Spectral Variations in Rocks and Soils Containing 

Ferric Iron Hydroxide and(or) Sulfate Minerals as 

seen by AVIRIS and Laboratory Spectroscopy

Barnaby W. Rockwell1 

Open-File Report 2004-1431 

Version 1.0

2004 

Open-File Reports are used for the dissemination of information that must be released immediately to fill a public 

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U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY 

1

barnabyr@usgs.gov, Denver, Colorado
Abstract 

Analysis of Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data covering the

Big Rock Candy Mountain area of the Marysvale volcanic field, west-central Utah, identified 

abundant rocks and soils bearing jarosite, goethite, and chlorite associated with volcanic rocks 

altered to propylitic grade during the Miocene (23–21 Ma). Propylitically-altered rocks rich in 

pyrite associated with the relict feeder zones of convecting, shallow hydrothermal systems are 

currently undergoing supergene oxidation to natrojarosite, kaolinite, and gypsum. Goethite 

coatings are forming at the expense of jarosite where most pyrite has been consumed through 

oxidation in alluvium derived from pyrite-bearing zones. Spectral variations in the goethitebearing rocks that resemble variations found in reference library samples of goethites of varying 

grain size were observed in the AVIRIS data. Rocks outside of the feeder zones have relatively 

low pyrite content and are characterized by chlorite, epidote, and calcite, with local copperbearing quartz-calcite veins. Iron-bearing minerals in these rocks are weathering directly to 

goethite. 

Laboratory spectral analyses were applied to samples of iron-bearing rock outcrops and 

alluvium collected from the area to determine the accuracy of the AVIRIS-based mineral 

identification. The accuracy of the iron mineral identification results obtained by analysis of the

AVIRIS data was confirmed. In general, the AVIRIS analysis results were accurate in 

identifying medium-grained goethite, coarse-grained goethite, medium- to coarse-grained

goethite with trace jarosite, and mixtures of goethite and jarosite. However, rock fragments from

alluvial areas identified as thin coatings of goethite with the AVIRIS data were found to consist 

mainly of medium- to coarse-grained goethite based on spectral characteristics in the visible and 

near-infrared. 

To determine if goethite abundance contributed to the spectral variations observed in 

goethite-bearing rocks with AVIRIS data, a laboratory experiment was performed in which 

spectra were acquired of a goethite-bearing rock while progressively decreasing the areal 

abundance of the rock with respect to a background of white, fine-grained quartz sand. This 

experiment found that, with decreasing material abundance, the crystal field absorption feature of 

goethite near 1.0 micron decreases in depth and narrows more from the long wavelength side of 

the feature than from the short wavelength side, as is the case in goethite reference spectra as

grain size decreases from coarse to fine. 

In the Marysvale study area, goethite-bearing alluvium downgradient from source 

outcrops tends to be identified as finer-grained or thin coatings of goethite due to the mineral’s

presence in lesser abundance. The goethite-bearing alluvium is a closer match to reference

spectra of thin coatings of goethite even though the actual grain size of the contained goethite 

fragments is medium to coarse grained, the same on average as that from the source outcrops. 

Coarser-grained goethite most likely will be correctly identified in areas of greater goethite 

abundance proximal to jarosite-bearing source rock where the surface is relatively free of 

goethite-free soil components and vegetation that corrupt the goethite spectral response. 

When analysis of imaging spectroscopy data is performed using reference spectra of iron 

minerals of varying grain sizes and mixed compositions, the results are useful not only for 

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purposes of mineral identification, but also for distinguishing goethite-bearing outcrop from 

alluvial surfaces with similar mineralogy, providing valuable information for geologic, 

geomorphologic, mineral exploration, and environmental assessment studies. 

Introduction 

Iron oxides, hydroxides, oxyhydroxides, and sulfates are common on the earth’s surface 

and commonly are associated with hydrothermally altered rocks and weathered iron-bearing 

rocks. The character and surficial distribution of iron-bearing minerals often provide important 

keys for locating and understanding the nature of hydrothermally altered rocks and their 

associated mineral deposits. In addition, secondary iron-bearing minerals are important 

indicators of areas with possible acid producing potential (APP). The presence of jarosite, a 

relatively insoluble iron sulfate mineral that can be produced through supergene, subaerial 

weathering of pyrite and other iron-bearing sulfide minerals and is readily identifiable using 

spectroscopic remote sensing, has been used as an important indicator of sources of acid 

generation in mine waste and mineralized rocks in unmined areas (Swayze and others, 2000; 

Rockwell and others, 2004). Soluble sulfate salts precipitated from low pH waters associated 

with pyrite oxidation, including the hydrated ferric iron hydroxyl-sulfate mineral copiapite, the 

hydrated ferrous iron sulfate melanterite, and ferric and ferrous iron-bearing hydroxyl-sulfates 

such as schwertmannite, also are characterized by diagnostic electronic absorption features in the 

visible and near-infrared (NIR) that allow spectroscopic identification and differentiation 

