Pura Alfonso

Kinetics of dolomite-portlandite reaction: application to portland cement concrete

The dedolomitization reaction kinetics are studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with different alkalinity, temperature and silica content. The experimental results are reproduced through computer simulation, which allows the estimation of the apparent dissolution constant rates for dolomite. These are discussed together with other parameters influencing the kinetics, in particular the modification of the specific surface of dolomite. The parameters obtained make it possible to predict the behavior of the system beyond experimental periods. Both experimental and simulated results are discussed in connection with the expansion and cracking occurrences observed in portland mortars and concretes made with dolomitic limestone.

Dedolomitization in different alkaline media: Application to Portland cement paste

Abstract The dedolomitization reaction kinetics is studied through several long-term experiments consisting of an aqueous dispersion of fine powders of dolomite and portlandite with alkalinity between 0.1 and 1 M KOH, at 25 and 75 °C. The experimental results are numerically simulated to calculate the apparent dissolution constant rates for dolomite, k dol . At low temperature, two dissolution stages were observed. In an early stage, part of dolomite powder dissolves quickly until an apparent steady is reached. After several days, the reaction continues at a lower rate. The calculated dissolution rate for dolomite in the first stage is one order of magnitude higher than that of the second stage. At 75 °C, the k dol is two orders of magnitude higher than at 25 °C. The addition of alkali increases the k dol at high temperature, but reduces it at room temperature.

Geology, ore facies and sulphur isotopes of the Koushk vent-proximal sedimentary-exhalative deposit, Posht-e-Badam Block, Central Iran

The Koushk zinc–lead deposit in the central part of the Zarigan–Chahmir basin, central Iran, is the largest of several sedimentary–exhalative (SEDEX) deposits in this basin, including the Chahmir, Zarigan, and Darreh-Dehu deposits. The host-rock sequence consists of carbonaceous, fine-grained black siltstone with interlayered rhyolitic tuffs. It corresponds to the upper part of the Lower Cambrian volcano-sedimentary sequence that was deposited on the Posht-e-Badam Block due to back-arc rifting of the continental margin of the Central Iranian Microcontinent. This block includes the late Neoproterozoic metamorphic basement of the Iran plate, overlain by rocks dating from the Early Cambrian to the Mesozoic. Based on ore body structure, mineralogy, and ore fabric, we recognize four different ore facies in the Koushk deposit: (1) a stockwork/feeder zone, consisting of a discordant mineralization of sulphides forming a stockwork of sulphide-bearing dolomite (quartz) veins cutting the footwall sedimentary rocks; (2) a massive ore/vent complex, consisting of massive replacement pyrite, galena, and sphalerite with minor arsenopyrite and chalcopyrite; (3) bedded ore, with laminated to disseminated pyrite, sphalerite, and galena; and (4) a distal facies, with minor disseminated and laminated pyrite, banded cherts, and disseminated barite. Carbonatization and sericitization are the main wall-rock alterations; alteration intensity increases towards the feeder zone. The δ34S composition of pyrite, sphalerite, and galena ranges from +6.5 to +36.7‰. The highest δ34S values correspond to bedded ore (+23.8 to +36.7‰) and the lowest to massive ore (+6.5 to + 17.8‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulphides formed from the reduction of seawater sulphate by bacteriogenic sulphate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulphate reduction likely played an important role in the feeder zone. Sulphur isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal SEDEX ore deposit.

Surface alteration of dolomite in dedolomitization reaction in alkaline media

Abstract The phases precipitated on the dolomite surface and the influence of alkalis on the properties of this surface were characterized. Experiments consisted of the immersion of single crystals of dolomite in saturated portlandite solutions with different alkalinity, temperature, and silica content. Most calcite forms in the solution as {104} rhombohedra. Brucite crystallizes as platelets, piles of platelets, sponge-like and {001} truncated ditrigonal pyramids attached to the dolomite surface. The morphology of brucite crystals depends on the pH of the solution. Apparent dissolution rate constants for dolomite at 75 °C are one order of magnitude higher than at room temperature. Initially, calcite grows on the dolomite surface with the same structural orientation {104}. In a second stage, when calcite has reached equilibrium with the solution, the new-formed {104} calcite surface starts playing an important role on the precipitation of brucite, while calcite continues crystallizing in the bulk solution. The calcite surface, negatively charged, can adsorb specifically metal cations. Mg2+, liberated by the dissolution of dolomite, would move attached to the surface, until they are trapped on these negatively charged surfaces, while OH− supplied by the alkaline solution favors the nucleation and growth of brucite. The attachment of Mg2+ to the surface controls the place where brucite grows.

