Iron Mineralogy and Bioaccessibility of Dust Generated 

From Soils as Determined by Reflectance Spectroscopy 

and Magnetic and Chemical Properties—Nellis Dunes 

Recreational Area, Nevada

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5054
Front cover. Windy conditions produce dust emission from a silty surface at Nellis Dunes recreational area. Photo by 

Dirk Goossens.

Back cover. Photograph showing off-road-vehicle tracks at various landscape positions in the western portion of the 

Nellis Dunes recreational area. Photo by Dirk Goossens.
Iron Mineralogy and Bioaccessibility of 

Dust Generated From Soils as Determined 

by Reflectance Spectroscopy and Magnetic 

and Chemical Properties—Nellis Dunes 

Recreational Area, Nevada

By Harland L. Goldstein, Richard L. Reynolds, Suzette A. Morman, 

Bruce Moskowitz, Raymond F. Kokaly, Dirk Goossens, Brenda J. Buck, 

Cody Flagg, Jessica Till, Kimberly Yauk, and Thelma S. Berquó

Scientific Investigations Report 2013–5054

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Goldstein, H.L., Reynolds, R.L., Morman, S.A., Moskowitz, Bruce, Kokaly, R.F., Goossens, Dirk, Buck, B.J., Flagg, Cody, 

Till, Jessica, Yauk, Kimberly, and Berquó, T.S., 2013, Iron mineralogy and bioaccessibility of dust generated from soils 

as determined by reflectance spectroscopy and magnetic and chemical properties—Nellis Dunes recreational area, 

Nevada: U.S. Geological Survey Scientific Investigations Report 2013–5054, 15 p., http://pubs.usgs.gov/sir/2013/5054/.
iii

Contents

Acknowledgments........................................................................................................................................vi

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Methods...........................................................................................................................................................3

Dry Fractionation and Particle Size Analysis...................................................................................3

Reflectance Spectroscopy..................................................................................................................3

Magnetic Properties.............................................................................................................................4

Magnetic Susceptibility..............................................................................................................5

Laboratory-Induced Magnetization at Room Temperature ..................................................5

Laboratory-Induced Magnetization at Low Temperature.....................................................6

Mössbauer Spectroscopy..........................................................................................................6

Chemical Properties.............................................................................................................................6

Total Chemical Composition.......................................................................................................6

In Vitro Bioaccessibility Analysis..............................................................................................7

Results ...........................................................................................................................................................10

Reflectance Spectroscopy................................................................................................................10

Magnetic Properties...........................................................................................................................10

Hysteresis and Low-Temperature Results.............................................................................10

Mössbauer Spectroscopy........................................................................................................11

In Vitro Bioaccessibility .....................................................................................................................12

Discussion.....................................................................................................................................................12

References Cited..........................................................................................................................................12

Figures

1. Map showing location of the Nellis Dunes recreation area in Nevada; modified 

from Goossens and Buck (2009a)...............................................................................................2

2. Chart showing absolute reflectance spectra obtained from averaged reflectance

measurements made relative to Labsphere, Inc. Spectralon white reference 

panel for samples from the Nellis Dunes recreation area.....................................................4

3. Chart showing temperature dependence of room-temperature saturation 

isothermal remanence magnetization (RTSIRM) in zero field on cooling (ZFC) to 

10 kelvin (K) for samples from the Nellis Dunes recreation area. The magnetizations during ZFC (RTSIRM(T)) are normalized to the initial magnetization at 300 K 

(RTSIRM(300)), which was imparted at 300 K in a 2.5-tesla field. The drop in 

magnetization near 120 K is the Verwey transition in magnetite........................................10

4. Charts showing thermal demagnetization of low-temperature isothermal 

remanent magnetization (LTIRM) imparted at 10 K after cooling in a zero field 

(ZFC) and in a 2.5-tesla magnetic field (FC) for samples from A, unit 2.2, B, unit 3.1, 

and C, unit 3.2 from the Nellis Dunes recreation area. The remanence is measured 

on warming to 300 K...................................................................................................................11
iv

Conversion Factors and Datum

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2

) 0.3861 square mile (mi2)

Flow rate

meter per second (m/s) 3.281 foot per second (ft/s) 

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8.

273.15 kelvin (K) is equal to 0.01 °C or 32.018 °F.

Tables

1. Spectral reflectance data ...........................................................................................................3

2. Magnetic properties.....................................................................................................................4

3. Mössbauer spectroscopy ...........................................................................................................6

4. Geochemical data determined by inductively coupled plasma–mass spectrometry.......7

5. Total bulk sample concentration and leachate concentration, percent 

bioaccessibility and milligram leached per kilogram of solid for selected

elements as measured in simulated gastric, lung, and phagolysosomal fluid...................8
v

Abbreviations

χhf high-field susceptibility

σhys hysteresis shape

ARM anhysteretic remanent magnetization

Av vesicular A horizon

Bc coercivity

Bcr coercivity of remanence

BHF magnetic hyperfine field

DC direct current

FC field cooling

FDMS frequency-dependent magnetic susceptibility

HIRM hard isothermal remanent magnetization

ICP–MS inductively coupled plasma–mass spectrometry

IRM isothermal remanent magnetization

IS isomer shift

IVBA in vitro bioaccessibility analysis

LTIRM low-temperature isothermal remanent magnetization

MICA material identification and characterization algorithm

Mr saturation remanence

MS magnetic susceptibility

Ms saturation magnetization

MShf magnetic susceptibility high frequency

MSlf magnetic susceptibility low frequency

NDRA Nellis Dunes recreational area

ORV off-road vehicle

PRISM Processing Routines in IDL for Spectroscopic Measurements (software)

QS quadrupole splitting

RLT ratio between a low temperature SIRM imparted in a FC and a ZFC

RTSIRM room-temperature saturation isothermal remanent magnetization

SGF simulated gastric fluid

SIRM saturation isothermal remanent magnetization

SLF simulated lung fluid

SLyF simulated phagolysosomal fluid

SOFIPEX soil fine particle extractor technique

USGS U.S. Geological Survey

ZFC zero-field cooling
vi

Acknowledgments

The Bureau of Land Management provided funding and granted permission to Dirk Goossens 

of the University of Nevada Las Vegas and the University of Leuven and Brenda J. Buck of the 

University of Nevada Las Vegas to conduct the original study for which these samples were collected. The analytical work was supported by the U.S. Geological Survey Climate and Land Use 

Change Program. Jiang Xiao of ADC Management Services provided laboratory assistance with 

reflectance spectroscopy and magnetic measurements. Jeremy Havens of ADC Management 

Services provided graphic assistance.

The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities 

Program of the National Science Foundation Division of Earth Science.

We thank Gary Skipp and Frank Urban for their review of this report and helpful suggestions for 

improvement.
Abstract

Atmospheric mineral dust exerts many important 

effects on the Earth system, such as atmospheric temperatures, marine productivity, and melting of snow and ice. 

Mineral dust also can have detrimental effects on human 

health through respiration of very small particles and the 

leaching of metals in various organs. These effects can be 

better understood through characterization of the physical 

and chemical properties of dust, including certain iron oxide 

minerals, for their extraordinary radiative properties and 

possible effects on lung inflammation. Studies of dust from 

the Nellis Dunes recreation area near Las Vegas, Nevada, 

focus on characteristics of radiative properties (capacity of 

dust to absorb solar radiation), iron oxide mineral type and 

size, chemistry, and bioaccessibility of metals in fluids that 

simulate human gastric, lung, and phagolysosomal fluids. In 

samples of dust from the Nellis Dunes recreation area with 

median grain sizes of 2.4, 3.1, and 4.3 micrometers, the ferric oxide minerals goethite and hematite, at least some of it 

nanosized, were identified. In one sample, in vitro bioaccessibility experiments revealed high bioaccessibility of arsenic 

in all three biofluids and higher leachate concentration and 

bioaccessibility for copper, uranium, and vanadium in the 

simulated lung fluid than in the phagolysosomal fluid. The 

combination of methods used here to characterize mineral 

dust at the Nellis Dunes recreation area can be applied to 

global dust and broad issues of public health.

