Activity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica. D E This is our activity diagram plotted to correct scale. The dashed lines represent the solubilities of quartz and amorphous silica; we will see how to plot these in the next slide. Note that no phase boundaries intersect at angles greater than 180°. If we now have analyses of a natural water, we can convert the measured concentrations to activities and plot them on the diagram to predict which aluminosilicate phase we would expect to be in equilibrium with the water. Note that, because the diagram does not involve the activity of Al, we cannot be certain, if a water plots in the K-feldspar stability field, for example, that the water is actually saturated with respect to K-feldspar. To know this for sure, we would have to accurately determine the aluminum activity and calculate the ion activity product, and then the saturation index. However, because the solubility of Al is low, many natural waters probably are saturated with some aluminosilicate, and the activity diagram can tell us which aluminosilicate is the stable one. Another point to make is that, we can only make conclusions about minerals we have chosen to plot on the diagram. If a water plots in the muscovite field, we can say that muscovite is the most stable of the phases we chose to include on our diagram. However, there may be a more stable phase that we did not include on our diagram. Of course, we cannot plot a phase unless we have thermodynamic data for it. C B A

A to B Infinite amount of microcline comes in contact with a solution of composition A in a closed system . Formation of gibbsite KAlSi3O8 + 7H2O + H+ = Al(OH)3 + 3H4SiO4 + K+ Increasing ratio of [K+]/[H+] and [H4SiO4]

Activity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica. D E This is our activity diagram plotted to correct scale. The dashed lines represent the solubilities of quartz and amorphous silica; we will see how to plot these in the next slide. Note that no phase boundaries intersect at angles greater than 180°. If we now have analyses of a natural water, we can convert the measured concentrations to activities and plot them on the diagram to predict which aluminosilicate phase we would expect to be in equilibrium with the water. Note that, because the diagram does not involve the activity of Al, we cannot be certain, if a water plots in the K-feldspar stability field, for example, that the water is actually saturated with respect to K-feldspar. To know this for sure, we would have to accurately determine the aluminum activity and calculate the ion activity product, and then the saturation index. However, because the solubility of Al is low, many natural waters probably are saturated with some aluminosilicate, and the activity diagram can tell us which aluminosilicate is the stable one. Another point to make is that, we can only make conclusions about minerals we have chosen to plot on the diagram. If a water plots in the muscovite field, we can say that muscovite is the most stable of the phases we chose to include on our diagram. However, there may be a more stable phase that we did not include on our diagram. Of course, we cannot plot a phase unless we have thermodynamic data for it. C B A

B to C Point B: Kaolinite becomes stable phase Gibbsite reacts with silicic acid to kaolinite and microcline continues to dissolve, now into kaolinite 2Al(OH)3 + 2H4SiO4 = Al2Si2O5(OH)4 + 5 H2O 2 KAlSi3O8 + 9 H2O + 2H+ = Al2Si2O5(OH)4 + 2K+ + 4H4SiO4 KAlSi3O8 + 2 Al(OH)3 + H+ - 1.5Al2Si2O5(OH)4 + K+ + 0.5 H2O

Activity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica. D E This is our activity diagram plotted to correct scale. The dashed lines represent the solubilities of quartz and amorphous silica; we will see how to plot these in the next slide. Note that no phase boundaries intersect at angles greater than 180°. If we now have analyses of a natural water, we can convert the measured concentrations to activities and plot them on the diagram to predict which aluminosilicate phase we would expect to be in equilibrium with the water. Note that, because the diagram does not involve the activity of Al, we cannot be certain, if a water plots in the K-feldspar stability field, for example, that the water is actually saturated with respect to K-feldspar. To know this for sure, we would have to accurately determine the aluminum activity and calculate the ion activity product, and then the saturation index. However, because the solubility of Al is low, many natural waters probably are saturated with some aluminosilicate, and the activity diagram can tell us which aluminosilicate is the stable one. Another point to make is that, we can only make conclusions about minerals we have chosen to plot on the diagram. If a water plots in the muscovite field, we can say that muscovite is the most stable of the phases we chose to include on our diagram. However, there may be a more stable phase that we did not include on our diagram. Of course, we cannot plot a phase unless we have thermodynamic data for it. C B A

