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MDAG.com Internet Case Study 25

A Case Study of Important Aluminosilicate Neutralization

by K.A. Morin and N.M. Hutt

© 2007&2008 Kevin A. Morin and Nora M. Hutt

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Abstract

            At a molybdenum-related site with more than 2 km of underground workings and 60,000 tonnes of waste rock, acid-base accounts say the drainage waters should be acidic, but several decades of monitoring shows no ARD. We looked more closely at the Neutralization Potential (NP), particularly the aluminosilicate-based (“silicate”) NP, based on (1) the mathematical conversion of aluminosilicate minerals to NP, (2) the dissolution-precipitation of minerals to explain the water chemistry at the portal and in humidity cells, and (3) the dissolution (neutralization) rates of aluminosilicate minerals based on measured pH values.

            Silicate NP values were calculated from solid-phase levels of aluminosilicate minerals in the five humidity-cell samples. As a result, all except the already acidic metasedimentary Cell 1 sample would be net acid neutralizing, as long as a substantial portion of this silicate NP successfully neutralized pH to above 6. However, this was not true for the Cell 2, because it was already acidic, so most of its silicate NP derived from biotite, magnetite, and epidote cannot neutralize fully. The remaining three cell samples were still near neutral (as of Week 105), contained minor carbonate, and derived most of their silicate NPs from plagioclase. Their mineralogy-based Sulphide Net Potential Ratios indicate they will not become acidic, if much of their silicate NP can neutralize fully.

            The U.S. Geological Survey SpreadBal-2002 software estimated the amounts of silicate minerals dissolving and precipitating to explain water chemistries seen at the portal and in recent weeks of the five humidity cells. Epidote, plagioclase, calcite, and biotite were often calculated as major dissolving components to create the observed water chemistries, with substantial precipitation of quartz, ferric oxides, and aluminum oxides also needed to explain the chemistries. Subsequent conversions to neutralization potentials and alkalinities showed that silicate neutralization played a major role in the partial (below pH 6) to full neutralization in these humidity cells. This work also showed that calcite was not needed to explain the water chemistry, with atmospheric CO2 potentially supplying the carbon for alkalinity. However, slightly better agreement was obtained when calcite was included.

            Literature-derived, pH-dependent rates of neutralization by aluminosilicate minerals in the near-neutral humidity cells were similar to, or substantially greater than, the measured neutralization rates in the three near-neutral cells. The substantially larger measured rates apparently reflect (1) the additional contribution of calcite and/or (2) micro-scale acidic conditions (pH < 6) around the aluminosilicate mineral grains which would cause them to react faster. However, such a small-scale pH cannot be estimated from available information.

1. Introduction

            At some minesites, standard interpretations of acid-base accounting (ABA) can predict widespread ARD when none has been observed for decades (Morin et al., 2001; Eary and Williamson, 2006). This contradiction is especially seen at some molybdenum-related sites.

            The contradiction can be traced to two factors associated with the surrounding rock, which is usually similar to granodiorite. First, sulphide minerals at some minesites react relatively slowly, producing acidity at a low rate. Second, slow-neutralizing aluminosilicate minerals, which are not detected in the hours-long Neutralization Potential (NP) test, can successfully neutralize the low acidity rate.

            For one site with more than 2 km of underground workings and approximately 60,000 t of waste rock (not discussed by Morin et al., 2001), we looked more closely at three aspects of rock’s capacity to neutralize acidity:

1) The amount of NP represented by the aluminosilicate minerals in the rock, which cannot be detected in the standard, short, hours-long acid bath (Section 2). In this study, this is called “silicate NP”, and will provide a better estimate of NP than ABA for this site.

2) The amounts of aluminosilicate minerals and carbonate minerals that must dissolve or precipitate to explain the water chemistry from the humidity cells and the underground drainage (Section 3).

3) The rate at which the aluminosilicate minerals dissolve and neutralize aqueous acidity and pH (Section 4), which in this study is labelled “rate of silicate neutralization”. This may indicate to what rate the sulphide oxidation would have to increase before the silicate minerals could no longer keep pH above 6.

            Ideally, the silicate NP and the rate of silicate neutralization should come from site-specific testwork. For this site, the testwork included expanded acid-base accounts, total-element contents, mineralogy including types and amounts of silicate minerals, and long-term kinetic tests for bulk reaction rates including small-scale laboratory tests and large-scale monitoring of the existing minesite components.

