Types Of Gypsum Research Articles

Slaking durability and its effect on the doline formation in the gypsum

Texture and grain size is of great importance in understanding the mechanical properties of rocks. The aim of this study was to investigate textural and slaking durability characteristics, and correlate them with the gypsum types where the dolines occurred, with particular reference to the texture and crystal size. The investigation comprised two stages: field work and laboratory testing. Initially, rock samples were obtained from various representative karstified and non-karstified locations. Thereafter, mineralogical, physical and slaking durability characteristics of the gypsum samples were determined by means of laboratory testing. After the field and laboratory works, the geological, mineralogical and slaking durability characteristics of the study area gypsum were reviewed and discussed from the point of view of doline formation. Results showed that fine grain sized alabastrine gypsum tends to be karstified and leads to doline formation. This is closely related to the crystal size, texture and attributable to the slaking durability of the gypsum. Consequently, the results demonstrate that the texture, especially crystal size, and effective porosity are important parameters controlling the slake durability of the gypsum and doline formation.

微酸性電解水の歯科的応用―練和水として使用した石膏の硬化膨張と圧縮強さ―

This study investigated both the setting expansion and compressive strength of dental gypsum mixed with slightly acidic hypochlorous acid water for the prevention of hospital infection. Slightly acidic hypochlorous acid water (PURESTER Mp-240B, Morinaga Milk Industry) was used to mix each of three types of gypsum (model plaster, dental stone and high strength dental stone), and both the setting expansion and compressive strength of these gypsum products were analyzed in comparison with those mixed with tap water. 1. With regard to the setting expansion, a significant difference was not observed between gypsum products mixed with the two types of water, except for both the model plaster and dental stone 30 minutes after the start of mixing. 2. Regarding the compressive strength, a significant difference was not observed between gypsum products mixed with the two types of water, except for the model plaster 90 minutes after the start of mixing. From the results of the study, it was revealed that both the setting expansion and compressive strength of gypsum products using the slightly acidic hypochlorous acid water showed almost the same characteristics as those using tap water.

Optimization of SO3 content in blended cementitious materials

Experimental investigation was conducted on the effects of gypsum types and SO3 content on the fluidity and strengths of different cementitious systems. The experimental results show that influences of gypsum in various cementitious materials are different. For cementitious materials blended with various proportions of slag-fly ash and 5% gypsum content, influences of gypsum and calcined gypsum on the fluidity and flexural/compressive strength are similar. It is revealed that “combination effect” and “synergistic effect” of slag and fly ash play an important role during hydration. For cementitious materials with 45% clinkers, 30% slag, 20% fly ash and 5% limestone, the optimized SO3 contents in gypsum and calcined gypsum are 3.13% and 3.51% respectively and the optimized gypsum content is 6.5%. While both of them are blended, the optimum ratio of gypsum to calcined gypsum is 40%:60% (total gypsum content 6.5%), correspondingly the optimum ratio of SO3 is 19.3%:32. 4%.

Trace elements and environmental significance of Messinian gypsum deposits, the Nijar Basin, southeastern Spain

The Messinian evaporites in Nijar Basin, southeastern Spain are composed of coarse-crystalline Yesares selenites and Feos laminated fine-crystalline gypsum. These primary Yesares selenites, Feos gypsum, and secondary vein gypsum were analyzed for trace elements (Na, Mg, and Sr) in order to understand gypsum origin. Trace-element concentrations from the three types of gypsum differ significantly. The consistency of the Yesares data with natural marine and modeled marine results suggests that Yesares selenites were deposited from marine-dominated environments. Within the entire Yesares Formation or within one single-gypsum bed, Sr, Mg, and Na concentrations increase upward while crystal size decreases, probably indicating an increase of brine concentration as Yesares selenites were accumulating. The Sr, Na, and Mg concentrations from the Feos gypsum are partially overlapped with, but also significantly higher than those of the Yesares selenites. These Feos data are inconsistent with natural marine and modeled marine data. One interpretation for the inconsistency is that diagenetic alteration occurred to primary marine Feos gypsum. An alternative scenario is that the Feos gypsum was deposited from mixed nonmarine fluids with marine brine during the late Messinian. The latter interpretation is consistent with the stratigraphic, sedimentary, and O, S, and Sr isotopic data.

