03 was flocculated with HC1, a stiff silica examine with a microscope. gel was formed with the clay dispersed through- The gray clay had a strong preferred orienta- out the gel. Upon drying, a hard chertlike tion of the clay particles parallel to the bottom mass was formed. The basal portion contained of the tube. Incipient horizontal cracks were a concentration of clay that was strongly present. The black clay had a strong preferred oriented parallel to the bottom. The rest of the orientation parallel to the bottom with fine clay had a random orientation. laminae of black organic matter. The red clay Powdered Fe203, A12O3, and SiO2, obtained had a random orientation of the clay minerals by drying gels of Fe(OH) , A1(OH) , and but with a slight lamination caused by the con- 3 3 Si(OH)4, were added separately to three gray- centration of iron oxide in layers. Horizontal clay suspensions. Flocculation of these sus- partings tended to occur along these laminae. pensions produced mudrocks with random Apparently red fissile shales with a random orientations of the clay particles. orientation of the clay minerals could be formed in this way, but none were observed in the Discussion of Results samples studied. Gravity settling.—Gravity settling or evapora- Flocculation tion of a dispersed clay results in a parallel arrangement of the flaky clay particles. Hauser Clay suspensions were flocculated with elec- (1939, p. 214) described this disposition as the trolytes under a variety of conditions to de- one which gives the minimum free energy to

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the system or, as stated in a later paper (Hauser, tion. However, he neglected to consider drying 1941, p. 188), "charged particles, if forced to or compaction forces. settle, have the ability to pack as closely to- Effect of organic matter.—The writer's ex- gether as possible." The hydrosphere surround- periments with dispersed clay and organic ing each clay particle acts as a lubricant per- matter obtained from black shales show that mitting adjustment of the particles. either gravity settling or flocculation gives a Hauser has done much work on the forma- parallel orientation of the clay particles. Anal- tion of clay films by the evaporation of clay yses for organic content also show that fis- suspensions. Careful evaporation of an ex- sility and the presence of organic matter are tremely fine bentonite suspension resulted in closely related. The mechanism for this is not gelation at a certain concentration (Hauser, clear. Perhaps the organic particles are flaky, 1939, p. 224). Ultramicroscopic examination or perhaps the organic matter is in solution or showed that there was no geometrical arrange- is a fine colloid so that the adsorption of the ment of the particles. Continued evaporation organic matter does not prevent or disturb resulted in an alignment of the particles into the parallel arrangement of the clay particles. larger "crystals" (Hauser and LeBeau, 1938, Sideri (1936, p. 471; 1938, p. 131) showed that p. 965) with individual particles aggregated the moistening of a clay with a humate solution into threads and filaments which maintained resulted in the formation of a number of parallel a crystallographic continuity. laminae. Other workers have obtained similar results. In some situations organic matter may hin- Bray, Grim, and Kerr (1935, p. 11) and Grim der the development of a platy structure (Peter- (1934, p. 45-47) were able to make optical son, 1944, p. 44). However, of the mudrocks measurements on clay by allowing it to settle studied, none of the massive ones had even a on a glass slide, the particles settling on one moderate amount of organic matter. Peterson another with their c axes parallel. Kerr (1937, worked with puddled clays. As his clays were p. 545) obtained a parallel arrangement by the not dispersed, perhaps there was no oppor- settling of a montmorillonitic clay. White and tunity for the humus to be adsorbed, and the Jackson (1946, p. 152) found that settling of excess combined as a cement around clay- clay in water or any other dipolar liquid re- humus aggregates. sulted in a parallel arrangement of the clay The flaggy shales contain by far the largest particles. Chang (1941, p. 220-221), in work- amount of organic matter. Humus is almost ing on the structure of adobe, came to the same irreversibly adsorbed by clay. The resulting conclusion concerning a parallel arrangement cementing action enables the shale to break resulting from gravity settling of dispersed into large sheets without falling apart. Camp- clay. bell's observation (1946, p. 832), that in the Flocculation.