2. Calculation of Silicate, Slow-Reacting, and Effective Neutralization Potential

            As part of acid-base accounting (ABA), a small amount of pulverized sample is placed in an acid bath and heated until the reaction (visible bubbling, usually created by fast-reacting carbonate minerals) ceases. A subsequent titration with hydroxide then shows how much of the original acid had been neutralized by minerals in the sample. This is known as bulk or “Sobek” Neutralization Potential (NP; Sobek et al., 1978). It can also be considered fast-reacting “short-term NP”, often representing calcium-magnesium-bearing carbonate minerals.

            It is important to note that, despite the abundance of aluminosilicate minerals, fast-reacting carbonate minerals can still be found in rock types like granodiorite. White et al. (2005) frequently identified calcite, in granitoid rocks around the world, at levels of 0.028 to 18.8 kg/tonne, with a mean of 2.52 kg/t. Jambor et al. (2006) also detected carbonate minerals in rocks ranging from granitic to ultramafic.

            Bulk NP often represents the actual amount of potential neutralization to a pH greater than 6.0, which is important for in situ control of ARD. However, theoretical cases (Eary and Williamson, 2006) and actual cases (Morin et al., 2001) show measured short-term NP can substantially underestimate total neutralizing capacity (called “Effective NP” here to distinguish it from measured NP). This is due to the presence of aluminosilicate minerals that can neutralize water to pH above 6.0 under certain conditions.

            Morin and Hutt (1997 and 2001) provided the following equation for Effective NP:

            Effective NP = measured (short-term Sobek) NP + Slow-Reacting NP - Unavailable NP

This equation recognized that additional, unmeasured NP (SRNP) is not detected by the NP procedure, and that some measured NP (UNP) is “unavailable” for neutralization. SRNP in rock is the focus of this case study, with SRNP assumed to be the same as silicate NP. UNP, often around 5-15 kg/t, has been discussed elsewhere (Morin and Hutt, 1997 and 2001).

            Because the additional silicate NP (SRNP) provided by aluminosilicate minerals cannot be detected using standard ABA procedures, the NP represented by these minerals can be calculated. This is not a simple task, because several assumptions must be made about the minerals and about the water into which they are dissolving (Morin and Hutt, 2006). Nevertheless, a general estimate of silicate NP and SRNP can be obtained by setting the final neutralized pH to near-neutral values (around 7-8) so that simplistically all iron precipitates as Fe(OH)₃, all aluminum precipitates as Al(OH)₃, and all silicon forms aqueous H₄SiO₄⁰.

            Detailed mineralogy of rock in this study is available from five samples tested in humidity cells (Table 1, and Figures 1 to 3) and from hundreds of related historical thin sections. The five humidity-cell samples contained mixtures of minerals that can partially or fully neutralize acidity, including:

- carbonates mostly as calcite (calcium carbonate) detected at levels below 1% in the three cell samples that are currently near neutral (Cells 3, 4, and 5),

- plagioclase containing more sodium than calcium in the granodiorite,

- potassium feldspar (K-feldspar),

- muscovite (mostly in metasedimentary Cell 1 and lesser amounts in the granodiorite) and biotite (mostly in the volcanics), which in their ideal forms release potassium,

- biotite (mostly in the volcanics) and chlorite and clinochlore (both at low levels in four cell samples), which in their ideal form release magnesium, and

- clinozoisite and epidote (seen visually in both volcanics cell samples and one granodiorite cell sample), which in their ideal forms release calcium.

Figure 1. Weekly Effluent pH from the Five Humidity Cells.

Figure 2. Weekly Sulphate-Production Rate from the Five Humidity Cells.

Figure 3. Temporal Trends of Weekly Mole Ratios of Calcium, Magnesium, Sodium, and Potassium from the Five Humidity Cells.

            Based on more than two years of humidity-cell testing (Figures 1 to 3), aqueous calcium concentrations dominated over those of magnesium, sodium, and potassium in most humidity cells and in drainage from the full-scale underground portal. The major exception was acidic Cell 2, with elevated potassium. Thus, calcite, plagioclase, and clinozoisite-epidote appear to be the most important minerals.  However, ion exchange and secondary-mineral precipitation (like smectites) can remove potassium, sodium, and/or magnesium from solution and thus mask their contributions to neutralization.

