Surface Area Of Quartz Research Articles

Comprehensive parametric study of CO2 sequestration in deep saline aquifers

Carbon dioxide injection in deep saline aquifers is a key method for permanently sequestering anthropogenic CO2. This study employs a reactive transport model to explore mineral precipitation/dissolution and its impact on reservoir properties in deep saline aquifers. We also assess capillary pressure and relative permeability hysteresis on various CO2 trapping mechanisms. Results from this study reveal the significant influence of initial brine composition on mineral precipitation/dissolution. The dissolution and precipitation of minerals have different effects around the wellbores compared to the overall reservoir. Additionally, salt concentration (Ca++ and Mg++) and quartz surface area affect CO2 mineralization, while Na+ impacts halite precipitation, altering reservoir flow properties. The effect of capillary pressure is significant, as including the capillary pressure in the simulation case resulted in significantly improved CO2 trapping, achieving almost total dissolution of the injected CO2 in around 300 years. This study offers novel insights into the interactions of reservoir minerals, brine properties, and the injected CO2.

The importance of facies, grain size and clay content in controlling fluvial reservoir quality – an example from the Triassic Skagerrak Formation, Central North Sea, UK

Clay-coated grains play an important role in preserving reservoir quality in high-pressure, high-temperature (HPHT) sandstone reservoirs. Previous studies have shown that the completeness of coverage of clay coats effectively inhibits quartz cementation. However, the main factors controlling the extent of coverage remain controversial. This research sheds light on the influence of different depositional processes and hydrodynamics on clay-coat coverage and reservoir quality evolution. Detailed petrographic analysis of core samples from the Triassic fluvial Skagerrak Formation, Central North Sea, identified that channel facies offer the best reservoir quality; however, this varies as a function of depositional energy, grain size and clay content. Due to their coarser grain size and lower clay content, high-energy channel sandstones have higher permeabilities (100–1150 mD) than low-energy channel sandstones (<100 mD). Porosity is preserved due to grain-coating clays, with clay-coat coverage correlating with grain size, clay-coat volume and quartz cement. Higher coverage (70–98%) occurs in finer-grained, low-energy channel sandstones. In contrast, lower coverage (<50%) occurs in coarser-grained, high-energy channel sandstones. Quartz cement modelling showed a clear correlation between available quartz surface area and quartz cement volume. Although high-energy channel sandstones have better reservoir quality, they present moderate quartz overgrowths due to lesser coat coverage, and are thus prone to allowing further quartz cementation and porosity loss in ultra-deep HPHT settings. Conversely, low-energy channel sandstones containing moderate amounts of clay occurring as clay coats are more likely to preserve porosity in ultra-deep HPHT settings and form viable reservoirs for exploration. Supplementary material: of data and technique used in this study are available at https://doi.org/10.6084/m9.figshare.c.6438450

Kinetic insight on CO2 hydrate formation and dissociation in quartz sand in presence of brine

CO2 storage as hydrates in porous media is a promising method to sequester carbon dioxide (CO2) that need to be comprehended to guide its potential site selection prospects. In this study, CO2 hydrate formation and dissociation were studied in three quartz sand particles with varying petrophysical properties such as surface area, pore size, and porosities. The studied quartz sand particle sizes are QS-1 (<0.6 mm), QS-2 (0.6–0.8 mm), and QS-3 (0.8–2.0 mm). The hydrate formation experiments were conducted in a high-pressure hydrate reactor with and without brine (3.3 wt.% NaCl) at 4 MPa and temperatures of 274.15 K and 277.15 K in 100% sample saturation. The hydrate dissociation kinetics were studied at two different temperatures of 277.15 K and 279.15 K. The results show that hydrates form faster in deionized water (DI) than brine solution. The presence of brine reduced the CO2 storage capacity by 30% compared with deionized water. In addition, the presence of brine reduces the stability of CO2 hydrates. CO2 hydrate formation and dissociation behavior in porous media is highly controlled by the quartz sand porosity, pore size, and surface area. The quartz sand particles with low porosity were found to efficiently store more CO2 with a fast onset hydrate formation time of 6 hr and high gas to hydrate conversion of 55%. However, the CO2 hydrates in the sand particles with low porosity were less stable and averagely prolonged the time take to release 90% of CO2 of the systems with high porosity by 39%.

