Anhydrite Crystals Research Articles

Stability of surface atomic structures of ionic crystals studied by atomic force microscopy observation of various faces of CaSO4 crystal in solutions

Dissolution processes on three low index faces of anhydrite (CaSO4) crystal in water were monitored in situ with atomic force microscopy. Well-controlled layer-by-layer dissolution was observed on the most stable (010) surface, where rectangular pits with monolayer depth grew in the directions of the crystal axes until they met each other forming an atom-flat surface. The shape of the pits suggests that the dissolution proceeds row by row, by formation of vacancies at the step edge followed by rapid removal of kink ions. Speeds of the step retreat decreased markedly when sulfate and calcium ions were added to water. The steps got saturated at different concentrations. Stabilities of the steps are discussed in relation to the ionic arrangement. On a highly directional (100) face, marked anisotropy was observed in step stabilities. In addition to the numbers of ionic bonds and charge neutrality as a whole, local polarity greatly affects the stabilities of the step structures. The speeds of the step retreat vary by five orders of magnitude depending on the structures, but they all approach zero toward the bulk saturation concentration of CaSO4 in water.

Deep‐water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

ABSTRACTThe Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault‐bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north‐western Red Sea has cored, below a massive salt unit, an anhydrite‐bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca‐sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine‐grained laminated sediments to coarse‐grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken‐wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom‐rich oozes formed in marine, shallow‐water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca‐sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope‐foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine‐grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.

Emplacement and diagenesis of gypsum and anhydrite in the late Permian Raisby Formation, north-east England

SUMMARY Cavities, pores and crystal pseudomorphs which are lined or filled by calcite are ubiquitous in Raisby Formation dolostones at outcrop in north-east England. Their distribution in most exposures is random on a metre scale. On a smaller scale, cavities, pores and crystal pseudomorphs are most numerous in bioclastic and resedimented beds. Deep borehole cores of the Raisby Formation contain nodules and euhedral crystals of gypsum and anhydrite of very similar size and shape to the cavities and pseudomorphs. This indicates that calcium sulphates were once abundant in the Raisby Formation at outcrop, but almost all have been removed subsequently. Calcium sulphate minerals were formed early in the diagenetic history of the Raisby Formation, predominantly by selective replacement of carbonate rocks. Gypsum was precipitated first, probably as a by-product of dolomitization, by brines refluxing basinwards through the Raisby Formation from the hypersaline Zechstein Sea during deposition of the Edlington Formation and the Hartlepool Anhydrite Formation. Later, replacive anhydrite formed following both pressure-solution and dehydration of the gypsum. During further burial, some anhydrite was replaced by barite, dolomite and sphalerite. Upon uplift of the Raisby Formation during the Tertiary, anhydrite rehydrated to porphyroblastic gypsum and this was then dissolved by meteoric groundwaters. The resultant pores were lined or filled by two generations of calcite and also, locally, by kaolinite. The earlier calcite generation is luminescent and was precipitated partly as a passive cement and partly by replacement of surviving gypsum and anhydrite, a reaction thought to have been mediated by sulphate-reducing bacteria. The later calcite generation is predominantly non-luminescent and was precipitated from near-surface meteoric groundwaters.

Carbonated, alkaline hybridizing melts from a sub-arc environment: Mantle wedge samples from the Tabar-Lihir-Tanga-Feni arc, Papua New Guinea

