THE "IDAHO METHOD": SETTING THE SCALE FOR IDENTIFICATION OF FIBROUS AND ACICULAR ZEOLITES

The need for a correct identification method for the fibrous/acicular zeolite erionite has become more in demand due to erionite’s ties to mesothelioma in Turkey, and now in the western USA. The regulatory community still struggles to correctly identify potentially asbestiform minerals when they occur in the natural rather than built environment. Erionite presents a unique challenge because there is a lack of regulatory method for the correct identification of these fibrous/acicular particles in nature.

By definition zeolites are framework silicates with “open” structures composed of channels and cages that house extraframework cations and water. While different amphibole mineral species are mainly determined by composition instead, zeolites are first based on different structure types and second on composition. To further complicate matters there are three general methods to describe the structure: 1) framework codes, 2) secondary building units and 3) a combination of secondary building units and historical context. Simplified identification techniques can be obtained largely by the use of SAED on the TEM and PLM. It is also important to note the associated environments of formation for these zeolites, such as altered volcanic tuff vs. vesicle.

The use of TEM and PLM for the identification of these fibrous and acicular zeolites is largely scale-dependent. Due to the morphology of these elongate zeolites, the c crystallographic axes will lie in the plane of view, making particle orientation simpler. Smaller TEM-scale provides direct structural data in the form of diffraction patterns, and gives compositional data in the form of EDS (i.e. diffraction patterns of erionite along [uv0] will show constraints of 00l = 2n along c* due to space group symmetry, while offretite will show no diffraction constraints along c*). For larger PLM-scale, zeolites can be distinguished based on their low birefringence and refractive index values (i.e. there is no refractive index overlap for erionite and offretite, and erionite is one of the only length slow fibrous/acicular zeolites). Fibrous and acicular zeolite identification does not need to be overcomplicated. By using scale dependent identification methods such as TEM and PLM, these zeolites can be easily distinguished from one another.