Computational modelling of interactions between gold complexes and silicates

The interactions of gold complexes including gold chloro-hydroxy species, gold thiosulphate and gold thiourea, with protonated and deprotonated silicate monomers, are studied using density functional theory (DFT) methods. The previously published optimal geometry of gold complexes is used here, together with the geometry of silica monomers that is optimised and compared with experimental and available theoretical data. COSMO (COnductor like Screening Model) solvation simulation of different systems (for different pH and gold complexes) is also used to represent the surrounding aqueous environments. The interaction energy of gold complexes with silica species based on theoretical studies has been shown to correlate well with the extent of silica preg-robbing (sorption) per surface area of quartz determined experimentally. The ability of DFT to compute the interactions of different gold complexes involving significant relativistic effects, with other species, has been demonstrated in this study. This work allows us to explain and control the chemical processes which result in loss of gold from solution in hydrometallurgical extraction.

Effect of reactive surface area of minerals on mineralization and carbon dioxide trapping in a depleted gas reservoir

In this study, a long-term (up to 1000 years) geochemical modelling of subsurface CO2 storage was carried out on sandstone reservoirs of depleted gas fields in northeast Netherlands. It was found that mineral dissolution/precipitation has only a minor effect on reservoir porosity. In order to validate this, we focused specifically on the reactive surface area of minerals which we measured by Scanning Electron Microscopy. In this way we obtained distributions for the measured reactive surface areas of each individual mineral. Subsequent parameter analysis and Monte Carlo sampling of these distributions revealed that in the Rotliegend sandstones, the surface area of quartz has by far the largest effect on SMCO2 (total amount of CO2 sequestered as mineral). The proportional relation of SMCO2 and quartz reactive surface area leads to the conclusion that CO2 injection in a sandstone reservoir with fine grained quartz has a higher potential for mineral trapping of CO2.In addition, using parameter analysis we also could determine the effect of surface area of each mineral on its own dissolution/precipitation mechanisms as well as on the other minerals. For example, the results showed that dawsonite precipitation is proportional to kaolinite and K-feldspar surface area.

Mechanochemical polymerization of methyl methacrylate initiated by the grinding of quartz in an n-heptane solvent

Abstract The information on mechanoradicals produced by grinding quartz in an n-heptane solvent was examined by means of electron spin resonance spectroscopy (ESR). The wet grinding of quartz was carried out by using a vibrating ball mill with a laboratory scale. The data of the ESR measurement revealed that E′ center radicals ( Si ) primarily produced from ground quartz in an n-heptane solvent and that the concentration of the mechanoradicals was approximately proportional to the specific surface area of ground quartz. The polymerization of methyl methacrylate (MMA) mechanochemically initiated by the grinding of quartz in the mixture solvent of n-heptane and MMA was also attempted by using the vibrating ball mill. The effect of the concentration of MMA monomer on the polymerization was investigated. As a result, the existence of the induction period to be observed well in radical polymerization was confirmed in all of the concentrations of MMA. It was found that the conversion of MMA increased with the decrease of MMA monomer and that the largest polymer yield was obtained at the concentration where the amount of monomer and solvent was the same in volume.

Sandstone diagenesis and reservoir quality prediction: Models, myths, and reality

Models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration. Not all of these are equally supported by quantitative data, observations, and rigorous hypothesis testing. Simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones. Calibrated numerical models that simulate compaction and quartz cementation, when linked to basin models, have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture, composition, and quartz surface area is available. Analysis of global, regional, and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality. (1) The effectiveness of grain coatings on quartz grains (e.g., chlorite, microquartz) as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results. (2) Vertical effective stress, although a fundamental factor in compaction, cannot be used alone as an accurate predictor of porosity for lithified sandstones. (3) Secondary porosity related to dissolution of framework grains and/or cements is most commonly volumetrically minor (2%). Exceptions are rare and not easily predicted with current models. (4) The hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model. (5) Heat-flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure, thereby slowing the rate of quartz cementation in some subsalt sands.

Thermally-responsive surfaces comprising grafted poly(N-isopropylacrylamide) chains: Surface characterisation and reversible capture of dispersed polymer particles

In this study we investigate thermally-responsive surfaces prepared by grafting PNIPAm from a cationic macroinitiator (MI) that was adsorbed onto a range of anionic substrates. The substrates used were mica, glass, quartz and high surface area carbon foam. The carbon foam was rendered thermally responsive by first coating it with a layer of calcined laponite particles. PNIPAm brushes were grown from the substrates using surface-initiated atom transfer radical polymerisation. The thermally-responsive PNIPAm layers were characterised in detail at room temperature and 50 °C using atomic force microscopy (AFM) and contact angle measurements. The surfaces changed from being non-adhesive to adhesive when the temperature was increased to 50 °C. Young’s modulus values and adhesive force values are reported. Particle capture experiments involving dispersed polystyrene or poly(BD/MAA) (butadiene and methacrylic acid) particles were conducted. High extents of particle capture were observed. It was shown that the highest extents of thermally-triggered particle capture at 50 °C occurred for surfaces that exhibited the largest increases in contact angle upon increasing the temperature. Importantly, thermally-triggered capture for both anionic polystyrene and poly(BD/MAA) particles was shown to be partially reversible with up to 30% of the captured particles released during cooling. This is the first time that significant reversibility of thermally-triggered capture of polymer particles has been reported.