            For the five humidity-cell samples (Table 1), petrographic-based values were considered more representative for most minerals, and any minerals at levels considered “trace” were ignored (Table 1). Also,

- all illite, sericite, and muscovite were combined as muscovite;

- gypsum, actinolite, and rutile were ignored as sources of neutralization;

- all sulphides were combined as pyrite although they might not all be acid generating;

- due to the significant discrepancies between petrographics and XRD for K-feldspar and plagioclase, the XRD proportions of K-feldspar and plagioclase which appear more reasonable for a granodiorite were applied to the petrographics values (e.g., for Cell 2, its petrographics-based 5% K-feldspar was adjusted to 0.5% K-feldspar and 4.5% plagioclase);

- plagioclase was assigned an Anorthite ratio of An20 (20% calcium and 80% sodium), which has slightly less neutralizing capacity than the rough average of An30 reported in the historical thin sections, and

- weight-percent was assumed equal to volume-percent, which may underestimate sulphide levels but is offset by assuming all sulphides were acid-generating pyrite.

            This resulted in mineralogy-based NP values of 54 to 230 kg CaCO₃ equivalent/ tonne based on mineralogy (Table 2), compared to measured Sobek NP values of 5-15 kg/t. Silicate NP represented most to all of the mineralogy-based NP values. The samples with the highest and lowest NPs were already acidic, so their mineralogy-based NPs derived mostly from muscovite and biotite were not applicable. The remaining three near-neutral cells drew most of their mineralogy-based NP from plagioclase.

            For the Cell 1 metasedimentary sample, its 40% illite-sericite (considered as muscovite) provided most of the silicate NP of 54 kg/t, with biotite supplying the remainder. This failed to raise its SNPR values above 1.0 and thus it is still considered net acid generating. More important, it became acidic quickly (Figure 5-1), so the silicate minerals provided virtually no NP to maintain near-neutral conditions.

            For the high-NP volcanics sample (Cell 2), roughly half its silicate NP of 230 kg/t was derived from biotite, with most of the remainder from epidote and magnetite. This raised its mineralogy-based SNPR above 2.0. Thus, it would be net acid neutralizing using standard SNPR criteria, but it became acidic relatively quickly during testing (Figure 1), so its actual effective NP was very low. Nevertheless, unlike the other four cells, weekly effluents show that potassium has been dominant, with calcium not far less on a mg/L basis. The elevated potassium would be consistent with dissolution of biotite and K-feldspar, but much lower magnesium would rule out biotite as the primary source of potassium. This is discussed further in Section 3 of this case study.

             The other, low-NP volcanics sample (Cell 3) had a silicate NP of 138 kg/t, which was derived mostly from plagioclase, biotite, and K-feldspar. When combined with a relatively small contribution from calcite, this yielded a mineralogy-based SNPR of 2.14. Thus, it would be considered net acid neutralizing, and its dominant calcium concentrations may be derived from dissolution of calcite, plagioclase, and/or clinozoisite-epidote. The lack of high sodium concentrations suggests plagioclase is not a major contributor, although loss of sodium from solution cannot be ruled out. If plagioclase is ruled out, then the mineralogy-based SNPR would fall to around 0.4 which would be considered net acid generating. The most recent data to Week 105 of testing indicated the rate of sulphide oxidation was decreasing towards non-detectable levels in this cell, while neutralization of weekly rinse water was still continuing.

            Based on mineralogy, the two granodiorite samples contain roughly 5-10 kg/t of carbonate-based NP. They also contain 101-122 kg/t of silicate NP, derived mostly from plagioclase and K-feldspar. Thus, Cell 4 would remain net acid neutralizing, if a large portion of its silicate NP (primarily from plagioclase) were fully neutralizing. In contrast, Cell 5 has less sulphide and thus only a small proportion of its silicate NP is needed, as long as that NP was fully neutralizing.

            In summary, silicate NP values were calculated from solid-phase levels of aluminosilicate minerals in the five humidity-cell samples. As a result, all except the already acidic metasedimentary Cell 1 sample would be net acid neutralizing, as long as a substantial portion of this silicate NP successfully neutralized pH to above 6. However, this is not true for the high-NP Cell 2 sample, because it is already acidic, so most of its silicate NP derived from biotite, magnetite, and epidote cannot neutralize fully. The remaining three cell samples are still near neutral (as of Week 105), contain minor carbonate, and derive most of their silicate NPs from plagioclase. Although one may become acidic, their mineralogy-based Sulphide Net Potential Ratios indicate they will not become acidic if much of their silicate NP can neutralize fully.

