Geochemistry of Uranium 

in Apatite and Phosphorite

GEOLOGICAL SURVEY PROFESSIONAL PAPER 314-D

This report concerns work done on behalf 

of the U. S. Atomic Energf Commission 

and is published with the permission 

of the Commission

Geochemistry of Uranium 

in Apatite and Phosphorite

By Z. S. ALTSCHULER, R. S. CLARKE, JR., and E. J. YOUNG

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 314-D

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission 

of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1958
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

Altschuler, Zalman Samuel, 1919-

Geochemistry of uranium in apatite and phosphorite, by 

Z. S. Altschuler, R. S. Clarke, Jr., and E. J. Young. Washington, U. S. Govt. Print. Off., 1958.

iv, 45-90 p. illus., plates, tables. 29 cm. ([U. S.j Geological 

Survey. Professional Paper 314-D. Shorter contributions to general 

geology)

Bibliography: p. 85-87.

1. Uranium ores. 2. Apatite. i. Title. (Series: U. S. 

Geological Survey. Prosessional Paper 314-D. Series: U. S. Geological Survey. Shorter contributions to general geology)

553.49

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

Washington 25, D. C. - Price 60 cents (paper cover)
CONTENTS

Introduction _______________________________________

Acknowledgments_________________________________

Mineralogy of apatite_______________________________

Crystal chemistry_______________________________

Composition of the sedimentary carbonate-apatites. _ 

Petrographic terminology._______________________

Uranium content of apatite._________________________

Factors affecting equivalent uranium determinations. 

Uranium content of representative apatites. ________

Island phosphorites secondary uptake of uranium 

and fluorine_____________.___.________________

Problems of occurrence of uranium in phosphorite ______

Possible modes of occurrence____-___-_-________

Absence of uranium minerals___________________._

Relation of uranium content to composition of 

phosphorite._________________________________

Fluorescence and uranium content.___________

Lack of preferential solution of uranium_______

Relation of uranium to P.Os_______________

Marine versus ground-water origin of uranium in 

phosphorite__________________________________

General statement and review_______________

Evidence of young phosphorites and submarine 

nodules________________________________

Necessity of geologic interpretation.___________

Principles governing uranium distribution in phosphorites________________________________________

Uranium in phosphorite during weathering__ _____

Florida Land Pebble phosphates general geology __ ______

Illustration of leaching___________________.__

Postdepositional enrichment___ ____________

Aluminum phosphate zone lateritic weathering and supergene enrichment. _______

The Cooper marl residual enrichment. ___

Contrasting modes of enrichment in mild

versus intense weathering_________

Brown-rock phosphates of Tennessee residual concentration_________________

Secondary emplacement of uranium in bones_______

Marine enrichment and reworking ________________

Page 

45 

45 

47 

47

47

48

49

50

50

50

51

52

52

52

53

53

53

53

55

55

55

56

57

57

57

57

59

59

62

64

65

66

67

Page 

Crystallo-chemical nature of uranium in apatite _________ 68

General statement___________________________.__ 68

Substitution of uranium for calcium in apatite. ______ 69

Theoretical considerations ___________________ 69

Relation of uranium to phosphorus______ 69

Oxidation state of uranium in apatite _________ 69

Chemical problems____________________ 69

U(IV) content of sedimentary apatites__.. 71 

U (IV) /U per cents reflect geologic history.. _ _ 71 

Primary marine uranium in young phosphorites is mainly U(IV)__-----------_- 72

Subaerial weathering alters U(IV) to

U (VI) in marine phosphorite ___________ 72

Possible relation of U(VI) content to radioactivity and age_________-____________ 73

