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The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota
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Regulatory Toxicology and Pharmacology 52 (2008) S43–S50 www.elsevier.com/locate/yrtph
The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota Malcolm Ross *, Robert P. Nolan, Gordon L. Nord Center for Applied Studies of the Environment, The Graduate School and University Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA Received 25 September 2007 Available online 16 October 2007
Abstract Asbestos crystallizes within rock formations undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Some of these conditions have prevailed during formation of the taconite iron ore deposits in the eastern Mesabi Iron Range of Minnesota. This range includes the Peter Mitchell Taconite Mine at Babbitt, Minnesota. The mine pit is over 8 miles long, up to 1 mile wide. Fifty three samples were collected from 30 sites within areas of the pit where faulting, shearing and folding occur and where fibrous minerals might occur. Eight samples from seven collecting sites contain significant amounts of ferroactinolite amphibole that is partially to completely altered to fibrous ferroactinolite. Two samples from two other sites contain ferroactinolite degraded to ropy masses of fibers consisting mostly of ferrian sepiolite as defined by X-ray diffraction and TEM and SEM X-ray spectral analysis. Samples from five other sites contain unaltered amphiboles, however some of these samples also contain a very small number of fiber bundles composed of mixtures of grunerite, ferroactinolite, and ferrian sepiolite. It is proposed that the alteration of the amphiboles was caused by reaction with water-rich acidic fluids that moved through the mine faults and shear zones. The fibrous amphiboles and ferrian sepiolite collected at the Peter Mitchell Mine composes a tiny fraction of one percent of the total rock mass of this taconite deposit; an even a smaller amount of these mineral fragments enter the ambient air during mining and milling. These fibrous minerals thus do not present a significant health hazard to the miners nor to those non-occupationally exposed. No asbestos of any type was found in the mine pit. 2007 Elsevier Inc. All rights reserved. Keywords: Taconite; Asbestos; Amphibole; Reserve mining; Tailings; Mesabi iron range; Mineral fibers
1. Introduction 1.1. Origin of the asbestos Asbestos crystallizes under very special conditions that occur within rock formations that are undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Such deformations are often accompanied by the intrusion of magmatic fluids that solidify to form dikes and sills. The fibers crystallize in high strain environments, such as within folds, shear planes, faults, dilation cavities, and at intrusion-host rock boundaries. Fibers
*
Corresponding author. Fax: +1 800 709 0028. E-mail address: [email protected] (M. Ross).
0273-2300/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.yrtph.2007.09.018
can crystallize from solutions moving within the fault and shear zones; the fibers being oriented parallel to the two rock faces that compose the shear or slip plane—thus the term slip fiber. Slip fiber growth can also occur during folding of layered rocks for the folding process causes a shearing between adjacent layers. Fibers can also crystallize from a fluid phase within cracks formed when the rock undergoes dilation due to tectonic stress, a process in which parallel cracks and fissures form open spaces within the rock. The process of folding in layered rocks can also produce openings or dilation cavities between adjacent layers. Such fibers nucleate on a wall of the crack or cavity and grow across to the opposite wall—thus the term crosses fibers (Ross and Nolan, 2003). Cross and slip fiber growth of crocidolite and amosite asbestos occurs in the intensely folded Precambrian banded
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ironstones of the Transvaal and Cape Provinces of South Africa. The asbestos deposits in the Cape Province are generally found within monoclinal folds. The crocidolite asbestos deposits of Western Australia are also found Precambrian banded ironstones, the deposits generally confined to the northern limb of a broad syncline of the Hamersley Range. Here, crocidolite asbestos grew within dilation cavities between folded iron formation bands. Asbestos-bearing serpentinites, such as those found in Quebec, Canada, commonly contain cross-fiber chrysotile asbestos that crystallized within dilation cavities. Slip fiber chrysotile asbestos is less common but is ubiquitous in the serpentinite located in Eden Mills, Vermont (Ross, 1981; Ross and Nolan, 2003). In our examination of numerous mines and construction sites within igneous and metamorphic terrain located in the eastern United States, Arkansas, Michigan, California, Cyprus, and Ontario and Quebec, Canada, we observe that asbestos occurs within faults and shear zones, in folds, or at contacts between igneous intrusions and the host rock. In any of these geologic features it is possible that other fibrous minerals can form as describe above. However, in most rock formations, other than those exploited for asbestos production, such non-asbestos fibers are restricted to a very small volume of rock.
1.2. Survey of the Peter Mitchell Mine for fibrous minerals With the above mentioned criteria in mind we made a detailed survey of the rocks within the Peter Mitchell Mine to see if any rocks contained asbestos or asbestos-like minerals; our results are reported below. The history of the asbestos controversy over exposure to mineral particulates, including those suspected to be asbestos, released during
the mining and milling of taconite iron ore is given by Berndt and Brice (2008) and Wilson et al. (2008). This asbestos controversy is particularly related to litigation involving the Reserve Mining Company which previously operated the Peter Mitchell Mine (Reserve Mining Company v USEPA, 514 F.2d 492, 8th Cir. 1975).
