Geology and Origin of the \i*J W > V lil^

Chilean Nitrate Deposits

0£0C0&'ICAi. SURVMV PROfESSIOHAX PAPER 1188

Prepared in cooperation with the 

Insiiiuto de Ihvesiigaciohes 

ieas de Chile,
Geology and Origin of the 

Chilean Nitrate Deposits

By GEORGE E. ERICKSEN

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1188

Prepared in cooperation with the 

Instituto de Investigaciones 

Geologicas de Chile

A discussion of the characteristic features

of the nitrate deposits and the sources and modes

of accumulation of their saline components

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Ericksen, George Edward.

Geology and origin of the Chilean nitrate deposits.

(Geological Survey professional paper ; 1188)

Bibliography: p.

Supt. of Docs, no.: 119.16:1188

1. Nitrates. 2. Duricusts Chile Atacama Desert. 3. Geology Chile Atacama Desert. I. Chile. Institute

de Investigaciones Geologicas. II. Title. III. Series: United States. Geological Survey. Professional paper ;

1188. 

QE471.E75 553.6'4'0983 80-607111

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page

Abstract ____________________________________________________________________ 1

Introduction __________________________________________________________________ 1

Location _______________________________________________________ 2

Acknowledgments ___________________________________________________________ 3

Geomorphology _____________________________________________________________ 3

Climate, vegetation, and soil microorganisms ______________________________________________ 8

Geology of the nitrate deposits ______________________________________________________ 12

Topographic setting1 __________________________________________________________ 12

Structural features ___________________________________________________________ 14

Chemical and mineralogical composition ______________________________________ 19

Origin of the nitrate deposits ___________________________________________________________ 20

Discussion of theories ____________________________________________________ 21

Sources of the saline components ____________________________________________ 23

Nitrate _______________________________________________________________ 24

Halogens _______________________________________________________________ 26

Sulfate ________________________________________________________________ 28

Borate, chromate, and dichromate __________________________________________________ 29

Alkali and alkaline-earth metals _______________________________________________ 30

Accumulation and enrichment of the saline components ______________________________________ 30

Conclusions _______________________________________________________________________ 31

References cited _____________________ _ _ _ _____________________________________ 32

ILLUSTRATIONS

Page

FIGURE 1. Index map of northern Chile, showing location of nitrate deposits ______________—————— 2 

2-4. Landsat images showing—

2. Nitrate region in northern Antofagasta and southern Tarapaca Provinces and the major

physiographic units of northern Chile _________________-____________— 4

3. Distribution of principal nitrate workings in the Tocopilla and Baquedano districts _____ 5

4. Deeply incised transverse valleys of probable late Miocene to Quaternary age cutting across

the northern Pampa del Tamarugal ____ ___________________________— 7 

5, 6. Aerial photographs of nitrate workings—

5. In the northern part of the Tarapaca district __________________________— 9

6. At Oficina North Lagunas __________________________________________ 12 

7. Photograph showing underground nitrate workings at the northern end of the Tarapaca district — 13 

8, 9. Topographic maps of—

8. Northern Tarapaca district, showing distribution of nitrate workings along the eastern margin of the Coastal Range ___________________________________—————— 14

9. Baquedano district, showing areas of exploitable or potentially exploitable nitrate deposits __ 14 

10. Diagram showing characteristic layering of an alluvial-type nitrate deposit --_-__—-_————- 15 

11-17. Photographs of—

11. Layer of friable high-purity humberstonite at Oficina Alemania, Taltal district _____—_ 16

12. Rajo San Martin, a nitrate mine in the Taltal district ___________________—- 16

13. Bedrock caliche, consisting of impregnations and veins of caliche blanco in rhyolite tuff — 17

14. Polygonal surface of costra, Tarapaca district _______________________——— 17

15. Saline-cemented, nitrate-rich sand dike in alluvial-type caliche, northern Tarapaca district _ 18

16. Newly formed desiccation polygons in nitrate ore, Tarapaca district ____________—— 18

17. Losa cakes embedded in powdery chuca, Tarapaca district _______________— 19

in
IV CONTENTS

TABLES

Page

TABLE 1. Chemical composition of water-soluble saline components in selected nitrate ores and high-purity veins

from the nitrate deposits in northern Chile __—_—————__———_————————————————— 20 

2. Saline minerals of the Chilean nitrate deposits ______________________________________ 21
GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

By GEORGE E. ERICKSEN

ABSTRACT

The unique nitrate-rich caliche deposits in the Atacama 

Desert of northern Chile owe their existence to an environment favorable to accumulation and preservation of the deposits, rather than to any unusual source of the saline 

materials. The essential conditions are: (1) extremely arid 

climate similar to that of the present-day Atacama Desert; 

(2) slow accumulation during late Tertiary and Quaternary 

time; and (3) paucity of nitrate-utilizing plants and soil 

microorganisms. The wide distribution of the deposits without 

systematic relationship to topography or rock type can be 

explained only by atmospheric transport and deposition. The 

saline constituents are chiefly those that have been found 

widely in the atmosphere in other parts of the world; in the 

nearly rainless Atacama Desert they have accumulated at or 

near the sites of the deposits chiefly as dry fallout and as 

solutions in condensate droplets from low clouds or fogs and 

dew prevalent during the winter. These saline materials were 

later leached, redistributed, and enriched in the highly soluble components, such as nitrate, at deeper soil levels and in 

soils on lower hillsides by fog condensate and water from 

infrequent rains. Ore-grade nitrate deposits evidently formed 

in three ways: (1) slow accumulation on old land surfaces 

that have had little or no modification since the Miocene;

(2) accumulation on lower hillsides and at breaks in slopes 

as the result of leaching and redeposition by rainwater; and

(3) accumulation in saltpans and saline ponds.

Although some of the saline materials in the nitrate deposits may have come from distant sources, I believe that 

most were from local sources, chiefly spray and evaporation 

from the nearby Pacific Ocean and from volcanic emissions 

in the nearby Andes Mountains. The nutrient-rich Humboldt 

Current may have been a major source of ammonia, oxides 

of nitrogen, and iodine, which were transformed to nitrate 

(N03 ") and iodate (I03~) by photochemical reactions, particularly ozone reactions, in the lower atmosphere and at soil 

surfaces. Similar reactions may have transformed chloride 

to perchlorate (C104~). an ubiquitous minor constituent of 

the nitrate ore. The soils of the nitrate fields, though having 

an extremely low level of biological activity, do have populations of microorganisms capable of nitrogen fixation, nitrification, and ammonification. These organisms may have contributed to the formation of some of the nitrate, particularly 

in near-surface soils where condensate from winter fogs is 

sufficient to support growths of blue-green algae and other 

microorganisms. Chloride and sulfate, which together with 

nitrate are the dominant anions of the nitrate deposits, also

are chiefly of oceanic origin, but significant amounts may 

have been supplied during times of intense volcanism in the 

nearby Andes. The principal cations, sodium, potassium, magnesium, and calcium, were supplied by sea spray, by windblown dust from saline soils and playas in the Atacama 

Desert, and by volcanic emissions in the Andes. Small 

amounts of these cations and widespread trace amounts of 

chromate (CrOr2 ) may have been derived by chemical 

weathering of soils and rocks in the nitrate fields. Boron, 

which is a widespread minor element, is of volcanic origin.

INTRODUCTION

The origin of the Chilean nitrate deposits has 

been a recurrent subject of discussion for more 

than 100 years, but none of the many theories proposed has found general acceptance. The proliferation of theories has been chiefly the result of three 

factors: (1) a scarcity of accurate geological descriptions of the deposits; (2) a tendency on the 

part of some authors to ignore geological data that 

would make their theories untenable; and (3) scarcity of information about processes that might form 

or supply large quantities of nitrate in a desert environment. Disagreement about the origin has persisted, as is shown by different theories published 

since 1960: (1) Mueller (1960, 1968) reproposed 

the theory of Singewald and Miller (1916) that 

postulates accumulation by capillary migration and 

evaporation of ground water at the margins of 

salars *; (2) Wetzel (1961) reaffirmed his proposal, 

published in several reports (for example, Wetzel, 

1932), that the deposits accumulated as saline-rich 

mudflows; (3) Fiestas (1966) reaffirmed his proposal 

(Fiestas, 1922) that the saline components were 

formed by reaction of clouds of volcanic acids with 

the rocks and soils of the nitrate fields; and (4) 

Claridge and Campbell (1968) and Ericksen (1975, 

1979) proposed the deposition of atmospheric saline 

materials at or near the sites of the deposits.

1 Saltpans or salt-encrusted playas are called "salars" in Chile and 

neighboring countries, a usage followed in this report
GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

The single most important factor in the accumulation of the nitrate deposits in the Atacama Desert 

has been the exceptional long-term aridity of this 

region. As will be shown, the region has been a 

desert at least since the middle Tertiary, and some 

of its landforms may date from early Tertiary. 

Nitrate-rich soils occur locally in some of the other 

deserts of the world, but the presence of widespread 

exploitable nitrate deposits is unique to the Atacama 

Desert. This special feature of the Atacama Desert 

undoubtedly reflects the long period of time available 

for accumulation of the deposits. In contrast, nitrate 

deposits that might have formed in other deserts 

during intervals of extreme aridity would have 

tended to be destroyed during intervals of increased 

rainfall.

This report will show that the Chilean nitrate deposits probably formed by slow accumulation of the 

saline components by deposition from the atmosphere of materials from diverse sources. However, 

local rather than distant sources predominated, and 

among these, the nearby ocean was probably the 

most important. Emanations and rocks from widespread volcanism in the northern Chilean Andes 

Mountains were also important sources of some 

saline components. Photochemical reactions in the 

atmosphere probably played a role in the formation 

of nitrate, sulfate, iodate, and perchlorate, all of 

which are constituents of the deposits.

LOCATION

The nitrate deposits are in the Atacama Desert of 

northern Chile, which lies west of the Andes Mountains and which is said to be the driest desert of the 

world. Most of the deposits are in the two northernmost provinces of Tarapaca and Antofagasta (fig. 

1). During the heyday of the nitrate industry, in the 

latter part of the 19th century and early part of the 

20th century, the nitrate deposits were exploited in 

five major centers or districts (fig. 1): Tarapaca, 

Tocopilla, Baquedano, Aguas Blancas, and Taltal.

Most of the commercial nitrate deposits are along 

the eastern side of the Coastal Range, in a band 

generally ranging from a few kilometers to about 

30 km in width, between long 69°30' W. and 70° W. 

(fig. 1). They extend from about lat 19°30' S. to 

26° S., a distance of about 700 km. The northernmost deposits in Tarapaca are largely restricted to a 

narrow band along the east side of the Coastal Range, 

whereas to the south, they spread out more extensively in the Coastal Range, Central Valley, and 

Andean Front. Noncommercial nitrate deposits and 

nitrate-rich soils are much more widespread than the 

commercial or potentially commercial deposits shown

Pedro de 

Valdivia /

O Sierra Gorda

Salar de 

Pampa Blanca

FIGURE 1.—Index map of northern Chile, showing location of 

nitrate deposits, as shown by mining-claim maps of the 

Servicio de Minas del Estado, and areas covered by Landsat 

images (figs. 2-4).
GEOMORPHOLOGY

in figure 1. Much of the Coastal Range is encrusted 

with nitrate-bearing- saline-cemented regolith, commonly ranging- from a few tens of centimeters to a 

meter or two in thickness. Near-surface fractured 

bedrock also contains veinlets and impregnations of 

saline minerals. Low-grade nitrate ore, containing 

1-7 percent NaN03 , probably forms an almost continuous band, 10-30 km wide, along the center and 

eastern side of the Coastal Range throughout the 

region of nitrate fields shown in figure 1. The only 

major gap in this band of nitrate deposits is in 

southern Tarapaca Province, where Salar de 

Llamara and associated clay playas interrupt the 

Coastal Range.

Low-grade nitrate deposits are known in the 

coastal desert of Peru several hundred kilometers 

north of the Chilean border (Portaro, 1935, 1936), 

and saline-rich soils probably occur elsewhere in the 

coastal desert of southern Peru. Commercial deposits 

have not been found there, nor have they been found 

in Chile north of Pampa de Tana (fig. 4). The reason 

for an absence of commercial deposits in these areas 

is not known, but climatic variations and the ages 

of the land surfaces could have been factors that 

inhibited accumulation of large amounts of nitraterich saline materials.

Most of the commercial nitrate deposits are at 

altitudes of less than 2,000 m, but those east of 

Aguas Blancas (fig. 1) extend upwards to altitudes 

of about 3,000 m. The deposits in the vicinity of 

Salar de Maricunga, which have not been exploited, 

are near'4,000 m in altitude and are the highest 

known nitrate deposits in northern Chile. This sector of the high Andes is exceptionally arid, having 

an annual precipitation of about 50 mm, mostly as 

snow; it undoubtedly has more extensive nitrate 

deposits and nitrate-rich soils than have been reported.

ACKNOWLEDGMENTS

The field and laboratory investigations on which 

this report is based were carried out as part of a 

cooperative project between the Institute de Investigaciones Geologicas (IIG) of Chile and the U.S. 

Geological Survey. The work began in 1961 under 

the auspices of the United States Technical Aid Program (AID) of the U.S. Department of State. The 

principal field investigations were made in 1962, 

with the support of the United Nations Special Fund, 

and led to a report on the saline resources of northern Chile (Ericksen, 1963). Many other short-term 

field studies from 1964 to 1979 were made in cooperation with the IIG and other government agencies 

in Chile, including the Corporation de Fomento and 

the Universidad del Norte in Antofagasta. The field 

studies have been facilitated in many ways by the 

nitrate-producing companies, especially the AngloLautaro Nitrate Corp., the former owner of Oficinas 

(Oficina is the name applied to the beneficiation 

plant and town site) Maria Elena and Pedro de 

Valdivia, the Cia. Salitrera de Tarapaca y Antofagasta, the former operator of Oficina Victoria, and 

the Sociedad Quimica y Minera de Chile, the government corporation that took over these three oficinas 

in the early 1970's. These three plants are the largest of the more than 100 plants that formerly operated in northern Chile and are the only ones still 

in operation in 1979. Oficina Victoria closed at the 

end of 1979.

Many people have assisted or contributed to the 

field and laboratory investigations. Special thanks 

are given to Carlos Ruiz F., former Director of the 

Institute de Investigations Geologicas of Chile and 

to Enrique Colzani, General Manager, and Henry B. 

Suhr, Chief Experimental Chemist, Oficinas Maria 

Elena and Pedro de Valdivia, all of whom have given 

enthusiastic support to the investigationes. Special 

thanks also are given to Roy E. Cameron, formerly 

of the Jet Propulsion Laboratory, California Institute of Technology, for information about his studies 

of soil microorganisms in the Atacama Desert. Several geologists of the IIG made notable contributions 

to the investigations. Nelson Bravo and Jaime Sayez 

assisted during early fieldwork. Guillermo Chong, 

now Professor of Geology, Universidad del Norte, 

prepared the nitrate claim maps on which figure 9 

of this report is based and collaborated in field investigations in 1967 and 1977. Maurico Tabac, Fernando Munizaga, and Hernan Cusicanqui collaborated 

in laboratory studies of the nitrate ores. The many 

other people in Chile who contributed information 

or helped to make the field studies pleasant and 

efficient are too numerous to be mentioned individually.

