Embedded Files
Classification of minerals
i. Based on the mode of origin
Primary minerals: The minerals formed cooling and solidification of original molten material is known as the primary minerals eg. Feldspar, Hornblende, Mica, Quartz etc;
1. Quartz: SiO2
• Most common soil forming mineral
• Make up 13% of earth's crust and from 30 to 40% of the average soil
• Commonly a translucent milky-white color
• Hard enough to scratch glass
• Resistant to weatherin • Present in granite; absent from basalt
• Present in almost all sandstone • Does not contribute plant nutrients to the soil
B. Plagioclase feldspar--Na AlSi308↔Ca Al2Si2O8
• Slightly harder than glass
• Common gray color (from almost white to dark bluish gray)
• Commonly has striations (flat faces within crystals seen as straight lines On surface)
2. Feldspar - alumino-silicates with bases of K, Na, and Ca
• Account for 60% of the earth's crust
A. Orthoclase Feldspar---KA1Si3O8
• Slightly harder than glass
• Commonly white, orange, or pink in color
• Fine wavy lines may occur within crystals
• Flat surfaces are common (intersecting at 88-90° angles)
• The most abundant mineral in granite
• Is an important source of potassium
3. Horneblende --- NaCa2 (Mg, Fe, Al)5 (Si, Al)8 O22 (OH)2
• Slightly harder than glass
• Black, dark brown or dark green in color
• One of the dark-colored minerals in granite
• Weathers more rapidly than feldspar, but persist in soils as dark colored gravel
4. Micas- alumino-silicates with K, Mg, and Fe basic components
• Easily spilt into thin flexible elastic plates
• Has shiny surface
• Present in granite, basalt, loess, and glacial till
• Muscovite (white Mica) KAl3Si3O10(OH)2 : is Al mica and is colorless
Contains more Potassium than Biotite
• Biotite (black mica) KA1(MgFe)3Si3O10(OH)2 : is Mg, Fe mica and is black
Has more Iron and Magnesium
Secondary minerals: Minerals that are formed, by the weathering of primary minerals are known as secondary mineral. Eg: Limonite, Gibbsite etc., and clay minerals like Kaolinite, Montmorillonite etc.,
1. Gypsum - CaSO4 2H2Q
• Forms from evaporating calcium sulfate-bearing waters
• Very soft and weathers fairly readily
• Accumulate in large quantities in semi-arid regions
• Can be both a Primary and Secondary mineral
2. Calcite - CaCO3(carbonates)
• Commonly found in limestone and Marble
• Much softer than glass; harder than fingernails
• White or colored by impurities
• Slightly soluble in water
• Effervesces in dilute HCI (release bubbles of CO2)
3. Dolomite - CaCO3 MgCO3
• Most common liming material in NC
• Similar to calcite
• Contains Mg
4. Iron oxides
• Formed through chemical weathering
• Geothite (FeOOH): gives yellow color in soils
• Hematite (Fe2O3): responsible for red coloration in soils
5. Clay Minerals (kaolinite)
• Highly colloidal
• Formed primarily from chemical weathering of primary minerals
• Ability to absorb or hold nutrient ions on their surfaces
ii. Based on its importance or amount
Essential mineral: Those minerals which form the chief constituents of a rock are known as essential minerals. They are present in large quantities varying from 95-98% eg. Calcite and Silicate minerals.
Accessory minerals: Those minerals which occur only in small quantities and whose presence / absence is of no consequence as far as the character of the rock is concerned are called accessory minerals. Eg. Tourmaline, magnetite, pyrites etc.
Silicate minerals Ferro magnesian silicate minerals
Inosilicates (Pyroxenes and amphiboles)
The pyroxenes and amphiboles are two groups of ferromagnesian minerals. The structure consists of long chains of linked silica tetrahedra. The pyroxenes consist of a single chain (2 oxygen shared in each tetrahedron) whereas amphiboles consist of double chains (alternately 2 and 3 oxygen atoms share the successive tetrahedra).
Phyllosilicates
The phyllosilicates are an important group of soil – forming minerals and are represented by micas (biotite, muscovite). They have sheet structure of tetrahedra where each silicon ion shares three oxygen ions with adjacent silicon ion to form a honey – comb like pattern. The fourth unshared oxygen ion of each tetrahedron stands above the plane of all others. The basic structural unit of phyllosilicates is formed by the condensation of two sheets of silicon-tetrahedra with one sheet of aluminum or magnesium octahedron.
Non Ferro magnesian minerals --Tectosilicates: The most common minerals of this group are feldspars and quartz.
Non Ferro magnesian minerals --Tectosilicates: The most common minerals of this group are feldspars and quartz.
Feldspars:
Feldspars are aluminosilicates of K, Na and Ca. The feldspar structure consists of tetrahedral which are attracted by sharing each oxygen atom between neighbouring tetrahedra.
The tetrahedra contain mainly Si ions with sufficient Al substitution. It belongs to the group of minerals that are light in weight.
There are two groups of feldspars: (i) Potassium feldspars (KAlSi3O8) include orthoclase and microcline. Orthoclase and microcline are more common in the plutonic and metamorphic rocks.
The potassium feldspars occur commonly in the silts and sands of soils and also abundant in clay-size fractions of soil (ii) plagioclase feldspars- a series consisting of solid solution of albite (NaAlSi3O8) high in sodium and anorthite (CaAl2Si3O8) high in calcium.
Plagioclase weathers more rapidly than orthoclase.
Quartz
It is very densely packed and occurs in a high degree of purity. It is strongly resistant to weathering as the structure is densely packed, and free from any substitution. It is the most abundant mineral next to feldspars.
Non-silicate minerals
Oxides: Haematite (Fe2O3) ,Limonite (Fe2O3, 3H2O), Goethite (FeO (OH) H2O)
Gibbsite (Al2O3H2O)
The red, yellow or brown colours in soils are due to the presence of goethite and haematite, which occur as coatings on the surface of soil particles.
Carbonates : Calcite (CaCO3) Dolomite (CaMgCO3)
Sulphates : Gypsum (CaSO4.2H2O)
Phosphates : Apatite (Rock phosphate Ca3 (PO4)2 - primary source of phosphorus
1. Quartz: SiO2
• Most common soil forming mineral
• Make up 13% of earth's crust and from 30 to 40% of the average soil
• Commonly a translucent milky-white color
• Hard enough to scratch glass
• Resistant to weathering
• Present in granite; absent from basalt
• Present in almost all sandstone
• Does not contribute plant nutrients to the soil
Page updated
Google Sites
Report abuse