AMPHIBOLES FROM NEW IDRIA, CALIFORNIA
Chrysotile asbestos was mined at New Idria until 1993. The serpentinite lies in a remote area that has also been a mecca for ATV riders and hunters, as well as prospectors and mineral collectors. The district lies on public land, managed by the BLM. In May, 2008, an EPA study reported trace amounts of tremolite asbestos in dust samples, and the BLM closed the district to public access forthwith, citing increased health risks. While the health risks of incidental or non-occupational exposure to chrysotile asbestos are extremely low, there is general consensus that amphibole asbestos exposure presents a greater risk.
A field study in May, 2010, resulted in 63 georeferenced samples: hard rock samples from resistant outcrops and dust samples from unconsolidated material that is typical of the area. Control samples were gathered outside the serpentinite itself. Hard rock samples were analyzed by electron microprobe. Forty two dust samples were analyzed using XRD to identify the presence of amphiboles. Ten of the 42 samples contained detectible quantities of amphiboles. The majority of the control samples analyzed by XRD contained trace levels of amphiboles. The presence of amphiboles did not correlate with major quantities of serpentine group minerals. However the presence of amphiboles did correlate with quartz, plagioclase, and potassium group feldspars. This suggests that amphiboles observed in the area are not necessarily related to the serpentinite and may come from an external source.
The New Idria serpentinite is a tectonic melange containing blocks of several types, most commonly Franciscan blueschists, greenschists and greywackes. There are also intrusive kaersutite syenites. The spatial occurrence of amphiboles at New Idria correlates directly with these features. Amphiboles associated with tectonic blocks include crossite and non-fibrous actinolite. Riprap used for road maintenance contains hornblende. No naturally occurring fibrous amphiboles were found in this study. However, amphibole asbestos insulation was found in old mining furnaces, a legacy of 150 years of mining.