Xenoliths and xenocrysts from a high-K, calc-alkaline arc volcano on Simberi Island, Papua New Guinea contain alkali-rich aluminosilicate glasses with crystals of calcite, anhydrite, sodalite and fluorapatite. The glasses are quenched equivalents of a Sulfate-, Carbonate-, H 2O- , Alkali- Rich aluminosilicate Melt (SCHARM), similar to phonolite in composition, which is believed to have been derived from a subducted slab. At mantle pressures, SCHARM contained substantial dissolved F, Cl, Sr, Ba, H 2O, CO 3 2− and SO 4 2−, and had a high oxygen fugacity ( ƒO 2 ≈ FMQ + 4 ). SCHARM hybridized mantle peridotite, reacting with mantle minerals to produce a vertically zoned mantle wedge containing phlogopite-clinopyroxenite at P > 2GPa, and amphibole-clinopyroxenite at lower pressure. REE patterns of the Simberi basalts show a negative Ce and a positive Eu anomaly, with an overall depletion in the HREE relative to LREE. Partial melting of a seawater-altered basalt slab, in which a feldspathic phase (possibly scapolite) contributes significantly to an early incremental liquid, could produce a sulfate-, carbonate-, H 2O- and alkali-rich melt with a negative Ce and positive Eu anomaly. Preliminary results of experiments with anhydrous, calcite-bearing basalt, analogous to altered oceanic crust, shows that melting occurs at temperatures of 875°C at 1 GPa, and 975°C at 2 GPa to produce a carbonated, nepheline-normative melt with major element and dissolved CO 2 contents similar to SCHARM, but more enriched in Fe and Mg [1].

Diagenesis of Low-mobility Elements (Ti, Rees, Th) and Solid Bitumen Envelopes in Permian Kennedy Group Sandstone, Western Australia

The Permian Kennedy Group, Carnarvon Basin, Western Australia, is a shallow-marine shelf deposit of sandstone and interlaminated siltstone. The original sand was apparently arkosic but was altered by dissolution of feldspar and by other diagenetic processes into a kaolinitic, sideritic, silicified quartz arenite. After lithification, hydrocarbons moved through the sandstone, leaving microscopic solid bitumen envelopes that had formed around radioactive detrital minerals when irradiation polymerized, crosslinked, and locally solidified the oil. Most radioactive cores are monazite; a few are zircon and xenotime. Some monazite grains contain minute, elongate, randomly oriented perforations that almost certainly represent fission tracks, and are filled with bitumen. The diagenetic sequence can be divided into three phases. Phase 1, the earliest, is an episode of complex diagenesis. Phase 2 covers the time when pores were filled with hydrocarbons and the solid bitumen envelopes formed but other diagenetic reactions were inhibited. Phase 3 begins with the entry of formation waters. Phase 1 authigenic minerals comprise anatase, anhydrite, brookite, florencite, thorite, halite, muscovite, pyrite, quartz, possible rutile, siderite, cryptocrystalline silica, sphalerite, and xenotime. Rare thorite (ThSiO 4 ) locally replaces monazite. Siderite crystals contain nine zones, the main variations being in Fe and Mg, and contain solution cavities that may have formed in Phase 1. Two minerals not commonly reported as authigenic are florencite (Ce, La, Al phosphate), mainly as minute pseudocubic euhedra replacing feldspar and mica, and xenotime (YPO 4 ) as outgrowths on detrital zircon grains. Phase 2 diagenesis includes formation of the solid bitumen envelopes, and Phase 3 includes authigenesis of quartz, kaolinite, and zoned calcite, and perhaps solution of some florencite. The solid bitumen envelopes surrounding the radioactive accessory minerals enclose rare minute authigenic halite and anhydrite crystals whose absence elsewhere suggests that the envelopes preserved them from solution during Phase 3. Titanium dioxide, probably from ilmenite or titano-magnetite, or both, partly replaces feldspar and mica. Some aspects of the diagenesis are particularly significant. (1) Solid bitumen envelopes not only indicate the former presence of hydrocarbons but also enable the complex diagenetic sequence to be divided into three well-defined phases. (2) The envelopes can trap and preserve minerals apparently unable to resist solution outside the envelopes, i.e., minerals that would otherwise have disappeared. (3) The zoned siderite, calcite, and florencite indicate changes in pore-water composition in the first and third diagenetic phases. (4) Ti and some REE were mobile, at least on the scale of a thin section, during early diagenesis.

Tetrad effect in lanthanide partitioning between calcium sulfate crystal and its saturated solution

The distribution of rare-earth elements (REE's) between anhydrite crystal of calcium sulfate and its saturated solution was investigated at different temperatures. The lanthanide tetrad effect was observed as the apparent distribution coefficients. Furthermore, from the dependence of the apparent distribution coefficients on the growth of the anhydrite crystal, it was found that the process of the REE incorporation into the anhydrite crystal could be divided into two stages, namely, nonselective incorporation followed by thermodynamically controlled differential incorporation. It can be presumed that the factor affecting the extent of the tetrad effect depends effectively on the relative intensity of the two processes of incorporation of REE's into the anhydrite. This result would throw light on understanding of tetrad effects observed in various natural environments.