GEOMORPHOLOGY

The dominant physiographic provinces of northern Chile are, from west to east: 1) a relatively low 

Coastal Range, whose peaks are generally less than 

2,000 m in altitude; 2) a longitudinal depression or 

Central Valley, called the Pampa del Tamarugal 

where it is broadest and deepest in northern 

Antofagasta and southern Tarapaca provinces; and 

3) the high Andes Mountains (fig. 1). The elongate 

Coastal Range has a maximum width of about 50
GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

km; the western margin is marked by a wave-cut 

cliff having a maximum height of about 1,000 m

(fig. 2). The Central Valley consists of two major 

segments, the Pampa del Tamarugal in the north

Tocopilla

FIGURE 2.—Landsat image of the nitrate region in northern Antofagasta and southern Tarapaca Provinces, showing the 

major physiographic units of northern Chile: the Coastal Range, which has low relief and subdued topography; the 

broad Central Valley, which here is called the Pampa del Tamarugal; and the high Andes Mountains, including one 

of the high front ranges, the Cordillera Central. The coast is marked by a prominent wave-cut cliff and the Andean 

Front, by thick piedmont sediments. Principal nitrate workings (stippled), salars (outlined by dashed line), and selected 

recent faults are shown. Figure 1 shows area covered by image. Image No. 1244-14060, March 24, 1973.
GEOMORPHOLOGY

and a longitudinal basin to the south, centering at 

Aguas Blancas, west of the Cordillera Domeyko 

(fig. 1). The two segments are separated by a broad 

transverse alluvial valley, herein referred to as the 

Baquedano valley, extending from Sierra Gorda

through Baquedano, and ending at Salar del Carmen 

(figs. 1 and 3). High-grade nitrate deposits have 

been worked extensively in this valley between 

Sierra Gorda and Baquedano (fig. 3).

, .'*:::::::::x-lVtena Etena

'••; .-X'XvvI' •J'XvXv •'•*»'•

FIGURE 3.—Landsat image showing distribution of principal nitrate workings (stippled) in the Tocopilla and Baquedano 

districts. Unnamed salar south of Baquedano is in northernmost part of the Aguas Blancas basin, a segment of the 

longitudinal valley separated from the Pampa del Tamarugal by the oblique-trending Baquedano valley. Area covered by 

image is shown in figure 1. Image No. 1244-14062, March 24,1973.
GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

The Central Valley generally shows a rounded 

profile, having margins that merge with piedmont 

aprons along the Coastal Range and the Andean 

Front (fig. 3). However, in the latitude of the 

Tarapaca nitrate district (fig. 1), the Pampa del 

Tamarugal slopes westward and ends abruptly at 

the Coastal Range. The Andes Mountains consist of 

a plateau about 4,000 m in altitude, on which are 

hundreds of well-preserved volcanic cones. Only a 

few of the volcanoes are known to have erupted in 

historic time; some of these and a few others, not 

known to have erupted in historic time, now show 

fumarolic activity. In Tarapaca Province, the Andean Front is marked by gentle slopes that merge 

with the Pampa del Tamarugal, whereas southward, 

longitudinal front ranges—Cordillera Central (fig. 

2) and Cordillera Domeyko (fig. 1)—and valleys 

separate the Andean plateau from the Central 

Valley.

Northern Chile is underlain chiefly by sedimentary, plutonic, and volcanic rocks of Jurassic and 

Cretaceous age, covered extensively by volcanic 

rocks and associated unconsolidated sediments of 

Tertiary and Quaternary age. The older rocks are 

best exposed in the Coastal Range and in the longitudinal ranges along the Andean Front. Upper Tertiary and Quaternary volcanic rocks are most extensive in the Andean region. The gently sloping 

Andean Front has a thick cover of piedmont sediments (fig. 2) and interbedded ash-flow tuffs. The 

Central Valley contains a fill of alluvium and interbedded lacustrine sediments that is as much as 900 

m thick in the Pampa del Tamarugal, southern 

Tarapaca Province (Mordojovich, 1965). The 

Coastal Range is mantled with saline-cemented regolith consisting chiefly of fragmental rock material 

admixed with windblown silt and sand. Some of the 

windblown material is characteristically reddishbrown and consists chiefly of fine volcanic ash and 

eroded playa sediments. Closed basins in the Coastal 

Range contain fill consisting of alluvium and interbedded lacustrine sediments. The only known saline 

deposits in the older rocks of the region are anhydrite beds in marine sedimentary rocks of Late 

Jurassic age exposed in the Coastal Range, and 

layers and domal masses of rock salt in a sequence 

of terrestrial clastic sedimentary rocks of Cretaceous to early Tertiary age in the basin of Salar de 

Atacama (fig. 1).

The Atacama Desert has had an extremely arid, 

nearly rainless climate at least since Miocene time, 

and some of the Miocene and even earlier Tertiary 

landforms have been preserved with little modification. For example, the Coastal Range north of Antofagasta (fig. 3) is considered to be an early Tertiary 

erosion surface, the Coastal Tarapaca Pediplain 

(Mortimer and Saric, 1972, 1975), that has been 

somewhat modified by late Tertiary and Quaternary 

block faulting. Additional modification by stream 

erosion has been slight, attesting to the aridity of 

the climate since early Tertiary time. Infilling of 

the Central Valley, which became a depression as 

the result of uplift of the Coastal Range and Andes, 

began in Oligocene time and was largely completed 

by the end of middle Miocene time (Mortimer and 

Saric, 1975). The Miocene surface of the Central 

Valley is well preserved as the broad flat pampas— 

Pampa de Tana and others (fig. 4)—between the 

deep transverse valleys in northern Tarapaca Province. In southern Tarapaca, this surface is covered 

with alluvial fans from the Andes and by salars 

and playas crowded along the western side of the 

Pampa del Tamarugal (fig. 2). The deep transverse 

valleys in northern Tarapaca (fig. 4) were formed 

after middle Miocene time by streams from the 

Andes that cut across areas where the fill in the 

Central Valley overtopped the Coastal Range (Mortimer and Saric, 1975). The Andean uplift, which 

began in the Oligocene, has continued until the 

present. It has been accompanied by ejection of huge 

amounts of volcanic material, chiefly of rhyolitic 

composition during the Miocene and Pliocene, and 

of andesitic composition during Quaternary time.

Northern Chile has been a region dominated 

by internal drainage during late Tertiary and 

Quaternary time, and saline lakes formerly existed 

in many of the closed basins. Such lakes were most 

prevalent in the Pampa del Tamarugal and the 

Aguas Blancas basin, where they evidently formed 

at times of exceptionally high rainfall and runoff 

from the Andes, but small saline lakes and ponds 

also occurred in closed basins of the Coastal Range 

that were interconnected by recent faults serving 

as conduits carrying water from the Central Valley. 

The lacustrine and playa sediments of these former 

lakes include layers of diatomite and anhydrite. Diatomite is particularly widespread in the Pampa del 

Tamarugal in northern Antofagasta and southern 

Tarapaca Provinces, and a thick sequence of interbedded clastic lacustrine sediments and diatomite is 

well exposed along the Rio Loa. This region was the 

site of a large perennial lake of probable late Tertiary age that Briiggen (1950) named Lago Soledad.

About 100 closed basins now exist in northern 

Chile, many of which contain salars or clay playas 

in their lowermost parts (Stoertz and Ericksen,
GEOMORPHOLOGY

FIGURE 4.—Landsat image showing deeply incised transverse valleys of probable late Miocene to Quaternary age cutting 

across the northern Pampa del Tamarugal (flat pampas between the valleys are remnants) and the Coastal Range. Lowgrade unworked nitrate deposits occur in the western part of the Pampa de Tana, a gentle, westward-sloping surface 

of probable Miocene age. Area covered by image is shown in figure 1. Image No. 1155-14102, Dec. 25, 1972.

1974). Only a few of these basins in the Andean 

Mountains now contain perennial saline lakes, but 

many others now having salars contained lakes

during the Pleistocene, as indicated by prominent 

shoreline features. Shallow playa lakes now form in 

the Atacama Desert only at places along the western
GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

margin of the Pampa del Tamarugal in southern 

Tarapaca Province at times of exceptionally heavy 

rainfall in the Andes. These lakes may form only 

once or twice in a given 10-year period and last from 

a few days to several weeks.

CLIMATE, VEGETATION, AND SOIL 

MICROORGANISMS

The Atacama Desert is the driest of the world's 

deserts; average annual rainfall is probably less 

than a millimeter in the areas where the nitrate 

deposits are most prevalent, and in any given part 

of the desert, measurable rainfall (1 mm or more) 

may be as infrequent as once every 5 to 20 years. 

Heavy rainfalls of a centimeter or more may take 

place only a few times each century. Such rains 

may continue intermittently for several days and 

have an aggregate precipitation of several centimeters. Because rainwater soaks into the ground 

with little or no runoff, gullying is minimal, and 

even some of the large valleys lack well-defined 

stream courses. The absence of recent stream erosion has given rise to the smoothed or rounded 

topography (fig. 5), and hills, gullys, and valleys of 

an earlier erosion cycle are now covered or choked 

with debris from nearby bedrock, mixed or covered 

with windblown dust and sand. Curiously, even the 

dust-covered surfaces are relatively stable and give 

rise to dense dust clouds only during very strong or 

turbulent winds. Sand dunes are absent in most of 

the desert; a single dune field occurs east of Salar 

de Pintados (fig. 1), and a few large dunes are 

found along the coast and in the Coastal Range. 

Most of the region is devoid of plant growth; plants 

are found only along streams in areas in the Central 

Valley where the ground-water table is near the 

surface, and on hills along the coast that receive 

abundant fog condensate.

Rainfall shows a general increase with altitude 

along the Andean Front. The lowest limit of regular 

or seasonal rainfall is at altitudes of 2,500 to 3,000 

m along the Andean Front east of the Pampa del 

Tamarugal (Fuenzalida, 1965). In this region, the 

rains take place chiefly during the summer months 

of January-March and commonly are associated 

with violent thunderstorms. The maximum rainfall 

is in the Andean Highlands at altitudes of 4,000 m 

and above. Rainfall is greatest in the northern part 

of the province of Tarapaca, decreasing southward 

into Antofagasta, and then again increasing southward into Atacama. Fuenzalida (1965) reported the 

average annual rainfall in the northeasternmost

part of the Tarapaca Province (above 4,000 m) to 

be 276 mm, whereas that in the Andes of central 

Antofagasta does not exceed 50 mm. In southern 

Antofagasta and northern Atacama Provinces, precipitation is chiefly snow during the winter months 

of June-September, and annual precipitation is probably not significantly greater than that in central 

and northern Antofagasta Province. However, because of the prevalence of snow, which tends to 

sublimate rather than melt, this region of the Andes 

appears to be nearly as arid as the Atacama Desert. 

This arid climate favored the accumulation of the 

nitrate deposits known to exist in the Andean Highlands near Salar de Maricunga (fig 1).

Infrequent heavy rains in the Atacama Desert 

result in flash floods that may cause local flooding 

of low areas, mudflows, cutting of sharp gullies, and 

damage to buildings and roads. Hawxhurst (1926) 

reported torrential rains and floods in the Pampa 

del Tamarugal in the years 1819, 1823, 1852, 1859, 

1878, 1884, 1905, and 1911. Evidently, flooding during some of those years was due to runoff from the 

Andes, which Hawxhurst does not distinguish from 

flooding due to rains in the Atacama Desert. He 

further stated that heavy rainfall and flooding in 

1911 caused suspension of railroad service and operation of nitrate plants, and that considerable 

amounts of sodium nitrate in open storage piles 

were washed away. No other records of heavy rains 

in the Atacama Desert are available, but rains are 

reported to have destroyed unprotected stocks of 

nitrate in the Taltal district at least once during the 

present century.

One of the unusual features of the Atacama 

Desert is the presence of widespread, relatively 

abundant ground-water supplies, commonly not 

more than a few meters to a few tens of meters 

below the surface. The ground water occurs in the 

alluvium of the Central Valley as well as in that of 

dry valleys along the Andean Front and in the 

Coastal Range. This ground water, which is recharged chiefly by runoff from the Andes after 

heavy rains and by slow infiltration from valleys 

along the Andean Front, has been a major source 

of water for nitrate-beneficiation plants since the 

earliest days of the nitrate industry. The ground 

water in dry valleys and closed basins in the Coastal 

Range, particularly in the region of the nitrate deposits of southern Tarapaca Province (fig. 1), evidently is recharged by inflow along recent faults 

that connect the basins to the Pampa del Tamarugal.

Dense coastal fogs or low clouds, locally known 

as camanchaca, sweep inland from the Pacific
CLIMATE, VEGETATION, AND SOIL MICROORGANISMS

FIGURE 5.—Aerial view of nitrate workings in the northern part of the Tarapaca district. Nitrate ore mined in this area 

was chiefly of bedrock type, consisting of veins and impregnations of saline minerals in strongly fractured andesite. 

Waste dumps (D) of ore treated at Oficina Barcelona can be seen in left foreground.

Ocean, covering the Coastal Range nearly every 

night during the winter months of June to August, 

at times precipitating considerable amounts of water 

as condensate. The camanchaca of the nitrate region 

is typically a distinct cloud layer a few hundred 

meters thick, the bottom of which is several hundred 

meters above sea level. At times of the most prevalent fogs, the condensate may be sufficient to moisten 

soil to depths of a millimeter or two, and exceptionally heavy fogs cause deeper moistening of soil and 

the formation of tiny rivulets of water on hard-rock 

surfaces and paved roads. The amount of water precipitated in this way is perhaps a few millimeters 

per month. Relative humidity is high in the Coastal 

Range, and considerable quantities of water also are 

precipitated as dew during the winter nights. Even 

though moisture precipitated at night is largely 

evaporated the following day, the near-surface soil

moisture probably remains relatively high during 

the cool winter months. The effect of such moisture 

is greatest near the coast, where it is sufficient to 

support local sparse plant growth, particularly on 

high hills facing the ocean. The fogs generally do 

not extend east of the Coastal Range, and the Central Valley is singularly free of fog or of cloud cover. 

The coastal fogs of northern Chile contain saline 

material and they may have been important sources 

of constituents of the nitrate deposits. A 2-liter 

sample of fog condensate 2 from the crest of the

2 Collected in July 1962, on frames having tightly strung nylon fibers, 

during an investigation by the Universidad del Norte into the feasibility 

of extracting water from fog for domestic and agricultural uses. It •was 

found that frames 1-2 m on the side and strung with closely spaced nylon 

fibers were able to collect several liters of water from dense, wind-driven 

fog in a single night. Condensation was due chiefly to vibration of the 

nylon fibers as the fog moved across them.
10 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

coastal range, a few kilometers east of the city of 

Antofagasta, was found to contain 162 mg/1 of dissolved salts, as follows (Ericksen, 1963) :

mg/l

46

32

Cl _________________________________________

so,-2 _________________.

NO3- _____________________________ 19 

HCQr ____________________________ 15 

Na+ ___________________ __

Ca+

K+ 

SiOa

30

12

6

1

1

These constituents are considered to be chiefly of 

oceanic origin, having been ejected into the atmosphere in sea spray (see'Blanchard and Woodcock, 

1957), but part may have been contaminants from 

the atmosphere over Antofagasta and saline dust 

from the Atacama Desert. Although this single 

sample does not justify an estimate of quantities of 

saline material that might be precipitated in the 

nitrate fields, it does show that the fog contains the 

major constituents of nitrate ore in quantities sufficiently large to make it a potential major source 

of these constituents.

Temperatures are low in the Atacama Desert, in 

comparison with those in other deserts at low latitudes and altitudes. They also are relatively uniform 

throughout the year. An isothermal map by Almeyda Arroyo (in Fuenzalida, 1966) shows the 

average annual temperature of most of the Coastal 

Range in the region of the nitrate deposits to be 

18° to 20°C. The average annual temperature in 

Iquique is 17.9°C, in Antofagasta, 16.6°C, and in 

Taltal, 17.7°C (Fuenzalda, 1965). The average daily 

temperature in Iquique ranges from about 21 °C in 

January, the hottest, month, to 15.5°C in August, the 

coldest month, a difference of only 5.5°C. In contrast to the Coastal Range, the Central Valley shows 

greater fluctuation in daily temperature and has 

lower relative humidity. For example, at Canchones, 

a government experimental farm at the northeast 

corner of Salar de Pintados (fig. 1), the mean annual temperature is 16.7°C, whereas the hottest 

month, February, averages 21.2°C, and the coldest, 

June, 12.9°C (Fuenzalida, 1965). The greatest fluctuation in daily temperatures here is during the 

period July to September, when minimum night 

temperature is near freezing and maximum day temperature is near 30°C.