Secondary sources of U (VI) in phosphorite. _ 73 

Relative structural favorability of U(IV) and

U(VI) in apatite________...._____ 74

Test of uranium and fluorine interdependence in 

apatite.________________________________... 75

Effects of reworking on U(IV) content of apatite.... 75

Summary of evidence bearing on the primary state of 

uranium in marine phosphorites_________________ 77

Uranium in igneous apatite__--_--__-_-_------_-----_ 79

General statement and analyses __________________ 79

Variation of uranium in apatite with uranium in host

rock.______________________ 80 

Oxidation state of uranium in igneous apatite. _____ 80

U(IV)/U(VI) ratio reflects magmatic equilibria.--.. 81

Relation of U(IV)/U(VI) ratio to age_____ ._ 81

Auto-oxidation. ____________________________ 81

External weathering controlled by radioactive

decay...... ____ ._ 82

Conclusions._______-__-_--_-_---_-_-_--_-----_----- 82

Summary.. _______ _____________________________ 82

Nature of uranium fixation in phosphorites_______ 83

Regenerative capture________________ ______ 84

Equilibrium fixation of uranium in igneous apatite. _ 84 

Selected references. ____------_-_-_-_-_-----_----_--- 85

Index..__--.....-.----_.__-_-_____-----_-___-_---_ 89

m
IV CONTENTS

ILLUSTRATIONS

	[Plates follow page 60]

PLATE 11. Sections and autoradiographs of sandstone, bone, and pebbles.

12. Contact of aluminum phosphate zone and overlying sand mantle, and mined-out area of Tennessee brown-rock phosphate.

13. Photomicrographs of arkosic sandstone with carbonate-fluorapatite cement from the Gas Hills area of Wyoming.

	Page 

FIGURE 3. The structure of fluorapatite____________________-_______________-_______----_________-__-_-__-______ 48

4. Relation of uranium to PaQs in Mona Island cave phosphorites----____-----------_----------__-_-----__- 51

5. Relation of uranium to fluoridation in Mona Island cave phosphorites._______--__--_______-__-_---_______ 52

6. Relation of uranium to P2Os in phosphorites from Oulad-Abdoun Basin, French Morocco ___________________ 53

7. Stratigraphic relations in the Land Pebble phosphate field, Florida_____________________________________ 58

8. Sampling scheme used for study of uranium distribution in pebbles. _____________________________________ 59

9. Distribution of oxides through aluminum phosphate zone at Homeland, Fla_____-_-___-____-_-_--_--__-.__ 61

10. Uranium distribution in various textural components at Homeland mine, Fla____________________________ 61

11. Contrasting modes of uranium enrichment and weathering in phosphorites-_______________________________ 64

12. Sample locations for comparison of brown-rock phosphate with parent limestone in Tennessee.._____________ 65

13. Uranium versus excess fluorine in Phosphoria formation._______________________________________________ 76

14. U(IV) versus excess fluorine in various phosphorites-__--_---------_---_-_--_----------_-_-----_-----_-_ 77

15. Total uranium in rock versus total uranium in apatite for rocks from Boulder Creek batholith_____________ 80

TABLES

TABLE 1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

Resources of some well-known uraniferous phosphorites. ________________________________________________

Chemical analyses of sedimentary apatite pellets from the Bone Valley formation. _ _______________________

Semiquantitative spectrographic analyses of sedimentary apatite pellets ___________________________________

Thorium content of Florida pebble phosphorite __ _____________________________________________________

Uranium content of representative apatites-____--____--__-_____---_--------------_------------------_-

Partial analyses of cave phosphorite from Mona Island, Puerto Rico_ _____________________________________

Comparison of fluorescence and uranium content in composite samples of apatite nodules from the Bone Valley 

formation, Boyette mine, Florida_____--____-_---_---_--------------------------_------------------_

Size distribution and viranium contents of pelletal phosphates from Khouribga, French Morocco_____ _________

Correlation coefficients among eU, PzQs, F, CO2 and organic matter in samples of the Phosphoria formation _

46

49

49

50

50

51

53

54 

54 

"Inferred uranium contents" of Polish phosphorites----------------------------------------------------- 55

Phosphate and uranium contents of bottom samples from the Gulf of Mexico ______________________________ 56

Relative contents of uranium and PaOs in marine phosphorites. __________________________________________ 56

Chemical analyses showing distribution of uranium within phosphate pebbles ______________________________ 60

Examples of phosphate- and uranium-enriched hardpan in Cooper marl near Charleston, S. C____ ____________ 62

Comparison of phosphatized and primary Cooper marl from Lambs, South Carolina ________________________ 63

Variations of enrichment with weathering in phosphatized Cooper marl, South Carolina. ____________________ 63

Semiquantitative spectrographic analyses of phosphatic hardpan from the Cooper marl __ __________________ 64

P2O5 and uranium content of weathered and fresh Tennessee phosphates, Akin mine, Columbia, Tenn__ _______ 65

Uranium content of representative clastic apatite from the land-pebble phosphate field, Florida. ____ ______ 67