Introduction

Atmospheric dust can affect atmospheric temperatures, 

melting of snow and ice, marine productivity, and human 

health. Among mineral particles in dust, ferric oxide minerals possess important radiative properties, can stimulate 

phytoplankton productivity, and may contribute to inflammation in the human lung, but these minerals remain poorly 

described with respect to presence in far-traveled aerosol, 

specific mineralogy (hematite or goethite), particle size, and 

surface area (Formenti and others, 2010). The diverse effects 

of atmospheric dust can be better comprehended by characterization of iron minerals in dust. The distinction between 

hematite and goethite is useful for modeling the effects of 

dust on radiative forcing (that is, how dust affects the energy 

balance between incoming and outgoing solar radiation) in 

the atmosphere and in mountain snow and ice.

Results from studies of dust emission in the Nellis Dunes recreation area (NDRA; fig. 1) northeast of Las 

Vegas, Nevada, have been recently published by Goossens 

and Buck (2009a,b, 2011a,b) and Goossens and others 

(2012). This 37-square-kilometer (km2

) area is the most 

popular site in southern Nevada for off-road vehicle (ORV) 

driving. In addition to natural dust emissions caused by 

wind erosion, large amounts of dust are produced by ORV 

activity. The situation has attracted concern with respect to 

human health and air quality because of the proximity of 

the NDRA to Las Vegas (the NDRA is about 6 kilometers 

(km) from the northeastern margin of the city) and because 

the regional wind regime sends NDRA-emitted dust into the 

city during most of the winter.

Iron Mineralogy and Bioaccessibility of Dust Generated 

From Soils as Determined by Reflectance Spectroscopy 

and Magnetic and Chemical Properties—Nellis Dunes 

Recreational Area, Nevada

By Harland L. Goldstein,1 Richard L. Reynolds,1 Suzette A. Morman,1 Bruce Moskowitz,2 Raymond F. Kokaly,1

Dirk Goossens,3,4

 Brenda J. Buck,3 Cody Flagg,1 Jessica Till,2 Kimberly Yauk,2 and Thelma S. Berquó5

1

U.S. Geological Survey.

2

University of Minnesota.

3

University of Nevada Las Vegas.

4

University of Leuven.

5

Concordia College.
2 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

Four major types of surfaces occur in the NDRA: sand 

and sand-affected areas, silt and clay deposits, rock-covered 

sands and silts, and bedrock. Most ORV-produced dust from 

the NDRA is generated from the silt and clay and the rockcovered areas, the latter of which may contain a silty substratum—the vesicular A (Av) horizon—with a thin, discontinuous 

cover of rock fragments. Much less ORV-produced dust comes 

from the sand areas, and almost no dust comes from the bedrock areas (Goossens and Buck, 2011a,b).

Analytical results were obtained from three samples from 

the very uppermost soil for three units. The samples were 

obtained from bulk sediment dry fractionated to particle sizes 

4.3 micrometers (μm) or less. According to the International 

Standards Organization’s respirable convention curve, this 

size corresponds to a probability of 50 percent of penetration 

of respirable particles within workers’ airways when working under a typical workload (Lee and others, 2011). Samples 

were collected from the following units:

• a silt and clay unit (described by McLaurin and others 

(2011b) as unit 2.2), rich in arsenic, with some surface 

gravel that forms a portion of badlands topography in 

the NDRA; median diameter after dry fractionation is 

4.3 μm

36°00’

36°30’

115°30’ 115°00’

NELLIS DUNES

RECREATION

AREA

Las Vegas

LAKE MEAD

CALIFORNIA

UTAH

NEVADA

ARIZONA

Location

map area

0

0 10

10 20 KILOMETERS

20 MILES

15

15

95

95

93

93

Figure 1. Map showing location of the Nellis Dunes recreation area in Nevada; modified from Goossens 

and Buck (2009a).
Methods 3

• a well-developed desert pavement (described by 

McLaurin and others (2011b) as unit 3.1) with underlying silty Av soil horizon; median diameter after dry 

fractionation is 2.4 μm

• a rock-covered surface with silt and clay zones 

(described by McLaurin and others (2011b) as unit 3.2) 

that composes the greatest amount of surface area in 

the NDRA; median diameter after dry fractionation is 

3.1 μm

All analyses were performed on less than a total of 

2.5 grams of each dry fractionated sample. The purpose of 

our analyses was to characterize reflectance spectroscopy, 

magnetic, and chemical properties and bioaccessibility as a 

basis for better understanding (1) the relations between dust 

compositions and radiative properties and (2) potential health 

effects of dust emitted from the NDRA. The results presented 

here complement ongoing studies by others on NDRA dust 

(Goossens and Buck, 2011a,b; McLaurin and others, 2011a; 

Goossens and others, 2012; Soukup and others, 2012), and 

they add to the body of expanding information and data about 

the properties and effects of atmospheric dust (Reynolds and 

others, 2011).

Methods

Dry Fractionation and Particle Size Analysis

Samples were collected from the upper 2 to 3 centimeters 

(cm) of the topsoil, which produces the most dust when being 

disturbed by wind erosion or ORV driving. The samples were 

first dry sieved at 125 μm. The fraction that was less than 

125 μm was then further dry fractionated to collect the fraction 

less than 4 μm using the soil fine particle extractor (SOFIPEX) technique described by Goossens (2012). This technique 

allows dry extraction of large volumes (dozens of grams and 

more) of fine particulate matter from bulk soil samples in a 

short time, usually in the order of minutes. After collection, 

the extracts were stored in plastic vials and then sent to the 

laboratory for further analysis.

Particle size distribution was determined with a Malvern 

Mastersizer S laser particle size analyzer. All analyses were 

done in water. No dispersion techniques were applied during 

the tests.

Reflectance Spectroscopy

Reflectance spectra of samples were measured to evaluate their mineral composition and to determine average 

reflectance values (table 1). Reflectance spectra were measured in the laboratory using an Analytical Spectral Devices, 

Inc., FieldSpec3 spectrometer, covering the wavelength 

range of 0.35 to 2.50 μm in 2,151 channels. A bidirectional 

reflectance measurement setup was used, with the light 

source and the fiber optic head of the spectrometer at fixed 

angles above the sample. For each sample, multiple reflectance measurements were made relative to a Labsphere, Inc., 

Spectralon white reference panel. The individual measurements were averaged, and the average spectrum (fig. 2) was 

converted to absolute reflectance using the U.S. Geological Survey (USGS) Processing Routines in IDL (interface 

description language software) for Spectral Measurements 

(PRISM; Kokaly, 2011) software.

Table 1. Spectral reflectance data.

[Sample ID, unique sample identification number. Total reflectance is average reflectance over entire wavelength range (0.35 to 2.50 micrometers (μm)). Visible reflectance is average reflectance over visible wavelengths (0.40 to 0.70 μm). Iron mineral fit, 1-μm minerals; dominant iron-bearing mineral identified. 

Fit values measure from 0 to 1 of the match of the identified mineral; highest values represent best match. Other mineral fit, 2-μm minerals; dominant other 

minerals identified. Other mineral fit results were derived from a different measurement than the iron mineral fit measurements due to potential influence from 

the background material in the 2- to 2.5-μm wavelength region. n.d.: not determined; —, none]

Sample ID Total

reflectance

Visible

reflectance

Iron

mineral fit

Iron mineral

fit values Other mineral fit Other mineralfit values

Unit 2.2 0.620836 0.391134 — n.d. Montmorillonite_Ca 0.952

Montmorillonite_Na 0.8541

Unit 3.1 0.646428 0.392579 Goethite 0.7591 Montmorillonite_Ca 0.9591

Montmorillonite_Na 0.7941

Muscovite_lowAl 0.7730

Illite_gds4 0.7730

Calcite.8+montmorillonite_Na.2 0.7503

Unit 3.2 0.584185 0.32341 Goethite 0.8031 Calcite.8+montmorillonite_Ca.2 0.9139

Calcite.8+montmorillonite_Na.2 0.8174

Muscovite_lowAl 0.8044

Illite_gds4 0.7833

Illite 0.6905
4 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

The mineral compositions of samples were characterized 

by analyzing their reflectance spectra with the material identification and characterization algorithm (MICA), a module of 

the PRISM software, which uses continuum removal to isolate 

diagnostic absorption features and linear regression to compare 

spectral features. MICA was used to compare the spectra of the 

Nellis Dunes samples to reference spectra of minerals and other 

materials in the USGS spectral library (version 6; Clark and others, 2007). Mineral spectra can be divided into two classes that 

are naturally separated in the wavelength regions of their primary absorption features: iron-bearing minerals with electronic 

absorption features centered primarily in the visible and nearinfrared wavelengths (0.4 to 1.0 μm; commonly referred to as 

1-μm minerals) and clays, carbonates, sulfates, and alterationrelated minerals with vibrational absorption features centered 

primarily in the shortwave-infrared wavelengths (2.0 to 2.5 μm; 

commonly referred to as 2-μm minerals). The MICA analysis 

computes a fit value, ranging from 0 to 1, between the sample 

spectrum and each reference material (Kokaly, 2011). The reference spectrum with the highest fit value is termed the best fit.

