Soil Mineralogy
Soil Mineralogy a soil has three phases - solid, liquid and gas. The solid phase of a mineral soil is composed of 95% inorganic materials and 5% organic materials by volume. In the inorganic fraction of the soil a large number of minerals occur as a mixture. Minerals in soils occur in three size fractions, the diameter of which are as follows: sand (2.0-0.02 mm), silt (0.02-0.002 mm) and clay (<0.002 mm). The minerals that occur in the sand and silt fractions are mostly primary minerals formed at elevated temperatures and inherited from the igneous and metamorphic rocks (sometimes through a sedimentary cycle). The minerals that occur in the clay fraction are considered to be secondary minerals formed by low temperature reactions and are inherited by soils from sedimentary rocks or formed in soils during weathering or pedogenesis. The mineral composition of soils has an important bearing on the fertility of soils. Minerals in soils are divided into two classes, crystalline, and amorphous or non-crystalline. Most of the minerals in soils are crystalline. There are, however, some amorphous minerals in a few soils. Allophane is an important amorphous mineral in soil formed from volcanic ash. Minerals present in the soil can further be divided into two kinds: light and heavy. Again, minerals may be of three types: essential, accessory and secondary.
Several investigators have studied the mineralogy of the soils in Bangladesh at different times since the sixties of the last century. There are three broad geological formations in Bangladesh. These are tertiary sediments in the northern and eastern hills; the madhupur clay of the pleistocene terraces in the Madhupur and barind tracts; and the recent alluvium on the floodplain and estuarine areas which occupy around 80% of the total area of Bangladesh.
The sand fraction of the Tertiary deposits is dominated by quartz (70-90%). In most places, contents of feldspars (mainly alkali) and mica are low. The heavy mineral fraction does not exceed 3%. These minerals include varying proportions of zircon, tourmaline, kyanite, staurolite and epidote. The total contents of feldspars and biotite are usually less than 10%. Exceptionally in some younger rocks adjacent to the northern border of Mymensingh and Sylhet districts higher contents of feldspars (up to 15%) are found. The total content of easily weatherable minerals are encountered, the contents of which sometimes exceed 15%. Clay mineral analysis available for some soils of the chittagong hill tracts indicated the dominance of kaolinite but there are small amounts of vermiculite and fine-grained mica (illite) as well.
The sand mineralogy of the Madhupur Clay is broadly similar to that of the Tertiary deposits. The madhupur tract and the eastern part of the Barind Tract are similar in mineral content to most of the Tertiary deposits, they are high in quartz and relatively low in feldspars and micas. Like the Tertiary sediments, zircon, tourmaline, kyanite, staurolite, sillimanite and epidote dominate the heavy mineral fraction. The content of easily weatherable minerals ranges from 4-9%. The western half of the Barind Tract has a slightly higher content of easily weatherable minerals (8-14%) and a different heavy mineral assemblage. In the south, garnet occurs in addition to kyanite, staurolite, sillimanite and epidote. In the north, sillimanite is dominant and epidote contents are small. Clay mineral data for unweathered Madhupur Clay near Dhaka contains illite, kaolinite and trace amounts of smectite. The foregoing discussion leads one to conclude that the Madhupur Clay is similar in origin to that of the Tertiary hill rocks. Further, the low contents of mica and feldspars are distinctly different from adjoining floodplain sediments. This indicated that the Madhupur Clay was formed either before the major uplift of the himalayas to the north or more likely originated from the crystalline rocks similar to those of the Rajmahal hills or the shillong plateau. The homogenous nature of the Madhupur Clay vertically and horizontally over a long distance, indicates that it might have been of marine or estuarine origin.
The mineralogy of recent alluvium in Bangladesh is far from being homogenous as it has been deposited under different geomorphologic conditions in different areas: piedmont plains near the foothills (including the Himalayas); river meander floodplains; estuarine floodplains and tidal floodplains. They occupy about 80% of Bangladesh. The situation becomes complicated due to the fact that new alluvium is still being deposited near the river channels, but most floodplains have received little or no new fresh alluvium for several hundred years and moreover, rivers have changed their courses from time to time in the past. Most floodplain deposits have a high silt content. This is specially so in the brahmaputra, jamuna and meghna deposits. On the old Himalayan Piedmont Plain, north Tista Floodplain, some parts of the Old Brahmaputra Floodplain and the west Ganges River Floodplain sandy materials occur widely in the soil substratum. Clay deposits occur widely on the surface of the Ganges River Floodplain, Sylhet Basin, Atrai Beel and the lower Atrai Basin.
The ganges, Brahmaputra and tista rivers and the rivers draining the northern and eastern hills carry deposits from areas of diverse geology. The Ganges, Brahmaputra and Tista rivers bring large amounts of mica (both muscovite and biotite). On the other hand, the northern and eastern hill sediments are much less micaceous. The Ganges and the Lower Meghna sediments are calcareous. The sand fraction of the Tista, the Brahmaputra and the Ganges river sediments contain 15-30% feldspars equally divided between alkali and plagioclase varieties and 5-10% micas. The heavy mineral content varies from 2 to 9% while the contents of easily weatherable minerals vary from 25 to 40%. The amount of mica varies with particle size. Some silty deposits may contain as much as 80% mica in the sand fraction. In the Brahmaputra and the Meghna (both river and estuarine) floodplains, part of the Karatoya-Bangali floodplain and beach sand deposits in the southern part of Patuakhali district amphiboles (45-70%) and epidote (15-30%) predominate in the heavy mineral assemblage. Biotite accounts for approximately 65-75% of the total mica content.