D, O and C isotopes in podiform chromitites as fluid tracers for hydrothermal alteration processes of the Mayari-Baracoa Ophiolitic Belt, eastern Cuba

The Mayari — Baracoa Ophiolitic Belt (MBOB, eastern Cuba) is composed of two large, chromite-rich massifs: Mayari —Cristaland Moa-Baracoa. The chromitites and hosting dunites were firstly affected by a regional serpentinization event, a subsequent episode of hydrothennal alteration (chloritization mainly) and, finally, these already altered bodies were crosscut by thin calcite-dominated veins. Analysed serpentines from serpentinized chromitites and dunites present very similar isotopic compositions (δ18O=+4.7 to +6.3 and δD= −67 to −60 , suggesting that the serpentinization process took place at moderate temperatures, in an oceanic environment. Serpentine formation by interaction with ocean water is also supported by the isotopic composition of chlorite and calcite. These results suggest that the serpentinization, chloritization and fracture filling processes of the Mayari — Baracoa Ophiolite Belt took place in a subocean floor scenario and, thus, that the Mayari — Baracoa serpentines represent a good example of serpentine formed during interaction with seawater. The oceanic origin of the serpentines from serpentinized chromitites and dunites from the MBOB indicate that the serpentinization of the mantle sequence occurred pre-thrusting (pre-emplacement in age).

The Catanda extrusive carbonatites (Kwanza Sul, Angola): an example of explosive carbonatitic volcanism

Carbonatite lavas and pyroclastic rocks are exposed in the volcanic graben of Catanda and represent the only known example of extrusive carbonatites in Angola. A new detailed geological map of the area is presented in this study as well as six different stratigraphic sections. Pyroclastic rocks, apparently unwelded, are dominant in the area and represented in all the stratigraphic columns. They form shallowly to moderately inclined layers, mostly devoid of internal structures, that range in thickness from several centimetres to metres. They are dominantly lapilli tuffs and minor tuffs occasionally comprising pelletal lapilli. Based on their different features and field relationships, at least five different pyroclastic lithofacies have been distinguished in the area. Carbonatitic lavas outcrop in the external parts of the Catanda graben, forming coherent layers interbedded with pyroclastic rocks. Calcite is the most common mineral in the lavas, but other accessory minerals such as fluorapatite, titaniferous magnetite, phlogopite, pyrochlore, baddeleyite, monticellite, perovskite, cuspidine and periclase have also been identified. At least four different types of lavas have been distinguished based on their mineral associations and textural features. This study reveals an overall abundance of pyroclastic material in comparison to lava flows in the Catanda area, suggesting that eruptive processes were dominated by explosive activity similar to what has been described in other carbonatite and kimberlite localities. The Catanda carbonatitic volcanism was associated with monogenetic volcanic edifices with tuff ring or maar morphologies, and at least seven possible eruptive centres have been identified in the area.

Sulfur isotope geochemistry of the submarine hydrothermalcoastal vents of Punta Mita, Mexico

Abstract Coastal hydrothermal submarine vents occur near Punta Mita (Mexico) through a seafloor fissure hosted by basaltic rocks and partially covered by recent sediments. Hydrothermal venting produces calcareous tufa mounds, with minor sulfides, barite and apatite. Pyrite is the most abundant sulfide mineral that precipitates around the submarine hot springs. It occurs as framboids, euhedral crystals, thin botryoidal coatings interbedded with calcite, and as replacement of detrital magnetite grains. Vent fluids have temperature below 100 °C, and methane is abundant. Sulfur isotopic composition of pyrite predominantly has δ 34 S values from − 10.7 to +4.9 suggesting that the source of sulfur is microbially reduced sulfate seawater.

The role of cassiterite controlling arsenic mobility in an abandoned stanniferous tailings impoundment at Llallagua, Bolivia.