Magnetic Properties

Magnetic properties provide information about the 

types, amounts, and grain sizes of magnetic iron oxide minerals. Magnetic properties at room temperature and at low 

temperatures (10–300 kelvin (K)) were determined on the 

dry-fractionated size fractions (table 2).

0.7

0.8

0.6

0.5

0.4

0.3

0.2

0.1

0 0.5 1.0 1.5 2.0 2.5

Wavelength, in micrometers

Reflectance

Unit 2.2

Unit 3.2

Unit 3.1

EXPLANATION

Figure 2. Chart showing absolute reflectance spectra obtained 

from averaged reflectance measurements made relative to 

Labsphere, Inc., Spectralon white reference panel for samples 

from the Nellis Dunes recreation area.

Table 2. Magnetic properties.—Continued

[Sample ID, unique sample identification number. HIRM is calculated as (IRM + IRM )/2. HIRM* is calculated as HIRM except that in place of a 1.2-T 1.2 -0.3

(tesla) field (IRM ) a field of 1.0 T was used (IRM ; data not shown). S ratio calculated as IRM /IRM ; n.d., not determined] 1.2 1.0 -0.3 1.2

Sample ID

Unit 2.2 Unit 3.1 Unit 3.2

Median grain size, in micrometers

Magnetic susceptibility, in cubic meters per kilogram:

Low frequency

High frequency

Frequency-dependent magnetic susceptibility:

Percent

Cubic meters per kilogram

Anhysteretic remanent magnetization, in ampere square meters per kilogram

Isothermal remanent magnetization (IRM) from induction at room temperature, in ampere square 

meters per kilogram:

In a 1.2-tesla (T) field, (IRM ) 1.2

In a 0.3-T field, (IRM ) –0.3

Hard isothermal remanent magnetization, in ampere square meters per kilogram:

For a 1.2-T field (HIRM)

For a 1.0-T field (HIRM*)

S ratio

Saturation remanence (Mr), in ampere meter per kilogram

Saturation magnetization (Ms), in ampere meter per kilogram

Mr/Ms

Coercivity (Bc), in milliteslas

4.3

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

5.48E-05

n.d.

8.73E-04

5.67E-03

0.15

11.56

2.4

3.87E-07

3.74E-07

3.33

1.29E-08

3.13E-04

5.84E-03

4.99E-03

4.25E-04

2.77E-04

0.85

5.81E-03

3.28E-02

0.18

12.01

3.1

3.31E-07

3.25E-07

1.82

6.03E-09

2.88E-04

5.58E-03

4.84E-03

3.71E-04

2.52E-04

0.87

5.62E-03

3.02E-02

0.19

13.01
Methods 5

Magnetic Susceptibility

Magnetic susceptibility (MS) was measured by using a 

susceptometer with a sensitivity higher than about 2 × 10–9

cubic meters per kilogram. Samples were measured in a 

0.1-millitesla (mT) induction at an MS low frequency (MSlf) 

of 600 hertz (Hz) and MS high frequency (MShf) of 6,000 Hz. 

For each sample, the MS value was calculated as the mean of 

four measurements. Frequency-dependent magnetic susceptibility (FDMS) was calculated as follows:

FDMS =

MS – MS

MS

lf hf

if

. (1)

Laboratory-Induced Magnetization at Room 

Temperature

Measurements from anhysteretic remanent magnetization 

(ARM) and isothermal remanent magnetization (IRM) experiments were made by using a high-speed spinner magnetometer. Anhysteretic remanent magnetization was imparted in a 

decreasing alternating field from a peak induction of 100 mT 

and a direct current (DC) bias of 0.1 mT. IRM was generated 

at room temperature by using an impulse magnetizer. First, 

IRM was imparted in a 1.2-tesla (T) induction (IRM1.2T). The 

samples then were magnetized in the opposite direction by 

using an induction of 0.3 T (IRM-0.3T). 

Hard isothermal remanent magnetization (HIRM; a measure of ferric oxide content) and the S-ratio (a measure of the 

relative amounts of low-coercivity phases (for example, magnetite and maghemite) to high-coercivity phases (for example, 

hematite and goethite)) were calculated as follows (King and 

Channel, 1991):

HIRM= IRM +IRM

2

1.2T -0.3T , and (2)

S=

IRM

IRM

-0.3T

1.2T

. (3)

S-ratio values near 1 indicate a high proportion of magnetite to hematite. Goethite is extremely hard to magnetize at 

room temperature in fields below 2.0 T (Rochette and others, 

2005) and will contribute a negligible amount to HIRM or the 

S-ratio value even though it may be the abundant phase.

Hysteresis loops and backfield remanence curves at 

room temperature were measured on a vibrating-sample 

magnetometer using a saturating field of 1.0 T. Magnetic 

parameters determined from the hysteresis data include 

saturation magnetization (Ms), saturation remanence (Mr), 

coercivity (Bc), coercivity of remanence (Bcr), and high-field 

susceptibility (χhf). The shape of the loop was quantified 

using the hysteresis shape (σhys) parameter of Fabian (2003). 

Hysteresis shape parameters were determined after the loops 

were corrected for high-field slopes using the approach to 

saturation fitting described in Jackson and Solheid (2010). 

Coercivity of remanence was determined from DC backfield 

remanence curves. Samples were first given a remanence in 

a positive 1.0-T field and subsequently exposed to negative 

fields of increasing strength to 300 mT. The field where the 

remanence is zero is the coercivity of remanence. The magnetite weight percent was determined from the room-temperature bulk saturation magnetization values and the known 

value for pure magnetite (Ms=92 ampere square meters per 

kilogram (Am2/kg)) as follows:

Magnetite weight percent = M

92 Am

kg

 × 100 percent s

2 . (4)

Table 2. Magnetic properties.—Continued

[Sample ID, unique sample identification number. HIRM is calculated as (IRM1.2 + IRM-0.3)/2. HIRM* is calculated as HIRM except that in place of a 1.2-T

(tesla) field (IRM1.2) a field of 1.0 T was used (IRM1.0; data not shown). S ratio calculated as IRM-0.3/IRM1.2; n.d., not determined]

Sample ID

Unit 2.2 Unit 3.1 Unit 3.2

Coercivity or remanence (Bcr), in milliteslas 35.54 34.91 37.00

Bcr/Bc 3.08 2.91 2.84

High field magnetic susceptibility, in cubic millimeters per kilogram 4.64E-08 7.08E-08 5.42E-08

Weight percent of magnetite 0.01 0.04 0.03

Hysteresis shape parameter (shys) 0.18 0.14 0.14
6 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

Laboratory-Induced Magnetization at Low 

Temperature

Low-temperature (10–300 K) magnetic properties were 

measured using a Quantum Design, Inc., Magnetic Properties 

Measurement System. Low-temperature isothermal remanent 

magnetization (LTIRM) was imparted using a 2.5-T field on 

samples that had been cooled to 10 K in the absence (zerofield cooling, ZFC) or presence (field cooling, FC) of a 2.5-T 

magnetic field. LTIRM was measured on warming in zero 

field to 300 K in 5-K increments. Samples were also given an 

IRM in a field of 2.5 T at 300 K (room-temperature saturation 

isothermal remanent magnetization; RTSIRM), and magnetization was measured during cooling to 10 K and during warming back to 300 K in zero field at 5-K increments. The low 

temperature remanence ratio (RLT) was calculated to describe 

the shapes of the FC and ZFC curves (Liu and others, 2006; 

Smirnov, 2009), as follows:

R = IRM(10K)

IRM(10K) LT

FC

ZFC

, (5)

where

 IRM(10K)FC is field cooling IRM at 10 K and

 IRM(10K)ZFC is zero-field cooling IRM at 10 K. 