Wind patterns and drainage are significant, in 

that they favor retention of saline material carried

into the Atacama Desert by wind and streams. This 

accounts in part for the exceptional saline accumulations that are more prevalent here than in other 

deserts. Wind is now the dominant agent of erosion 

and deposition, and wind transport and deposition 

of saline dust probably have been important processes in formation of the nitrate deposits. The Atacama Desert is characterized by onshore westerly 

winds during the day that alternate with, easterly 

breezes from the Andes during the night. Along the 

coast, south to southwesterly winds dominate, 

whereas inland, the winds are more variable in direction. Because of variable wind directions, windtransported material tends to be spread rather 

evenly over the desert, instead of forming dunal 

accumulations.

Vegetation is absent in much of the Atacama 

Desert, and the typical barren landscape is very 

different from that of most other deserts. Hardy 

shrubs grow sparsely only on hillsides where fog 

condensate is heaviest, particularly near the tops of 

the highest ridges and hills along the coast. Some 

areas near the coast have a sparse cover of Tillandsia, a Spanish moss that spreads over the otherwise barren soil. Tillandsia is an air plant, having 

rootlets that function as anchorage in young plants 

but that generally are absent in older plants. Shrubs 

and salt grass grow in areas in the Pampa del 

Tamarugal subject to occasional flooding by runoff 

from the Andes or where the ground-water table is 

within a few tens of centimeters of the surface. 

Tamarugo trees, a type of mesquite (Prosopis), are 

reported to have once covered large areas of the 

Pampa del Tamarugal where the ground-water table 

is shallow, but during the early days of the nitrate 

industry they were used as fuel, and the pampa was 

almost completely stripped of the trees. In the late 

1930's or early 1940's, the government of Chile 

started a very successful reforestation program by 

planting Tamarugo and Algarrobo (also a type of 

mesquite) trees in the Salar de Pintados. The trees 

are planted in holes dug through the thin hard salt 

crust into underlying moist sediments.

The recent discovery that soils in the Atacama 

Desert have microflora populations capable of nitrogen fixation as well as nitrification and ammonification 3 (Cameron and others, 1965, 1966; Cameron,

3 Nitrogen fixation is the conversion of elemental atmospheric nitrogen 

to ammonia and other nitrogen compounds, chiefly by activity of specialized 

bacteria and blue-green algae in soils; ammonification is the conversion 

of proteins and other organic compounds to ammonia, -water, and carbon 

dioxide by many types of bacteria, actinomycetes, and molds; nitrification 

is the oxidation of ammonium to nitrite and nitrate by specialized bacteria.
CLIMATE, VEGETATION, AND SOIL MICROORGANISMS 11

1969, and Cameron, written communs., 1966, 1967, 

1968) invites speculation about the role of microorganisms in the formation of the nitrate deposits. 

Unfortunately, the investigations are not sufficiently 

detailed to support quantitative estimates of the 

significance of microorganisms in formation of the 

nitrate, but they do show that microbial activity in 

the dry soils of the Atacama Desert is exceedingly 

low, a requisite for preservation of the nitrate. On 

the other hand, activity might be great at soil surfaces subject to frequent wetting by fog condensate 

during winter months, and during such periods of 

high microbial activity, significant quantities of 

nitrate could have been formed or consumed.

Investigation of soil microflora of the Atacama 

Desert was undertaken as part of a more extensive 

study of desert microorganisms supported by the 

National Aeronautics and Space Administration 

(NASA) and carried out under the direction of Roy 

E. Cameron who was then with the Jet Propulsion 

Laboratory (JPL), California Institute of Technology. I had the opportunity to accompany Cameron 

during part of a field investigation in northern Chile 

in 1965 and through correspondence with him was 

able to keep abreast of information gathered in subsequent laboratory studies. The information presented here is based on published and unpublished 

data of Cameron and his colleagues at JPL and on 

studies undertaken under JPL contract by W. B. 

Bollen (unpub. data, 1967, 1968) of Oregon State 

University and by E. E. Staff eldt (unpub. data, 1967, 

1968) of New Mexico State University. Prior to the 

work of Cameron and his colleagues, relatively little 

was known about soil microorganisms in the Atacama Desert. Darwin (1890) mentioned the presence 

of the yellow lichen Cladonia on loose sand of the 

Coastal Range near Iquique, where winter fogs are 

prevalent. Schwabe (1960a, b) reported on bluegreen algae in soils near Antofagasta.

The dry soils of the Atacama Desert contain a 

variety of species of bacteria, actinomycetes, molds, 

and algae, but populations are generally several 

orders of magnitude less than in soils of milder 

deserts. In this respect, the soils are most similar 

to the relatively barren soils of Antarctica, where 

nitrate-rich saline complexes are known to have accumulated (Claridge and Campbell, 1968). Laboratory cultures of soils of the Atacama Desert show 

considerable diversity in types and growth rates of 

microflora populations and capacity for ammonification and nitrification. Bacteria and actinomycetes

are generally the most abundant microorganisms 

present. Incubation periods in most laboratory cultures of soils from this area were many days rather 

than the 48 hours or less for soils from less arid 

deserts (Cameron and others, 1966). Some of the 

soils were found to have high nitrifying capacity 

but little or no ammonifying capacity, whereas 

others show the reverse (W. B. Bollen, unpub. data, 

1967). Presumably, some other soils have both ammonifying and nitrifying capacity. Cameron (written commun., 1967) reported that some of the soils 

tested had the capacity for sulfate reduction.

The filamentous blue-green algae are among the 

more intriguing of the microflora in the soils of the 

Atacama Desert because they are photoautotrophic— 

requiring only oxygen, light, and inorganic substances for their life processes—an ability that is 

essential for significant microbial activity in the 

organic-deficient soils of this region. Furthermore, 

many species have the ability to fix atmospheric 

nitrogen. The favored habitats of these algae are 

the surfaces of wet or moist soils, and they may 

exist symbiotically with fungi in lichens. Nostoc 

muscorum, a well-known nitrogen-fixing blue-green 

alga, was found in saline material and mud from 

the edge of a saline pond in Salar de Bellavista near 

Oficina Victoria (figs. 1 and 2) (R. E. Cameron, 

written commun., 1966). It has been found in the 

coastal desert of Peru where winter fogs are most 

prevalent (Dawson, 1963) and where fog condensate 

is somewhat greater than it is in the Atacama 

Desert. Nostoc has not been reported from the desert 

soils of northern Chile, but it could occur undetected 

locally and might have been more widespread in 

times past, when fog condensate may have been 

greater.

Cameron and Blank (1966) summarized information about desert habitats of algae, indicating that 

favored sites for growth of algae in deserts were 

the undersides of translucent rock fragments, such 

as chalcedony, and beneath thin salt crusts where 

soil moisture is retained and yet light is available 

for photosynthesis. Algae are known to occur in such 

habitats in the Atacama Desert (Schwabe, 1960a, 

b). Residual algal colonies such as these serve as 

reservoirs from which spreading could take place to 

form extensive algal crusts on soil surfaces after 

rains (Cameron and Blank, 1966) and perhaps at 

times of high moisture due to accumulation of fog 

condensate.

Blue-green algae also are abundant in saline ponds 

associated with salt pans in the nitrate region of 

northern Chile, and we may assume that they also
12 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

were abundant in former saline lakes and ponds in 

this region. The two saline ponds for which chemical 

analyses are listed on page 31 have abundant algae 

and associated microflora in a nitrate-rich brine. In 

addition to Nostoc muscorum, samples of saline material and mud from the pond in Salar de Bellavista, 

collected in 1965, were found to contain Schizothrix 

ccHcicola, an alga having the ability to utilize ammonia as its sole source of nitrogen (R. E. Cameron, 

written commun., 1966). Whether these species and 

associated microorganisms have had any significant 

effect on the formation of the nitrate in these ponds 

is not known. However, the presence of a nitrogenfixing species suggests the possibility of significant 

amounts of nitrate being formed in such ponds and 

in moist soils at margins of ponds and lakes at 

present and in the past.

GEOLOGY OF THE NITRATE DEPOSITS

TOPOGRAPHIC SETTING

The nitrate fields (fig. 1) are typically in areas 

of low relief characterized by rounded hills and 

ridges and by broad, shallow, debris-filled valleys. 

Nitrate deposits occur in all topographic positions 

from tops of hills and ridges to the centers of the 

broad valleys, but the richest deposits that have 

been worked most extensively tend to be on the 

lower slopes of hills (fig. 5). Nevertheless, highgrade deposits also are found in broad debris-filled 

valleys and basins (fig. 6) and in nearly flat to

FIGURE 6.—Aerial view of nitrate workings at Oficina North 

Lagunas (ruins of former beneficiation plant can be seen 

at base of hill in background), near Salar de Lagunas at 

the southern end of the Tarapaca district. Nitrate ore here 

is chiefly saline-cemented regolith.

gently undulating terrain (fig. 7). Most of the 

nitrate fields in the Tarapaca and Tocopilla nitrate 

districts (fig. 1) are in gently inclined piedmont 

plains and steeper rocky slopes along the eastern 

side of the Coastal Range. They either end abruptly 

near the margin of a salar or playa along the western side of the Pampa del Tamarugal, typical of 

the Tarapaca district (fig. 8), or merge with valley 

fill without a perceptible break, as in the Tocopilla 

district (fig. 3). However, the topographically lowermost deposits in the Tocopilla district tend to occur 

along the zone where the lacustrine deposits that 

extend over the Pampa del Tamarugal intersect or 

interfinger with the piedmont deposits covering the 

gentle eastern slopes of the Coastal Range.

On the whole, the richest nitrate ore in northern 

Chile is in the fields of the Tarapaca district. These 

fields were the chief source of nitrate produced prior 

to the 1870's. The first nitrate-beneficiation plants 

were built in the vicinity of Zapiga (fig. 8) between 

1810 and 1812 (Bermudez, 1963). The most extensively worked deposits in this district include those 

in a narrow band along the eastern margin of the 

Coastal Range (fig. 8) and those within the Coastal 

Range south of this area (fig. 1), where a generally 

undulating terrain is marked by several closed 

basins. In the latter area, the richest nitrate deposits 

tend to be at the margins of the basins, but rich 

deposits also occur on the floors of some basins and 

in the nearly flat uplands between basins. Salars 

are found in the central parts of a few of these 

basins, and the nitrate deposits are on the hillsides 

marginal to them. Rich deposits also occur in several 

islandlike bedrock hills, 10-20 m high, within but 

near, the western sides of playas and salars along the 

western side of the Pampa del Tamarugal in the 

northern part of the Tarapaca district. Some of these 

hills have been worked extensively for nitrate ore 

from near the level of the playa or salar to the hilltop.

The nitrate deposits of the Baquedano nitrate district are distinctly different topographically from 

those in Tarapaca, as can be seen in figure 9. These deposits are nearly continuous along the broad debrisfilled valley from Baquedano to Sierra Gorda, a distance of about 60 km. They occur along the valley 

bottom, where the nitrate layer is locally exposed in 

the 1-3-m-high walls of a recent stream channel, and 

in the gently sloping valley sides for horizontal distances of as much as 10 km and vertical distances 

of 100-200 m from this stream channel. Scattered 

deposits also are found in an undulating upland area 

between this valley and the north-trending valley to
GEOLOGY OF THE NITRATE DEPOSITS 13

the west (fig. 9), which connects with the Pampa 

del Tamarugal. Both the salars shown in figure 9— 

Pampa Blanca and Lina—contain nitrate-rich saline 

crusts that formerly were exploited as nitrate ore. 

These ores evidently were relatively low grade and 

generally less than 50 cm thick but because they 

lacked overburden, they could be mined at low cost. 

Nitrate deposits also have been found on hilltops 

and in high valleys far from other known deposits 

and from any possible source of saline materials in 

ground water or in former lakes. Some of the deposits at the east side of the Aguas Blancas district 

(fig. 1) are of this type. They are high on the west 

flank of the Cordillera Domeyko, and seem isolated 

from the known deposits to the west. However, this 

isolation may be more apparent than real; deposits 

in this poorly explored area probably are much more

widespread than indicated in figure 1, and nitraterich soils may be even more widespread. Cerro 

Peineta, a flat-topped mountain in the southern part 

of the Taltal district, is unusual in having an isolated 

nitrate deposit at its top, several hundred meters 

above the surrounding terrain. This deposit has been 

cited as an example of a nitrate deposit that could 

have formed only by atmospheric deposition of the 

saline components ( Bruggen, 1938).

Several salars, in addition to those in the Baquedano district, have nitrate-rich crusts, and the 

crusts of at least two—Salar del Carmen and Salar 

de Lagunas (fig. 1)—are actively receiving increments of new nitrate-rich saline material by capillary evaporation of near-surface saline ground 

water. Salar del Carmen was mined in the latter 

part of the 19th century, when nitrate-rich efflores-

• ? r? r r ~ i r r

FIGURE 7.—Underground nitrate workings at the northern end of the Tarapaca district. Near-horizontal discontinuous layers of relatively pure saline material occur in limestone and interbedded shale 5-10 m below the surface. Shallow layers 

and veins of this material were mined in opencut in foreground; tunnels from this cut connect shafts marked by 

isolated waste piles.
14 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

o 10 KILOMETERS

Contour interval 250 meters

FIGURE 8.—Topographic map of the northern Tarapaca district, showing distribution of nitrate workings along the 

eastern margin of the Coastal Range. Deposits here were 

the first to be worked in the early 19th century and on 

the average contain the richest nitrate ore in northern 

Chile. From unpublished map prepared by the Institute de 

Investigaciones Geologicas of Chile.

cences that reformed in some old mine areas were 

harvested periodically (Whitehead, 1920). The 

nitrate-rich crusts of two or three small salars in 

the Aguas Blancas district also have been mined.

STRUCTURAL FEATURES

The nitrate deposits show characteristic features 

that reflect original structures in the host rocks and 

other structures that formed later, during accumulation and enrichment of the deposits. The deposits 

are in all types of rocks and unconsolidated sediments in the Coastal Range and Central Valley 

without showing any systematic variation in content of the saline components or of trace elements in 

these components. Nitrate and other saline minerals 

occur as veins or impregnations in such diverse rock 

types as granitic intrusions, andesite and rhyolite

Contour interval 100 meters

FIGURE 9.—Topographic map of the Baquedano district, showing areas of exploitable or potentially exploitable nitrate 

deposits. Most of the nitrate workings, indicated by distribution of beneficiation plants, are in the broad valley extending from Baquedano to Sierra Gorda. Prepared from 

topographic maps of the Institute Geografico Militar (scale 

1:50,000) and unpublished nitrate mine-claim maps of the 

Servicio de Mineria of Chile.

flows and tuff layers, limestone, sandstone, and 

shale. The most widespread deposits are those in 

which the saline minerals form a cement in unconsolidated regolith. Saline minerals were deposited in 

original openings—pores, cavities, and fractures— 

in the host rock and in new openings formed by 

force of crystallization. Features such as compositional layering, veins of high-purity saline minerals, 

sand dikes, solution cavities, desiccation polygons, 

and gypseous cakes (losa) resulted from processes 

of solution and redeposition. The nitrate fields are in 

a region of active faults, and nitrate layers have been 

extensively fractured in the vicinities of faults.