Experimental evaluation of the effects of quench rate and quartz surface area on homogeneous mercury oxidation

This paper presents a mercury oxidation data set suitable for validation of fundamental kinetic models of mercury chemistry and for mechanism development. Experimental facilities include a mercury reactor fitted with a 300-W, quartz-glass burner and a quartz reaction chamber. While operated with a temperature profile representative of a typical boiler, a residence time of 6 s was achieved. Participating reacting species (chlorine, mercury) were introduced through the burner to produce a radical pool representative of real combustion systems. Speciated mercury measurements were performed using a Tekran 2537A Analyzer coupled with a conditioning system. Homogeneous mercury reactions involving chlorine have been investigated under two different temperature profiles producing quench rates of −210 K/s and −440 K/s. The larger quench rate produced 52% greater total oxidation than the lower quench at chlorine concentrations of 200 ppm. The effect of reactor surface area on oxidation was also investigated. The quartz surfaces interacted with mercury only in the presence of chlorine and their overall effect was to weakly inhibit oxidation. The extent of oxidation was predicted using a detailed kinetic model. The model predicted the effects of quench rate and chlorine concentration shown in experimentation.

Coupled thermohaline groundwater flow and single-species reactive solute transport in fractured porous media

A 3D numerical model has been developed to solve coupled fluid flow, heat and single-species reactive mass transport with variable fluid density and viscosity. We focus on a single reaction between quartz and its aqueous form silica. The fluid density and viscosity and the dissolution rate constant, equilibrium constant and activity coefficient are calculated as a function of the concentrations of major ions and temperature. Reaction and flow parameters, such as mineral surface area and permeability, are updated at the end of each time step with explicitly calculated reaction rates. Adaptive time stepping is used to increase or decrease the time step size according to the rate of temporal variation of the solution to prevent physically unrealistic results. The time step size depends on maximum changes in matrix porosity and/or fracture aperture. The model is verified against existing analytical solutions of heat transfer and reactive transport in fractured porous media. The complexity of the model formulation allows studying chemical reactions and variable-density flow in a more realistic way than done previously. The newly developed model has been used to simulate illustrative examples of coupled thermohaline flow and reactive transport in fractured porous media. Simulations indicate that thermohaline (double-diffusive) transport impacts both buoyancy-driven flow and chemical reactions. Hot zones correspond to upwelling and to quartz dissolution while in cooler zones, the plume sinks and silica precipitates. The silica concentration is inversely proportional to salinity in high-salinity regions and proportional to temperature in low-salinity regions. Density contrasts are generally small and fractures do not act like preferential pathways but contribute to transverse dispersion of the plume. Results of a long-term (100 years) simulation indicate that the coexistence of dissolution and precipitation leads to self-sealing of fractures. Salt mass fluxes through fractures decrease significantly due to major fracture aperture reduction in the precipitation zone. The system is the most sensitive to temperature because it impacts both the dissolution kinetics (Arrhenius equation) and the quartz solubility. The system is least sensitive to quartz surface area in the fracture because the volumetric fraction of a fracture is small compared to the volumetric fraction of the porous matrix.

Kaolin-Coating of Stylolites, Effect on Quartz Cementation and General Implications for Dissolution at Mineral Interfaces

Abstract The deeply buried (4 km, 160°C) quartz-rich sandstones of the Middle Jurassic Garn Formation in well 6507/7-1 offshore mid-Norway typically contain 20–30% quartz cement and only 3–9% porosity. The exception is a less than 1.5 m-thick interval at the top of the formation where quartz cement volumes typically drop to 8–16% and porosities are in the range 7–19%. The uppermost part of the Garn Formation is not isolated from the rest of the formation by any type of impermeable barrier; the sandstones are water-filled indicating that the change in cementation does not correspond to a hydrocarbon contact; and, there are no significant changes in grain size, grain coating abundance, grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1.5 m of the sandstones. However, in the samples from the less cemented interval, illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete. We propose that the cause of reduced quartz cementation in the top of the Garn Formation may be the coating of stylolites developed from primary illitic clay-rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin-rich rock adjacent to the stylolites is dissolved. Although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality, our observations may help to unravel the mechanisms of quartz dissolution at stylolites. Specifically, we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin-quartz contacts. Similarly, lack of dissolution at clean quartz-quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other, whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica, which may explain the extensive dissolution often seen at stylolites in limestones.

