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Mineral replacement reactions mediated by fluids are common in sedimentary basins, where they influence geochemical cycles. Phosphorous (P) pollution of soils, sediments and water bodies is currently a widespread problem. Some apatite accumulations in sediments may have formed through the interaction of P-bearing aqueous solutions with mineral surfaces that result in mineral replacement reactions. Here, we investigate the pseudomorphic replacement of anhydrite single crystals by aggregates of β-tricalcium phosphate and hydroxyapatite upon interaction with a P-bearing solution at temperatures between 120 and 200 °C. SEM imaging is used to study the texture of the aggregates. Rietveld refinement of the X-ray diffraction patterns and Raman spectra analysis of the reacted samples provide information on the kinetics of the replacement. At all temperatures β-tricalcium phosphate forms alongside hydroxyapatite at early stages of the replacement reaction. At T ≥ 180 °C, hydroxyapatite/β-tricalcium phosphate ratio rapidly increases, and hydroxyapatite is the only phase in fully replaced samples. At T < 180 °C hydroxyapatite/β-tricalcium phosphate ratio increases slowly and fully replaced samples still contain significant amounts of β-tricalcium phosphate. The progress of the replacement is facilitated by the formation of porosity. The evolution of the hydroxyapatite/β-tricalcium phosphate ratio and the crystal habit of both phases strongly influence the arrangement of this porosity. The empirical activation energy Ea (kJ/mol) of the replacement reaction is determined by the Avrami and the iso-conversion methods. Both approaches yield an Ea of ~40 kJ/mol. Anhydrite dissolution appears as the rate-limiting process and the overall kinetics of the replacement reaction is controlled by the rate diffusion of dissolved species through the porosity network. The ripening of the metastable β-tricalcium phosphate into hydroxyapatite affects the characteristics of the porosity network and further modulates the kinetics of the replacement. These results may improve the understanding of the mechanisms of P-sequestration by mineral surfaces through coupled dissolution–precipitation reactions and shed light on the origin of apatite accumulations associated to evaporitic sedimentary rocks. [ABSTRACT FROM AUTHOR]

Many studies have improved our understanding of the mode of preservation at the Crato fossil Lagerstätte. The high degree of preservation of the Crato mineralized insects is thought to be a consequence of the diffusion of ions through carcasses and envelopment by bacteria that, in turn, created microenvironmental conditions that led to mineralization, mainly pyritization. Pyritized insects have been oxidized by in situ weathering to more stable oxide/hydroxy minerals during Quaternary time. This transformation is essential to maintain the palaeontological information acquired during microbially induced pyritization in an oxidizing atmosphere. However, intense weathering can diminish or obscure the morphological fidelity, and little attention has been paid to the post-diagenetic processes experienced by these fossils. Here, we aim to determine the degree of alteration undergone by Crato pyritized insects using the following combination of analytical tools: scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared and Raman spectroscopy. Our results show that well-preserved insects are preferentially replaced by haematite and poorly preserved fossils are replaced by goethite. In addition, we recorded three types of post-diagenetic alteration: insects with iron-oxide overgrowths; insects associated with black coatings, sometimes with the formation of dendrites; and insects preserved as an impression, where only the outline of the body remains. All of these alterations have the potential to distort or tarnish palaeontological information. Here, we measured the effects of such telodiagenetic alterations at macro and micro scales. Therefore, this taphonomic approach has wide applicability wherever fine-grained deposits bearing mineralized insects are found. [ABSTRACT FROM AUTHOR]

Over the almost 190 years-long research of the Ditrău Alkaline Massif (Eastern Carpathians, Romania), felsic rocks have been regarded as homogeneous, uniform units of the igneous complex. Nevertheless, our detailed textural study revealed that the felsic suite (diorite–alkaline feldspar syenite and nepheline-bearing syenite–granite series) cropping out north of the Jolotca Creek valley is more heterogenous at micro-scale than previously thought. This heterogeneity partly derives from abundant mafic mineral-rich clusters; nevertheless, felsic minerals also exhibit various, remarkable textural features. Outcrop to micro-scale traits of felsic crystal settling, mafic mineral aggregates and flow fabrics along with metamorphic country rock xenoliths suggest that the studied rocks crystallized under dynamic magmatic conditions. Cumulate formation, shear flow, convection currents as well as various open-system magmatic processes (e.g., magma recharge, magma mixing and mingling, crystal or mush transfer and recycling, country rock assimilation) played a significant role in the petrogenesis of the examined felsic suite. Based on field observations as well as on the microtextural relationship of the minerals, two major groups of felsic rocks were distinguished: (1) felsic rocks (lacking or containing sparse mafic minerals) spatially associated with mafic rocks and (2) felsic rocks (with mafic minerals and clots) spatially unassociated with mafic rocks. Rocks of the former group are dominated by plagioclase, accompanied by minor alkaline feldspar, biotite and accessory titanite. Distinct structural and textural features suggest the physical accumulation of the rock-forming phases. Such textural properties can also be observed in some rocks of the second group. Isolated mafic minerals are rather scarce in the latter; nevertheless, different types of aggregates made up of either identical or various mafic phases are more common. Clustered minerals are either intact or show different stages of alteration. A detailed petrographic study of the above-mentioned peculiarities has been implemented in order to define their potential origin(s) and petrogenetic significance. [ABSTRACT FROM AUTHOR]