Two major types of nitrate ore (called caliche 4 

in Chile) can be recognized, one in which the saline 

minerals occur chiefly as a cement in regolith and 

the other in which they form impregnations, veins, 

and irregular masses in porous or fractured bedrock. For convenience of discussion, the two types

4 Caliche is now considered to be a nitrate ore having a minimum grade 

of about 7 percent NaNOa, the approximate cutoff grade for the operating 

beneficiation plants. Formerly, higher grade nitrate ore was mined 

(earlier in this century minimum grades were generally about 15 percent 

NaNOs), and lower grade material was not considered to be caliche.
GEOLOGY OF THE NITRATE DEPOSITS 15

are referred to as alluvial caliche and bedrock 

caliche. Alluvial caliche generally rests on poorly 

cemented or uncemented rock debris similar to that 

within the caliche itself, whereas bedrock caliche 

gives way downward to less fractured or more impervious rock relatively free of saline minerals. The 

typical alluvial-type nitrate deposit is considered to 

be made up of several layers (fig. 10)—chuca, 

costra, caliche, conjelo, and coba—each having 

characteristic chemical and physical features. Distribution of the saline minerals in these layers is in 

part determined by their solubilities and is consistent with selective leaching of surface material 

and reprecipitation at deeper soil levels.

Chuca and costra constitute the overburden of 

the alluvial-type nitrate deposit. Chuca is the powdery to poorly cemented surface layer, generally 

10-30 cm thick, consisting of silt, sand, and rock 

fragments, variable though generally abundant 

gypsum or anhydrite, and small amounts of other 

saline minerals. It consists of the surface soil, from 

which most of the water-soluble saline minerals have 

been leached, and admixed windblown material. 

Saline materials leached from this layer have been 

redeposited to enrich underlaying or downslope 

caliche and to form veins and mantos of high-purity 

saline materials in preexisting caliche. At many 

places, the chuca contains a layer of hard to soft 

white saline material a few centimeters to about 50 

cm thick (fig. 11), which consists chiefly of either 

relatively insoluble gypsum and anhydrite or of 

soluble sulfates such as' thenardite, bloedite, and 

humberstonite. The hard material is brittle and 

breaks with a conchoidal fracture; it has the appearrance of having formed as a gel. It weathers to a 

soft friable material. Costra consists either of a 

hard and brittle material similar to the underlying 

caliche, or of poorly cemented friable material that 

appears transitional between chuca and caliche. 

Contacts between costra and chuca are generally 

sharp, and although those of costra and caliche tend 

to be gradational, they also may be sharp, as shown 

in figure 12. Costra is generally 1/2-2 m thick, but 

locally it may be thinner or even absent. Some 

nitrate fields characteristically have poorly cemented 

costra several meters thick, and thicknesses of as 

much as 12 m have been observed.

Alluvial-type caliche is generally 1-3 m thick, 

but material as thin as 50 cm and as thick as 5 m 

has been mined locally. Veins and layers of highpurity, white nitrate-rich saline material (called 

caliche blanco by the miners) are widespread in the 

alluvial-type caliche of some fields or areas, but

relatively rare in others. They generally range from 

10-50 cm in thickness. Layers of caliche blanco are 

generally at the base of the caliche layer, perhaps 

the level of deepest penetration by rainwater. Most 

alluvial-type caliche is somewhat porous and contains irregular to slitlike cavities as much as 20 cm 

in longest dimension. Such cavities commonly contain well-developed crystals of saline minerals. 

Pebbles may have halos of fine-grained, white, saline 

material, as much as a centimeter thick, some of 

which show desiccation cracks. The caliche layer 

typically shows considerable variation in physical 

character and chemical composition in short horizontal distances. These variations seem to be the result 

of a combination of factory including the physical

Powdery

Friable to powdery

Moderately to 

firmly cemented.

Firmly cemented

Conjelo, 0-2+m < 

firmly cemented

Uncemented

- Chuca, 10-30 cm

 Costra, 50 cm-2 m

 Caliche, 1-3 m

- Coba

EXPLANATION

Layer of thenardite or humberstonite

Salt-cemented regolith

FIGURE 10.—Characteristic layering of an alluvial-type 

nitrate deposit.
16 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

nature and porosity of the host rock, configuration 

of the land surface, and degree of erosion, leaching, 

and redeposition of the saline minerals by rainwater.

Alluvial-type caliche either grades downward into 

conjelo, which is saline-cemented regolith containing little NaNO3 , or into coba, which is loose uncemented regolith (fig. 10). Conjelo, where present, 

grades downward into coba. Conjelo was not observed in this study, and thicknesses are not reported in the literature. Presumably, maximum 

thicknesses are not more than a meter or two. Isolated lenses and pods of firmly cemented conjelo 

or caliche have been found in the coba below the 

caliche, but evidently these are relatively rare. Both 

coba and conjelo are dry, although some coba has 

been reported to be slightly moist (Briiggen, 1938). 

The contact between caliche and coba is relatively 

sharp, generally changing from hard nitrate ore to 

soft uncemented material in a few centimeters.

Bedrock caliche consists of saline impregnations 

in porous rock and of veins and layers of caliche 

blanco along fractures in otherwise massive or 

poorly bedded rocks such as ash-flow tuffs (fig. 13) 

and along bedding planes and in crosscutting fractures in layered sedimentary rocks. Such veins and 

layers, which generally range from a few centimeters to several tens of centimeters in thickness, 

formed along incipient cracks and narrow fissures

FIGURE 11.—Layer of friable high-purity humberstonite 

10-30 cm thick, overlain by 10-30 cm of sandy rocky soil 

(chuca). Oficina Alemania, Taltal district. Ore pile to 

right contains large chunks of humberstonite.

that were gradually forced open by the crystallizing 

saline minerals. Where carried to the extreme, such 

a process transforms strongly fractured bedrock 

into a mosaic of isolated rock fragments in a matrix 

of caliche blanco.

Layers of caliche blanco in bedrock are generally 

at depths of 3-6 m. They are most extensive in thinbedded, near-horizontal, stratified rocks such as a 

sequence of Jurassic limestone and interbedded shale 

and sandstone that occurs in the northern Tarapaca 

district (fig. 7). In a given nitrate field, the layers 

tend to be at a relatively constant depth and, like 

layers of caliche blanco in alluvial-type caliche, tend 

to be parallel or subparallel to the land surface. Because of the depth and difficulty of removing the 

bedrock overburden, much of this type of nitrate 

ore was formerly mined in shafts and interconnecting tunnels, such as those shown in figure 7.

The nitrate fields are characterized by large and 

small desiccation polygons that formed and continue 

to form in response to volumetric changes related to 

slow leaching and redeposition of the saline minerals. 

Small polygons generally do not penetrate deeper 

than the upper part of the caliche layer, as shown in 

figure 14, but the large polygons, which are several 

meters in diameter, may extend to the bottom of 

the layer. Marginal fractures of the large polygons 

generally have a filling of saline-cemented sand and 

silt a few centimeters to several tens of centimeters

FIGURE 12.—Rajo San Martin, a nitrate mine in the Taltal 

district that was active at the time this photograph was 

taken in 1962. The working face shows a sharp contact 

(marked by hammer) of hard massive caliche and overlying costra, which here is slightly porous and relatively soft. 

A 20-30-cm layer of unconsolidated chuca on top of the 

costra was removed before mining.
GEOLOGY OF THE NITRATE DEPOSITS 17

thick, which have been called sand dikes (Ericksen, 

1963). The sand dikes (fig. 15) commonly are richer 

in nitrate than is the alluvial caliche in which they 

occur, which reflects enrichment due to selective 

leaching of nitrate from near-surface soil or caliche 

and redeposition as cementing material in the dikes. 

Some of the sand dikes and narrow fissures without 

dikes contain narrow veins of caliche blanco. Large 

polygons, which continue to form in the nitrate fields 

today, are marked by depressions in soft chuca 

(fig. 16).

One of the unusual surface manifestations of prolonged slow leaching is the formation of hard spheroidal cakes of anhydrite, called losa, embedded in 

powdery chuca (fig. 17). These cakes evidently are 

the end product of processes of desiccation and 

leaching that first formed small polygons such as 

those shown in figure 14.

Abundant chalcedony nodules in some of the 

nitrate fields, notably those of the Taltal district, 

have been the subject of much discussion and specur1" J*P • • ' -JSF fyw** v " «F**' . * -- !? M - «V *

•' ",' ' j *'. %S-":i ,'•»<* i"*'*

<•-/ -» * - ^sifcfc-"*'^^**-^^ *" -!L:~» * ^

**^ $ ' £~?**' » "'— :i *.** - *i*' ^fr •*

FIGURE 13.—Bedrock caliche, consisting of impregnations and 

veins of caliche blanco in rhyolite tuff, exposed in an opencut nitrate mine in the Taltal district. The veins from this 

locality were found to contain about 99 percent NaNO3, 

the highest nitrate content reported from northern Chile.

FIGURE 14.—Polygonal surface of costra, Tarapaca district, which has been swept free of the surface covering (chuca) 

along a mine-access road. Inset on left shows nodular uderside of 50-75-cm-thick slab of hard polygonal costra; inset 

on right shows section of leached soft polygonal costra in which polygonal fractures terminate in the underlying 

caliche.
18 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

ft -

FIGURE 15.—Saline-cemented, nitrate-rich sand dike in alluvial-type caliche, northern Tarapaca district. These dikes 

generally are somewhat richer in nitrate than is the enclosing caliche. Note parallel sides of dike indicating that 

the structure is a former open fissure; sides of other dikes 

are more irregular because of sloughing of material from 

the sides of the open fissure.

lation. They occur as small, convolute, flattened 

nodules and fragments of tan, pink, and white 

chalcedony and are locally so abundant that they 

form a desert pavement. Nitrate miners have long 

considered abundant chalcedony to be an indicator 

of rich nitrate ore. Several authors (Ochsenius, 

1887-88; Sundt, 1919; Wetzel, 1928) suggested that 

the chalcedony is the residual product of in situ 

decomposition of feldspathic rocks that liberated the 

sodium, potassium, magnesium, and calcium in the 

saline minerals of the nitrate deposits. The literature 

does not give a clue to any other source of these 

chalcedony nodules, but early in our studies we 

found veins of vuggy l^hiellar chalcedony in rhyolite 

ash-flow tuff in the eastern part of the Taltal district 

that were being weathered to form a desert pavement identical with the pavements of chalcedony 

fragments seen elsewhere in the district (Ericksen, 

1963). Evidently, these veins formed by thermalspring activity in the ash flows shortly after they 

were emplaced. Wind erosion of the relatively soft 

tuff exposed the veins which then disintegrated, and 

the fragments spread in their vicinity. In turn, the 

fragments were spread more widely by mudflows 

ahd runoff at times of exceptionally heavy rains. 

They accumulated on some old surfaces that were 

favorable sites for slow accumulation of rich nitrate

ore, which led the miners to believe that chalcedony 

was an indication of such ore.

Feathers and mummified bodies of birds, masses 

of bird guano, and even egg fragments have been 

found in the nitrate fields, some in the caliche layer, 

a meter or more below the surface. The remains of 

both land birds and sea birds have been found. The 

presence of masses of guano and of eggs indicates 

that some areas of the nitrate fields were favored 

nesting or roosting grounds for certain birds. Some 

guano appears to be very old and has been cemented 

by saline minerals of the nitrate ore. Other guano 

is uncemented and obviously more recent. Masses of 

saline-cemented guano a meter or two in diameter 

and as much as 50 cm thick are particularly abundant in the nitrate fields near Oficina Alemania, 

Taltal district (fig. 1). Wetzel (1925) described 

bird remains in the region of Toco (Tocopilla district) and identified a mummified sea bird as being 

of the genus Thalassidroma. Nitrate miners have 

reported that small land birds have been blasted 

alive out of nitrate ore during mining. They evidently live in crevices and cavities in the ore. Sea 

birds now occasionally fly into the nitrate region, 

and Briiggen (1938) mentioned sea birds as having 

nesting grounds in some of the nitrate fields. It seems 

unlikely that sea birds would now occupy nesting 

grounds far from the coast, but they may have done

. "ff-tr .-• -.f-'-J'J- >.»«« .-"-.**•--,«-. -^r ; &«t&t&&f-3>. *•£•*£'2*-"^?J.>~" ix*z:>&& :**•*&'%j&i !^^;; .-.

FIGURE 16.—Newly formed desiccation polygons in nitrate 

ore, Tarapaca district; fissures at the margins of the 

polygons are marked by depressions in the chuca.
GEOLOGY OF THE NITRATE DEPOSITS 19

.:*'«••** »- /^^^-^fe"£Jft^5i>il^>^^ xv - Vl«?fi¥ -f^SIviJfes. -»-*!*"*?>.ST.- .SsSk**...-^/ r ."1Illrdrr^l. .»„ -

^

Mt_v».

%•- v,-- , -^i

^.•\- ^

?j ':' - V ' ^

FIGURE 17.—Losa cakes embedded in powdery chuca, Tarapaca district. Cakes show typical oblate spheroidal form, and 

consist chiefly of tan anhydrite cut by veinlets of white anhydrite deposited in shrinkage cracks, as can be seen in 

broken cakes in inset. Tops of cakes tend to be smoothly rounded, whereas bottoms are nodular or jagged.

so in the past when lakes were present in the Atacama Desert (see p. 6).

CHEMICAL AND MINERALOGICAL COMPOSITION

The nitrate deposits are unique not only in having 

a high concentration of nitrate (table 1), but also 

in having exploitable iodate, an extremely high 

iodine/bromine ratio, and the only known naturally 

occurring perchlorate ion (ClOr) . 5 The several iodate 

minerals are unique to the deposits, as are the potassium chromate and dichromate minerals (table 2).

6 The perchlorate ion has an exceptionally high oxidation potential 

and is not known to form by any natural chemical process, which has led 

some reviewers of this report to express doubt that it exists in the Chilean

nitrate deposits. However, the evidence for widespread perchlorate in 

nitrate ore is overwhelming, and it cleaH^.does exist, even though neither 

its mode of formation nor its mineral identity is known. Conventional 

chemical analyses of nitrate ore, in which v perchlorate is determined by 

an oxidation-reduction reaction, show ubiquitous perchlorate in amounts 

similar to those shown by the analyses in table 1. Also, end-product nitrate 

produced from the ore contains perchlorate, and one nitrate plant reportedly experimented in the production of potassium perchlorate earlier in 

this century. To test the validity of the conventional chemical analyses 

of nitrate ore and to determine possible interference of the other highly 

oxidized ions nitrate and iodate, John Marinenko of the U.S. Geological 

Survey devised a colorirnetric method of determining perchlorate in the 

presence of these ions. He found that a sample of rich ore from a 

nitrate field in northern Antafagasta Province, a region known for exceptionally high perchlorate, contains 0.73 percent C1O4-. An on-going mineralogical study of this sample and other samples of nitrate ore from the 

same region has not yet shown the presence of a perchlorate mineral, 

but potassium perchlorate was formed during fractional crystallization 

experiments, by Mary E. Mrose (U.S. Geological Survey) and by me, of 

solutions of these ores. We now can only estimate that the perchlorate is 

present either as finely crystalline potassium perchlorate or as a contaminating ion in the crystal structure of other saline minerals.
20 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

The nitrate ores show great local and regional 

variations in chemical composition, so it is difficult 

to estimate the composition of an average or typical 

ore. Perhaps the best estimates are those based on 

large tonnages of ore treated by the nitrate-beneficiation plants (table 1). Higher grade nitrate ores, 

mined selectively during earlier days, contained two 

or three times as much nitrate as do the present 

ores, and some contained 50 percent or more sodium 

nitrate.

One of the problems of estimating chemical composition of the nitrate ores is the lack of complete 

chemical analyses of the saline constituents. Nearly 

all analyses, including those cited in table 1, show 

only the saline components that are dissolved by hot 

water. As a consequence, the analyses show low 

values for constituents such as sulfate, calcium, 

borate, and iodate, which occur in part in minerals 

of low solubility.

Of the many saline minerals present (table 2), 

halite, the only halide mineral, and soda niter are 

by far the most abundant, being the dominant minerals in nearly all nitrate ore. Although the total 

sulfate may equal or exceed nitrate (table 1), it is 

contained in several minerals, and any single sulfate 

mineral is rarely more abundant than soda niter. 

The most abundant sulfate minerals are glauberite, 

bloedite, gypsum, and anhydrite. Also abundant are 

the sulfate-nitrate minerals, darapskite, and humberstonite. Thenardite, which is an abundant mineral 

in other desert saline deposits, is rare or absent in 

the nitrate ore because it is unstable in solutions

from which soda niter precipitates (Ericksen and 

Mrose, 1970). Ulexite is the dominant boron mineral.

ORIGIN OF THE NITRATE DEPOSITS

The diversity of ideas about the origin of the 

Chilean nitrate deposits may be taken as an indication of the complexity of the problem and the scantiness or obscurity of direct evidence about the sources 

of the saline components and their modes of accumulation and concentration. The origin is neither 

simple nor obvious. Although the saline materials 

in the deposits clearly accumulated as the result of 

atmospheric transport and deposition, the major 

sources or modes of formation of these materials 

remain obscure. This is particularly true for the 

nitrate, iodate, perchlorate, and chromate ions. 

Nevertheless, the presence of widespread nitraterich saline soils in the Atacama Desert and their 

scarcity or absence in other deserts should be a 

result of the unique characteristic of this desert— 

the long existence of a nearly rainless climate. This 

factor has favored the accumulation and preservation of the highly soluble saline minerals in the 

nitrate deposits. Other factors of importance are 

wind and hydrologic regimens that favored accumulation and retention of saline materials within the 

Atacama Desert, the proximity of the Pacific Ocean, 

a probable source of significant amounts of saline 

materials, and widespread volcanic activity in the 

nearby Andes during late Tertiary and Quaternary 

time, a source of additional saline material.

TABLE 1.—Chemical composition of water-soluble saline components in selected nitrate ores and high-purity veins from the

nitrate deposits in northern Chile

[Composition in weight percent. Based on total sample; insoluble residue not cited in analyses. Recalculated to facilitate comparison of analyses from 

different sources. Ions shown are the principal chemical components of nitrate ore, except for boron, which exists largely as ulexite NaCaBr,O9-8HaO]

NOsci- _ _ _ _.

Sor2 _ _ .

IO3- __ _ _ __. ClOr _____ . B _ ____ __ . Na+* _ __ __ . K* _ _ __ _ . Mer+2

Ca+2

H20 ______

i

. — _ 6.35

. _ _ 3.87

. _ __ 6.60

— __ .061

- __ .035

. — _ .10

6.2

.56

.15

_ _ __ 1.15

1.74

2

6.66

7.07

12.43

.068

.027

.17

8.9

.61

.73

2.27

1.08

3

17.67

11.11

3.47

.004

.57

.02

13.8

.95

1.06

1.87

4

8.77

4.73

2.69

.04

.4

.06

6.4

.28

1.00

4.87

5

72.15

.07

.10

.04

.01

.05

1.52

6

32.63

21.84

8.30

.04

.25

.05

.32

.93

.32

3.40

7

• 44.46

14.97

2.38

.14

1.12

.28

.36

.79

8

49.65

12.92

2.72

.08

.56

.12

6.89

.27

.14

——

1. Average grade of 5.97 million tons of nitrate ore treated at Oficina Pedro de Valdivia during 12-month period, July 1, 1935-June 30, 1936. Furnished by Anglo-Lautaro Nitrate Corp.

2. Average grade of 6.27 million tons of nitrate ore treated at Oficina Maria Elena, July 1, 1951-June 30, 1952. Furnished by Anglo-Lautaro Nitrate

Corp. 

3-4. Channel samples of selected parts of nitrate ore layer from two localities at Oficina Humberstone. Analyses by Empresa Salitrera Victoria of

samples collected by Institute de Investigaciones Geologicas of Chile.

5. Oficina Lautaro; specimen of caliche blanco analyzed at Oficina Victoria, 1964.

6. Oficina Santa Fe; specimen of caliche blanco analyzed at Oficina Maria Elena, 1966.

7. Oficina Argentina; specimen of caliche blanco (Semper and Michels, 1908).

8. Tarapaca district; specimen of caliche blanco (Penrose, 1910).

* Not determined in conventional analyses by nitrate companies of northern Chile. Calculated by author on basis of equivalent weights of reported 

components. Estimated to be within 1 percent of correct value.
ORIGIN OF THE NITRATE DEPOSITS 21

TABLE 2.—Saline minerals of the Chilean nitrate deposits

HALIDES:

Halite, NaCl

NITRATES:

Soda niter, NaN03

Niter, KN03

Darapskite, NastSO*) (N03 ) -H2 0

Humberstonite, K3Na7Mg2 (SO*) 8 (N03 ),• 6H2 0

IODATES AND CHROMATES: 

Lautarite, Ca(IOs )2 

Briiggenite, Ca(I03 )2-H2 0 

Dietzeite, Ca- (I03 ) 

Tarapacaite, K2CrO* 

Lopezite, K2Cr2O7

BORATES:

Ulexite, NaCaB5O9 -8H2 0 

Probertite, NaCaB5 09 -5H2 0 

Ginorite, Ca2B14023 -8H20 

Hydroboracite, CaMgBaOn-6H2 0 

Kaliborite, HKMgoB]2Oi6 (OH) 10 -4H20

SULFATES:

Thenardite, Na2SOt 

Glauberite, Na2Ca(S04) 2 

Bloedite,Na2Mg (SO*) 2 - 4H2 0 

Kieserite, MgSOt-H20 

Epsomite, MgS04-7H2 0 

Gypsum, CaS04-2H2 0 

Anhydrite, CaSO* 

Bassanite, 2CaSOt - H20

DISCUSSION OF THEORIES

Discussions of origin have been concerned largely 

with sources of the nitrate and with the mode of 

accumulation of the deposits. The sources of the 

other saline components generally have been assumed to be those most compatible with the presumed source of nitrate.

The following sources of nitrate have been proposed : (1) Decay of seaweed and other marine vegetation in waters and marshes of partially cutoff inland arms of the sea (Forbes, 1861; Noellner, 1867; 

Flagg, 1874; Miintz, 1887; Miintz and Marcano, 

1885) or of vegetation in saline lakes of continental 

origin (Sieveking, 1887); (2) nitrification and 

leaching of seabird guano at the margins of saline 

lakes (J. C. Hillinger, I860 6 ), inland arms of the 

sea (Penrose, 1910), or salars (Gautier, 1894), or 

accumulation of windblown ammoniacal particulate 

matter and gas from extensive guano deposits that 

were presumed to have existed formerly along the 

coast of northern Chile (Ochsenius, 1887-88, 1888, 

1903; Briiggen, 1928, 1938); (3) bacterial decay of 

plant and animal remains during Tertiary and Qua8 Referred to in many publications about the nitrate deposits as a 

report in the "Annario Nacional de Peru." The original report concerning 

mineral deposits in Tarapaca was published in the newspaper "Comercio" 

and cited by A. G. Leubel in his book "El Peru en 1860 o sea anuario 

nacional." No further information is available on the book or the 

newspaper.

ternary time when a less arid climate supported 

abundant vegetation in northern Chile (Plagemann, 

1897-98), or of ancient plant material in soils (Newton, 1896); (4) nitrification and fixation of atmospheric nitrogen by bacteria in soil (Gale, 1912, 1917; 

Briiggen, 1938); (5) reaction of atmospheric nitric 

acid with feldspathic igneous rocks of the nitrate 

fields (Pissis, 1878; Sundt, 1904, 1911, 1917, 1921; 

Wetzel, 1924, 1926, 1928, 1932, 1961; Knoche, 1930, 

1939). Pissis and Wetzel suggested that the nitrate 

ion formed by electrical discharge in the dense winter fogs (camanchaca); Sundt proposed diverse 

sources for nitrate and ammonia in the atmosphere, 

and Knoche suggested formation by reaction of atmospheric nitrogen with ozone; (6) accumulation 

of nitrogen compounds of volcanic origin (ammonia, 

nitrogen oxides, or nitric acid) in gases associated 

with intense volcanism in the Andes (de Kalb, 1916; 

Feistas, 1922, 1966; Steinman, 1925), and nitrate 

salts leached either from volcanic rocks of Jurassic 

age (Whitehead, 1920) or from rhyolithic ash-flow 

tuffs of late Tertiary and Quaternary age (Ericksen, 

1961); (7) nitrate from diverse sources accumulated 

in the subsurface saline waters and brines of salars 

(Singewald and Miller, 1916; Mueller, 1960, 1968) 

or in the soils of the nitrate fields (Hofseth, 1931; 

Claridge and Campbell, 1968; Ericksen, 1975, 1979). 

According to these theories, the saline constituents 

of the deposits were supplied by: (1) Evaporation
22 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

of saline waters at the margins of inland arms of 

the sea or saline lakes, or within such bodies of 

water or marshes that subsequently evaporated to 

dryness. Charles Darwin, who visited the nitrate 

fields of Tarapaca in July 1835 during the famous 

voyage of the Beagle, was the first to suggest that 

the deposits were formed at the margin of an inland 

arm of the sea (Darwin, 1890); (2) capillary migration of subsurface saline water and brine from 

salars into soils and fractured rock of nearby hillsides where the saline materials were deposited by 

capillary evaporation, first proposed by Singewald 

and Miller (1916) and then reproposed by Mueller 

(1960); (3) accumulation in soils of the nitrate 

fields by fallout or washout from the atmosphere 

and by reactions of atmospheric constituents with 

these soils, first proposed by Pissis (1878); and (4) 

accumulation of saline-rich mudflows, first proposed 

by Wetzel in 1924 (Wetzel, 1924) and discussed in 

several subsequent papers (Wetzel, 1926, 1928, 1932, 

1961). Wetzel has been the only adherent of this 

proposed mode of accumulation of the nitrate deposits.

Many authors have proposed special chemical or 

biological reactions in the soils of the nitrate fields 

to form the saline constituents of the deposits, as 

well as special mechanisms for concentrating the 

widespread saline materials to form commercial 

nitrate deposits. The reaction of nitrate and other 

atmospheric constituents with f eldspathic rocks was 

favored by many authors (for example, see Pissis, 

1878; Sundt, 1919; Wetzel, 1928) as the chief source 

of the principal cations Na, K, Mg, and Ca in the 

saline minerals of the deposits. Briiggen (1928, 

1938) suggested that the deposits formed at a time 

when rainfall was sufficient to support widespread 

though sparse plants in the Atacama Desert and 

that symbiotic bacteria associated with roots of 

legumes caused fixation of atmospheric nitrogen and 

nitrification of ammonia from guano deposits. Most 

of the theories for accumulation by deposition or 

reaction of atmospheric constituents at the sites of 

the deposits also proposed that leaching by rainwater 

and movement of solutions downslope enriched the 

nitrate to produce the high-grade deposits that are 

so common on the lower parts of hills (for example, 

see Briiggen, 1938).

Theories suggesting formation of the deposits at 

the margins of salars, lakes, or inland arms of the 

sea are based chiefly on the distribution of nitrate 

workings in Tarapaca Province, which tend to be 

along the eastern side of the Coastal Range near 

salars and playas in the Pampa del Tamarugal (fig.

1). Indeed, the positions of these workings are suggestive of accumulation at a lake margin or within 

the capillary fringe of near-surface saline water or 

brine at the margins of salars. However, moist capillary fringes are not evident at the margins of 

present-day salars, even where the water level is 

virtually at the surface, and there is no reason to 

suspect that deposition of saline material in capillary-fringe zones of salars has been a significant 

process in the formation of the nitrate deposits. 

Moist capillary-fringe zones may have existed at the 

margins of former saline lakes in the Atacama 

Desert, but they would have been restricted to relatively narrow zones at the lake margins, and capillary evaporation and deposition of saline material 

would have been restricted to the soils of these narrow zones. Most of the nitrate deposits are far from 

such zones. On the other hand, former saline lakes 

in the Pampa del Tamarugal (see p. 6) might have 

contributed significant quantities of saline material to 

nitrate deposits by evaporation of shallow marginal 

waters and of windblown spray. Isolated small saline 

lakes and ponds in the Coastal Range, which were 

fed by ground-water inflow along faults connecting 

them to the Pampa del Tamarugal (see p. 6), also 

may have received saline materials that infrequent 

rains leached from nearby nitrate-rich saline soils 

and preexisting nitrate deposits. Such nitrate-rich 

saline materials would have been redeposited in the 

ponds as they evaporated to dryness. The nitrate 

region has been above sea level since before the 

nitrate deposits began to form; consequently, none 

of the saline materials in the deposits originated in 

inland arms of the sea.

Pissis (1878) was the first to suggest that an 

atmospheric source of the saline components was 

the only way the geographic and topographic distribution of the deposits could be adequately explained. His arguments are as valid today as they 

were a century ago. His evidence for an atmospheric 

source was chiefly the presence of nitrate deposits 

on hills where they could not have been formed in 

or near a lake, arm of the sea, or marsh. Sundt 

(1911) and Briiggen (1928) also noted that the 

existence of these deposits presented insurmountable 

difficulties to any mode of accumulation of the saline 

material other than deposition from the atmosphere. 

They both cited nitrate fields high on hills and at 

great distances from salars as examples of accumulations that could not have formed by migration and 

evaporation of subsurface saline water at margins 

of salars (Singewald and Miller, 1916). Rich (1941, 

1942) noted that the distribution of the nitrate de-
ORIGIN OF THE NITRATE DEPOSITS 23

posits could not be adequately explained by the 

theory of Singewald and Miller. Bruggen (1938) 

pointed out that Wetzel (1932) was in error in supposing that mudflows had anything to do with accumulation of the nitrate deposits. Wetzel had suggested accumulation of saline mudflows in an attempt to explain the fragmental nature of the saline 

components of nitrate ore, which he discovered in 

microscopic examination of thin sections (Wetzel, 

1924), and a peculiar layering that he observed in 

alluvial-type nitrate deposits (Wetzel, 1926, 1928). 

Evidently, Wetzel failed to recognize that the fragmental texture could be caused by repeated partial 

solution and redeposition of the saline components 

as they migrated through the soils to form the enriched nitrate deposits, and that the layering was a 

primary structure in the alluvium rather than a 

feature related to accumulation of the nitrate deposits.

Some of the proposed sources of the nitrate are 

untenable on the basis of present-day geological and 

chemical knowledge or did not yield a significant 

part of the nitrate in the deposits. The soils of the 

nitrate fields do not show any widespread concentration of phosphatic material indicating the presence 

of former guano deposits that might have been the 

source of nitrate. Some nitrate may have originated 

as ammonia released from seabird guano along the 

coast, as suggested by Bruggen (1938), but the 

amount probably was small because the guano deposits are small in comparison with the nitrate deposits. Ammonia and ammonium compounds are 

known products of volcanic emissions, but the 

amounts are small compared with those of sulfate, 

sulfur dioxide, and chloride (for example, see White 

and Waring, 1963). Nitrate has been found in 

water-soluble saline material in ash-flow tuffs of 

Pliocene age in northern Chile (Ericksen, 1961), 

but whether this nitrate is an original constituent, 

perhaps formed by oxidation of primary ammonia, 

or a later contaminant is uncertain. Whitehead 

(1920) suggested that volcanic rocks of Jurassic age 

in northern Chile contained concentrations of nitrate 

that were leached and redeposited to form the 

nitrate deposits. His theory was based on the occurrence of rich nitrate ore along certain tuffaceous 

horizons in a Jurassic volcanic sequence exposed in 

nitrate fields in the eastern part of the Aguas Blancas district. However, later work (Bruggen, 1938; 

Ericksen, 1963) has shown that the nitrate deposits 

are unrelated to rock type, and that the fortuitous 

relationship observed by Whitehead was probably 

due to porosity of the tuff rather than to its composition.

Feistas (1922) first proposed that the nitrate deposits formed by reaction of soil and rock in the 

nitrate fields with acids in clouds of volcanic gases. 

This proposal was based on his spectrographic 

tudies of nitrate ores, which showed a wide variety 

and great abundance of rock-forming elements. Evidently Feistas failed to recognize that these elements were contained in the rock and soil of the 

nitrate ore as well as in the saline components. 

Feistas (1966) reiterated his theory after a visit to 

southern Peru and proposed that the acid-bearing 

volcanic clouds originated near the city of Arequipa 

in southern Peru and swept northward in Peru and 

southward into Chile. In both papers, Feistas proposed some rather fanciful geological processes. For 

example, in the 1966 paper, he concluded that dune 

fields in southern Peru mark the paths of the corrosive acid clouds and that the famous sillar, or ashflow tuff, near Arequipa was the final product of 

the most intensive leaching by volcanic acids.

SOURCES OF THE SALINE COMPONENTS

In a recent paper (Ericksen, 1979), I suggested 

that the nitrate deposits could have formed by normal atmospheric deposition of the major constituents 

in periods of 100,000 to 200,000 years, if it were 

assumed that none of these constituents was removed from the nitrate fields by rainwater leaching 

or was destroyed by microbial activity. These estimates were based on investigations of atmospheric 

trace constituents conducted by many investigators in the 1950's and 1960's (for example, see 

Eriksson, 1960; Robinson and Robbins, 1970a, b). 

Additional investigations in the 1970's cast doubt 

on earlier estimates of atmospheric concentrations of 

natural particulate and reactive gaseous compounds 

of nitrogen and sulfur, indicating that these estimates are too high by at least an order of magnitude 

(for example, see Noxon, 1975, 1978; Granat and 

others, 1976; Soderlund and Svensson, 1976; Busenberg and Langway, 1979; Crutzen, 1979). As a consequence, it does not seem possible that the nitrate 

deposits could have accumulated by atmospheric deposition of saline constituents whose concentrations 

here are not greater than world averages for clean 

air; therefore, local sources must increase these constituents.

The local sources of the saline constituents are 

those that supply these constituents to the atmosphere elsewhere; they include evaporation and spray 

from the ocean surface and from former saline lakes 

in the Atacama Desert, gaseous and particulate matter from volcanic activity in the Andes, windblown
24 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

dust, biological activity in soils, and photochemical 

reactions or electrical discharge in the atmosphere. 

I assume that saline constituents from these sources 

have been accumulating at the sites of the nitrate 

deposits at least since mid-Miocene time, 10-15 million years ago, when infilling of the Central Valley 

had been largely completed (see p. 6). Although 

the climate of the Atacama Desert has been extremely arid throughout late Tertiary and Quaternary time, there undoubtedly have been intervals 

of climatic change when increased rainfalls greatly 

modified or destroyed preexisting nitrate deposits.

Although some of the saline material in the deposits may have been transported from distant 

sources, as has been suggested for saline materials in 

dry Antarctic soils (Claridge and Campbell, 1968), 

most probably came from local sources. Sea salts, 

ejected from the ocean surface chiefly in bubble 

spray (Blanchard and Woodcock, 1957; Bloch and 

Luecke, 1972), are carried inland as solutions in fog 

droplets and as dry particulate matter remaining 

after evaporation of the spray. The amounts deposited in dry fallout and in rainwater may be considerable (for example, see Junge and Gustafson, 

1957; Eriksson, 1952a, b, 1959, 1959-1960). As I 

have pointed out (p. 10), the coastal fog of northern Chile contains saline materials and may be an 

important source of some of the saline constituents 

of the nitrate deposits. The hundreds of volcanoes 

that were active in the northern Chilean Andes at 

various times during late Tertiary and Quaternary 

time may have supplied large amounts of gaseous 

materials and dust containing compounds such as 

S02 , H2S04 , NaCl, NH3 , and NH4C1 that could have 

been important sources of sulfate and chloride. The 

associated volcanic rocks contain considerable 

amounts of saline materials that are leached by 

rainwater and carried by ground water and streams 

into the Atacama Desert (Ericksen, 1963); there, 

they accumulated in former lakes and today are 

being deposited in playa lakes and salars. Rocks of 

the nitrate fields show evidence of corrosion by the 

saline minerals in the soils; such evidence chiefly 

consists of pits and cavities on the undersides of 

rock fragments that have lain in one position on 

saline soil surfaces for many thousands or tens of 

thousands of years. However, chemical weathering 

is extremely slow in the Atacama Desert, and it is 

unlikely that it produced more than small amounts 

of the saline constituents of the nitrate deposits.

Recent investigations undertaken primarily to 

evaluate the effects of atmospheric anthropogenic 

contaminants such as flurochloromethanes and

oxides of nitrogen on stratospheric ozone (McElroy 

and McConnell, 1971; Crutzen, 1974a; Wofsy and 

McElroy, 1974; Cicerone and others, 1974; Molina 

and Rowland, 1974; Rowland and Molina, 1975; 

McElroy and others, 1976) show many reactions in 

the atmosphere that could have formed significant 

amounts of nitrate and other oxidized saline components of the nitrate deposits. Tropospheric reactions involving ozone (03 ) and photochemically dissociated water (H02 ~, OH-) probably have been 

of prime importance in the formation of nitrate 

from gaseous nitrogen oxides and ammonia (Levy, 

1972; Crutzen, 1974b; Chameides, 1975, 1978; 

Simonaitis and Heicklen, 1975; Chameides and Stedman, 1977). Similar reactions involving gaseous 

chlorine and iodine in the atmosphere may have 

formed perchlorate and iodate, although such reactions have not yet been identified as occurring in 

the atmosphere. Ozone and other gas-phase reactants are formed in the atmosphere by ultraviolet 

radiation, the dominant process in the stratosphere, 

by a variety of photochemical reactions involving 

oxides of nitrogen, methane, and carbon monoxide, 

and by lightning during thunderstorms (for example, see Crutzen, 1979; Chameides, 1975, 1978; 

Noxon, 1976; Griffing, 1977). A discussion of these 

and other atmospheric photochemical reactions is 

beyond the scope of this report, but their diversity 

has been indicated by Anderson (1976) who cited 

250 gas-phase reactions of possible importance in 

the atmosphere.

NITRATE

Nitrogen (N2 ) is a chemically unreactive atmospheric gas that is fixed or combined into natural 

organic and inorganic compounds by microbial activity in soils and water and by photochemical processes in the atmosphere (for example, 'see Soderlund 

and Svensson, 1976). Of the global nitrogen cycle, 

90-97 percent involves exchange of organically fixed 

nitrogen in soil and water, whereas only 3-10 percent involves fixation of atmospheric nitrogen (Rosswall, 1976). Microbial fixation of atmospheric nitrogen is chiefly by symbiotic bacteria of certain 

leguminous plants, by autotrophic bacteria, and by 

certain blue-green algae (for example, see Soderlund 

and Svensson, 1976; Griffing, 1977). Fixation of 

nitrogen in the troposphere is chiefly due to thunderstorm activity, wherein temperatures in lightning 

flashes are high enough to cause formation of nitrogen oxides (Noxon, 1976, 1978; Crutzen, 1979).

Fixed nitrogen compounds that have been detected 

in the atmosphere including the gaseous forms ni-
OEIGIN OF THE NITEATE DEPOSITS 25

trous oxide (N2 0), nitric oxide (NO), nitrogen 

dioxide (N02 ), nitric acid (HN03 ), and ammonia 

(NH3 ); compounds such as (NH4 ) 2S04 , NaN03 , and 

HN03 occurring as particulate material and as solutions in rainwater; and nitrogen-bearing organic 

substances such as proteins, amines, and amino acids 

(for example, see Eriksson, 1952a; Junge, 1958b; 

Visser, 1961; Jones 1971; Levy, 1972; Soderlund and 

Svensson, 1976; Griffing, 1977; Noxon, 1978). Nitrous oxide is formed by anaerobic decay of organic 

materials on the earth's surface, chiefly on continents but also at the ocean surface (KoblentzMishke and others, 1970; Junge and Hahn, 1971; 

Hahn, 1974; McElroy and others, 1976; Griffing, 

1977). Nitric oxide and nitrogen dioxide, which are 

the principal reactive odd nitrogen oxide molecules 

(NOX ) in the atmosphere, are formed by photochemical reactions involving ozone (for example, 

see Georgii, 1963; Nicolet, 1970) and by electrical 

discharge in thunderstorms (Noxon, 1976; Griffing, 

1977; Dubin and Zipf, 1977). Nitrate is concentrated 

in thin organic films at the ocean surface (Wilson, 

1959; Williams, 1967; Barger and Garrett, 1970; 

Barker and Zeitlin, 1972), which are ejected into the 

atmosphere in bubble-spray (Maclntyre, 1970). The 

chief source of ammonia is considered to be biological processes in continental soils (for example, 

see Georgii and Miiller, 1974), but the sea surface 

may be the source of considerable amounts of ammonia or ammonium compounds in areas of high 

biological activity. For example, Bloch and Luecke 

(1970) reported a relatively high concentration of 

ammonium (NH4 +) in seawater bubble-spray, showing as much as 100 ppm compared with 0.1 ppm in 

seawater. They suggested that nitrate in the Chilean 

deposits formed from such oceanic ammonium that 

was oxidized photochemically in the atmosphere. 

Photochemical oxidation of NH3 in the troposphere, 

chiefly by reaction with OH~ molecules produced by 

reaction of ozone with water vapor or by other photochemical reactions, may be an important source of 

tropospheric NOX (Junge, 1958b; Levy, 1972; Crutzen, 1974b; Chameides, 1975).

Nitrous oxide (N2 0), which is the most abundant 

of the fixed nitrogen compounds in the atmosphere 

(about 250 ppbv), is a relatively unreactive gas in 

the troposphere, but is the major source of NOX in 

the stratosphere (for example, see Griffing, 1977). 

Reaction of N2 0 in the stratosphere with the electronically excited oxygen atom 0( 1D) is rapid and 

probably is the major process in the formation of 

stratospheric NOX (Nicolet, 1970; McElroy and 

McConnell, 1971). NOX formed in the stratosphere

may be carried to the troposphere by mixing of 

iropospheric and stratospheric air. The reaction of 

N2 0 and 0( 1D) is of little importance in the troposphere because 0 CD), which forms by photodissociation of ozone by ultraviolet light, is sparse in the 

troposphere (Levy, 1972).

The principal source or sources of the nitrate in 

the Chilean nitrate deposits is uncertain, because 

average concentrations of NOX , HN03 , and NH3 in 

the atmosphere appear to be too low for these constituents to be the sole sources of the nitrate as I 

had formerly proposed (Ericksen, 1979). Estimates 

that indicated background concentrations of the 

NH3 and NOX in clean unpolluted air of 6 ppbv 

(parts per billion, vol/vol) and 3 ppbv, respectively 

(Robinson and Robbins, 1970a), were the basis of 

the estimate (McConnell, 1973) that worldwide annual deposition of NH3 -N and N03 -N is about 

2 x 101 * g. At this rate of deposition, and assuming 

that NO, N02 , and NH3 were oxidized to N03 in the 

troposphere or at ground level, the nitrate in the 

deposits could have accumulated in about 200,000 

years (Ericksen, 1979).

New estimates indicate that concentrations of 

NOX in clean air are <0.1 to 1 ppbv (Chameides, 

1978; Noxon, 1978; Huebert and Lazrus, 1978) and 

that removal rates are on the order of 1012 to 1013 g 

N yr- 1 (Galbally and Roy, 1978; Crutzen, 1979). 

Concentrations of particulate nitrate in the troposphere are similar to NOX (Huebert and Lazrus, 

1978), and the natural global source of ammonia 

in the atmosphere is about 5 X 1013 g yr- 1 (Dawson, 

1977). Crutzen (1979) suggested that gas-phase 

reactions producing NOX from NH3 in the atmosphere would yield only about 1013 g NOX -N yr- 1 , 

and that most NH3 is removed from the troposphere 

by processes other than gas-phase reactions. On the 

basis of these estimates, the deposition rate of 

NOX -N from the atmosphere would be at least an 

order of magnitude slower than that estimated by 

McConnell (1973), and the time required to form 

the nitrate deposits would be nearer to 2 million 

years than 200,000 years.

If, as I have suggested (p. 24), the nitrate deposits were formed during the past 10-15 million 

years, and if they have had a complex history of repeated deposition and destruction, a rate of deposition whereby the nitrate theoretically might accumulate in 200,000 years is reasonable, whereas I believe that a rate requiring 2 million years is not. 

Two factors must be considered: (1) Only part of 

the nitrate-bearing saline material that falls to the
26 GEOLOGY AND ORIGIN OF THE CHILEAN NITEATE DEPOSITS

ground surface moves downward into the soil—some 

may be carried away by wind, some may evaporate 

as a gas, HN03 or N02 , for example, and some may 

be washed away by rainwater. (2) Nitrate deposits 

once formed have probably been partly destroyed 

many times by exceptionally heavy rains or during 

periods of climatic change when rainfall was greater 

than it is at present. Also, as will be shown, the 

chloride in the deposits, which shows an abundance 

similar to that of the nitrate, has a rate of atmospheric deposition whereby it might accumulate in 

less than 200,000 years. I believe that the rates of 

accumulation of the chloride and the nitrate were 

similar. Therefore, local rich sources of fixed nitrogen compounds are probably more important in 

northern Chile than are the distant or normal sources 

of these compounds that account for average global 

concentrations. The most likely local source for NOX, 

NH3 , and organically fixed nitrogen, which through 

photochemical reactions or other processes might be 

transformed to nitrate, is the nutrient-rich water of 

the nearby Pacific Ocean. In addition, some ammonia 

may have been supplied by erupting volcanoes in 

the Andes, but to judge from the small amounts 

present in volcanic gases in other regions (for example, see Alien and Zies, 1923; White and Waring, 

1963), the amounts transformed to nitrate in northern Chile are correspondingly small. Rhyolite ashflow tuffs in the north Chilean Andes, which apparently contain small amounts of water-soluble 

nitrate (see p. 23), are evidently not sources of any 

significant amount of nitrate in the deposits. If these 

tuffs were important sources of nitrate or other 

water-soluble nitrogenous material, one would expect 

some of the many salars in this part of the Andes, 

which contain saline materials leached from these 

tuffs, to show abnormal concentrations of nitrate. 

Neither the saline crusts nor associated saline waters 

or brines in these salars contain unusual amounts 

of nitrate. Considerable amounts of nitrogen oxides 

may have been formed in Andean thunderstorms and 

carried westward to the Atacama Desert by wind 

and as solutions in streams and ground water.

