Growth and early diagenetic changes in artificial gypsum crystals grown within bentonite muds and gels

ROBERT D. CODY Department of Earth Science, Iowa State University, Ames, Iowa 50010

ABSTRACT marizes observations on the primary growth and early diagenetic alteration of gypsum grown in bentonite gels and muds at different Experiments designed to simulate muddy environments failed to temperatures and with different inorganic chloride salts. nucleate anhydrite or cause replacement of initially formed gypsum crystals by anhydrite during five months at temperatures to 80°C PREVIOUS EXPERIMENTAL STUDIES and with pore solutions containing as much as 20 percent sodium chloride. Gypsum and bassanite (at higher temperatures and Among the earliest systematic studies of crystallization in the sys- salinities) were the only species produced. In addition, gypsum tem CaS04-H20 published in English was that of Lambert and crystals experimentally grown for several months in Wyoming ben- Schaffer (1926). They found that either gypsum or the metastable tonite gels and pastes and in the presence of dissolved monovalent hemihydrate CaS04 • V2H2O precipitated, depending on tempera- salts underwent major habit changes. The habit produced by these ture and solution composition. The transition temperature for the early diagenetic changes is characteristic of many natural crystals two phases depended on the ionic constituents in solution. The found in saline sediments and probably is diagnostic of evaporite lowest transition temperature, 76°C, occurred with 2 N solutions sedimentation. of calcium acetate and sodium sulfate; the highest temperature, 96°C, occurred with 2 N calcium chloride and sulfuric acid solu- INTRODUCTION tions. In addition to the specific effects of solution constituents, the effect of ionic strength on the activity of water in these concen- Thermodynamic calculations have shown that anhydrite should trated solutions must also have influenced the system. In relatively be the stable phase in many natural environments, but laboratory dilute solutions, Nancollas and others (1973) found that the solu- experiments have been unable to precipitate anhydrite under condi- bility of gypsum and the hemihydrate phase are equal at 102°C tions similar to those prevailing in sedimentary environments. under 4 atm pressure; this indicates that hemihydrate will normally These experiments, together with the extreme scarcity of primary crystallize above about 100°C in dilute solutions and at lower anhydrite in modern environments, have led many geologists to temperatures in more concentrated ones. In neither study did conclude that reaction kinetics, rather than thermodynamic stabil- anhydrite nucleate, although thermodynamically it should be the ity, is the major control on the nucleation and growth of this min- stable form at higher temperatures. Most workers accept a temper- eral and that only gypsum will precipitate in sedimentary environ- ature of about 40°C as the temperature at which gypsum and ments. Recently, however, studies of the Trucial Coast sabkha sed- anhydrite can coexist, with gypsum becoming unstable at higher iments have revealed nodules composed of masses of small tabular temperatures. This conclusion is based both on heat capacity anhydrite crystals that might be primary (Kinsman, 1969; Shear- measurements (Kelley and others, 1941) and on solubility studies man, 1966). If they are, it is reasonable to conclude that primary by several workers, beginning with Posnjak (1938). Recently, how- anhydrite may be more common than previously assumed. Further, ever, the 40°C transition temperature was challenged by Hardie if primary anhydrite does occur in modern sediments, the inability (1967) and Blount and Dickson (1973), who found a transition of experiments to nucleate anhydrite at relatively low temperatures temperature of 56° to 58°C. All workers have agreed that anhydrite may be due to a failure to simulate natural conditions. is exceptionally difficult to nucleate under experimental conditions Relatively few experiments have been designed to understand the that approximate near-surface aqueous environments. In dilute processes leading to authigenic mineral formation within sedi- solutions, even at temperatures of 125°C, Nancollas and others ments. Theoretical approaches can often provide useful informa- (1973) found no anhydrite in experimental runs lasting for several tion on the limiting conditions of mineral growth, but such calcula- days, although one can speculate that the initially precipitated gyp- tions cannot always predict which mineral will actually nucleate, sum or hemihydrite might have converted to anhydrite if their runs grow, and persist as a metastable phase; this is because of the com- had continued for many months. Apparently, a high activation plex interactions between numerous sytem variables and the differ- energy is required for complete dehydration of calcium ions in solu- ing rates of several possible reactions. Experimental studies, in tion in order for the anhydrous critical nucleus to form; this energy many cases, supply this information, provided that close simulation barrier allows metastable forms such as gypsum and hemihydrate of natural conditions is achieved. to nucleate and grow. In natural environments, the only possibility of direct anhydrite nucleation may be under conditions of ex- Gypsum is ideal for this type of study because of the relative sim- tremely high ionic strength and high temperature, unless there are plicity of the CaS0 -H 0 system, and because it can be easily 4 2 undiscovered activating substances that could decrease the kinetic grown under a variety of experimental conditions. Many details of barriers to nucleation. gypsum nucleation, growth, stability, and crystal morphology in

