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Distinct pseudomorphs of displacive pseudohexagonal aragonite crystals are found in organic-rich lake deposits of the rift basins of the east coast of North America (Triassic-Jurassic). These crystals developed while the sediments were relatively uncompacted and soft, and hence are displacive, readily mistaken for molds and casts of pre-existing halite and/or thinolite (ikaite). A second form of early aragonite is found in more evaporative sediments of the Newark Basin (Passaic Formation), developed as pseudomorphic replacement of precursor, more soluble minerals such as glauberite and/or gypsum that formed during deposition. In all instances the aragonite is now replaced by calcite and dolomite, but still preserves much of the original and unusual crystal form and associated inter al structure. This aragonite is probably formed due to release of CO2 by bacterial breakdown of organic matter in bottom sediments. Elevated Mg/Ca in the pore waters may also have promoted preferential formation of aragonite over calcite.

The fossil-bone mineral in dinosaurs from the Morrison Formation is well-crystallized, stoichiometric francolite. The closely packed, subparallel francolite crystals have equidimensional cross sections mostly 10-40 nm wide. In contrast, crystallites in unaltered mammal bone tissue are poorly crystallized, nonstoichiometric carbonate hydroxyapatite, a few nanometers thick, and plate-, rod- or needle-shaped. The diagenetic francolite crystals are elongated in the c axis direction, as are bone-tissue crystallites. Along permeable cracks, francolite crystals grew to 250 nm wide. Compared to unaltered crystallites in mammal bones, the francolite has higher concentrations of Ba, Ce, Cr, F, La, Mn, Ni, Pb, Rb, Sr, Th, U, V, and Y, and perhaps Zr. Na and Mg are lower in concentration. Diagene ic francolite grew from groundwater enriched in ions from silicic ashes in the Brushy Basin Member and from dissolution of bone-tissue crystallites. After burial, francolite grew on crystallite seeds, filling the space formerly occupied by collagen, perhaps half the tissue volume, and dissolved crystallites. The micron-scale structures such as Haversian canals and laminae are preserved because the diagenetic francolite retains the orientations of the seeds. The laminae, 1-10 µm thick, commonly preserve an orthogonal plywood structure. The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature. Trace-element concentrations in the francolite may reflect groundwater composition rather than dinosaur diet. Pore-filling cements precipitated in the sands and bones at shallow burial depths, as evidenced by the 35-52% minus-cement porosities of the sandstones and numerous uncompacted oversize pores filled by cements. Gypsum and calcite were precipitated from shallow alkaline groundwater. As the groundwater became increasingly dominated by ions from silicic ash, argentite, chalcopyrite, native gold, native silver, tiemannite, and uraninite replaced areas of bone a few microns to tens of microns in size along cracks. A thin chlorite crust commonly mantled francolite crystals along bone cracks and lined openings in bones and sandstones. Pervasive precipitation of a chalcedony crust followed, with pseudomorphs of chalcedony after gypsum. In larger cancellous openings, the remaining space was fi led by the various forms of silica. The bones are brown due to goethite-filled cracks that cut pore-filling minerals.

Gypsum deposits in the Carpathian Foredeep of southern Poland were altered daring burial to anhydrite. This alteration did not completely destroy primary lithologic features. Different gypsum lithotypes have equivalents in the altered sulfate sections. Relics of, and pseudomorphs after, crystalline and sedimentary structures indicate that extensive replacement and cementation occurred early during burial diagenesis. The replacement of gypsum by anhydrite was probably initiated in the shallow subsurface by high temperature and/or reaction with brines of high salinity. Accordingly, the importance of a deep burial for gypsum diagenesis in the studied area may have been overestimated.

Carbonate rhombohedra have been Found in a majority of chert nodules examined. In limestones, they are calcite, sometimes accompanied by a small quantity of dolomite, Fe-dolomite and ankerite; in dolostones only dolomite was found within the cherts. The presence of syntaxial rims on the corroded cores of rhombohedra having a different composition and rhombic pores after dissolved rhomdohedra and silica pseudomorphs suggests changing conditions during the growth of rhombohedra. Syntaxial overgrowth on echinoderm ossicles can be older or younger than silicification. Rhombohedra were formed during the early stages of silica diagenesis. They preserved relics of microspherules (lepispheres ?) which are totally absent in other parts of the chert nodules.