The surface water contamination by potentially toxic elements (PTE) leached from mine tailings is a major environmental concern. However, the formation of insoluble solid phases can control the mobility of PTE, with subsequent decrease of the risk that tailings suppose to the environment. We characterized the tailings from a tin inactive mine in Llallagua, Bolivia in order to assess the risk for surface water quality. These tailings contain high concentrations of PTE, with up to 94,344 mg/kg Fe, 9,135 mg/kg Sn, 4,606 mg/kg As, 1,362 mg/kg Cu, 1,220 mg/kg Zn, 955 mg/kg Pb and 151 mg/kg Cd. Oxidation of sulfide minerals in these tailings generates acid leachates (pH=2.5-3.5), rich in SO4(2-) and dissolved PTE, thereby releasing contaminants to the surface waters. Nevertheless, the concentrations of dissolved Sn, As and Pb in acid leachates are low (Sn<0.01 mg/L; As=0.25-2.55 mg/L; Pb<0.05 mg/L). This indicates that, for the most part, Sn, As and Pb are being retained by the solid phases in the impoundment, so that these elements are not reaching the surface waters. Fe-bearing cassiterite-an insoluble and weathering-resistant oxide mineral-is abundant in the studied tailing deposits; it should be the main solid phase controlling Sn and As mobility in the impoundment. Additionally, jarosite and plumbojarosite, identified among the secondary minerals, could also play an important role controlling the mobility of As and Pb. Taking into account (a) the low solubility constants of cassiterite (Ksp=10(-64.2)), jarosite (Ksp=10(-11)) and plumbojarosite (Ksp=10(-28.66)), and (b) the stability of these minerals under acidic conditions, we can conclude that they control the long-term fate of Sn, As and Pb in the studied tailings.

Kaolin from Acoculco (Puebla, Mexico) as raw material: Mineralogical and thermal characterization

Abstract The present study determined the mineralogy and thermal properties of kaolin from Acoculco (Puebla), at the eastern Trans-Mexican Volcanic Belt and compared it with the nearby deposits of Agua Blanca (Hidalgo) and Huayacocotla (Veracruz). The mineralogy of the kaolins was determined by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. Thermal behaviour was studied by differential thermal analysis, dilatometry and hot-stage microscopy. The Acoculco deposit is composed mainly of kaolinite and SiO2 minerals. In the case of Agua Blanca and Huayacocotla, alunite is abundant in places and minor anatase is also present locally. The Acoculco kaolins are Fe-poor and relatively rich in some potentially toxic elements (Zr, Sb, Pb). They undergo a relatively small amount of shrinkage (~3-4 vol.%), during firing at 20-1300°C and cooling down to 20°C, except when >10 wt.% alunite is present. These kaolins are a suitable raw material for the ceramics industry. Other applications (pharmaceuticals, cosmetics) would require an enrichment process to eliminate impurities such as Fe oxides.

Source of ore-forming fluids in El Cobre VHMSdeposit (Cuba): evidence from fluid inclusions and sulfur isotopes

Abstract The El Cobre deposit, east of Cuba, lies in the intermediate volcanosedimentary sequence of the Sierra Maestra intraoceanic island arc. The structure of the deposit corresponds to that of a volcanogenic-hosted massive sulfide (VHMS) model. It comprises (a) thick stratiform bodies (baryte and anhydrite), (b) three stratabound bodies (formed by silicification and sulfidation of limestones or sulfate strata), (c) stockwork zones, an older anhydrite stockwork and a younger quartz-pyrite stockwork grading downwards to (d) simple veins (quartz with sulfide ores). Pyrite, chalcopyrite and sphalerite are the most abundant sulfides. Fluid inclusions from this deposit have a salinity between 2.3 and 5.7 wt.% NaCl eq., homogenization temperatures range between 177 and 300 °C. Sulfur exhibits a range of δ 14 S values from − 1.4 to +7.3 for sulfides and from + 16 to +21 for sulfates. Fluid inclusions and sulfur isotope data at El Cobre deposit indicate that the hydrothermal fluid from which the sulfide precipitated was seawater, modified by reaction with volcanic host rocks during hydrothermal circulation.