natural environments are not well understood; therefore, I have Crystallization studies of the system CaS04-H20 are also com- performed a series of experiments designed to determine the effects plicated by the possibility that other unstable and metastable of a wide variety of environmental conditions on the precipitation phases may nucleate under various conditions. In addition to gyp- and characteristics of gypsum. A previous report (Cody and sum, possible compounds and polymorphs are the a and ¡3 forms of Shanks, 1974) summarized the differences between gypsum crys- the hemihydrate, the a and ¡3 forms of soluble anhydrite, and insol- tals grown in silica gels and bentonite clay gels; this report sum- uble anhydrite (Power and others, 1966). Each of these species has

Geological Society of America Bulletin, v. 87, p. 1163-1168, 8 figs., August 1976, Doc. no. 60810.

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its own dissolution behavior in water, and apparently each is a dis- nongelling clay pastes contain more clay. Sodium silicate gels facili- tinct phase that can be prepared in the laboratory either by crystal- tate crystal growth (Henisch, 1970); therefore, it was anticipated lization from solution at various temperatures or by dehydration of that experiments utilizing gelling clays might also facilitate growth. gypsum under controlled atmosphere and temperature. It is not If crystals grown in the gelled bentonite did not closely resemble known, however, if any of the forms except gypsum, a hemihy- gypsum grown in natural muddy environments, crystal characteris- drate, and insoluble anhydrite will precipitate from aqueous solu- tics might be altered by preventing gelling with pretreatments such tions in natural environments. as soaking the clay in concentrated salt solutions. Finally, the In nearly all of these studies, except that of Nancollas and others availability of two types of bentonite provides opportunities to (1973), the crystalline species were precipitated by mixing two rela- compare the results of crystal growth in clays that are quite similar tively concentrated solutions. Additionally, in many cases, solu- in many respects, but that are different in exchangeable bases and tions were used that were unusual by comparison to natural wa- particle size distributions. ters; for example, relatively concentrated calcium acetate solutions Two types of bentonite were used in this study: sodium (or were used in one study, and in other studies, relatively concentrated Wyoming) bentonite obtained from Wyo-Ben Company of sulfuric acid solutions were the source of sulfate. With sulfuric acid Greybull, Wyoming, and calcium (or southern) bentonite obtained solutions, the pH of the system would be much lower than is com- from American Colloid Company. Both materials are composed mon in natural environments. almost entirely of dioctahedral smectite clays with traces of quartz, feldspar, and mica. According to the American Colloid Company, EXPERIMENTAL PROCEDURES the southern Panther Creek bentonite has the following size dis- tribution: 30 percent, >5 /xm; 35 percent, 5 to 0.5 /xm; and 35 Relation to Natural Environments percent, <0.5 /xm. Typical Wyoming bentonite is much finer, with about 89 percent that is <0.5-/u.m equivalent spherical diameters. Experiments in the study reported here closely approximated Panther Creek bentonite contains an average of 0.34 to 0.46 per- conditions usually found in lake and sea-floor muds. Nucleation cent Na and 1.35 to 3.34 percent Ca, whereas Wyoming bentonite and crystal growth were induced in smectite muds and gels utilizing contains 2.49 to 2.56 percent Na and 0.58 to 1.28 percent Ca. diffusion of nutrient ions in U-shaped growth cells. The chemistry Wyoming bentonite differs from southern clay in that a small quan- of the solution phases is consistent with many natural waters, al- tity of the Wyoming clay forms a rigid gel in distilled water, though lower concentrations of the nutrient ions and co-ions exist whereas the southern bentonite does not gel. One experimental run in most natural systems. Temperatures in most of the experiments was performed in duplicate, using Yara kaolinite obtained from the corresponded to those in natural muds, although the highest tem- Yara Engineering