—Coagulation of a dispersed New Albany shale, laminae are thinnest where clay by the addition of an electrolyte results the amount of carbon is the greatest, sub- in the formation of a gel or a gelatinous pre- stantiates this view. cipitate with the clay particles coalescing in a The black flaggy shales show a of random manner (Hauser, 1939, p. 212-213). flattened white clay or clay-calcite lenses in an This gives a loose packing and a high porosity. opaque organic matrix. Why these clay lenses Because of the high porosity of flocculated are unstained is not apparent. clay, it should be expected that compaction Effect of iron.—Gravity settling of a red- or drying will result in the flaky particles being clay suspension, flocculation of clay and oriented parallel to the bottom of the container. Fe(OH)3 together, and flocculation of clay with This explains why Kerr (1937, p. 545) obtained powdered Fe2Os all produce a random orienta- a parallel orientation of clay in both fresh and tion of the clay particles. Apparently the salt water. Fe(OH)3 and the Fe2O3 enmesh or adsorb the Chang (1941, p. 220-221) suggested that, clay particles on irregular surfaces hindering because flocculation resulted in a random ar- their normal orientation. rangement of the particles, dried electrolyte- A parallel orientation was obtained in a red flocculated clays should show a random orienta- mudrock by flocculating a clay suspension with

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a FeCls solution. The ferric ions could have been early during compaction and is usually com- adsorbed uniformly by the clay and later plete in the top 5-10 cm. of a mud deposit. oxidized forming small grains of Fe203 which He made no mention of the influence of im- would not disturb the orientation. However, purities in the clay. other explanations for red fissile shales are not excluded. If only a small amount of Fe(OH)3 Penecontemparaneous Clay Minerals were flocculated with the clay, orientation might not be disturbed. It is also possible that Keller (1946, p. 70), Payne (1942, p. 1766), sufficient iron to give a red color upon oxida- and others suggested that clay minerals may tion might diffuse into a fissile shale. form in a mud during diagenesis. Keller used Other workers have observed the disturbing the mechanism of random growth of clay min- influence of iron on the structure of mudrocks. erals in a gel to explain massive fireclays in Keller (1936, p. 55) saw only a slight orienta- Missouri. Allen's observations (1945, p. 265- tion in a flocculated lateritic soil. Sideri (1938, 266) on the migration of clay in colloidal sus- p. 132-134) reported that "anisotropic colloidal pension or in solution increase the possibility particles of kaolinite lose their ability to dis- of such a mechanism. tribute themselves in parallel rows" when Payne (1942, p. 1717) and Grim and Cuth- sesquioxides are present. After the removal bert (1945, p. 94) inferred that the diagenetic of iron oxides with Tamm's Reagent, the forma- formation of illite in marine sediments explained tion of parallel structure began. Peterson's the fissility of most shales. As the writer found experiments (1944, p. 45) with puddled kaolinite no relationship between fissility and the pres- showed a gradual decrease in the platiness of ence of illite, this view cannot be substantiated. the dried residue as the amount of Fe£>a in- creased. Wave Action Effect of aluminum.—The influence of alu- minum on the structure of mudrocks is similar Weller (1930, p. 729) suggested that repeated to that of iron; the only difference is that a stirring of bottom muds by wave action may explain some thick-bedded, structureless mud- flocculated Al(OH)3-clay system gave a parallel orientation of the clay. The alumina gel did stones. In the writer's flocculation experiments, not trap the clay so as to prevent its orienta- repeated stirrings of the flocculated clays pro- duced no effect on the final structure. But if tion upon drying. Flocculation of an A1203- clay system gave a random structure similar consolidated or partially consolidated shales were broken by wave action and reassembled to the Fe2O3-clay system. Effect of silica.—The drying of a silica gel so as to break up the common orientation of in which clay had been dispersed produced a the clay particles, a mudstone might be pro- hard, chertlike mass in which much of the clay duced. The probability of this happening ap- had been expelled to the bottom where the pears to be small though such a fabric was orientation was parallel to the bottom. Ap- observed in one of the samples studied. parently silica gel exerts a force which tends to clear it of impurities as it dries. Powdered Weathering amorphous Si02 in a clay suspension produces the same random structure as Fe O and A1 O . The weathering of mudrocks determines to 2 3 2 3 a large degree the way a potential fissility will be expressed. In general, weathering of a given POST-DEPOSITIONAL CHANGES shale increases the platiness of frag- Compaction ments. This relation was noted in one sample that was flaggy when weathered and more Flocculated clays can hold up to 85-90 per massive when unweathered. McKelvey (1946, cent water by volume. Hedberg's detailed p. 28-29) observed the same phenomena in work (1936, p. 269) on the compaction of clay the Phosphoria formation of Wyoming and showed that a mechanical rearrangement of the Utah. Soft, black fissile shales at the surface clay particles to a parallel position takes place became hard, massive siltstones at a depth of