Sedimentological and Petrographical Aspects of Upper Miocene Evaporites in the Beypazari and Çankiri-Çorum Basins, Central Anatolia, Turkey

Upper Miocene evaporites in the Beypazari and Cankiri-Corum basins were deposited in an inter-montane playa–lake complex environment and are intercalated with gypsiferous mudstones and clay-stones. In the Beypazari basin, these evaporites are part of the Kirmir Formation, and in the Çankiri-Çorum basin they occur in the Bozkir Formation. In these two basins, the evaporite minerals comprise mainly Ca-sulfate (gypsum and anhydrite) and Na-sulfate minerals (glauberite and thenardite). The Kirmir Formation consists of a cyclic alternation of gypsiferous claystone, carbonate mud-stone, gypsum-cemented sandstone, alabastrine gypsum rosettes after glauberite, and laminated-banded pseudomorphic alabastrine secondary gypsum after Na-sulfate minerals; at the top, a thick-bedded porphyroblastic secondary gypsum lithofacies is present. The Na-sulfate lithofacies in the Kirmir Formation is dominated by lens-shaped idiomorphic glauberite crystals, which are cemented and/or replaced by thenardite. In the weathering zone of the Na-sulfate deposit, glauberite minerals are transformed to the secondary gypsum that shows a very typical crystalline texture. The main crystalline textures of this secondary gypsum are reticulate, acicular, spindle-shaped acicular, and zigzag textures with acicular crystalline fabric in alabastrine and porphyroblastic secondary gypsum matrix. The Bozkir Formation in the Cankiri-Corum basin comprises, in ascending order, claystone, gypsum-cemented sandstone, laminated to thin-bedded limestone and claystone alternations, nodular alabastrine and porphyroblastic secondary gypsum after anhydrite, laminated secondary gypsum (after gypsarenite?), and thick-bedded alabastrine secondary gypsum after glauberite (at some localities) and claystone alternations, alabastrine micro-nodular secondary gypsum after anhydrite and claystone with displacively grown discoidal gypsum crystal alternation, partly primary and partly secondary gypsum with claystone, and, at the top, selenitic gypsum, a lithofacies of the gypsarenite-gypsiferous claystone alternation. The evaporite sequences of the Bozkir Formation can be divided simply into two parts—a lower part represented by predominantly secondary gypsum and an upper part consisting of mainly primary gypsum (selenites and gypsarenites). Geochemical study of the secondary gypsum after glauberite shows that this type of gypsum has less than 1% Na2O, which is attributed to weathering of the outcrop. Thus, typical secondary gypsum textures after glauberite may be used as a guide in Na sulfate exploration.

Ground reflectance of natural bare and pure surfaces as an aid in interpretation of Landsat Thematic Mapper data in southern Tunisia

Reflectance measurements with a Thematic Mapper (TM)-compatible instrument on artificial plots of pure gypsum, quartz and carbonate in a South-Tunisian desert showed that these materials can be satisfactorily separated using TM bands 5 and 7. Gypsum types differing in grain size and habit show different reflectance characteristics. Gypsum contents of natural eolian dune surfaces can be determined from satellite data provided dune areas have been delineated previously by physiographic interpretation.

The effect of pozzolan additions on the shrinkage of cement pastes and mortars during their first hours of age

The traditional favour enjoyed by cement including additions and by their diverse uses in Chile calls for an extense investigation of their behaviour in order that the results may justify their utilization in specific projects. This works studies volume changes occurring in cement pastes and mortars containing pozzolan additions during their first hours of age. This investigation used cements made in the laboratory from raw materials supplied by chilean manufacturers. Two types of clinkers were used, namely; a gypsum type and a natural pozzolan type, added in proportions ranging from 0 to 30%.Tests were conducted to ascertain the shrinkage of cement pastes and mortars since the first moments following their mixing operation, employing there for moulds fitted with a special device designed and implemented at the laboratory of the IDIEM Department of Agglomerants. The results thus gathered corroborated the fact that independently of cement characteristics, in general the deformation of pastes and mortars exhibits successive periods of first shrinkage swelling, and second shrinkage. The first shrinkage is affected by the ambient conditions of humidity, temperature, and wind (evaporation), but also in a preponderant way by cement specific surface, which allows higher velocity in the chemical reactions occurring during that period. Moreover the compactation degree is also affecting shrinkage, here. On the other hand, it was confirmed that with cements of like fineness, those having higher C 3 A contents exhibit an overall shrinkage larger than that of cements having low contents. At last it was possible to ascertain that an increase in pozzolan contents does not affect shrinkage directly, but that its presence may eventually modify the gypsum/clinker ratio and thus give rise to changes, specially in the two States of swelling and second shrinkage.