The Effect of Stylolite Spacing on Quartz Cementation in the Lower Jurassic Sto Formation, Southern Barents Sea

ABSTRACT The shallow marine Lower Jurassic quartz arenites of the Sto Formation in the southern Barents Sea comprise (1) intervals where dispersed detrital clay is absent, and where the spacing between clay-rich laminae that evolved into stylolites upon burial is exceptionally large, up to several meters, and (2) intervals where minor detrital clay matrix occurs, clay laminae are very common, and stylolite spacing is typically less than a centimeter. Point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large, 4-11%, versus 10-20% outside these intervals. There is also a correlation between distance to nearest stylolite and volume of quartz cement. Samples located a centimeter or less from a stylolite contain 10-20% quartz overgrowths, for distances of 3-20 cm quartz cement content is 4-10%, and only 3-8% when the closest stylolite is more than 20 cm distant. Modeling of quartz cementation with the ExemplarTM diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size, degree of grain coating, or content of quartz grains, i.e., the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites. On the contrary, the modeling suggests that the samples situated more than 20 cm from stylolites contain 5-8% less quartz cement than what would have been the case given a more normal stylolite abundance. This study indicates that sandstones with exceptionally few clay-rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories. However, such sandstones seem to be highly unusual on the Norwegian continental shelf. This suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures, and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation.

Nitrate-Cancrinite Precipitation on Quartz Sand in Simulated Hanford Tank Solutions

Caustic NaNO3 solutions containing dissolved Al were reacted with quartz sand at 89 degrees C to simulate possible reactions between leaked nuclear waste and primary subsurface minerals at the U.S. Department of Energy's Hanford site in Washington. Nitrate-cancrinite began to precipitate onto the quartz after 2-10 days, cementing the grains together. Estimates of the equilibrium constant for the precipitation reaction differ for solutions with 0.1 or 1.0 m OH- (log Keq = 30.4 +/- 0.8 and 36.2 +/- 0.6, respectively). The difference in solubility may be attributable to more perfect crystallinity (i.e., fewer stacking faults) in the higher-pH cancrinite structure. This is supported by electron micrographs of crystal morphology and measured rates of Na volatilization under an electron beam. Precipitate crystallinity may affect radionuclide mobility, because stacking faults in the cancrinite structure can diminish its zeolitic cation exchange properties. The precipitation rate near the onset of nucleation depends on the total Al and Si concentrations in solution. The evolution of experimental Si concentrations was modeled by considering the dependence of quartz dissolution rate on AI(OH)4- activity, cancrinite precipitation, and the reduction of reactive surface area of quartz due to coverage by cancrinite.

The effect of liquid additives on dry ultrafine grinding of quartz

The dry ultrafine grinding of quartz with liquid additives was carried out using a vibration rod mill with a laboratory scale. Liquid additives used were mainly seven alcohols and three glycols with different alkyl groups. All of the experiments were carried out by batch operation, and the variations of specific surface areas of products with grinding time were measured by BET adsorption method. The results showed that alcohol and glycol additives were satisfactorily effective for the ultrafine grinding of quartz. The maximum specific surface area of quartz obtained with an additive was found to increase in proportion to the amount of the additive. The additive molecules of multimolecular layers were supposed to adsorb on the quartz surface when the specific surface area reached the maximum value. The principal effect of additives in ultrafine grinding is considered to be the increase in flowability and the control of reagglomeration of quartz powder due to the alkoxylation of hydroxyl groups on the quartz surface.