3. Theoretical Neutralization of Aqueous Acidity by Silicate Minerals

            Part of the complexity for decoding neutralization at this site is that both fast-neutralizing carbonate minerals and slow-neutralizing silicate minerals are present. For example, aqueous concentrations of calcium represent neutralization by some combination of fast-neutralizing calcite (CaCO₃) and slow-neutralizing minerals like plagioclase. Plagioclase ranges from Albite (NaAlSi₃O₈) to Anorthite (CaAl₂Si₂O₈), with Oligoclase (Ca_0.2Na_0.8Al_1.2Si_2.8O₈) representing this site’s plagioclase.

            Therefore, the first step is to estimate how much of each mineral is dissolving to create the observed aqueous concentrations. A mass-balance spreadsheet, called “SpreadBal-2002” (Bowser and Jones, 2002), was used for this purpose. SpreadBal-2002 is a free download from the U.S. Geological Survey (http://water.usgs.gov/ software/spreadbal/).

            SpreadBal-2002 accepts a water analysis as input, and then calculates quantities of various minerals dissolved or precipitated to create the input water analysis. The input and output units are µmoles/kg H₂O (basically µmoles/L H₂O for this site). The minerals allowed to dissolve and precipitate were chosen from the SpreadBal-2002 database, based on observed mineralogy including a plagioclase Anorthite ratio of 20% (An 20) representing Oligoclase. Because formulas for minerals can vary from site to site, and even from rock unit to rock unit, the results should be taken only as general indicators and not 100% accurate estimates. For example, SpreadBal-2002 contains six varieties of biotite, all with differing stoichiometries, so biotite/chlorite was chosen because chlorite was also often identified in the cell samples (Table 3).

            According to SpreadBal-2002, the water draining from the portal at this site on December 5, 2005, was created partly by dissolving silicate minerals, primarily epidote and plagioclase (An 20) (Table 3). This was driven by the calculated dissolution (oxidation) of pyrite.

            There are two Spread-Bal simulations for each date in Table 3. The difference is the source of carbon found in the portal drainage for alkalinity. One source is solid-phase calcite, which is known to exist in the rock, and thus its dissolution neutralizes acidity and yields alkalinity to the water.

            The second source is atmospheric CO₂. This second source is important, because aluminosilicate dissolution can produce calcite at near-neutral pH. Using anorthite for simplicity, this reaction would be:

CaAl₂Si₂O₈ + 7H₂O + CO_2(gas) → CaCO₃ + 2Al(OH)₃ + 2H₄SiO₄⁰

SpreadBal-2002 calculations show epidote dissolution can actually account for the formation of calcite. In any case, this calcite could then dissolve later to neutralize acidity.

            This raises the issue of how much calcite at this site is a secondary product of aluminosilicate weathering, rather than a primary mineral, and thus may be replenished through time. Petrographic work found in the near-neutral cell samples:

“Fine anhedral patches, occurring with epidote”

“Fine anhedral patches occurring as selective replacement of K-feldspar and typically associated with muscovite-sericite.”

“Fine anhedral patches occurring as selective replacement of K-feldspar and typically associated with muscovite-sericite or epidote-clinozoisite.”

The ongoing replenishment of calcite would explain why Sobek NP values do not seem to decrease significantly through time at some molybdenum minesites (Morin et al., 2001).

            According to SpreadBal-2002, potassium feldspar (K-feldspar) may have precipitated from portal drainage (Table 3). However, portal concentrations were less than detection (2 mg/L) and thus the use of one-half the detection limit (1 mg/L) may distort the potassium balance. Similarly, iron was below detection, which affects the calculated dissolution of some aluminosilicate minerals and the precipitation of ferric oxide.

            Table 3 estimated that large amounts of aluminum (gibbsite), silica (quartz), and iron (goethite) precipitated from the waters. This would explain the extensive staining and accumulation of secondary minerals observed on the walls inside the portal, and thus the formation of these secondary minerals is consistent with SpreadBal simulations.

            There must be some type of geochemical “sink” to explain the loss of sodium from the portal waters, which is released by the dissolution of plagioclase. SpreadBal-2002 could not identify this sink.

            For the five humidity cells, the composite average chemistries through Weeks 37 to 41, and through Weeks 101 to 105, were evaluated by SpreadBal-2002 (Tables 4 and 5). As with the portal drainage, there were some complexities, such as (1) using one-half detection for some concentrations and (2) the lack of identification of the sodium sink.