Mössbauer Spectroscopy

Mössbauer spectra were measured at 4.2 K and 300 

K on dry-fractionated size fractions for two samples (from 

units 2.2 and 3.2) to characterize iron oxide mineralogy 

and to determine iron phases present. Measurements were 

made at 4.2 K using a conventional constant-acceleration 

spectrometer in transmission geometry with a cobalt-57 

and rhodium (57Co/Rh) source. An alpha-iron foil at room 

temperature was used to calibrate isomer shifts and velocity scale. The magnetic hyperfine parameters (table 3) 

including the hyperfine field (BHF), isomer shift (IS), and 

quadrupole splitting (QS) were fit using the NORMOS 

program (Brand, 1987).

Chemical Properties

Total Chemical Composition

Dry-fractionated samples were analyzed to determine 

their chemical composition using inductively coupled plasma–

mass spectrometry (ICP–MS) following acid digestion (Briggs 

and Meier, 2002). Chemical composition is represented by 37 

major, minor, and trace elements (table 4).

Table 3. Mössbauer spectroscopy.

[Sample ID, unique sample identification number. K, kelvin; BHF, magnetic hyperfine field, expressed in tesla (T). IS, isomer shift, expressed in millimeters per 

second (mm/s). QS, quadrupole splitting, expressed in mm/s. Iron phase, iron-bearing phases. %, percentage of total iron that the iron phases compose. —, not 

available]

Sample ID

4.2 K 300 K

BHF IS QS Iron phase % BHF IS QS Iron phase %

Unit 2.2 49.8 0.50 –0.24 Goethite 32 — — — Goethite —

— 0.50 0.54 Fe3+ 63 — 0.38 0.58 Fe3+ 90

— 1.30 2.97 Fe3+ 5 — 1.1 2.59 Fe2+ 10

Unit 3.2 53.5 0.50 –0.03 Hematite 11 50.3 0.33 –0.16 Hematite 13

49.1 0.50 –0.20 Goethite 26 — — — Goethite —

— 1.18 3.05 Fe2+ 8 — 1.00 2.86 Fe2+ 14

— 0.52 1.11 Fe3+ 22 — 0.42 1.10 Fe3+ 26

— 0.50 0.43 Fe3+ 33 — 0.37 0.45 Fe3+ 47
Methods 7

In Vitro Bioaccessibility Analysis

In vitro bioaccessibility analyses (IVBAs) are physiologically based extraction tests used to estimate the bioaccessibility of metals and metalloids along human exposure pathways. 

Bioaccessibility is defined as the amount of an element soluble 

in a simulated gastric fluid and available for uptake by the 

body (Ruby and others, 1996). Our IVBA tests simulated 

gastric, lung, and phagolysosomal fluids. The methodology of 

leaching Earth materials with fluids compositionally similar 

to those of the human body is used widely to examine the 

effects of contaminants on human health. All extraction solutions were analyzed by ICP–MS (table 5; Lamothe and others, 

2002). Duplication of IVBA extractions was not possible due 

to the very small amount of sample available.

The simulated gastric fluid (SGF; with a pH of 1.5) 

extraction method (Morman and others, 2009) used here is 

based on a method developed by Drexler and Brattin (2007) 

and approved by the U.S. Environmental Protection Agency 

(2008) to measure bioaccessibility of lead. This method is 

currently being reviewed for use in determining arsenic bioaccessibility (Bradham and others, 2011). Simulated gastric fluid 

extraction samples are typically sieved to less than 250 μm 

because particulates of this size fraction or smaller are most 

likely to adhere to the hands of children and be ingested (Van 

Wijnen and others, 1990); however, our tests were performed 

on the dry-fractionated samples of much smaller particle sizes.

Application of IVBA tests to inhalation pathways is a 

relatively new technique, and these methods have not been 

accepted for regulatory purposes. Nevertheless, simulated 

lung (SLF) and phagolysosomal (SLyF) fluids (Herting and 

others, 2006; Stefaniak and others, 2005, 2006) permit modeling of in vivo solubility, an important physiochemical factor. 

The SLF has neutral pH (7.4), whereas the SLyF simulates 

the pH (4.5) that is present when particles are engulfed by 

pulmonary alveolar macrophages, which are the specialized 

lung cells involved in particle removal from the lung. Samples 

for SLF and SLyF extractions are usually sieved to less than 

20 μm. This fraction is an approximation of the size deemed 

to be respirable (generally less than 5 μm). Our tests were 

performed on the dry-fractionated samples with median grain 

sizes comparable to respirable size fractions.

Table 4. Geochemical data determined by inductively coupled 

plasma–mass spectrometry.

[Values are in parts per million. Sample ID, unique sample identification]

Sample ID Unit 2.2 Unit 3.1 Unit 3.2

Aluminum 48,300 73,200 58,700

Antimony 0.743 1.02 1.08

Arsenic 145 22.7 23.4

Barium 921 521 465

Beryllium 1.38 1.78 1.62

Bismuth 0.422 0.463 0.394

Cadmium 0.263 0.39 0.619

Calcium 151,000 75,700 124,000

Cerium 54.2 71.7 62.7

Cesium 14.5 10.1 7.6

Chromium 34.6 56.1 60.7

Cobalt 6.79 11.8 9.75

Copper 75.9 41.2 48.4

Gallium 11.3 17.3 13.5

Iron 21,900 29,700 23,800

Lanthanum 34.1 38.8 35.2

Lead 19.7 21.6 18.4

Lithium 50.8 53.6 39.9

Magnesium 12,800 25,400 24,200

Manganese 259 570 498

Molybdenum 4.76 2.61 2.47

Nickel 7.61 24.5 19.1

Niobium 13 16.4 14.4

Phosphorus 525 1,070 786

Potassium 13,800 21,900 18,900

Rubidium 75.4 108 85.5

Scandium 7.44 11.3 9.35

Silver 0.085 0.098 0.071

Sodium 1,300 2,480 2,380

Strontium 549 245 404

Thallium 0.963 0.831 0.647

Thorium 9.33 13.5 11.2

Titanium 2,110 2,890 2,530

Uranium 6.16 3.03 3.37

Vanadium 94.1 80 73.4

Yttrium 24.9 26.5 25.4

Zinc 71.7 109 96.8
8 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

Table 5. Total bulk sample concentration and leachate concentration, percent bioaccessibility, and milligram leached per kilogram of 

solid for selected elements as measured in simulated gastric, lung, and phagolysosomal fluid.—Continued

[Sample ID, unique sample identification number. mg/kg, milligrams per kilogram; μg/L, micrograms per liter; n.d., no value could be calculated because 

leachate concentration was less than method detection limit; ppm, parts per million; <, value was less than the method reporting limit; *, calculated value 

resulted in a negative number because of blank correction; %, percent]

Sample ID Total,

in ppm 

Leachate,

in μg/L % bioaccessible

Leached,

in mg/kg

of solid

Total,

in ppm

Leachate,

in μg/L % bioaccessible

Leached,

in mg/kg

of solid

Aluminum Arsenic

Gastric Gastric

Unit 2.2 48,300 10,700 2.21 1,069 145 739 50.6 73.4 

Unit 3.1 73,200 24,700 3.37 2,469 22.7 59.1 23.8 5.41 

Unit 3.2 58,700 18,100 3.08 1,809 23.4 61.2 24.0 5.62 

Lung Lung

Unit 2.2 48,300 <20 n.d. n.d. 145 119 7.86 11.4

Unit 3.1 73,200 24.6 * * 22.7 <10 n.d. n.d.

Unit 3.2 58,700 175 0.022 12.9 23.4 17.0 5.13 1.20

Phagolysosomal Phagolysosomal

Unit 2.2 48,300 <20 n.d. n.d. 145 285 19.7 28.5

Unit 3.1 73,200 <20 n.d. n.d. 22.7 20.3 8.94 2.03

Unit 3.2 58,700 <20 n.d. n.d. 23.4 23.6 10.1 2.36

Barium Cadmium

Gastric Gastric

Unit 2.2 921 650 7.01 64.6 0.263 2.34 86.2 0.224 

Unit 3.1 521 2,560 49.1 256 0.390 3.43 85.4 0.333 

Unit 3.2 465 2,420 52.0 242 0.619 6.49 103 0.639 

Lung Lung

Unit 2.2 921 29.4 0.319 2.94 0.260 <0.2 n.d. n.d.

