INDUSTRY GUIDANCE

A. Erionite is a naturally occurring zeolite mineral. Zeolites are a family of hydrated aluminum-silicates. They form at low temperatures, generally at less than 150°C. Zeolite minerals are constructed from alumina and silica molecules linked together to form open cages which enclose cavities of various sizes and shapes. A characteristic feature of all zeolite minerals is that their cavities are interconnected to form channels that run through the mineral structure. Because of this property zeolites are commonly known as “molecular sieves”. In scientific terminology zeolites are classified as framework silicates.

The silica and alumina frameworks of zeolites have a negative charge which in nature is neutralised by hydrated sodium, potassium and/or calcium ions loosely held (adsorbed) onto the surfaces of the framework cages and cavities. In all zeolites, alkalis are exchangeable, that is they can move freely in and out of the cavities and/or be replaced by other molecules. The cavities also hold water and other natural fluids.

Erionite is a zeolite with a framework that has a high silica : alumina ratio, so it is most commonly formed from the alteration of felsic-siliceous (light coloured silica rich) tuffs and volcanic rocks (dacites and rhyolites) that have been deposited either in marine or saline lake environments, and then subject to low temperature (less than 100° C) alteration. Erionite is also found as crystals growing in vesicles and other cavities in basaltic and andesitic rocks. Erionite has, as far as we are aware, not been recorded in soils in New Zealand. Its colour varies from white to clear, and fibrous erionite looks like microscopic transparent chains of flexible glass-like fibres.

A. Like many minerals, erionite exists in different forms. When it is in amorphous or acicular (wooly) in form it is not considered to be hazardous. But fibrous forms (when the needle-like crystals are so fine as to be flexible) which fit the criteria of elongated mineral particles (or EMP), are a known type 1 carcinogen.

Like asbestos, erionite may pose health risks if inhaled and is associated with an increased risk of fibro-genic lung disease, lung cancer and malignant mesothelioma (a rare type of respiratory cancer usually related to asbestos exposure). Toxic effects have been documented in studies overseas.

Erionite can only affect human health when rocks or soils containing erionite are disturbed and erionite-bearing dust is inhaled. Therefore, erionite is hazardous if the fibres are small enough in size and shape to be respirable.

​​​A. Asbestos is a term used to refer to six naturally occurring silicate minerals, including the serpentine mineral, chrysotile (white asbestos), and the amphiboles, amosite (brown asbestos), crocidolite (blue asbestos), tremolite, anthophyllite and actinolite. Like the fibrous form of erionite, all six are composed of long and thin fibrous crystals, which are hazardous when breathed in. Chrysotile (white asbestos) was heavily used in industry in the past, but asbestos is now highly regulated globally due to its toxicity. Amosite and crocidolite asbestos are considered the most hazardous asbestos fibre types, but none of the 6 types of asbestos fibres are as potentially carcinogenic as erionite fibres.

In the past, occupational exposure to erionite has occurred during mining and other production operations overseas but unlike asbestos it had limited practical applications. Erionite is no longer mined or marketed for commercial purposes anywhere in the world.

The risk of exposure to erionite is therefore more similar to the risk of exposure to naturally occurring asbestiform minerals (the generic name for a group of minerals with similar morphology to asbestos but which are not classified as the 6 recognised as ‘asbestos’ minerals).

A. Fibrous erionite is believed to be very rare in Auckland, but has been identified in some tuff layers and basalt cavities.

Where erionite has been identified in Auckland it is either associated with volcanism or is a precipitate in hot springs. Tuff layers is thought to be an example of a former hot spring environment. The tuff layer was deposited toward the base of the Waitemata Group rocks (which are some 20 million years old).

Most of urban Auckland is underlain by Waitemata Group rocks. Although it is possible that the tuff beds that are occasionally found in the Waitemata Group sediments might contain erionite, any zeolitesed material in the majority of Waitemata sediments is primarily composed of the zeolite minerals phillipsite, clinoptilolite and analcime. 

When associated with volcanism, erionite is usually in cavities in marine basalts where volcanic glass has been devitrified and hydrated. The reaction products then crystalise (deposit) as zeolites.

Whichever erionite mode of formation has occurred, erionite is rare in the Auckland setting, and erionite so far appears to be very localised (but concentrated) rather than being widespread.

Greywacke rocks, the major aggregate resource for the Auckland region, are rocks that have been metamorphosed to temperatures around 200ºC, which is well above the stability limit for zeolites. However, zeolites do sometimes occur as infill of fractures within greywacke rocks. These zeolites are not known to include fibrous erionite.

This research project will help to confirm where erionite is present in the Auckland region.

A. The usual methods to identify erionite are lab-based and typically involve using X-Ray Diffraction (XRD) on a rock or soil sample. Then, scanning electron microscopy (SEM) with EDS or transmission electron microscopy (TEM) can be used to investigate the minerals further, including their shape, size and chemical composition. This is necessary to distinguish erionite from other zeolites which are more common, less toxic but have similar characteristics, such as mordenite. Field-based erionite testing apparatus for on-site use has yet to be developed for research or commercial purposes. We are currently working on low-cost screening tools to aid sample selection.

To identify which rocks may be worth testing in a laboratory, visual screening by a geologist can be appropriate. Zeolitised tuff horizons in the Waitemata Group rock are easy to recognise. They are cream coloured, fine-grained and massive and they are also more strongly cemented (i.e. harder than non-zeolitised rock types). These properties are in striking contrast to the enclosing “normal” Waitemata rocks which are grey or brownish-grey coloured, have alternating beds with variable grainsizes. We are currently preparing technical guidance to assist with accurate and effective sampling for erionite in the field. Please contact the research team for further information and guidance.

A. Fibrous erionite may be hazardous if disturbed, typically by being crushed or excavated (e.g. quarrying). This is particularly problematic in dry, dusty conditions. Where the ground is damp, fibres are less likely to enter the air and be breathed in. People who work in construction and who are disturbing erionite-bearing rock without stringent dust control measures would be at higher risk (fugitive dust from all sources is a generic and an intrinsic occupational health and safety risk in construction). In the USA, erionite exposure risks have been documented among off-road vehicle users, and for workers during tasks involving dirt road maintenance and forestry activities in lands known or suspected to have erionite. In the United States, erionite was found inside buses, although exposure risk is mainly thought to be for outdoor workers.

A. Because erionite is not regulated there is no formal list of appropriate individuals or organisations who can evaluate the presence of erionite. Please contact us at erionite@auckland.ac.nz if you suspect that you have erionite on your site and we will try to respond or direct you to suitable testing organisations.

A. There are no regulatory or consensus standards or occupational exposure limits for airborne erionite fibres. WorkSafe NZ guidance or the U.S. Occupational Safety and Health Administration’s guidance for working with asbestos could serve as a model for limiting the generation and inhalation of dust known or suspected to contain erionite.