Atomic force microscopic study of structures of cleaved surfaces of CaSO 4 after wet chemical etching

Cleaved surfaces of natural anhydrite (CaSO 4) crystal were observed with atomic force microscopy (AFM) before and after its partial dissolution in aqueous solutions of (NH 4) 2SO 4. After the dissolution, rectangular etch pits were observed on the (010) face, which grew faster along the a-axis than along the c-axis. This contradicts with a prediction based solely on the number of bonds broken upon forming steplines. It is concluded that smooth and “non-polar” step-edges are more stable against etching. On the (100) face, steplines were observed only along the c-axis, which is in accordance with the prediction by the number of bonds breaking. On the (001) face, steps along the a-axis developed in the etching.

Sulfate Platform-Basin Transition of the Lower Werra Anhydrite (Zechstein, Upper Permian), Western Poland: Facies and Petrography

ABSTRACT The peripheral evaporite platform of the Zechstein (Upper Permian) Werra Anhydrite of western Poland comprises a series of shoals (with thick sulfate deposits) and lows (with thin sulfate and thick halite deposits). Three cores were selected to examine facies variations from a sulfate platform, slope, and basin. The basal unit of the Lower Werra Anhydrite is represented by different varieties of nodular, bedded-nodular, fluidal-nodular, and brecciated facies. Some nodular fabrics probably originated within sabkha and/or gypsumpond environments, and the deposit was then mechanically redistributed. The absence of nodular structures in units other than the lower one in two of the wells suggests that the origin of these structures cannot be related to deep burial. Other units of the Lower Werra Anhydrite represent subaqueous deposits. The facies in the middle and upper units have no recent analog in coastal salina environments. Turbidity currents displaced sulfates from the platform to the adjacent basin. The origin of lenticular-bedded anhydrite may be related to deformation by compaction or by ravitational instability and slumping, which may have initiated crystallization or recrystallization of anhydrite. The vertical sequence of facies shows a distinct deepening-upward trend starting from the top of the basal nodular units. Deposition in shallow (salina) environments dominated the early history of the Lower Werra Anhydrite basin, although it was probably preceded by transgressive sabkha deposits. Relatively thick nodular anhydrites probably formed by syndepositional diagenesis of probable salina deposits and minor sabkha sediments. Gypsum was the original sedimentary mineral; it was later transformed into anhydrite, during either early or late (burial) diagenesis. We show that in deeply buried evaporites not only some macroscopic primary textures remain but also even microscopic details, which allow reconstruction of depositional environments and primary mineralogy of ancient anhydrites.

Vendian-Cambrian tidal and sabkha facies of the Siberian platform

The Vendian-Cambrian interval on the Siberian Platform contains thick carbonate and evaporite sequences formed in extensive shallow-water basins. The carbonate sequences are characterized by a cyclic composition. Finegrained dolomites, undulated algal dolomites, flat pebble conglomerates, storm breccias and solution-collapse breccias form the base of each cycle. These rocks are characterized by an increased clay content which can be high enough to form argillites. Short sedimentation breaks reflected by mudcracks or silicification horizons are present as well as small cross-bedded tidal channels. Peloidal grainstones with algal overgrowth dominate in the central parts of the cycles. These members are often recrystallized and dolomitized. Micritic dolomites, undulous laminated dolomites, storm breccias and columnar stromatolites with abundant mud cracks form the upper members of the cycles. These sequences are free of clay but contain abundant anhydrite crystals and nodules. In the uppermost parts of some cycles massive layered anhydrite beds are present. The cycles vary in thickness, but usually they are between 15 and 20 m thick.

