SSC 100 Lab 8

Lab 8. Soil genesis and weathering Concepts Soil genesis proceeds too slowly to measure in a few short weeks in a laboratory. We can, however, illustrate some of the processes of soil genesis that transform geologic material into soils. Part of soil formation is the vertical differentiation of the parent material. The most obvious result is the development of a profile, consisting of horizons, representing the redistribution, concentration, and depletion of soil constituents from the surface to some depth below the surface. Soil formation includes addition of organic matter from plant growth, dissolution of minerals, synthesis of new minerals, especially clay minerals, and movement of weathering products up or down the profile or out into the groundwater. For this exercise, there are demonstrations to let you observe some physical weathering stages, products, and chemical reactions. The separation between physical and chemical weathering is often somewhat arbitrary, as the two can proceed simultaneously. Physical weathering is the disintegration of rocks without changing chemical composition. Chemical weathering further reduces the size of the rock and mineral particles, and changes their chemical composition. 1. Chemical weathering Chemical weathering includes dissolution and alteration of minerals, and formation of secondary products. The chemical processes important in weathering include dissolution, hydration, hydrolysis, oxidation and reduction, and chelation. You will do some simple experiments that demonstrate some of these processes. Dissolution occurs when water molecules are attracted to ionic constituents in solids. The water forms a solvation complex with the ion. The ion and water detach from the mineral structure and diffuse into the bulk solution. If the bonds in the mineral have little covalent character, the formation of solvation complexes occurs rapidly. An example is when solid table salt (NaCl) is added to water. The sodium and chloride atoms readily form solvation complexes with the water and the solid NaCl "dissolves". Every material in soil has some solubility in water, although some are vanishingly small. Dissolution of solids with covalent bonds (such as aluminosilicate minerals) occurs more readily if hydrogen ions (protons) are abundant. An example of a dissolution reaction of a primary (inherited from the parent material) mineral is the reaction of biotite (mica) with the soil solution to form gibbsite and goethite: KMgFe 2 AlSi 3 O 10 (OH) 2 + 1/2 O 2 + 3CO 2 + 9H 2 O Al(OH) 3 + 2FeOOH + K + + Mg 2+ + 3HCO 3 - + 3H 4 SiO 4

Biotite gibbsite + goethite Note the role of carbonic acid in this dissolution and the release of plant nutrients. Hydration occurs when a water molecule or hydroxyl (OH) group is added to a mineral structure. The addition occurs primarily on surfaces and edges of mineral grains. With some simple salts, water may enter the entire mineral structure. Examples include: CaSO 4 + H 2 O CaSO 4 . H 2 O 5Fe 2 O 3 + 9H 2 O Fe 10 O 15 . 9H 2 O (transformation of hematite to ferrihydrite) Hydrolysis means "splitting of water". When water reacts with a mineral, H + (or H 3 O + ), a small and highly charged ion (relative to its size), replaces a cation in the structure. The replacement causes distortion of the crystal structure that leads to further breakdown. The equation is shown here with an “M” representing a cation such as K + , Ca 2+ , or Na + in a feldspar, for example. The pH of the solution must increase as H + from the split water molecule is consumed in the reaction and OH - from the split water molecule is produced. MAlSi 3 O 8 + H 2 O HAlSi 3 O 8 + M + + OH - Oxidation and reduction reactions occur frequently in soils. An oxidation-reduction reaction is a chemical reaction in which one or more electrons are transferred completely from one element or molecule to another. Oxidation of an element or molecule occurs when an electron is transferred from the atom or molecule. Concurrently, the electron is accepted by another atom. The atom that accepts the electron is reduced. An example of a reduction reaction is shown below. FeOOH (goethite, Fe 3+ ) + H + + e - Fe 2+ + H 2 O In this reaction, the iron in the solid mineral, goethite, accepts an electron and is reduced from Fe 3+ to Fe 2+ . Hydrogen ions are consumed in the reaction. At the same time, the goethite dissolves. This is a reduction half-reaction that must be accompanied by a second half-reaction that indicates the source of the electrons. In soils, organic matter oxidation is often the source of electrons for reactions such as this. Once in solution, the solubility of the Fe 2+ is affected by solution pH. If solution pH increases, the Fe 2+ will precipitate as Fe(OH) 2 . In his 1989 textbook on soil chemistry, Sposito shows the oxidation of formate as an example of the other half reaction. The total or coupled redox reaction is FeOOH + ½ COOH- + 5/2 H + Fe 2+ + ½ CO 2 + 2H 2 O

