PRISMATIC TO EXTREMELY FIBROUS OFFRETITE FROM NORTHERN ITALY: MORPHOLOGICAL AND CHEMICAL DATA OF A POTENTIALLY HAZARDOUS ZEOLITE

Offretite, a quite rare zeolite, is hexagonal with space group symmetry P6m2 and unit-cell parameters a = 13.27-13.32 Å, c = 7.56-7.61 Å, and has a chemical formula of KCaMg[Al5Si13O36]·16H2O. Offretite forms simple hexagonal prisms with pinacoid terminations and the habit is commonly acicular (Passaglia et al., 1998). Due to the structural and chemical similarities with erionite (a mineral recognized to be highly carcinogenic), and because of the possibility of intergrowth of these two species within each crystal, their distinction can be very difficult. The most significant discrimination is based on the Mg/(Ca+Na) cation ratio. Notwithstanding offretite has been reported in various localities from Italy (Passaglia & Tagliavini, 1994; Passaglia et al., 1996, 1998; Boscardin et al., 1998; Guastoni et al., 2002; Mattioli et al., 2016a), the morphologies of offretite crystals have not yet been fully understood and many mineralogical aspects are still unknown. Here we present new morphological and chemical data of offretite from Northern Italy with two main different habits. The type-1 habit ranges from stocky-prismatic to very thin, needle-like prisms, and thus very similar to those described in the literature for the other Italian offretite. The type-2 habit is very different and is constituted by extremely thin fibers, often with asbestiform appearance, that show a strong tendency to break down into small fibrils, which could potentially be of inhalable size. Preliminary ESEM/EDS analysis on the offretite fibers revealed the occurrence of Mg, K, and Ca as extra-framework cations, and a Mg/(Ca + Na) cation ratio of ≈ 1. Recent researches on the surface properties of fibrous zeolites (Mattioli et al., 2016b) showed that interaction ability of offretite surface is much lower than that found for erionite, but we can not exclude the fibers of offretite from those able to cause some toxic effect on health. In fact, it is unclear whether the mineralogical distinction between erionite and offretite has any health implications. However, as already seen for the case of asbestos minerals, codification of nomenclature such as specific mineral names or habits into laws or regulations may have consequences in the application of health and legal policy. These data suggest the need for a better understanding of the potential toxicity across the range of erionite and offretite compositions