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30 feet at Sublette Ridge, Utah. Investigations shales predominate among flaggy shales; other in mines of this area showed all rocks to be well-cemented shales may be somewhat flaggy. harder and more massive with depth. Mudrocks (7) Other factors being equal, most cement- with a parallel arrangement of the clay particles ing agents of mudrocks cause a decrease in have a potential fissility which may not be fissility. realized until weathering has weakened the (8) The influence of composition on the type cementing materials. of fissility is reflected in the color. Flaggy shales It is also entirely possible that extreme are predominantly black or gray black; flaky weathering may destroy fissility by changing a shales, gray or grayblack; and mudstones, shale into a soft clay. The differential swelling white, gray, red, yellow, purple, etc. and shrinking of a clay caused by alternate (9) Except where the clay has obtained a wetting and drying can disrupt the orientation random orientation by adsorption on particles pattern. Mohr (1932, p. 358-359) described of sesquioxides or silica, compaction of muds this condition in the weathering of marine results in a rotation of the particles to a posi- shales in the Netherlands East Indies. This tion perpendicular to the force of compaction. may explain Bain's statement (1896, p. 293) (10) Moderate weathering increases the that "the absence of shaly parting is often a fissility of a shale by the partial removal of the matter of weathering as shown by the greater cementing agents along planes parallel to the predominance on fresh exposures." bedding or by expansion caused by hydration Compaction and later elastic recovery caused of the clay particles. Intense weathering pro- by erosional unloading undoubtedly aids the duces a soft massive clay. development of fissility. (11) Given a certain critical, but small, amount of clay, the type of fissility is inde- CONCLUSIONS pendent of the particle sizes in a mudrock. (12) With a random collection of mudrocks, (1) The breaking characteristics of mudrocks the type of fissility is uncorrelatable with the can be described as massive, flaky, flaggy, or kind of clay minerals present. Possibly, how- some combination of these three pure types. ever, under certain conditions, the kind of clay (2) Fissility is associated with a parallel mineral may determine the type of fissility. arrangement of the clay particles; massiveness, with a random arrangement. REFERENCES CITED (3) Organic matter increases the tendency Alexander, Jerome, Editor, 1946, Colloidal chem- toward parallel arrangement of the clay istry: vol. 6, Reinhold Pub. Co., N. Y., 1215 p. Allen, Victor T., 1945, Effects of migration of clay particles. minerals and hydrous aluminum oxides on the (4) Colloidal sesquioxide and silica particles complexity of clay: Am. Cer. Soc. Tour., vol. increase the tendency toward a random ar- 28, p. 265-275. Ailing, Harold L., 1945, Use of microlithologies as rangement of the clay particles. Ferruginous illustrated by some New York sedimentary fissile shales can be explained by the oxidation rocks: Geol. Soc. America Bull., vol. 56. p. of iron ions entrapped during settling or floccu- 737-756. , 1946, Quantitative study of the Genessee lation or introduced epigenetically by solutions. Gorge sediments: Rochester Acad. Sci. Proc., (5) Keller's explanation (1946, p. 70) of some vol. 9, p. 5-63. Bain, H. F., 1896, of Polk County: Iowa mudstones as originating by the random growth • Geol. Survey, vol. 7, p. 263-417. of clay minerals in a gel is a probable one. Bradley, W. H., 1931a, Origin and microfossils of (6) The nature of the cementing agent de- the oil shales of the Green River formation of Colorado and Utah: U. S. Geol. Survey Prof. termines whether a shale will be flaky or flaggy. Paper 168, p. 1-58. If the cementing agent can hold the material , 1931b, Non-glacial varves: Am. Jour. Sci., together in a large slab, the shale will be vol. 22, p. 318-330. Bray, R. H., Grim, R. E., and Kerr, P. F., 1935, flaggy. If the amount or the tenacity of the Application of clay mineral technique to Illi- cementing agent is small, the shale will be nois clays and shales: Geol. Soc. America Bull., vol. 46, p. 1909-1926. flaky. As colloidal organic matter is almost ir- Brindley, G. W., Robinson, K., and Goodyear, J., reversibly adsorbed and dehydrated, black 1948, The effect of temperature and pressure

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