2. The Peter Mitchell Taconite Mine 2.1. Mine survey The Peter Mitchell Taconite Mine is owned by the Northshore Mining Corporation and is located approximately 3 miles south of Babbitt, Minnesota. The mine pit is over 8 miles long, up to 1 mile wide, and one to two hundred feet deep (Fig. 1). The mine is located within the Biwabik iron formation. In Table 1 are listed the minerals commonly found in this formation, especially important are the amphibole minerals which can sometimes occur in fibrous form. For detailed descriptions of the Minnesota iron formations see Jirsa et al. (2008) and McSwiggen and Morey (2008). Mark J. Severson (Technical Report NRRI/TR-93/24) states the at least 17 faults, five basaltic dikes, and several large-scale folds occur in the Peter Mitchell Mine pit. Over a five day period we surveyed and then collected samples from the areas within the pit where fibrous minerals might occur. Geological maps and structural analysis of the mine pit made by Mark J. Severson, augmented by a detailed map of the current working area, was used as a base to select and mark the sampling sites. The absence of any fibrous mineral growth in undeformed rock permitted us to sample only in those zones showing some form of rock deformation. As far as we know, ours is the only publication to make a detailed min-
Fig. 1. View of a portion of the Peter Mitchell Taconite Mine, Babbitt, Minnesota.
M. Ross et al. / Regulatory Toxicology and Pharmacology 52 (2008) S43–S50 Table 1 Names and composition of the major minerals and selected minor found in the Biwabik iron formation Mineral name
Chemical composition
Amphiboles Cummingtonite Grunerite Actinolite Hornblende
(Mg,Fe2+)7Si8O22(OH)2 (Fe2+,Mg)7Si8O22(OH)2 Ca2(Mg, Fe2+)5Si8O22(OH)2 Ca2(Na,K)(Mg,Fe2+,Fe3+Al)5(Si,Al)8O22(OH)2
Pyroxenes Diopside Pigeonite Hedenbergite Ferrohypersthene
CaMgSi2O6 (Mg,Fe2+,Ca)2Si2O6 CaFe2+Si2O6 (Fe2+,Mg)2Si2O6
Other silicates Quartz Wollastonite Vesuvianite Andradite Almandine Fayalite Olivine Biotite Plagioclase Greenalite Chamosite Minnesotaite Talc Stilpnomelane Ferrian sepiolite
SiO2 CaSiO3 Ca10(Mg,Fe)2Al4[Si2O7]2[SiO4]5(OH,F)4 Ca3(Fe3+,Ti)2Si3O12 (Fe2+)3Al2Si3O12 (Fe2+,Mg)2[SiO4] Mg2[SiO4] K2(Fe,Mg,Al)6[Si,Al]8O20(OH,F)4 (Na,Ca)Al(Al,Si)Si2O8 (Fe2+,Fe3+)2–3Si2O5(OH)4 (Fe2+,Mg,Fe3+)5Al(Si3Al)O10(OH,O)8 (Fe2+,Mg)3Si4O10(OH)2 Mg3Si4O10(OH)2 K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27 (Fe3+, Fe2+, Mg)8(Si, Fe3+)12(OH)4(H2O)n
Non-silcates Calcite Siderite Ankerite Magnetite Hematite Pyrite
CaCO3 FeCO3 Ca(Mg,Fe2+,Mn)(CO3)2 Fe2+(Fe3+)2O4 Fe2O3 FeS2
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in the area of site 10 (personal communication, 1998, Dennis Wagner Northshore’s environmental engineer who was present at the time of collection). We were able to obtained a portion of the original EPA sample from Dr. P. Cook for our present study. Site 10 is a particularly distinctive shear zone with much soft rock gouge lying at the base of the cliff face (Fig. 2). • Type 1 samples contain three distinct stages of crystal alteration. (1) Little altered composite ferroactinolitegrunerite crystals (Fig. 3) contain (1 0 0) and (1 0 1) exsolution lamellae of grunerite within the ferroactinolite portion of the crystal and (1 0 0) and (1 0 1) lamellae of ferroactinolite within the grunerite portion. (2) Unmixing and alteration can produce crystals in which the grunerite areas are relatively unaltered whereas the ferroactinolite areas are very fibrous in appearance (Figs. 4 and 5). (3) Ferroactinolite that is almost completely altered to a fibrous habit is shown in Fig. 6. (2) Type 2 samples. Type 2 samples, represented by 5C and 13E, were collected from sites 5 and 13. In sample 5C the original amphibole, ferroactinolite, is degraded to a ropy mass (Fig. 7), only small amounts of the original amphibole are left. This ropy material is a magnesium, iron and silica-rich mineral belonging to the clay mineral group. The X-ray powder pattern (Fig. 8) and transmission electron microscope X-ray spectra showing major Mg, Fe and Si identify
eralogical survey of the mine and to discover the source of the mineral particulates released from the Peter Mitchell Mine ore that are suspected to be asbestos. 2.2. Sample descriptions Fifty-three samples were collected at thirty sites from the Peter Mitchell Mine. Sampling was made particularly in fault and shear zones and within folds. In these areas the amphibole minerals within the rock were particularly susceptible to low temperature alteration due to infusion of rain water acidified by oxidation of sulfide minerals, these chemical processes being contemporaneous with rock shearing. We separated the 53 samples into four categories or types: (1) Type 1 samples. Type 1 samples are represented by 10B, 10C, 20A, 20B, 21A, 24A, and 24B (from sites 10, 20, 21 and 24) and the U.S. EPA’s ferroactinolite sample which was collected by IIT Research Institute (for the EPA) from the eastern end of the mine in 1975
Fig. 2. Site 10 shear zone within rock face of the Peter Mitchell pit. Soft rock gouge from this shear zone lies at the base of the cliff face (dark area upon which the mining geologist is standing). The gouge contains fibrous ferroactinolite and ferrian sepiolite.