Iron and uranium content, in weight percent, of representative apatites from the Bone Valley formation, Florida. 69 

The recovery, in weight percent, of uranium (IV) from 1.5 M phosphoric acid solutions of natural and synthetic 

apatite ______________ _________ ______ ____ _ __ __.__- _ _ 70

Percentages of uranium and of U(IV) in sample B.L.-3 before and after oxidative heating_______________ __ 71

Total and tetravalent uranium content of sedimentary apatites _ ________________________________ ______ 72

Experiment showing reduction of U(IV) by phosphoric acid solutions of hematite-rich apatite samples. _______ 73

Data on excess fluorine and U(IV) in miscellaneous apatite and phosphorite, _ ____________________________ 77

Relative contents of U, U(IV), and PaOs in phosphatic terrace gravel in the vicinity of Charleston, S. C_______ 77

Uranium and U(IV) content of igneous apatites- _____________ _________________ ___________________ 79
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE

By Z. S. ALTSCHULEK, K. S. CLAKKE, JR., and E. J. YOUNG

ABSTRACT

Apatite contains only traces of uranium, yet as apatite is a 

minor constituent in most rocks and the major constituent of 

a few very large deposits, it accounts, paradoxically, for both 

dispersal and concentration of uranium in nature.

Uranium typically makes up 0.001 to 0.01 percent of primary 

igneous apatite and 0.005 to 0.02 percent of sedimentary marine 

apatite. Marine reworked apatite becomes enriched in uranium 

to as much as 0.1 percent. This is demonstrated by the greater 

uranium content of the texturally more complex phases within 

a single deposit.

Uranium can be secondarily leached from or introduced into 

apatite by ground water. These secondary changes are indicated by pronounced concentration gradients within single pebbles of apatite as well as by the redistribution of uranium 

among different mineral hosts in leached and altered deposits 

of phosphorite. The postdepositional enrichment of uranium in 

phosphorite may be entirely residual as in the Tennessee 

"brown-rock" deposits, or greatly enhanced by ground-water 

additions as in the South Carolina phosphates. Moreover, the 

pattern of enrichment reflects the conditions and intensities 

of weathering. Highly acid lateritic weathering has caused 

supergene enrichment of uranium in the aluminum phosphate 

zone of the Bone Valley formation. In contrast, surficial enrichment characterizes the moderately weathered Cooper Marl 

of South Carolina. Isolated fossil bones or phosphate pebbles 

may contain almost one percent of uranium as a result of 

ground water enrichment. Such enrichment is comparable to 

the postdepositional uptake of fluorine by bone or insular 

phosphorite, and in some places the processes are synchronous 

and show good mutual correlations as they are codependent on 

the some ground water source.

It is proposed that uranium replaces calcium in the apatite 

structure. This is indicated by several lines of investigation. 

Uranium and calcium contents are parallel in sections of leached 

and altered phosphorite. Ionic radii of tetravalent uranium 

(0.97 A) and divalent calcium (O.99 A) are virtually identical, 

and much of the uranium in igneous, sedimentary, and bone 

apatite is found to be tetravalent. Petrographic and chemical 

analyses and nuclear emulsion studies have shown that uranium 

in apatite is disseminated rather than locally concentrated. 

In addition, phosphate deposits are essentially devoid of 

uranium minerals.

The igneous apatites contain from 10 to 66 percent of their 

uranium as U(IV). In apatite from a suite of related igneous 

rocks both the total uranium and the U(IV)/U(VI) ratio vary 

as the total uranium in the rock. Thus, the U(IV)/U(VI) ratio 

in igneous apatite reflects the prevalent equilibrium conditions 

in the crystallizing magma.

In marine phosphorite thus far investigated the tetravalent 

uranium ranges from a few percent to more than 90 percent 

of the total uranium. Taken alone such statistics suggest a 

great variation in the initial U(IV)/U(VI) ratio of uranium 

emplaced in apatite. Thus, experimental evidence that bone and 

apatite pellets can remove uranyl uranium from solution suggests that much of the U(VI) found in natural apatite may be 

of primary origin, fixed as chemically adsorbed uranyl anions. 