Isotopic constraints on the origin of salts in Australian playas. 1. Sulphur

The δ 34S values of a variety of gypsum types from the moderm playa lakes of southern Australia show little relationship with underlying bedrock types or the δ 34S values of the bedrock sulphur. On a regional scale, in both Western Australia (Yilgarn Block) and South Australia, the δ 34S values of surficial gypmsum have a regular pattern over distances of 500 to 1000 km with highest values ( ≈ +21%‰) near coastlines decreasing to δ 34S values of ∼ 14% further inland. Sea-salt sulphate is the dominant source of sulphur to the lakes investigated, although the proportion decreases from ∼ 100% near coastlines to ∼ 55% in some inland areas. A secondary source of sulphate is also airborne, but derives from volatile biogenic sulphur compounds of largely marine origin. Sulphur derived from rock weathering is a minor component, except in areas where the bedrock contains abundant sulphur. Thus Lake Cadibarrawirracanna receives ∼ 10% of its sulphur from weathering of the pyritic Bulldog Shale (Cretaceous), and Lake Amadeus, possibly up to one-third of its sulphate from evaporites of the Bitter Springs Formation (Late Proterozoic). Our δ 34S measurements are the first tracers to directly establish the marine origin of components in Australian surface brines. The δ 34S analyses indicate the accession of sulphur from both sea-salt and marine biogenic sulphur and clearly support delivery of salts to the Australian landscape as aerosols following established wind patterns. The data negate the possibility of derivation of the lacustrine “seawater-like” brine chemistry from either marine transgression or weathering of connate salts from marine strata alone. The occurrence of non-marine evaporites exhibiting largely marine-like chemistry and isotopic signatures is probably common to low-latitude tectonically stable areas and will make the distinction between some marine and non-marine evaporites from the geological record more difficult. The regular pattern of δ 34S values of surficial sulphate in lakes and groundwaters in southern Australia provides an ideal baseline against which to search for anomalous δ 34S values associated with base-metal or gold mineralization.

New Interpretation of Vertically Aligned Gypsum Fabrics: Implications for Gypsum Depositional Environments and Diagenesis: ABSTRACT

Gypsum and anhydrite fabrics in trenches and deep 500-m cores from Bristol Dry Lake, California, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentologic evidence indicates that the gypsum formed in Bristol Dry Lake precipitated as a diagenetic displacive mineral within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater seepage gypsum). Evidence for displacive growth of gypsum comes from (1) the geometry of the deposit, (2) stable isotopic data and the water chemistry of the brine, and (3) inclusions of matrix that follow twin planes and completely surround crystals as they grow. Because the fabrics and textures of this diagenetic gypsum formed by groundwater in playa settings are similar to those of primary gypsum formed in a brine pond, it is necessary to refine the criteria for the distinction between subaerial, groundwater, and subaqueous gypsum. When compared to the features observed in a Holocene subaqueous gypsum deposit (Marion Lake, Australia) and a Holocene subaerial deposit (Abu Dhabi sabkha), the following criteria can be used to distinguish between the three different types of gypsum deposits: (1) amount of matrix, (2) geometry of themore » deposit, (3) gypsum crystal size and orientation, (4) distribution of matrix and fluid inclusions within individual crystals, and (5) fabric of the matrix around the gypsum. Distinguishing between subaqueous and groundwater gypsum may be difficult. However, sufficient mesoscale differences exist such that, in most cases, the different gypsum types can be resolved from core slabs and thin sections. These criteria may be applied in ancient examples even after gypsum has been converted to anhydrite.« less

Evolution of Gypsum Karst in the Kirschberg Evaporite Member Near Fredericksburg, Texas

