The Mineral Characteristic of Tropical Residual Soil using X-Ray Diffraction (XRD) and Scan Electron Microscopy (SEM)

: Volcanic residual soil has unique characteristics due to weather and morphological influences. High temperatures and large amounts of rainfall made the rock weathering intense. Landslide events in volcanic residual soil of West Java are quite serious, more than 100 incidents in total a year. Landslides that occur due to unsaturated conditions on the top of the slope become saturated, so it is very important to conduct research on these tropical residual soil weathering zones to define its mineral characterization using Scan Electron Microscopy (SEM) and X-Ray Diffraction (XRD). From the research it was found that the dominant mineral types and contents were Halloysite and Kaolinite minerals. A typical mineral profile is found that is Goethite and Hematite minerals at the soil surface, Carbonate and Chlorite minerals in deeper soils and large amounts of Feldspar minerals in more deep layers. Because there are unique minerals at each depth of soil weathering, zone classification can be done easily starting from the Initial Leaching Zone at the deepest part where there is Feldspar, the Mineral Washing Zone there is Carbonate and Chlorite and the Oxidation Zone on the surface (there are Oxide and Hydroxide minerals). So that it can form a Residual Tropical Volcanic Weathering Profile.


Introduction
Residual soil is formed due to weathering of rocks and is above the original rock, while tropical soil is soil that lies between the Tropic of Cancer (23.5 o ) in the North and the Tropic of Capricorn (23.5 o ) in the South. The stage of weathering from the existing rock to the residual soil is an independent variable that determines the other residual soil characteristics. Landslides in West Java occur quite frequently, more than 100 incidents per year. Most of landslides are caused by a heavy rainfall where the shear strength at the top of the slope surface reduced as the surface zone become saturated from unsaturated stage. So that at the beginning of the research it is necessary to determine the composition of the residual soil material to determine the characteristics of the soil weathering stages [1]. The following is a Geological map of Sheet Cianjur, West Java.  [2] where the formation describe as Qob, Old Volcanic Soil. Wesley (1973) on residual tropical volcanic soils of West Java, concluded that the soil has specific characteristics in which high shear strength parameters are obtained, even though the soil has a large clay mineral content but it is included in the specification of soil with low activity [3]. Tuncer and Lohnes (1977) published a parameter profile of mineral content, physical properties and shear strength of weathered residual soils based on a research of residual soils in Hawaii and Puerto Rico. This weathering profile is very interesting because it shows very different conditions in each weathering zone. By compiling a profile of the same parameters, the deeper knowledge will be obtained regarding the mechanism of the two factors, structure and suction matrix in controlling shear strength on slopes [4].
Soil properties depend on two main factors, namely composition and structure. The composition includes the properties of the grains themselves, namely their size, shape and type of mineralogy. The composition of tropical residual soils, high temperatures and large amounts of rainfall make rock weathering intensive. Weathering characteristics of the rock consist of Feldspar and ferromagnesian which are fragmented, Silica and Bases (Na2O, K2O, CaO, MgO) which are leached (disappear) and iron and aluminium oxides which are concentrated.

Results of Residual Soil Mineral Structure by XRD method
In determining the dominant type of mineral, an X-Ray Diffraction test is performed. With this test it can be determined what minerals are found in the soil sample. In the test, a relatively small sample volume was taken in the test. The minerals expected to be found in the residual soil are montmorillonite, kaolinite and gibbsite minerals [5].
The measurement result obtained is the amount of intensity per step (time step) by carrying out a time series statistical analysis. The measurement results per step are converted into a graph of intensity peaks which is also called a diffractogram. The following is the result of a sample diffractogram. Peak width is the width of a peak on the diffraction diagram. Peak intensity is the magnitude of the intensity of a peak (counts). Background intensity is the intensity of the background in a sample. Relative intensity is the intensity of a peak to the highest peak measured in the sample.
The results of the tests were obtained at a depth of 0.50-1.00. The minerals found were Goethite/Hematite, Halloysite and Kaolinite. Goethite and Hematite minerals are produced from the oxidation process of iron minerals. Halloysite and Kaolinite are minerals which are groups of silica tetrahedral layers connected to octahedral aluminium layers.   Figure 3.
If we only consider outside Silicate Group, except Feldspar we have the composition as follow:

The Mineral Characteristic of Tropical Residual Soil using X-Ray Diffraction (XRD) and Scan Electron Microscopy (SEM)
Based on the BH04 profile of the Neglajaya slope (slope to the right from Jakarta) there are Carbonate, Chlorite and a little Feldspar at a depth of 0.50-1.00 m and not found at a depth of 4.00-5.00 m indicating that the piled up or colluvial soil has been transported.

Research Result of Residual Soil Mineral Structure with SEM Method
The test of the soil minerals composition using the Scan Electron Microscopy (SEM) method with the results of photomicrograph testing with magnifications of 2500 x and 5000 x there are soil profile samples of BH02 Cijengkol as follows:  In the SEM photo, the 2.00 -2.50 m layer of soil can be seen that there are still stacked plateshaped layers, euhedral (hexagon) kaolin minerals but not perfect. The size of the mineral plate is smaller than the previous layer.