It is proposed, however, that uranium in marine apatite is 

emplaced primarily as U(IV), structurally fixed. This follows 

from the fact that the higher U(IV)/U(VI) ratios are found in 

the younger unweathered marine apatites, and in apatites recently reworked by marine transgression. U(IV) is readily 

oxidized to U(VI), and postdepositional weathering, facilitated 

by radioactive decay, has most probably lowered the initially 

high U(IV)/U(VI) ratio in many older phosphorites.

Apatite, by effectively removing the small amounts of U(IV) 

produced in sea water by reduction of (UOa)*s, causes more 

U(IV) to be produced for its own uptake. As the marine apatite 

is far from saturated with respect to uranium, it thus interferes 

with the attainment of equilibrium while fixing an unusual 

quantity of U(IV). The name regenerative capture is proposed 

for this type of concentration in which the fixation of an insignificant valence species interferes with the equilibrium producing the ion and thus generates a continuing supply for further 

uptake and results, ultimately, in unexpectedly large build-ups 

of the insignificant trace element in the host mineral.

INTRODUCTION

Apatite contains only trace quantities of uranium, 

typically several thousandths to a few hundredths of 

one percent. However, as average rock has at most a 

few parts per million and average ocean water only one 

or two parts per billion, it is immediately apparent that 

apatite is an important concentrator of uranium in 

nature. The ubiquitous occurrence of apatite as an 

accessory mineral in igneous rocks establishes its role 

as an important agent, along with a few other minerals, 

notably zircon, monazite, and sphene, in dispersing 

uranium in rocks during primary differentiation. This 

is particularly true because apatite, unlike many other 

accessory minerals, is prominent in mafic as well as 

felsic rocks. Thus, whereas the uranium content may 

be low in the gabbros, the contribution of apatite to the 

total budget of uranium in the early differentiates may

45
46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

be correspondingly higher. As apatite is also rather 

common in shales and limestones in the form of phosphatic nodules, concretions, and fossil replacements, 

it is evident that apatite plays a major role in the 

geochemical cycle of uranium.

Beside contributing to the dissemination of uranium, 

apatite paradoxically is also one of very few minerals 

causing immense localizations of this element. It does 

so by virtue of its occurrence in vast accumulations of 

marine sedimentary phosphate composed essentially of

the mineral carbonate-fluorapatite (Altschuler and 

others, 1953). Table I illustrates the magnitude and 

economic potential of these deposits. Thus, as we annually produce more than ten million tons of phosphate 

fertilizer in the United States and as an appreciable 

part of this is processed chemically, an opportunity is 

afforded to mobilize and extract many hundreds of tons 

of uranium each year. Indeed, uranium as a byproduct 

of triple superphosphate production is now recovered in 

several plants in the United States (Barr, 1955).

TABLE 1. Resources of some well-known uraniferous phosphorites

Area and formation

Florida Land Pebble field, Bone Valley formation, 

matrix. ___ ___ ___ __ _ _______ _ ______

Western field, Phosphoria formation ___ _ _ ______

French Morocco _____ _--_-_ __ _____ _ ._ ___

Reserves minable at present

P.06 » Apatite '

Millions of long tons

330

870 

4,800

870

2,300 

13, 000

U3

Thousands 

of long 

tons

87

230 

1,300

Additional resources minable under 

changed conditions

PaOt' Apatite '

Millions of long tons

600 

4,000 

5,800 

6,420

1,600 

11, 000 

15, 000 

16, 900

Us

Thousands 

of long 

tons

160 

1, 100 

1, 500 

1, 690

Total resources

Apatite 3

Millions 

of long 

tons

2,500 

11, 000 

17, 000 

30, 000

U3

Thousands 

of long 

tons

250 

1, 100 

1, 700 

3, 000

1 Data on PjOs from Le Cornec (1951) for Moroccan deposits and McKelvey and both fictitious and misleading. This table gives PjOs and apatite that is minable,

Others (1953), for domestic deposits.

* Apatite data have been computed on the basis of an assumed average PzOs conteat of 38 percent in the carbonate-fluorapatite comprising these deposits. Generally 

such compilations show BPL (bone phospnate of lime or tri-calcium phosphate) 

and as the BPL is in turn based on theoretical fluorapatite of 42 percent PzOs, it is

The principal aims of this report are three: to survey 

the uranium content of a variety of apatite materials 

and deposits; to portray the geologic and chemical controls of uranium distribution in phosphorites of various 

origins; to attempt an explanation of the occurrence of 

uranium in apatite.