The pseudomorphous replacement of gypsum by halite at the transition between bedded sulfate and chloride in cyclic evaporite sequences of Permian age is interpreted as a thermal disequilibrium feature. In solar salt ponds, the hot brines at halite saturation are undersaturated in gypsum and oversaturated in halite with respect to the cooler, previously accumulated gypsum substrate and will cause the replacement to occur when in contact with the substrate. Solar heating of the brines and production of porosity within the gypsum crystals, both of which are key features of the replacement mechanism, are materially aided by bacterial activity. The large-scale transition of gypsum to anhydrite cannot be directly involved in this process because the resulting diluted brines would corrode ha ite rather than precipitate it. Modern examples of this process appear to be rare, perhaps because the appropriate brines are often decanted for commercial collection of halite. The process is unlikely to occur in deep water masses that do not experience sufficient superheating during evaporation-concentration processes.

Polygonal fractures in eolian sandstones occur along the upper surface of the Navajo Sandstone and along four additional surfaces within the Page Sandstone, near Page, Arizona. The polygons are largely rectangular and range from 1 to 6 m across. Most of the fractures are less than 1.5 m deep, and fracture widths range from "hairlines" to tens of centimeters or more. Fracture fill most commonly shows vertical layering, and, in some cases, a series of layers is evident symmetrical about the midline of the fracture. Other fractures are filled with contorted or structureless deposits, subhorizontal laminae, and displaced or brecciated host strata. In some cases, surfaces marked by polygonal fractures can be shown to have been topographically low at the time of deposition, and the fracture sandstone below these low areas contains calcite or siliceous nodules and siliceous cement. Interpreted as contractional in origin, the polygonal fractures formed on an exposed evaporite-encrusted surface in a manner similar to ice wedges. Irregularities in these fractures are attributed to erosion by wind or water. Evaporites, evidenced by anhydrite inclusions, pore shapes, megaquartz replacement, and pseudomorphs of evaporitic minerals, provided the cohesion and contraction that caused the tensional stresses necessary for polygonal fracturing. Stresses were the result of thermal contraction. Such surfaces are thought to represent hiatuses in eolian sand-sea development, and probably represent deflation to near the water table during net deflationary conditions.

Quartz geodes from the dolostones of the Fort Payne and "Warsaw" formations near Woodbury, Tennessee have proved to be pseudomorphs after early diagenetic anhydrite nodules. Their lithological association suggests that the anhydrite developed in an arid tidal-flat environment by a process similar to that currently operating in the sabkhas of the Persian Gulf. Silicification took place prior to compaction and lithification of the sediments, the most likely source of silica solutions being the abundant sponge spicules which characterize the peritidal dolostones. Similar geodes from other Mississippian localities share the same lithological association and mode of origin. They help to define a recurrent sabkha facies and serve as important shoreline indicators which may be used in the reconstruction of regional patterns of marine transgression and regression.

A chert bed in the Whitehall Formation (Cambrian and Ordovician) of eastern New York State is strikingly similar in petrography and inferred origin to Australian and South African silcretes. The chert in the Whitehall, like its Australian and South African counterparts, occurs along an erosion surface that formed subarially, and it contains colloform chalcedony and abundant ferruginous minerals. This chert also contains pseudomorphs and ghosts of halite. Silica precipitated from a solution that became enriched in electrolytes as a result of dissolving halite. Sand-size chert grains in the Whitehall are petrographically like the Whitehall silcrete and are probably grains of reworked silcrete.

A lower Arenigian limestone bed contains the first Lower Ordovician tephra layer to be found in northern Europe outside of the Caledonian fold-belt. The tephra is unusual insofar as the volcaniclastic fragments are preserved as calcite pseudomorphs surrounded by a thin skin of mixed-layer illite/beidellite. Structures occurring in the limestone beneath the tephra are interpreted as due to percolation of gas. These structures include forcibly injected sedimentary sills and pipes. The pipes functioned as micro-vents after sedimentation of the tephra layer and incipient sedimentation had stopped free percolation of gas through the calcareous sediment. Spar filled cavities contain evidence of early cementation. The succession of events has created structures some of which might incorrectl be regarded as intertidal.

