Precipitation Of Mg Research Articles

Intrinsic mechanical and interfacial characteristics of precipitates contributing to the room and elevated temperature strength in Mg–Sn–Y alloys

The intrinsic mechanical and interfacial characteristics of precipitates in Mg–Sn–Y alloys were calculated to predict strengthening precipitates and to design novel Mg alloys. The first-principle calculations results indicated that the Sn3Y5 phase had a higher bulk and shear moduli than the Mg24Y5 and Mg2Sn phases. The Mg/Sn3Y5 interface was more stable and possessed a higher adhesive strength than the Mg/Mg2Sn and Mg/Mg24Y5 interfaces. Four as-cast alloys (S10Y3 (Mg-0.5Sn-0.16Y), S5Y3 (Mg-0.43Sn-0.25Y), S1Y1 (Mg-0.36Sn-0.37Y) and S3Y5 (Mg-0.27Sn-0.48Y) alloy (at.%)) were then designed according to the different Sn to Y atomic ratios (10:3, 5:3, 1:1 and 3:5). The lower Sn to Y atomic ratio promoted the formation of the Sn3Y5 phase in the S3Y5 alloy, whereas the S10Y3 alloy with a higher Sn to Y atomic ratio mainly consisted of the Mg2Sn phase. Compared with the S10Y3 alloy, the yield strength and ultimate tensile strength of the S3Y5 alloy were increased by 70% and 16% without sacrificing the elongation at room temperature, which was greatly improved by 127% and 104% at elevated temperatures simultaneously. These results suggested that the calculations of intrinsic mechanical and interfacial characteristics of precipitates may provide an effective tool to design high-strength Mg alloys.

Effect of seawater as mixing water on the hydration behaviour of tricalcium aluminate

This study explored the mechanism of the effect of seawater on tricalcium aluminate (C3A) hydration. The results showed that seawater retarded C3A hydration and reduced the reaction degree of C3A. The co-existence or ion pairing of Ca2+ and SO42− onto the surface of C3A poisoning the reactive sites is the main reason for this retardation. Besides, the precipitation of Mg(OH)2 on the surface of C3A would prolong the induction period for another 30 min and then consequently decrease the dissolution rate of C3A hydration. Trace amounts of MgSO40 and Mg2+ present in the alkaline solution had little retardation effect. It was also found that Cl− would preferentially react with C3A to form Friedel's salt, rather than forming hydroxy-AFm as that in the C3A-deionized water paste. The direct formation of Friedel's salt resulted in an accumulation of Al3+ in solution, which would also hinder the subsequent dissolution of C3A.

New Composite Sorbent for Removal of Sulfate Ions from Simulated and Real Groundwater in the Batch and Continuous Tests.

The evaluation of groundwater quality in the Dammam formation, Faddak farm, Karbala Governorate, Iraq proved that the sulfate (SO42−) concentrations have high values; so, this water is not suitable for livestock, poultry and irrigation purposes. For reclamation of this water, manufacturing of new sorbent for permeable reactive barrier was required through precipitation of Mg and Fe hydroxides nanoparticles on the activated carbon (AC) surface with best Mg/Fe molar ratio of 7.5/2.5. Mixture of 50% coated AC and 50% scrap iron was applied to eliminate SO42− from contaminated water with efficiency of 59% and maximum capacity of adsorption equals to 9.5 mg/g for a time period of 1 h, sorbent dosage 40 g/L, and initial pH = 5 at 50 mg/L initial SO42− concentration and 200 rpm shaking speed. Characterization analyses certified that the plantation of Mg and Fe nanoparticles onto AC was achieved. Continuous tests showed that the longevity of composite sorbent is increased with thicker bed and lower influent concentration and flow rate. Computer solution (COMSOL) software was well simulated for continuous measurements. The reclamation of real contaminated groundwater was achieved in column set-up with efficiency of 70% when flow rate was 5 mL/min, bed depth was 50 cm and inlet SO42− concentration was 2301 mg/L.

