CaK-clinoptilolite, KNa-chabazite, KNa-heulandite, KNa-erionite and Na-phillipsite from tuffaceous rocks, Province of the Mesa Central, Mexico

Abstract

This paper presents the occurrence, mineralogy and process of formation of CaK-clinoptilolite, KNa-chabazite, KNa-heulandite, KNa-erionite and Na-phillipsite in pyroclastic rocks from the CalvilloJalpa-Nochixtlán area in the Province of the Mesa Central, Mexico. CaK-clinoptilolite, (Si30.600-30.150Al5.760-5.220)(Ca1.890-1.800K1.080-0.810Mg0.540-0.450) O72, of 5.04–5.86 Si/Al, 0.83-0.86 Si/(Si+Al) and 0.26–0.31 (Na+K)/ (Na+K+Ca+Mg) ratios, free of Na, crystallized from siliceous alkaline pore fluids and glasses of composition (Si0.890-0.825Al0.202-0.102)(K0.072- 0.020Ca0.020-0.010Fe0.050-0.010)O2 in tuffaceous rocks northeast of Jalpa. KNaheulandite, (Si28.080Al8.010)(K4.680Na2.250Ca0.270Fe0.180)O72, KNa-chabazite, (Si9.210-9.000Al3.090-2.940)(K0.750-1.180Na0.360-1.260Ca0.150-0.030Fe0.210-0.090Mg0.380-0.210) O24, NaK-erionite, (Si27.990Al7.830)(K2.000)(K4.210Na1.620Ca0.090Fe0.090 Mg0.180)O72, and Na-phillipsite, (Si12.080Al3.920)(Na3.160K0.240Ca0.160Fe0.040) O32, of 2.91-3.57 Si/Al, 0.74-0.78 Si/(Si+Al) and 0.72-0.97 (Na+K)/ (Na+K+Ca+Mg) ratios, dominant exchange cations K and Na, formed from siliceous alkaline high-pH fluids and glasses of composition (Si0.910-0.833Al0.170-0.092)(K0.101-0.050Na0-0.033-0.027)O2 in tuffs northwest of Nochixtlán. The zeolites were formed in the Jalpa area by diagenetic alteration of the tuff; in the Nochixtlán area, enrichment of Na and formation of alkali-rich zeolites was attributed to alkaline saline brines in an open hydrologic system. They occur in Oligocene – early Miocene tuffaceous rocks associated to the volcanism of the Sierra Madre Occidental, exposed in the Calvillo–Jalpa–Nochixtlán area in the states of Aguascalientes and Zacatecas, in the Province of the Mesa Central, Mexico.