Unit 3.1 521 330 6.33 33.0 0.390 <0.2 n.d. n.d.

Unit 3.2 465 79.6 1.71 7.96 0.620 <0.2 n.d. n.d.

Phagolysosomal Phagolysosomal

Unit 2.2 921 184 1.88 17.4 0.260 0.330 12.7 0.033

Unit 3.1 521 932 17.7 92.2 0.390 0.400 10.3 0.040

Unit 3.2 465 491 10.3 48.1 0.620 0.680 11.0 0.068

Chromium Copper

Gastric Gastric

Unit 2.2 34.6 102 * * 75.9 436 57.1 43.4 

Unit 3.1 56.1 122 1.16 1.00 41.2 62.4 14.5 5.99 

Unit 3.2 60.7 148 5.35 3.25 48.4 188 38.3 18.6 

Lung Lung

Unit 2.2 34.6 <10 n.d. n.d. 75.9 394 51.6 39.2

Unit 3.1 56.1 <10 n.d. n.d. 41.2 55.4 12.8 5.29

Unit 3.2 60.7 14.8 1.61 0.980 48.4 180 36.7 17.8

Phagolysosomal Phagolysosomal

Unit 2.2 34.6 <10 n.d. n.d. 75.9 257 33.9 25.7

Unit 3.1 56.1 <10 n.d. n.d. 41.2 29.4 7.14 2.94

Unit 3.2 60.7 10.1 * * 48.4 99.2 20.5 9.92

Iron Lead

Gastric Gastric

Unit 2.2 21,900 892 0.293 64.2 19.7 105 52.6 10.4 

Unit 3.1 29,700 1,240 0.333 99.0 21.6 74.1 33.7 7.28 

Unit 3.2 23,800 1,380 0.475 113 18.4 77.8 41.6 7.65 
Methods 9

Table 5. Total bulk sample concentration and leachate concentration, percent bioaccessibility, and milligram leached per kilogram of 

solid for selected elements as measured in simulated gastric, lung, and phagolysosomal fluid.—Continued

[Sample ID, unique sample identification number. mg/kg, milligrams per kilogram; μg/L, micrograms per liter; n.d., no value could be calculated because 

leachate concentration was less than method detection limit; ppm, parts per million; <, value was less than the method reporting limit; *, calculated value 

resulted in a negative number because of blank correction; %, percent]

Sample ID Total,

in ppm 

Leachate,

in μg/L % bioaccessible

Leached,

in mg/kg

of solid

Total,

in ppm

Leachate,

in μg/L % bioaccessible

Leached,

in mg/kg

of solid

Iron Lead

Lung Lung

Unit 2.2 21,900 <500 n.d. n.d. 19.7 <0.5 n.d. n.d.

Unit 3.1 29,700 <500 n.d. n.d. 21.6 <0.5 n.d. n.d.

Unit 3.2 23,800 <500 n.d. n.d. 18.4 <0.5 n.d. n.d.

Phagolysosomal Phagolysosomal

Unit 2.2 21,900 <500 n.d. n.d. 19.7 0.670 * *

Unit 3.1 29,700 <500 n.d. n.d. 21.6 <0.5 n.d. n.d.

Unit 3.2 23,800 <500 n.d. n.d. 18.4 <0.5 n.d. n.d.

Manganese Uranium

Gastric Gastric

Unit 2.2 259 1,110 42.8 111 6.16 24.6 39.1 2.41 

Unit 3.1 570 1,210 21.2 121 3.03 4.49 13.2 0.399 

Unit 3.2 498 1,410 28.3 141 3.37 9.66 27.2 0.916 

Lung Lung

Unit 2.2 259 24.0 0.888 2.30 6.16 5.02 7.34 0.452

Unit 3.1 570 14.2 0.232 1.32 3.03 <1 n.d. n.d.

Unit 3.2 498 13.2 0.245 1.22 3.37 2.53 6.02 0.203

Phagolysosomal Phagolysosomal

Unit 2.2 259 201 0.008 20.1 6.16 1.97 0.003 0.197

Unit 3.1 570 86.9 0.002 8.69 3.03 <1 n.d. n.d.

Unit 3.2 498 67.9 0.001 6.79 3.37 <1 n.d. n.d.

Vanadium Zinc

Gastric Gastric

Unit 2.2 94.1 204 18.9 17.8 71.7 113 11.3 8.11 

Unit 3.1 80.0 49.4 2.88 2.30 109 83.6 4.74 5.17 

Unit 3.2 73.4 62.6 4.93 3.62 96.8 110 8.07 7.81 

Lung Lung

Unit 2.2 94.1 75.9 7.80 7.34 71.7 <5 n.d. n.d.

Unit 3.1 80.0 11.7 1.15 0.920 109 <5 n.d. n.d.

Unit 3.2 73.4 30.7 3.84 2.82 96.8 <5 n.d. n.d.

Phagolysosomal Phagolysosomal

Unit 2.2 94.1 52.8 0.006 5.28 71.7 <30 n.d. n.d.

Unit 3.1 80.0 6.30 0.001 0.630 109 <30 n.d. n.d.

Unit 3.2 73.4 14.3 0.002 1.43 96.8 <30 n.d. n.d.
10 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

Results

Reflectance Spectroscopy

Average reflectance values range from 0.5842 to 0.6464 

(table 1) across the entire wavelength range of the instrument (average total reflectance), and from 0.3234 to 0.3926 

across the visible wavelength region, 0.40 to 0.70 μm (average visible reflectance). Diagnostic absorption features arising 

from the minerals in the samples were centered near 0.51 μm, 

0.90 μm, and 2.21 μm. Generally, the absorption features in 

the sample spectra were weak, as determined by the shallow 

band depth of the absorption features (see the discussion of 

band depth in Kokaly, 2011). The weak absorption features 

near 2.3 μm may be caused, in part, by absorption by the plastic background behind the samples.

The absorption features in the 1-μm region of the spectra 

of the samples from units 3.1 and 3.2 were matched to the 

reference spectrum of a thin coating of goethite on quartz 

(table 1; Clark and others, 2007). The wavelength positions of 

these weak absorption features indicate that the goethite is in 

low abundance and fine-grained or possibly nanocrystalline. 

Absorption features in the sample from unit 2.2 were even 

weaker, and the spectrum was not matched to any of the reference spectra of iron-bearing minerals.

The absorption features in the 2-μm region indicate the 

presence of montmorillonite clay in low abundance in the 

sample from unit 2.2 (table 1); the sample’s spectrum matched 

the calcium-rich montmorillonite reference spectrum with 

a higher fit value as opposed to the sodium-rich montmorillonite reference spectrum. Samples from units 3.1 and 3.2 

are best matched by the reference spectrum of calcium-rich 

montmorillonite clay and a mixture of calcite (80 percent) and 

calcium-rich montmorillonite clay (20 percent), respectively 

(calcite.8+montmorillonite_Ca.2, table 1). Lower fits to other 

reference spectra indicate the possibility that trace amounts of 

illite and (or) muscovite may be present in the samples from 

units 3.1 and 3.2. Reference spectra of calcite have a 2.3-μm 

absorption feature. An absorption feature near this position is 

present in the sample from unit 3.1 but does not have a high fit 

to the reference feature. Therefore, the weak spectral feature 

in this sample indicates calcite is present in a lower abundance 

than the other minerals.

Magnetic Properties

Hysteresis and Low-Temperature Results

At high fields, the hysteresis loops show a positive-slope 

characteristic of paramagnetic or antiferromagnetic minerals. 

After correction for the high-field susceptibility (Jackson and 

Solheid, 2010), the hysteresis loops display magnetic behavior 

typical of ferrimagnetic minerals like magnetite. However, 

the σhys parameters were all greater than zero, indicating a 

slight wasp-waisted shape due to a possible mixture of lowand high-coercivity minerals (Fabian, 2003). Magnetite and 

titanomagnetite are common low-coercivity minerals, and 

ferric oxide minerals, such as hematite and goethite, are common high-coercivity minerals. The presence of magnetite was 

confirmed by the RTSIRM curves showing the characteristic 

Verwey transition (Verwey, 1939) of magnetite (Verwey transition temperature (TV) is about 120 K) in all three samples 

(fig. 3), but the abundance of magnetite is extremely low. 

