Petrography and geochemistry of zeolite minerals in mafic alkaline lavas from Hormak area, north of Zahedan, SE Iran

Mafic alkaline lavas in Hormack area، North of Zahedan، SE Iran، are the host of zeolites and secondary minerals. Zeolites have been formed in three forms: amygdaloidal، vein-type and as replacement of both primary phases and groundmass. Based on field، petrographic، X-Ray diffraction (XRD) and backscattered secondary electron (BSE) studies and electron microprobe analyses، the studied zeolites were identified as natrolite (the most abundant)، thomsonite (in three blocky، bladded and waxy-growth habits)، analcime (as amygdaloidal and direct alteration of glass)، mesolite and trivial stellerite. The average chemical formulae، calculated for the zeolite minerals are as follow: [Na14. 967Ca0. 331K0. 062Ba0. 025Mg0. 013] [Al15. 893Si24. 098O80]. nH2O (natrolite) [Ca6. 008Na4. 731Sr1. 208K0. 066Ba0. 035Mg0. 023] [Al18. 453Si21. 292O80]. nH2O (thomsonite) [Na18. 256Ca14. 98K0. 103Mg0. 082Ba0. 046] [Al46. 413Si72. 964O240]. nH2O (mesolite) [Na13. 755Ca0. 362K0. 262Ba0. 105Mg0. 026] [Al15. 118Si32. 732O96]. nH2O (analcime) The mean Si/Al ratios calculated for natrolite، thomsonite، mesolite and analcime are 1. 52، 1. 16، 1. 57 and 2. 17، respectively. The zeolites most probably have formed after cooling of the host lava flows and as the result of precipitation of saline، alkaline fluids along fissures and in cavities of porous lavas at low temperatures.