Modelling precipitation hardening in an A356+0.5 wt%Cu cast aluminum alloy

The behavior of a A356+0.5 wt%Cu alloy used to manufacture cylinder heads was studied. Samples were solutionized, quenched and aged at 200 °C for 0.1, 1, 10 and 100 h. TEM characterization showed that for the short aging durations (up to 10 h), the dominating hardening precipitates were β″ rods, while for the long aging duration (100 h), the dominance shifted to the Q-phase (Q′, Q″ precipitates). The length and diameter of the β″ rods were measured to produce size distributions which were later used to calibrate and validate the precipitation model. The physics-based precipitation kinetics model relies on classical nucleation/growth/coarsening equations adapted for the precipitation of Mg–Si precipitates in the aluminum matrix. Indirect coupling to Thermo-Calc software was used in order to determine the essential thermodynamic variables such as the driving force for precipitation and the solubility product for the model. Recent developments regarding the correction of the growth rate equations and the curvature effect were used to take into account the elongated morphology of precipitates. A Kampmann-Wagner Numerical (KWN) based model was used to track the evolution of the size distributions during nucleation, growth and coarsening of the β precipitates. The yield strength of the alloy was modelled using the Pythagorean sum of the contributions of intrinsic strength, solid solution strengthening and precipitation hardening. Both models showed good accuracy when compared to experimental results.

A novel tetragonal precipitate with multiple orientations in Mg–La alloy: An atomic scale investigation using HAADF–STEM

During the process of precipitation in aged magnesium rare earth (Mg–RE) alloys, the subsequent structural transformation of the β″ precipitate is still controversial. In this study, the sequential precipitates of the β″ were investigated in Mg–3.0 wt% La binary alloys by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM). Distinguished from the β′, the β″ precipitate in Mg–La alloy is transformed preferentially into a La-rich FCC structure with the two crystallographic orientations to α-Mg matrix, and then evolved into a hitherto unreported tetragonal phase. It is confirmed that the crystal structure of tetragonal precipitate has a symmetry of space group I4/mmm and lattice parameters of a = 0.426 nm and c = 1.080 nm. Meanwhile, there exist three categories of orientation relationships (Type-І, Type-П and Type-Ш) between the tetragonal precipitate and α-Mg matrix in the microstructure. Based on it, the tetragonal precipitate exhibits three typical precipitation morphologies, which are elucidated reasonably according to the interfacial misfit degree. The aim of this study is to provide some efficient subsequent precipitation strengthening phases of β″ precipitate.

Finite Element Modeling of Chemical and Electrochemical Protection Mechanisms Offered by Mg-Based Organic Coatings to AA2024-T351

A finite element model of the protection mechanisms offered by Mg-based organic coatings was developed. The model predicted the change in the corrosion potential of AA2024-T351 as a function of pH, water layer thickness, and the inhibition of oxygen reduction reaction. The pH in the solution was calculated taking into account Mg dissolution, precipitation of Mg(OH)2, Al dissolution, and hydrolysis of Al3+ ions. The predicted critical pH value at which the corrosion potential of AA2024-T351 sharply decreases to values below pitting and pit repassivation potentials under full immersion conditions was in accordance with experimental observations. A limiting water layer thickness below which the pH-induced pit repassivation mechanism is not predicted to occur was calculated. If the inhibition of oxygen reduction reaction by Mg(OH)2 is considered, the pH-induced repassivation mechanism becomes feasible at thinner water layers. Cathodic protection offered by Mg-rich primers was modeled as a function of coating resistance, water layer thickness, and electrolyte chemistry. The magnitude of the resistance of the film in which Mg pigments are embedded mitigates the extent of the cathodic protection. The change in local pH due to corrosion reactions affected the galvanic potentials obtained. The framework developed can be used to help identify chemical inhibitors that can operate by the chemical protection mode described in this work.

Precipitation Behavior of Mg-7Gd-3Y-2Zn-0.5Zr Alloy during Isothermal Aging.

Precipitate phases in an Mg–7Gd–3Y–2Zn–0.5Zr alloy aged isothermally at 240 °C were examined using high-resolution transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM. The two types of precipitation sequence that involve Mg–Gd and long period stacking ordered (LPSO) type were found. The LPSO type sequence consisted of the precipitation of γ′′, γ′, 14H-LPSO/18R-LPSO. The Mg–Gd type precipitation sequence involved the formation of β′(b.c.o.) and β1(f.c.c.). The sequence, morphology, distribution, and crystal structure of these precipitates formed during isothermal aging were investigated. The results indicated that the priority precipitation of Mg–Gd and LPSO type sequences during aging can be affected by Nd, which has a higher diffusion coefficient than Gd and Y. The dislocation structures and strengthening mechanism were also discussed.