The Mineral Characteristic of Tropical Residual Soil using X-Ray Diffraction (XRD) and Scan Electron Microscopy (SEM)
In this layer, the size of the kaolin mineral plate is the smallest compared to other layers.  In layers of 8.50 -9.00 m it appears that block mineral forms are more visible and scattered compared to layers of 7.00 -7.50 m. Testing with Energy Dispersive X-Ray was carried out to obtain the dominant elements found in each soil layer. The results obtained are X-Ray graphs with energy units as follows. -Mineral O is the mineral with the highest average mass percentage in each layer except in the 8.50 -9.00 m layer where the highest average mass percentage is Fe minerals -Mineral Si is a mineral that is not easily dissolved (leached) has a consistent average mass percentage in each layer, as well as Zr minerals -Mineral Ca2+ which is the most easily leached mineral after Na+, K+, Mg2+ appears to be remaining at a depth of 8.50 -9.00 m -Mineral AI3+ from 1.71% in the soil layer 8.50 -9.00 m increased to approximately 18% at a depth of 4.00 -4.50 m to a depth of 0.50 -1.00 m -Mineral Fe from the large rock elements (such as ferrohornblende from granite) at a depth of 8.50 -9.00 m decreases at a depth of 4.00 -4.50 m then increases again at a depth of 2.00 -2.50 m to 0.50 -1.00 m due to the oxidation process -Mineral C as a weathering agent from rainwater content appears to have the largest mass percentage at a depth of 8.50 -9.00 m indicating that the weathering process has started at this considerable depth.

1 Weathering/laterization processes
The comparison found with the Energy Dispersive X-Ray method from the SEM tool shows in outline the laterization characteristics where the addition of iron oxide minerals occurs. It is

The Mineral Characteristic of Tropical Residual Soil using X-Ray Diffraction (XRD) and Scan Electron Microscopy (SEM)
confirmed that the results of the mineral composition obtained by the X-Ray method as shown in the following comparison tables and figures.

Early Weathering Zone
Based on the following table, the large FeO/Al2O3 ratio is found at a depth of 8.5 -9.0 then it is greatly reduced at a depth of 4.0 -4.5 m, and then the percentage increases slightly towards the surface. The value of FeO/Al2O3 and decreasing percentage indicates that the original rock is mafic or ultramafic rock, which in accordance with the reality in the resulting laterite field is a fertile residual soil.      It can be concluded in general that the alkaline mineral leaching zone is the zone where the highest pH and CEC values are obtained for a residual soil profile. While the leaching zone above is a zone where the pH value and CEC value are relatively smaller.

Iron Mineral Oxidation Zone (Fe)
From   The results of the mineral tests on the three samples showed an increase in the percentage of iron oxide and silica to quartz along with the high combustion temperature. This explains that when the soil drying process occurs, the mineral composition of the soil changes, this will affect the physical properties of the soil such as soil plasticity.

The Mineral Characteristic of Tropical Residual Soil using X-Ray Diffraction (XRD) and Scan Electron Microscopy (SEM)
From the test results with SEM, there is a significant difference in the size of the kaolin minerals from 1.00 -5.00 μm at 4.0 -4.5 m to 7.5 μm to 15 μm in size at 0.5 -1.0 m. This is related to the property of kaolin which can flocculate, which is a process in which a solid form suspension in the form of flakes (pieces with larger dimensions).
The results of measuring the total pore diameter of the soil from the SEM photomicrograph on the BH02 soil profile of the Cijengkol slope are shown in the following figure. The pore diameter and the number decreases with the depth of the soil layer. The results of measuring the total pore diameter of the soil from the SEM photomicrograph on the BH02 soil profile of the Cijengkol slope are shown in the following figure. The pore diameter and the number decrease with the depth of the soil layer.

Conclusions
Research on the composition of residual trophic soil minerals carried out by XRD and SEM testing resulted in the following conclusions: -In general, the same dominant type and mineral content are found on the slopes of volcanic tropical residual soils, namely Halloysite and Kaolinite -Typical mineral profiles encountered along the soil profile are as follows: a. On the surface of the soil there are minerals Goethite and Hematite b. In deeper soil layers, alkaline minerals are found, namely carbonate and chlorite c. In the next layer of soil, Feldspar minerals are found in large quantities. -Because there are unique minerals at each depth of soil weathering, zone classification can be done easily starting from the Initial Leaching Zone at the deepest part where there is Feldspar, the Mineral Leaching Zone there is Carbonate and Chlorite and the Oxidation Zone on the surface (there are Oxide and Hydroxide minerals). So that it can form a Residual Tropical Volcanic Weathering Profile -The SEM study showed that early signs of differences in the physical properties of the residual soil, namely differences in the shape and size of the soil chips and differences in the size of the soil pores.