Asbestiform Minerals of the Franciscan Assemblage in California with a Focus on the Calaveras Dam Replacement Project

R. MARK BAILEY* Asbestos TEM Laboratories, Inc., 600 Bancroft Way, Suite A, Berkeley, CA 94710

Key Terms: Asbestos, NOA, Glaucophane, Blueschist, Amphibole, Franciscan

ABSTRACT

The San Francisco Bay Area is underlain by bedrock of the Franciscan Assemblage, which outcrops in numerous places. A significant portion of these outcrops consists of rock types that contain both regulated and unregulated asbestiform minerals, including ultra-mafic serpentinites, various greenstones, amphibolites, blueschist, and other schists (talc-tremolite, actinolite, etc.). These rocks are a legacy of tectonic activity that occurred on the west coast margin of the North American plate ∼65–150 MY ago during subduction of the East Pacific and Farallon plates. The Calaveras Dam Replacement Project (CDRP), located in Fremont, California, is an example of an area within the Franciscan Assemblage that is substantially underlain by metamorphosed oceanic sedimentary, mafic, and ultra-mafic rocks in a tectonic subduction zone mélange with highly disrupted relationships between adjoining rock bodies with different pressure/temperature metamorphic histories. In order to protect the health of workers and residents in the surrounding area, an extensive effort was taken to identify, categorize, and monitor the types, locations, and concentrations of naturally occurring asbestos at the site. Using a combination of geologic field observations and transmission electron microscopy, energy dispersive X-ray, and selected area electron diffraction analysis of airborne particulate and rock/soil samples, the CDRP was discovered to contain chrysotile-bearing serpentine. It also had as a range of amphibole-containing rocks, including blueschist, amphibolite schist, and eclogite, with at least 19 different regulated and non-regulated fibrous amphibole minerals identified. The extensive solid solution behavior of the amphiboles makes definitive identification difficult, though a scheme was created that allowed asbestos mineral fingerprinting of various areas of the project site.

*Corresponding author email: mark@asbestostemlabs.com

INTRODUCTION The Calaveras Dam site lies at the south end of the Calaveras Valley (Figure 1) in Fremont, California, within the Diablo Range, one of the major California coastal ranges. Bedrock at the site consists of Franciscan Assemblage and Great Valley Sequence rocks. In 2001, the existing dam, which was not engineered for earthquake safety, was found by the California Department of Water Resources, Division of Safety of Dams, to have an unacceptably high potential to fail during a maximum seismic event from nearby faults to the west: the Calaveras (∼¼ mi), the Hayward (∼5 mi), or the San Andreas (∼20 mi). As a result, the water level at the dam was lowered to 40 percent of capacity, a considerable loss of water storage capacity for the San Francisco Public Utility Commission, particularly as the Calaveras Reservoir is the second-largest source of freshwater for the city of San Francisco. In 2011, the Calaveras Dam Replacement Project (CDRP) was undertaken to build a new dam downstream of the old dam and minimize the risk of potential dam failure.

Prior to dam construction, a geotechnical drilling program was performed to determine the rock types and competency of the proposed dam bedrock, which generated a large amount of drill core for study. Because Franciscan Assemblage serpentine was observed on geologic maps of the nearby area (Figure 2), indicating the potential presence of chrysotile naturally occurring asbestos (NOA), approximately 50 sections of drill core were submitted to Asbestos TEM Labs in Berkeley, California. Testing was conducted by transmission electron microscopy (TEM) bulk sample analysis to determine the asbestos content if any.

As expected, chrysotile asbestos was detected within several of the drill cores (Figure 3). However, an unexpected finding was that numerous samples were found to contain significant concentrations of a suite of highly fibrous amphiboles (Figure 4) with chemistry, determined by energy dispersive X-ray (EDX) analysis (Figure 5), to be similar to riebeckite but with an exceptionally high aluminum and sometimes calcium content. Fibrous amphiboles of these chemical compositions are not classified by the

Figure 1. Geologic map of California showing distribution of Franciscan Assemblage (light blue) (Irwin, 1990), modified.