(Crowley and others, 2003; Cunningham and others, 2004). These minerals are difficult to 

remotely identify using imaging spectroscopy, however, because of their limited areal 

occurrence with respect to the often genetically related hematite, goethite, and(or) jarosite that 

are more common and stable and tend to dominate the spectral signature of a remotely-sensed 

image pixel. Iron hydroxides, such as goethite, are abundant on the earth’s surface and are 

commonly associated with more neutral surface hydrological conditions in zones of 

hydrothermally altered rocks and mine waste and tailings. Rocks and soils associated with iron 

sulfate and hydroxide minerals in these environments often have anomalously high 

concentrations of toxic metals available for release into ground- and surface-water systems. 

Potentially toxic metals, such as arsenic, may preferentially adsorb to poorly crystalline 

(amorphous) iron hydroxides and oxyhydroxides in near neutral pH environments (Bowell, 

1994). Spectroscopy is an accurate and efficient tool for locating and identifying iron-bearing 

minerals, especially considering that X-ray diffraction (XRD) generally is unreliable in detecting 

amorphous iron minerals even when iron is present in high abundance (Swayze and others, 2000; 

Rockwell and others, 2004). 

Where pyrite occurs in altered rocks or in piles of mine waste, subaerial oxidation 

generates copiapite and(or) melanterite, which eventually will dehydrate to the more stable and 

insoluble jarosite. As the pyrite is consumed through oxidation, local acidity will decrease, and 

coatings of goethite will be formed, gradually replacing and covering the coatings of jarosite. 

Therefore, a gradational outward transition from a central zone of pyrite to minerals formed in 

progressively more neutral pH conditions will develop (Swayze and others, 2000). Due to its 

low reflectance, weak (saturated) crystal field absorptions, and frequent masking by coatings of 

secondary iron sulfate minerals, pyrite itself often is difficult to remotely identify using imaging 

spectroscopy, although the mineral was directly detected using data acquired by the Airborne 

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Visible Infrared Imaging Spectrometer (AVIRIS) at an abandoned sulfide ore mill near Stockton, 

Utah (Rockwell and others, 2004). With imaging spectroscopy, the mineralogic signature of 

ongoing pyrite oxidation usually is observed as a central zone of fine-grained jarosite, which 

grades outwards to a jarosite + goethite areal mix, and finally to goethite. Additional zonation 

within the zone of goethite-bearing rocks also has been observed surrounding unmined and 

anthropogenic sources of pyrite in semiarid climates (Rockwell and others, 2004; Livo and 

Clark, 2002). The purposes of this report are to document and to explain these spectral 

variations observed using AVIRIS data and to verify the iron mineral identification results 

obtained from AVIRIS data through the use of an expert system for semiautomated 

spectroscopic analysis. 

Identification and Mapping of Iron-Bearing Minerals Using AVIRIS Data and 

an Expert System for Spectral Analysis 

Background 

An expert system for spectral analysis developed at the United States Geological Survey 

(USGS) (Clark and others, 2003a) compares an “unknown” input spectrum from a laboratory or 

imaging spectrometer to a subset of spectra derived from a large spectral library of minerals, 

mineral mixtures, man-made materials, volatiles, and vegetation (Clark and others, 2003b). A 

modified least-squares curve-fitting algorithm is used to perform the spectral comparisons. The 

basic principle behind the expert system is that the spectral library upon which the system is 

based represents close to the full range of spectral variability present on the earth’s surface. As 

iron-bearing minerals exhibit profound spectral variations related to particle grain size, grain size 

series of several iron minerals were included in the expert system. Previous studies have shown 

that the distance (or path length) a photon may travel within a mineral grain increases with 

effective grain size, as larger grains offer increased volume-to-surface ratios and reduced 

chances for surface reflection (Clark, 1999). Therefore, in the visible and NIR, as grain size 

increases, path length and absorption increase while reflectance level decreases. Figure 1 shows 

spectra of a hematite grain-size series and illustrates how larger grain sizes exhibit increased 

saturation of the crystal field (Laporte-forbidden transition) absorption feature at 0.9 μm, 

indicated by the broadening of the feature and shifting of the apparent reflectance minima to 

longer wavelengths. In order to compare absorption features by their wavelength position and 

shape, the features must be isolated from other effects, such as level changes and slopes due to 

other absorbing (or emitting) materials (Clark and others, 2003a). A convex hull that fits the 

“highpoints” of a spectrum often represents these effects. A process named continuum-removal 

removes these extraneous effects by 1) computing a slope between two endpoints surrounding an 

absorption feature, and 2) removing this slope by dividing it into the original spectrum. The 

result of the continuum-removal procedure is a spectrum with reflectance values of 1 where the 

original spectrum coincides with the convex hull that fits the original spectrum. 

