Antigorite from Rowland Flat, South Australia: asbestiform character JOHN D. FITZ GERALD 1, * ,RICHARD A. EGGLETON 1 and JOHN L. KEELING 2 1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia *Corresponding author, e-mail: john.fitzgerald@anu.edu.au 2 Department of Primary Industries & Resources, Geological Survey, South Australia, PO Box 1671, Adelaide, SA 5001, Australia Abstract: Asbestiform antigorite from Rowland Flat near Adelaide in South Australia is shown by scanning and transmission electron microscopy to consist of bundles of [010] fibres. Defects combine with grain boundaries to define flattened fibres from 20 nm to 10 mm in width. Splitting along these microstructures leads to the asbestiform character. Within any individual crystal there are commonly two or more polysomes, indicating slight compositional variation within the crystal. Intergrowths of different polysomes are also associated with characteristic microstructures including [100] modulation dislocations. Also, the antigorite structures are everywhere incommensurate with polysome M ranging from 13 to 17. The fibrous habit appears related to the replacement reaction through which the antigorite phase has grown. Key-words: antigorite, asbestiform, modulation, SEM, TEM, microstructure, defect. 1. Introduction From the late 1940s to 1978, rock containing white asbes- tos, calcite and talc was extracted in a small quarry opera- tion near Rowland Flat, north-east of Adelaide in South Australia. Quarry product was sold to a specialist mineral milling company to produce fibrous filler for industrial use. The asbestos was originally described as amphibole (Ward, 1937) and subsequently as chrysotile (Hiern, 1976). During routine assessment of factors affecting rehabilita- tion of abandoned mine sites, a preliminary mineralogical report was prepared, which identified the asbestos as anti- gorite (Keeling et al., 2006). This was confirmed, and the geological setting, mineralogy and health implications were considered in Keeling et al. (2008). In this paper we report further electron microscope investigation aimed at understanding the nature and origin of the asbestiform habit of Rowland Flat antigorite. At Rowland Flat, antigorite is present as cross-fibre veins and as replacement of blocky aggregates of amphibole crys- tals throughout a former calcsilicate marble, now largely altered to serpentine and talc, with patchy calcite and chal- cedonic silica. The zone of alteration is 40–50 m wide, at least 750 m long and concordant with surrounding fine- grained quartzite and mica schist that strike north–south and dip steeply to the east. The calcsilicate is equated to silty, dolomitic carbonate lenses within Neoproterozoic Woolshed Flat Shale, an offshore marine sequence deposited during early sedimentation in the Adelaide Geosyncline. The sediments were folded and recrystallised under lower amphibolite metamorphic conditions during the Late Cambrian – Early Ordovician Delamerian Orogeny (Priess, 1993). The asbestos deposit is close to the Williamstown Fault where biotite-grade metamorphic rocks are faulted against andalusite–staurolite zone rocks that include the altered calcsilicate. Fluids expelled along the fault during latter stages of the orogeny are probably responsible for the serpentine-talc alteration (Keeling et al., 2008). Early workings on vein asbestos were abandoned in favour of an area of weathered calcsilicate where asbesti- form antigorite is partially leached of secondary calcite to give masses of separable fibres in a clayey, talcose matrix. Keeling et al. (2008) showed that the asbestos from this site was composed of antigorite and concluded that it formed by hydrothermal alteration of tremolite. The appearance of this material, in hand specimen, under optical microscopy and in scanning and transmission electron microscopy all confirm its asbestiform habit, a morphology rarely reported for anti- gorite. However, Viti & Mellini (1996) characterised vein antigorite from Elba which they described as of fibrous habit with ‘‘the splintery texture direction, that corresponds to [010]’’. The material, when cut perpendicular to the fibre axis, was bladed with ‘‘maximal dimensions in the order of 8  60 mm’’ and ‘‘randomly rotated about [010]’’. Viti and Mellini also summarised a few reports of asbestiform habit in other antigorites. Most recently, Groppo & Compagnoni (2007) and Cardile et al. (2007) described asbestiform anti- gorite, also sourced from veins. Ribeiro Da Costa et al. (2008) described fibrous-antigorite-bearing serpentinites from ocean-ridge shear zones. 0935-1221/10/0022-2045 $ 4.05 DOI: 10.1127/0935-1221/2010/0022-2045 # 2010 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart Eur. J. Mineral. 2010, 22, 525–533 Published online June 2010