B.Sc.Ag. (Hons) Part-I
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| Soil minerals. https://cststudy.blogspot.com/ |
SOIL FORMING MINERALS
A mineral is an
inorganic substance that occurs in nature with distinct physical and chemical
properties. It usually consists of two or more elements in chemical
combination in certain definite proportions.
A mineral that forms the original component of a rock is
known as a primary mineral, e.g., feldspar, mica etc., while that which has
been formed, deposited or introduced as a result of subsequent changes in rocks
is known as a secondary mineral. Secondary minerals are limonite, gibbsite
etc., and clay minerals like kaolinite, montmorillonite, etc. Those minerals
which form the chief constituents of a rock are known as essential minerals
while those which occur only in small quantities and not much important, are
called accessory minerals, e.g. apatite, pyrites, zircon etc.
A. Primary Minerals
1. Feldspars. Feldspars belong to the group of minerals
that are light in weight, and are divided into orthoclase and plagioclase.
In orthoclase, potassium is prominent. Plagioclase is divided into albite (high
in sodium), anorthite (high in calcium), and oligoclase which consists of both
albite and anorthite. Plagioclase weathers more rapidly than orthoclase. Potash
feldspars are present in granite and similar other igneous rocks while
plagioclase feldspars form important constituents of basic igneous rocks like
basalt, gabbro etc. In the soil,
feldspars are usually present in the sand and silt fractions.
2. Quartz. It is
another mineral which is widespread. It is one of the most resistant minerals. Quartz is the silica minerals. Serpentine,
a hydrous magnesium silicate, occurs more commonly as a secondary product.
Garnets are characteristic of metamorphic rocks and are very hard and very
resistant to weathering. Quartz is an important constituent of all the three
classes of rocks and is the dominant mineral in granite, sandstone, rhyolite
etc. Among metamorphic rocks, gneiss and quartzite contain appreciable
quantities of this mineral. It is present in sand and silt fractions. Even the
clay fraction of some soils contains quartz.
3. Micas. Micas
occur both as primary minerals in igneous rocks and as secondary minerals in
altered products of feldspars and other minerals. The two important types are
muscovite (white mica) a potassium aluminium silicate, and biotite (black mica)
a potassium, aluminium, magnesium, iron silicate. Biotite weathers more rapidly
than muscovite.
4. Hornblende and
Actinolite. These are the two important minerals of the amphibole group of
silicate, arc also known as ferromagnesium minerals. These minerals contain a
number of bases like calcium, magne-sium, iron (ferric), manganese, titanium,
sodium, potassium etc. Hornblende is more abundant in acid as well as basic
igneous rocks. It is found in soil, especially in sand and silt fractions.
B. Secondary Minerals
Important secondary non-silicate minerals are calcite
(CaCO3), magnetite (MgCo3). Dolomite (CaCO3.MgCO3) and gypsum (CaSO4
.2H2O) (See Table).
TABLE: Important
primary and secondary minerals found in soils
Primary minerals
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Secondary minerals
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Resistance to weathering
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Quartz
Muscovite
Microctine
Orthoclase
Biotite
Albite
Hornblende
Augitc
Anorthitc
Olivine
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Geothite (FeOOH)
Hematite (Fe2O3)
Gibbsite (Al203. 3H20)
Clay minerals (Al-silicates)
Dolomite (CaCO3. MgCO3)
Calcite (CaCO3)
Gypsum (CaSO4.2H20)
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Most resistant
Most resistant
Most resistant
Most resistant
Most resistant
Most resistant
Least resistant
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Clay minerals are the most important secondary minerals.
These have been synthesized from residual products of Weathering, are colloidal
in nature. Clay minerals are crystalline in nature and are very important
constituents of soils. Most of the physical, chemical and morphological
properties of soils are influenced by these clay minerals. Clay minerals are
hydrous aluminium silicate, frequently with some replacement of aluminium by
iron and magnesium. The three most important groups of clay minerals are
kaolinite (non-expanding type with two layers), montmorillonite (expanding type
with three layers) and illite.
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