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Fig. 3. Sample 10B-left: thick pristine grunerite lamella (arrow) oriented on the (10–1) plane of a relatively non-fibrous ferroactinolite crystal host. Nearly horizontal striations within the host ferroactinolite are due to (1 0 0) exsolution of grunerite. Sample 10B-right: Composite crystal of grunerite with (10–1) ferroactinolite exsolution lamellae trending N–S (area Y) and ferroactinolite (area X).
Fig. 4. Sample 10B. Scanning electron microscope photomicrograph of a composite crystal of grunerite (area X at lower left) and fibrous ferroactinolite (area Y at upper right). Note (10–1) exsolution lamellae of ferroactinolite within the grunerite area (arrows pointing to thin gray lines trending NW–SE). SEM X-ray spectra from areas X and Y are shown in Fig. 5.
this mineral as ferrian sepiolite (Fig. 11). The ropy masses, when subjected to lengthy ultrasonic treatment, are seen to be composed of a myriad of very thin ferrian sepiolite fibers as demonstrated by the TEM photomicrograph presented in Fig. 9. The chemical composition and crystal structure of ferrian sepiolite are not well understood, but an approximate chemical formula is: ðFe3þ ; Fe2þ ; MgÞ8 ðSi; Fe3þ Þ12 ðOHÞ4 ðH2 OÞn (3) Type 3 sample. The type 3 sample 30B from site 30 contains well crystallized ferroactinolite and grunerite and but also contains small amounts of brownish fibrous actinolite and brown mats of completely decomposed amphibole.
(4) Type 4 samples. Type 4 samples include 13C, 13F, 21B, and 29 and were collected from sites 13, 21, and 29. The amphiboles in these samples appear as blocky prismatic crystals with no fibrous structure. No fibers were detected in these samples. Table 2 gives a summary of these four sample types.
2.3. Geologic processes within the Peter Mitchell Mine We propose that water-rich fluids, acidified by oxidation of sulfide minerals, moved through the active mine faults and shear zones. Here chemical reactions, including oxidation of ferrous to ferric iron, occurred between the fluids and some of the constituent minerals, particularly with the grunerite and ferroactinolite amphiboles. In rocks con-
M. Ross et al. / Regulatory Toxicology and Pharmacology 52 (2008) S43–S50
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Fig. 5. SEM X-ray spectra analysis of grunerite taken in area X of Fig. 4 and of ferroactinolite taken in area Y of Fig. 4.
Fig. 6. Two photomicrographs showing mats of fibrous ferroactinolite (sample 24A). Small amounts of ferrian sepiolite and grunerite may also be present in these mats.
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Fig. 7. Two photomicrographs of sample 5C showing ropy masses resulting from the nearly complete alteration of primary grunerite and ferroactinolite amphiboles to ferrian sepiolite. The X-ray powder diffraction pattern of this sample is shown in Fig. 8.
Fig. 8. X-ray diffraction powder pattern of sample 5C (Fig. 7) identifying the sample as ferrian sepiolite. Copper ?a radiation. Note the characteristic 12.3 (2h = 7.2 degrees) and 7.5 (2h = 11.8 degrees) Angstrom lines of sepiolite. This X-ray pattern is a good match to that given by Brindley (1959, p. 498) for sepiolite from four localities.
taining the two amphibole assemblage ferroactinolitegrunerite, exsolution of grunerite on (10–1) and (1 0 0) planes of host ferroactinolite and of ferroactinolite on (10–1) and (1 0 0) planes of host grunerite are commonly observed. The very thick (10–1) lamellae of grunerite within the host ferroactinolite crystals of sample 10B (Fig. 3) indicates that this amphibole crystallized under high temperature conditions near the top of the ferroactinolite–grunerite solvus and thus contained a large grunerite component within the crystal structure (Ross et al., 1969, p. 291; Fig. 6). On slow cooling the ferroactinolite and grunerite components may segregate to form a crystal, such as shown in Figs. 3 and 4, where one end of the crystal is mostly grunerite, the other, mostly ferroactinolite. If the amphiboles had originally crystallized as true asbestos, textures such as seen in Figs. 3 and 4 would not have developed. For a review of exsolution processes occurring in amphiboles (see Ross et al., 1968, 1969). The transmission electron microscope photomicrograph presented in Fig. 10 reveals that (1 0 0) grunerite exsolution lamellae (now partly to completely altered to ferrian sepio-
lite) are present within a single fiber of the host ferroactinolite, the fiber being derived from a composite ferroactinolite–grunerite crystal such as that shown in area Y of Fig. 4. The TEM X-ray spectra of the interleaved ferroactinolite host and (1 0 0) grunerite /ferrian sepiolite lamellae (Fig. 10) are shown in Fig. 11. We suggest that the interfaces between the ferroactinolite host and the (1 0 0) exsolution lamellae of grunerite are places where reaction with fluids within the fault or shear zone fluids can readily occur. Decomposition of the grunerite lamellae within the ferroactinolite host (area Y, Fig. 4) to ferrian sepiolite could promote the formation of the fibrous textures seen in (Figs. 4 and 6). Where alteration is incomplete, some of the pristine amphibole remains. It also appears that some iron was removed from the amphibole grains during this weathering process to recrystallize as iron oxide, probably in the form of goethite [FeO(OH)] and appearing as brown masses within the ropy mats of ferrian sepiolite. A complete alteration of both grunerite and ferroactinolite could produce ferrian sepiolite and iron oxides such as found in the Type 2 samples.