4
Figure 1. Hematite grain-size series. All spectra are in continuum-removed format. Larger 

grain sizes show increased saturation of the crystal field absorption at 0.9 μm, as 

well as broadening and shifting of the apparent reflectance minima to longer 

wavelengths. These spectra are included as reference spectra in the USGS expert 

system for spectral analysis. From Clark (1999). 

A series of goethite spectra exhibiting similar spectral variations were added to the expert 

system and labeled as a goethite grain-size series (Clark and others, 2003a). Some of these 

goethite spectra were produced from crushed, purified, and sieved botyroidal goethite, and others 

were included in the grain-size series based on a visual estimation of grain size according to the 

shape and position of the crystal field absorptions at 0.485, 0.66, and 0.9 μm (Clark and others, 

2003b). Figure 2 shows the spectra of the “virtual” goethite grain size series in addition to 

spectra of jarosite and a goethite + jarosite mixture from the reference spectral library. Figure 3 

shows the same spectra in continuum-removed format in the vicinity of the crystal field 

electronic transition feature near 1.0 μm. It is evident from figure 3 that the absorption feature of 

goethite near 1.0 μm exhibits similar changes in shape and position with apparent grain size as 

does the feature in the hematite spectra shown on figure 1. This feature is used as a diagnostic 

feature for the identification of goethite, jarosite, and hematite by the expert system. Figure 4 

shows the same goethite and jarosite spectra in continuum-removed format in the vicinity of the 

crystal field electronic transition feature near 0.5 μm that is also used as a diagnostic feature by 

the expert system for the goethite series, goethite + jarosite mixture, and “coarse-grained” 

jarosite (sample BR93-34A2). The shape and position changes evident on figures 1 and 3 are not 

5
Figure 2. Reference spectra of goethite, jarosite, and a goethite + jarosite mixture. The 

spectrum of sample MPCMA2-C (shown in red), labeled as a “medium to coarse” 

grained goethite, may contain a trace amount of jarosite, as indicated by a small 

Fe-OH vibrational feature in the shortwave-infrared (SWIR) near 2.27 μm (not 

shown). The VIS-NIR portion of the spectrum shown here shows little, if any, 

evidence of jarosite, which may be indicative of a goethite coating which is 

opaque in the VIS-NIR but partly transparent in the SWIR (Clark and others, 

2003b). Click HERE for a larger image. 

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Figure 3. Reference spectra of goethite, jarosite, and a goethite + jarosite mixture. Spectra 

shown on figure 2 are shown here in continuum-removed format around the 

crystal field electronic transition feature near 1.0 μm. Click HERE for a larger

image. 

7
Figure 4. Reference spectra of goethite, jarosite, and a goethite + jarosite mixture. Spectra 

shown on figures 2 and 3 are shown here in continuum-removed format around 

the crystal field electronic transition feature near 0.50 μm. Click HERE for a 

larger image. 

8
exhibited for the feature near 0.5 μm displayed on figure 4. The crystal field electronic transition 

feature near 0.66 μm (not shown) normally is not used as a diagnostic feature by the expert 

system because this feature overlaps with the feature caused by chlorophyll in green plants. 

AVIRIS Data Analysis 

AVIRIS data acquired from the high-altitude ER-2 aircraft on August 5, 1998, and from 

the low-altitude Twin Otter turboprop aircraft on October 17, 1999, over the Big Rock Candy 

Mountain area of the Marysvale volcanic field in west-central Utah were analyzed using the 

USGS expert system (Rockwell and others, 2000; Rockwell and others, 2004; Cunningham and 

others, 2004). Figure 5 is a map of iron-bearing minerals and water interpreted from the lowaltitude AVIRIS data, which has a ground instantaneous field of view (GIFOV) of ~3 m/pixel. 

Pixels identified as iron-bearing minerals are shown in solid colors (see map explanation of 

mineralogic color coding), except for pixels that matched the reference spectra of goethite thin 

coating or fine-grained hematite, which are each shown in a range of colors (a “continuous 

stretch”) whose brightness is proportional to the fit × depth (fit times depth) values of that 

material (Rockwell and others, 2004; Clark and others, 2003a). Pixels with high fit × depth 

values are those where the AVIRIS spectrum had a high degree of least-squares fit to the 

reference spectrum and large absorption feature depths. Pixels with high fit × depth values for a 

given reference material depicted on the map with a continuous stretch are colored as shown on 

the map explanation, and pixels with lower fit × depth values are colored with successively 

darker shades of that color. 