and ferric- oxyhydroxides decreases as soil pH increases, although above about pH 10, ferric hydroxide solubility increase as pH increases. Chelating Fe makes it more soluble and more mobile. This mobility is important for both soil genesis and plant uptake. A chelate is a molecule (usually organic, in soils) that forms several bonds with one metal atom. Other important chelates include chlorophyll (Mg and four molecules of porphyrin) and hemolglobin, which is a chelate of iron. 2. Physical weathering Physical weathering includes (1) cracking and fracturing, including differential expansion and contraction of minerals due to temperature variations, expansion of freezing water in crevices, expansion of growing roots in rock and mineral crevices, expansion of salt crystals as they form in cracks; and (2) abrasion by particles carried by wind or running water, and by glaciers. 3. Rocks Rocks and the erosional products of rock weathering are the parent materials for soils. A rock is usually made up of several different minerals. Rocks are classified according to: - origin: igneous, metamorphic, or sedimentary - texture and structure (size and shape of crystals) - chemical and mineralogical composition Igneous rocks: Igneous rocks solidified from a molten state. They consist of a mixture of primary minerals. Igneous rocks are subdivided into two main groups according to where the molten material, magma, cooled and solidified. These two groups are: - Plutonic or intrusive rocks - These form from magma intruded into the earth's crust. One general characteristic of these rocks is that they cooled slowly, allowing large, easily recognizable crystals to form. - Volcanic or extrusive rocks - These form from magma poured out on the earth's surface or ejected explosively into the air. Magma at the earth’s surface cools rapidly relative to intrusive rocks, forming smaller less-easily recognized crystals. These rocks are said to be fine-grained crystalline, or in extreme cases, glassy.

The igneous rocks are further classified according to composition. The main criterion is the type of feldspars present and the proportion of Si relative to Na, Ca, Mg, and Fe. The difference is expressed as the proportion of quartz compared to ferromagnesian minerals. Sedimentary rocks: Sedimentary rocks formed by compaction and cementation of materials deposited by water, wind, glaciers, and biological action. They are usually stratified and are often porous enough to be important as aquifers (subsurface water bearing layers). Rocks formed as chemical precipitates are also classified as sedimentary. Sedimentary rocks may contain both primary and secondary minerals, although many are almost entirely made of secondary minerals such as calcite, dolomite, iron and aluminum oxides, clays, and secondary silica. Quartz is present in many sedimentary rocks, usually as a primary mineral that has survived from the weathering of igneous rocks. Some examples of sedimentary rocks: - Limestone and dolomitic limestone. These form from deposites of calcite, CaCO 3 , or dolomite, (CaMg)(CO 3 ) 2 , usually with impurities such as quartz, iron oxides, or phosphates. These rocks are usually softer than igneous rocks and effervesce with acid. - Shales. These are compacted clay and mud. They are typically fine-grained and often laminated. There are calcareous shales (grading into clayey limestones); and sandy shales (grading into clayey sandstones). - Sandstones. These are cemented grains of sand. Sandstone does not imply mineralogy, and any mineral can be part of sandstone. Quartz sand is frequently abundant in sandstones because quartz is resistant to weathering, but many other minerals such as feldspar and mica, or sand-sized rock fragments, are also found in sandstones. The cementing materials may be silica, calcite, iron oxides, or clay. Sandstones usually fracture and weather along the cement, crumbling to sandy debris that often produces sandy textured soils. Metamorphic rocks: Igneous and sedimentary rocks may become altered (metamorphosed) by heat, pressure, chemical action of gases and liquids, and movements of the earth's crust. Once altered, the rocks become metamorphic rocks. They are classified by degree of metamorphism and mineralogy. There are various grades of metamorphic rocks, from those just slightly changed, to those that have been so thoroughly metamorphosed that they have entirely new structure or mineral composition. Some examples of metamorphic rocks include:

Procedures 1. Chemical weathering Dissolution *Wear goggles when using hydrochloric acid (HCl). Add one scoop (about 50 mg) of CaCO 3 to a 50-mL beaker containing approximately 25 mL of de-ionized water. Add one drop of phenolphthalein indicator. Note the color of the solution. Recall that phenolphthalein indicator turns red above pH 8.3. From a dispenser of 0.1 M HCl, add 5 mL and swirl the beaker. Let the reaction proceed for 2 minutes and note the color of the solution and the presence or absence of a solid. Next, add 10 mL of 0.1 M HCl and let the reaction proceed for another 3 minutes noting the solution color and presence or absence of a precipitate. Record your observations on the data sheet. Discard solutions in the waste container. Hydrolysis The goal of this exercise is to observe the change in pH of a solution as mineral particles undergo the process of hydrolysis. Add one scoop (about 50 mg) of ground rock to 25 mL of water in a 50-mL beaker. This rock contains a significant fraction of Plagioclase Feldspar with the general formula (Ca, Na)Al 1-2 Si 2-3 O 8 . The mineral is continuously variable from 100% Ca-feldspar to 100% Na-feldspar, hence the variable contents of Al and Si. Add three drops of phenolphthalein indicator solution and note the color. Continue to note color changes for at least 30 minutes. What, if any, are the color differences? Answer the question on the datasheet. Oxidation/reduction, hydrolysis, and chelation One goal of this exercise is to determine how the oxidation state of iron changes during some simple chemical reactions. A second goal is to observe the effect of chelation on the iron in these reactions.

Part 1 : For observation only. Fresh 0.5 M FeSO 4 was poured into a beaker. Another sample of 0.5 M FeSO 4 was poured into a beaker at least 3 hours before lab. Record the appearance of the "old" solution compared to fresh FeSO 4 on the datasheet. Part 2 : Add 30 mL of 1.6 mM FeSO 4 to each of three empty beakers labeled 1, 2, 3. To beaker #1 add a scoop of sodium citrate, a chelating agent, and swirl until the crystals have dissolved. Next add 10 drops of hydrogen peroxide (an oxidizing agent) to the Na-citrate-treated beaker (#1) and 10 drops to one of the untreated beakers (#2). Note any changes on the datasheet. Set beakers #1 and #2 aside for 15 minutes. To the third beaker (#3), that has only FeSO 4 in it, add 10 mL of 0.015 M NaOH and observe the reaction. After 15 minutes, measure and record the pH of beakers #1 and #2. Next add 10 mL of 0.015 M NaOH to these two beakers (#1 and #2). Measure and record these pH values again. Complete the table in the data sheet and answer the questions. Discard solutions in the waste container. 2. Physical weathering The goal of this exercise is to observe examples of the products of physical and chemical weathering of parent materials. View the pans on the lab bench that demonstrate different stages of rock alteration to soil. Note that physical alteration produces a diminution in size and an increase in surface area. Chemical alteration causes changes in color as new minerals are formed. View the horizon samples and note the changes in color and structure with depth. Answer the questions on the datasheet. 3. Rock demonstration This demonstration consists of examples of different common rock types. The purpose of the demonstration is to give you some experience recognizing different kinds of rocks and appreciating differences in their main properties. View the rock samples and answer the questions.

Lab 8 datasheet and report Chemical weathering Treatment Observation CaCO 3 + HCl Additional 5 mL HCl Additional 10 mL HCl Explain the chemistry of what happened in each phase of the dissolution experiment. In the hydrolysis exercise with plagioclase feldspar, describe how the color changes over the duration of the experiment, and describe the chemistry of hydrolysis that causes the color change. Treatment Beaker contents Observations pH Fresh FeSO 4 solution -- Aged FeSO 4 solution -- Beaker 1 FeSO 4 , citrate, peroxide Beaker 2 FeSO 4 , peroxide Beaker 3 FeSO 4 , NaOH -- Beaker 1 after NaOH addition FeSO 4 , citrate, peroxide, then NaOH Beaker 2 after NaOH addition FeSO 4 , peroxide, then NaOH

Briefly explain the observations in the following comparisons. If you use chemical equations, explain them in words. a. Fresh vs aged FeSO 4 solution b. Beaker 1 vs Beaker 2 c. Beaker 3 vs Beaker 2 after NaOH addition d. Beaker 1 after NaOH and Beaker 2 after NaOH Physical weathering What morphologic characteristics distinguish the following horizons in the Aiken soil: litter layer, A1, Bt2? What process causes the red color of the Bt horizons of the Musick soil? Rocks How might the mineralogy of a rock affect soil texture? How might the crystal size of a rock affect soil texture? How might rock mineralogy influence plant nutrition?