Polycrystalline calcite pseudomorphs after dolomite can exhibit mosaic or drusy fabric caused by replacement of the dolomite by calcite (one-step dedolomitization) or dolomite dissolution followed by calcite cementation (two-step dedolomitization), respectively. This study discusses the differences between the two types and interprets the characteristics of the fluid causing them. Conclusions drawn here are founded on a microscopic and petrophysical examination of an outcrop of Tuwaiq Mountain Formation, central Arabian Peninsula. Petrographic examination shows that dedolomite has distinct microfabrics in each part of the section. The lower part of the section is not affected by dolomite calcitization, the middle part is dominated by replacement fabrics, and the upper part is dominated by dissolution and/or cementation fabrics. Petrophysical examination shows a wider range of porosity and permeability values for the upper part of the section, relative to the middle and lower parts. One-step dedolomitization is characterized by preservation of the external morphology of the parent dolomite, confinement to the volume previously occupied by the dolomite, development of intracrystalline porosity and permeability that allows the fluid to maintain contact with the reaction front, and no significant effect on porosity of the original rock. Two-step dedolomitization is characterized by corrosion of the external morphology of the parent dolomite, association with other forms of calcite cementation, development without intracrystalline porosity and permeability, and positive or negative effects on the porosity. While one-step dedolomitization appears to occur within a stagnant zone of fluid circulation, the two-step dedolomitization is considered to reflect an active fluid-flow system. [ABSTRACT FROM AUTHOR]

Moving from the problem of defining how medieval speculation conceived the aesthetic dimension of art, this essay purposes an insight into the aspects that describe the peculiarity of the Byzantine conception of beauty and art. Surpassing the noetic perspective established by Platonic thought – shared also by Western medieval philosophy – according to which beauty is an intelligible model subsisting in itself as an autonomous entity, the Byzantine proper vision conceives beauty as a divine energy. The implications of this perspective lead us to investigate its connection with some of the most original achievements of Byzantine speculation, such as hypostatic ontology, theology of deification, eikonic thinking, and especially sophianic gnoseology, which permit us to overcome the dichotomy of the intelligible and the sensible domains of reality.

Ancient evaporite deposits are geological archives of depositional environments characterized by a long‐term negative precipitation balance and bear evidence for global ocean element mass balance calculations. Here, Cretaceous selenite pseudomorphs from western Anatolia ('Rosetta Marble') — characterized by their exceptional morphological preservation — and their 'marine' geochemical signatures are described and interpreted in a process‐oriented context. These rocks recorded Late Cretaceous high‐pressure/low‐temperature, subduction‐related metamorphism with peak conditions of 1·0 to 1·2 GPa and 300 to 400°C. Metre‐scale, rock‐forming radiating rods, now present as fibrous calcite marble, clearly point to selenitic gypsum as the precursor mineral. Stratigraphic successions are recorded along a reconstructed proximal to distal transect. The cyclical alternation of selenite beds and radiolarian ribbon‐bedded cherts in the distal portions are interpreted as a two type of seawater system. During arid intervals, shallow marine brines cascaded downward into basinal settings and induced precipitation. During more humid times, upwelling‐induced radiolarian blooms caused the deposition of radiolarite facies. Interestingly, there is no comparable depositional setting known from the Cenozoic world. Meta‐selenite geochemical data (δ13C, δ18O and 87Sr/86Sr) plot within the range of reconstructed middle Cretaceous seawater signatures. Possible sources for the 13C‐enriched (mean 2·2‰) values include methanogenesis, gas hydrates and cold seep fluid exhalation. Spatially resolved component‐specific analysis of a rock slab displays isotopic variances between meta‐selenite crystals (mean δ13C 2·2‰) and host matrix (mean δ13C 1·3‰). The Cretaceous evaporite‐pseudomorphs of Anatolia represent a basin wide event coeval with the Aptian evaporites of the Proto‐Atlantic and the pseudomorphs share many attributes, including lateral distribution of 600 km and stratigraphic thickness of 1·5 to 2·0 km, with the evaporites formed during the younger Messinian salinity crisis. The Rosetta Marble of Anatolia may represent the best‐preserved selenite pseudomorphs worldwide and have a clear potential to act as a template for the study of meta‐selenite in deep time. [ABSTRACT FROM AUTHOR]

Zinc oxide (ZnO) is one of the main functional materials used to realize chemiresistive gas sensors. In addition, ZnO can be grown through many different methods obtaining the widest family of unique morphologies. However, the relationship between the ZnO morphologies and their gas sensing properties needs more detailed investigations, also with the aim to improve the sensor performances. In this work, seven nanoforms (such as leaves, bisphenoids, flowers, needles, etc.) were prepared through simple wet chemical synthesis. Morphological and structural characterizations were performed to figure out their growth mechanisms. Then, the obtained powders were deposited through screen-printing technique to realize thick film gas sensors. The gas sensing behavior was tested toward some traditional target gases and some volatile organic compounds (acetone, acetaldehyde, etc.) and compared with ZnO morphologies. Results showed a direct correlation between the sensors responses and the powders features (morphology and size), which depend on the specific synthesis process. The sensors can be divided in two behavioral classes, following the two main morphology kinds: aggregates of nanocrystals (leaves and bisphenoids), exhibiting best performances versus all tested gases and monocrystal based (stars, needle, long needles, flowers, and prisms).