We shall be concerned largely with the migration and 

fixation of an element that seldom exceeds 0.01 percent 

in the rocks studied. However, it is best to begin with 

a discussion of some of the restrictions which this low 

concentration imposes on the approach taken. Owing 

to its unusual properties of radioactivity and conferred 

fluorescence, uranium, unlike most trace elements, may 

often be followed in the field virtually as one follows 

a visible, major, mineral component. The same properties have allowed the development of analytical 

methods (Grimaldi and others, 1952) that permit accurate detection of uranium at extremely low concentrations. Unlike an investigation of a major element, 

however, a study of a trace constituent, in an impure 

material like phosphorite, is handicapped by lack of 

information on bulk composition. Whereas it is true

rather than tons of minable rock. The latter term does not correctly reflect absolute 

composition due to its dependence on a complex of external factors such as grade 

(amount of diluent), thickness, and overburden depth. 

1 Based on the conservative estimate of 0.01 percent U in the pure apatite.

that one may obtain the general composition of the 

mineral host with moderate accuracy and even here 

the carbonate-apatite problem is a vexing intrusion 

the tiny quantities of uranium dealt with suggest that 

most of the trace element assemblage be quantitatively 

determined. Otherwise, the immediate environment of 

each uranium sample is but poorly known. Obviously, 

it is impossible to overcome this difficulty in most cases 

because of the prohibitive time and money requirements, and the fact that adequate analytical methods 

are not available for many minor and trace elements. 

Instead, reliance must be placed on a reconstruction of 

conditions from field and petrographic facts and on 

selection of several elements for specific analysis. Thus, 

in studying the phosphorites, uranium contents are 

evaluated mainly in relation to PgOs content. Where 

important, fluorine, iron, calcium, aluminum or organic 

matter may also be studied. However, implicit in some 

of the work that follows is the assumption that uranium 

in phosphate nodules is associated mainly with the 

apatite composing the nodules, and that trace to minor 

amounts of included carbonates, clays, iron-oxides,
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 47

quartz, pyrite, and organic matter contain only insignificant amounts of uranium. 

Three lines of evidence validate this assumption:

1. Many phosphorite samples have been analyzed by 

two different methods, one, using weak acids, in 

which apatite dissolves readily, and clays, pyrite, 

iron-oxides or quartz would remain insoluble or 

dissolve only on prolonged treatment; the other, 

using a mixture of hydrofluoric and nitric acids in 

which all of the rock dissolves. In all samples the 

uranium determined was essentially the same by 

both methods.

2. Chemical analyses of the minerals associated with 

apatite in the Florida and Moroccan deposits showed 

their uranium content to be insignificant relative 

to that of apatite.

3. Nuclear emulsion studies reveal that most of the 

radioactivity in phosphate nodules comes from 

apatite.

ACKNOWLEDGMENTS

The field and laboratory studies which this paper 

summarizes have been part of the U. S. Geological 

Survey's program of investigation of radioactive raw 

materials for the Division of Kaw Materials of the 

U. S. Atomic Energy Commission. We have benefited 

greatly from the unpublished studies of many of our 

colleagues in the Geological Survey engaged in complementary and parallel studies of phosphorite. We 

are especially indebted to V. E. McKelvey and J. B. 

Cathcart, whose regional studies have contributed much 

important information on the geologic occurrence of 

phosphorite and uranium; to W. W. Kubey, whose 

earlier unpublished investigations and recommendations were largely instrumental in implementing the 

Survey's program of phosphorite investigations; and 

to the late John C. Kabbitt, whose reviews and suggestions in the senior author's unpublished studies are 

reflected in much that follows. We have also benefited 

greatly from the advice and interest of K. M. Garrels 

and F. S. Grimaldi in many helpful discussions of the 

chemical problems, from the stimulating discussions of 

regional weathering patterns with C. B. Hunt, and 

from the thoughtful and constructive reviews of E. S. 

Larsen, 3d, and K. P. Sheldon. The following Geological Survey colleagues furnished analyses: A. B. 

Caemmerer, Frank Cuttitta, G. J. Daniels, G. Edgington, Katherine Valentine Hazel, C. L. Johnson, Harry 

Levine, A. K. Schrenk, and Helen W. Worthing.