Preparation and application of Mg–Al composite oxide/coconut shell carbon fiber for effective removal of phosphorus from domestic sewage

A simple and cheap adsorbent using waste coconut shell fiber (CSF) as the biomass carbon precursor was successfully prepared with the in-situ growth of Mg–Al composite oxide and the resulting Mg–Al/CSF with fractional structure was used for the removal of phosphorus from wastewater. The fractional precipitation of Mg–Al composite oxide on the carbon fiber was achieved by simple immersion method and calcination method. The morphology, composition analysis and specific area of the raw and modified CSF was characterized by SEM, XRD and BET. The maximum adsorption capability of Mg–Al/CSF towards phosphorus was fitted out to be 60.39 mg/g from the Langmuir isotherms model with pH 6, temperature 45℃. The phosphorus removal mechanism was verified by the kinetics and thermodynamics results, which was the synergistic combination of surface adsorption and chemical adsorption. In addition, the Mg–Al/CSF adsorbent worked well over the total phosphorus (TP, 0.8–1.1mg/L) in actual domestic sewage. The results indicated that the prepared Mg–Al/CSF achieved over 50% TP removal in a wide range from 50 to 200min, which is obviously higher than that of commercial activated carbon. Furthermore, the average TP removal rate of Mg–Al/CSF could reach 46% after recycling for 3 times, which is still much higher than that of commercial activated carbon (28%). The superior recycles results demonstrated the excellent regeneration performance and stability of the prepared sample. The interactions between biomass material with large surface area and metal oxide could be a promising candidate for the other pollutant treatment.

Microstructural evolution and enhanced mechanical properties of Mg–Gd–Y–Zn–Zr alloy via centrifugal casting, ring-rolling and aging

A ring-shaped Mg–8.5Gd–4Y–1Zn–0.4Zr (wt%) alloy was manufactured via centrifugal casting and ring-rolling process. The effects of accumulative ring-rolling reduction amount on the microstructure, texture, and tensile properties of the alloy were investigated. The results indicate that the microstructure of centrifugal cast alloy consists of equiaxed grains and network-like eutectic structure present at grain boundaries. The ring-rolled alloy exhibits a characteristic bimodal microstructure composed of fine dynamic recrystallized (DRXed) grains with weak basal texture and coarse un-DRXed grains with strong basal texture, along with the presence of LPSO phase. With increasing amount of accumulative ring-rolling reduction, the coarse un-DRXed grains are refined via the formation of increasing amount of fine DRXed grains. Meanwhile, the dynamic precipitation of Mg 5 RE phase occurs, generating a dispersion strengthening effect. A superior combination of strength and ductility is achieved in the ring-rolled alloy after an accumulative rolling reduction of 80%. The tensile strength of this ring-rolled alloy after peak aging is further enhanced, reaching 511 MPa, while keeping a reasonable ductility. The salient strengthening mechanisms identified include the grain boundary strengthening of fine DRXed grains, dispersion strengthening of dynamic precipitated Mg 5 RE phase, short fiber strengthening of LPSO lamellae/rods, and precipitation strengthening of nano-sized prismatic β′ precipitates and basal γ′ precipitates.

