Formation of Secondary Minerals across Topogeothermal Sequences on Ratu Crater of Tangkuban Parahu Volcano, West Java
Abstract
The internal environment of volcanic craters is influenced by chemical content contained in geothermal resources, in the form of fumarola and solfatara, as well as the topography and extreme fluctuating temperatures can create a specific phenomenon to the formation of secondary minerals.
There were 5 represent profiles samples i.e. Profile A (toeslope), Profile B , Profile D (backslope), G Profile and Profile J (summit), plotted based on the topographic gradient of Ratu Crat er, Tangkuban Parahu Volcano (topogeothermal), with a steep to very steep slope grade (van Zuidam , 1985). Samples on each profile were analyzed on its mineralogical properties.
Mineralogical analysis showed that the sand fraction of heavy minerals (density > 2.87) are opaque, augite, and hiperstin with varying percentages, while light mineral (density < 2.87) are a volcanic glass, zeolite, andesin, labradorite, bytownite and rock fragments. Extraction with oxalic and pyrophosphate acid showed the highest m ineral content of allophane (1.414 %), imogolite (0.391 %), and ferrihydrite (2,091 %) contained in the profile D (backslope). The lowest content founded in the Profile A (toeslope), which has a smaller content than the Profile J (summit).
XRD analysis results (without treatment) showed that in each profile (A, B, D, G, J) have almost the same reflection pattern that consists of gypsum (4.27Å), feldspar (3.1 -3.25 Å, cristobalite (4.04 Å), gibsite (4.85 Å), calcite (3.03 Å), kaolinite (7.1 Å) and quartz (3.3 4 Å).
XRD analysis (Mg+glycol) on each profile mostly dominated by non -crystalline minerals (amorphous), however the Profile J (Summit) and the Profile A (toeslope) are dominated by crystalline minerals that have been developed from amorphous minerals, i.e . mineral 2:1 (smectite and chlorite) and mineral 1:1 (halloysite and kaolinite).
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Introduction
Tangkuban Perahu volcano is located in West Java, it is one of the active volcanoes in Indonesia. Tangkuban Perahu is a post-caldera volcano situated on the eastern rim of Sunda caldera. Some previous researcher conducted research on Tangkuban Perahu volcano. However, they only explained the geologic condition of the volcano, and the soil genesis develoved in the external system of the volcano. Crater is one of the volcanic depression form as result of the explosion, a part of volcano internal system. The mineral development inside the crater still remains unclear.
The clay mineralogy of soils formed in volcanic materials varies widely depending on such factors as the composition of the parent material, stage of soil formation, pH, soil moisture regime, and the accumulation of organic matter (Shoji et al., 1993). Poorly ordered materials such as allophone, imogolite, ferryhydrite, and Al- and Fe-humus complexes often dominate the clay size fraction of volcanic soils.
Generally, Climatic conditions and their effects on degree of leaching and soil solution chemistry also play an important role in volcanic material weathering pathways and secondary mineral neogenesis. Volcanic materials may weather directly to Short Range Order (SRO) materials or kaolins, depending on the amount of rainfall and solution silica activity (Parfitt et al., 1983). Kaolin minerals can show a wide range of structural disorder (Churchman, 1990; Soma et al., 1992; Newman et al., 1994) due primarily to Al-vacancy displacements in the octahedral sheet (Plancon and Tchoubar, 1977a, 1977b; Soma et al., 1992). These vacancies may originate from nonstoichiometric substitution of Fe3+ for Al3+ in the octahedral sheet (Soma et al., 1992).
Indeed, some studies have shown that crystalline clays, such as halloysite, form initially without a SRO precursor in weathering systems that exhibit high solution silica activity (McIntosh, 1979; Singleton et al., 1989). Variable effects of hydration might also add to degree of disorder in halloysite. Low rainfall or leaching promotes high solution silica activity, facilitating halloysite formation, whereas high precipitation or leaching promotes low silica activity, favoring SRO material formation (Parfitt et al., 1983). Coupled with precipitation, temperature also plays a role in the formation of SRO materials or crystalline minerals, with crystallization promoted as the soil climate becomes warmer and drier (Talibudeen, 1981; Schwertmann, 1985). Short-range ordered materials are more persistent under cool soil conditions because crystallization is hindered by low input of thermal energy, therefore we hypothesize that thermal energy radiated by sulphide existance in crater basis will affect the transformation of the mineral surrounding in the crater, beside effected by translocation process.
Therefore, the study will investigate the formation of secondary mineral across topogeothermal sequences on ratu crater of Tangkuban Parahu volcano, west java.
Conclusion
Based on the results of this study concluded the topography of the crater (topogeotermal) influence on the formation of secondary miner al soil with specific content indicated on each profile.
Temperature and content of the geothermal source (fumarole and solfatar) influence on the formation of secondary minerals crater, secondary mineral formation in the lower profile (toeslope) dominated crystalline minerals as the effects of temperature and specific content of the crater geothermal environment.