Corporation. peratures (80°C) would be characteristic only of diagenetic condi- tions. The effects of ionic strength and several foreign cations were Preparation of Support Media evaluated by mixing chloride salts with the muds and solution phases. The sodium and calcium smectite clays composing the sed- To make a clay gel or paste, powdered dry clay was slowly sifted iment phase are common in many natural muds. In addition, into distilled water in a blender to achieve a moderately rigid mix- diffusion-controlled growth, as in U-tube growth cells, probably is ture. Approximately 9 percent sodium bentonite, or about 42 per- the type of growth most common for authigenic minerals in natural cent calcium bentonite, was required. In a few experiments, gelling muds. Experiments of long duration identified those mineral phases of the Wyoming clay was prevented by using a solution of 10 per- most likely to persist in muds, although the nature of the original cent sodium chloride in place of distilled water; about 39 percent primary precipitates cannot be evaluated from the experiments. clay was required for a rigid paste. Various amounts of sodium chloride or other soluble chlorides, usually from 5 to 10 wt percent Support Media of the water in the paste, were then mixed into the paste. Clays were not usually pretreated, since it was believed inadvisable to change The crystal growth technique used in these and related experi- the natural characteristics of the clays for these initial experiments. ments was slightly modified from the U-tube silica gel system dis- After mixing was complete, 100 ml of gel or paste were poured into cussed by Henisch (1970); the major modification was the substitu- the bottom of a U-shaped glass tube 1 in. in diameter and 8V2 in. tion of natural clay-water mixtures for sodium silicate gel as a sup- high. Each tube was filled to about 6 in. from the top. One arm of port medium. Gypsum has been grown in sodium silicate gels by each tube was then filled with 50 ml of 1 N CaCl2 • 2H20 solution the U-tube method (Barta and others, 1971), but this medium does containing the same amount of soluble salt as did the clay pore sol- not closely approximate natural sediments either in composition or ution; the other arm was filled with 50 ml of 1 N (NH4)2S04 solu- texture, and the crystals so grown were unlike those most com- tion, also containing an equal amount of salt. monly found in natural muds and sands. Nearly monomineralic muds were used to reduce the surface Experimental conditions property, composition, and particle-size variables of the system. Several types of relatively pure clay minerals are available commer- The growth cells were placed in a refrigerator, oven, or oil bath, cially. These include gelling clays such as Wyoming bentonite, hec- depending on the temperature and number of samples for each ex- torite (a trioctahedral lithium smectite), and palygorskite and periment; experiments were performed at 8°, 20°, 40°, 50°, 60°, and nongelling clays such as southern bentonite, many different types 80°C, with temperatures held to within ±1°C. A few duplicate runs of kaolinites, and a few mica-clays. were made to assess the effects of random crystal growth processes, Bentonites were chosen for several reasons. They have much but little significant variation was observed. Most experiments larger cation exchange capacities than do the majority of clay min- were 4V2 to 6 months in length, a few were as long as 11 months, erals; this allows the effects of these characteristics on authigenic and others were of much shorter duration (Table l1). The shorter mineral growth to be evaluated in experiments of reasonably short duration. Also, bentonites are composed of clay minerals com- monly found in modern marine muds, so the experimental muds 1 closely approximate natural muds both chemically and mineralogi- Table 1 (Data and results of experimental gypsum syntheses in Wyoming bentonite and other clay matrices) is available as supplementary material 76-16 and may be or- cally. In addition, gels of Wyoming bentonite resemble sodium sili- dered from Documents Secretary, Geological Society of America, 3300 Penrose Place, cate gels in their relatively large ratio of liquids to solids, whereas Boulder, Colorado 80303.