Since Spengler borrowed from the mineralogical terminology the term “pseudomorphism” to apply it to the morphology of cultures and to the interpretation of the relationship between different cultural traditions, this concept has widely been used in the vocabulary of philosophy and human sciences. This article intends to reconstruct some milestones in the history of the effects of such a loan, while investigating the oscillations of its semantic spectrum and its methodological and epistemological implications. Spengler employed the concept in a negative sense, to describe a phenomenon of morphological constraint: a new culture is compelled by the inertial force of a previous culture to take on alien forms to express its own contents. Among Spengler’s readers, Jonas had recourse to the notion of “pseudomorphosis” in order to explore the links between the Gnosis and the Greek, Oriental and Christian traditions; Mumford used it to analyse the connections between recent technological inventions and the old forms of power and economy; Adorno conceived pseudomorphosis as a pernicious mimesis (philosophy mimicking sciences; art imitating the conceptual language; music imitating painting). All share the idea that pseudomorphism consists of a false manifestation of form, which ends up masking the expression of an authentic and original content. Moreover, the adoption of old forms to communicate new contents encourages merely superficial and largely unfounded analogies and similarities. Panofsky also adopts the notion in his iconology, impressing however to the concept different semantic nuances at different moments of his work. In Studies of Iconology (1939) he recognizes the heuristic value of pseudomorphism, which is capable of bringing to expression, during the Renaissance, contents that had remained unexpressed in ancient times thanks to the mediation of surviving medieval elements: a complex articulation of the pseudomorphic function in three times, that highlights a poietic retroactive movement exerted by the present on the remote past thanks to the metabolic activity of the recent past, in the wake of Warburg’s investigations on the Nachleben der Antike. Such acknowledgment is however lost in the subsequent reflections: in Tomb Sculpture (1964) Panofsky reaffirms the negative and simplistic sense of pseudomorphism as an apparent but groundless formal analogy. Taken in this pejorative sense, the term is assumed by art historians like Rosenblum and Bois. But the revaluation, in recent times, of the phenomena of anachronism (Didi-Huberman, Nagel, Wood, Powell) is promoting a new sensibility for the heuristic potentials of the pseudomorphic retroaction. To be fully accomplished, this requalification must be accompanied by a coherent theory of similarities.

Carbonated hydroxyapatite (CHAp) and β-tricalcium phosphate (β-TCP) have been employed for decades as constituents of scaffolds for bone regeneration because they chemically resemble bone mineral. In this study, the feasibility to manufacture CHAp/β-TCP scaffolds by pseudomorphic transformation of casted blocks of gypsum was investigated. The transformation was carried out by immersing the precursor gypsum block in 1 M (NH4)2HPO4/1.33 M NH4OH solution with liquid/solid ratio of 10 mL/g and autoclaving at 120 °C and 203 kPa (2 atm) for 3 h at least. Neither shape nor dimensions significantly changed during transformation. The composition of scaffolds treated for 3 h was 70 wt.% CHAp and 30 wt.% β-TCP, and their compressive and diametral compressive strengths were 6.5 ± 0.7 and 5.3 ± 0.7 MPa, respectively. By increasing the time of treatment to 6 h, the composition of the scaffold enriched in β-TCP (60 wt.% CHAp and 40 wt.% β-TCP) but its compressive and diametral compressive strengths were not significantly affected (6.7 ± 0.9 and 5.4 ± 0.6 MPa, respectively). On the basis of the results obtained, it was concluded that this route is a good approach to the manufacturing of biphasic (CHAp/β-TCP) scaffolds from previously shaped pieces of gypsum.

In this work, pseudomorphic transformation is introduced as an effective method to prepare mesoporous silica in the shape of millimeter sized disc from a biosilica precursor. The samples were prepared by two approaches. In the former method, pre-shaped silica discs were autoclaved in a basic solution containing cethyltrimethyl ammonium bromide (CTAB) and trimethyl benzene (TMB) to perform pseudomorphic transformation (S1) and in the latter case, silica powders were initially transformed by a pseudomorphic reaction following by shaping them before the removal of CTAB and TMB in calcination and sintering step (S2). The textural properties of the obtained mesoporous silica discs present that S2 sample results in larger pore size and also higher pore volume. The prepared samples have been utilized for preparation of Ni impregnated-silica catalysts in steam reforming of methane. Ni/S2 sample has been shown superior catalytic activity in comparison with other samples (methane conversion of 75% and hydrogen yields more than 40% at 550 °C). The measured physicochemical properties of the samples have been implemented to describe the catalytic results and possible coking mechanism on different samples. [ABSTRACT FROM AUTHOR]

Moving from the problem of defining how medieval speculation conceived the aesthetic dimension of art, this essay purposes an insight into the aspects that describe the peculiarity of the Byzantine conception of beauty and art. Surpassing the noetic perspective established by Platonic thought - shared also by Western medieval philosophy - according to which beauty is an intelligible model subsisting in itself as an autonomous entity, the Byzantine proper vision conceives beauty as a divine energy. The implications of this perspective lead us to investigate its connection with some of the most original achievements of Byzantine speculation, such as hypostatic ontology, theology of deification, eikonic thinking, and especially sophianic gnoseology, which permit us to overcome the dichotomy of the intelligible and the sensible domains of reality. [ABSTRACT FROM AUTHOR]

The crystal structure of [NiEn]CrO (En is ethylendiamine), CHCrNNiO is studied: a = 9.0408(5) Å, c = 9.7466(4) Å, V = 689.92(8) Å, space group P6/ mmc, Z = 2, d = 1.704 g/cm, Ni-N 2.133(9) Å, ∠N-Ni-N 82.4(5)°. The thermal properties are determined. When it is heated in a hydrogen atmosphere, a mixture of nanocrystalline Ni and CrO powders with coherent scattering regions of 59 nm and 90 nm respectively is formed. It is demonstrated that thermal decomposition products consist of pseudomorphic particles that inherit the sizes and shapes of initial [NiEn]CrO single crystals. [ABSTRACT FROM AUTHOR]