Figure 6 shows an enlargement of a portion of Figure 5 for the area from which rock 

samples were acquired for this study. This area is underlain by andesitic to dacitic volcanic

rocks of the Miocene/Oligocene Bullion Canyon Volcanics (33 Ma) that were altered by one of 

several convective, near-surface, H2S-bearing hydrothermal systems that developed around a

composite intracaldera intrusion of quartz monzonite (the Central intrusion) soon after its

emplacement around 23 Ma (Cunningham and others, 1984; Rowley and others, 2002). The 

Hoover pluton, a quartz monzonite stock comprising the southwestern section of the Central 

intrusion, is indicated on figure 5 immediately east of the northward-flowing Sevier River. 

Immediately adjacent to the feeder zones of the hydrothermal systems, the volcanic and intrusive 

rocks have been propylitically altered and are rich in calcite, epidote, and chlorite. 

Sample MV03-10 (fig. 6) was collected from propylitically-altered rocks in which 

muscovite and chlorite are abundant locally. In these rocks, local pyrite in trace amounts and(or) 

other iron-bearing minerals have weathered to goethite. From the AVIRIS data, the area 

immediately surrounding the location of sample MV03-10 mapped as coarse-grained goethite, 

“generic” ferrous iron minerals (including chlorite), or ferrous iron with goethite and 

illite/muscovite. Veins of calcite and quartz with greenish specks of copper-bearing minerals 

formed from volatile-rich fluids within the carapace of the stock cut the outcrop. At the Trinity 

mine approximately 300 m to the southeast (fig. 5), similar veins were mined for copper from

skarn deposits within a flap of Triassic and Permian metasedimentary rocks that was tilted 

upward at the edge of the stock. 

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10
Figure 5. Map of iron-bearing minerals and water generated from AVIRIS data of the Big 

Rock Candy Mountain area, Marysvale volcanic field, Utah. Area from which 

rocks were sampled for this study (fig. 6) is indicated by the blue box. On this 

map, two spectrally identified mineral types are displayed through the use of 

continuous stretches in which the brightness level of the color selected for that 

assemblage decreases with decreasing spectral fit × depth of the material 

identification. Continuous stretches (marked “C” in map key) have been applied 

to fine-grained hematite and thin coatings of goethite. White arrows with black 

outlines show several examples of the gradation in mineralogy and(or) grain size 

associated with the weathering of pyrite-bearing rock mentioned in the text. Click 

HERE for a larger image (JPEG format). Click HERE for the map in highresolution, zoomable PDF format (use Acrobat 5+ to view). 

The other sample locations shown on figure 6 are within narrow zones of intense 

propylitic alteration from the feeder zone of the Big Star hydrothermal system (Rockwell and 

others, 2004). These altered rocks are rich in pyrite and illite and were formed in reducing 

conditions well below the paleo-water table and the lower limit of boiling. The pyrite presently 

is oxidizing to natrojarosite, and acidic fluids generated by the oxidation process are currently 

forming supergene kaolinite, gypsum, and minor smectite similar to the exposed, yellow-colored

flanks of Big Rock Candy Mountain less than a kilometer to the southwest. The most intensely 

altered rocks are found along WNW-striking fractures that cross the small N-S-trending ridge 

shown in the AVIRIS color composite image at the top of figure 6. Jarosite occurs along the 

fractures near the top of the ridge where pyrite is continually being exposed by erosion. Areas 

with abundant jarosite and(or) goethite on the surface appear as yellow to white areas on the 

AVIRIS color composite. Lower down the flanks of the ridge (sample MV03-4), goethite 

becomes more abundant at the expense of jarosite in coarse-grained alluvium derived from the

altered rocks above. Sample MV03-11 was taken from the south-facing slope of an adjacent EW-trending ridge that was sparsely covered with scattered cobbles of goethite-coated altered 

rock up to 20 cm in size, chips and angular cobbles of chlorite-bearing propylite, and relatively 

unaltered volcanic rocks. Sparse sage and dry grass were present in greater abundance there than 

on the more altered N-S-trending ridge, although light gray soil was extensively exposed 

between the cobbles and vegetation on the slope. The alluvium on this south-facing slope was a 

spectral match to the reference library spectrum of thin coatings of goethite, whereas samples 

from the more altered areas along and adjacent to the N-S-trending ridge were taken from areas 

that were spectral matches to coarse-grained goethite, coarse-grained goethite ± trace jarosite 

(reference library sample MPCMA2-C, figs. 2–4), medium-grained goethite, and coarse-grained 

jarosite from the reference library. 