Theoretical equilibrium and growth morphology of anhydrite (CaSO 4) crystals

Theoretical equilibrium and growth morphology of CaSO 4 (anhydrite) are obtained by applying the Hartman-Perdok method and computing surface and attachment energies with a Born interatomic potential (electrostatic + dispersive + exponential repulsive terms). Dispersive coefficients derive from experimental refractivity data and the other parameters have been fitted to structural and elastic properties of the anhydrite crystal. A comparison is made with the morphology of natural samples, where the three fundamental pinacoids {100}, {010} and {001} prevail on other S forms. Finally the equilibrium shape of 2D nuclei which can form on F faces is calculated.

Structure of cleaved surfaces of anhydrite (CaSO4) studied with atomic force microscopy

A natural anhydrite crystal (CaSO4, orthorhombic, space group Amma, a0= 7.006 A, b0= 6.998 A, c0= 6.245 A, Z= 4, A. Kirfel and G. Will, Acta Crystallogr. Sect. B, 1990, 36, 2881) was cleaved in air in three pinacoidal directions at right angles and the surface structures were studied with atomic force microscopy (AFM). The arrangement of the outermost oxygen atoms of sulfate groups was clearly observed on the (010) and (100) faces. The (010) surface was atomically flat on the scale of 0.1 µm, although steplines with monolayer height were observed in the [101], [102] and [203] directions. On the (100) surface, a prominent feature was densely populated ridges of monolayer height running in the [011] direction. The (001) surface did not show and atomically flat surface. The reasons for the different cleavability and the step structures are discussed qualitatively. It is confirmed that smaller numbers of bonds ruptured per unit area is a condition of good cleavage. The arrangement of dipoles formed by ion pairs at the surface greatly affects stabilities of crystal faces and steplines.

Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction

Primary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclogitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite. This daughter mineral suite indicates high solubilites of Na, K, Ca, Mg, Fe, Zr, Ti, P, Ba, Ce, La, Th, and S species and provides direct evidence for transport of high-fieldstrenght, large-ion-lithophile, and light-rare-earth elements as dissolved species during subduction. Fluid-inclusion heterogeneities preserved within and between adjacent grains in the veins, however, suggest that the scale of fluid equilibration was small. A crack-seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins. From these observations, we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure; the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow. Vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites, Small-scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow, and hence limited metasomatism. We conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase. At Monviso, despite evidence for high trace element solubilities in saline brines, the elements were not removed from the downgoing slab prior to teaching depths of ∼40 km.

Bedded Cherts in the Early Ordovician Arbuckle Group of Southwestern Oklahoma

Bedded cherts are a rarely occurring but environmentally significant facies in the Early Ordovician Arbuckle Group. Two such units have been identified: one in the Cool Creek Formation and one in the Kindblade Formation. In each, microcrystalline calcite and dolomite alternate in thin laminae with microcrystalline silica in units up to 25 cm in thickness. The areal extent of the bedded cherts encompasses more than 130 km{sup 2}. It is hypothesized that the interlaminated cherts and micrites were deposited in very shallow marine conditions. A relatively small part of the Oklahoma Aulacogen was cut off from open ocean circulation, resulting in an isolated lagoon. The chert/micrite couplets may represent seasonal deposits in the lagoon. During seasonal flooding, slightly deeper water resulted in algal growth with accompanying precipitation of carbonates and solution of detrital quartz sand. During the dry season, the dissolved silica precipitated, resulting in thin layers of microcrystalline chart. Some carbonate layers contain small pseudomorphs after what may have been gypsum and anhydrite crystals and nodules, indicating that salinities increased enough to permit formation of evaporites. At least ten couplets occur in the thickest units, suggesting that the lagoon existed for ten wet-dry seasons.

A fluid-inclusion investigation of the tongonan geothermal field, philippines

At least 660 fluid-inclusion homogenization temperature (Th) and 44 freezing temperature (Tm) measurements, mainly on anhydrite crystals sampled to 2.5 km depth from 28 wells, record thermal and chemical changes in the Tongonan geothermal field. Interpretations of the Th (175–368°C range). Tm (-0.3 to -12.7°C range) and crushing stage observations indicate that early trapped fluids contained up to ≈2 mol% CO 2 (now measured at <0.4 mol%). reservoir temperatures have decreased by ≈20°C overall (−88 to + 145° C range) compared with minimum Th temperatures, but the reservoir salinity has remained constant at ≈1.7 weight% NaCl equivalent (or 10,000 ppm Cl). In present boiling zones, NaCl dominates the contribution to Tm, and salinity here has increased to 6 weight% NaCl equivalent.