M. Ross et al. / Regulatory Toxicology and Pharmacology 52 (2008) S43–S50
Fig. 9. Transmission electron micrograph of sample 5C showing ferrian sepiolite fibers that have formed from the alteration of primary amphiboles (width of field is 0.04 mm). The thinnest fibers are less than 0.1 lm in width.
Table 2 Summary description of the samples collected from various sites within the Peter Mitchell Taconite Mine (1) Type 1 samples. Type 1 samples are represented 10B, 10C, 20A, 20B, 21A, 24A, 24B and the EPA ferroactinolite. These samples contained ferroactinolite amphibole and composite grunerite-ferroactinolite amphibole that have partially or completely altered to very fibrous crystallites (2) Type 2 samples. Type 2 samples represented by 5C and 13E In sample 5C the original amphibole, ferroactinolite, is degraded to a ropy mass composed mostly of the clay mineral ferrian sepiolite. TEM examination of sample 5C shows to be composed of a myriad of very thin fibers (3) Type 3 sample. The type 3 sample 30B from site 30 contains well crystallized ferroactinolite and grunerite and but also contain small amounts of brownish fibrous actinolite and brown mats of completely decomposed amphibole (4) Type 4 samples. Type 4 samples include 13C, 13F, 21B, and 29. The amphiboles in these samples appear as blocky prismatic crystals with no fibrous structure
3. Conclusions The fibrous amphiboles and ferrian sepiolite collected in the Peter Mitchell pit composes a tiny fraction of one percent of the total rock mass within the mine. This fibrous ferroactinolite is a low temperature alteration product of non-fibrous amphibole; it does not occur in the manner of true asbestos which crystallizes as a primary mineral from hydrothermal solutions into open
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Fig. 10. Transmission electron microscope photomicrograph of a single fiber from a composite crystal of ferroactinolite and grunerite (sample 21A), This fiber is composed of alternating lamellae of ferroactinolite (lamella at X) and grunerite/ferrian sepiolite (lamella at Y) as identified by the respective TEM X-ray spectra. Lamellae within the fiber vary from 300 to 700 nm in thickness. Total thickness of the fiber is about 3000 nm. Width of field is about 6500 nm. TEM X-ray spectra collected from areas X and Y are shown in Fig. 11.
veins within deformed rock. The ropy mats of ferrian sepiolite are composed of inter-woven sub-micrometer sepiolite fibers. Due to inter-weaving of the fibers within the mats, individual fibers will be released only with great difficulty. Ultrasound was used to disperse the fibers for TEM analysis. Ferrian sepiolite has not appeared in the air samples collected in the taconite mill area and in the nearby town of Silver Bay (Wilson et al., 2008). The fibers collected during ambient air sampling at the taconite mill where the Northshore iron ore is processed and at the town of Silver Bay are not asbestos, but rather are non-asbestiform ferroactinolite and grunerite. The mean air concentration in Silver Bay is less than 0.00036 fibers per milliliter, a value within the expected background for airborne asbestos reported by the World Health Organization and the U.S. Environmental Protection Agency. The risk-related cancer to such an exposure is less than 0.77 excess cancer cases in 1,000,000 lifetimes (Wilson et al., 2008). Conflict of Interest The authors declare that they have no conflicts of interest.
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Fig. 11. SEM X-ray spectra analysis of ferroactinolite taken in area X of Fig. 10 and of grunerite/ferrian sepiolite taken in area Y of Fig. 10.
Acknowledgments We thank Dr. Ronald G. Graber, Dennis M. Wagner and Dennis Wagner for their help with the geological survey. We acknowledge support from a Higher Education Advance Technology grant from New York State and the International Environmental Research Foundation (www.ierfinc.org) of New York, New York and assistance from Cleveland-Cliffs, Cleveland, Ohio. References Berndt, M.E., Brice, W.C., 2008. The origins of public concern with taconite and human health: Reserve mining and the asbestos case. Reg. Tox. Pharm. 52, S31–S39. Brindley, G.W., 1959. X-ray and electron diffraction data for sepiolite. Am. Mineral. 44, 495–500. Jirsa, M.A., Miller, J.B., Morey, G.B., 2008. Geology of the Biwabik Iron Formation and Dulth Complex. Reg. Tox. Pharm. 52, S5–S10.
McSwiggen, P.L., Morey, G.B., 2008. Overview of the Mineralogy of the Biwabik Iron Formation, Mesabi Iron Range, Northern Minnesota. Reg. Tox. Pharm. 52, S11–S25. Ross, M., 1981. The geological occurrences and health hazards of amphibole and serpentine asbestos. In: Veblen, D.R. (Ed.), . In: Amphiboles and Other Hydrous Pyriboles-Mineralogy. Reviews in Mineralogy, vol. 9A. Mineralogical Society of America, Washington, DC, pp. 279–323. Ross, M., Papike, J.J., Weiblen, P.W., 1968. Exsolution in clinoamphiboles. Science 159, 1099–1102. Ross, M., Papike, J.J., Shaw, K.W., 1969. Exsolution textures in amphiboles as indicators of subsolidus thermal histories. Mineralogical Society of America Special Paper #2, 275–299. Ross, M., Nolan, R.P., 2003. History of asbestos discovery and use and asbestos-related disease in context with the occurrence of asbestos within ophiolite complexes. In: Dilek, Y., Newcomb, S. (Eds.), . In: Ophiolite Concept and Evolution of Geologic Thought, 373. Geological Society of America Special Publication, pp. 447–470. Wilson, R., McConnell, E.E., Nolan, R.P., Axten, C.W., Ross, M., 2008. Risk assessment due to environmental exposures to fibrous particulates associates with taconite ore. Reg. Tox. Pharm. 52, S232–S245.