            For the two acidic cells (Table 4), there was no alkalinity and thus no estimated involvement with calcite or atmospheric CO₂. SpreadBal-2002 indicated both acidic cells experienced significant dissolution of biotite, epidote, and plagioclase for neutralization, and pyrite for acid generation. Volcanics Cell 2 experienced dissolution of K-feldspar, due to its relatively high effluent potassium concentrations. In contrast, Cell 1 theoretically formed some K-feldspar, but this probably represented one or more geochemical “sinks”.

            For the three cells currently near neutral pH (Table 5), epidote was the major mineral dissolving from all three samples according to SpreadBal-2002, with Cell 4 having the largest epidote dissolution of all five cells. Compared with the acidic cells (Table 4), the calculated dissolutions of plagioclase, K-feldspar, and biotite were notably smaller. Calcite dissolution was higher than plagioclase, K-feldspar, and biotite in the two granodiorite samples, but similar in the lower-pH Volcanics Cell 3. Nevertheless, calcite was not needed to explain the average weekly chemistry, with atmospheric CO₂ and additional dissolution of epidote replacing calcite dissolution.

            Each mineral that dissolved and precipitated in SpreadBal-2002 simulations for the portal (Table 3) and humidity cells (Tables 4 and 5) can be associated with a certain amount of acidity neutralization or generation (Section 2 and Morin and Hutt, 2006). The amount depends on the stoichiometry of the mineral, the amount of mineral dissolved or precipitated, and the fate of various reaction products. Based on this, the amount of neutralization by the silicate minerals, minus the amount of acidity by the sulphide assumed to be all pyrite, will provide a net alkalinity (Table 6).

   For the acidic cells, this approach correctly yielded negative values for mineralogy-based net alkalinity, for both Weeks 37-41 and Weeks 101-105 (Table B2-6). This was consistent with the lack of measured alkalinity. The mineralogy-based total (but not complete) neutralizations were typically higher than measured total neutralizations, but within 20%. The exception was Cell 1 at Weeks 101-105, where measured neutralization was about one-half of the calculated mineralogy-based value. Nevertheless, the partial neutralization in the two acidic cells could generally be explained by SpreadBal-2002 calculations considering the generalizations.

                For the near-neutral cells, the mineralogy-based total neutralization typically overestimated the measurement-based neutralization (Table B2-6). However, this was often within 20% for Cells 4 and 5, but 40-100% for Cell 3. In general, discrepancies between calculated and measured total neutralizations were lower for the calcite-based scenario than the CO2-based scenario, although the differences were often less than 10% and thus may not be significant.

            In summary, the USGS SpreadBal-2002 software estimated the amounts of silicate minerals dissolving and precipitating to explain water chemistries seen at the portal and in the recent weeks of the five humidity cells. Epidote, plagioclase, calcite, and biotite were often calculated as major dissolving components to create the observed water chemistries, with substantial precipitation of quartz, ferric oxides, and aluminum oxides also needed to explain the chemistries. This software also showed that calcite was not needed to explain the water chemistry, with atmospheric CO₂ potentially supplying the carbon for alkalinity. Based on these amounts of reacted minerals, stoichiometric calculations showed that neutralization in the acidic cells (only partial neutralization below pH 6) and total neutralization in the near-neutral cells, as well as alkalinity, could be generally explained using mineralogy.

4. Theoretical Rates of Neutralization by Silicate Minerals

            The preceding sections examined neutralization based on bulk amounts of minerals. This section looks at theoretical kinetic rates of neutralization by various aluminosilicate minerals. An aluminosilicate mineral at a low solid-phase concentration could have a relatively fast reaction rate, despite its bulk amount, and thus account for much of the weekly neutralization observed in the near-neutral humidity cells.

            A major problem here is that reaction rates are often dependent on pH, and the pH around the reacting mineral grains could be very different from the composite pH of water draining from humidity cells and the portal (e.g., Li et al., 2006). Thus, identifying the small-scale pH at which the silicate minerals are actually dissolving and neutralizing is not possible. In turn, this means that using near-neutral pH to calculate reaction rates could underestimate actual silicate neutralization rates at small-scale acidic pH.