From the Ms values at room temperature, the weight percent 

of magnetite was found to vary from 60 parts per million 

(ppm; sample from unit 2.2) to 360 ppm (samples from units 

3.1 and 3.2). The remanence (Mr/Ms) and coercivity (Bcr/Bc) 

ratios have values typical of pseudo-single-domain magnetite 

particles (magnetic grain sizes about 1–10 μm). HIRM values 

were larger in units 3.1 and 3.2 than in unit 2.2, indicating 

higher concentrations of high-coercivity minerals (like hematite) in units 3.1 and 3.2.

The FC and ZFC remanent magnetization curves show 

continuous decreases in magnetization with increasing temperature coupled with FC curves displaying stronger magnetizations than the corresponding ZFC curves until converging near 

300 K (fig. 4). There was no indication of the Morin (1950) 

transition (temperature = 250–260 K) associated with coarsegrained, crystalline hematite in any of the three samples, but 

there was a minor expression of the Verwey transition associated with magnetite in samples 3.1 and 3.2. The RLT ratio 

was 2.7 for sample 2.2 and 1.4 for samples 3.1 and 3.2. The 

FC–ZFC behavior represented by the RLT ratio values confirms 

0 50 100 150 200 250 300

RTSIRM(T)/RTSIRM(300)

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

Temperature, in kelvin

Unit 2.2

Unit 3.2

Unit 3.1

EXPLANATION

Figure 3. Chart showing temperature dependence of roomtemperature saturation isothermal remanence magnetization 

(RTSIRM) in zero field on cooling (ZFC) to 10 kelvin (K) for samples 

from the Nellis Dunes recreation area. The magnetizations during 

ZFC (RTSIRM(T)) are normalized to the initial magnetization at 

300 K (RTSIRM(300)), which was imparted at 300 K in a 2.5-tesla 

field. The drop in magnetization near 120 K is the Verwey transition 

in magnetite. 
Results 11

the presence of a mineral that has high coercivity and is not 

saturated in a 2.5-T field, which is consistent with a significant 

fraction of goethite in all three samples (Guyodo and others, 

2003; Liu and others, 2006; Smirnov, 2009). Furthermore, the 

higher RLT ratio in the sample from unit 2.2 compared with 

those in the samples from units 3.1 and 3.2 indicates that the 

sample from unit 2.2 has a larger fraction of goethite relative 

to magnetite. In pure end-member cases, magnetite has RLT 

ratios less than 1.4 (Smirnov, 2009), but goethite can have 

RLT ratios greater than 2.0 (Guyodo and others, 2003; Liu and 

others, 2006).

Mössbauer Spectroscopy

The Mössbauer spectra at 4.2 K and 300 K for the samples from units 2.2 and 3.2 were fit to multiple doublets and 

sextets representing magnetically ordered goethite and hematite, and paramagnetic ferric (Fe3+) and ferrous (Fe2+) phases. 

The hyperfine parameters are listed in table 3. At 4.2 K, the 

spectra are dominated by Fe3+ paramagnetic doublets (more 

than 50 percent of total iron). However, due to the broadened 

resonant peaks and multiple components, it was not possible 

at the time of the study to assign specific mineral phases to the 

paramagnetic doublets. Goethite was detected in both samples 

and composed approximately 30 percent of the iron-bearing 

phases. A smaller amount of hematite (11 to 13 percent) was 

also detected in the sample from unit 3.2 at 4.2 K and 300 K. 

It was not possible to obtain reliable fits for magnetite in either 

sample because it occurred in such low concentrations (less 

than 0.1 percent, as determined from magnetization experiments) that it was well below the detection limits (about 

1 percent of total iron) for Mössbauer spectroscopy.

Mössbauer analysis for samples from units 2.2 and 

3.2 was also carried out at 300 K. The characteristic sextet 

associated with goethite found at 4.2 K was missing at 300 

K and reflects complete magnetic order for goethite only at 

very low temperatures even though the Néel temperature 

for pure goethite is about 373 K. The difference between 

the 4.2 K and 300 K spectra is due to magnetic relaxation 

and shows that goethite is superparamagnetic at 300 K and 

exhibits a doublet (unblocked and cannot carry remanence) 

but becomes blocked at temperatures higher than 300 K. 

This confirms that a significant fraction of goethite occurs as 

nanogeothite with particle sizes less than about 20 nanometers (nm). This size fraction will not contribute to remanence 

properties (saturation isothermal remanent magnetization 

(SIRM), HIRM) at 300 K.

A magnetically ordered sextet for hematite is observed at 

4.2 K and 300 K in the sample from unit 3.2, indicating that 

most of the hematite is not superparamagnetic for temperatures less than or equal to 300 K and will contribute to roomtemperature remanence properties (SIRM, HIRM). Moreover, 

the quadrupole splitting (QS) parameter at 4.2 K is negative 

(table 3), indicating that a significant fraction of the hematite 

phase in the sample from unit 3.2 is still above the Morin transition even at 4.2 K and is still in its canted antiferromagnetic 

Figure 4. Charts showing thermal demagnetization of lowtemperature isothermal remanent magnetization (LTIRM) imparted 

at 10 K after cooling in a zero field (ZFC) and in a 2.5-tesla 

magnetic field (FC) for samples from A, unit 2.2, B, unit 3.1, and C, 

unit 3.2 from the Nellis Dunes recreation area. The remanence is 

measured on warming to 300 K. 

A. Unit 2.2

B. Unit 3.1

C. Unit 3.2

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

Temperature, in kelvin

0 50 100 150 200 250 300

Magnetization, in ampere square meters per kilogram

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

FC 

ZFC 

EXPLANATION

FC 

ZFC 

EXPLANATION

FC 

ZFC 

EXPLANATION
12 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

state. This is consistent with the absence of the characteristic 

Morin transition for hematite in the LTIRM remanence curves. 

Suppression of the Morin transition in hematite can occur 

due to reduced crystallinity, cation substitution (for example, 

titanium (IV) and aluminum (III)), or small particle effects 

(diameter (d) less than 100 nm; Murad and Johnston, 1987). 

The presence of hematite in unit 3.2 is also consistent with 

HIRM values.

In Vitro Bioaccessibility

Leaching results for selected elements vary in solubility relative to the pH of the IVBA fluids (table 5). In general, 

solubility and bioaccessibility of elements were lowest in the 

neutral pH SLF and highest in the SGF. The elevated bioaccessibility and total concentration of arsenic in all fluids in the 

sample from unit 2.2 indicate large concentrations of leached 

arsenic per kilogram of solid. Three metals (copper, uranium, 

and vanadium) have a higher leachate concentration and 

bioaccessibility in the SLF than in the SLyF. Despite total iron 

concentrations from nearly 22,000 to 29,000 ppm, iron bioaccessibility is minimal in the SGF (less than 0.5 percent), and 

leach concentrations were below the detection limit in the SLF 

and the SLyF.

Discussion

Iron oxide minerals, and especially the ferric oxides 

such as hematite and goethite, are important for issues of 

atmospheric dust. The iron oxide minerals in dust are capable 

of absorbing solar radiation. Whereas strongly magnetic (ferrimagnetic) magnetite (Fe3

O4) depresses reflectance across 

the full spectrum of solar radiation, the weakly magnetic 

(antiferromagnetic) ferric oxide minerals have capacity to 

absorb strongly in the visible and near-infrared wavelengths. 

Radiative properties of iron oxide minerals thus have an 

effect on the ways by which dust can affect weather and 

climate (Gassó and others, 2010) and the melting of snow 

and ice (Painter and others, 2007). Furthermore, ferric oxide 

minerals have the capacity to absorb metals and thereby have 

relevance to human-health issues approached through our 

bioaccessibility methods.

A central contribution of this study is a demonstration of 

combined techniques to determine with high specificity the 

iron oxide minerals in atmospheric dust. Most importantly, 

we identified by multiple, independent methods the presence 

of goethite, possibly nanosized. The presence of hematite 

was detected using Mössbauer spectroscopy and magnetization measurements but was not convincingly supported by 

other methods.

There are too few samples from the NDRA to draw 

any inferences about relations among magnetic properties, 

iron mineralogy, and radiative properties. Nevertheless, the 

data for these samples contribute to data accumulated for 

atmospheric dust originating from source regions elsewhere 

in North America, Australia, as well as southern and northern 

Africa (Moskowitz and others, 2011; Reynolds and others, 

2011). The values for reflectance and magnetic properties of 

the samples from the NDRA are very tightly grouped relative 

to the ranges of values obtained from the much larger, global 

dust suite.