Proton-induced X-ray and gamma-ray emission analysis of unopened fluid inclusions

An external beam (avg diam 20 mu m) of protons from a 4.0-MV Van de Graaff accelerator has been used to excite, simultaneously, characteristic X-rays and gamma rays from a wide range of elements contained in individual, unopened, fluid inclusions in doubly polished mineral plates. We report qualitative and semiquantitative compositional data obtained using the proton-induced X-ray (PIXE) and gamma-ray (PIGE) emission techniques for selected, near-surface (10-40 mu m deep) inclusions from three major, well-studied ore deposits.Proton-induced X-ray emission analysis of numerous low-salinity, CO 2 -bearing inclusions in quartz from the Panasqueira wolframite-quartz vein system, Portugal, did not detect Na, Cl, or K, presumably because of X-ray attenuation, whereas proton-induced gamma-ray emission analysis revealed Na in all inclusions. The gamma-ray spectrum yielded by an inclusion containing a daughter or accidentally trapped crystal resembling muscovite confirmed the presence of F, Na, and Al; and several X-ray spectra recorded K, Ti, Mn, Fe, Cu, Zn, Rb, and Sn.Proton-induced X-ray emission analyses of multiphase fluid inclusions in quartz from a molybdenite-rich veinlet in the core of the Bingham, Utah, porphyry Cu(-Mo) deposit confirmed the presence of the S, Ca, Cl, K, and Fe, expected from the occurrence of anhydrite, sylvite, and hematite daughter crystals. Quantification of the abundances of the lighter elements was subject to considerable error, but the data permit reasonable estimates of several heavier elements: Mn, 1,700; Fe, 28,000; Cu, 700; Zn, 1,800; Pb, 700; and Br, 200 (all in ppm).Two fluid inclusions were examined in secondary spodumene from the Tanco, Manitoba, rare element pegmatite: both contained a highly birefringent daughter crystal initially assumed to be the new mineral diomignite (Li 2 B 4 O 7 ). The proton-induced gamma-ray emission spectra were compared to those obtained from Li 2 B 4 O 7 sealed within SiO 2 glass capillaries. Boron was clearly detected in the capillaries but not in two large daughter crystals, one of which contains major amounts of Ca (tens of wt %) and is tentatively identified as calcite.Although these data are preliminary and mainly qualitative, they demonstrate that the combined proton-induced X-ray and gamma-ray emission method has great potential for fluid inclusion studies. Considerable improvements in instrumentation and techniques for fluid inclusion analysis are anticipated, as are more accurate determinations through precise standardization employing synthetic inclusions of known composition.

A tempestite-stromatolite-evaporite association (late Vendian, East Greenland): a shoreface-lagoon model