It is apparent from figures 5 and 6 that the number of pixels identified as thin coatings of

goethite is far greater than the number of pixels identified as coarse- or medium-grained goethite, 

reflecting a greater areal extent of finer-grained, “distal” alluvium bearing only small quantities

of goethite-bearing fragments as compared to goethite-bearing outcrop surface and coarser11
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Figure 6. Enlargement of rock sample collection area from map of iron-bearing minerals 

shown on figure 5. In the AVIRIS color composite shown at top, the N-Strending ridge underlain by jarosite- and goethite-bearing altered rocks mentioned 

in the text is located between points A and B, and the E-W-trending ridge 

underlain by less-altered rocks is located between points B and C. Click HERE

for a larger image. 

grained, “proximal” alluvium. The “distal” alluvium mapped as thin coatings of goethite is 

comprised of soil, sparse vegetation, and fragments of both unaltered and goethite-bearing 

altered rock. Where pyritic rock is exposed, figure 5 generally shows a local outward gradation 

from outcrops with fine-grained coatings of jarosite, to aerial mixtures of jarosite and goethite, to

coarse-grained goethite with trace jarosite, to coarse-grained goethite, intermediate-grained 

goethites, and finally to thin coatings of goethite. Some examples of this gradation are indicated 

on figure 5 by white arrows with black outlines. This gradation may be interrupted by local 

perturbations in lithology and(or) geomorphology, including where goethite is present that was

formed directly by weathering of iron-bearing minerals (for example biotite or hornblende) in 

propylitically-altered volcanic rocks. Jarosite with “coarse-grained” features (match to spectrum

of reference library sample BR93-34A2-r95 shown on figs. 2–4) may be identified instead of 

fine-grained jarosite coatings in areas where pyrite is less abundant and goethite has begun to 

replace and(or) cover older jarosite coatings, as is the case in the study area shown on figure 6. 

In general, both jarosite and coarser-grained goethite tend to occur closer to areas where active 

pyrite oxidation and acid generation are occurring. 

Figure 7 shows single-pixel high altitude AVIRIS spectra sampled from altered rocks of

the Big Star cell. The colors of the spectra on figure 7 are similar to those of the goethite mineral 

classes on the mineral maps that they represent (figs. 5 and 6). Like the spectra in the hematite 

and goethite grain size series shown on figures 1–4, the goethite spectra on figure 7 show a

general broadening and shift of the absorption position to longer wavelengths with increasing 

grain size. The spectrum identified as coarse-grained goethite with trace jarosite (spectrum #5, 

shown in magenta) shows a small shift of the absorption minimum back toward shorter 

wavelengths and a small feature at 0.43 μm, both indicative of a trace content of jarosite. 

However, the overall shape of the absorption feature near 0.9 μm is quite similar to that of the 

coarse-grained goethite spectrum shown in dark green (spectrum #4). A positive spectral feature 

near 1.1 μm in the spectrum of fine-grained goethite (spectrum #2) is a residual artifact caused 

by atmospheric water vapor. 

The AVIRIS spectrum that was a match to thin coatings of goethite (shown in yellow on 

fig. 7) is representative of the AVIRIS spectra of the south-facing slope from which sample

MV03-11 was collected. Although the AVIRIS spectrum is dominated by the goethite from the 

rock fragments scattered across the surface, the extent to which the AVIRIS spectrum is affected

by the small, dark fragments of chlorite bearing, propylitically altered rock and possible trace 

amounts of fine-grained ferric and(or) ferrous iron-bearing minerals (including goethite, chlorite, 

and epidote) in the light-gray colored soils is unknown. Further work is needed to sample and 

13
Figure 7. Single-pixel reflectance spectra of altered rocks sampled from high-altitude 

AVIRIS data at the southwestern edge of the Big Star hydrothermal cell. Upper 

plot shows AVIRIS reflectance spectra. Lower plot shows same spectra in 

continuum-removed format in vicinity of crystal field absorption caused by ferric 

iron near 1.00 μm. Click HERE for a larger image. 

14
characterize the soils from this slope to determine what effect they may have on the AVIRIS

spectra. 

Laboratory Analysis of Rock Samples 

Rock samples were characterized spectrally in the laboratory with an ASD FieldSpec Pro 

FR spectrometer over the 0.35–2.50 μm range. Spectra of individual rock fragments were 

measured from sample locations MV03-11 (a–c) and MV03-10b (fig. 6). Rock fragments from

the remaining locations (fig. 6) were measured in piles about 15 cm across and up to 5–6 cm in 

height. For each sample, 60 spectral measurements were integrated to generate a saved 

spectrum, and from 10 to 15 saved spectra were averaged together. The ASD FR spectrometer 

has three separate detector arrays that acquire data from different wavelength regions of the 

electromagnetic spectrum (array 1: VNIR, 0.35–1.05 μm; array 2: SWIR1, 1.00–1.80 μm; and 

array 3: SWIR2, 1.80–2.50 μm). In the averaged spectra, offsets in reflectance level between 

data acquired by each of the three detector arrays were computed and removed by extrapolating 

the linear trends from several channels at each end of the more stable SWIR1 array, calculating

the offsets between the extrapolated trends and the first channels of the adjacent arrays, and 

adding the offsets to adjust all data from the adjacent arrays. The adjusted spectra then were 

converted to absolute reflectance through multiplication by the reflectance spectrum of the

Spectralon white reference standard. The spectra in absolute reflectance format are shown on 

figure 8. 