Modeling Quartz Cementation and Porosity in Middle Jurassic Brent Group Sandstones of the Kvitebjørn Field, Northern North Sea

Petrographic study of deeply buried Middle Jurassic Brent Group sandstones from the Kvitebjorn gas field in the Norwegian sector of the North Sea shows that quartz cement volumes range from less than 1% to almost 30% over short distances, and porosity ranges from 5 to 30%. A clear correlation between quartz surface area and quartz cement volume indicates that this variation is due to differences in quartz surface area available for quartz overgrowth formation, which, in turn, is a function of grain size, abundance of grain coatings, and quartz clast abundance. The correlation between quartz surface area and quartz cement volume also suggests that the quartz cementation process is a strongly precipitation rate-controlled process, and that quartz cementation can indeed be modeled quantitatively by modeling the precipitation step in the quartz cementation process. Using temperature history, detrital mineralogy, grain size, and grain coating abundance as input, quartz cement volumes were for 90% of the samples modeled to within 4% or less of observed values with the EXEMPLAR® diagenetic modeling program. Modeled porosities deviate from measured values by less than 3% for 75% of the samples, and the difference between measured and modeled porosities exceeds 5% for only two of the 40 samples. Deviations between modeled and measured quartz cement volumes do not correlate with distance to nearest stylolite, but a tendency for underestimating quartz cement in samples with low quartz surface areas may possibly be present. Comparison with results from modeling of quartz cementation in other sandstones shows that optimal fit between measured and modeled quartz cement volumes is not always obtained with the same values for the kinetic parameters controlling quartz precipitation rate per unit surface area as a function of temperature. The variation in optimal kinetics between data sets is probably partly due to inaccurate temperature histories, but improving the quartz surface area function may also reduce the range of optimal values for the kinetic parameters. Olav Walderhaug is employed in Statoil's exploration technology division in Stavanger, Norway, where diagenesis and reservoir quality prediction are his main areas of responsibility. Olav holds an M.Sc. degree in petroleum geology from the University of Bergen and a D.Sc. degree in sandstone diagenesis from the University of Oslo. His research interests are mainly within sandstone diagenesis and related topics, including the development of quantitative models of cementation, porosity evolution, and basin subsidence.

AUTOCLAVED CEMENT-QUARTZ PASTES: THE EFFECTS ON CHEMICAL AND PHYSICAL PROPERTIES WHEN USING GROUND QUARTZ WITH DIFFERENT SURFACE AREAS PART II: RESULTS OF ACCELERATED CARBONATION

Previously saturated and oven dried, autoclaved cement-quartz prisms, made with quartz of varying fineness, were exposed to carbon dioxide for 24 hours. The extent of carbonation and drying shrinkage, as well as the degree of cracking were found to increase with increasing surface area of quartz. The binder became progressively more like SiO2-gel in character. Thermal analysis revealed both low- and high-temperature decarbonation steps, the mass loss increasing, in both decarbonation steps, with the surface area of quartz used. Difficulties encountered in interpreting the results, attributed to a multiplicity of factors involved, are highlighted. © 1997 Elsevier Science Ltd

Autoclaved cement-quartz pastes: The effects on chemical and physical properties when using ground quartz with different surface areas Part I: Quartz of wide particle size distribution

The effects of different surface area of ground quartz in 24 hour precured and autoclaved cement-quartz pastes, at C S of 0.5, were investigated. Portlandite production and calcium silicate hydrate formation increased during procuring in the presence of ground quartz, while the compressive strength increased with increasing quartz surface area. Autoclaving for 8 hours at 177 °C under saturated steam yielded calcium silicate hydrates, belonging to the tobermorite group, of decreasing crystallinity with increasing surface area of quartz. The drying shrinkage increased with increasing surface area of quartz corresponding with the decrease in crystallinity of the binder, while the compressive strength reached an optimum value at a quartz surface area comparable to that of the cement used.

Kinetic Modeling of Quartz Cementation and Porosity Loss in Deeply Buried Sandstone Reservoirs

A mathematically simple kinetic model simulates quartz cementation and the resulting porosity loss in quartzose sandstones as a function of temperature history. Dissolved silica is considered to be sourced from quartz dissolution at stylolites or individual quartz grain contacts containing clay or mica, and diffuses short distances to sites of precipitation on clean quartz surfaces. The modeled sandstone volume is located between stylolites, and no quartz dissolution or grain interpenetration takes place within this volume. After quartz cementation starts, compactional porosity loss is typically minor, and porosity loss within the modeled sandstone volume is therefore considered to be equal to the volume of precipitated quartz cement. The quartz cementation process is mod led as a precipitation rate-controlled reaction where quartz precipitation rate per unit time and surface area can be expressed by an empirically determined logarithmic function of temperature. When the sandstone's temperature history is known, precipitation rate per unit time and surface area can be expressed as a function of time, and the amount of quartz cement precipitated within a certain time interval can be calculated by multiplying the precipitation rate function with the surface area available for quartz precipitation and integrating with respect to time. Because quartz surface area will change as quartz cement precipitation proceeds, the calculations are performed for short time steps, and quartz surface area is adjusted after each time step. The total amount of quartz cement p ecipitated during a sandstone's burial history and the corresponding porosity loss are found by taking the sum of the increments of quartz cement precipitated during each time step. The effect of variation in parameters such as grain size, detrital quartz content, abundance of clay or other grain coatings, prequartz cementation porosities, and temperature history is easily simulated with the presented algorithm. This flexibility is illustrated by presenting calculated histories of quartz cementation and porosity loss for sandstones with a range of grain sizes, framework grain compositions, degree of clay coat development, prequartz cementation porosities, and temperature histories.