CM chondrites form the largest group of hydrated meteorites and span a wide range of alteration states, with the Paris meteorite being the least altered CM described to date. Ca-Carbonates are powerful proxies for the alteration conditions of CMs because they are direct snapshots of the chemical and isotopic compositions of the parent fluids. Here, we report a petrographic and a C isotope and O isotope survey of Ca-carbonates in Paris in order to better characterize the earliest stages of aqueous alteration. Petrographic observations show that Paris contains two distinct populations of Ca-carbonates: Type 1a Ca-carbonates, which are surrounded by rims of tochilinite/cronstedtite intergrowths (TCIs), and new Type 0 Ca-carbonates, which do not exhibit the TCI rims. The TCI rims of Type 1a Ca-carbonates commonly outline euhedral crystal faces, demonstrating that these Ca-carbonates were (i) partially or totally pseudomorphosed by TCI and (ii) precipitated at the earliest stages of aqueous alteration, before Type 0 Ca-carbonates. Isotopic measurements show that Paris' Ca-carbonates have δ 13 C values that range from 19 to 80‰ (PDB), δ 18 O values that range from 29 to 41%, and δ 17 O values that range from 13 to 24‰ (SMOW). According to the δ 13 C–δ 18 O values of Paris' Ca-carbonates, we developed a new alteration model that involves (i) the equilibration of a primordial 17,18 O-rich water (PW) with 16 O-rich anhydrous silicates and (ii) varying contribution of 12 C- and 13 C-rich soluble organic matter (SOMs). It also suggests that many parameters control the C and O isotopic composition of Ca-carbonates, the principles being the degree of isotopic equilibration between the PW and the anhydrous silicates, the respective contribution of 12 C and 13 C-rich SOMs as well as the thermal evolution of CM parent bodies. Consequently, we suggest that CM Ca-carbonates could record both positive and negative δ 13 C–δ 18 O relationships, but a systematic correspondence is probably absent in CM chondrites due to the large number of factors involved in generating the isotopic characteristics of Ca-carbonates. From recent reports of the C-isotopic compositions of SOM in CM chondrites, we propose that water-soluble organic compounds were the most probable source of 13 C enrichment in the majority of CM carbonates. [ABSTRACT FROM AUTHOR]

The CM chondrites represent the largest group of hydrated meteorites and span a wide range of conditions, from less altered (i.e., CM2) down to heavily altered (i.e., CM1). The Paris chondrite is considered the least altered CM and thus enables the earliest stages of aqueous alteration processes to be deciphered. Here, we report results from a nanoscale study of tochilinite/cronstedtite intergrowths (TCIs) in Paris—TCIs being the emblematic secondary mineral assemblages of CM chondrites, formed from the alteration of Fe-Ni metal beads (type-I TCIs) and anhydrous silicates (type-II TCIs). We combined high-resolution transmission electron microscopy, scanning transmission X-ray microscopy and electron diffraction tomography to characterize the crystal structure, crystal chemistry and redox state of TCIs. The data obtained are useful to reconstruct the alteration conditions of Paris and to compare them with those of other meteorites. Our results show that tochilinite in Paris is characterized by a high hydroxide layer content ( n = 2.1–2.2) regardless of the silicate precursors. When examined alongside other CMs, it appears that the hydroxide layer and iron contents of tochilinites correlate with the degree of alteration experienced by the chondrites. The Fe 3+ /ΣFe ratios of TCIs are high: 8–15% in tochilinite, 33–60% in cronstedtite and 70–80% in hydroxides. These observations suggest that alteration of CM chondrites took place under oxidizing conditions that could have been induced by significant H 2 release during serpentinization. Similar results were recently reported in CR chondrites (Le Guillou et al., 2015), suggesting that the process(es) controlling the redox state of the secondary mineral assemblages were quite similar in the CM and CR parent bodies despite the different alteration conditions. According to our mineralogical and crystallographic survey, the formation of TCIs in Paris occurred at temperatures lower than 100 °C, under neutral, slightly alkaline conditions that favored the formation of both tochilinite and cronstedtite. During the course of alteration, the reduction in sulfur activity and/or the decrease of temperature prevented tochilinite crystallization and favoured the formation of cronstedtite and iron hydroxides. We suggest that iron hydroxides probably formed as ferrihydrite and then progressively converted to goethite between 50° and 80 °C, a temperature range that is also favorable for cronstedtite formation. The presence of cronstedtite plays a key role in the reconstruction of the alteration history, demonstrating that the alteration of Paris took place by way of serpentinization processes similar to those described on the Earth. [ABSTRACT FROM AUTHOR]