Final spectra were convolved to the wavelength centers and bandpass of the 1998 

AVIRIS instrument by using a spline algorithm assuming Gaussian bandpass distributions and 

normalized to unity. These convolved spectra were analyzed with the expert system to 

determine iron mineral composition. The iron minerals identified by the expert system in each 

input spectrum are shown on figure 8 at lower right. 

Rock fragments from sample location MV03-11, which mapped as thin coatings of 

goethite from the AVIRIS data, were identified as coarse- (two fragments) or medium-grained 

(one fragment) goethite from the laboratory spectra. Single-spectrum analysis by the expert

system of rock fragments from the other sample locations generally agreed with the iron mineral 

and grain size identifications from the AVIRIS mapping (fig. 6). 

The laboratory spectra of most of the rocks identified with AVIRIS as having spectral

characteristics similar to those of medium- or coarse-grained goethite contained some spectral 

evidence of jarosite, although they were identified as goethite-bearing (± trace jarosite) with the 

expert system. The AVIRIS mapping identified coarse-grained jarosite in the immediate 

vicinities of sample locations MV03-4, MV03-5, and MV03-9. Field checking identified dark 

orange-yellow (typical of coarse-grained jarosite or a jarosite-goethite mixture) coatings of

jarosite on pyritic rock at these locations, in addition to abundant goethite. The laboratory 

spectra of these samples were all identified as medium- to coarse-grained goethite ± trace jarosite 

(reference library sample MPCMA2-C, figs. 2–4) with the expert system, and evidence of 

jarosite is present in the spectra at 0.43 μm, 0.66 μm, 1.47 μm, and weakly at 2.27 μm. These 

15
Figure 8. Laboratory reflectance spectra of rocks sampled from the Big Star hydrothermal 

cell. Refer to figure 6 for sampling locations. Iron mineral identifications made 

by the expert system are shown at lower right. Click HERE for a larger image. 

16
spectra are similar to a reference spectrum of an intimate mixture of natrojarosite + illite +

gypsum sampled from the pyritic feeder zone of the Big Rock Candy cell that is undergoing 

supergene weathering (sample BRCM-1; Rockwell, 2002), although the spectra presented here 

show no obvious evidence of gypsum. Several pixels matching the BRCM-1 reference spectrum 

were identified at the location of sample MV03-9 with the AVIRIS mapping in the SWIR region 

(Rockwell and others, 2004). 

At sample location MV03-8, the AVIRIS mapping identified medium- to coarse-grained

goethite ± trace jarosite (reference library sample MPCMA2-C, figs. 2–4), medium-grained 

goethite (reference library sample GDS240, figs. 2–4), and coarse-grained goethite (reference

library sample WS222, figs. 2–4). The laboratory spectra of samples from this location were 

identified as either medium- to coarse-grained goethite ± trace jarosite (reference library sample 

MPCMA2-C, figs. 2–4) or medium-grained goethite (reference library sample GDS240, figs. 2–

4). Fragments of propylitically-altered rocks near sample location MV03-10 were identified in 

the laboratory as bearing “generic” ferrous iron plus either chlorite (MV03-10a, pile) or goethite 

(MV03-10b, single rock fragment). Absorptions diagnostic of chlorite and muscovite are evident 

in the SWIR regions of these spectra. 

Spectral Analysis of Goethite Abundance

To determine if goethite abundance contributed to the spectral variations observed in 

goethite-bearing rocks with AVIRIS data, a laboratory experiment was performed in which 

spectra were acquired of a goethite-bearing rock while progressively decreasing the areal 

abundance of the rock with respect to a background of white, fine-grained quartz sand. 

The fiber-optic cable of the ASD FieldSpec Pro FR spectrometer with a 25o field of view 

was positioned to view directly downward at a spot with approximately 10 cm diameter. A 

large, shallow tray wrapped in aluminum foil was covered with a layer of fine-grained white 

quartz sand approximately 2 cm in thickness. A single rock approximately 20 cm by 11.5 cm in 

size sampled from an altered fracture zone on Jupiter Hill southwest of Park City, Utah was 

placed on the sand and spectrally characterized for this experiment. Spectra were measured first 

with the rock sample covering 100 percent of the spot. This spectrum was named “spotfill.” The 

rock sample was then moved in 2.5 cm increments across the spot, so that progressively less of 

the sample filled the spot relative to the background substrate of quartz sand. These spectra were 

named “spotfill_0.75,” “spotfill_0.5,” and “spotfill_0.25,” respectively. For the next to last

measurement, the sample was moved only 1.25 cm, generating the spectrum named

“spotfill_0.125.” Lastly, a spectrum of only the quartz sand was measured, generating a 

spectrum titled “nofill-sand.” For each spectrum, 60 spectral measurements were integrated to 

generate a saved spectrum, and from 10 to 15 saved spectra were averaged together. The 

resultant spectra were reduced to absolute reflectance format as described above and are shown 

on figure 9. Final spectra were convolved to the wavelength centers and bandpass of the 1998 

AVIRIS instrument by using a spline algorithm assuming Gaussian bandpass distributions and 

normalized to unity. These convolved spectra were analyzed using the expert system to 

determine iron mineral composition. The results of these analyses are shown on figures 9 and 10 

at lower right. 