In vitro bioaccessibility tests are becoming more commonly used in the United States, especially to examine contaminated, urban, and geogenic or naturally produced Earth 

materials. Controls on element mobility and bioaccessibility 

may include particle size, soil pH, and the presence and types 

of clays and (or) iron or manganese oxides. Mineral phase 

is an important control on lead and arsenic bioaccessibility 

(Ruby and others, 1996). In these experiments on the samples 

from the NDRA, solution pH was evaluated as the primary 

variable on element mobility because particle sizes are nearly 

the same for all samples and because of a lack of information 

on mineral forms. Extrapolation of the IVBA results to human 

health risk is difficult because many physiological factors control uptake of elements by the body. With the exception of lead 

(U.S. Environmental Protection Agency, 2008), little work has 

been done to evaluate bioaccessible concentrations for other 

elements or their application in determining relative bioavailability values in an environmental setting where exposures are 

generally low dose but chronic. Nonetheless, for some metals 

(arsenic, cadmium, copper, manganese, lead, and uranium), 

upwards of 50 percent of the total concentrations are soluble 

in the simulated body fluids, particularly in the SGF. Despite 

the uncertainties with factors that control the uptake of these 

metals by the body, the elevated solubilities of these metals 

may lead to associated health risks.

The methodology and data presented in this report not 

only complement ongoing studies but also prescribe techniques and provide information for future studies of the effects 

of dust within the NDRA and globally. The ability to characterize atmospheric dust in terms of iron phases, iron oxide 

mineralogy, radiative properties, and bioaccessibility is essential for understanding dust effects from local to global scales.

References Cited

Bradham, K.D., Scheckel, K.G., Nelson, C.M., Seales, P.E., 

Lee, G.E., Hughes, M.F., Miller, B.W., Yeow, Aaron, Gilmore, Thomas, Serda, S.M., Harper, Sharon, and Thomas, 

D.J., 2011, Relative bioavailability and bioaccessibility and 

speciation of arsenic in contaminated soils: Environmental 

Health Perspectives, v. 119, no. 11, p. 1629–1634. (Also 

available at http://dx.doi.org/10.1289/ehp.1003352.)

Brand, R.A., 1987, Improving the validity of hyperfine field 

distributions from metallic alloys—Part I—Unpolarized 

source: Nuclear Instruments and Methods in Physics 

Research, section B, v. 28, no. 3, p. 398–416.
References Cited 13

Briggs, P.H., and Meier, A.L., 2002, The determination 

of forty-two elements in geological materials by inductively coupled plasma-mass spectrometry, chap. I of

Taggart, J.E., Jr., ed., Analytical methods for chemical 

analysis of geologic and other materials: U.S. Geological Survey Open-File Report 02–223, p. I–1—I–14. 

(Also available at http://pubs.usgs.gov/of/2002/ofr-02-

0223/OFR-02-0223.pdf.)

Clark, R.N., Swayze, G.A., Wise, R.A., Livo, K.E., Hoefen, 

T.M, Kokaly, R.F., and Sutley, S.J., 2007, USGS digital 

spectral library splib06a: U.S. Geological Survey Data 

Series 231, available at http://speclab.cr.usgs.gov/spectral.

lib06/ds231/index.html.

Drexler, J.W., and Brattin, W.J., 2007, An in vitro procedure 

for estimation of lead relative bioavailability; with validation: Human and Ecological Risk Assessment, v. 13, 

p. 383–401. (Also available at http://www.tandfonline.com/

doi/abs/10.1080/10807030701226350.)

Fabian, Karl, 2003, Some additional parameter to estimate 

domain state from isothermal magnetization measurements: 

Earth and Planetary Science Letters, v. 213, nos. 3–4, 

p. 337–345. (Also available at http://www.sciencedirect.

com/science/article/pii/S0012821X03003297.)

Formenti, P., Schuetz, L., Balkanski, Y., Desboeufs, K., 

Ebert, M., Kandler, K., Petzold, A., Scheuvens, D., 

Weinbruch, S., and Zhang, D., 2010, Recent progress in 

understanding physical and chemical properties of mineral 

dust: Atmospheric Chemistry and Physics Discussions, v. 10, 

p. 31187–31251.

Gassó, Santiago, Grassian, V.H., and Miller, R.L., 2010, 

Interactions between mineral dust, climate, and ocean ecosystems: Elements, v. 6, no. 4, p. 247–252. (Also available 

at http://elements.geoscienceworld.org/content/6/4/247.

abstract.)

Goossens, Dirk, 2012, A method for dry extracting large 

volumes of fine particulate matter from bulk soil samples: 

Air Quality Atmosphere and Health, v. 5, no. 4, p. 425–431. 

(Also available at http://rd.springer.com/article/10.1007/

s11869-011-0142-7.)

Goossens, Dirk, and Buck, Brenda, 2009a, Dust dynamics in off-road vehicle trails—Measurements on 16 

arid soil types, Nevada, USA: Journal of Environmental Management, v. 90, p. 3458–3469. (Also available 

at http://www.sciencedirect.com/science/article/pii/

S0301479709001972.)

Goossens, Dirk, and Buck, Brenda, 2009b, Dust emission 

by off-road driving—Experiments on 17 arid soil types, 

Nevada, USA: Geomorphology, v. 107, nos. 3–4, p. 

118–138. (Also available at http://www.sciencedirect.com/

science/article/pii/S0169555X08005266.)

Goossens, Dirk, and Buck, Brenda, 2011a, Effects of wind erosion, off-road vehicular activity, atmospheric conditions and 

the proximity of a metropolitan area on PM10 characteristics 

in a recreational site: Atmospheric Environment, v. 45, no. 1, 

p. 94–107. (Also available at http://www.sciencedirect.com/

science/article/pii/S1352231010008228.)

Goossens, Dirk, and Buck, Brenda, 2011b, Gross erosion, net 

erosion and gross deposition of dust by wind: field data 

from 17 desert surfaces: Earth Surface Processes and Landforms, v. 36, no. 5, p. 610–623. (Also available at http://

onlinelibrary.wiley.com/doi/10.1002/esp.2080/abstract.)

Goossens, D., Buck, B., and McLaurin, B., 2012, Contributions to atmospheric dust production of natural and anthropogenic emissions in a recreational area designated for 

off-road vehicular activity (Nellis Dunes, Nevada, USA): 

Journal of Arid Environments, v. 78, p. 80–99. (Also available at http://www.sciencedirect.com/science/article/pii/

S0140196311003168.)

Guyodo, Yohan, Mostrom, Alison, Penn, R.L., Banerjee, S.K., 2003, From nanodots to nanorods—Oriented 

aggregation and magnetic evolution of nanocrystalline goethite: Geophysical Research Letters, v. 30, 

1512, 4 p. (Also available at http://www.agu.org/pubs/

crossref/2003/2003GL017021.shtml.)

Herting, Gunilla, Wallinder, I.O., and Leygraf, Christofer, 

2006, Factors that influence the release of metals from 

stainless steels exposed to physiological media: Corrosion Science, v. 48, no. 8 p. 2120–2132. (Also available 

at http://www.sciencedirect.com/science/article/pii/

S0010938X05002465.)

Jackson, Mike, and Solheid, Peter, 2010, On the quantitative analysis and evaluation of magnetic hysteresis data: 

Geochemistry Geophysics Geosystems, v. 11, Q04Z15, 

25 p. (Also available at http://www.agu.org/pubs/

crossref/2010/2009GC002932.shtml.)

King, J.W., and Channel, J.E.T., 1991, Sedimentary magnetism, environmental magnetism, and magnetostratigraphy: 

Reviews of Geophysics, v. 29, supplement, p. 358–370.

Kokaly, R.F., 2011, PRISM—Processing routines in IDL for 

spectroscopic measurements (installation manual and user’s 

guide, version 1.0): U.S. Geological Survey Open-File 

Report 2011–1155, 431 p., available at http://pubs.usgs.gov/

of/2011/1155/.