C to D Point B: Kaolinite becomes stable phase microcline continues to weather into kaolinite 2 KAlSi3O8 + 9 H2O + 2H+ = Al2Si2O5(OH)4 + 2K+ + 4H4SiO4 Permanent increase in K+/H+ ratio and H4SiO4 activity

Activity diagram showing the stability relationships among some minerals in the system K2O-Al2O3-SiO2-H2O at 25°C. The dashed lines represent saturation with respect to quartz and amorphous silica. D E This is our activity diagram plotted to correct scale. The dashed lines represent the solubilities of quartz and amorphous silica; we will see how to plot these in the next slide. Note that no phase boundaries intersect at angles greater than 180°. If we now have analyses of a natural water, we can convert the measured concentrations to activities and plot them on the diagram to predict which aluminosilicate phase we would expect to be in equilibrium with the water. Note that, because the diagram does not involve the activity of Al, we cannot be certain, if a water plots in the K-feldspar stability field, for example, that the water is actually saturated with respect to K-feldspar. To know this for sure, we would have to accurately determine the aluminum activity and calculate the ion activity product, and then the saturation index. However, because the solubility of Al is low, many natural waters probably are saturated with some aluminosilicate, and the activity diagram can tell us which aluminosilicate is the stable one. Another point to make is that, we can only make conclusions about minerals we have chosen to plot on the diagram. If a water plots in the muscovite field, we can say that muscovite is the most stable of the phases we chose to include on our diagram. However, there may be a more stable phase that we did not include on our diagram. Of course, we cannot plot a phase unless we have thermodynamic data for it. C B A

D to E Point D: Muscovite becomes stable phase Kaolinite reacts with K+ to muscovite and microcline continues to dissolve, now into muscovite 3 Al2Si2O5(OH)4 + 2 K+ = 2 KAl3Si3O10(OH)2 + 3 H2O + 2H+ 3 KAlSi3O8 + 12 H2O + 2H+ = KAl3Si3O10(OH)2 + 2K+ + 6H4SiO4 Transformation of kaolinite and dissolution of microcline (Increase in K+/H+ ratio and [H4SiO4] activity) stops at point E = equilibrium between microcline, muscovite and kaolinite

Activity diagram for anorthite and secondary minerals. This diagram plots analyses of waters in crystalline rocks. It is reprinted in Kehew (2001) from Tardy (1971) Chem. Geol. v. 7, p. 253-271. The first thing to note from this diagram is that, although most of the waters are supersaturated with quartz, none are supersaturated with amorphous silica. The second is that, the primary control on the stable weathering product of anorthite feldspar is the dissolved silica activity. At the lowest silica activities, gibbsite is the stable weathering product. Gibbsite is usually only found in tropical soils where the amount of rainfall is high enough to keep dissolved silica activities low. With increasing silica activities, kaolinite, and then Ca-montmorillonite become the stable phases. The transition from kaolinite to Ca-montmorillonite also depends on the activity ratio aCa2+/(aH+)2. Higher values of the latter activity ratio favor Ca-montmorillonite over kaolinite.

THE CHEMICAL INDEX OF ALTERATION It is predominantly feldspars that weather to clays. We can thus base a measure of the degree of weathering on how far the composition is from that of an ideal feldspar. During weathering, Al and Fe are insoluble as oxides or oxyhydroxides. Other cations and Si are quite soluble. The concentrations are in molecular proportions. CaO* is CaO in silicates (excluding that in carbonates and phosphates).

CIA values of  100% are typical of heavily leached materials such as topical laterites and bauxites. Kaolinite and gibbsite occur in well-drained, heavily leached soils. Smectites form in poorly drained soils.

Figure 19.1: A. Variation of the chemical composition of saprolites representing increasing intensity of chemical weathering of granitic gneisses from Minnesota. B. Variation of the measured abundances of minerals in the saprolites shown above. From Faure (1997).

SOLID PRODUCTS OF WEATHERING The final stable products of weathering consist of quartz and clay minerals. Clay minerals: Hydrous sheet silicates (phyllosilicates) with a grain size < 4 m. Clays are constructed of two major structural components: 1) Sheets of SiO44- tetrahedra sharing three oxygens with neighbors. 2) Sheets of Al, Fe and/or Mg in octahedral coordination with O2- and/or OH-.