Physico-chemical aspects in silicosis

Silicosis is a pulmonary disease which afflicts persons who inhale over long periods of time freshly-ground silica-containig dust. Its mechanism of action at the molecular level is not yet understood. The majority of crystalline SiO 2 forms are pathogenic whereas the amorphous silicas are largely inactive. The primary cause of silicosis has thus to be sought in the structural and surface characteristics of the SiO 2 particles. Recent biological work envisages in the engulfment of the SiO 2 particles into a macrophage the first step ending up with the formation of the silicotic nodule in the lung [1–3]. Intermediate stages of the process are illustrated in Fig. 1. Any silica particle ▪ exhibits a membranolytic action on the phagolysosome, probably related to the surface hydroxyls configuration, with consequent release of lytic enzymes into the cytoplasm, death of the macrophage and release of the free SiO 2 particle in the lung tissue where it can be phagocytosed by another macrophage. This does not imply any fibrotic action per se. If the process takes place with fresh ground, crystalline SiO 2 , namely quartz, tridymite and cristobalite, within the phagolysosome a fibrogenic factor is also formed, which, when released in the lung tissue, stimulates fibroblasts to an abnormal production of collagen, and hence the formation of the silicotic nodule. Membranolysis can be reduced or blocked by chemical modification of the surface, however the intimate cause of silicosis relies on the catalytic role of crystalline SiO 2 in the production of the ‘factor’. Virtually any difference in surface properties between amorphous and crystalline silicas may account for their different biological activities. We have investigated, so far, the formation of free radicals at the crushed surface (E.S.R.), the heat of interaction with water molecules (adsorption microcalorimetry) and the kinetics and energy of interaction with some aminoacids (immersion calorimetry) on amorphous and crystalline silicas of comparable size [4]. Free radicals are produced by mechanical grinding of quartz, and readily react with various atmospheric components yielding paramagnetic species such as SiO • 2 and SiCO • 2, possible intermediates in the formation of the fibrogenic factor. Water vapour reacts with silicas in various ways depending on the dehydration degree of the surface. The heat of adsorption on micronized quartz (∼ 5 M 2 g −1), low and high surface area amorphous silicas (porasil ∼ 16 m 2 g −1), (aerosil ∼ 380 m 2 g −1 all out-gassed in vacuo below 423 K (in order to prevent elimination of silanols), are reported in Fig. 2 as a function of water uptake. Micronized quartz exhibits at low coverages an interaction energy (∼ 210 kJ mol −1) which is much higher than corresponding one on the two amorphous silicas (∼ 125 kJ mol −1). ▪ Crystalline structure rather than particle size thus dictates the binding energy of surface sites. Aminoacids are adsorbed at the quartz surface [5] but when the process occurs from aqueous solution their interaction energy is competitive with water itself. In the case of proline, however, a specific interaction with quartz was observed: in that case the heat released upon contact with quartz was 30 times higher than on amorphous silica and the interaction lasted many hours indicating an activated process, possibly the oxidation to hydroxyproline specifically catalyzed by the quartz surface. All these findings may be regarded as a first step in the investigation of the particular reactivity of crystalline SiO 2 within the cells.

Influence of Surface Area on the Density of Quartz heated at 1,300°–1,500° C

DURING an investigation of silica brick it soon became apparent that the extent of the thermal alteration of quartz was dependent on its initial particle size. A search of the literature revealed only a few fragmentary pieces of information1–5. Most of this information merely compared the extent of alteration of two fractions, a fine and a coarse, and stated that the finer fraction is altered to the greatest extent. The finer fraction was coarse compared with the quartz used here. Therefore a systematic, qualitative investigation of the influence of the initial surface area of quartz on the extent of its thermal alteration, as measured by density changes and X-ray diffraction, was undertaken. The purpose of this communication is to emphasize graphically the extent of this surface area effect.