Regulatory Toxicology and Pharmacology 52 (2008) S43–S50 www.elsevier.com/locate/yrtph
The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota Malcolm Ross *, Robert P. Nolan, Gordon L. Nord Center for Applied Studies of the Environment, The Graduate School and University Center of the City University of New York, 365 Fifth Avenue, New York, NY 10016-4309, USA Received 25 September 2007 Available online 16 October 2007
Abstract Asbestos crystallizes within rock formations undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Some of these conditions have prevailed during formation of the taconite iron ore deposits in the eastern Mesabi Iron Range of Minnesota. This range includes the Peter Mitchell Taconite Mine at Babbitt, Minnesota. The mine pit is over 8 miles long, up to 1 mile wide. Fifty three samples were collected from 30 sites within areas of the pit where faulting, shearing and folding occur and where fibrous minerals might occur. Eight samples from seven collecting sites contain significant amounts of ferroactinolite amphibole that is partially to completely altered to fibrous ferroactinolite. Two samples from two other sites contain ferroactinolite degraded to ropy masses of fibers consisting mostly of ferrian sepiolite as defined by X-ray diffraction and TEM and SEM X-ray spectral analysis. Samples from five other sites contain unaltered amphiboles, however some of these samples also contain a very small number of fiber bundles composed of mixtures of grunerite, ferroactinolite, and ferrian sepiolite. It is proposed that the alteration of the amphiboles was caused by reaction with water-rich acidic fluids that moved through the mine faults and shear zones. The fibrous amphiboles and ferrian sepiolite collected at the Peter Mitchell Mine composes a tiny fraction of one percent of the total rock mass of this taconite deposit; an even a smaller amount of these mineral fragments enter the ambient air during mining and milling. These fibrous minerals thus do not present a significant health hazard to the miners nor to those non-occupationally exposed. No asbestos of any type was found in the mine pit. 2007 Elsevier Inc. All rights reserved. Keywords: Taconite; Asbestos; Amphibole; Reserve mining; Tailings; Mesabi iron range; Mineral fibers
1. Introduction 1.1. Origin of the asbestos Asbestos crystallizes under very special conditions that occur within rock formations that are undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Such deformations are often accompanied by the intrusion of magmatic fluids that solidify to form dikes and sills. The fibers crystallize in high strain environments, such as within folds, shear planes, faults, dilation cavities, and at intrusion-host rock boundaries. Fibers
*
Corresponding author. Fax: +1 800 709 0028. E-mail address: [email protected] (M. Ross).
0273-2300/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.yrtph.2007.09.018
can crystallize from solutions moving within the fault and shear zones; the fibers being oriented parallel to the two rock faces that compose the shear or slip plane—thus the term slip fiber. Slip fiber growth can also occur during folding of layered rocks for the folding process causes a shearing between adjacent layers. Fibers can also crystallize from a fluid phase within cracks formed when the rock undergoes dilation due to tectonic stress, a process in which parallel cracks and fissures form open spaces within the rock. The process of folding in layered rocks can also produce openings or dilation cavities between adjacent layers. Such fibers nucleate on a wall of the crack or cavity and grow across to the opposite wall—thus the term crosses fibers (Ross and Nolan, 2003). Cross and slip fiber growth of crocidolite and amosite asbestos occurs in the intensely folded Precambrian banded
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ironstones of the Transvaal and Cape Provinces of South Africa. The asbestos deposits in the Cape Province are generally found within monoclinal folds. The crocidolite asbestos deposits of Western Australia are also found Precambrian banded ironstones, the deposits generally confined to the northern limb of a broad syncline of the Hamersley Range. Here, crocidolite asbestos grew within dilation cavities between folded iron formation bands. Asbestos-bearing serpentinites, such as those found in Quebec, Canada, commonly contain cross-fiber chrysotile asbestos that crystallized within dilation cavities. Slip fiber chrysotile asbestos is less common but is ubiquitous in the serpentinite located in Eden Mills, Vermont (Ross, 1981; Ross and Nolan, 2003). In our examination of numerous mines and construction sites within igneous and metamorphic terrain located in the eastern United States, Arkansas, Michigan, California, Cyprus, and Ontario and Quebec, Canada, we observe that asbestos occurs within faults and shear zones, in folds, or at contacts between igneous intrusions and the host rock. In any of these geologic features it is possible that other fibrous minerals can form as describe above. However, in most rock formations, other than those exploited for asbestos production, such non-asbestos fibers are restricted to a very small volume of rock.
1.2. Survey of the Peter Mitchell Mine for fibrous minerals With the above mentioned criteria in mind we made a detailed survey of the rocks within the Peter Mitchell Mine to see if any rocks contained asbestos or asbestos-like minerals; our results are reported below. The history of the asbestos controversy over exposure to mineral particulates, including those suspected to be asbestos, released during
the mining and milling of taconite iron ore is given by Berndt and Brice (2008) and Wilson et al. (2008). This asbestos controversy is particularly related to litigation involving the Reserve Mining Company which previously operated the Peter Mitchell Mine (Reserve Mining Company v USEPA, 514 F.2d 492, 8th Cir. 1975).