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runs were performed to evaluate changes in crystal size and mor- modification in habit of primary gypsum crystals grown in Wyom- phology as a function of time. ing bentonite, which normally did not occur in the southern clay. After long contact with the mud pastes, the pH of the solutions Crystals grown in the southern bentonite are generally charac- in the U-tubes was measured with a Leeds and Northrup meter. terized by smooth faces with only traces of corrosion (Fig. 1). There Values were consistent at pH = 6.8 in solutions containing 5 per- are two distinct types of crystals that grow at 60°C in the presence cent NaCl, but these values could be considerably in error because of 5 percent dissolved sodium chloride. One type consists of radial of the dependence of glass electrode response on the high ionic clusters of small crystals that form after a few days close to the in- strength of the solutions. Hydrogen ion concentration was not con- terface with the sulfate solution reservoir. These crystals result trolled by buffers in these experiments, because nearly all buffers from the reaction of diffusing sulfate with calcium ions that have introduce unusual molecules or ions; any supposed correlation of been displaced from clay surfaces by the ammonium that diffuses gypsum growth with solution pH could then have been equally well with the sulfate; these crystals do not reach large sizes (Fig. 1). The attributed to the effects of the foreign dissolved substances. second type is composed of relatively large single crystals com- Under the conditions of this study, calcium ions diffuse in one monly longer than 1 cm, which grow more nearly equidistant from direction into the sediment at the base of the U-tube, and sulfate the two solution reservoirs and result from diffusion mixing of the ions diffuse from the other direction into the sediments. The two two solutions. These crystals generally become visible after about 2 ionic species eventually meet within the mud, where their concen- weeks and reach maximum size in about 6 to 12 weeks; no further trations gradually increase; eventually, a sparingly soluble calcium changes can be seen up to 11_ months. The crystals are_ charac- sulfate species nucleates and grows. An advantage of such a system terized by the forms {010}, {111}, {120}, and a few {103}; by is that nucleation occurs at precisely the critical supersaturation re- elongation parallel to [001]; and by relatively wide {010} faces. quired for the species in question, avoiding excessively large con- Crystals grown in Wyoming bentonite gels initially are similar in centrations during nucleation. The more limited supersaturation habit to the single crystals grown in southern clay pastes at the favors the formation of only a few critical nuclei, which then gen- same temperature and salt content, except that corrosion occurs in erally develop into macroscopic crystals. In these experiments visi- crystals grown at higher temperatures in the presence of dissolved ble crystals developed in the muds within about two to three weeks, sodium chloride. At 60°C and 5 percent admixed sodium chloride, except in the samples at 8° and 20°C, where as much as three these crystals also become visible in about two weeks, and by four months were required. In nearly all runs, 5 ml of 1 N CaCl2 -2H20 to six weeks they have reached their maximum length. Stability of were added to the calcium side of the tube after one month; con- habit is not achieved in this time, however, and the crystals con- trols showed that the only effect of the additional calcium was to tinue to change for several months. For about six weeks, nucleation produce larger crystals. and growth similar to that in southern bentonite systems seem to predominate, but corrosion accompanies growth, so that well- Treatment and Examination of formed crystals are not present in the gels. After this time, corro- Experimental Products sion and growth modification predominate over primary growth. These modifications proceed at different rates, producing wide Upon completion of a run, the gel or mud paste was extruded variation in the extent of corrosion. For example, the crystal shown from the tubes onto a 100 mesh/in. nylon sieve and promptly in the lower right of Figure 2 shows relatively little corrosion, washed with distilled water, leaving the larger gypsum crystals on whereas that in the lower left exhibits resolution such that two sec- the sieve. To assess the possibility of rehydration of lower hydrate tions of an originally elongate crystal are nearly separated. In the crystals during the washing step, crystals grown at 60°C for 4Vi same photograph, the crystal shown in the upper left has separated months in Wyoming bentonite gel containing 5 percent NaCl were from other portions of a primary crystal; this segment is nearly washed with acetone instead of water. There was no difference, as disc-shaped, with the smallest dimension parallel to [001]. In the shown by x-ray diffraction analysis, in crystals washed in acetone original crystal, [001] was the direction of elongation, similar to compared to those washed in water. Once freed from the mud ma- crystals grown in calcium bentonite pastes. trix, the crystals were dried at 40°C on the sieve; drying on the sieve Figures 3 to 8 show the effect of temperature on the habit of prevented much wash water from remaining in contact with the crystals during drying. X-ray diffraction and optical microscopy were used to identify the mineral species.

