Prismatic to Asbestiform Offretite from Northern Italy: Occurrence, Morphology and Crystal-Chemistry of a New Potentially Hazardous Zeolite

Abstract

A multi-methodological approach, based upon field investigation, morphological characterization, chemical analysis and structure refinement was applied to different samples of fibrous offretite, a new potentially hazardous zeolite recently discovered in northern Italy. Their morphology ranges from stocky-prismatic to asbestiform. All the investigated fibers may be considered as "inhalable", and they are well within the range of the "more carcinogenic fibers" regarding diameter. As regards the length, the main mode observed in the asbestiform samples is 20-25 µm, and~93% of the measured fibers are >5 µm and may be significantly associated with carcinogenesis also in terms of lengths. The chemical-structural features of the investigated fibers are comparable: the extra-framework cations K + , Mg 2+ and Ca 2+ are present in all samples in similar proportions, and refined cell parameters are similar among the samples. Offretite occurs in 60% of the investigated sites, with an estimated amount up to 75 vol % of the associated minerals. The presence of this mineral could be of concern for risk to human health, especially if one considers the vast number of quarries and mining-related activities that are operating in the zeolite host rocks.

Real-Time Observation of Fibrous Zeolites Reactivity in Contact with Simulated Lung Fluids (SLFs) Obtained by Atomic Force Microscope (AFM)

“…The average chemical composition of the natural offretite sample (FF), based on 7 points analysis, is (K 1.85 Mg 1.55 Ca 2.03 )·(Al 8.99 Si 27.01 O 72 ]·28.57H 2 O. Our new data acquired on the FF sample are consistent with the data reported by [15] for the same sample. The point analyses performed on visible particles (at SEM images) on the surface of natural crystals showed the presence of Fe, P and Cl.…”

“…The FF sample is offretite originating from Nogare (VI), in the Lessini Mountains (northern Italy), where it occurs as a secondary mineral in vesicles of basalts and basanites [41]. Morpho-chemical and mineralogical characteristics of the FF sample were preliminarily studied by X-ray powder diffraction and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) by [15]. However, due to the abundance of other mineralogical phases (mainly chabazite and calcite), an accurate structural refinement through X-ray powder diffraction could not be obtained.…”

“…To minimize the alkali migration, as suggested by [47], chemical data were acquired using a low counting time, up to 30 s for each measurement. The reliability of the chemical analyses of both offretite and erionite crystals were evaluated by using the charge balance error formula E%, [48], the K-content test [15,49], and the Mg-content test [50].…”

“…The FF sample is composed of prismatic crystals of offretite with different sizes, as described in [15], with compact well-shaped hexagonal or pseudo-hexagonal individuals, while others show a complex network of fractures. The crystal size ranges from~50 µm to~150 µm in diameter and from 100 µm to~300 µm in length.…”

“…Offretite typically occurs as fine microscopic crystals of hydrothermal origin clustered in vugs and veins of basic volcanic rocks, but it Minerals 2019, 9,83; doi:10.3390/min9020083 www.mdpi.com/journal/minerals Minerals 2019, 9,83 2 of 23 has also been found in cavities of arenaceous metasiltites or hornfels [14]. Along with commonly observed prismatic habit, asbestiform offretite has also been found [15]. Due to chemical and structural similarities and possible intergrowth, the distinction between erionite and offretite can be challenging.…”