International Mineralogical Association (Leake et al., 1997) as minerals that are regulated as asbestos per both U.S. and California regulations, even though they are in a solid solution series with them and have similar morphology. Following this discovery, a careful review of the scientific literature on the glaucophane-riebeckite solid solution series (Deer et al., 1997; Leake et al., 1997; and National Institute for Occupational Safety and Health [NIOSH], 2005), as well as the documented

Figure 2. Geologic map of Calaveras Dam site (Dibblee and Minch, 2005). Jurassic/Cretaceous Franciscan rocks. sp = serpentinite; f= shale; fm = mélange; gl = blueschist mixed with greenstone; fs = greywacke sandstone; Kp/Kps = Cretaceous Panoche formation (marine shale and sandstone); Tt = Tertiary Temblor sandstone; Tso = Tertiary Sobrante sandstone; Tm = Tertiary Monterey/Claremont formation; Tbr = Tertiary Briones sandstone; Qls = Quaternary landslide; Qa = Quaternary alluvium; af = artificial fill. Approximate new dam location in red.

widespread occurrence of glaucophane blueschists in the Franciscan ranges, led to the determination that the dominant material was either a highaluminum riebeckite or a mix of glaucophane and Feglaucophane. These amphiboles had not been previously recognized as occurring in an asbestiform habit in the literature on NOA occurrences (CARB, 1991; Van Gosen, 2007; and Van Gosen and Clinkenbeard,

Figure 3. TEM photomicrograph of a bundle of chrysotile asbestos fibers.

Figure 4. TEM photomicrograph of glaucophane asbestos fibers. Note slender width (∼0.25 μm), curvature, and high aspect ratios >10:1.

Figure 5. EDX spectra collected from amphibole fibers. Note the lack of Al in (a) NIST crocidolite standard but its presence in all CDRP spectra; apparent solid-solution substitution of Fe 3+ and Al between (b) glaucophane and (c) Fe-glaucophane; presence of significant Ca in (d) winchite.

2011). Later, with further analysis, aluminum-rich varieties of the sodic-calcic amphibole mineral species winchite and Fe-winchite were also identified though in significantly lower concentrations. Unlike glaucophane and Fe-glaucophane, winchite and Fe-winchite have been found to occur in an asbestiform habit at the EPA Superfund site at Libby, Montana, within the vermiculite ore body of the former W.R. Grace mine.

Because EDX is not considered a fully quantitative technique for detailing amphibole mineral identification and the exact ratio of Fe 3+ to Al was unknown, the choice was made by the lab and the consulting geologist on the project to initially designate the fibrous glaucophane/Fe-glaucophane material as “high-aluminum crocidolite”. This decision was based on the International Mineralogical Association (IMA) (IMA 1978) definition of crocidolite as any asbestiform sodic amphibole (Leake, 1978). TEM quantitative bulk concentrations of these sodic amphiboles were found to range to over 10 wt.%. This was of considerable concern and led to a number of studies in the field and in the lab to determine its extent at the site. One particularly interesting finding was the presence of greenstone and blueschist intermixed on all scales, making it virtually impossible to separate the two (Figures 6 and 7).

As excavation for the new dam proceeded, it became evident that the rocks of the right abutment and

Figure 6. Blueschist and greenstone boulders used as waterside armoring of old Calaveras Dam.

base of the new dam consisted of Franciscan mélange block-in-matrix rocks of non–NOA-containing rocks (greywacke, siliceous schist, and eclogite) and potentially NOA-containing rocks (greenstone, blueschist, serpentinite, actinolite amphibolite, and actinolite schist) in a shale matrix (Figure 8). Within the literature on the Franciscan Assemblage, it has been observed that tectonic mélange zones frequently contain rocks such as those found at the CDRP dam site that have been subducted to considerable depth (Figure 9) under high-pressure/low-temperature conditions (Wakabayashi and Unruh, 1995). At the CDRP dam site, greenstone and blueschist are the hosts that commonly contain the fibrous minerals glaucophane, Fe-glaucophane, winchite, and Fewinchite. Notably, most of the glaucophane/Feglaucophane fibers would fall into the pre-1994 IMA amphibole mineral crossite solid-solution field designation, which included parts of the current glaucophane, Fe-glaucophane, riebeckite, and Mg-riebeckite chemical solid-solution fields (Leake, 1978).

