Indonesian Journal on Geoscience

8

Authors: Sufriadin, A. Idrus, S. Pramumijoyo, I W. Warmada, I . Nur, A. Imai, A.M. Imran, and Kaharuddin

Mineralogical characterization of some garnierite samples from Soroako have been conducted using X-ray diffraction, thermal analysis, and infrared spectroscopy methods. XRD patterns reveal the samples mainly containing the mixture of kerolite (talc-like phase) and serpentine with minor smectite, sepiolite, and silica. Thermal analyses of garnierite samples indicated by DTA curves are in good agreement with patterns that have been reported in literature. Three endothermic peaks normally occur in the ranges between 58º C and <800º C illustrating three steps of weight losses: adsorbed, bound, and hydroxyl/crystal water. One additional weight loss in low temperature region of sepiolite is corresponding to the lost of zeolitic water. Infrared spectra appeared in 3800 - 3200 cm-1 region generally exhibit broad absorption bands, indicating low crystallinities of studied samples and can be assigned to the presence of hydroxyl group bonded to octahedral coordination mainly Mg atom. The bands observed at 1660 cm-1, 1639 cm-1, 1637 cm-1, and 1633 cm-1 in all samples indicate water molecules. FTIR spectra displaying the strong bands at 1045 cm-1, 1038 cm-1, and 1036 cm-1 could be related to the presence of Si-O-Si bonds linking to tetrahedral coordination. The strong absorption bands appeared at 511 cm-1, 505 cm-1, 499 cm-1, and 496 cm-1 in respective samples are attributed to divalent cation bonds (e.g. Mg, Ni-O). Both TG/DTA and FTIR seem to be the powerful tool in diagnosing the crystal chemistry of garnierite which is mainly composed of phyllosilicate minerals. Keywords: infrared, thermal analysis, kerolite, sepiolite

9

Authors: Premonowati, B. Prastistho, and I. M. Firdaus

Lithologically, Punung Formation as a paleoreef comprises coral boundstone rhodolith, algal grainstone, algal packstone, algal wackestone, algal floatstone, and algal rudstone. It is dominated by red algae and had formed a fringing reef in a warmly shallow marine environment. They built seven phases of paleoreef complex. Each paleoreef complex has been bounded by a local unconformity that is characterized by caliche. The Jaten Formation has becomes the base of the Punung paleoreef which build up by an angular unconformity contact on uppermost part. It consists of tuffaceous wacky sandstone with silicate cement. The formation as the reef base indicates two factors. The external factor because of the decrease of a volcanic activity and the internal one was caused by the depositional environment of the Jaten Formation becoming shallower. The subsurface runoff systems in many caves (like: Jaran cave and others) have the same southward direction to the dipping direction of algal grainstone lithofacies of Punung Formation. The vertical caves are formed by a jointing system. Keywords: Punung Formation, paleoreef complex, allostratigraphy, caves

12

Authors: S. Maryanto

Limestone diagenetic records of Sentolo Formation have been studied in order to trace the history of the geological processes after the deposition of the rocks. A stratigraphic measure traverse was made in Hargorejo Village, Kulonprogo District, to identify the petrological characteristics of the Sentolo Formation. Limestone samples were taken along this traverse, and were used for a petrographic analysis. The analysis of thirtyeight limestone samples shows that the type is dominated by bioclastic grainstones which have been affected by various kinds of diagenetic processes after the deposition. The diagenetic processes recorded under the polarization microscope include cementation, replacement, bioturbation, micritization, recrystallization, dolomitization, compaction, fracturing, and leaching. Keywords: grainstone, cementation, bioturbation, micritization, compaction