Formation of secondary minerals,clay minerals and atmospheres minerals

Formation of secondary minerals, clay minerals and atmospheres minerals

The secondary minerals are formed at the earth’s surface by weathering of the preexisting primary minerals under variable conditions of temperatures and pressure. During weathering, water accompanied by CO₂, from the atmosphere plays an important role in processes, such as hydrolysis, hydration and solution. As a results the primary minerals are altered or decomposed.

Feldspar+water – clay mineral + cations + anions + soluble silica

Because of weathering, many elements are released into solution; apart of which may be used as a source of plant nutrients, a part may be leased out into the groundwater; still another part together with other constituents of the environment (like CO₂ and H₂O) may recombine to form secondary minerals. The most commonly formed secondary minerals are clay minerals (e.g. illite, montmorillonite, kaolinite etc.) are iron and aluminum oxides. Other secondary minerals observed in soils, especially in arid and semi-arid regions are gypsum, calcite, attapugite and apatite.

Silicates: -  clay minerals : hydrous alluminosilicates, with layer structure similar to micas, e.g. illite, montomorillonite, kaolinite, etc

 Non- silicates

Oxides, hydroxides or oxyhydrates of Sl, A1, and Fe

Haematite           Fe₂O₃

LIMONITE            FeO(OH)n H₂O

Gibbsite               A1(OH)₃

Clay minerals: -  clay minerals in soils are formed from primary minerals due to weathering processes. These clay minerals are of size <0.002mm and are considered to be the most reactive part of soil. Important soil properties like nutrients and water holding capacity are controlled by clay minerals. These mineral are layered silicates consisting of silica tetrahedron and aluminium octahedron.

1.     1 silicon tetrahedron + 1 aluminum octahedron = 1:1 clay mineral (kaolinite)

2.    2:1 non-expanding clay mineral

:black mica(biotite)

:white mica(muscovite)

:weathered mica (illite)

3.    2:1 expanding clay mineral                     : partially expanding

Of the naturally occurring inorganic minerals found in the clay fraction of soils, the most commonly observed are layer silicates (illite, montmorillonite, chlorite, vermiculite, kaolinite). Beside O, OH, A1 and Si, they contain Mg, Fe and K in large amounts. They are variable in color (white, grey, light yellow) depending on their chemical composition. In soil, the clays and oxihydrates of iron which form coatings on mineral grains impart shades of yellow, brown or red color to soils.

The clay minerals carry a significant negative electrical charge on their surfaces and have a structure like that of mica. In some cases, the groups of sheets are not firmly bounded together and water molecules can enter in their crystal lattice. This can cause considerable swelling due to change in soil moisture content. This is case in vertisols (black cotton soils) of India and NE Iraq, where deep and wide cracks on the surfaces are suggestive of the shrink-swell characteristics of soil clays.

Owing to the negative electrical charge on the clay surfaces, the cations are attracted to regions of electrical charge around the caly minerals this cation do not get bounded permanently and can be exchanged for other cations. The amount of charge varies depending upon the type of calay mineral and it is referred to as the cation exchange capacity. Because of this exchange, these are always a balance between the concentration of cations in soil water and those adsorbed on the surfaces of the particles. Rain water percolating through the soil leaches out many meta contain (K, Na, Ca, Mg) together with the existing soil water and replaces it with new water containing H⁺ ions and may render the soils acidic in reaction. The H⁺ concentration is expressed in terms of pH. A soil with high pH is alkaline while a soil with low pH is acidic in nature.

·       pH is expressed in terms of negative log to the base 10 of H⁺ ion concentration.

Distribution:-

·       While primary minerals are observed in all rocks  and in sand and silt fractions of soils, the secondary minerals dominantly occur in the clay fraction of almost all soils and in sedimentary rocks, especially shales. The kind and proposition of minerals observed in a soil depend on the kind of parent material and weathering intensity. The most common clay minerals observed is illite. Apart from illite, smectite predominates in the cracking clay soil, kaolinite in the highly-weathering soils of the inter tropical zones and southern Iraq,western india. In view of their high surface area and negative charge on them, they are considerable as a source of cations adsorption and cations release which are so important in acidic soil fertility.

Non-silicates

Oxides, hydroxides or hydrous-oxides group

We have already seen that oxygen is present in great abundance (46.7%) in the earth’s crust. The oxide minerals are found by the direct combination of elements (present in the earth’s crust) with oxygen.

The oxides are usually harder than any other mineral, except the silicates. The most important soils-forming oxide minerals are:

Haematite       :        Fe₂O₃

Limonite          :        Fe₂O₃3H₂O

Goethite          :        FeO(OH).nH₂O

Gibbsite           :        A1₂O₃.H₂O

Haematite (Fe₂O₃) – it varies in color from red to blackish and has reddish streak. It has a metallic luster and hardness (H) of about 5. Its presence in rocks is indicative of quick chemical change. Haematite alters to limonite, magnetite, pyrite and siderite. It occurs as coatings on sand grains and acts as a cementing agent. It swells on absorbing water to form hydrated iron oxide, i.e. limonite, 2 Fe₂O₃, 3H2O and goethite,

Limonite or bog iron – it is hydrated ferric oxide, yellow to brown in color and is of wide occurrence. It is the final product of most iron minerals and hence is resistant any further change, except for absorption of water. It is an important coloring and cementing agent in soils, iron. Limonite is a common alteration product of pyrite, magnetite, hornblende and pyroxene. It may be present in the form of iron concentration.

Goethite –

Most materials, called limonite, are goethite with some absorbed water. It is usually white but May pink or grey in color. Its hardness is 5.2

Gibbsite (hydragillite) –

It is the most common aluminum compound in soils. Its natural color is white.  It is abundantly observed in highly-weathered soils of the tropical environment, supporting laterites. Its present in soil suggest extreme degree of weathering and leaching under well drained conditions.

The red, yellow or brown color in soils is due to the presence of goethite and hematite which occur as coating on the surfaces of soil particles, especially clay.

Carbonate group –

The basic compounds, like Mg (OH)₂, and Ca(OH) combine with CO₂ or carbonic acid to form carbonates as under:

Calcite

 A white mineral, with hardness of 3, is widely distributed in sedimentary rocks, like limestone and decomposes easily to calcium bicarbonate as:

CaCO₃ +CO₂ + H₂O – Ca(HCO₃)₂ (soluble in water)

Dolomite –

Dolomite is less-readily decomposed than calcite; it is the chief source of Mg in soils.

Sulphate group –

Sulphate is a complex group formed by the combination of 1 sulphur and 4 oxygen ions, which further reacts with Ca to form calcium sulphate on hydration it forms gypsum

Gypsum –

 It is a common mineral in deserts soils and in sedimentary rocks having a hardness of 2. It is slightly soluble in water and gets most-easily leached. It precipitates as very fine, powdery mycelium from ground waters rich in Ca and SO₄ ions. India, it is used as an amendment to reclaim sodic soils and also act as a source of Ca and S for plants. Under the hot aridic climatic environment of Iraq, the presence of gypsum in high amounts in a problem, as it cause civil structures to collapse and makes sink-holes in soils, resulting in loss of irrigation water.

Phosphate group

Apatite, rock phosphate – it is a primary source of phosphorous in soils. Its hardness is 5 in mho’s scale. It decomposes readily under the influence of carbonic acid. It becomes immobile in calcareous soils as it readily combines with clays, with clays, Fe-A1 hydrous oxide, calcium carbonate to form rock phosphate. It also precipitates under acidic environment, as Fe and /or A1- phosphate