RESULTS

Single or multiple crystals of gypsum represented the only crys- talline phase found at the end of all experiments in the Wyoming bentonite, southern bentonite, and Yara kaolinite systems; the crystals contained from 0 to 20 percent admixed sodium chloride at temperatures from 8° to 60°C. Many of these experiments in- cluded conditions under which gypsum would be thermodynami- cally unstable. Although anhydrite might not have spontaneously nucleated and grown as a primary crystal, one might have sup- posed that primary gypsum would have dehydrated to anhydrite over the long experiments, but this did not happen. In Wyoming bentonite at 80°C, without admixed salt, gypsum was the only crystalline phase detectable by x-ray diffraction, but at 10 percent salt content, minor amounts of the hemihydrate (bassanite) de- veloped, together with gypsum; at 20 percent salt, bassanite was Figure 1. Gypsum crystals grown in southern bentonite with 5 percent the dominant species. In these experiments, bassanite occurred as NaCl at 60°C for 8 weeks. Crystals were coated with ammonium chloride; small and poorly developed equant crystals, much different in habit scale represents 1 mm. Small radiating clusters grew close to interface of than the large gypsum crystals. bentonite mud with sulfate solutions, and larger crystals grew nearly mid- An unusual aspect of the experiment was the relatively rapid way between the two nutrient solutions.

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Figure 3. Gypsum crystals grown in Wyoming bentonite gel with 5 percent NaCl at 8°C for 24 weeks. Ammonium chloride coated; scale represents 1 mm. Tendency for radiating clusters was common at this temperature. Note nearly complete absence of corrosion and more elongate habit compared to crystals grown at higher temperatures.

modifications for every crystal growing within the gel at any tem- perature. For example, in Figure 8 the crystal partly exposed in the upper right has smooth faces that apparently are original. This well-formed type of crystal, however, was a rare exception whenever Wyoming bentonite was used as the support medium. The characteristics of these experimentally grown tabular crys- tals occur widely in nature. Elongate crystals with corrosion re- entrants similar to those shown in Figure 2 were reported by Figure 2. Gypsum crystals in Wyoming bentonite gel with 5 percent Broughton (1934) from Tertiary strata in Texas. Goldschmidt NaCl at 60°C for 3 weeks and 2 days. Ammonium chloride coated; scale (1913-1923, v. 4, PI. 68, fig. 51) illustrated a crystal from represents 1 mm. Crystals shown in Figures 1 and 2 were grown under simi- Bohman, Germany, with a disc-shaped habit similar to the one lar environmental conditions, differing only in clay type. shown here in Figure 2, upper left. Similar disc-shaped crystals have been reported by Kinsman (1969) and Shearman (1966) in crystals grown in Wyoming bentonite gels containing either 5 or 10 modern carbonate sediments of the Trucial Coast. Shearman percent admixed sodium chloride. A more elongate crystal habit (1966) presented a lengthy summary of tabular crystals flattened predominates in the lower temperature experiments (Fig. 3). Cor- parallel to the c axis, which are similar to those shown in Figures 6 rosion and lateral growth perpendicular to [001] are strongly and 7. In the sabkha sediments of the Trucial Coast area, tabular temperature dependent. Crystals grown at 8°C for 24 weeks show crystals occur in intertidal zone sediments, where crystals ranged in no obvious corrosion, whereas corrosion becomes more evident at size from <1 mm to >1 cm across, and in supratidal sediments, higher temperatures; the difference in corrosion between crystals where these crystals commonly contained abundant inclusions of grown at 20° and 40°C is much less than that between crystals carbonate sand arranged in a herringbone pattern. Shearman grown at 40° and 60°C. The crystal shown in Figure 6 was origi- (1966) also summarized many previously reported occurrences of nally elongate parallel to [001], but it was dissolved along planes similar crystals, as, for example, those in sediments of Laguna perpendicular to that direction until it was separated into small Madre, Texas (Masson, 1955). Gypsum crystals in Laguna Madre disc-shaped segments (see Fig. 2). These segments have grown to- sediments have undergone progressive modification as a result of gether by lateral growth perpendicular to [001], resulting in a tabu- corrosion and lateral growth, similar to the experimentally grown lar polycrystalline mass. Similar masses grown at 80°C for 17 crystals. Although the initial gypsum crystals of Laguna Madre weeks are shown in Figures 7 and 8; the major difference between were different in habit from those obtained experimentally, some the masses grown at 60° and 80°C is the better preservation of the of the older crystals were similar to those shown in Figure 2, upper crystal forms {010} and {120} at the lower temperature. At the left. higher temperature, lateral growth is characterized by a lack of face In the experiments reported here, the extent of corrosion and lat- differentiation, resulting in nearly oval cross sections. However, eral growth depended on several factors. Wyoming bentonite pro- corrosion and lateral growth do not produce major habit moted the reactions, whereas southern bentonite nearly prevented them. In an attempt to evaluate the effects of clay species on corro- sion and habit modifications, one experiment using Yara kaolinite and 5 percent sodium chloride at 60°C with duplicate runs was per- formed. There was no evidence of progressive corrosion or lateral growth in the kaolinite-grown crystals. An experiment to evaluate the effect of exchangeable bases on the reactions in bentonite clay was then performed. Calcium bentonite was first treated with di-

