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 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 OC 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.