Sections of the Canyon Formation (310 m) and overlying Spiral Creek Formation (25 m) have been studied on Ella Ø (Ella Island), central East Greenland. These strata cap Vendian glacial deposits and are overlain disconformably by probable basal Cambrian sandstones. The Canyon Formation is informally divided from bottom to top into mudrock (250 m), siltstone (15 m), flakestone (25 m) and stromatolitic (27 m) members. Overlying a thin basal sandstone interpreted as littoral facies, the main bulk of the mudrock member consists of variably dolomitic silt-shales (offshore deposits below wave base). Bituminous films in the upper half of the member are thought to represent microbial mats. The overlying units represent a major shallowing-upwards sequence. The siltstone and flakestone members consist largely of sharp-based coarse dolomitic siltstones (0.05-0.6 m thick) containing combinations of: gutter casts, parallel and undulatory lamination, wave-ripple lamination and flexed dolostone intraclasts. A tempestite origin is indicated. Dolostone clasts in the flakestone member are analogous to shell fragments in Phanerozoic tempestites and are derived by reworking of thin dolomite crusts within the background sediments of the flakestone member. There is a transitional boundary to the overlying stromatolitic member which consists dominantly of finely crystalline dolostones with thin (<0.05 m) storm layers and stromatolite biostromes and bioherms. The lower half of the member lacks evidence of desiccation, but does contain subaqueous shrinkage cracks. The upper half contains a number of scoured and desiccated dolostone surfaces, abundant rigid intraclasts and several thin oolitic beds. Stromatolite morphology is characteristically broad (metre-scale), fairly smooth domes, whose synoptic relief tends to decrease upwards in individual beds and overall in the member. Scattered anhydrite crystal and nodule pseudomorphs are common, varying in abundance at different horizons. The Spiral Creek Formation contains a basal cross-stratified sandstone, with desiccated dolostone layers, interpreted as a tidal sandflat deposit. The central part of the Spiral Creek Formation consists of centimetre-scale alternations of dolomitic sandstones with short-wavelength wave ripples, and mudrocks with desiccation cracks and halite pseudomorphs interpreted as playa lake deposits. The overall facies assemblage above the mudrock member is interpreted as representing the progradation of a mudrock-dominated evaporative lagoon-sandflat-mudflat-playa assemblage over a shoreface dominated by storm action. The lagoon may have been partly barred by stromatolites, but is seen to be transitional with the shoreface. This requires a new facies model in which muddy coastal and lagoonal carbonate facies are not bounded seaward by a continuous reef or sandbody. This is indicative of low wave energy, except during infrequent storms, and a low tidal range, so that strong tidal currents are only locally developed.

Diagenesis of sabkha-related, sulphate nodules in the early Proterozoic Gordon Lake formation, Ontario, Canada

Unusually old (c. 2.4 Ga) loosely-packed anhydrite nodules and replacement nodules of gypsum, quartz and jasper are well preserved in transgressive marine siliciclastic sabkha sediment of the Huronian Gordon Lake Formation. There is no evidence of a gypsum precursor. The nodules are commonest in mud chip breccia at the base of sandstone, siltstone and mudstone upward-fining storm cycles. Anhydrite nodules are composed of a mosaic of blocky crystals. An earlier lath form is preserved in silicified outer rims. Its texture is similar to that of Recent displacive sabkha anhydrite nodules. Siliceous nodules are composed of megaquartz, some calcite-cored, or of jasper. Megaquartz is normally anhedral with strongly undulose (flamboyant) extinction. Crystals may be columnar or crudely spheroidal. Some megaquartz crystals are lath-shaped with the oblique extinction of lutecite. Rarer euhedral megaquartz in calcite nodule cores may have simple extinction but was probably not formed by cavity filling. Megaquartz crystals and anhydrite inclusions may preserve the texture of pre-existing early anhydrite laths. Smaller “jasper” nodules are composed of hematite-bearing fibrous fine-grained quartz, that is coarser than true jasper. Fibrous structure in this jasper may have oblique, length fast or length slow extinction, suggestive of recrystallization after lutecite, length fast and length slow chalcedony. True chert and chalcedony (<20μ) are absent. The jasper of the smaller nodules may contain ghost laths, after anhydrite? and scattered anhydrite and carbonate crystals. The jasper of larger nodules is granular. The jasper nodules contain rare gypsum crystals as well as anhydrite crystals which have been replaced to varied degrees by pseudomorphous quartz mosaics and carbonate rhombs. Some jasper nodules have an outer rim of megaquartz with anhydrite inclusions and associated carbonate. Some larger jasper nodules may have been formed in subaqueous sediments. Some weakly-silicified anhydrite nodules have been hydrated to gypsum nodules that are connected with subhorizontal gypsum veinlets. The hydration postdates silicification and conversion of anhydrite laths to blocky anhydrite. The veinlets, probably related to late unroofing, are a possible access route for water of hydration. They are later than host-rock lithification and calcite formation in the anhydrite nodules. Where the Gordon Lake Formation is metamorphosed megaquartz replacement nodules can be recognized by inclusions of anhydrite and interstitial calcite patches. These nodules are a useful indicator of vanished evaporites in such rocks. If the evaporite nodules of the Gordon Lake Formation, and the replaced evaporites of the Kona Dolomite of Michigan and the Peribonca Formation of Quebec are stratigraphically equivalent they indicate a cupriferous sabkha with an original extent of at least 1400 km.

