The structure of volcanic cristobalite in relation to its toxicity; relevance for the variable crystalline silica hazard

Abstract

Background: Respirable crystalline silica (RCS) continues to pose a risk to human health worldwide. Its variable toxicity depends on inherent characteristics and external factors which influence surface chemistry. Significant population exposure to RCS occurs during volcanic eruptions, where ashfall may cover hundreds of square km and exposure may last years. Occupational exposure also occurs through mining of volcanic deposits. The primary source of RCS from volcanoes is through collapse and fragmentation of lava domes within which cristobalite is mass produced. After 30 years of research, it is still not clear if volcanic ash is a chronic respiratory health hazard. Toxicological assays have shown that cristobalite-rich ash is less toxic than expected. We investigate the reasons for this by determining the physicochemical/structural characteristics which may modify the pathogenicity of volcanic RCS. Four theories are considered: 1) the reactivity of particle surfaces is reduced due to co-substitutions of Al and Na for Si in the cristobalite structure; 2) particles consist of aggregates of cristobalite and other phases, restricting the surface area of cristobalite available for reactions in the lung; 3) the cristobalite surface is occluded by an annealed rim; 4) dissolution of other volcanic particles affects the surfaces of RCS in the lung.

Methods: The composition of volcanic cristobalite crystals was quantified by electron microprobe and differences in composition assessed by Welch's two sample t-test. Sections of dome-rock and ash particles were imaged by scanning and transmission electron microscopy, and elemental compositions of rims determined by energy dispersive X-ray spectroscopy.

Results: Volcanic cristobalite contains up to 4 wt. % combined Al(2)O(3) and Na(2)O. Most cristobalite-bearing ash particles contain adhered materials such as feldspar and glass. No annealed rims were observed.

Conclusions: The composition of volcanic cristobalite particles gives insight into previously-unconsidered inherent characteristics of silica mineralogy which may affect toxicity. The structural features identified may also influence the hazard of other environmentally and occupationally produced silica dusts. Current exposure regulations do not take into account the characteristics that might render the silica surface less harmful. Further research would facilitate refinement of the existing simple, mass-based silica standard by taking into account composition, allowing higher standards to be set in industries where the silica surface is modified.

Figure 1

Backscattered electron (BSE) images showing cristobalite textures in thin section. a) Cristobalite crystal showing typical ‘fish-scale’ cracking in MVO1236. The boundary between the fish-scale cristobalite and surrounding groundmass is unclear. One platy crystal is protruding from a pore to the right, centre of the image; b) ‘Feathery’ groundmass texture (indicated by arrows) in MVO617, composed of cristobalite, glass and plagioclase feldspar (Horwell et al. [12]) with associated fish-scale cristobalite which, in this case, is probably a product of extensive devitrification and also has a diffuse boundary.

Figure 2

Al₂O₃vs. SiO₂(wt. %) for prismatic and platy cristobalite, and devitrified and magmatic quartz. Data derived from electron microprobe analyses of individual crystals for all dome-rock samples. No data for devitrification cristobalite as ‘feathery’ crystals were too small for the resolution of the microprobe.

Figure 3

BSE images of resin mounted and polished cristobalite particles separated from volcanic ash sample MBA12/7/03.a) A particle composed solely of cristobalite; b) A particle composed of a cristobalite crystal still attached to the groundmass patch from which it nucleated; c) A particle of ‘feathery’ devitrified groundmass with a totally-devitrified cristobalite patch in the centre; d) A particle of cristobalite with a ‘rim’ of glassy groundmass.

Figure 4

FEG-TEM image of thinned edge of a cristobalite crystal from volcanic ash sample MRA5/6/99. The platinum (Pt) strip shows as dark grey on the right hand side of the image with the mottled area being attributed to crystal thinning or background scattering from the Pt strip. The dashed black line represents the approximate boundary between the crystal and the strip. Superimposed on the image are the EDS results from a 50 nm transect from rim towards the core of the crystal. Pt = dark blue; Si = red; Al = green; K = pink; Fe = turquoise; Cl = light blue; O = yellow.