Figure 5. Gypsum crystals grown in Wyoming bentonite gel with 5 percent NaCl at 40°C for 24 weeks. Ammonium chloride coated; scale represents 1 mm. Corrosion is evident. Figure 4. Gypsum crystals grown in Wyoming bentonite with 10 per- cent NaCl at 20°C for 24 weeks. Corrosion is apparent in larger crystal. Ammonium chloride coated; scale represents 1 mm.

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monovalent salt systems); and magnesium reduced but did not eliminate the progressive habit modifications. A third influencing factor was the percentage of Wyoming ben- tonite in the support media. Corrosion was much more rapid in ungelled mud pastes containing 39 percent clay compared to that in gels with 9 percent clay. In runs lasting 11 weeks at 60°C with 10 percent sodium chloride, corrosion in the ungelled clay separated nearly every initially elongate crystal into small disc-shaped seg- ments, whereas the elongate habit was largely preserved in crystals grown under identical conditions in the gelled clays. A potentially important factor influencing corrosion, treated only briefly in these experiments, is the presence of natural organic material. In calcium bentonites, at 60°C and regardless of added Figure 6. Gypsum crystals grown in Wyoming bentonite gel with 5 per- salt concentration, corrosion is nearly absent. However, in two ex- cent NaCl at 60°C for 24 weeks. Ammonium chloride coated; scale repre- periments in which plant leaves were ground in a blender with sents 1 mm. Smaller crystal is oriented with [001] parallel to the plane of the photograph, whereas larger one has [001] perpendicular to photograph water and added to calcium bentonite muds without added salt, plane. Larger crystal is thin in the [001] direction and is apparently com- corrosion was prominent on crystals grown at 60°C for several posed of originally small crystals that were separated by corrosion of a weeks. Another factor shown to influence diagenetic modification crystal similar to those shown in Figure 2; small crystals apparently grew of gypsum habits was temperature. together to form tabular crystal shown here. DISCUSSION AND CONCLUSIONS