We are deeply grateful to the mining and administration staffs of the many companies engaged in phosphate mining in central Florida for their wholehearted

cooperation in allowing access to operating quarries 

during fieldwork from 1949 to 1954.

MINERALOGY OF APATITE 

CRYSTAL CHEMISTRY

It is important in considering the nature of uranium 

occurrence in apatite and phosphorite deposits to understand the possible variations in apatite composition. 

Basically, apatite is a hexagonal network structure 

composed of PO* tetrahedra, a simple anion (usually 

fluorine) and a divalent cation, calcium. Beevers and 

Mclntyre (1946) liken the structure to a honeycomb 

(fig. 3) with the vertices made up of vertical Ca-O 

columns in which the oxygens are supplied by PO* 

tetrahedra intervening between columns and linking 

adjoining columns together in a continuous structure. 

The fluorine or hydroxyl ions occur vertically above 

each other within the channels of the honeycomb, where 

they are coordinated to additional calciums (three 

about each fluorine) that occur in "caves" within the 

irregular walls of the network (Beevers and Mclntire, 

1946).

Apatite is host to many substitutions by cations, 

anions, and anionic radicals that resemble its normal 

constituents in size and charge (McConnell, 1938). As 

a result it may depart significantly from the composition of simple igneous fluorapatite, depending on its 

environment of genesis. The fluorine position may be 

occupied wholly or in part by hydroxyl. (Chlorine may 

also substitute for fluorine; however, as their size difference is appreciable, a structural rearrangement 

occurs in chlorapatite and it has only limited miscibility with fluorapatite.) In fossil bone, hydroxyl is 

eventually replaced by fluorine through ion exchange. 

Minor to major amounts of Sr, Mn, Mg, and Pb are 

known to replace calcium in apatite, and VO* and AsO4 

occur as traces substituting for PO* in apatite and form 

analogues of normal apatite in the pyromorphite series 

(Palache and others, 1951).

Additional variety is effected by the opportunity for 

coupled diadochy in which cationic or anionic replacements causing loss or gain in valence are balanced by 

replacements of opposite kind. Thus, in apatites containing rare earths the imbalance created by the substitution of tervalent cerium for calcium is quantitatively 

compensated by substitution of tetravalent silicate for 

phosphate or by the monovalent sodium for calcium 

(Borneman-Starinkevitch, 1938; Volodchenkova and 

Melentiev, 1943). An analogous situation exists in 

many other phosphate mineral groups. For example, 

monazite, the cerium phosphate, contains thorium and 

silicate in roughly equivalent amounts.
48 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

 9

&L>2>

9

'o H \ o

\/ r-&\£

> a. \ I p 

P0 4 tetrahedron, Oj 

superimposed

Fluorine, singly 

and superimposed

Calcium Relation of unit cell 

to hexagonal network

FIGURE 3. The structure of fluorapatite showing progressive stages of completion as viewed looking down the C axis. (Based on drawings of 

Beevers and Mclntyre, 1946.) a, Incomplete structure showing hexagonal framework and unoccupied channels; b, addition of 3 channel 

calciums and a fluorine occupying same plane; c, position of second planar group of 3 channel calciums and 1 fluorine, at a different vertical 

height in the structure; d, complete structure.

COMPOSITION OF THE SEDIMENTAEY 

CAEBONATE-APATITES

The fine-grained microcrystalline carbonate-apatites 

of the insular and marine phosphorites are best understood in terms of such coupled substitutions. This 

apatite is generally characterized relative to fluorapatite by a deficit in PgOs content of 3 to 6 percent, 

an excess of F, OH (or both) of 0.5 to 1.0 percent, and 

by the presence of 2 to 3 percent of carbonate. The 

exact mode of occurrence of carbonate in apatite is a 

controversial question. Hendricks and Hill (1950) have 

proposed that the carbonate is adsorbed on the surfaces

of discontinuities within the apatite crystals. It is significant, however, that carbonate fluorapatite is demonstrably smaller in unit-cell dimensions than fluorapatite 

(Altschuler and others, 1953). It is felt therefore that 

the structural difference revealed by the characteristically smaller cell must reflect the major and equally 

characteristic chemical deviations, and that carbonatefluorapatite is a structurally distinct species as proposed 

by Gruner and McConnell (1937). Lacking a precise 

determination of the position of carbonate in the structure we shall adopt provisionally the structural formula 

which best rationalizes the chemical composition, as
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 49

follows: Caio(PO4,CO3) 6F2 -3. Thus excess fluorine 

(or hydroxyl) serves to balance the charge difference 

created by the substition of (CO3 )-2 for (PO4 )-3 .