The role of microorganisms in the formation of 

the nitrate deposits is difficult to evaluate, except 

that a general low level of microbial activity was 

essential to the preservation of the deposits. As has 

been pointed out (p. 11), microorganisms identified 

in the soils of the nitrate fields include species capable of nitrification and nitrogen fixation, and 

although these organisms are sparse in the dry 

desert soils today, they would be expected to multiply 

at times of greater-than-normal rainfall or coastal

fog. At such times, significant amounts of nitrate 

or organic nitrogen compounds, later oxidized to 

nitrate, may have been formed, particularly at soil 

surfaces where the content of soluble saline material 

is low and where autotrophic bacteria and blue-green 

algae would be most active. For example, more prevalent fogs in times past may have created conditions 

in the nitrate region similar to those of the wellknown present-day lomas of the coastal desert of 

Peru where the condensate from winter fogs (June^- 

August) is sufficient to support growth of a variety 

of lichens, mosses, grasses, tuberous plants, and 

drought-resistant bushes in areas that are essentially 

devoid of plants (except for sparse bushes) the rest 

of the year. Furthermore, during times of the 

heaviest fogs, the nitrogen-fixing blue-green alga 

Nostoc locally forms extensive mats on soil and dune 

sand (Dawson, 1963; Grolier and others, 1974). 

Although lomas do not exist today in the nitrate 

region of northern Chile, they probably have existed 

in past times of more prevalent coastal fogs. At such 

times, the higher plants and microorganisms both 

consumed and formed soil nitrates. Unfortunately, 

little is known about the salt content of the loma 

soils of Peru, so that direct comparison with soils 

in northern Chile is not possible.

HALOGENS

The only halogens occurring as distinct chemical 

components in the nitrate deposits are chloride 

(C1-), perchlorate (C104~), and iodate(I03 -). Only 

trace amounts of bromide (Br~) have been reported. 

The presence of perchlorate, the relative abundance 

of iodate, and the paucity of bromine are unique 

features of the deposits.

Chlorine, bromine, and iodine occur in the atmosphere as the molecular gases C12 , Br2 ,12 ; as the gaseous compounds HC1, CIO, HBr, and BrO; as inorganic compounds such as NaCl and NH4C1; and as 

halogen-bearing organic compounds (Eriksson, 

1952b; Miyake and Tsunogai, 1963; Duce and others, 

1963; Moyers and Duce, 1972a, b; Maclntyre, 1974; 

Wofsy and McElroy, 1974; Wofsy and others, 1975). 

Investigators generally agree that the ocean is the 

dominant source of these halogens, but considerable 

amounts of chlorine also are injected into the atmosphere by .major volcanic eruptions (Ryan and Mukherjee, 1975). Both these sources may have contributed chloride to the nitrate deposits, but the 

oceanic source probably was dominant.

The relatively high I/C1 and I/Br ratios in marine 

aerosols and rainwater, much greater than those in
ORIGIN OF THE NITRATE DEPOSITS 27

seawater (Duce and others, 1963; Miyake and 

Tsunogai, 1963; Moyers and Duce, 1972a, b; Seto and 

Duce, 1972), indicate somewhat different processes 

by which each element is added to the atmosphere. 

The chief source of chlorine in the atmosphere is 

probably NaCl in the sea spray (Blanchard and 

Woodcock, 1957), whereas iodine is released by 

evaporation of iodine gas from the ocean surface 

and as iodine-rich organic material of sea-surface 

films ejected in sea spray (Miyake and Tsunogai, 

1963; Maclntyre, 1974). The ratio of Br/Cl in the 

marine atmosphere is similar to that in seawater 

(Moyers and Duce, 1972a), so that sea spray is 

probably the chief source of the bromine.

The global flux of gaseous chlorine and of chlorine 

as NaCl in sea spray to the atmosphere is much 

greater than that of the other principal anions, sulfate and nitrate, in the nitrate deposits. Wofsy and 

McElroy (1974) estimated a net global production 

of gaseous chlorine, chiefly of marine origin, to be 

about 2 x 10" g yr-1, whereas production of particulate chloride, chiefly as NaCl in ocean spray, is 

about 6 X 1015 g yr- 1 . These authors suggested that 

HC1 and CIO are the dominant gaseous compounds 

at ground level. Eriksson (1959) found a relatively 

high concentration of chloride in rainfall and dry 

fallout along continental margins, where annual 

rates of chloride deposition from the atmosphere are 

commonly greater than 2 g per m2 and at places may 

exceed 10 g per m2 . Values are much lower in the 

interiors of continents, an indication of the importance of the ocean as a source of chloride. At rates 

such as these, the chloride in a typical nitrate field 

could have accumulated by atmospheric deposition in 

less than 200,000 years.

The oceans are the chief source of atmospheric 

iodine and probably the chief source of the iodine in 

the nitrate deposits. Iodine also occurs in volcanic 

gases and sublimates (Honda and others, 1966; 

Honda, 1970), but the amounts are small compared 

with those emitted from the ocean surface. Maclntyre (1974) suggested that organically bound 

iodine is the chief form of iodine in the atmosphere 

and that the principal source of this iodine is films 

on the sea surface that are rich in organic matter. 

He also pointed out that evaporation of iodine takes 

place at the sea surface but that the relative importance of this source of iodine is not known. Blanchard (1969) and Moyers and Duce (1972b) also 

suggested that organic-rich films on the sea surface 

were major sources of iodine in the atmosphere. 

Miyake and Tsunogai (1963) estimated that 99 percent of the iodine in the atmosphere was iodine that

vaporated from the ocean surface. According to 

these authors, the flux of iodine from the ocean to 

he atmosphere is about 5 X 1011 g I yr- 1 , which 

would also be the rate of deposition if the observed 

concentration of gaseous iodine in the atmosphere 

were to be maintained. At such a rate of deposition, 

the iodine in the Chilean nitrate deposits would require about 3 million years to accumulate. As in the 

case of the nitrate, I believe this rate of deposition 

to be too slow and that organically bound iodine, 

ejected in bubble spray from the nutrient-rich 

coastal waters of northern Chile, is the principal 

source of the iodine in the nitrate deposits. Furthermore, the iodine, which occurs chiefly in calcium 

iodate minerals of low solubility, would not be as 

readily leached at times of relatively heavy rainfall 

and would tend to remain in the soils of the nitrate 

fields, whereas more soluble saline minerals would be 

leached.

Virtually all the iodine in the nitrate deposits 

occurs as the iodate ion (I03 ~); iodine as iodide 

(I~) is absent or occurs only in traces. Iodine released from the ocean surface, either as a gas or in 

organic compounds, probably is oxidized to iodate by 

photochemical reactions in the atmosphere and at 

soil surfaces in the nitrate fields. Iodate also occurs 

in seawater and is released to the atmosphere along 

with NaCl in bubble spray, but the amount is insignificant in comparison with the amount of iodine 

found in maritime air (Moyers and Duce, 1972b).

The absence of bromine in the nitrate deposits is 

difficult to explain. Unfortunately, only a few determinations of bromine have been made for saline 

materials of northern Chile, so that little is known 

about bromine distribution and variations in concentration. Several semiquantitative X-ray spectrographic analyses of nitrate ore by Harry J. Rose 

of the U.S. Geological Survey (written commun., 

1968) showed all to contain less than 100 ppm bromine. Mueller (1960) reported his analysis of a 

specimen of nitrate ore from near Oficina Victoria 

(fig. 1) as showing 0.19 percent iodine (1,900 ppm) 

and 0.007 percent (70 ppm) bromine, giving an I/Br 

ratio of 27/1. Mueller also reported an analysis of 

a sample of saline crust (evidently high in NaCl) 

from a nearby salar as containing 0.005 percent 

(50 ppm) iodine and 0.011 percent (110 ppm) 

bromine, giving an I/Br ratio of 0.45. This I/Br 

ratio is considerably higher than ratios for continental evaporites and brines elsewhere and is several orders of magnitude higher than those for marine evaporites, (for example, see White and others, 

1963; Stewart, 1963).
28 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

Bromine occurs in nature chiefly as bromide salts 

of magnesium, sodium, and potassium (Pourbaix 

and others, 1966). It occurs in the atmosphere as 

the gaseous form Br2, HBr, and BrO, and in association with particulate matter such as aerosols, 

cloud droplets, and sea salts (Wofsy and others, 

1975), where these same gases are adsorbed or dissolved, or where the bromine occurs as the bromide 

ion (Br~). Bromate, the analogue of iodate, is not 

known in nature, and perbromate, the analogue of 

perchlorate, is not known to form as a chemical compound (Latimer, 1952; Goldschmidt, 1958; Cotton 

and Wilkinson, 1966). Bromine occurs in soils in 

amounts averaging 6.5 ppm, chiefly in organic complexes ( Rankama and Sahama, 1950).

Bromine is much more abundant than iodine in 

nearly all natural materials, so bromine must be 

eliminated during formation of the nitrate deposits. 

This elimination could be effected by processes at 

the sources of the saline materials, probably the 

ocean for iodine and bromine, in the atmosphere, and 

in the soils of the nitrate fields. Processes at the 

ocean surface cause a great enrichment of iodine in 

the marine atmosphere, relative to bromine and 

chlorine (Duce and others, 1963; Moyers and Duce, 

1972a, b; Seto and Duce, 1972; Bloch and Leucke, 

1972; Maclntyre, 1974). However, as indicated by 

the I/Br ratios for seawater and marine rainwater 

and aerosols (Duce and others, 1963), the bromine 

in the marine atmosphere is still more abundant than 

iodine, and additional bromine would have to be 

eliminated to account for the high I/Br ratios of the 

nitrate deposits, if the ocean is to be considered a 

major source of the iodine. The most likely means of 

separating bromine from iodine is through atmospheric photochemical processes that cause oxidation 

of iodine to iodate, which then may be dissolved in 

rain and fog droplets and removed from the atmosphere, whereas the bromine remains as a gas that 

becomes dispersed in the atmosphere. Wofsy and 

others (1975) suggested that HBr is the dominant 

bromine compound in the atmosphere and that it is 

produced by hydrolysis of bromine, chiefly in marine 

aerosols. These authors estimated annual emission 

of bromine to the atmosphere to be 1.1 X 1012 g Br 

yr- 1 . They also suggested that the gaseous forms of 

bromine act as catalysts in the destruction of ozone 

and, consequently, remain unchanged in ozone reactions. Duce and others (1965) suggested that photochemical oxidation of the Br~ ion in the aerosols 

formed Br2 , released to the atmosphere as a gas.

The perchlorate ion (C104~) is the most enigmatic 

component of the nitrate deposits because it has not

been detected in nature elsewhere and is not known 

to form by any natural process. However, perchlorate 

is an ubiquitous constituent that is reported in 

nearly all conventional chemical analyses of nitrate 

ore; in this study it has been identified as a chemical 

component in nitrate ore from northern Antofagasta 

province (see p. 20). The perchlorate in the nitrate 

deposits most probably formed in the atmosphere by 

photochemical reactions involving chlorine and ozone 

(Motoako Sato, oral commun., 1978). It has been 

proposed that chlorine-ozone reactions produce 

oxides of chlorine (C10X ) in the stratosphere (for 

example, see Nicolet, 1975). Simonaitis and Heicklen 

(1975) proposed that perchloric acid could form in 

the stratosphere by the following photochemical reactions:

Cl + 03 + 02 -* C103 

C103 + OH -* HC104

02

Gaseous chlorine, ozone, and OH~ from photodissociation of H2 0 are present in the troposphere (for 

example, see Levy, 1972; Crutzen, 1974b; Chameides, 

1975), and the same reactions might produce HC104 

in the troposphere or even at ground level. Unfortunately, neither C103 nor HC104 have been detected 

in the atmosphere, so their formation by photochemical reactions remains speculative.

SULFATE

Sulfate compounds such as sulfuric acid (H2S04 ) 

and ammonium sulfate ((NH4 ) 2S04 ), occurring 

chiefly as particulates and as solutions in rainwater, 

and gaseous sulfur dioxide (S02 ) and hydrogen sulfide (H2S) are trace constituents of the atmosphere. 

The chief natural sources of gaseous sulfur compounds are biological decay of organic matter and 

volcanic emissions (see Granat and others, 1976), 

whereas most of the sulfate in the atmosphere is 

formed by photochemical oxidation and hydrolysis 

of the gaseous sulfur compounds and is supplied by 

sea spray (Eriksson, 1960; Gambell and Fisher, 

1964; Goldhaber and Kaplan, 1974; Granat and 

others, 1976). Sulfate is removed from the atmosphere as dry fallout and as rainwater solutions of 

H2S04 and sulfate compounds. Granat and others 

(1976) estimated the flux of natural sulfur compounds through the atmosphere to be 84 X 1012 g 

S yr- 1 as compared with that of present-day anthropogenic sources of 65 X 1012 g S yr- 1 . These authors 

suggested that H2S formed by biological decay of 

plant material in the oceans, especially in estuaries 

and marshes at continental margins, and sulfur 

compounds in sea spray are the dominant natural
ORIGIN OF THE NITRATE DEPOSITS 29

sources of atmospheric sulfur, contributing 34 

X 1012 g S yr- 1 and 44 X 1012 g S yr- 1 , respectively. They suggested that S02 and other compounds of sulfur originating in volcanic eruptions 

and fumaroles contribute only about 3 X 1012 g S 

yr- 1 to the atmosphere, and that other natural 

sources contribute an additional 3 X 1012 g S yr- 1 . 

These estimates of the flux of sulfur through the 

atmosphere are less than previous estimates that 

natural emissions of gaseous sulfur compounds 

ranged from a low of 89 X 1012 g S yr- 1 to a high of 

267 X 1012 g S yr- 1 (Eriksson, 1960; Junge, 1963; 

Robinson and Bobbins, 1970b; Kellogg and others, 

1972; Friend, 1973). Most investigators agree that 

the ocean is the major source of atmospheric sulfur 

and that volcanic sources are small by comparison 

(for example, see Eriksson, 1963; Friend, 1973; 

Stoiber and Jepson, 1973; Cadle, 1975).

Granat and others (1976), after reviewing sources 

and fluxes of sulfur through the atmosphere, concluded that atmosphere deposition of sulfur compounds on land areas are equivalent to about 25 X 

1012 g S yr~S which would be equivalent to about 

half a gram S04 per square meter of land surface. 

At such a rate of deposition, the accumulation of 

sulfate in the nitrate deposits would probably require at least a million years, rather than 100,000- 

200,000 years that I previously estimated (Ericksen, 

1979), on the basis of a rate of deposition 

on the land of 165 X 1012 g S yr- 1 proposed by 

Eriksson (1963). The figure by Eriksson (1963) includes anthropogenic sulfur in the atmosphere, 

which he estimated to be only 1/10 of the emissions 

of sulfur from natural sources. Accumulation of 

sulfate over a time span of a million years is perhaps 

not unreasonable when it is considered that 10-15 

million years have been available for formation of 

the nitrate deposits. However, the rate of accumulation of sulfate in the Atacama Desert probably has 

been much greater than the world average because 

of a greater local flux of sulfur to the atmosphere, 

chiefly H2 S gas and sea spray from the nearby nutrient-rich coastal waters of the Pacific ocean, and 

S02 and other sulfur compounds from the hundreds 

of volcanoes in the nearby Andes that were active at 

various times during late Tertiary and Quaternary 

time. Furthermore, the amount of sulfate in the 

nitrate deposits is similar to that of chloride, which 

might have accumulated in less than 200,000 years; 

these two components probably had similar rates of 

accumulation.

BORATE, CHROMATE, AND DIGHROMATE

Boron in the nitrate deposits, occurring chiefly as 

ulexite, is probably of volcanic rather than oceanic 

origin. Gast and Thompson (1959) noted that the 

B/C1 ratio of rainwater was several times that of 

seawater, and they concluded that boric acid in the 

atmosphere was evaporated from the ocean surface. 

However, Nishimura and Tanaka (1972) presented 

experimental data that indicate the atmosphere to 

be supersaturated in boron with respect to seawater; 

these authors concluded that the oceans are sinks 

rather than sources of atmospheric boron. Perhaps 

the best evidence for a volcanic source of boron in 

the Chilean nitrate deposits is the abundant ulexite 

in salars of the Andean highlands, which consist 

chiefly of saline constituents supplied by recent volcanic emissions and thermal springs as well as those 

leached from nearby volcanic rocks of late Tertiary 

and Quaternary age. Salars in the Atacama Desert 

also contain local concentrations of ulexite deposited 

by water draining from the Andes (Ericksen, 1963).