17
Figure 9. Laboratory reflectance spectra of an altered rock (sample PC-JH-GOE1) 

measured in progressively decreasing abundance relative to a background 

substrate of white quartz sand. The lowermost spectrum (black) was acquired 

with the rock covering 100 percent of the viewed spot, and the uppermost 

spectrum (cyan) represents only the background substrate. A weak Al-OH 

vibrational absorption doublet near 2.2 μm is evidence of trace amounts of 

kaolinite in the quartz sand. Click HERE for a larger image. 

18
It is evident from figure 9 that the quartz sand contains trace amounts of kaolinite, but no 

strong electronic absorptions due to iron are present in the VIS/NIR spectral region. The expert 

system identified no iron minerals in the quartz sand spectrum (“nofill-sand”). A bulk XRD 

analysis of the rock (sample PC-JH-GOE1) found major (>25%) quartz and trace (<5%) 

montmorillonite, jarosite, goethite, and kaolinite. Although weak absorptions caused by jarosite 

are present in the spectrum of sample PC-JH-GOE1 at 0.43 μm, 1.47 μm, and 2.27 μm, there 

appears to be little evidence of jarosite in the vicinity of the large crystal field absorption near 1.0

μm. The spectrum of the rock filling the spot (“spotfill”) was identified as coarse-grained

goethite (reference library sample WS222, figs. 2–4) by the expert system. Spectral 

measurements across the width of the rock did not reveal substantial variations in spectral shape

that could reflect local changes in grain size or mineral composition on the rock surface. It is not 

known if multiple scattering effects and(or) shadowing affected the resultant spectra in this

experiment. Shadowing was minimized by the vertical alignment of the light source directly

above the rock/sand surface. Figure 10 shows the spectra in continuum-removed format in the

vicinity of the goethite absorption feature near 1.0 μm. 

Figure 10 shows that the spectra of the rock in high areal abundance relative to the quartz 

sand were identified as coarse-grained goethite by the expert system, and a spectrum of the 

sample covering less than one third of the quartz sand-covered spot was identified as a mediumgrained goethite. As the areal coverage of rock in the spot viewed by the spectrometer

decreased, the widths and depths of the goethite feature near 1.0 μm also steadily decreased. The

band centers of the feature remained nearly constant at first, but then increased by 0.0085 μm 

(“spotfill_0.5”) before decreasing back to nearly the original values in the spectrum of 

“spotfill_0.25.” The center value of the weak feature present in the “spotfill_0.125” spectrum 

shifted strongly to shorter wavelengths. Whereas the continuum endpoints at the long 

wavelength side of the feature were located between 1.29 and 1.32 μm for the first three spectra

of goethite in substantial abundance (“spotfill,” “spotfill_0.75,” and “spotfill_0.5”), the 

endpoints shifted substantially to shorter wavelengths in the “spotfill_0.25” (1.16 μm) and 

“spotfill_0.125” (0.99 μm) spectra. In the reference library spectra (fig. 3), the long wavelength 

continuum endpoint is located at longer wavelengths for medium- and coarse-grained goethite 

than it is for the thin coatings of goethite. Values of modified least-squares curve fit between

measured and reference spectra generally remained constant until the rock filled only a small 

portion of the spot (“spotfill_0.25”), at which point the fit substantially decreased because of the 

increased influence of noise. 

19
Figure 10. Crystal field-absorption features caused by goethite near 1.0 μm in spectra shown 

on figure 9. Feature widths, depths, and center wavelengths are shown, as are 

degrees of least-squares fit of each spectrum to the identified reference spectra 

indicated at lower right. Click HERE for a larger image. 

Conclusions

Rock samples were obtained from rock outcrops and alluvium identified as containing 

goethite and(or) jarosite with AVIRIS data. Most samples were taken from areas differentiated 

by the expert system as having spectral properties similar to goethites of varying grain size. 

20
These samples were characterized with a laboratory spectrometer, and the resultant spectra were

analyzed by the expert system to determine iron mineral composition. 