Mineral replacement reactions are common in the various environments where rocks have undergone re-equilibration with geologic fluids. Replacement reactions commonly take the form of fluid-aided, coupled dissolution-precipitation and often result in pseudomorph formation. One class of environment that frequently shows significant examples of mineral replacements is hydrothermal ore deposit systems. The goal of this study was to test the simultaneous reactivity of fluorapatite and monazite in Na- and Si-rich hydrothermal fluids, which partially mimic the mineralogy and fluid chemistry of the Llallagua tin deposit in Bolivia. A series of experiments were performed at 300 to 600 °C and 100 MPa, utilizing various combinations of monazite, fluorapatite, and H2O + Na2Si2O5. Reaction products were evaluated using scanning electron microscopy, electron microprobe analysis, and single-crystal X-ray diffraction. The results of this experimental study show that fluorapatite and monazite are differentially reactive under the conditions studied. The reaction products, pathways, and kinetics have a large temperature dependence. The 300 and 400 °C experiments show variable amounts of monazite replacement and only minor, if any, dissolution or reactivity of fluorapatite. The high-temperature 500 and 600 °C experiments are characterized by massive replacement of monazite by vitusite and britholite. Exclusively at 600 °C, monazite alteration takes the form of symplectite development at the reaction front as vermicular intergrowths of vitusite and britholite. The higher-temperature experiments also show substantially more reactivity by fluorapatite, which is partially pseudomorphically altered into britholite. This is an example of regenerative mineral replacement where both fluorapatite and britholite share the same atomic structure and are crystallographically coherent after the partial replacement. The britholite replacement is characterized by the presence of oriented nanochannels, which facilitate fluid-based mass transfer between the bulk solution and the reaction front. The fluorapatite replacement is enhanced by monazite alteration through a self-perpetuating, positive feedback mechanism between these two reactions, which enhance the REE mobility in alkali-bearing fluids and further drives bulk re-equilibration. These results have potential geochronologic implications and may be significant in the evaluation of monazite and fluorapatite as potential solid nuclear waste forms. They also give us deeper insights into the mechanism of mineral replacement reactions and porosity development. [ABSTRACT FROM AUTHOR]

Texture encompasses ‘ the overall appearance a rock has because of the size , shape , and arrangement of its constituent mineral grains ’. Textural observations are crucial for deciphering the origin and geological history of rocks and their constituting minerals. In metamorphic and hydrothermal settings, textural observations hence serve to reconstruct the P,T path and the compositions and origins of the parent fluids. Over the past 13 years, a number of studies have emphasized the role of ‘coupled dissolution reprecipitation reactions’ (CDR) in geological systems. In these fluid-driven reactions, the replacement of one phase by another occurs via coupling between the dissolution of the parent and the precipitation of the product. In this paper we review the diversity of textures that arise from the CDR mechanism. The great diversity of textures relates to the diversity of mechanisms responsible for the coupling between dissolution and precipitation. Key parameters defining textures include volume change, the rate-limiting process, and the local composition at the mineral–fluid interface. In many of the reviewed examples, reaction mechanisms, rather than intensive properties such as P–T history, control the textures in the products, and far-from-equilibrium or local equilibriums at the mineral–fluid interface play a key role in controlling the final textures and mineral assemblages. These processes can also lead to the scavenging of trace elements from hydrothermal fluids. Because by nature CDR reactions are interface-controlled, many of the products are metastable, which further drives the reactions. These subsequent reactions can add to the textural complexity, or on the contrary obscure the original reaction mechanism. This review emphasizes the need to improve our understanding of reaction mechanisms, especially in systems containing even minor amounts of fluids (ore systems; metasomatic and metamorphic systems). Such a process-driven understanding is vital to supporting the petrological interpretation of textures. [ABSTRACT FROM AUTHOR]

Calaverite, krennerite, and sylvanite are Au-Ag-tellurides with close compositions and related crystal structures. Previous experimental studies show that both calaverite and sylvanite transform to porous "mustard" gold under hydrothermal conditions; however the transformation of sylvanite follows a complex reaction path with several intermediary products, contrasting with the simple replacement of calaverite by gold. Here we report results of an experimental study of the transformation of krennerite, a phase with Ag contents intermediate between those of calaverite and sylvanite. Krennerite was replaced by Au-Ag alloy under all experimental conditions explored (160 to 220 °C; pHT ~ 3 and 9; varying availability of oxygen). No reaction was observed at the same temperature under dry conditions. The replacement was pseudomorphic and the resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 µm. The replacement of krennerite proceeds via an interface coupled dissolution-(re)precipitation (ICDR) reaction mechanism. Tellurium is lost to the bulk solution as Te(IV) complexes, and may precipitate away from the dissolution site. In contrast, Au-Ag alloy precipitates locally near the krennerite dissolution site. Overall, the hydrothermal alteration of krennerite is very similar to that of calaverite, but differs from the alteration of sylvanite, for which multi-step reaction paths led to complex products and textures under similar conditions. These striking differences are driven by the competition between solid-state reactions and ICDR reaction in sylvanite. This reflects the fact that a metastable, Ag-rich calaverite nucleates on sylvanite during the early steps of its dissolution, as a result of the close relationship between the structures of these two minerals and the enrichment in Au, Ag, and Te in solution at the reaction front. In contrast, for calaverite and krennerite, no such phase precipitates, and both minerals are transformed in a pseudomorphic manner into Au-Ag alloy. [ABSTRACT FROM AUTHOR]