ABSTRACT At the Fredericksburg Gypsum Quarry in Gillespie County near Fredericksburg, Texas the quarry operations expose modern karst features in a 10-m (30 ft) thick gypsum bed that is part of the Cretaceous Kirschberg Evaporite Member. The following sequence of events is documented: 1) Formation of nodular daisy-head gypsum. Replacement by gypsum was at first slow but pervasive, with phreatic fluids oozing through the calcium sulfate bed. The main permeability pathways were provided by fractures and the intercrystalline porosity in dolomite films surrounding the calcium sulfate nodules and along thin dolomite beds. Large aligned gypsum crystals nucleated off these flow paths and grew by replacement of the earlier calcium sulfate phase. A change to more active hydrologic conditions modified the rate of secondary gypsum growth, so that the remaining gypsum grew rapidly as a microcrystalline alabastrine form. This adjustment in gypsum type was due to the relative uplift of the bed into more active phreatic conditions closer to the water table. There the bed was subjected to more pronounced temperature changes and increased groundwater flow. 2) Karst Structures and Speleothems. The gypsum bed was introduced into the vadose zone and the process of karstification of the bed began. Dissolution took place forming vertical pipes or caves in the Fredericksburg gypsum bed. Gypsum and calcite speleothems, mainly in the form of popcorn and flowstone, were deposited in the pipes and caves. Extreme dissolution of the gypsum bed produced zones of collapse breccia. This collapse occurred gradually. Locally, the overlying beds were disrupted and folded; regionally, the overlying beds were minimally disrupted by widespread lowering due to complete dissolution and gentle collapse folding. Coarse, botryoidal calcite grew in void spaces in the breccia as it was collapsing. The effects of the vadose regime on the Fredericksburg gypsum bed suggest that the end result of such an unroofed evaporite bed, even if it is never subaerially exposed, is a stratiform layer that has associated with it a breccia consisting of calcite popcorn, boxwork and botryoidal calcite, mixed with fragments of the overlying strata. Little if any evidence of the gypsum remains in the breccia. Textures and structures described herein are used to interpret similar features in other ancient evaporite beds.

Gypsum material effects on peanut and soil calcium

Abstract Adequate availability of calcium (Ca) in the upper 7–10 cm of soil is extremely important for pod development and therefore for production of quality peanut (Arachis hypogaea L.). Supplemental Ca is usually applied as gypsum, however, availability of Ca may depend on the type of gypsum. The objective of a laboratory study was to evaluate recovery of Ca in Mehlich I, 0.01 M NaNO3 and deionized water extractants from seven gypsum materials which varied physically from fine and coarse powders to crystals, granules and pellets. Overall, recovery of Ca was much greater in Mehlich I (89.5–99.6% of total Ca) than in either 0.01 M NaNO3 (81.0–98.4%) or deionized water (78.7–97.5%). However, for 3 sources, recovery of Ca was very similar in NaNO3 solution and deionized water. Field experiments were conducted on Lakeland sand (Mehlich I Ca = 127 kg/ha, 0–15 cm) and Tifton loamy sand (Mehlich I Ca = 665 kg/ha) soils to study the effects of the gypsum materials on Florunner peanut grade and yield. Effects of...

S-isotope studies of shallow water, laminated gypsum and associated evaporites, Upper Permian, East Greenland

Three types of primary subaqueous gypsum occur in the Karstryggen Formation, Upper Permian of East Greenland: (1) planar laminated gypsum, having a narrow δ 34S range of 10.2 to 10.4ℵ CDT, (2) algal laminated gypsum (9.2 to 12.0ℵ), and (3) bottom nucleated gypsum crystals (11.6 to 11.7ℵ). Type 3 was deposited from a residual brine in the basin while types 1 and 2 consist of seawater or mixed fresh water-seawater-derived sulphate with varying contribution of reoxidized sulphur originally deposited as sulphide or organic sulphur. Diagenetic nodular gypsum (7.3 to 10.8ℵ) was formed by redistribution of marine or mixed fresh water-seawater-derived brines which precipitated the laminated gypsum with contribution of sulphate originally precipitated as sulphide or organic sulphur in the organic-rich sediments.