The accuracy of the iron mineral identification results obtained by expert system analysis 

of the AVIRIS data was confirmed. Rocks identified as medium- to coarse-grained goethite ±

trace jarosite from the AVIRIS data were found to contain mainly goethite with trace jarosite, 

and were characterized by a primary crystal field absorption feature near 1.0 μm similar to that

of coarse-grained goethite, with weak absorption features caused by jarosite present elsewhere in 

the spectra. Alluvial surfaces identified as thin coatings of goethite using the AVIRIS data were 

sparsely covered with coarse-grained chips and angular cobbles up to 20 cm in size of goethitecoated altered rock, chlorite-bearing propylite, and relatively unaltered volcanic rocks on a

surface of light-gray soil with sparse dry vegetation. In general, the expert system was accurate 

in identifying medium-grained goethite, coarse-grained goethite, medium- to coarse-grained 

goethite with trace jarosite, and mixtures of goethite and jarosite. 

Rock fragments from areas identified as coarser-grained goethite and thin coatings of

goethite with the AVIRIS data were found to consist mainly of medium- to coarse-grained 

goethite and have similar spectral characteristics. Given the geochemical rationale for the 

development of goethite coatings at the expense of jarosite coatings over time from waters of

decreasing acidity adjacent to outcrops of oxidizing pyritic rocks proposed by Swayze and others 

(2000), it follows that the thickness of goethite coatings could increase with increased distance

from exposed pyritic rocks that are weathering to jarosite. Assuming that new goethite will 

accumulate as growths on existing goethite grains, the grain size of the goethite coatings would

be expected to increase as the coating thickness increased. Thus, it appears unlikely that changes 

in crystal grain size alone are responsible for the spectral variations in goethite-bearing rocks and

alluvium observed using AVIRIS data that resulted in the identification of thin coatings of

goethite. 

Spectroscopic principles suggest that, assuming constant material composition and grain

size, variation in the abundance of a material should affect mainly the depth of a diagnostic

absorption feature, and not its shape (Clark, 1999). The experiment to determine the spectral

effects of variation in material abundance found that, with decreasing material abundance, the 

absorption feature of goethite near 1.0 μm decreases in depth and narrows more from the long 

wavelength side of the feature than from the short wavelength side. This phenomenon also is 

observed in reference library samples of goethite (Clark and others, 2003b). The reference 

library spectra of fine-grained goethite and thin coatings of goethite are characterized by

electronic crystal field absorptions near 0.9 μm that are narrower, shallower, and located at 

shorter wavelengths than the absorptions of coarser-grained goethites which are broader and 

more saturated. Therefore, it appears that, in the Marysvale study area, goethite-bearing

alluvium downgradient from source outcrops tends to be identified as finer-grained or thin 

coatings of goethite due to the mineral’s presence in lesser abundance. Coarser-grained goethite, 

with deep, broad, and often saturated features, will most likely be correctly identified in areas of 

greater goethite abundance proximal to jarosite-bearing source rock where the feature is 

relatively free of corruption by goethite-free soil components and vegetation. For these reasons, 

goethite-bearing alluvium is a closer match to reference spectra of thin coatings of goethite even 

21
though the actual grain size of the contained goethite is the same on average as that from the

source outcrops. 

Further work is needed to understand the effects of soil mineralogy and grain size on the 

AVIRIS spectra of the primarily alluvial surfaces identified as thin coatings of goethite. The

widespread occurrence of alluvium identified as thin coatings of goethite using AVIRIS data 

adjacent to altered rocks has been observed in a variety of geologic terrains and deposit types

(Rockwell and others, 2004; Livo and Clark, 2002). Thus, it appears unlikely that the alluvial 

soils at the Marysvale study area identified as thin coatings of goethite contain a distinct 

assemblage of minerals (for example goethite ± chlorite ± epidote) and grain sizes that mimic the

spectral shape of the reference spectrum of thin goethite coatings. However, as chlorite is a

common constituent of propylitic alteration assemblages that surround areas affected by 

advanced argillic, argillic, and(or) phyllic alteration, its influence on the spectral properties of 

alluvium derived from the altered areas should be explored further. 

When analysis of imaging spectroscopy data is performed using reference spectra of iron 

minerals of varying grain sizes and mixed compositions, the results are useful not only for 

purposes of mineral identification, but also for distinguishing goethite-bearing outcrop from 

alluvial surfaces with similar mineralogy, providing valuable information for geologic, 

geomorphologic, mineral exploration, and environmental assessment studies. 

Acknowledgements 

The author would like to send a special thanks to Robert O. Green and the rest of the 

AVIRIS team at NASA’s Jet Propulsion Laboratory for providing the high-quality remote 

sensing data upon which this research was based. He also would like to thank Robert J. Horton 

of the USGS for providing the quartz sand used for the goethite abundance experiment. Daniel 

H. Knepper, Jr. and Trude V. V. King of the USGS supported this research as a part of the 

Remote Sensing Project. Special thanks also go to Daniel H. Knepper, Jr. and K. Eric Livo of

the USGS for their helpful reviews of this manuscript. 

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