Cherty iron-formations (= ferriferous cherts) have sedimentary textures analogous to the sedimentary textures of limestones; it is possible to define textural rock types analogous to the micrites, intramicrites, oosparites, etc. of limestone classification. The close analogies between limestone textures and the textures of cherty iron formation permit paleogeographic interpretation of Precambrian iron formation by comparison with the textural facies of Recent limestones. The Sokoman Iron Formation of the Central Labrador trough, Northeastern Quebec, Canada, is subdivided in five members on the basis of composition; these can be subdivided in sub-members distinguished by their textural properties. Comparison of the distribution of facies types of the Sokoman Iron Formation with Recent limestone environments suggests deposition in two basically different environments: (1) A lagoonal platform probably composed of rapidly shifting banks of oolitic and intraclastic sand, of mud banks, and of small, lagoonal basins. (2) A basinal environment where relatively monotonous fine grained intraclastic sands, and rocks analogous to micrites were deposited. A narrow domain of oolite shoals separates both environments. Traces of evaporites (length-slow chalcedony and pseudomorphs after gypsum) prove deposition of the Sokoman Iron Formation in a warm and comparatively dry climate.

Mining operations in the Permian Salado Formation for the Waste Isolation Pilot Plant (WIPP) reveal filled pits in halite beds which are interpreted as synsedimentary dissolution features. The pits range from a few centimeters to as much as 1 m wide and 1 m deep, and they clearly truncate thin, underlying beds. Halite hopper crystals and possible chevron halite structures occurring in the pits and continuous, undeformed bedding in overlying units demonstrate that the pits formed and filled before deposition of the overlying unit. Pseudomorphs of halite after gypsum swallow-tail crystals are part of the evidence that the dissolution pits formed in shallow water.

The origin of albitized grains in sandstones has been the topic of considerable discussion in recent years. Some workers believe that the albitization of feldspar can occur in situ during burial diagenesis, but others argue that albitization is a feature inherited from source clastic grains. Our petrological and chemical studies of subsurface samples of the Jurassic, Lower Cretaceous, and Tertiary reservoir sandstones from offshore Norway show that detrital grains of potassium feldspar have been albitized during burial diagenesis. Criteria suggesting that the albitization has occurred during burial diagenesis include 1) euhedral habit of albite crystals with sharp edges and comers and markedly smooth crystal faces; 2) generally untwinned albitized grains, mostly riddled with minute br wnish inclusions; 3) lack of cathodoluminescence in albite; 4) homogeneous and pure albite composition (> 99% Ab); 5) absence of albitized grains in carbonate-cemented zones; and 6) increase in the percentage and degree (partial to complete) of albitized K-feldspar with depth. Under SEM, albitized grains display two types of fabrics. Type I albitized grains are composed of numerous tiny albite crystals (1-30 µm) growing along cleavage planes in parent grains, and they have abundant dissolution porosity, while Type II albitized grains show blocky albite crystals (5-80 @m) forming pseudomorphs of detrital K-feldspar, and they lack any dissolution porosity. Type I albitized grains are found commonly at shallower depths (between 2.2 and 3 km and temperatures varying from 65° to 90°C), whereas Type II albitized grains dominate at greater depths (> 3.5 km and at temperatures > 90°C). Albitization proceeds by a dissolution-precipitation mechanism. At shallower depth, dissolution of the parent K-feldspar is more rapid than the precipitation of albite, and rates of albite precipitation increase with depth, favoring pseudomorphic replacement of K-feldspars. Other factors that influence albitization processes are aNa+/aK+, permeability reduction, and surface-dissolution kinetics of the parent grain.

Conversion of carbonate grains to fluorite gives a microporous pseudomorph in which the contained organic material may be dyed and observed. Photographs show the application of this technique to in-situ observation of perforating algae and other organisms in Bahama oolites. Other possible applications are to ancient carbonates and to carbonates trapped by blue-green algae.

Magadi-type cherts occur in lacustrine rocks of the Middle Devonian (Middle Old Red Sandstone) Orcadian Basin, Scotland. Chert nodules are replacive within organic-rich laminites, the deposits of a stratified lake. They exhibit soft-sediment deformation, including sediment injection along shrinkage fractures within the nodules. The thefts preserve stellate quartz pseudomorphs after an evaporite mineral, which may have been a sodium carbonate mineral (?trona). These are features common to other documented occurrences of Magadi-type cherts. Nodules were nucleated about vertical cracks in the host sediment and show polygonal or parallel-aligned patterns. Precipitation of the magadiite precursor occurred from alkaline groundwaters rather than in the lake water column.

The paper describes some sedimentary structures, found beautifully preserved in the rocks of the great Vindhyan System of India. These consist of discoidal ferruginous markings, algal structures, salt pseudomorphs, conical structures, stylolites, load-casts, flute-casts, gas-pits, and worm-tracks. The conditions for the deposition of the Vindhyan sediments are briefly described. This study is probably the first of its type of the sedimentary formations of India.