Under hydrothermal conditions, diffusion-driven solid-state reactions can compete with fluid-mediated reaction mechanisms. We have obtained an insight into the complex textures resulting from this competition by studying experimentally the transformation of Au-Ag-telluride sylvanite to Au-Ag alloy under hydrothermal conditions, and exploring the effects of temperature (160--220 °C), pH (2--10), and redox conditions on the sample textures and the reaction kinetics. Sylvanite transformed to Au-Ag alloy over all hydrothermal conditions investigated, but not under dry conditions. The replacement was pseudomorphic, as the Au-Ag alloy preserved the external dimensions of the sylvanite grains. The resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 µm. In addition to Au-Ag alloy, a range of other phases were observed as intermediate products, including petzite (Ag3AuTe2), hessite (Ag2Te), and two compositions of calaverite: an Ag-rich-Te-depleted composition, (Au0.78Ag0.22)Te1.74, and a normal calaverite, (Au0.93Ag0.07)Te2.The transformation of sylvanite to Au-Ag alloy follows a complex reaction path, with competing reactions proceeding either via interface-coupled dissolution and reprecipitation (ICDR) mechanism or via solid-state exsolution. Initially, sylvanite was replaced by an Au-Ag alloy following an ICDR mechanism, with sylvanite dissolution being the rate-limiting step relative to Au-Ag alloy precipitation. Tellurium was lost to the bulk solution as tellurite or telluride complexes, depending on the redox conditions. Once the concentration of Te in solution reached a critical state, the reaction switched and sylvanite dissolution was coupled to the precipitation of an Ag-rich-Te-depleted calaverite. This Ag-rich-Te-depleted calaverite decomposes via exsolution to calaverite and phase X (Ag3+xAu1-xTe2 with 0.1 < x < 0.55), which in turn breaks down to a mixture of low petzite and low hessite below 120 °C via exsolution. As the reaction continues, the calaverite and phase X are all transformed to Au-Ag alloy via ICDR. In the ICDR reactions the Au-Ag alloy precipitated locally near the telluride dissolution site. Such local Au-Ag alloy precipitation is facilitated by fast heterogeneous nucleation onto the sylvanite, calaverite, and petzite surfaces. The dissolution of sylvanite and of the intermediate telluride species, and the overall reaction, are oxidation reactions. The diffusion of oxygen through the porous Au-Ag alloy layer plays an important role in sustaining the reaction.A similar combination of dissolution-reprecipitation and solid-state processes may be responsible for the formation of some of the Au and Au-Ag telluride assemblages observed in Nature. These processes may also play a role in the formation of mineral assemblages in Cu-Fe sulfide systems, where the solid-state mobility of Cu+ ions is relatively high at moderate temperatures. The interplay of different reaction mechanisms results in complex textures, which could easily be misinterpreted in terms of complex geological evolution. At 220 °C, solid-state replacement of sylvanite by Au-Ag alloy is slow (months), but under hydrothermal conditions sylvanite grains ~100 µm in size can be fully replaced in as little as 96 h, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores. [ABSTRACT FROM AUTHOR]

We examine the partial survival of high-temperature mantle microstructures throughout multi-stage hydration- and dehydration-mediated pseudomorphism at differing pressure–temperature–fluid conditions. Throughout the harzburgitic mantle section of the Leka Ophiolite Complex (Norway), finite domains of parallel olivine encompassed by mesh-textured olivine resembling ‘perfectly cleaved’ olivine grains were identified. Crystallographic orientation mapping, combined with micro-computed tomography, reveals that the parallel olivine grains are highly misoriented (up to 90°) with no crystal-preferred orientation, despite remaining parallel in three dimensions. Parallel olivine grains exhibit free dislocations with low dislocation density, whereas within mesh-textured olivine dislocations are aligned into walls. MnO is enriched (up to 1·8 wt %) and NiO depleted (0·21 ± 0·24 wt %) within parallel olivine grains compared with mesh-textured olivine (0·29 ± 0·14 wt % MnO; 0·38 ± 0·19 wt % NiO). Clinopyroxene lamellae that are crystal-plastically deformed occur sandwiched in lizardite layers between every parallel olivine grain or fully enclosed within olivine. Al2O3 and Cr2O3 concentrations of clinopyroxene lamellae (2·09 ± 0·88 wt % Al2O3; 0·79 ± 0·27 wt % Cr2O3) overlap with those of primary clinopyroxene grains (2·43 ± 0·69 wt % Al2O3; 0·83 ± 0·36 wt % Cr2O3) and are distinctly different from those of secondary diopside found within the parallel olivine domains. Intragranular serpentine inclusions (XMg = 0·95 ± 0·01), displaying elevated Al2O3 (3·92 ± 4·10 wt %) and Cr2O3 (0·78 ± 0·82 wt %) concentrations, are exclusively found within parallel olivine grains. Lizardite (XMg = 0·92 ± 0·02) within the domains originates from hydration of parallel olivine and compositionally overlaps with mesh-texture lizardite. Antigorite (XMg = 0·95 ± 0·01) replaces both types of olivine grains. Whole-rock compositions indicate a harzburgitic composition; however, microstructural and chemical observations and the current absence of primary orthopyroxene suggest that the precursor silicate of every parallel olivine domain was a single orthopyroxene grain that was initially serpentinized and later dehydrated to result in the present microstructure. Although desilicification is necessary during the transformation of orthopyroxene to olivine via a bastite stage, calculations based on whole-rock compositions imply that the released SiO2(aq) was mobile only over micrometer to centimeter scales, reacting with the surrounding olivine directly to form serpentine. Crosscutting relationships and serpentine compositions imply that dehydration occurred prior to the now evident lizardite- and antigorite-serpentinization. Comparison with the regional geological setting indicates that dehydration may have occurred transiently within the oceanic lithosphere prior to obduction. [ABSTRACT FROM AUTHOR]