Holocene sedimentology of hypersaline Lake Urmia, Northwestern Iran

A limnogeological expedition examined the sedimentary record of Lake Urmia, one of the world's largest saline lakes, located in a tectonically active, arid, intermontane plateau in northwestern Iran, near the cradle of western civilization. Lake Urmia resembles the Great Salt Lake, U.S.A. in many respects of morphology, water chemistry, and sediments. The present lake area is about 5000 km 2, shallow (8–12 m), and a perennial sodium chloride—sulfate system (22% salts). High resolution (3.5 kHz) seismic profiles show morphological relicts from a desiccation playa phase buried below only a few meters of sediments from lacustral phases. In places, around the deeper areas, the lake floor is not smooth but covered with mounds aligned in prevailing current directions. These are erosional relicts rather than bioherms. Sediments comprise aragonite chemical precipitates, fecal-pellet sands, detrital clayey muds, thin aragonite crusts, oolites, various gypsum types and occurrences of dolomite in peripheral areas. Organic matter, characteristic of the present shorelines, is not preserved at depth in basinal areas. Piston cores up to 6 m long penetrated a Holocene sequence of hypersaline facies with laminated aragonite—fecal-pellet muds and bottomed in a playa—mudflat facies with central gypsiferous muds and marginal terrigenous to dolomitic muds. Preliminary radiocarbon and pollen analyses suggest a playa stage prior to 9000 yr B.P. with saline lake deposition in a cool arid climate. From 9000 to 7500 yr B.P., higher-energy facies prevailed in a shallow saline lake and, since then, present-day saline environments have persisted with evidence of numerous second-order water-level fluctuations. The preliminary results suggest a lake system out-of-phase with North African and some other Middle Eastern lakes.

The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia

ABSTRACTWell exposed, often laminated gypsum sequences occur in many Quarternary salt lakes in southern South Australia. The gypsum in the salt lakes is classified by increasing grain size into gypsite, gypsarenite and selenite. The salt lakes are classified by age and hydrological setting into coastal salinas which are Holocene sea‐water fed groundwater lakes, and continental playas which are late Pleistocene endorheic basins. A study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes. As the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited. Application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non‐salina gypsum deposits.

Effect of gypsum fineness on the reclamation of sodic soils

The effect of gypsum, ground to varying degrees of fineness, on the properties of two soils having exchangeable sodium percentages of 35 and 80, respectively, was studied in the laboratory. Two types of gypsum were used, one having both upper and lower size limits of the grades defined (2.0-1.0, 1.0-0.6, 0.6-0.25, 0.25-0.125 and < 0.125 mm) and the other having only the upper size limit defined (2.0, 1.0, 0.6, 0.25 and 0.125 mm). After mixing with gypsum, the soils were leached with distilled water under a constant water head. Treatments with the finest gypsum had the highest initial hydraulic conductivity which decreased sharply with time. Treatments with coarser particles had a lower initial hydraulic conductivity that was maintained or increased with time. The reason for this was higher solubility of finer particles followed by a sharp decrease in electrolyte concentration due to precipitation of dissolved calcium in the first case, and dissolution of coarser particles and little precipitation in the latter case. Inactivation of gypsum particles due to formation of CaCO 3 coatings over their surfaces has also been indicated in the case of the finer grades. These studies suggest that gypsum passed through a 2-mm sieve, having a wide particle size distribution, is likely to be more efficient for the reclamation of sodic soils with appreciable quantities of Na 2CO 3 in the soil solution.

Sedimentology and evaporite genesis in a Holocene continental‐sabkha playa basin—Bristol Dry Lake, California

ABSTRACTBristol Dry Lake, a 155 km2 continental‐sabkha playa basin in the Mojave Desert of south eastern California, is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. Evaporite facies conform approximately to a bull's eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan to the centre of the playa, reveal: (1) crudely‐bedded, alluvial fan clastics interfingering with (2) playa‐margin sand flat and wadi sand and silt, followed by (3) gypsum, anhydrite, chaotic mud halite, and clay of the saline mud flat, and (4) salt‐pan halite beds.Terrigenous clastics were deposited in Bristol Dry Lake by sheetflow and by suspension settling from ponded floodwater. Some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin. Thin nodular‐like beds of anhydrite and several types of gypsum occur across most of the playa. Giant hopper‐shaped halite cubes are suspended in saline mud flat facies, suggesting that they grew displacively in brine soaked sediment just below the surface. Thick beds (4 m) of halite, in the playa centre, may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods, intense evaporation and brine‐level lowering, and capillary discharge of brines.The stratigraphy in the playa centre is cyclic. An ideal cycle consists of: (1) chaotic mud halite at the base overlain by (2) green to red clay with abundant, giant hoppers, and at the top (3) red clay, gypsum, and anhydrite with flaser‐ to wavy‐bedded sand and silt. This type of cycle probably records a gradual progradation of mud‐flat facies over salt pans.Bristol Dry Lake sediments are nearly identical to some of the Permian evaporites of the Permian Basin region, U.S.A. and they can serve as modern analogues for ancient‐sabkha facies analysis.