DIOCTAHEDRAL VS. TRIOCTAHEDRAL Dioctahedral - Only two out of three octahedral sites are occupied by trivalent ions. Trioctahedral - All three out of three octahedral sites occupied by a divalent ion.

Serpentine-Kaolin Group 1:1 CLAY MINERALS Serpentine-Kaolin Group Kaolinite - Al2Si2O5(OH)4 1) Cations cannot get between layers. 2) Solid solution is limited. octahedral sheet hydrogen bonds tetrahedral sheet

2:1 CLAY MINERALS micas, illite, smectite, chlorite solid solution is quite common in the 2:1 clays. tetrahedral sheet octahedral sheet tetrahedral sheet

ILLITE Illite - A general term to describe clay-size, mica-type minerals. Generally the composition is similar to muscovite. One out of four Si4+ ions are replaced by Al3+ in the tetrahedral sheet. This leads to a strong net negative charge. Some octahedral Al3+ may be replaced by Fe2+ and Mg2+, which also leads to net negative charge. The charge is neutralized by large cations, usually K+, in the interlayer spaces.

ILLITE STRUCTURE tetrahedral octahedral Interlayer sites filled with K+. Strongly bonded, so cations cannot easily exchange with K+. tetrahedral K+ K+ K+ K+ tetrahedral In illite, although interlayer cations exist, the interlayer bonding is so strong that other cations cannot work their way into the interlayer and exchange with potassium ion. So the ability of illite to adsorb contaminant cations is higher than kaolinite, but not as high as clays in which the interlayer cations can be exchanged. octahedral tetrahedral

SMECTITE Smectite - similar structurally to illite. However, the 2:1 units are not as tightly bound. Water can penetrate the interlayer sites, causing them to swell. Cations such as H+, Na+, Ca2+ and Mg2+ also can enter the interlayer sites. Thus, the weak interlayer bonding makes smectites prone to replacement by other cations. This leads to a high cation exchange capacity (CEC).

MAJOR CLAY MINERAL GROUPS This table outlines the properties of five major clay mineral groups. The column titled “Layer Type” refers to the proportion of octahedral to tetrahedral sites in the structure. In the 1:1 layer type, the basic structural unit is composed of one tetrahedral sheet and one octahedral sheet that are tightly bound to one another. This basic unit is then repeated indefinitely. In the 2:1 layer type, we have one octahedral sheet sandwiched between two tetrahedral sheets as the basic structural unit. In some clay mineral groups, Al3+ may substitute for Si4+ in the tetrahedral sheets, and some divalent metal ions may substitute for Al3+ in the octahedral sheets. This results in a charge imbalance, with the layers having an excess negative charge. This negative charge can be balanced by interlayer cations such as NH4+, K+ and Na+. The column titled “Layer Charge” gives the amount of excess negative charge per formula unit that must be balanced by interlayer cations. Finally, the last column gives the chemical formula for each clay type, with some structural information. For example, the tetrahedral cations are given in square brackets, the interlayer cations are given in front of the square brackets, and the octahedral cations after the square brackets. Chlorite has an additional twist. Together with the tetrahedral-octahedral-tetrahedral sandwich, there is an aluminum hydroxide interlayer. an = 0 is kaolinite and n = 4 is halloysite; M = monovalent interlayer cation.

Clay-OH + K+  Clay-OK + H+ ION EXCHANGE Clay-OH + K+  Clay-OK + H+ Clays (smectites) can hold ions both on their surfaces, on their edges, and in interlayer sites. Clays can be used as adsorbents, e.g., as backfill in nuclear waste repositories. Natural clays in groundwater aquifers retard the migration of pollutants by adsorption. Clay surfaces may act as catalysts.

Figure 13.2: Stability of kaolinite, K-montmorillonite, and Na-montmorillonite in the presence of amorphous silica. From Faure (1998).

Figure 13.3: Stability of selected minerals in the system K2O-MgO-Al2O3-SiO2-H2O-HCl at 25°C and 1 atm in the presence of amorphous silica. From Faure (1998). microcline phlogopite illite muscovite kaolinite chlorite