Effect of Magnesium Species on Cationic Flotation of Quartz from Hematite

ABSTRACT In the Quadrilátero Ferrífero Region, located in the state of Minas Gerais – Brazil, the utilization of recycled water from the tailing dams in the reverse cationic flotation of siliceous iron ores is common practice. In such process, ore concentrates assigning dolomite (2.5% to 10%) besides quartz in the mineral gangue, do not reach the desired specification for the production of blast furnace pellets (≤2% SiO2) given the loss of selectivity occurring in the separation of quartz from the Fe-bearing minerals, which can be related to a rise in the ions Ca and Mg in an aqueous medium resulting from the dissolution of dolomite. With a view to improving the reverse cationic flotation route for this type of ore, fundamental studies (microflotation trials, zeta potential, adsorption/Fourier-transform infrared spectroscopy) into the effect of the Ca and Mg ions in the cationic flotation with the use of pure quartz and hematite mineral samples were carried out. The results attained with the Ca ions were presented in previous studies by the authors. The current investigation presents the results achieved with the Mg ions, added in water as MgCl2. In the microflotation trials carried out at pH 10.5, amine dosage for maximum recovery of quartz (2.5 ppm) and hematite (50 ppm) was applied. A strong depression of hematite by starch was confirmed, which did not occur with quartz. In the case of the Mg ions, a stronger depression effect was observed, if compared to starch, for the two minerals, starting from a dosage of 10 ppm of MgCl2. After the conditioning of both minerals with 10 ppm of MgCl2, followed by conditioning with 10 ppm of starch, the recoveries attained were negligible (3.5% to 4.4%). Based on thermodynamic data, zeta potential measurements, and infrared spectroscopy, it was concluded that the strong depression effect caused by the Mg ions on both minerals owes to the adsorption of the Mg2+ and MgOH+ species and also the precipitation of Mg(OH)2, where the starch is chemically adsorbed, preventing further amine adsorption. Only quartz recovery was reestablished after the complexation of the Mg ions in the water by ethylenediaminetetraacetic acid having close molar concentrations to the molar concentration of the Mg ions followed by starch conditioning. The results presented indicate the need for a reduction in the concentration of Mg in the water used, before flotation is conducted.

Effect of Ce-base mischmetal addition on the microstructure and mechanical properties of hot-rolled ZK60 alloy

Mg-Zn-Zr (ZK) alloys exhibit notably high mechanical strength amongst all magnesium alloy grades. However, due to the formation of low melting point Mg 3 Zn 7 -precipitates, these alloys are susceptible to hot cracking, thus compromising their metallurgical processing. The addition of rare earths to ZK alloys is an alternative to form higher melting point intermetallic compounds, speed up dynamic recrystallization, refine grain size, enhance corrosion resistance and extend the service temperature due to improved creep resistance. This work deals with the effect of Ce-base mischmetal addition on the hot rolling behaviour of as-cast ZK60 alloy. The microstructure investigation conducted using electron microscopy and X-Ray diffraction shows that precipitation of Mg 7 Zn 3 intermetallics occur during hot rolling, whereas no further precipitation is observed for the ZK60-Mm alloys. The fragmentation of the intermetallic compounds occur during hot rolling and finer particles of Mg 7 Zn 3 are observed for the ZK60, whereas Mg 7 Zn 3 and MgZn 2 Ce intermetallics are formed in the alloy modified with mischmetal addition. A higher fraction of dynamically recrystallized grains is observed for the ZK60-Mm in comparison to the ZK60. Continuous recrystallization takes place in ZK60 with the formation of sub-grains near to the intermetallics and the addition of mischmetal promotes the occurrence of discontinuous recrystallization with the nucleation of new grains close to the precipitates. The mechanical strength and, in particular, the ductility of the hot-rolled alloys are notably improved when compared to the same alloys in the as-cast condition. The mechanical strength is, however, higher for the ZK60 alloy. Less solid solution strengthening, softer MgZn 2 Ce intermetallics and more extensive recrystallization contribute to reduce the mechanical strength of ZK60-Mm. Failure in both alloys are initiated at coarse intermetallics and propagate through intermetallic-rich regions.

Differential Precipitation of Mg(OH)2 from CaSO4·2H2O Using Citrate as Inhibitor-A Promising Concept for Reagent Recovery from MgSO4 Waste Streams.

In hydrometallurgical processing and acidic wastewater treatment, one of the neutralizing agents employed is MgO or Mg(OH)2. At the end of this process, the resulting solution, which is rich in SO42− and Mg2+ is treated with lime to remove (or minimize the amount) of these ions via the precipitation of Mg(OH)2 and CaSO4·2H2O (gypsum). In our work, an attempt was made to separate the two solids by increasing the induction time of the gypsum precipitation, thus regenerating relatively pure Mg(OH)2 which could be reused in wastewater treatments or hydrometallurgical processing circuits, and in this way, significantly enhancing the economic viability of the process. During our experiments, the reaction of an MgSO4 solution with milk of lime prepared from quicklime was studied. The effects of a range of organic additives, which can slow down the precipitation of gypsum have been assessed. The process was optimized for the most promising inhibiting agent—that is, the citrate ion. The reactions were continuously monitored in situ by conductometric measurements with parallel monitoring of solution pH and temperature. ICP-OES measurements were also carried out on samples taken from the reaction slurry. The composition of the precipitating solids at different reaction times was established by powder XRD and their morphology by SEM. Finally, experiments were carried out to locate the additive after the completion of the precipitation reaction to get information about its potential reuse.

