Geology, characterization, quality improvement and recommended utilization of natural zeolite (zeolitic tuff) deposits from Gunung Kidul, Yogyakarta Special Teritory, Indonesia

The natural zeolite (zeolitic tuff) in Ginning 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 Na2O content (1.06-1.51 wt. %) is relatively higher than K2O (0.71-0.98 wt. %). Initial (un-activated) CEC of the zeolitic tuffs vary from 39.41 to 67.84 mg Na2O/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 degrees C during 1 hour heating, which is indicated by an increase of CEC from 67.37 to 88.26 mg Na2O/100 gr zeolite. The Gunung Kidul natural zeolites are recommended to be an additive material for a slow release fertilizer.