Chromate (Cr04 - 2 ) is a widespread trace constituent of nitrate ore, occurring sporadically 

throughout the nitrate fields in the so-called caliche 

azufrado (sulfurous caliche, in reference to its 

color) which together with small amounts in typical 

nitrate ore become concentrated and impart a yellow 

color to the leach solutions that are recycled in the 

processing of the nitrate ore. Maximum values for 

chromate in typical ore are probably a few parts 

per million (Ericksen, 1963), whereas the caliche 

azufrado may contain several percent of chromate. 

No obvious source exists for this chromate. For 

example, ultramafic igneous rocks, which are the 

richest sources of chromium, are absent in northern 

Chile. The most abundant igneous rocks of the area 

are granitic plutons and rhyolitic tuffs, which contain very little chromium (for example, see Krauskopf, 1967).

The most likely sources of the chromate are sedimentary rocks in the nitrate fields, principally shales 

that elsewhere commonly contain about 100 ppm Cr 

(Krauskopf, 1967; Connor and Shacklette, 1975). In 

the highly oxidizing environment of the nitrate deposits, trace amounts of chromium in these rocks 

may have been oxidized from the trivalent state 

(Cr+3 ), which tends to form insoluble oxides, to the 

relatively mobile hexavalent (Cr+6 ) state in CrCXr 2 . 

Rankama and Sahama (1950) suggested that formation of the relatively mobile chromate in the Chilean 

nitrate deposits was due to the high oxidation po-
30 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

tential of the deposits. Additional chromate might 

have been released to soils of the nitrate fields by 

decay of organic matter in sea spray or by decay 

of plants such as Tillandsia that exist in the nitrate 

fields (see p. 11). Shacklette and Conner (1973) reported Tillandsia ash as containing- as much as 1,500 

ppm Cr.

Bichromate (Cr2 07 - 2 ), which is extremely sparse 

in the nitrate deposits, evidently formed chiefly by 

alteration of chromate. The transition of chromate 

to dichromate can be observed in vug's in the chromate-rich caliche azufrado. Also, grains of brightorange dichromate slowly form in moist residues left 

by evaporation of solutions of caliche azufrado in the 

laboratory.

ALKALI AND ALKALINE-EARTH METALS

The alkali and alkaline-earth metals: sodium, potassium, magnesium, and calcium, which are the 

only cations in nitrate ore in more than trace 

amounts (table 1), show concentrations indicating 

that they come from diverse sources. The Na/Cl 

ratio of the nitrate ores is two to three times that 

of seawater, whereas the Mg/Cl ratio is only slightly 

greater than that of seawater. The K/C1 and Ca/Cl 

ratios are 10-30 times that of seawater, and although 

both these elements have been shown to be somewhat 

enriched in bubble-spray because of fractionation at 

the sea-air interface (for example, see Chesselet and 

others, 1972; Wilkniss and Bressan, 1972), the 

amount of enrichment is small compared with that 

required to form the nitrate deposits. On the other 

hand, Wilson (1959) suggested that an enrichment 

of K in organic films at the ocean surface, which are 

ejected by bubble-spray, might account for K in rainwater, known to be as much as 10 times the amount 

in seawater. Likens and others (1979) proposed that 

soil particles swept into the atmosphere by wind may 

release the cations Na+, K+, Mg+2, and Ca+2 to 

rainwater solutions.

It may be inferred that the dominant sources of 

the cations in the nitrate deposits probably were 

(1) sea spray; (2) windblown saline dust resulting 

from wind erosion in the Atacama Desert, including 

erosion of former playas and salars; and (3) windtransported ash and dust from volcanic eruptions 

in the Andes. Additional small amounts of these 

cations may have been derived from chemical weathering of rocks in the nitrate fields. Because of the 

enrichment processes that have affected the nitrate 

deposits, the varied solubility of the saline minerals 

present, and the paucity of analytical data for saline 

accumulations other than nitrate ore in northern

Chile, precise comparisons of the relative quantities 

of these cations from each of the above sources in 

the nitrate fields cannot be made. Nevertheless, the 

ocean clearly has been a major source of Na+ and 

Mg+2 , as indicated by comparison of the ratios of 

these elements, relative to chlorine in the nitrate 

deposits and in seawater, the probable source of 

most of the chlorine, and windblown dust and volcanic ash, the major sources of Ca+2 and K+.

ACCUMULATION AND ENRICHMENT OF 

THE SALINE COMPONENTS

Most of the ore-grade nitrate deposits apparently 

formed in three ways: (1) slow accumulation on 

very old, flat to gently sloping or undulating land 

surfaces that have had little or no modification since 

Miocene time; (2) accumulation on lower hillslopes 

and breaks in slopes as the result of leaching by 

rainwater and downslope migration of solutions to 

sites of redeposition; and (3) accumulation of saline 

minerals in saltpans and saline ponds. Examples of 

accumulation on old surfaces are the deposits on 

uplands of the Coastal Range in Tarapaca and Antofagasta Provinces, which have been little modified 

since early Tertiary time; on old surfaces of the 

Central Valley such as that of Pampa de Tana in 

northern Tarapaca Province (fig. 4), of probable 

Miocene age; and in the Baquedano valley, where 

some of the deposits are in old stream gravels that 

probably date from early Tertiary time. Downslope 

migration of saline material is perhaps the most important process in the formation of the richest 

nitrate deposits, and nitrate workings along lower 

slopes of hills and at breaks in hillslopes can be 

seen throughout the nitrate region. Unusual nitrate 

deposits are found along the courses of old streams 

in the low range of rocky hills in the vicinity of 

the southwesternmost nitrate beneficiation plants 

in the Baquedano district (fig. 9), which are downhill from a broad, gently sloping area of lower-grade 

nitrate deposits. The richest deposits here are in the 

stream channel rather than on the nearby hillsides.

Nitrate-rich brines are now accumulating in ponds 

and salars in the nitrate region of northern Chile, 

and similar accumulations may have formed rich 

deposits in the past. Several salar crusts have been 

worked as nitrate ore. The best-known nitrate-rich 

salar is Salar del Carmen (fig. 1) where efflorescences of nitrate-bearing salts were harvested periodically during the latter part of the 19th century 

(Whitehead, 1920). Other nitrate-bearing salars 

have been mined in the Tarapaca, Baquedano, and
ORIGIN OF THE NITRATE DEPOSITS 31

Aguas Blancas districts (fig. 1). Two ponds in the 

Tarapaca district, one in a valley at Oficina Jazpampa, about 2 km northwest of Zapiga (fig. 1), 

and the other in Salar de Bellavista near Oficina Victoria, were found to contain nitrate-rich brines, as 

shown by the following chemical analyses by the 

laboratory at Oficina Maria Elena, Sept. 1977:

N03- _ _ _ .

ci- __ _._ ___ .

SOr2 _ ___________ ios- __ __ _ _

cio4- ____________

B __ ______ __ K+

Ms-*2

Pond, Oficina 

Jazpampa (ppm)

13,700

28,700

19,400

140

270

370

1,550

5,080

Pond, Salar de 

Bellaxicta (ppm)

4,280

15,300

8,100

210

210

770

1,170

These brines are contaminated with old leach solutions draining from waste dumps at nearby nitrate 

beneficiation plants, which account for the exceptionally high nitrate, iodate, and perchlorate, admixed with brines or saline waters from salars. 

Although these ponds are anomalous in receiving 

these leach solutions, they indicate types of ponds 

that might have formed in the past in closed basins 

within the nitrate fields. Saline water and brine 

formed by rainwater leaching of surrounding nitrate 

deposits and saline-rich soils could have accumulated 

in such basins and evaporated to form nitrate-rich 

saline crusts. The nitrate efflorescences at Salar del 

Carmen evidently had such an orgin.

The role of former ephemeral saline lakes in formation of the nitrate deposits is difficult to evaluate. 

However, some deposits were destroyed or prevented 

from forming by such lakes. For example, the absence of nitrate deposits in southernmost Tarapaca 

Province, the region of Salar de Llamara (fig. 1), is 

considered to be due to the presence of the former 

large lake of probable Pleistocene age, Lago Soledad, 

and to playa lakes that still form periodically there 

(Bruggen, 1938). The high-grade nitrate deposits 

at the margins of some of the playas and salars in 

the Pampa del Tamarugal, northern Tarapaca Province, do not extend down to playa or salar levels; 

rather, they terminate a few meters above these 

levels. I believe that the absence of nitrate deposits 

in this narrow band is due to leaching at the margins of late Pleistocene and Holocene ephemeral 

lakes. On the other hand, the persistence and abundance of high-grade nitrate deposits along the eastern side of the Coastal Range in the Tarapaca and 

Tocopilla nitrate districts, the region marginal to 

these ephemeral saline lakes, suggests that some of

the saline material here came from the lakes. Evaporation of shallow marginal waters and windblown 

spray of the lakes might have contributed significant 

quantities of saline materials to the nearby nitrate 

deposits, and microbial activity in the shallow waters 

and moist soils at the margins of the lakes may have 

formed nitrate.

The position of the nitrate caliche layer between 

leached surface material and underlying salt-free 

rock or regolith indicates deposition of the saline 

material in a zone where capillary evaporation takes 

place. The bottom of the caliche layer is probably the 

level of deepest penetration of water from the 

heaviest rains, whereas the leached overburden 

(costra) is in the zone of average penetration by the 

more frequent light rains. Penetration of rainwater 

to the deepest parts of the caliche is probably along 

major desiccation cracks, such as those shown in 

figure 16. The powdery surface layer (chuca) is 

affected by both rain and fog. Although workable 

nitrate-rich layers are generally less than 5 m below 

the surface, some are as deep as 12 m. At a few 

places, layers of caliche blanco have been found in 

relatively salt-free to salt-cemented regolith (coba 

or conjelo) beneath the normal near-surface caliche 

layer. Such a layer of caliche blanco was found at 

Oficina Virginia in the southern part of the Tarapaca district, 2-3 m below the normal caliche (F. H. 

Humberstone, oral commun., 1962). This layer is 

probably a remnant of a former nitrate deposit that 

was partially destroyed during a period of greater 

than normal rainfall. Small amounts of nitrate 

(0.1-1.0 percent NaN03 ) and other saline components of the nitrate deposits have been found to 

depths of as much as 66 m in unconsolidated regolith, as shown by drill-hole logs from nitrate fields 

east of Iquique (logs were made available by Dr. 

Donald Garrett, President, Saline Processors, Inc., 

1977). These saline materials probably indicate 

deeper penetration by surface water or the former 

presence of saline ground water in areas where 

ground water does not now exist.

CONCLUSIONS

The geographic and topographic distribution of 

the Chilean nitrate deposits indicates atmospheric 

transport and deposition of the saline constituents 

and subsequent redistribution by rainwater at the 

sites of the deposits. The saline components in the 

deposits are chiefly those found worldwide in the 

atmosphere or those known to form by atmospheric 

photochemical processes; these components have been
32 GEOLOGY AND ORIGIN OF THE CHILEAN NITRATE DEPOSITS

deposited and preserved in the Atacama Desert because of the uniquely arid climate. Although some 

constituents may have been transported from distant sources, the greater part probably came from 

local sources such as the nearby Pacific Ocean and 

the volcanic rocks and the emissions from intense 

volcanic activity in the nearby Andean Highlands 

during late Tertiary and Quaternary time. A general 

low level of microbial activity in the dry soils of the 

Atacama Desert has been essential to preservation 

of the nitrate.

The following model for the formation of Chilean 

nitrate deposits is proposed:

(1) Long-term widespread accumulation of saline 

materials from the atmosphere in soils of the nearly 

rainless Atacama Desert, perhaps starting in middle 

Miocene time, 10-15 million years ago, and continuing until the present. Although a virtually rainless climate prevailed during this time, periodic 

heavy rains or rainy periods caused considerable 

leaching and redeposition of soluble saline materials 

and at times may have caused widespread destruction of former nitrate deposits.

(2) The saline components were transported by 

and deposited from the atmosphere chiefly as dry 

fallout and as fog-condensate solutions. These components included salts such as NaCl and NaN03 , as 

well as gaseous or water-soluble constituents such 

as NH3 , HC1, H2S04, and HN03 that reacted at the 

soil level to form the saline components in the deposits.

(3) Ore-grade nitrate deposits formed by: (a) 

accumulation of saline materials on very old, flat to 

gently inclined or undulating land surfaces, where 

rainwater dissolved the more soluble components 

such as nitrate and redeposited them at deeper soil 

levels; (b) accumulation of saline materials on hills, 

where they were dissolved by rainwater and moved 

to lower levels on hillslopes, typically at slope breaks 

and bottoms of hills; and (c) accumulation of nitrate-rich saline materials in salt pans and saline 

ponds.

(4) The principal sources of the saline components were: (a) sea spray and gases such as ammonia and iodine from the nearby Pacific Ocean; (b) 

volcanic rocks and emanations of active volcanoes in 

the Andes; (c) photochemical reactions in the 

atmosphere; (d) wind-transported saline materials 

derived from erosion of saline soils, pre-existing nitrate deposits, and salars in the Atacama Desert; 

and (e) atmospheric constituents that reacted with 

soils and rocks in the nitrate fields or that were 

transformed by soil microorganisms.

Different sources dominated for each of the saline 

constituents, as follows: (1) Nitrate probably was 

formed chiefly by photochemical oxidation of ammonia (NH3 ) or the ammonium ion (NH4+), gaseous nitrogen oxides NO and N02 , and nitrogeneous 

organic materials. The chief source of the ammonia 

and nitrogeneous organic material probably was sea 

spray from the nearby Pacific ocean, whereas the 

nitrogen oxides were formed by oxidation of these 

substances, by nitrogen fixation during thunderstorms in the nearby Andes, and by mixing of stratospheric air containing NO and N02 formed by reaction of relatively abundant nitrous oxides (N2 0) 

with electronically excited oxygen atoms OCD); additional nitrate may have been formed at soil surfaces of the nitrate fields by nitrification of NH3 and 

NH4 + and nitrogen fixation by photoautotrophic bacteria and blue-green algae. (2) The principal source 

of NaCl was probably spray from the nearby Pacific 

Ocean, but significant amounts of NaCl or HC1 may 

have been supplied by volcanic emanations in the 

Andes. (3) Sulfate originated as S02 in volcanic 

emanations, as water-soluble sulfate minerals in volcanic rocks of the Andes, and as S04 ~2 or H2 S from 

seawater. (4) Essentially all the borate came from 

volcanic emanations and volcanic rocks in the Andes. 

(5) Perchlorate is considered to have formed by 

reactions of chlorine with ozone (03 ) in the troposphere and at ground level. (6) Iodine is chiefly of 

oceanic origin, as iodine-rich organic material 

ejected in bubble spray and as gaseous iodine formed 

by photochemical oxidation of iodide at the ocean 

surface; iodine was oxidized to iodate by photochemical reactions in the troposphere and at ground level. 

(7) The anomalously low bromine content of the 

nitrate deposits is attributed to atmospheric photochemical processes that favored, formation of the 

gaseous forms of bromine, Br2 , HBr, BrO, which 

were lost in the atmosphere rather than accumulating as saline components of the nitrate deposits. (8) 

The alkali and alkaline-earth elements, sodium, potassium, calcium, and magnesium, which are the 

dominant cations of the saline minerals of the nitrate 

deposits, had diverse sources, including sea spray, 

airborne particulate matter related to Andean volcanism, and wind-transported saline dust from soils, 

salars, and playas in the Atacama Desert.

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* U.S. GOVERNMENT PRINTING OFFICE: 1981-341-614/109