Electrochemical and corrosion behaviors of the wrought Mg–Y–Zn based alloys with high Y/Zn mole ratios

The electrochemical behaviors and corrosion resistance of the wrought Mg–Y–Zn based alloys with high Y/Zn mole ratio have been investigated in details. The results show that the corrosion resistance of the investigated Mg–Y–Zn based alloys are dependent on the modified arrangement of LPSO phase by adjusting Y/Zn mole ratios. Increasing the Y/Zn mole ratio not only greatly decreases the size of LPSO phase plates, but also leads to the precipitation of Mg 24 Y 5 phase. The corrosion rate of Mg–Y–Zn based alloys greatly increases from 7.4 mg·cm −2 ·day −1 to 11.3 mg·cm −2 ·day −1 with increasing the Y/Zn mole ratio up to 3. It should be attributed to the decreasing size of LPSO phase plates as cathodes, further increasing the hydrogen evolution kinetics. The related corrosion mechanism is discussed in details.

Evolution of precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy with fine plate-like 14H-LPSO structures aged at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr (wt.%) alloy with fine plate-like 14H-LPSO structures aged at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Fine plate-like 14H-LPSO structures precipitate after heat treatment at 500 °C for 2 h, and β-type phases precipitate after the alloy is aged at 240 °C. The long-period atomic stacking sequence of 14H-LPSO structures along the [0001]α direction is ABABCACACACBABA. After being aged at 240 °C for 2 h, the β-type phases are the ordered solution clusters, zig-zag GP zones, and a small number of β′ phases. The peak hardness is obtained at 240 °C for 18 h with a Brinell hardness of 112, the β-type phases are β’ phases and local RE-rich structures. After being aged at 240 °C for 100 h, the β-type phases are β’, β1 and β’F phases. β′ phases nucleate from the zig-zag GP zones directly without β″ phases, and then transform into β1 phase by β’→β’F→β1 transformations. The Zn not only can form LPSO structure, but also is the constituent element of β1 phases. LPSO structures have a certain hindrance to the coarsening of β’ and β1 along 〈0001〉α.

Electrochemical removal of phosphate in the presence of calcium at low current density: Precipitation or adsorption？

Phosphorus removal and recovery from waste streams are crucial to prevent eutrophication and sustain fertilizer production. As has been shown in our previous papers, electrochemical treatment has the potential to achieve this goal. However, the adoption of electrochemical approach is limited by its high energy consumption. Here, we investigate the possibility of electrochemical phosphorus removal at extremely low current density using graphite felt as the cathode. We found a current density as low as 0.04 A/m2 can enhance the removal of phosphate in our electrochemical system. The removal of phosphate at extremely low current density resulted from electrochemical induced calcium phosphate precipitation and not by electrochemical adsorption. Electrochemical treatment of real domestic wastewater at 0.2 A/m2 almost eliminates the precipitation of Mg(OH)2 and limits the formation of CaCO3. The recovered precipitates are dominated by calcium phosphate (59%), followed by 35% CaCO3 and 6% Mg(OH)2. The specific energy consumption of this newly electrochemical system is between 4.4 and 26.4 kW h/kg P, which is 2 orders of magnitude lower than our previous system (110–2238 kW h/kg P). Key factors for this improvement prove to be enlarged precipitation area and hydroxide flux retardation by graphite felt. Practically, our study offers a potential way to reduce the energy consumption in electrochemical removal of phosphate by using a graphite felt cathode and at a current density below 0.2 A/m2. Fundamentally, our study contributes to the understanding of adsorption and precipitation in electrochemical removal of phosphate at an extremely low current density and with carbon-based electrodes.