2. The Peter Mitchell Taconite Mine 2.1. Mine survey The Peter Mitchell Taconite Mine is owned by the Northshore Mining Corporation and is located approximately 3 miles south of Babbitt, Minnesota. The mine pit is over 8 miles long, up to 1 mile wide, and one to two hundred feet deep (Fig. 1). The mine is located within the Biwabik iron formation. In Table 1 are listed the minerals commonly found in this formation, especially important are the amphibole minerals which can sometimes occur in fibrous form. For detailed descriptions of the Minnesota iron formations see Jirsa et al. (2008) and McSwiggen and Morey (2008). Mark J. Severson (Technical Report NRRI/TR-93/24) states the at least 17 faults, five basaltic dikes, and several large-scale folds occur in the Peter Mitchell Mine pit. Over a five day period we surveyed and then collected samples from the areas within the pit where fibrous minerals might occur. Geological maps and structural analysis of the mine pit made by Mark J. Severson, augmented by a detailed map of the current working area, was used as a base to select and mark the sampling sites. The absence of any fibrous mineral growth in undeformed rock permitted us to sample only in those zones showing some form of rock deformation. As far as we know, ours is the only publication to make a detailed min-
Fig. 1. View of a portion of the Peter Mitchell Taconite Mine, Babbitt, Minnesota.
M. Ross et al. / Regulatory Toxicology and Pharmacology 52 (2008) S43–S50 Table 1 Names and composition of the major minerals and selected minor found in the Biwabik iron formation Mineral name
Chemical composition
Amphiboles Cummingtonite Grunerite Actinolite Hornblende
(Mg,Fe2+)7Si8O22(OH)2 (Fe2+,Mg)7Si8O22(OH)2 Ca2(Mg, Fe2+)5Si8O22(OH)2 Ca2(Na,K)(Mg,Fe2+,Fe3+Al)5(Si,Al)8O22(OH)2
Pyroxenes Diopside Pigeonite Hedenbergite Ferrohypersthene
CaMgSi2O6 (Mg,Fe2+,Ca)2Si2O6 CaFe2+Si2O6 (Fe2+,Mg)2Si2O6
Other silicates Quartz Wollastonite Vesuvianite Andradite Almandine Fayalite Olivine Biotite Plagioclase Greenalite Chamosite Minnesotaite Talc Stilpnomelane Ferrian sepiolite
SiO2 CaSiO3 Ca10(Mg,Fe)2Al4[Si2O7]2[SiO4]5(OH,F)4 Ca3(Fe3+,Ti)2Si3O12 (Fe2+)3Al2Si3O12 (Fe2+,Mg)2[SiO4] Mg2[SiO4] K2(Fe,Mg,Al)6[Si,Al]8O20(OH,F)4 (Na,Ca)Al(Al,Si)Si2O8 (Fe2+,Fe3+)2–3Si2O5(OH)4 (Fe2+,Mg,Fe3+)5Al(Si3Al)O10(OH,O)8 (Fe2+,Mg)3Si4O10(OH)2 Mg3Si4O10(OH)2 K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27 (Fe3+, Fe2+, Mg)8(Si, Fe3+)12(OH)4(H2O)n
Non-silcates Calcite Siderite Ankerite Magnetite Hematite Pyrite
CaCO3 FeCO3 Ca(Mg,Fe2+,Mn)(CO3)2 Fe2+(Fe3+)2O4 Fe2O3 FeS2
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in the area of site 10 (personal communication, 1998, Dennis Wagner Northshore’s environmental engineer who was present at the time of collection). We were able to obtained a portion of the original EPA sample from Dr. P. Cook for our present study. Site 10 is a particularly distinctive shear zone with much soft rock gouge lying at the base of the cliff face (Fig. 2). • Type 1 samples contain three distinct stages of crystal alteration. (1) Little altered composite ferroactinolitegrunerite crystals (Fig. 3) contain (1 0 0) and (1 0 1) exsolution lamellae of grunerite within the ferroactinolite portion of the crystal and (1 0 0) and (1 0 1) lamellae of ferroactinolite within the grunerite portion. (2) Unmixing and alteration can produce crystals in which the grunerite areas are relatively unaltered whereas the ferroactinolite areas are very fibrous in appearance (Figs. 4 and 5). (3) Ferroactinolite that is almost completely altered to a fibrous habit is shown in Fig. 6. (2) Type 2 samples. Type 2 samples, represented by 5C and 13E, were collected from sites 5 and 13. In sample 5C the original amphibole, ferroactinolite, is degraded to a ropy mass (Fig. 7), only small amounts of the original amphibole are left. This ropy material is a magnesium, iron and silica-rich mineral belonging to the clay mineral group. The X-ray powder pattern (Fig. 8) and transmission electron microscope X-ray spectra showing major Mg, Fe and Si identify
eralogical survey of the mine and to discover the source of the mineral particulates released from the Peter Mitchell Mine ore that are suspected to be asbestos. 2.2. Sample descriptions Fifty-three samples were collected at thirty sites from the Peter Mitchell Mine. Sampling was made particularly in fault and shear zones and within folds. In these areas the amphibole minerals within the rock were particularly susceptible to low temperature alteration due to infusion of rain water acidified by oxidation of sulfide minerals, these chemical processes being contemporaneous with rock shearing. We separated the 53 samples into four categories or types: (1) Type 1 samples. Type 1 samples are represented by 10B, 10C, 20A, 20B, 21A, 24A, and 24B (from sites 10, 20, 21 and 24) and the U.S. EPA’s ferroactinolite sample which was collected by IIT Research Institute (for the EPA) from the eastern end of the mine in 1975
Fig. 2. Site 10 shear zone within rock face of the Peter Mitchell pit. Soft rock gouge from this shear zone lies at the base of the cliff face (dark area upon which the mining geologist is standing). The gouge contains fibrous ferroactinolite and ferrian sepiolite.
