Cronstedt’s zeolite

Axel F. Cronstedt (1722-1765), famous Swedish mineralogist, was the first scientist to describe,250 years ago, the distinctive property of zeolites, i.e., the unique frothing characteristics whenheated in a blow-pipe flame. Cronstedt examined two specimens: one from Svappavaara inNorthern Sweden and one that was said to come generically from Iceland. From Cronstedt’sindications, the occurrence of the first specimen was near the mining area of Kiruna. Themorphological characteristics of the specimen suggested the zeolite species is stilbite which wouldmake it the first discovered zeolite mineral.This paper is devoted to the description of the structure and microstructure of that first discoverednatural zeolite from Svappavaara (Northern Sweden). The SEM investigation of all the sample rockcavities filled with the zeolite crystals and the optical observation of the separated zeolite crystalsrevealed that the sample is mainly composed of stellerite with subordinate stilbite crystals which arepresent only in a few cavities together with stellerite. The chemical formula of the stellerite crystalsderived from the EPMA and TG analyses is (Ca4.03Mg0.01Na0.03K0.11)[Si27.81Al8.19O72]×28.6H2O. Theresults of the structure refinement confirm that the investigated specimen is actually stellerite. Thestructure refinement evidenced no ordering in the tetrahedral sites. The extraframework cation Ca islocated in the centre of the main channel parallel to the a axis, on the mirror plane and issurrounded by water molecules with no contact with framework oxygen atoms. The coordination ofCa is 6-fold. A number of short water-water distances are possible by considering the differentpossible schemes around the Ca atom. Considering the short water-water distances and the partialoccupancy of water sites, a number of octahedral coordination around Ca are possible.