The volume change of solid phases associated with dissolution and precipitation reactions during mineral replacement is a critical factor for the advancement of the reaction boundary. Contributing parameters to the overall volume change of a replacement reaction are the molar volume of parent and product and their solubility ratio within a given solution. Based on simple model salt systems, the contribution of solubility to volume change is quantitatively determined. For NaCl-KCl as an example of a binary salt system without solid solution, the relative volume changes can be calculated for various reaction paths using the slope of the solubility from a simple solubility diagram. For KBr-KCl as an example of a binary salt system with complete solid solution, the determination of the solubility curve is based on a modified Lippmann phase diagram called a solubility phase diagram. It allows a quantitative calculation of the relative volume change based on the solid solution-aqueous solution (SS-AS) relationships for variable solution compositions and reaction paths in the salt-water system. Reaction kinetics, textures and the compositional evolution of replacements in both salt systems can be conclusively explained by the relative volume change on the basis of experimentally constrained reaction paths. The analogy from simple model system to replacement reactions at the Earth's surface and crustal conditions (for example in apatites or feldspars) may offer insights to successfully describe volume changes and porosity generation in mineral reactions on the basis of solubility data towards a more quantitative modeling of interface-coupled dissolution-precipitation reactions. [ABSTRACT FROM AUTHOR]

Thermolysis of the complex salts (NH4)2[OsCl6] x[PtCl6]1− x ( x = 0.25−0.9) formed nanocrystalline Os xPt1− x phases. Pseudomorphism has been found: the habit of the single crystals of the starting salts is preserved during thermolysis, and the ∼10–20 nm metal particles are agglomerated into octahedral structures sized 5–10 μm. [ABSTRACT FROM AUTHOR]

Thermolysis of the complex salts (NH4)2[OsCl6] x[PtCl6]1− x ( x = 0.25−0.9) formed nanocrystalline Os xPt1− x phases. Pseudomorphism has been found: the habit of the single crystals of the starting salts is preserved during thermolysis, and the ∼10–20 nm metal particles are agglomerated into octahedral structures sized 5–10 μm. [ABSTRACT FROM AUTHOR]

• CaCO 3 is rapidly converted to CaF 2 by the action of a KF solution at room temperature. • The volumetric reaction deficit is about 1/3. • About 2% of potassium enters into CaF 2 lattice. • The KCaF 3 compound is unstable at temperatures below 500 °C. Conversion of microcrystalline calcium carbonate powder to fluorite by reacting it with potassium fluoride solution is studied. The reaction occurs at room temperature and yields a white flowing powder. Rhombohedral CaCO 3 microcrystals are replaced by an ensemble of CaF 2 nanoparticles sized ca. 20 nm, which in part fill in the volume of the precursor microcrystals. The product calcium fluoride contains about 2 mol% KF. KCaF 3 is not produced by the reaction, and KCaF 3 is a high-temperature compound stable above 500 оС. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The potential of zero charge (PZC) of a pseudomorphic silver monolayer electrodeposited onto Au(111), Pt(111), Pd(111), Ir(111) and Rh(111) has been determined in diluted perchlorate solution by measuring capacitance–potential curves. The PZC of these Ag overlayers varies in the potential range between −0.8V and −0.45V versus SCE depending on the nature of the substrate material and thus reveals significant shifts compared to −0.7V versus SCE for a massive Ag(111) electrode. This bulk value for the PZC has also been obtained for two monolayer thick Ag overlayers, demonstrating that mainly the first monolayers are electronically altered by an underlying foreign substrate. In addition, the PZC of thicker deposits has always been found to be identical to the PZC of Ag(111), showing an epitaxial growth of Ag on the electrodes under study. While no dependence on the lattice constants of the substrates for the PZC of the pseudomorphic Ag monolayers was found, it has been observed that calculated shifts in the d-band centre show the same systematic variation. Work function data for Ag monolayers on Au(111) and Pt(111) are in good agreement with the measured PZC values. Consequences of altered double-layer properties on electrocatalytic reactions are briefly addressed. [Copyright &y& Elsevier]

Abstract: The initial stages of palladium deposition onto Au(111) from 0.1M H2SO4 +0.1mM PdSO4 have been studied by cyclic voltammetry and in situ scanning tunnelling microscopy. While Pd is commonly deposited from chloride solutions, the effect of sulphate adsorption is considered in this work. A Pd monolayer is formed at underpotentials before a second monolayer grows at overpotentials. There is strong evidence that these two Pd layers are pseudomorphic with the Au(111) substrate. Sulphate is adsorbed on the pseudomorphic Pd layers over a wide potential range in a ()R19.1° structure like in the case of massive Pd(111). The deposition process changes to three-dimensional growth with the third Pd layer, which has already bulk properties. This is indicated by the appearance of a voltammetric peak in the hydrogen adsorption region, which is characteristic for the behaviour of massive Pd(111). Differences to Pd deposition from chloride solutions are discussed. [Copyright &y& Elsevier]

Abstract: Isothermal treatments of the ammonium-exchanged Na–zeolite A were carried out at 400°C for different durations up to 1000min in the air. The products were characterized by XRD, SEM, AFM, FT-IR, DTA and nitrogen adsorption measurement. The results confirmed that the amorphous pseudomorphism occurred, showing the tweed-like patterns on the surface of cubic solid. These characterizations supported the existence of thin surface layer of zeolite crystal as a microvessel, which was consistent with the previous hypothesis. The amorphous transformation was taking place by the mechanism, in which the zeolite framework reacted with hydrogen decomposed from ammonium ions. The obtained micro-cubic glass should be useful as an eco-functional material, such as the fixation of harmful ions. [Copyright &y& Elsevier]

Categorical structures and their pseudomaps rarely form locally presentable 2-categories in the sense of Cat -enriched category theory. However, we show that if the categorical structure in question is sufficiently weak (such as the structure of monoidal, but not strict monoidal, categories) then the 2-category in question is accessible. Furthermore, we explore the flexible limits that such 2-categories possess and their interaction with filtered colimits. [ABSTRACT FROM AUTHOR]