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Fig. 3. Sample 10B-left: thick pristine grunerite lamella (arrow) oriented on the (10–1) plane of a relatively non-fibrous ferroactinolite crystal host. Nearly horizontal striations within the host ferroactinolite are due to (1 0 0) exsolution of grunerite. Sample 10B-right: Composite crystal of grunerite with (10–1) ferroactinolite exsolution lamellae trending N–S (area Y) and ferroactinolite (area X).
Fig. 4. Sample 10B. Scanning electron microscope photomicrograph of a composite crystal of grunerite (area X at lower left) and fibrous ferroactinolite (area Y at upper right). Note (10–1) exsolution lamellae of ferroactinolite within the grunerite area (arrows pointing to thin gray lines trending NW–SE). SEM X-ray spectra from areas X and Y are shown in Fig. 5.
this mineral as ferrian sepiolite (Fig. 11). The ropy masses, when subjected to lengthy ultrasonic treatment, are seen to be composed of a myriad of very thin ferrian sepiolite fibers as demonstrated by the TEM photomicrograph presented in Fig. 9. The chemical composition and crystal structure of ferrian sepiolite are not well understood, but an approximate chemical formula is: ðFe3þ ; Fe2þ ; MgÞ8 ðSi; Fe3þ Þ12 ðOHÞ4 ðH2 OÞn (3) Type 3 sample. The type 3 sample 30B from site 30 contains well crystallized ferroactinolite and grunerite and but also contains small amounts of brownish fibrous actinolite and brown mats of completely decomposed amphibole.
(4) Type 4 samples. Type 4 samples include 13C, 13F, 21B, and 29 and were collected from sites 13, 21, and 29. The amphiboles in these samples appear as blocky prismatic crystals with no fibrous structure. No fibers were detected in these samples. Table 2 gives a summary of these four sample types.
2.3. Geologic processes within the Peter Mitchell Mine We propose that water-rich fluids, acidified by oxidation of sulfide minerals, moved through the active mine faults and shear zones. Here chemical reactions, including oxidation of ferrous to ferric iron, occurred between the fluids and some of the constituent minerals, particularly with the grunerite and ferroactinolite amphiboles. In rocks con-
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Fig. 5. SEM X-ray spectra analysis of grunerite taken in area X of Fig. 4 and of ferroactinolite taken in area Y of Fig. 4.
Fig. 6. Two photomicrographs showing mats of fibrous ferroactinolite (sample 24A). Small amounts of ferrian sepiolite and grunerite may also be present in these mats.
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Fig. 7. Two photomicrographs of sample 5C showing ropy masses resulting from the nearly complete alteration of primary grunerite and ferroactinolite amphiboles to ferrian sepiolite. The X-ray powder diffraction pattern of this sample is shown in Fig. 8.
Fig. 8. X-ray diffraction powder pattern of sample 5C (Fig. 7) identifying the sample as ferrian sepiolite. Copper ?a radiation. Note the characteristic 12.3 (2h = 7.2 degrees) and 7.5 (2h = 11.8 degrees) Angstrom lines of sepiolite. This X-ray pattern is a good match to that given by Brindley (1959, p. 498) for sepiolite from four localities.
taining the two amphibole assemblage ferroactinolitegrunerite, exsolution of grunerite on (10–1) and (1 0 0) planes of host ferroactinolite and of ferroactinolite on (10–1) and (1 0 0) planes of host grunerite are commonly observed. The very thick (10–1) lamellae of grunerite within the host ferroactinolite crystals of sample 10B (Fig. 3) indicates that this amphibole crystallized under high temperature conditions near the top of the ferroactinolite–grunerite solvus and thus contained a large grunerite component within the crystal structure (Ross et al., 1969, p. 291; Fig. 6). On slow cooling the ferroactinolite and grunerite components may segregate to form a crystal, such as shown in Figs. 3 and 4, where one end of the crystal is mostly grunerite, the other, mostly ferroactinolite. If the amphiboles had originally crystallized as true asbestos, textures such as seen in Figs. 3 and 4 would not have developed. For a review of exsolution processes occurring in amphiboles (see Ross et al., 1968, 1969). The transmission electron microscope photomicrograph presented in Fig. 10 reveals that (1 0 0) grunerite exsolution lamellae (now partly to completely altered to ferrian sepio-
lite) are present within a single fiber of the host ferroactinolite, the fiber being derived from a composite ferroactinolite–grunerite crystal such as that shown in area Y of Fig. 4. The TEM X-ray spectra of the interleaved ferroactinolite host and (1 0 0) grunerite /ferrian sepiolite lamellae (Fig. 10) are shown in Fig. 11. We suggest that the interfaces between the ferroactinolite host and the (1 0 0) exsolution lamellae of grunerite are places where reaction with fluids within the fault or shear zone fluids can readily occur. Decomposition of the grunerite lamellae within the ferroactinolite host (area Y, Fig. 4) to ferrian sepiolite could promote the formation of the fibrous textures seen in (Figs. 4 and 6). Where alteration is incomplete, some of the pristine amphibole remains. It also appears that some iron was removed from the amphibole grains during this weathering process to recrystallize as iron oxide, probably in the form of goethite [FeO(OH)] and appearing as brown masses within the ropy mats of ferrian sepiolite. A complete alteration of both grunerite and ferroactinolite could produce ferrian sepiolite and iron oxides such as found in the Type 2 samples.