lute cold HC1 to remove soluble carbonate or sulfate and to aid The experiments were unable to nucleate and grow primary base exchange; it was then treated with concentrated sodium anhydrite under a wide range of conditions designed to closely chloride (5 M) at 60°C with three changes of solution. After this simulate natural environments. Temperatures of the growth cells treatment, the southern bentonite should be saturated with sodium ranged from 8° to 80°C; the latter temperature is certainly higher in exchange sites and should be more similar to Wyoming benton- than that found in natural muds near the Earth's surface, except ite than before saturation. Crystals grown in the sodium-form near volcanic or hydrothermal sources. Salinity in the cells during southern bentonite did not exhibit corrosion re-entrants after sev- crystal nucleation ranged from conditions of nearly fresh water to eral months, but the habit of the crystals was markedly different highly saline water containing more than 200 per mil total dis- from those grown in untreated clay. solved salts. The combinations of high temperatures and salinities Another important factor in the diagenetic reactions is the pres- assure that some of these systems were well within anhydrite's ence and concentration of admixed monovalent chlorides in the thermodynamic stability field, but anhydrite failed to nucleate Wyoming clay systems. Corrosion and habit modifications were under these conditions. Even more surprising than the lack of nearly absent in crystals shown in Wyoming clay without added anhydrite nucleation was that initially precipitated gypsum was not sodium chloride, whereas 5 or 10 percent salt promoted the reac- replaced by anhydrite after several months under these rigorous tions. There was no obvious difference between crystals grown in conditions. the 5 percent and 10 percent salt concentrations, but where the sys- These results better define the conditions under which anhydrite tems contained 20 percent salt, there were no habit modifications. could form in natural environments, either by direct nucleation In these systems, numerous small, slowly growing, well-formed from solution and subsequent growth or by replacement of primary crystals were produced that were different in habit from those gypsum. Since temperatures higher than 80°C cannot be expected found in lower salt concentrations. to occur within sedimentary environments, the only other condi- To determine if sodium was unique in its effects, 1 M solutions of tion seems to be that of salinity in excess of 200 per mil, as from other chloride salts (Table 1, see footnote 1) were added to Wyo- extreme evaporation and concentration of saline waters. One envi- ming clays and nutrient solutions, and new growth experiments ronment in which this high salinity could develop is the zone of were run at 60°C. Lithium, ammonium, and potassium had effects capillary rise within muds bordering saline water bodies in arid nearly identical to those of sodium; manganese, nickel, and zinc climates, as, for example, in the Trucial Coast area. The experi- eliminated corrosion and lateral growth (although these salts pro- ments reported here do not disprove the possibility that primary duced crystals with a slightly different habit from those grown in anhydrite can occur in arid environments, but only that extremely saline conditions will be necessary if the reaction occurs.

Figure 7. Gypsum crystal grown in Wyoming bentonite gel with 5 per- Figure 8. Gypsum crystal grown in Wyoming bentonite gel with 5 per- cent NaCl at 80°C for 17 weeks. Ammonium chloride coated; scale repre- cent NaCl at 80°C for 17 weeks. Side view, with [001] parallel to plane of sents 1 mm. Modification of habit by corrosion and simultaneous crystal photograph, of a tabular crystal similar to that in Figure 7; scale represents growth has proceeded further than in Figure 6. 1 mm.