Precipitate evolution in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C

The morphology and crystal structure of the precipitates in Mg−7Gd−3Y−1Nd−1Zn−0.5Zr alloy during isothermal ageing at 240 °C were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). After under-ageing for 2 h, the precipitates in the alloy are ordered solute clusters with rare earth atomic columns exhibiting hexagonal ring structure, zigzag GP zones and β' in its early formation. After peak-ageing for 18 h, the precipitates are mainly β' and new rod-like β'p accompanied with β'. After over-ageing for 100 h, the precipitates are β', β1, long-period stacking ordered (LPSO) building block known as γ′ and 14H-LPSO. β' has the three-dimensional shape of convex lens with smaller length-to-width ratio viewed along 〈0001〉α than that in the EW75 alloy. The excellent thermal stability of this alloy can be attributed to the γ' and 14H-LPSO retarding the growth of β' and β1, low diffusion rate of rare earth atoms and physical character of β' and β1.

Enhanced age-hardening response in Mg–Zn–Co alloys with Bi additions

Effects of Bi additions on age-hardening response and precipitation of Mg–2.31Zn–0.31Co (at.%) alloy have been investigated. The addition of Bi can substantially promote the age-hardening response at 200 °C. The precipitates microstructure is characterized using high-angle annular dark-field scanning transmission electron microscope, and the results show that the remarkable improvement is attributed to a much higher number density of β1' rods and the formation of prismatic Mg3Bi2 plates. At the early stage of ageing, Mg3Bi2 precipitates out first and then Zn segregates to the Mg3Bi2/Mg interface, which could provide chemical environment and heterogeneous nucleation sites for the subsequent precipitation of the β1' phase.

Characterisation of structural similarities of precipitates in Mg–Zn and Al–Zn–Mg alloys systems

ABSTRACTHigh angle annular dark field scanning transmission electron microscopy has been employed to observe precipitate structures in Al–Zn–Mg and Mg–Zn alloys. precipitate structures in Al–Zn–Mg are commonly formed by MgZn2 Penrose bricks, but also frequently observed to incorporate Mg6Zn7 elongated hexagons via two different modes. Tilings of MgZn2 and Mg6Zn7 building blocks in both in Mg–Zn and in Al–Zn–Mg alloys, create overall patterns which deviate from the chemical and structural configuration of solely monoclinic Mg4Zn7 or MgZn2 unit cells. Precipitate morphologies were found to be correlated to their internal sub-unit cell arrangements, especially to Mg6Zn7 elongated hexagons. Systematic or random arrangements of Mg6Zn7 elongated hexagons inside precipitates and therefore compositional and structural patterns, were found to be strongly related to the aspect ratio of the precipitates and altering of the precipitate/matrix interfaces.

Functional Elements Coatings on the Plasma Electrolytic Oxidation-Treated Ti-6Al-4V Alloy by Electrochemical Precipitation Method.

In this study, functional elements coatings on the plasma electrolytic oxidation (PEO)-treated Ti-6Al-4V alloy by electrochemical precipitation method were investigated. Ti-6Al-4V ELI disks were used as specimens for PEO and HA coating. The applied voltage and time were selected to be 280 V and 3 minutes for PEO treatment, respectively. The electrochemical precipitation on the Ti-6Al-4V alloy was carried out using cyclic voltammetry with cycle of 3, 5, 10, and 20 from -1.5 V to 0 V (vs. SCE electrode) in electrolyte containing Ca, P, Mg, Mn, Sr, Zn, and Si ions by cyclic voltammetry after PEO treatment. The morphology changes of the coatings on the PEO treated Ti-6Al-4V alloy surface were observed using FE-SEM and EDS. PEO surface has a uniformly distributed circular shape and a porous surface and the deposition of low cycles in electrolyte containing Mg, Mn, Sr, Zn, and Si-HA coated surfaces show uniform circular and granular structures. The precipitates of Mg, Mn, Sr, Zn, and Si-HA on the PEO treated surface showed a large number of circular particles as the number of deposition cycles increases with a mixture of rod-shaped particles and petal-shaped particles around pores. The precipitate nucleates around the pore and grows rapidly as the cycles increase.

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

The Neoproterozoic sequence of volcanic rocks in the Camaquã Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment.