Quartz Dissolution Research Articles

The process of vanadium migration and silicon extraction by alkaline leaching from high silicon stone coal under reduction conditions

It is difficult to utilize stone coal with a vanadium grade of below 0.7%. With the aim of effectively utilizing the silicon and vanadium in stone coal, the vanadium concentration and silicon extraction processes by alkaline leaching under reducing conditions were investigated. The effect of NaOH mass fraction, temperature and the amount of hydrazine hydrate on the leaching process was researched. Moreover, the laws of phase transformation and the migration process of vanadium were revealed. The results show that the dissolution of quartz resulted in the exposure of muscovite and goldmanite, but the phase of muscovite and goldmanite was not damaged. Under the optimal operation conditions (25% mass fraction of NaOH, liquid-solid ratio 2:1 at 200 °C for 4 hours), 90.14% of silicon was removed. At the same time, the modulus of sodium silicate product reached 1.88 and the content of V2O5 enriched from 0.5% to 2.51%. In addition, V(IV) was partially reduced to V(III) by adding hydrazine hydrate in an amount of 10% by mass. V(IV) migrated from the liquid phase to the alkali leaching slag, and the recovery of vanadium increased from 89.48% to 94.43%. All the obtained results suggest an efficient process for separation and extraction of vanadium and silicon.

Creep Strain and Permeability Evolution in Cracked Granite Subjected to Triaxial Stress and Reactive Flow

Fluid flow and fluid-rock interaction mainly take place in fracture network, consequently resulting in deformation and permeability variation of rock and deterioration of the wellbore performance. Mechanical-reactive flow coupling creep tests are performed on cracked granite under various confining pressures and acid and alkaline solution flows. The testing results show that the confining pressure and solution pH significantly influence the creep deformation, creep strain rate, and permeability. A primary creep stage and secondary creep stage are observed in all creep tests in this study; notably, the sample under a confining pressure of 10 MPa and acid solution injection undergoes creep failure for over 2700 hours. The acid solution has a more obvious influence on the creep behavior than that of the alkaline solution. With an increase in confining pressure, the total creep strain and creep strain rate in the samples gradually decrease during the injection of either solution. The permeability of the samples injected with either solution gradually deceases during the testing process, and this deceasing rate increases with the confining pressure. The scanning electron microscopy observations on the crack surfaces after the creep tests show that the surfaces of the fractures injected with the acid solution are smooth due to the dissolution of the matrix, while those injected with the alkaline solution include voids due to the dissolution of quartz. These experimental results could improve the understanding of the long-term transport and mechanical behaviors of wellbore.

Dissolution pore types of the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, south China: Implications for shale gas enrichment

Abstract In shale gas reservoirs, scientific consensus indicates that the main pore types are organic matter pores, mineral pores and micro-fractures. The organic matter pores are crucial to porosity, and in many cases they form the main pore type favorable for shale gas enrichment. Dissolution pores can influence shale gas enrichment and preservation. Although the various types of dissolution pores have been reported, few studies have identified the characteristics of mineral dissolution pores, let alone the development of dissolution pores and their effect on shale gas enrichment. Based on observations from shale gas wells in the Sichuan Basin, this paper studied the types of dissolution pores in the Wufeng Formation and the Longmaxi Formation. The study shows that the types of dissolution pores are carbonate dissolution pores, feldspar dissolution pores, and quartz dissolution pores and that carbonate dissolution pores are the main type. The formation time of the dissolution pores can be identified through analyzing the time of the mineral generation and the time at which the solid bitumen and/or minerals filled in the dissolution pores. We found that the dissolution pores in the secondary enlargement of minerals formed later than those in the original minerals. The dissolution pores without solid bitumen may have formed later than the dissolution pores with solid bitumen filling. Given that the dissolution pores had formed before the peak stage of oil generation, the dissolution pores should be well connected through the dissolved trench, which would largely improve the reservoir volume for hydrocarbon storage and matrix permeability. If there is no solid bitumen filling in the dissolution pores and the formation of the dissolution pores began after the crude oil was charged but before the peak stage of gas generation, these conditions will have some destructive effects on shale gas enrichment. In the worst-case scenario, if the dissolution pores formed after the peak stage of gas generation, the development of the pores will likely destroy the shale gas enrichment and can convert reservoir pressure to a normal pressure. The new findings in this paper reveal that the formation time of the dissolution pores has a controlling effect on shale gas enrichment in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin; these findings also provide valuable guidance for future research on shale gas reservoirs around the world.

Interação entre magmas graníticos e a origem de epissienitos potássicos estaníferos do granito Madeira, mina Pitinga, Amazonas

O estudo petrográfico de amostras da zona de contato entre as fácies albita-granito de borda e feldspato alcalino-granito hipersolvus porfirítico, na borda oeste do plúton Madeira, identificou rochas com características texturais e composicionais herdadas das duas fácies. Tal feição é interpretada como um processo de interação entre os dois magmas graníticos, colocados e cristalizados contemporaneamente. Em condições subsolidus, fluidos hidrotermais causaram alteração potássica pervasiva, ou epissienitização, caracterizada pela dissolução de quartzo das fácies graníticas, gerando cavidades e aumentando a permeabilidade das rochas alteradas. Paralelamente, a albita da fácies de borda do albita-granito foi desestabilizada e substituída por microclínio hidrotermal e, nos estágios mais tardios, as cavidades foram preenchidas por quartzo, microclínio, fluorita, hematita, cassiterita, fengita, clorita, pirita, esfalerita, galena e calcopirita. Os epissienitos potássicos são rochas de coloração vermelha escura a marrom, granulação fina a média, ricas em microclínio hidrotermal, apresentando-se ora porosas, ora com cavidades preenchidas por quartzo e sulfetos. A cassiterita das rochas epissienitizadas é mais pura e relativamente mais pobre em Nb do que aquela presente no albita-granito magmático. Os fluidos que desencadearam a epissienitização devem ter sido oxidados, ricos em K, alcalinos e subsaturados em sílica. Propõem-se duas hipóteses para a sua origem: 1) por separação de uma fase fluida a partir do líquido magmático formador do granito hipersolvus; 2) pela ação do mesmo fluido que desencadeou o processo de autometassomatismo do albita-granito de núcleo, gerando sua fácies de borda.

Dissolution and Thermal Spallation of Barre Granite Using Pure Water Hydrothermal Jets

Spallation induced by rapid hydrothermal heating was investigated as a possible method of drilling rock. In this study, an electrically-heated hydrothermal jet was impinged on the surface of cylindrical Barre Granite specimens (basement rock) contained in an autoclave reactor to induce localized thermal stress. Comminution of the rock surfaces was achieved at supercritical water conditions, temperatures from 535 to 580 °C and pressures of 22.5–27 MPa. These conditions simulate those encountered in drilling deep, water-filled wells at depths greater than about 2300 m. Preferential removal of quartz grains from the rock matrix was observed. This comminution cannot be attributed to erosion by either the jet’s momentum, or by differential pressure forces. Additionally, silica removal (primarily from quartz grains) was observed at rates greater than those that could be attributed to dissolution of quartz alone. This implied a secondary comminution mechanism associated with spallation caused by the local thermal stresses from the impinging hydrothermal jet. The experimentally determined heat flux and surface temperature measurements indicated that hydrothermal drilling occurred below the empirically determined minima for the onset of continuous thermal spallation reported in earlier studies for low-density, high-velocity and high-temperature flame jet drilling at temperatures in excess of 1000 °C jets impinging on rock surfaces at near-atmospheric pressures.

Quartz Solubility in the Vapor Phase of the H2O–HF System at 200°С and Saturated Vapor Pressure: Experimental Data

Abstract—This work was carried out to elucidate the silica transport in the presence of fluorine at the low-temperature hydrothermal stage. The liquid-vapor distribution of fluorine in the HF–H2O system under saturated vapor pressure at 150 and 200°С as well as the dissolution of quartz in the vapor phase of the same system at 200°С and under saturated vapor pressure were studied. It is established that fluorine at 150–200°С is mainly partitioned in a a liquid phase, while its liquid/vapor distribution coefficient (Kd) is lower at 200°С than at 150°С in the region of high mHF concentrations. Quartz solubility in the vapor phase HF–H2O as compared to the pure water vapor at 0.1 mHF increases by more than four orders of magnitude and approaches quartz solubility in liquid water.

Tiny particles building huge ore deposits – Particle-based crystallisation in banded iron formation-hosted iron ore deposits (Hamersley Province, Australia)

The world’s major source of iron ore is hosted in Precambrian banded iron formations. These chemical (meta-) sedimentary rocks are composed of alternating laminae of iron oxide minerals and chert. Despite the economic significance of high-grade iron ore deposits, controversy persists after decades of research on how banded iron formations became upgraded to form iron ore. The fundamental requirement for iron ore formation is the removal of vast amounts of chert coupled with an increased concentration of iron oxide minerals.Here, we assess the fate of colloidal hematite inclusions encapsulated in chert after quartz dissolution and examine their role in the formation of hematite ore. We have analysed hematite ores from the Hamersley Province (Australia) using a combination of petrography, high-resolution electron microscopy and X-ray diffraction. These techniques reveal the presence of abundant nano and microscale hematite particles that we suggest form the building blocks of larger hematite crystals within the iron ore. Our textural observations indicate that hematite colloids that were previously encapsulated inside the microcrystalline quartz grains in chert layers are released during quartz dissolution, and subsequently reassemble in a self-similar fashion to from new microplaty hematite crystals via non-classical crystallisation pathways. Progressive growth and fusion leads to the transformation of hematite microplates into hematite bands, which resemble pre-existing iron oxide laminae of banded iron formations. In contrast to previous models, we observe the direct transformation of banded iron formations to microplaty hematite and have not found evidence that significant amounts of hematite formed through metamorphism of goethite or that intermediate carbonate minerals were involved during the upgrading of banded iron formations to pure hematite ore.Given the strong evidence for hypogene fluids found in many deposits, we have also assessed the role that such warm, highly saline fluids may have played during the evolution of iron ore. Using insights from crystal chemistry we conclude that fluid infiltration impacts many aspects of iron ore formation by controlling hematite colloid liberation and aggregation, and finally controlling the transformation of the colloids into macroscopic crystals of hematite via non-classical crystallisation mechanisms. Our study underlines the significance of hypogene fluids in the upgrading of banded iron formation to iron ore, however, we suggest that their influence was mainly passive as hematite was not precipitated directly from these fluids.

Si cycling and isotope fractionation: Implications on weathering and soil formation processes in a typical subtropical area

The translocation of silicon (Si) is closely related to soil evolution. However, how to trace the transformation and migration paths of Si and further understand its effects on soil formation and evolution remains as a challenge in geochemistry and soil science. Here we studied the Si isotope (δ30Si) values and physical, chemical and mineralogical properties of rock, soil, plant and water in representative small watersheds in the south of Anhui province, subtropical China. The aims are to illustrate weathering and soil formation processes by tracing the fractionation of Si isotope among those ecosystem components as well as soil components and plant organs. Results show that the δ30Si values of bulk soil and clay are significantly related to many soil development indicators, such as total, free and amorphous Al oxides, as well as active Fe oxides, clay, silt and sand contents, Al/Si molar ratio and chemical index of alteration (CIA). These indicators evidence the relationships between Si isotope changes and mineral weathering and soil development degree. Biological resilication (accumulation of Si) by plant does not lower soil δ30Si because further fractionation occurs among plant organs, and soil phytoliths have significantly higher δ30Si than soil clay and silt particles. However, soil desilication (loss of Si) lowers soil δ30Si because much 30Si is transported to streams in runoff. The consistently positive δ30Si values of water and their relationships with Si concentrations and discharge suggest that dissolved Si (DSi) in stream water mainly comes from weathering of primary minerals; the contribution of dissolution of secondary minerals, quartz and phytolith is minor. Differential δ30Si values in clay, silt, sand, rock and water are the result of soil formation processes which directly evidence a dominate neoformation pathway of secondary clay minerals in this area. This study can help to further understand mechanisms and processes of Si translocation during soil evolution and, extend the application of Si translocation in soil genesis studies.

Adenine and Thymine Effect on Quartz Dissolution at Different Artificial Seawaters

According to Bernal (1951) minerals may have played important roles in the origin of life such as: protect biomolecules against degradation by UV radiation or hydrolysis, pre-concentrate biomolecules from prebiotic seas and formation of polymers. Today quartz is widely distributed on Earth as well as was widely distributed on the prebiotic Earth. There are several works showing the quartz dissolution, this work shows for the first time, quartz dissolution under prebiotic chemistry conditions. The experiments were carried out with and without adenine and thymine dissolved in several artificial seawaters, distilled water and alkaline distilled water (pH = 11.0). It should be noted that seawater 4.0 Ga resembles the major cations and anions of the sea of the prebiotic Earth. The quantification of dissolved quartz in different seawater, the measurement of pH and of conductivity were performed daily for a 35 day period, at a controlled temperature (30 °C). The dissolution was dependent of water salinity and especially of pH. Depending on the artificial seawater, adenine and thymine can either accelerate the quartz dissolution or protect it. DOI: http://dx.doi.org/10.17807/orbital.v10i6.1152

Aseismic Refinement of Orogenic Gold Systems

Abstract Orogenic Au deposits have contributed the majority of Au recovered globally throughout history. However, the mechanism that concentrates Au to extremely high bonanza grades in small domains within these deposits remains enigmatic. The volume of fluid required to provide extreme Au endowments in localized occurrences is not reflected in field observations (e.g., in the extent of quartz veining or hydrothermal alteration). Detailed optical, scanning and transmission electron microscopy, nanoscale secondary ion mass spectrometry, and 3-D neutron tomography have been used to investigate the processes responsible for development of anomalously high grade ore (upward of 3% Au) found in quartz veins at Fosterville gold mine (Victoria, Australia). Distinct textural settings of visible Au include (1) Au concentrated along pressure solution seams associated with wall-rock selvages, (2) as nano- to microscale dusty Au seams parallel to pressure solution seams, and (3) in microscale tension fractures perpendicular to stylolitic seams. The distribution of Au in arsenopyrite and pyrite hosted within pressure solution seams changes as a function of the extent of deformation. Sulfides in highly deformed pressure solution seams exclusively host Au as nano- to micrometer-sized clusters within features associated with corrosion and brittle failure, whereas sulfides in mildly deformed pressure solution seams have Au bound in the crystal structure. It is proposed that Au supersaturation in fluids introduced during seismic periods led to the deposition of abundant Au nanoparticles in quartz-carbonate veins. Subsequent pressure dissolution of vein quartz and carbonate during interseismic intervals allowed for episodic increase in the Au/quartz ratio and permitted liberation and migration of Au nanoparticles, promoting Au grain growth in favorable textural settings. Galvanic corrosion and brittle fracturing of auriferous sulfides during the interseismic period allowed additional remobilization and/or enrichment of sulfide-hosted Au. Repetition of this mechanism over the time scale of deposit formation acted to concentrate Au within the lodes. This Au ore upgrading model, referred to as “aseismic refinement,” provides a new insight for the genesis of ultrarich Au mineralization and, based on textures reported from many Au deposits, may be a globally significant component in the formation of orogenic Au deposits.

The application of quartz grain morphology measurements to studying iron-rich duricrusts

Iron-rich duricrust profiles are commonly located on old flat surfaces. They promote landscape stability by the protective action of ferricrete, providing important records of multiple soil and geomorphic processes, as well as paleoclimatic events. In the Southern Serra do Espinhaço (SSE) in Brazil, iron-rich duricrust profiles occur on flat to slightly convex hills of quartzite at elevations of mostly 1200–1400 m a.s.l. Some of these profiles may be remnants of old paleosurfaces. Here we describe an image analysis technique to measure morphological attributes of quartz grains in thin sections of iron-rich duricrusts; we present a way to investigate the genesis of these materials. Size and shape variations of quartz grains were measured for iron-rich duricrust profiles from four different locations (GS2, PF1, PF2 and PF3). Thin sections of 5 × 7.5 cm from surface horizons and iron-rich duricrusts were studied. Distinct morphological regions (iron concretion and matrix) of iron-rich duricrusts were separately analyzed to identify variations in regolith morphology. A TESCAN VEGA3 scanning electron microscope (SEM) was used to produce backscattered electron (BSE) images for regions of sufficient area to enable the use of image analysis software (ImageJ 1.50i). BSE images provide a clear contrast between quartz grains and iron-rich duricrust matrix allowing resolution of quartz grains for specific investigation. The size, circularity and roundness of many quartz grains were easily determined. This procedure enables the quantitative analysis of soil materials allowing comparisons of soils and the testing of hypotheses for soil genesis. Quartz grains within iron concretions of PF3 had different morphological properties from quartz in the surrounding ferricrete indicating that different processes had occurred during the genesis of the two materials. In contrast, PF1 which showed no evidence of parent material diversity, contained quartz grains that are indicative of a polycyclic character. We observed fragments of iron concretions with pores previously occupied by quartz grains indicating that extensive quartz dissolution had occurred during duricrust evolution.

Soluble and colloidal translocation of Al, Fe, Si and Mn in an artificially drained French Retisol

Soil evolution results from both solute and colloid/particle translocation, but their relative importance in terms of soil formation is still under debate. The genesis of albic material in Retisols represents an ideal case to quantify the relative contribution of these two forms of transport in soil evolution since it results from i) clay eluviation, ii) reductive solubilisation of Fe oxides, and possibly iii) silicate hydrolysis. This issue seems particularly challenging in cultivated Retisols in which detailed analysis of the soil solid phases has demonstrated that artificial drainage and liming alter the genetic pathway of albic material both qualitatively and quantitatively.In the present study, we characterised the chemistry (pH, Eh, Al, Si, Fe and Mn) and dynamics of soil water and its evolution with increasing distance from the drain. Rainfall water, soil solution at water saturation in the E&Bt horizon and drainage water were sampled by an open rainfall collector, piezometers located at different distances from the drain, and an automatic water collector located at the main drain outlet, respectively.An element budget at the profile level revealed that only Si was lost from the soil profile while the leaching losses of Al, Fe and Mn were overcompensated by deposition from the atmosphere. The constant Fe/Al concentration ratios in piezometers and drain waters as well as the breakthrough curves at the drain outlet for individual rainfall events suggested that Fe and Al were exported together and mainly in colloidal form. While this was also partly true for Si, a longer tailing of the Si than the Fe and Al breakthrough curves indicated that Si was additionally transported in soluble form. Lastly, the temporal variation of the Mn concentrations was unrelated to that of the other elements, pointing to the predominance of translocation in soluble form. All these results demonstrated that: i) particle/colloidal translocation is more important than translocation in soluble form in a drained Retisol; ii) reductive solubilisation of Fe and Mn oxides still occurred, in spite of the artificial drainage; iii) processes such as wet and dry deposition, dissolution of quartz or ferrolysis also contribute to element budgets in Retisols, at least at some microsites and in some years.

Kinetics of dissolution of sapphire in melts in the CaO–Al2O3–SiO2 system

The dissolution rate of sapphire in melts in the CAS system of varying silica activity, viscosity and degree of alumina saturation has been determined at 1600 °C and 1.5 GPa. After an initiation period of up to 1800 s, dissolution is controlled by diffusion of cations through the boundary layer adjacent to the dissolving sapphire. The dissolution rate decreases with increasing silica activity, viscosity and molar Al2O3/CaO. The calculated diffusion matrix for each solvent melt shows that CAS 1 and 9 which have molar Al2O3/CaO of 0.33 and 0.6 and dissolution rate constants of 0.65 × 10−6 and 0.59 × 10−6 m/s0.5 have similar directions and magnitudes of diffusive coupling: DCaO-Al2O3 and DAl2O3-CaO are both negative are approximately equal. The solvent with the fastest dissolution rate: CAS 4, which has a rate constant of 1.5 × 10−6 m/s0.5 and Al2O3/CaO of 0.31 has positive DCaO-Al2O3 and negative DAl2O3-CaO and the absolute values vary by a factor of 4. Although many studies show that aluminium is added to the melts via the reaction:Si4+=Al3++0.5Ca2+the compositional profiles show that this reaction is not the only one involved in accommodating the aluminium added during sapphire dissolution. Rather, aluminium is incorporated as both tetrahedrally coordinated Al charge balanced by Ca and as aluminium not charge balanced by Ca (termed Alxs). This reaction:AlIV–Ca=Alxs+CaNBOwhere CaNBO is a non-bridging oxygen associated with calcium, may involve the formation of aluminium triclusters. The shape of the compositional profiles and oxide-oxide composition paths is controlled by the aluminium addition reaction. When Alxs exceeds 2%, CaO diffusion becomes increasingly anomalous and since the bond strength of Alxs correlates with CaO/CaO + Al2O3, the presence of more than 2% Alxs leads to significantly slower dissolution than when Alxs is absent or at low concentration. Thus, dissolution is controlled by diffusion of cations through the boundary layer, but this diffusion is itself controlled by the structural modifications required by the addition of new components to the melt. Comparison of quartz dissolution rates in similar melts shows that dissolution is much faster for quartz than for sapphire and that dissolution rates show the same correlation with silica activity and viscosity. We suggest that diffusive fluxes are related to changes in melt structure and the nature of the reaction that incorporates the added component. For the slow eigendirection, SiO2 addition occurs by a single reaction whereas Al2O3 addition requires a more complex two part reaction in which Al is accommodated by charge balance with Ca until Al is in excess of that which can be charge balanced. The Alxs incorporation reaction, is slower than the Si incorporation reaction which inhibits sapphire dissolution relative to quartz in melts of the same composition.

