TO BE OR NOT TO BE A FIBROUS ZEOLITE: OPTICAL PROPERTIES, CRYSTAL STRUCTURE AND HEALTH CONCERNS

For the case of zeolites there can be confusion regarding the word “fibrous” when it comes to describing a mineral group (i.e., zeolites with the T₅O₁₀ groups as the main building bloc) vs. the mineral habit. The nomenclature distinction is that not all zeolites can be considered part of the fibrous zeolite group, but many zeolites can form in the fibrous habit. Zeolites are a diverse and structurally interesting group of silicate minerals. The basic building block of the zeolite is comprised of a 1:2 ratio of tetrahedral cations to oxygen, similar to quartz and feldspars, with the addition of H₂O into the crystal structure of the zeolite. The framework of the zeolite group is characterized as having open channels and cages which usually house H₂O molecules and framework cations. Erionite and offretite both belong to the 6-membered ring group of the zeolites and both of these minerals crystallize in the hexagonal system. One of the main differences between erionite and offretite is in the framework stacking sequences. Offretite contains infinite channels surrounded by 12-membered rings, while the framework of erionite includes stacking faults. The channel constituents of zeolites in relation to crystal structure play a large role in determining crystal optics (optic sign, 2V, refractive index, and sign of elongation). Because of the ability of zeolites to hold H₂O in their channels, there have been a few studies which have looked at the relationship of the dehydration and subsequent hydration of zeolites in relation to changes in refractive index. By further analyzing these changes in coincidence with changes in crystal chemistry, better constraints can be put on characterizing the differences between erionite and offretite. Erionite has gained attention in the news recently due to its ties with mesothelioma. It is important to gain a better understanding of both the chemical and physical properties of erionite and offretite in order to dispel any future mischaracterizations of these two minerals, since erionite is currently not regulated by the EPA.