Lamothe, P.J., Meier, A.L., and Wilson, S.A., 2002, The 

determination of forty-four elements in aqueous samples 

by inductively coupled plasma-mass spectrometry, chap. H

of Taggart, J.E., Jr., ed., Analytical methods for chemical 

analysis of geologic and other materials: U.S. Geological 

Survey Open-File Report 02–223, p. H–1—H–14. (Also 

available at http://pubs.usgs.gov/of/2002/ofr-02-0223/

OFR-02-0223.pdf.)
14 Iron Mineralogy and Bioaccessibility: Nellis Dunes Recreational Area, Nevada

Lee, A., Lee, E.G., Lee, T., Kim, S.W., Slaven, J.E., and 

Harper, M., 2011, Size-selective sampling of particulates 

using a physiologic sampling pump: Journal of Environmental Monitoring, v. 13, no. 3, p. 527–535. (Also available 

at http://www.ncbi.nlm.nih.gov/pubmed/21234496.)

Liu, Qingsong, Yu, Yongjae, Torrent, José, Roberts, A.P., 

Pan, Yongxin, and Zhu, Rixiang, 2006, Characteristic 

low-temperature magnetic properties of aluminous goethite [α-(Fe, Al)OOH] explained: Journal of Geophysical 

Research, v. 111, B12S34, 12 p. (Also available at http://

www.agu.org/pubs/crossref/2006/2006JB004560.shtml.)

McLaurin, B.A.T., Goossens, Dirk, and Buck, B.J., 2011a, 

Combining surface mapping and process data to assess, 

predict, and manage dust emissions from natural and 

disturbed land surfaces: Geosphere, v. 7, no. 1, February, 

p. 260–275. (Also available at http://geosphere.gsapubs.org/

content/7/1/260.abstract.)

McLaurin, Brett, Goossens, Dirk, and Buck, Brenda, 2011b, 

Physical settings of the Nellis Dunes Recreation Area, 

chap. 2 of Goosens, Dirk, and Buck, B.J., eds., Assessment 

of dust emissions, chemistry, and mineralogy for management of natural and disturbed surfaces at Nellis Dunes 

recreation area, Nevada: Las Vegas, Nevada, University 

of Nevada Las Vegas, p. 13–39. (Also available at http://

www.blm.gov/nv/st/en/fo/lvfo/blm_programs/more/nellis_dunes_dust.html.)

Morin, F.J., 1950, Magnetic susceptibility of α-Fe2

O3 and α-Fe2O3 

with added titanium: Physical Review, v. 78, no. 6, p. 819–820.

Morman, S.A., Plumlee, G.S., and Smith, D.B., 2009, Application of in vitro extraction studies to evaluate element 

bioaccessibility in soils from a transect across the United 

States and Canada: Applied Geochemistry, v. 24, no. 8, 

p. 1454–1463. (Also available at ftp://nrn1.nrcan.gc.ca/ess/

geochem/files/miscel/env_health/applied_geochem_v24_

issue8/p1454_1463.pdf.)

Moskowitz, B., Yauk, K., Till, J., Berquó, T., Banerjee, S.K., 

Reynolds, R.L., and Goldstein, H.L., 2011, Magnetic 

properties of iron oxide minerals in atmospheric dust and 

source sediments from western US: Eos, Transactions of 

the American Geophysical Union fall meeting supplement, 

abstract U11A–0002. (Also available at http://www.agu.org/

meetings/fm11/waisfm11.html.)

Murad, Enver, and Johnston, J.H., 1987, Iron oxides and oxyhydroxides, in Long, G.J., ed., Mössbauer spectroscopy applied 

to inorganic chemistry: New York, Springer, v. 2, p. 507–582.

Painter, T.H., Barrett, A.P., Landry, C.C., Neff, J.C., Cassidy, M.P., Lawrence, C.R., McBride, K.E., and Farmer, 

G.L., 2007, Impact of disturbed desert soils on duration of 

mountain snow cover: Geophysical Research Letters, v. 34, 

L12502, 6 p. (Also available at http://www.agu.org/pubs/

crossref/2007/2007GL030284.shtml.)

Reynolds, R.L., Goldstein, H.L., Moskowitz, B.M., Till, 

J.L., Flagg, C., Kokaly, R.F., Munson, S., Landry, C., 

Lawrence, C.R., Hiza, M.M., D’Odorico, P., and Painter, 

T.H., 2011, Iron oxide minerals in atmospheric dust and 

source sediments—Studies of types and properties to assess 

environmental effects: Eos, Transactions of the American 

Geophysical Union Fall Meeting Supplement Abstract 

U13B–01. (Also available at http://www.agu.org/meetings/

fm11/waisfm11.html.)

Rochette, Pierre, Mathé, P.-E., Esteban, Lionel, Rakoto, 

Harison, Bouchez, J.-L., Liu, Qingsong, and Torrent, José, 

2005, Non-saturation of the defect moment of goethite and 

fine-grained hematite up to 57 teslas: Geophysical Research 

Letters, v. 32, L22309, 4 p. (Also available at http://www.

agu.org/pubs/crossref/2005/2005GL024196.shtml.)

Ruby, M.V., Davis, Andy, Schoof, Rosalind, Eberle, Steve, 

and Sellstone, C.M., 1996, Estimation of lead and arsenic 

bioavailability using a physiologically based extraction 

test: Environmental Science and Technology, v. 30, no. 

2, p. 422–430. (Also available at http://pubs.acs.org/doi/

abs/10.1021/es950057z.)

Smirnov, A.V., 2009, Grain size dependence of low-temperature remanent magnetization in natural and synthetic magnetite—Experimental study: Earth Planets Space, v. 61, no. 

1, p. 119–124. (Also available at http://www.terrapub.co.jp/

journals/EPS/pdf/2009/6101/61010119.pdf.)

Soukup, Deborah, Buck, Brenda, Goossens, Dirk, Ulery, April, 

McLaurin, B.T., Baron, Dirk, and Teng, Yuanxin, 2012, 

Arsenic concentrations in dust emissions from wind erosion 

and off-road vehicles in the Nellis Dunes recreational area, 

Nevada, USA: Aeolian Research, v. 5, August, p. 77–89. 

(Also available at http://www.sciencedirect.com/science/

article/pii/S1875963711000851.)

Stefaniak, A.B., Guilmette, R.A., Day, G.A. Hoover, M.D., 

Breysse, P.N., and Scripsick, R.C., 2005, Characterization 

of phagolysosomal simulant fluid for study of beryllium 

aerosol particle dissolution: Toxicology in Vitro, v. 19, no. 

1, p. 123–134. (Also available at http://www.sciencedirect.

com/science/article/pii/S0887233304001250.)

Stefaniak, A.B., Day, G.A., Hoover, M.D., Breysse, P.N., 

and Scripsick, R.C., 2006, Differences in dissolution 

behavior in a phagolysosomal simulant fluid for singleconstituent and multi-constituent materials associated with 

beryllium sensitization and chronic beryllium disease: 

Toxicology in Vitro, v. 20, no. 1, p. 82–95. (Also available at http://www.sciencedirect.com/science/article/pii/

S0887233305001098.)

U.S. Environmental Protection Agency, 2008, Standard 

operating procedure for an in vitro bioaccessibility assay 

for lead in soil: U.S. Environmental Protection Agency 

EPA 9200.1–86. (Also available at http://www.epa.gov/

superfund/bioavailability/pb_ivba_sop_final.pdf.)
References Cited 15

Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Geosciences and Environmental Change

Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geosciences and Environmental Change Science Center 

Web site at: http://gec.cr.usgs.gov/

(303) 236-1562

Or visit the Central Mineral and Environmental Resources Science 

Center Web site at: http://minerals.cr.usgs.gov/

Van Wijnen, J.H., Clausing, P., and Brunekreef, B., 1990, Estimated soil ingestion by children: Environmental Research, 

v. 51, no. 2, p. 147–162. (Also available at http://www.sciencedirect.com/science/article/pii/S0013935105800854#.)

Verwey, E.J., 1939, Electronic conduction of magnetite 

(Fe3O4) and its transition point at low temperatures: Nature, 

v. 144, no. 3642, p. 327–328. (Also available at http://www.

nature.com/nature/journal/v144/n3642/pdf/144327b0.pdf.)
Goldstein and others—Iron Mineralogy and Bioaccessibility—Nellis Dunes Recreational Area, Nevada—Scientitic Investigations Report 2013–5054