Clay coating preserving high porosities in deeply buried intervals of the Stø Formation

The Stø Formation is the most important reservoir interval in the Norwegian Barents Sea, however the reservoir quality can be highly affected by the detrimental effects of quartz cement where there have been extensive post depositional burial. Core plug data from well 7219/8-2 in the Southwestern Barents Sea shows abnormally high porosity and permeability values in certain units of the deeply buried and otherwise highly quartz cemented Stø Formation. The amount of quartz cement in the samples is inversely proportional to the porosity. Samples with high and low porosities are similar texturally and mineralogically, but the high porosity samples have a layer of illitic clay coating the majority of the detrital quartz grains. Illitic clay coating present at grain contacts can result in a lowered IGV given they aid in the dissolution of quartz at interfaces, also creating a source of dissolved silica. Clay induced dissolution means that silica saturation is not a limiting factor in quartz cementation in these samples. The results show that the illitic clay coating is capable of limiting the amount of authigenic quartz overgrowth from 20 to 23% in samples with negligible grain coating to 5–11% in the intervals with high coating coverage. The illitic clay coating inhibits quartz overgrowth by limiting the surface area available for nucleation on detrital grains. The Stø Formation comprises mainly shallow marine deposits of highly reworked clean sandstone. Abnormally high porosities appear to be linked to settings where sediments of a more proximal location are preserved without extensive reworking. The grain coating clay is illitic and most likely originates from clay infiltration processes prior to final deposition. The difference in extent of clay coating in similar facies can mostly be correlated with varying amount of post depositional reworking. This study suggests that there is a potential for considerable porosity and permeability to be preserved in deeply buried sandstones in the Barents Sea. This study could be important in the future exploration activity of deeply buried structures in the area.

Subsolidus segregation of quartz in metamorphic differentiation of metasedimentary rocks

Thermodynamic modeling of the SiO2–TiO2–Al2O3–Fe2O3–MnO–MgO–CaO–Na2O–K2O–P2O5–H2O (STAFMMCNKPOH) system at 600°C, 5 kbar has been applied to investigate dissolution and re-precipitation of quartz. Comparing silica molality in the STAFMMCNKPOH and SiO2–H2O systems, there is seen to be no effect of mineral assemblage on quartz solubility. From quantitatively estimated water/rock ratio required to dissolve quartz completely, one can deduce that the segregation of quartz appears to be due to diffusive transport of silica in inner pore fluid rather than to advective transport (in fluid flow).

Numerical investigation of Lower Tuscaloosa Sandstone and Selma Chalk caprock under geological CO2 sequestration conditions: mineral precipitation and permeability evolution

AbstractA numerical model was developed using CrunchFlow to simulate reactive transport and porosity and permeability changes of sandstone and carbonate rock samples taken from the Lower Tuscaloosa Formation and the Selma Chalk Formation, Jackson County, MS, USA. The model predicted a permeability decrease from 2190 mD to 2038 mD for the Lower Tuscaloosa Sandstone sample in a static batch reactor after 180 days of exposure to CO2‐saturated brine, which is consistent with measured permeability results. The model predicted a negligible permeability change from 2.00 mD to 2.08 mD for the Selma Chalk carbonate sample after 180 days of exposure to CO2‐saturated brine. Based on model prediction, key mineral dissolution and precipitation reactions in the Lower Tuscaloosa Sandstone sample include dissolution of quartz, chlorite, and feldspar, as well as precipitation of amorphous silica and kaolinite. For the Selma Chalk carbonate sample, key predicted reactions include dissolution of calcite, quartz and chlorite, and precipitation of kaolinite and amorphous silica. Initial porosity, initial feldspar content and the exponent n value (related to pore structure and tortuosity) used in permeability calculations were three important factors affecting permeability evolution of sandstone samples under CO2 sequestration conditions. The small permeability change predicted for both the Lower Tuscaloosa Sandstone and the Selma Chalk caprock after exposure to CO2‐saturated brine suggests that poro‐permeability changes during CO2 injection into the Lower Tuscaloosa Formation are not likely to significantly affect reservoir and seal quality. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.

Core Flooding Experiments and Reactive Transport Modeling of Seasonal Heat Storage in the Hot Deep Gassum Sandstone Formation

Seasonal storage of excess heat in hot deep aquifers is considered to optimize the usage of commonly available energy sources. The chemical effects of heating the Gassum Sandstone Formation to up to 150 °C is investigated by combining laboratory core flooding experiments with petrographic analysis and geochemical modeling. Synthetic formation water is injected into two sets of Gassum Formation samples at 25, 50 (reservoir temperature), 100, and 150 °C with a velocity of 0.05 and 0.1 PV/h, respectively. Results show a significant increase in the aqueous concentration of silicium and iron with increasing temperature due to dissolution of silica and siderite. Increasing the reservoir temperature from 50 to 100 °C enhanced the naturally occurring weathering of Na-rich feldspar to kaolinite. Dissolution of quartz increased sharply above 100 °C and was the dominating process at 150 °C, resulting in a significant increase in the aqueous silicium concentration. At temperatures ≤100 °C, the silicium concentration ...

Physical and Chemical Evolution of the Dabaoshan Porphyry Mo Deposit, South China: Insights from Fluid Inclusions, Cathodoluminescence, and Trace Elements in Quartz

The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (V1), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit. V1 and V2 veins are dominated by low-salinity (1–6 wt % NaCl equiv), CO 2 -bearing (4–10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. V1 veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti than any other vein generation (24–89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10–65 ppm). V3 veins have a broader spatial distribution than V1 and V2 veins, extending from inside the porphyries out into the adjacent limestone. These veins are also dominated by low-salinity (2–6 wt % NaCl equiv), CO 2 -bearing (4–7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5–12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4–7 wt % NaCl equiv), CO 2 -bearing (4–5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all vein types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54–5.3 ppm). Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 650° ± 40°C for V1 veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system cooled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar. Unlike nearly all other reported porphyry-type ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two- and three-phase CO 2 -bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of V1 to V4 veins remained above the V-L surface in the H 2 O-NaCl-CO 2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack of fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone.

On the Mechanism of the Dissolution of Quartz and Silica in Aqueous Solutions.

Quartz and silica are common materials, and their dissolution is of significant interest to a wide range of scientists. The kinetics of the dissolution of quartz and silica have been measured extensively, yet no clear theory of dissolution is available. A novel theory of dissolution and crystallization has recently been proposed that envisages the removal of material from the surface to form ions in solution leaving behind a charged surface vacancy. These vacancies create a potential difference across the Stern layer that accelerates or retards the removal of ions. In this way, the surface potential difference is caused by and influences the rate of the removal of ions. From this theory, a model of quartz dissolution is derived that predicts the observed orders of reaction. This prediction of the orders of reaction fits a data set consisting of 285 experiments. The model also describes the effect of Na+, K+, and Li+ ions, as well as the effect of heavy water. A significant component of the model is its ability to describe the zeta potential of the quartz–water interface. The model successfully predicts a transient period at the beginning of the reaction when the rate could either increase or decrease.

Characterization of Alkali-Induced Quartz Dissolution Rates and Morphologies

A number of studies have investigated the impact of alkali cations on quartz dissolution to increase the understanding of natural rock weathering and enhance the predictability of silica behaviour in reservoir systems. However, there are few evidences for how alkali cations approach the quartz surface. Thus, this study is an attempt to provide empirical evidence on the interaction of electrolytes (\(\hbox {Na}^{+}, \hbox {K}^{+}, \hbox {Ca}^{2+})\) with quartz surface. Bulk quartz grains and clean-faced single quartz crystals were dissolved/etched in varying pH solutions of acidic, near-neutral/neutral and alkali solutions (KOH, NaOH, KCl, NaCl, \(\hbox {CaCl}_{2}, \hbox {CH}_{3}\hbox {COOH}\), HCl). The amount of dissolved silica was measured with molybdate spectrophotometry method, while variations in quartz surface morphology were studied using scanning electron microscopy attached with EDX. Quartz dissolution rates varied with pH, hydrated radius and hydrolysis constants of alkali cations in the order: \(\hbox {Ca}^{2+}<\hbox {Na}^{+}<\hbox {K}^{+}\). Relatively deeper triangular etch pits were observed in quartz dissolved in KOH and NaOH solutions, while shallower pits formed in neutral solutions, and exsolution faces and lamellae were formed in quartz dissolved in low-pH solutions. The results also showed that interfacial secondary phases of cationic silicates play a key role in quartz dissolution.