The Ganges sediments have lower amphibole contents (35-40%) and higher garnet contents (> 20%) in the heavy mineral assemblage. They also contain dolomite and calcite particles. The sediments of the Tista river possess high biotite contents, 45-75% in the heavy fraction; the Old Himalayan Piedmont plain deposits contain only 10-30% of this mineral. Tista sediments differ from those of the Brahmaputra in possessing a lower epidote and higher biotite contents. They however, differ from Ganges sediments in having higher contents of biotite and sillimanite. The sediments of the surma and other eastern rivers are more or less similar to those of Tertiary hill sediments. These are low in feldspars and micas. Easily weatherable minerals amount to approximately 10%. The sediments in the Sylhet basin are also richer in amphiboles (25-35% of the heavy fraction than most alluvium derived from Tertiary hill sediments). At the same time, they are richer in epidote (20-40%) and zircon than the Brahmaputra sediments.
The clay mineralogy of the floodplain sediments is distinctively different from that of the hill sediments and the Madhupur Clay. The Tista and Brahmaputra river sediments and the old Meghna estuarine deposits have a mixture of kaolinite, illite and chlorite minerals. The Ganges River floodplain and young Meghna estuarine deposits have the same clay minerals as above, but they also contain significant amounts of smectite minerals. Surprisingly, a small area of alluvium near the confluence of the Ganges and the Atrai floodplains in the Rajshahi area has a mixture of illite and chlorite, with either kaolinite or smectite. Data available from clay mineral analysis of 16 samples from Deep Red-Brown Terrace soils developed over Madhupur Clay indicate that kaolinite is the dominant mineral with illite contributing about 20% and vermiculite 5-20% of the total.
A total of 228 soil samples (composite, topsoil, subsoil and substratum) representing 8 parent materials, namely Tertiary Hill deposits; Madhupur Clays; Himalayan Piedmont alluvium; Meghna alluvium; Ganges alluvium; Brahmaputra alluvium; Tista alluvium; Surma-Kushiyara alluvium, and samples from five hydrological conditions, such as hilly land; well-drained highland; poorly drained highland; shallowly flooded land and deeply flooded land were collected by the soil resources development institute for mineralogical studies. The results indicated that kaolinite and illite minerals in composite samples were present in medium to large amounts (20 to 60% of total minerals). Similar results were also obtained in the topsoil, subsoil and substratum. However, only three soils of Tertiary hills contained lower amounts (5-20%) of illite and more than 60% of kaolinite mineral. In almost all the soils, irrespective of depth, chlorite and vermiculite were present either in trace or small amounts. The kaolinite and illite minerals were distributed almost in equal proportions throughout the soil profiles formed on the Madhupur Clay. Chlorite mineral was present in Tista alluvial soils in small amounts (5-20%) and only trace quantities in other soils of Bangladesh.
Gangetic alluvium had a small amount (5-20%) of smectite while all other soils of Bangladesh contained either traces or none at all. The sand fraction of all these soils contained more than 70% quartz. The K- bearing minerals in the sand fraction were mostly feldspar and mica. The sand fraction of the soils of the Tertiary Hills and the Madhupur Clay soils has less than 5% feldspar and mica. In the Floodplain areas, the Meghna Floodplain is richer in K- bearing minerals (feldspars or micas) than all other floodplain sediments. The sand fraction of Brahmaputra alluvium possesses proportions of orthoclase and plagioclase equal of those of the Himalayan Piedmont plain.
The Gangetic Floodplain, the Tista Floodplain and the Surma-Kushiyara Floodplain were dominated by orthoclase. In the floodplain soils, there is little evidence of any change in clay mineralogical composition caused by pedogenesis, except in the topsoils of some older non-calcareous soils where ferrolysis reactions appear to have destroyed some of the original clay minerals forming amorphous silica-alumina-iron gels. Extreme changes due to ferrolysis have taken place in the formation of shallow and deep grey terrace soils over Madhupur Clay. However, when used for agricultural purposes some soils of Bangladesh show problems. For example, development of acidity in acid sulphate soils due to presence of sulphur bearing mineral Zerosite (specially when oxidised); and development of alkalinity in some Gangetic Floodplain soils due to the presence of lime and smectite clay minerals. The major Floodplain soils in Bangladesh contain as high as 40% weatherable minerals at the initial stage. The soils of the Gangetic Floodplain and the red lateritic soils of the terraces and hills have high phosphate fixing capacity. The harmful radioactive mineral monazite was found in the black sands of the Cox's Bazar seacoast. [Aminul Islam and Md Sultan Hussain]
Bibliography HGJ Hizing, 'A Reconnaissance Study of the Mineralogy of Sand Fractious from East Pakistan Sediments and Soils', Geoderma (6), 1971; Soil Resources Development Institute, A report on potassium studies in soils and crops of Bangladesh, SRDI, 1990; Hugh Brammer, The Geography of the Soils of Bangladesh, UPL, Dhaka, 1996.
