Geology, Characterization, Quality improvement and Recommended Utilization of Natural Zeolite (Zeolitic Tuff) Deposits from Gunung Kidul, Yogyakarta Special Teritory, Indonesia ARIFUDIN IDRUS 1* , ANASTASIA DEWI TITISARI 2 , RAHMAN SUDIYO 3 AND R. SOEKRISNO 4 1, 2 Department of Geological Engineering, 3 Department of Chemical Engineering, and 4 Department of Mechanical Engineering, Faculty of Engineering, Gadjah Mada University, Yogyakarta, INDONESIA Abstract: - The natural zeolite (zeolitic tuff) in Gunung Kidul regency-Yogyakarta, Indonesia is the most promising deposit to be ranked as the most prominent commodity. Lithologically, the areas are occupied by interbedding green stone breccia, zeolitic tuff and sandstone tuff layers. Zeolitic tuff is characterised by green in colour, coarse – fine sand grain sized, with a variably thickness of up to 15 m. Zeolite minerals consist of clinoptilolite and mordenite with a range of 3.52-7.17 and 6.29-18.47 wt. %, respectively. Mordenite (Na-rich zeolite) is more abundant than clinoptilolite (K-rich zeolite). This agrees well with the bulk-geochemical composition of the rocks, where Na 2 O content (1.06-1.51 wt. %) is relatively higher than K 2 O (0.71-0.98 wt. %). Initial (un-activated) CEC of the zeolitic tuffs vary from 39.41 to 67.84 mg Na 2 O/100 gr zeolite. The measured resources of zeolitif tuffs in the studied areas are 451,248 tonnes. If the average percentage of zeolite minerals (clinoptilolite and mordenite) is 18.14 wt. %, the total amount of zeolite minerals within the rocks are 81,856 tonnes. The quality of zeolites (in term of CEC) was improved by thermal activation. The optimum operating condition was achieved at temperatures of 250 O C during 1 hour heating, which is indicated by an increase of CEC from 67.37 to 88.26 mg Na 2 O/100 gr zeolite. The Gunung Kidul natural zeolites are recommended to be an additive material for a slow release fertilizer. Key-Words: - Deposit geology, natural zeolite, characterization, quality improvement, slow release fertilizer. 1 Introduction Industrial minerals are abundant in the Gunung Kidul Regency, Yogyakarta, Indonesia, and are potentially can be a major income for the region. Among these industrial minerals, the natural zeolite is the most promising deposit to be ranked as the most prominent commodity. The development of zeolite resources will support the development of the local zeolite-based industry. In turn, this will stimulate the development of the economy growth of the local community. This research deals with the deposit geology and characterization of the natural zeolite deposit (in term of mineralogy, geochemistry and physical properties), quality and quantity (resources), conducting research on applications of zeolites for additive fertilizer materials for the small local enterprises as a pilot project for designated application. 2 Research Area Preliminary studies e.g. [1], [2], [3] and [4] revealed that the quality of natural zeolites from Gunung Kidul regency, Yogyakarta, is among the highest rank compared to other deposits have been developed in Indonesia (in term of cation exchange capacity). Therefore, in this project, we proposed four areas to be investigated including Serut, Sidomulyo, Hargomulyo and Jatirejo, which are situated in Gunung Kidul regency, Yogyakarta (Fig. 1). 3 Regional Geology Regionally, the studied area is situated in the northwestern part of the Southern Mountain. It is comprised of Oligocene-Miocene volcano- clastic sediments, which have the dips of layers relatively to south [5]. The regional stratigraphy of the Southern Mountain (from the oldest to the youngest) consists of the Kebobutak Formation, Semilir Formation, Nglanggran Formation, Sambipitu Formation, Wonosari Formation, Kepek Formation and Quartenary alluvium. The zeolitic tuff, in the research area, is a member of the Kebobutak Formation. The Kebobutak formation is covered conformably by the Semilir Formation, which is characterised by 2nd IASME / WSEAS International Conference on GEOLOGY and SEISMOLOGY (GES '08), Cambridge, UK, February 23-25, 2008 ISSN: 1790-2769 Page 21 ISBN: 978-960-6766-39-8