Zinc oxide (ZnO) is one of the main functional materials used to realize chemiresistive gas sensors. In addition, ZnO can be grown through many different methods obtaining the widest family of unique morphologies. However, the relationship between the ZnO morphologies and their gas sensing properties needs more detailed investigations, also with the aim to improve the sensor performances. In this work, seven nanoforms (such as leaves, bisphenoids, flowers, needles, etc.) were prepared through simple wet chemical synthesis. Morphological and structural characterizations were performed to figure out their growth mechanisms. Then, the obtained powders were deposited through screen-printing technique to realize thick film gas sensors. The gas sensing behavior was tested toward some traditional target gases and some volatile organic compounds (acetone, acetaldehyde, etc.) and compared with ZnO morphologies. Results showed a direct correlation between the sensors responses and the powders features (morphology and size), which depend on the specific synthesis process. The sensors can be divided in two behavioral classes, following the two main morphology kinds: aggregates of nanocrystals (leaves and bisphenoids), exhibiting best performances versus all tested gases and monocrystal based (stars, needle, long needles, flowers, and prisms). [ABSTRACT FROM AUTHOR]

In this paper, we suggest the synthetic routes to prepare octahedrally crystallized hematite particles. One of these routes allows stabilizing pseudomorphic nanoparticles. Calcination of iron (III) oleate at 300 ​°C in air gives triangular shaped magnetite particles. At higher temperatures (400–500 ​°C), the octahedrons, about 1 μm in size, with predominant hematite phase are obtained. Calcination of the same precursor in the presence of polymeric beads results in a decrease of particle size up to 20 ​nm while preserving the magnetic phase admixture. To achieve effective stabilization, swelling of polystyrene beads during the iron (III) oleate impregnation is required. Schematic illustration of crystals growth on the boundaries (1) and within the polymeric matrix (2). Image 1 • Fe x O y from iron (III) oleate precursor are significantly smaller if calcined within the polymeric network. • Octahedrally crystallized hematite particles are obtained from iron (III) oleate. • Calcination of iron (III) oleate at 300 ​°C in air gives triangular shaped magnetite particles. [ABSTRACT FROM AUTHOR]

Margarite- and muscovite-bearing pseudomorphs after topaz are described from the Juurakko pegmatite dyke, Orivesi, southern Finland. A supercritical vapour phase rich in calcium and alkalies caused alteration of topaz and some other silicates during final phase of crystallization of the dyke. The original columnar form of topaz crystals is characteristic in the pseudomorphs, although roundish or irregular mica aggregates are also common. The pseudomorphs are composed of fine-scaled, light-brown muscovite, but they may have a topaz-margarite or margarite core. Coarse-scaled pink, lilac or yellow muscovite forms a rim around the pseudomorphs. The muscovites are nearly ideal dioctahedral. The amount of paragonite and phengite substitution is minute. The pink muscovite is slightly enriched in Mn, but the mica is poor in Fe. Margarite is fibrous or massive, fine-scaled and white in colour. The fibre axis is either a crystallographic a or b axis. Microprobe analyses show that the composition of margarite varies largely from one crystal to the other. The mineral has appreciable paragonite and ephesite as solid solution. Fine-scaled muscovite is also a main mineral in the pseudomorphs after schorl and garnet. The pseudomorphs after topaz and tourmaline may be similar. The prismatic form and the hexagonal cross-section is, however, often well-preserved in the pseudomorphs after tourmaline and the muscovite is richer in Fe, Mg, Mn, and Ti. The muscovite in the pseudomorphs after garnet has also appreciable phengite component. Being composed of bertrandite or fine-grained bertrandite, chlorite and muscovite mass, the pseudomorphs after beryl differ from the pseudomorphs after topaz and tourmaline in mineralogy.

The transformation of calaverite to gold under hydrothermal conditions was studied experimentally by probing the effects of temperature (140 to 220 °C), pH (2-12), oxidant concentration, geometric specific surface area, and solid-weight to fluid-volume ratio on the sample textures and the reaction kinetics. Under all of the experimental conditions explored, calaverite transformed to various extents to metallic gold. The replacement is pseudomorphic, as gold preserves the external dimensions of calaverite. The resulting elemental gold is porous; consisting of filament-shaped aggregates with diameters ranging from 200 to 500 nm and lengths up to 25 μm. Gold crystals appear to be randomly oriented with respect to the twinned calaverite grains. The transformation proceeds via a coupled calaverite dissolution-gold precipitation mechanism, with calaverite dissolution being rate-limiting relative to gold precipitation. Tellurium is lost to the bulk solution as Te(IV) complexes, and may further precipitate away from the dissolution site (e.g., autoclave walls) as TeO2(s). In contrast, gold precipitates locally near the calaverite dissolution site. Such local gold precipitation is facilitated by fast heterogeneous nucleation onto the calaverite surface. The dissolution of calaverite and the overall reaction are oxidation reactions, and oxygen diffusion through the porous metallic gold layer probably plays an important role in sustaining the reaction. A similar dissolution-reprecipitation process may be responsible for the formation of mustard gold during the weathering of gold-telluride ores. At 220 °C, solid-state replacement of calaverite by gold is slow (months), but calaverite grains ~100 μm in size are fully replaced in <24 h under hydrothermal conditions, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores. [ABSTRACT FROM AUTHOR]