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The second result of the study was the formation of gypsum ACKNOWLEDGMENTS crystals that were tabular perpendicular to the crystallographic c axis. Although this type of crystal is common in saline environ- Bert Nordlie, Robert Palmquist, Lyle Sendlein, and Howard R. ments, experimental production of the habit has not been previ- Shanks contributed many helpful comments and suggestions for ously reported. The factors found experimentally to promote this this study. Gregory Thompson performed much of the x-ray dif- crystal habit substantiate Shearman's (1966) tentative conclusion fraction work. Anita Cody provided many helpful comments and that this morphology is an indicator of saline conditions and suggestions on the manuscript. W. E. Dean provided many useful perhaps of saline intertidal zones. Thus, evidence of this crystal and pertinent comments in his review of the manuscript. habit in sedimentary rocks — for example, as original gypsum crystals, as pseudomorphs after these crystals, or as casts or molds REFERENCES CITED — should indicate very saline conditions during growth of the gyp- sum. Barta, C., Zemlicka, J., and Rene, V., 1971, Growth of calcium carbonate The mechanism causing the progressive modifications in habit is and calcium sulfate dihydrate crystals in gels: Jour. Crystal Growth, still not clear. The fact that these reactions occurred almost exclu- v. 10, p. 158-162. Blount, C. W., and Dickson, F. W., 1973, Gypsum-anhydrite equilibria in sively in Wyoming bentonite and not in southern bentonite or Yara systems CaS04-H20 and CaS04-NaCl-H20: Am. Mineralogist, v. 58, kaolinite suggests that some property limited to the Wyoming clay p. 323-332. causes interaction with monovalent salt solutions to produce the Broughton, M. N., 1934, Secondary selenite crystals in Tertiary strata in habit changes. A property that could promote these reactions is the Texas: Am. Mineralogist, v. 19, p. 466-473. cation exchange capacity of the clay, which depends on many dif- Cody, R. D., and Shanks, H. R., 1974, A comparison of calcium sulfate ferent physical and chemical characteristics of each clay mineral dihydrate grown in clay gels and in sodium silicate gels: Jour. Crystal species, such as particle size distribution, clay layer and interlayer Growth, v. 23, p. 275-281. chemistry, and so forth. Specific characteristics of the Wyoming Goldschmidt, Victor, 1913-1923, Atlas der Krystallformen: Heidelberg, C. clay, however, cannot be responsible for the reactions, because the Winter, 9 vol. same tabular morphology occurs naturally in nearly pure carbon- Hardie, L. A., 1967, The gypsum-anhydrite equilibrium at one atmosphere ate muds containing only traces of silicate clays. Another possibil- pressure: Am. Mineralogist, v. 52, p. 171-200. ity that can be discounted is that the saline solutions were the sole Henisch, H. K., 1970, Crystal growth in gels: University Park, Pennsylvania State Univ. Press, 111 p. cause of the habit modifications. If this were true, similar reactions Kelley, K. K., Southard, J. C., and Anderson, C. T., 1941, Thermodynamic should occur in other clay muds such as the southern bentonite and properties of gypsum and its dehydration products: U.S. Bur. Mines Yara kaolinite. Tech. Paper 625, 73 p. Thin sections of experimentally grown gypsum crystals with in- Kinsman, D.J.J., 1969, Modes of formation, sedimentary associations, and termediate degrees of corrosion and lateral growth show fractures diagnostic features of shallow-water and supratidal evaporites: Am. with thin clay inclusions along planes perpendicular to the gypsum Assoc. Petroleum Geologists Bull., v. 53, p. 830-840. Lambert, B., and Schaffer, R. J., 1926, Studies on precipitated solids. Pt. II, c axes. Corrosion occurs most readily along these fractures. Similar Calcium sulfate: Chem. Soc. London Jour., p. 2648-2655. fractures are uncommon in crystals grown in southern bentonite, Masson, P. H., 1955, An occurrence of gypsum in southwest Texas: Jour. but where they do occur, there is the same alignment of corrosion Sed. Petrology, v. 25, p. 72-77. re-entrants with fracture planes. Nancollas, G. H., Reddy, M. M., and Tsai, F., 1973, Calcium sulfate dihy- One can speculate that perhaps these fractures are locations of drate crystal growth in aqueous solution at elevated temperatures: abundant lattice dislocations caused by inclusion of small amounts Jour. Crystal Growth, v. 20, p. 125-134. of clay during crystal growth. Such dislocations would increase the Posnjak, E., 1938, The system CaS04-H20: Am. Jour. Sci., v. 35A, solubility of the crystal slightly so that resolution could result under p. 247-272. favorable conditions. Lateral growth could result from the addition Power, M. H., Fabuss, B. M., and Satterfield, C. M., 1966, Transient sol- ubilities of the calcium sulfate-water system: Jour. Chem and Eng. of calcium and sulfate released by dissolution of the more soluble Data, v. 9, p. 437-442. portions of the crystals. The near absence of corrosion on crystals Shearman, D. J., 1966, Origin of marine evaporites by diagenesis: Inst. grown in other clay matrices could perhaps be a result of properties Mining and Metallurgy Trans., sec. B, v. 75, p. 208-215. of these clays that hinder dislocation development. Promotion of

corrosion reactions by monovalent salts could perhaps be due to a MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 10, 1975 change in the surface energy of the gypsum by these salts, which REVISED MANUSCRIPT RECEIVED JANUARY 1, 1976 would facilitate inclusion of clay by the growing crystals. MANUSCRIPT ACCEPTED FEBRUARY 13, 1976

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