Multi-method characterization of Asbestiform Antigorite from South Australia

Antigorite occurs in several locations worldwide and is defined as the simple serpentine Mg₃Si₂0₅(OH)₄ polymorph that compensates for the silica tetrahedral layer (Si₂O₅ sheet) misfit with the octahedral brucite Mg(OH)₂ layer by regular structural undulation down the “b” crystalline axis. The chrysotile polymorph of the same chemistry compensates for the misfit in full rolls or scrolls along “b” axis, which can form fine fibres, and is the most common mineral in commercial asbestos. Chrysotile is therefore always asbestiform, but antigorite can exhibit a variety of crystalline habits in hand sample and the microscope, from massive, to platy, to bladed, and (occasionally) fibrous, but is not regulated as asbestos.

Here, we performed a multi-method study in order to characterize a highly fibrous asbestiform occurrence of antigorite from Rowland Flat, South Australia. In comparison to HSE Canadian chrysotile, we show that optical (PLM) and transmission electron microscopy (TEM) allows to distinguish this antigorite from chrysotile. Based on PLM analysis with CS dispersion staining, chrysotile is magenta parallel (ǁ) to the polarizer and exhibits blue perpendicular (Ʇ) colours while this antigorite exhibits gold to golden magenta ǁ and blue-magenta Ʇ colours (1.550 HD RI at 25^◦C). Also, we demonstrate SAED-TEM patterns for the two specimens to distinguish chrysotile “enrolled” crystal structure from the undulating antigorite structure, focusing on the highly characteristic [110] zone rel-rod streaking versus systematic repeats.

Scanning electron microscopy (SEM) images were collected to further investigate the asbestiform nature of the antigorite from Rowland Flat. We show that the dominant morphology of Rowland Flat antigorite is of microscopic laths that split into very thin needles. In order to further investigate the morphometric parameters of this fibrous antigorite, individual fibres were measured at 20,000-25,000x by TEM. We show that the majority of these structures meet or exceed WHO fibres criteria, display high mean aspect ratios, ranging from 20:1 up to more than 100:1, and display widths lower than 1 µm, which in the literature is definition for asbestos fibres.

Finally, our data indicate that the antigorite from Rowland Flat exhibits all characteristics of the asbestiform habit as defined in international standards (i.e., EPA/600/R-93/116), supporting the need for regulation of asbestiform antigorite as asbestos.