Hydrologic Setting, Occurrence, and Significance of Gypsum in Late Quaternary Salt Lakes, South Australia: ABSTRACT

Well-exposed, commonly laminated gypsum sequences occur in many Quaternary salt lakes in southern South Australia. The gypsum in the salt lakes is classified by increasing grain size into gypsite, gypsarenite, and selenite. The salt lakes are classified by age and hydrologic setting into (1) coastal salinas which are Holocene seawater-fed ground water lakes, and (2) continental playas which are late Pleistocene endorheic basins. A study of the relations between coastal salina hydrology and the associated gypsum deposition has shown that different types of gypsum form under distinct hydrologic regimes. As the hydrology above a coastal salina depositional surface changes through time, so does the type of gypsum deposited. Application of a gypsum depositional model derived f om a study of the coastal salina gypsum to those parts of a continental-playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non-salina gypsum deposits. End_of_Article - Last_Page 640------------

Holocene Gypsum Types and Formative Processes in Tidal Flat Settings, Carnarvon Basin, Western Australia: ABSTRACT

Study of gypsum presently forming in terrigenous and carbonate tidal flats along the semiarid coastline of Western Australia indicates that gypsum crystal habits, textures, structures, and fabrics developed on or within host sediments can be correlated with the physical, chemical, and hydrologic conditions of formation. By treating the study of gypsum from a sedimentologic approach, a classification of gypsum types is developed which allows comparative analysis and appears to be applicable to the interpretation of ancient evaporitic sequences. Gypsum precipitated on a substrate from a free-standing body of water crystallizes in a habit dominated by the prism (110) and displays variations in texture, fabric, and structure that are related to the maintenance or progressive change in environmental conditions within the brine body. The action of physical and organic agencies is important in the genesis of the fabrics and structures displayed and in their destruction to form clastic gypseous sediments. Gypsum precipitated within a host sediment crystallizes in a habit dominated by the hemi pyramid (111) and displays textures, fabrics, and structures that are related to host sediment properties, brine chemistry, ground-water hydrology, and the mechanisms for maintenance of such environmental conditions. Gypsum emplacement acts to disrupt and modify sedimentary features within the host and to form new textures, fabrics, and structures which are related to, and overpoint, their precursors. End_of_Article - Last_Page 556------------

Gypsum-organic interactions in the marine environment: sorption of fatty acids and hydrocarbons

Free fatty acids make up the bulk (50–84% wt/wt) of lipid materials recovered from artificial gypsum precipitates and crystals collected from sea salt evaporation pans. Both the fatty acids and hydrocarbons associated with artificial gypsum tend to be of longer mean chain length than organic materials dissolved in the mother liquor. Increased specific adsorption of n-fatty acids at Ca 2+ sites on gypsum surfaces is positively correlated with alkyl chain length. Neutral lipids were not significantly enriched in either type of gypsum precipitates. The fatty acids associated with gypsum from sea salt evaporation pans are branched fatty acids in contrast to the predominantly normal acids associated with artificial gypsum samples. Differences in the two adsorbed acid distributions are attributed to the unique lipids of hypersaline biota.

Authigenic gypsum in marine sediments

Gypsum crystals of both clastic and authigenic origin are reported in marine sediments. Two types of authigenic gypsum, readily distinguishable from the clastic, reworked forms have been identified; one being tabular and the other prismatic to acicular. A detailed stratigraphic and quantitative study of Pleistocene Core V12-18, located on the Rio Grande Rise at a depth of 4021 m, has shown tabular gypsum crystals growing around fragments and enclosing planktonic foraminiferal tests. The crystals are particularly associated with high concentrations of solution-resistant benthonic and planktonic foraminifera. Their stratigraphic distribution and faunal association suggests that gypsum precipitation is intimately linked to episodic influx of Antarctic Bottom Water originating in the Weddell Sea. The oceanic regime occupying the core site provides the physical—chemical conditions necessary to produce the SO 2− 4 and Ca 2+ ionic concentration necessary to precipitate gypsum. In response to climatic shifts (cold phase), the subtropical convergence and associated water masses were displaced in a way to permit the intrusion of cooler more productive waters. This stimulated and increased organic deposition on the relatively stagnant sea floor. During climatic amelioration, an influx of oxygen-rich, undersaturated Antarctic Bottom Water sweeps over the basin floor, actively dissolving the foraminiferal ooze and thus supplying the necessary Ca 2+. At this point gypsum is produced. Climatic aridity and semirestricted basins are no longer considered limiting factors in the origin of marine gypsum, and the classical model for its origin must be extended to include conditions periodically obtained on the sea floor.