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Fig. 9. Transmission electron micrograph of sample 5C showing ferrian sepiolite fibers that have formed from the alteration of primary amphiboles (width of field is 0.04 mm). The thinnest fibers are less than 0.1 lm in width.
Table 2 Summary description of the samples collected from various sites within the Peter Mitchell Taconite Mine (1) Type 1 samples. Type 1 samples are represented 10B, 10C, 20A, 20B, 21A, 24A, 24B and the EPA ferroactinolite. These samples contained ferroactinolite amphibole and composite grunerite-ferroactinolite amphibole that have partially or completely altered to very fibrous crystallites (2) Type 2 samples. Type 2 samples represented by 5C and 13E In sample 5C the original amphibole, ferroactinolite, is degraded to a ropy mass composed mostly of the clay mineral ferrian sepiolite. TEM examination of sample 5C shows to be composed of a myriad of very thin fibers (3) Type 3 sample. The type 3 sample 30B from site 30 contains well crystallized ferroactinolite and grunerite and but also contain small amounts of brownish fibrous actinolite and brown mats of completely decomposed amphibole (4) Type 4 samples. Type 4 samples include 13C, 13F, 21B, and 29. The amphiboles in these samples appear as blocky prismatic crystals with no fibrous structure
3. Conclusions The fibrous amphiboles and ferrian sepiolite collected in the Peter Mitchell pit composes a tiny fraction of one percent of the total rock mass within the mine. This fibrous ferroactinolite is a low temperature alteration product of non-fibrous amphibole; it does not occur in the manner of true asbestos which crystallizes as a primary mineral from hydrothermal solutions into open
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Fig. 10. Transmission electron microscope photomicrograph of a single fiber from a composite crystal of ferroactinolite and grunerite (sample 21A), This fiber is composed of alternating lamellae of ferroactinolite (lamella at X) and grunerite/ferrian sepiolite (lamella at Y) as identified by the respective TEM X-ray spectra. Lamellae within the fiber vary from 300 to 700 nm in thickness. Total thickness of the fiber is about 3000 nm. Width of field is about 6500 nm. TEM X-ray spectra collected from areas X and Y are shown in Fig. 11.
veins within deformed rock. The ropy mats of ferrian sepiolite are composed of inter-woven sub-micrometer sepiolite fibers. Due to inter-weaving of the fibers within the mats, individual fibers will be released only with great difficulty. Ultrasound was used to disperse the fibers for TEM analysis. Ferrian sepiolite has not appeared in the air samples collected in the taconite mill area and in the nearby town of Silver Bay (Wilson et al., 2008). The fibers collected during ambient air sampling at the taconite mill where the Northshore iron ore is processed and at the town of Silver Bay are not asbestos, but rather are non-asbestiform ferroactinolite and grunerite. The mean air concentration in Silver Bay is less than 0.00036 fibers per milliliter, a value within the expected background for airborne asbestos reported by the World Health Organization and the U.S. Environmental Protection Agency. The risk-related cancer to such an exposure is less than 0.77 excess cancer cases in 1,000,000 lifetimes (Wilson et al., 2008). Conflict of Interest The authors declare that they have no conflicts of interest.
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Fig. 11. SEM X-ray spectra analysis of ferroactinolite taken in area X of Fig. 10 and of grunerite/ferrian sepiolite taken in area Y of Fig. 10.
Acknowledgments We thank Dr. Ronald G. Graber, Dennis M. Wagner and Dennis Wagner for their help with the geological survey. We acknowledge support from a Higher Education Advance Technology grant from New York State and the International Environmental Research Foundation (www.ierfinc.org) of New York, New York and assistance from Cleveland-Cliffs, Cleveland, Ohio. References Berndt, M.E., Brice, W.C., 2008. The origins of public concern with taconite and human health: Reserve mining and the asbestos case. Reg. Tox. Pharm. 52, S31–S39. Brindley, G.W., 1959. X-ray and electron diffraction data for sepiolite. Am. Mineral. 44, 495–500. Jirsa, M.A., Miller, J.B., Morey, G.B., 2008. Geology of the Biwabik Iron Formation and Dulth Complex. Reg. Tox. Pharm. 52, S5–S10.
McSwiggen, P.L., Morey, G.B., 2008. Overview of the Mineralogy of the Biwabik Iron Formation, Mesabi Iron Range, Northern Minnesota. Reg. Tox. Pharm. 52, S11–S25. Ross, M., 1981. The geological occurrences and health hazards of amphibole and serpentine asbestos. In: Veblen, D.R. (Ed.), . In: Amphiboles and Other Hydrous Pyriboles-Mineralogy. Reviews in Mineralogy, vol. 9A. Mineralogical Society of America, Washington, DC, pp. 279–323. Ross, M., Papike, J.J., Weiblen, P.W., 1968. Exsolution in clinoamphiboles. Science 159, 1099–1102. Ross, M., Papike, J.J., Shaw, K.W., 1969. Exsolution textures in amphiboles as indicators of subsolidus thermal histories. Mineralogical Society of America Special Paper #2, 275–299. Ross, M., Nolan, R.P., 2003. History of asbestos discovery and use and asbestos-related disease in context with the occurrence of asbestos within ophiolite complexes. In: Dilek, Y., Newcomb, S. (Eds.), . In: Ophiolite Concept and Evolution of Geologic Thought, 373. Geological Society of America Special Publication, pp. 447–470. Wilson, R., McConnell, E.E., Nolan, R.P., Axten, C.W., Ross, M., 2008. Risk assessment due to environmental exposures to fibrous particulates associates with taconite ore. Reg. Tox. Pharm. 52, S232–S245.