Numerous authors have studied the Franciscan blueschists, a few of whom are listed here (Smith, 1906; Ernst, 1984; Wakabayashi, 2011; Wassmann and Stöckhert, 2012; Kim et al., 2013; and Erskine and Bailey, 2018). While several have implied that the material in some cases might be fibrous, noting highaspect-ratio glaucophane particles (Kim et al., 2013) or making direct observations by scanning electron microscopy (SEM) of glaucophane needles (Wassmann and Stöckhert, 2012), the author is the first to have characterized it as asbestiform (Erskine and Bailey, 2018).

One of the first clues as to full the extent of glaucophane/winchite fibrosity occurred in 2013 when field emission scanning electron microscopy (FESEM)

Figure 7. Blueschist-greenstone mylonite cut parallel to foliation viewed in plane polarized light. Glaucophane fibers (pleocroic blue  γ) oriented approximately east to west intermixed with rounded gray-green greenstone fragments. Low magnification, fibers subvisible.

analysis was performed on fracture surfaces of several blueschist rock samples. The results were startling, showing virtually the entire rock surface covered with fibers (Figure 10), and the observed fibers showed all the classic characteristics of asbestos (Figure 11): High-aspect-ratio fibers Bundles with split ends Curving fibers Fibers/bundles with lengths >5 μm

In addition to fibrous glaucophane, low concentrations of other fibrous calcic amphibole species were identified, including asbestiform actinolite as well as elongate particles of hornblende that met the CARB 3:1 aspect ratio to be counted as asbestos. In particular, actinolite was found to occur as fibrous

Figure 8. Right dam abutment hillcut in riprap Franciscan mélange with three types of asbestos-bearing rocks underlined.

Figure 9. Cross section of hypothesized Pacific–North American plate margin ∼100 MY BP in the area of central California, looking north. Oceanic Pacific plate (left) is subducting under the continental North American plate (right) (National Park Service, 2015).

overgrowths on large columnar non-asbestiform actinolite within an amphibolite sample as determined by polarized light microscopy and FESEM analysis (Figure 12).

Due to the presence of this wide range of fibrous amphiboles at the CDRP dam site, a comprehensive air monitoring program was developed to monitor the presence of fibrous minerals. At the site fibrous minerals were observed in extreme cases to reach concentrations as high as ∼2 fibers/cc PCME determined by

Figure 10. FESEM photomicrographs of asbestiform CDRP glaucophane fibers from a blueschist/greenstone rock fracture surface.

NIOSH 7402 TEM analysis, which is well over regulatory permissible exposure limits if considered as asbestos. Furthermore, it became apparent that similar rocks also occurred off-site that at times contributed independently to high airborne fiber concentrations. This led to an effort to individually identify and speciate all fibrous amphiboles, a laborious analytical task, to allow tracking of both on-site and off-site NOA emissions.

CONCLUSION

A wide range of asbestiform minerals have been identified at the CDRP site (chrysotile, glaucophane, Fe-glaucophane, winchite, Fe-winchite, actinolite, CARB 435–countable hornblende, and others) with the highest concentrations being found in a previously unidentified source of asbestos: blueschist/ greenstone metabasalt. The presence of the widespread occurrence of these fibrous amphiboles led to an intensive effort to understand the nature and extent of their presence as well as an intensive dust control and air monitoring effort to protect human health, including that of workers at the site and the public nearby. Blueschists are widely observed throughout the Franciscan Assemblage and should be expected to be found at future construction sites. It is important that these sites be identified to minimize the NOA hazard potential they represent during ground-disturbing activities.

Figure 11. Glaucophane-winchite fibers in blueschist viewed by FESEM. Sample cut parallel (a) and perpendicular (b) to foliation/lineation. Note the high concentration of sub-parallel fibers and fiber bundles with consistent fiber width.

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