The chemical and spectrographic analyses in tables 2 

and 3 illustrate the chemical nature of the sedimentary 

carbonate-fluorapatites. Theoretical fluorapatite has 

the following composition:

CaO = 55.5 (includes F for O = 1.6) 

P2O5 = 42.3

F = 3.8

The analyses portray the deficiency of PzOa relative 

to CaO (augmented by Na, Mg, Sr and other 

divalent metals shown in table 3), and the excess of F 

plus (OH). It should be noted in table 2 that the carbonate is substantially all nonleached and therefore 

assignable to apatite.

Sedimentary carbonate-hydroxylapatite occurs as 

nodules or massive deposits in caves or islands and is 

indicative of continental origin. It usually originates 

as a replacement of limestone by guano solutions. In 

the presence of fluorine, carbonate-fluorapatite is the 

stable form and it precipitates as such in marine sediments (Kazakov, 1937). Furthermore, fluorapatites

TABLE 2. Chemical analyses of sedimentary apatite pellets from 

the Bone Valley formation

[Analyst, E. S. Clarke, Jr.]

TABLE 3. Semiquantitative spectrographic analyses of sedimentary 

apatite pellets

CaO 

P2 05 

CO2 

SiO2 (total) 

SO3 

AltOi....... ................... ..

FesOa - - -

NasO - . 

K2O 

H2 0 (+). -

H2 0 (-)_. _

u

F

Total... --------

F=O

B. L.-3, Bonny Lake 

mine, Eidgewood, Fla.

1

49.5

34.9

2.1

6.7

.3

1.4

.4

.1

.1

1.6

1.0 

.016

3.8

101.9

1.6

100.3

2

6.6

2.0

.8

.6

1,8

3

52.9 

37.3 

2.2

.9

.3

1.5 

.4 

.1 

.1 

1.7 

1.1 

.017 

4.1

102.6

1.7 

100.9

Wa.-lO, Watson mine, 

Fort Meade, Fla.

1

51.5 

36.6 

1.9 

2.9

.1 

1.1 

.9 

.2 

.2 

1.8 

.7 

.0075 

3.8

101.7

1.6 

100.1

2

?, 4

1.7

1.1

.5

1 6

3

52.7 

37.5 

1.9

1.1 

.1 

1.1 

.9 

.2 

.2 

1.8 

.7 

.0077 

3.9

102.1

1.6 

100.5

1. Analysis by complete decomposition of sample by solution in HNOs and fusion 

of insoluble residue with NagCOs.

2. Partial analyses of same material. Acid insoluble determined after boiling sample 

for 20 minutes with 1+3 HC1. Nonleachable carbonate determined after treatment in 0.5 M tri-ammonium citrate (Silverman, Fuyat, and Weiser, 1952).

3. Corrected analysis, free of insoluble residue. Microscopic examination and the 

two sets of SiO2 figures establish that acid insoluble is essentially quartz.

468547 58 2

Weight 

percent

Over 10.0 

10.0-5.0 -

5.0-1.0 

1.0-0.5. 

0.5-0.1 

0.1-0.05 

0.05-0.01 

0.01-0.005 

0.005-0.001 

0.001-0.0005-

0.0005-0.0001-

B. L.-3 i

Ca. P 

Si 

Al... ...

Mg, Fe, 

Na.

Pb, Mn, 

Or. 

Ba. _-

Cu, V, Y__ 

Zr 

Yb ---_

Wa.-lO »

Ca, P 

Si, Al, Fe-_.

Na, Mg _ .- 

Ti, V, Sr 

Mn, B, Y, 

Or.

Ba, La, NL_

Zr, Yb, Cu-

}

A rr Pp

Va.-7 »

Ca, P 

Si -

1 Al, Fe, Mg, 

J Na.

[sr, Y, La, 

[ Ti, Zr, B.

Mn, V, Ba,

Ni, Yb. 

Or, Pb, Cu, 

Sc.

ACT

K.-l*

Ca, P 

Na, Si, Al, 

Mg.

Cr, V, Y, 

Fe, Sr.

Ni, La, Ba,

Ti, Zr. 

