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 when heated in a blow-pipe flame. Cronstedt examined two specimens: one from Svappavaara in Northern Sweden and one that was said to come generically from Iceland. From Cronstedt’s indications, the occurrence of the first specimen was near the mining area of Kiruna. The morphological characteristics of the specimen suggested that the zeolite species is stilbite which would make it the first discovered zeolite mineral. This paper is devoted to the description of the structure and microstructure of that first discovered natural zeolite from Svappavaara (Northern Sweden). The SEM investigation of all the sample rock cavities filled with the zeolite crystals and the optical observation of the separated zeolite crystals revealed that the sample is mainly composed of stellerite with subordinate stilbite crystals which are present only in a few cavities together with stellerite. The chemical formula of the stellerite crystals derived from the EPMA and TG analyses is (Ca4.03Mg0.01Na0.03K0.11) [Si27.81Al8.19O72]Æ28.6H2O. The results of the structure refinement confirm that the investigated specimen is actually stellerite and evidenced no ordering in the tetrahedral sites. The structure refinement evidenced no ordering in the tetrahedral sites. The extraframework cation Ca is located in the centre of the main channel parallel to the a axis, on the mirror plane and is surrounded by water molecules with no contact with framework oxygen atoms. The coordination of Ca is 6-fold. A number of short water–water distances are possible by considering the different possible schemes around the Ca atom. Considering the short water–water distances and the partial occupancy of water sites, a number of octahedral coordinations around Ca are possible.