Thomsonite along with other zeolite minerals such as natrolite, mesolite, analcime and rarely stellerite in cavities and veins of mafic alkaline lavas in Hormak area, north of Zahedan, eastern Iran, is formed in there distinct habits of blocky, bladed, and waxy-growth. Chemical composition of Hormak thamsonite is reasonably comparable with those of other reported thomsonites. Si/Al ratio of the thomsonite is 1.16 on average. The minerals show high Sr contents, up to 2.94 atoms per formula unit. Na and Ca are the main extra-framework cations with average values of 4.73 and 6.01, respectively. Calcium was most probably provided after either alteration of groundmass component or dissolution of secondary calcite, while sodium was mainly provided from saline and alkaline lake waters where the magma was poured. Hormak thomsonite was formed at low temperatures, about 100 ± 40 ^oC.