Award Date

May 2023

Degree Type


Degree Name

Master of Science (MS)



First Committee Member

Brenda Buck

Second Committee Member

Rodney Metcalf

Third Committee Member

Pamela Burnley

Fourth Committee Member

David James

Number of Pages



The presence of naturally-occurring asbestos (NOA) is increasingly concerning for scientists, health and regulatory agencies, and citizens living in impacted areas. It is commonly believed that fibrous amphibole asbestos can only form through neocrystallization. In southern Nevada, NOA occurs as a result of hydrothermal alteration of granitic rock producing fibrous amphibole both as cross-cutting neocrystallized veins and via recrystallization of original magmatic hornblende crystals. Fibers with a greater aspect ratio are known to have increased toxicity. This study measured the maximum length and average width of both neocrystallized and recrystallized fibers to see if the morphologies were similar. Neocrystallized and recrystallized fibers from the McCullough Range, NV were identified using a petrographic microscope, extracted from polished thin sections using a motorized drill, and analyzed using the scanning electron microscope (SEM), and field emission scanning electron microscope (FESEM) with energy dispersive x-ray analysis (EDS). Neocrystallization produced fibers with an average width of 0.53 ± 0.018 µm, an average length of 4.26 ± 0.257 µm, and an average aspect ratio of 8.7 ± 0.38. Recrystallization produced fibers with an average width of 0.64 ± 0.023 µm, an average length of 7.31 ± 0.483 µm, and an average aspect ratio of 13. ± 0.86. Multiple statistical analyses, explicitly the Mann-Whitney U-test, confirmed that recrystallized particles more commonly had either statistically similar (p> 0.05) or statistically wider (p< 0.05) widths, significantly longer lengths, and significantly greater aspect ratios compared to neocrystallized particles of similar mineralogy and chemistry. Because the recrystallization process produces fibers that have greater average aspect ratios, they may be more toxic than those produced through neocrystallization. We hypothesize that the likely reason for this are increased chain width defects resulting from fluctuating conditions during recrystallization. Because hydrothermal alteration and recrystallization of primary minerals is a very common geologic process, this finding may significantly increase the estimated potential of rocks and soils that contain NOA.


Asbestos; fibrous amphibole; Naturally Occurring Asbestos; neocrystallization; NOA; recrystallization


Environmental Health | Environmental Health and Protection | Geology

Degree Grantor

University of Nevada, Las Vegas




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