            In order to calculate neutralization rates by aluminosilicate minerals, the pH-dependent dissolution rates of minerals from Palandri and Kharaka (2004) were compiled into a spreadsheet. Thus, silicate neutralization was calculated by applying a time-based aqueous rate instead of a bulk solid-phase level (Table 7). This rate was expressed in units of mg CaCO₃ equivalent / kg of sample / week, for comparison with measured rates of neutralization (sulphate production minus residual acidity) in the humidity cells.  Measured rates were calculated without including alkalinity as a rough estimate of silicate-only neutralization, and with alkalinity as a rough estimate including calcite neutralization.

            As a result, the silicate neutralization rate was generally similar to that of measured neutralization, with and without alkalinity, in the near-neutral volcanics cell. For Granodiorite Cell 4, with the highest rate of sulphide oxidation, its neutralization rates with and without including alkalinity exceeded the silicate rate substantially. For Granodiorite Cell 5, the measured rate without alkalinity was similar to the calculated silicate rate, but the measured rate including alkalinity to approximate calcite contributions was much greater. Again, micro-scale pH around the silicate grains might be more acidic than the composite effluent pH, especially in Cell 4 with the highest oxidation rate, which would lead to higher silicate neutralization rates than calculated here. However, such a small-scale pH cannot be estimated from available information.

            In summary, theoretical rates of neutralization by aluminosilicate minerals in the near-neutral humidity cells were similar to, or substantially greater than, the measured neutralization rates in the three near-neutral cells. The substantially larger rates apparently reflect (1) the additional contribution of calcite and/or (2) micro-scale acidic conditions (pH < 6) around the aluminosilicate mineral grains which would cause them to react faster. However, such a small-scale pH cannot be estimated from available information.

5. References

Bowser, C.J., and B.F. Jones. 2002. Mineralogical controls on the composition of natural waters dominated by silicate hydrolysis. American Journal of Science, 302, p. 582-662.

Eary, L.E., and M.A. Williamson. 2006. Simulations of the neutralizing capacity of silicate rocks in acid mine drainage environments. IN: R.I. Barnhisel, ed., Proceedings of the 7th International Conference on Acid Rock Drainage (ICARD), p.564-577, March 26-30, 2006, St. Louis, MO, USA.

Jambor, J.L., J.E. Dutrizac, and M. Raudsepp. 2006. Comparison of measured and mineralogically predicted values of the Sobek Neutralization Potential for intrusive rocks. IN: R.I. Barnhisel, ed., Proceedings of the 7th International Conference on Acid Rock Drainage (ICARD), p.820-832, March 26-30, 2006, St. Louis, MO, USA.

Li, L., C.A. Peters, and M.A. Celia. 2006. Upscaling geochemical reaction rates using pore-scale network modelling. Advances in Water Resources, 29, p. 1351-1370.

Morin, K.A., and N.M. Hutt. 2006. Conversion of Minerals into Neutralization Potentials with Units of CaCO₃ Equivalent. Internet Case Study 20 at www.mdag.com.

Morin, K.A., and N.M. Hutt. 2001. Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies - Digital Edition. MDAG Publishing, Vancouver, Canada. ISBN 0-9682039-1-4.

Morin, K.A., and N.M. Hutt. 1997. Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies. MDAG Publishing, Vancouver, Canada. ISBN 0-9682039-0-6.

Morin, K.A., N.M. Hutt, W.A. Price, and V. Coffin. 2001. Violation of common ABA prediction rules by molybdenum-related minesites in British Columbia. IN: Proceedings of Securing the Future, International Conference on Mining and the Environment, Skellefteå, Sweden, June 25-July 1, Volume 2, p. 566-575. The Swedish Mining Association.

Palandri, J.L., and Y.K. Kharaka. 2004. A Compilation of Rate Parameters of Water-Mineral Interaction Kinetics for Application to Geochemical Modeling. U.S. Geological Survey Open File Report 2004-1068, 70 p.

Sobek, A.A., W.A. Schuller, J.R. Freeman, and R.M. Smith. 1978. Field and Laboratory Methods Applicable to Overburdens and Minesoils. Report EPA-600/2-78-054, U.S. National Technical Information Report PB-280 495. 403 p.

White, A.F., M.S. Schulz, J.B. Lowenstern, D.V. Vivit, and T.D. Bullen. 2005. The ubiquitous nature of accessory calcite in granitoid rocks: Implications for weathering, solute evolution, and petrogenesis. Geochemica et Cosmochimica Acta, 69, p. 1455-1471.

© 2007&2008 Kevin A. Morin and Nora M. Hutt

For more case studies, see Environmental Geochemistry of Minesite Drainage: Practical Theory and Case Studies.

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