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      A Brief Insight on Factors Controlling Rate of Chemical Weathering of Minerals Existing in Soil

      Tejaswini Sahoo1, Rashmirekha Tripathy1, Jagannath Panda1,3, Madhuri Hembram1, Saraswati Soren1, Deepak Kumar Senapati1, C.K. Rath1, Sunil Kumar Sahoo2 and Rojalin Sahu1*

       1School of Applied Sciences, Kalinga Institute of Industrial Technology, Deemed to be University, Bhubaneswar, India

       2Health Physics Division, BARC, Mumbai, India

       3CSIR-IMMT, Bhubaneswar, India

       Abstract

      Mineral weathering relative to soil has two parts: weathering happened previously where hard rocks have been broken down into clay which ultimately forms soil, loams, and unconsolidated sands both chemically and physically and other is soil and mineral weathering happening currently which acts as vital source of crop nutrients. This chapter discusses background of chemical weathering of minerals, sequence of weathering of minerals from soil, mainly throwing light on the factors which controls the rate of chemical weathering like temperature and time factor, biotic process, oxidation, reduction, water, leaching, acidity, and many more.

      Keywords: Chemical weathering, time factor, temperature factor, leaching, oxidation, reduction

2.1 Introduction

Weathering of rocks can be defined as the modification in the rate of composition and consolidation, occurring in the crust of earth under the influence of hydrospheric and atmospheric factors [1]. It has two different categories: chemical and physical. Physical weathering is defined as formation of unconsolidated state of rock from consolidated rock. In case of chemical weathering, there is chemical composition modification either in the unconsolidated or consolidated rock. Merrill et al., in the year 1906 [2–4], elaborated weathering as insincere changes caused in masses of rock due to atmospheric conditions, thereby causing complete or partial damage of the rock.

      Physical weathering was referred as disintegration, whereas chemical weathering referred to as decomposition. As already discussed, in case of physical weathering, large masses of rock get converted into elastic or unconsolidated state, owing to the hydrospheric and atmospheric factors. Polynov et al., in the year 1937 [5–7], states that weathering is a cyclic, complicated procedure which involves breakdown of solid structure of rock and production of enhanced interfacial interaction between the rock and nearby surrounding.

      By mechanical conditions, in case of disintegration or physical weathering process, rocks break down into smaller form which retain both its composition as well its originality. For example, even though basalt gets broken down via physical weathering or mechanical means into its smaller form, it could be identified as basalt. Physical weathering of rock occurs either in place or by the media like wind, gravity displacement at steep slopes, moving water and glacial ice which carries the rock and then grind it. In place physical weathering is driven by five factors as stated by Rieche et al. in the year 1950 [8–10]. The factors include unloading, thermal expansion, crystal growth, colloid plucking, and pressure and abrasion due to animals and plants. When there is reduction in confining pressure due to joints, cracks, erosion, and uplift, unloading causes rock masses expansion. According to Polynov et al., in the year 1937 [5], thermal contraction and expansion is vital factor, but as per Origgs et al. and Blaclrwelder et al. [11, 12], it is comparatively not important. Crystal growth causes production of prying action on minerals and rocks. This involves activity due to frost and also up to some extent formation of crystals due to chemical weathering, for example, as per Humbert and Marshall, in the year 1943 [13–15], quartz physical weathering occurred due to films of iron oxides. Colloid plucking refers to the physical damage occurred owing to the reduction of colloidal matter, resulting in peeling of the rock surface. According to Kellog et al., in the year 1943 [16–18], destruction of rocks occurs due to overgrowth of roots of the plants.

On the other hand, chemical weathering refers to modifications in the rocks’ chemical properties, owing to the hydrospheric and atmospheric factors. It is also known as the process of decomposition of minerals and rocks. The substance formed after the process of chemical weathering must include new mineral (mechanism of synthesis) or after the weathering the products should exist as residue after the elimination of other constituents (mechanism of residue). According to Polynov et al., in the year 1937, the product obtained as per mechanism of synthesis is known as products of accumulation and the product obtained from mechanism of residue is known as ortho eluvium. Moreover, the solutes getting washed and accumulating in ground water are also considered as weathering products, but in the context of mineral content of soil, it is of least importance. This chapter discusses background of chemical weathering of minerals, sequence of weathering of minerals from soil, mainly throwing light on the factors which controls the rate of chemical weathering like temperature and time factor, biotic process, oxidation, reduction, water, leaching, acidity, and many more.

       2.1.1 Weathering Similar to Hydrothermal and Diuretic Alteration of Minerals

Some minerals and layer silicates also form part of weathering research which are obtained from hydrothermal and deuteric changes. Layer silicates, consequently, appear at more depth of geologic column in comparison to the weathering depths during initiation. Even though silicates find its way to soil during the exposure of formation of rock due to erosion at the surface, they does not represent example of c weathering process chemically. According to Bowen et al., in the year 1922 [19–21], occurrence of deuteric changes was observed later during rock crystallization. According to Harker et al., in the year 1932 [22–24], during the rock metamorphosis, from feldspar, sericite was formed and chlorite might formed from garnet-biotite schist. Shand et al., in the year 1944 [25–27], reported that hydrothermal alteration is modifications in rock occurring due to the hot solution movement through specified routes into the earth. Some examples of this type of weathering were reported by Meyer and Sales et al., in the year 1950 [1], sericite formation from hydrothermal alteration of quartz monazite up to 20 ft from the channel along which hot hydrothermal solution travelled. Kaolinite was formed further up to another 20 ft. Furthermore, hydrothermal alteration up to 60 ft formed montomorillonite. Variations in the rate of alterations depended on the distance of fracture or channels from the hydrothermal solutions. The examples explained above come in the category of alterations of rock owing to the movement of solutions in the depth of the earth. Moreover, according to Schmitt et al., in the year 1950 [1], there are cases of same kind of alterations arising due to movement of hydrothermal solutions close to the surface. Water of hot spring at Yellowstone Park altered rocks (volcanic) into calcite and zeolites, orthoclase and quartz, and kaolinite and beidellite containing rocks.

2.2 Comparitive Stability of Minerals on the Basis of Their Sequence of Weathering

      Depending on the factors initiating the chemical weathering process, weathering of minerals depends on their stability. Owing to the stability some gets weathered slower and some faster in comparison to other minerals. The process of weathering of minerals depending upon their stability is occurring in a sequence, often referred as weathering sequence. Establishment of sequence of weathering process occurs on the basis of certain criteria, comparative persistence with depth of formation, age of formation, size of particle, intensity factors of weathering, and geographical factors.

       2.2.1 Heavy Minerals

      According to Pettijohn