Development and Fabric Relationships of Anhydrite in Detroit River Group Sour Zone,"Ogemaw County, Michigan: ABSTRACT"

The study of diagenetic events within the Devonian Detroit River Group sour zone carbonates reveals several stages of anhydrite development: (1) massive, bedded anhydrite; (2) nodular anhydrite; (3) pore-filling anhydrite; and (4) postcompactional laths. The massive, bedded anhydrite was deposited as subaqueous gypsum. The nodular anhydrite developed in early stages of shallow burial diagenesis. Abundance of nodules decreases vertically, away from the subaqueous, massive anhydrite deposits. These anhydrite nodules, also originally gypsum, grew displacively within sabkha carbonate muds. Nodular anhydrite is visible in hand samples as individuals or as a network of reticulate ovate patches. Within pelletal micrites, dolomicrites, and oolitic packstones, abundant solution vugs have been greatly or entirely reduced by middle to late diagenetic precipitation of a meshwork of individual crystals or singular crystals of primary anhydrite. Euhedral, randomly oriented anhydrite laths are also present but are not precipitated within macropore spaces. These laths, which reveal a postcompactional origin, are often abundant and form a decussate pattern easily seen in hand sample.

A discussion of the paper “formation of soluble anhydrite by salicylic acid extraction of calcium silicosulphate” by F.J. Tang and E.M. Gartner

We describe a system for the in situ sulfur isotope analysis of small (>100 μm) anhydrite crystals and investigate its application to anhydrite-pyrite geothermometry. Anhydrite inclusions (<1 mm) have been analyzed in spectacular, museum quality pyrite crystals from the Mesozoic Cameros Basin in NE Spain. Some of the data yield isotopic equilibrium temperatures (367 ± 6°C) consistent with other geothermometric estimates of metamorphic temperature. This suggests that isotopic equilibrium was established between the host pyrite and anhydrite inclusions and was not affected by re-equilibration. However, other data points yield anhydrite compositions consistently too 34S-depleted, resulting in a geologically unrealistic temperature (610 ± 20°C). Experiments show that where pyrite becomes overheated by the laser during anhydrite decomposition, solid phase reaction can incorporate pyrite-sulfur into the sampled gas in a stoichiometric fashion, therefore, the consistency of the erroneous temperature estimates from this group. Successful analyses are only obtained when overheating of the pyrite is avoided during laser decomposition of anhydrite inclusions. The laser system allows isotopic measurement of anhydrite inclusions too small to be analyzed conventionally.

Recognition of Interstitial Anhydrite Dissolution: A Cause of Secondary Porosity, San Andres Limestone, New Mexico, and Upper Minnelusa Formation, Wyoming

Rectangular and stair-step pore reentrants in carbonate mudstones have been recognized previously as indirect evidence for anhydrite dissolution. In this study, direct evidence for subsurface dissolution of interstitial anhydrite in both dolomite grainstones and quartz sandstones includes: (1) cleavage-related dissolution fringe on anhydrite crystal surfaces, and (2) isolated remnants of optically continuous (formerly poikilotopic) anhydrite. Influenced by the prominent cleavages, the dissolution fringe on the surfaces of the anhydrite crystals consists of a series of sharp, right-angled projections and reentrants. Experimentally etched anhydrite surfaces exhibit features that directly compare to the dissolution fringe, whereas experimentally grown anhydrite does not. We deduced the following sequence of anhydrite dissolution within dolomite grainstones and quartz sandstones. Slow incipient dissolution began along the boundaries between anhydrite and adjacent minerals. From these intercrystalline boundaries, solutions penetrated anhydrite cleavages, leading to more rapid preferential dissolution perpendicular to the more prominent cleavage planes. The widened cleavage planes, together with intercrystalline boundaries, acted as conduits for the removal of dissolved ions. In the final stage, as dissolving anhydrite borders retreated toward pore throats, dissolution slowed and was, again, restricted to intercrystalline boundaries. This process was then repeated in adjacent interstices.