Cu, Mn, Yb, 

Yb, Ag.

K.-15 «

Ca, P.

Si.

Na, Mg, Al.

Fe, Sr, Cr, 

V. Y. Ni.

Mn, Ti, Ba,

La, Cu. 

Zr, Mo, Yb.

Ag.

1 Bone Vplley formation, Bonny Lake mine, Eidgewood, Fla.; analyst, Katberine 

V. Hazel.

1 Bone Valley formation, Watson mine, Fort Meade, Fla.; analyst, Katherine V. 

Hazel.

1 Bone Valley formation, Varn mine, Fort Meade, Fla.; analyst, Katherine V. 

Hazel.

4 Daily production samples, TOiouribga, French Morocco; analyst, Helen W. 

Worthing.

are less soluble than hydroxylapatites, and carbonatefluorapatite can replace previously formed hydroxyl 

varieties in bone and island phosphorites. This last 

fact was demonstrated in the classic investigations of 

Carnot (1893) and is the basis of the current interest 

in water fluoridation for the prevention of caries, as 

well as being partly responsible for the dramatic revelation of the Piltdown forgery (Oakley, 1955b; Weiner 

and others, 1953).

PETKOGKAPHIC TERMINOLOGY

Many of the petrographic terms common in phosphorite literature are ambiguous. The terms used in 

the present report are denned here. Most marine phosphate deposits contain oviform particles ranging in 

size from granules to fine sand (4.0 to 0.06 mm) and 

composed almost entirely of carbonate-fluorapatite. 

These may display oolitic structures; many of them are 

replaced fossils; many show no textures or structures. 

Reflecting the variety of possible origins of the deposits, 

the names oolites, nodules, pellets, and granules have 

been frequently employed for all of the ovules. The 

term "oolite" properly denotes only small round particles evincing accretionary textures; the term "granule" 

is primarily a size term; and the term "nodule" is too 

nonspecific to be useful here as it usually applies to 

round irregular masses of any size and may embrace 

rock as well as monomineralic particles. In keeping 

with similar usage by V. E. McKelvey and his colleagues (oral communication, 1950) the term "pellet"
50 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

is used as a general term for oviform sedimentary 

apatite particles.

The term "phosphorite" has gained increasing acceptance as a rock name synonymous with the cumbersome 

term "phosphate rock." It is used here to denote rock 

or specimen containing substantial sedimentary apatite. 

Thus, one may speak of a "phosphorite sample," a 

"phosphorite deposit," or a "phosphorite pebble." The 

last would denote a round pebble-size rock fragment, 

rich in apatite but containing other minerals such as 

quartz, clay, and calcite. In contrast the term "apatite 

pebble" denotes a pebble-size, round or rounded, fragment of virtually pure apatite.

URANIUM CONTENT OF APATITE

FACTORS AFFECTING EQUIVALENT URANIUM 

DETERMINATIONS

The occurrence of uranium in phosphate nodules, 

bones, and other apatite materials was first demonstrated by Strutt in a remarkable set of analyses based 

on radioactivity determinations (Strutt, 1906, 1908). 

Strutt pointed out that the determinations were not 

affected by thorium which is virtually absent in fossil 

bone or sedimentary apatite, as is borne out by the 

analyses of phosphorite pebbles given in table 4.

TABLE 4. Thorium contents of Florida pebble phosphorite 

[Analyses from U. S. Geological Survey laboratory]

Locality

South Pierce mine, Brewster ___ ______ ......

Percent Th

<0.0005

001

.0007

Percent U

in. d.

>n. d.

0.017

' Although uranium was not determined on these samples, similar materials from 

the same localities contain 0.01-0.02 percent of uranium.

Available analyses of igneous apatite show three or 

four times more thorium than uranium, and if rich in 

rare earths, igneous apatite may contain as much as 

0.38 percent of Th, as reported in apatite from Mineville, N. Y. (McKeown and Klemic, 1956). ' . v ..... Despite the lack of thorium in phosphorite, it is

nevertheless preferable to base a discussion of uranium

on chemical rather than radiometric determinations. 

r . . . T T , _.,!_ ._v i -, Ihis is particularly true in view of the ease with which

apatite can be postdepositionally leached or enriched

in uranium and thus be out of radioactive equilibrium

-n ... TJJ-.L' jj. A. ' regardless of its age. In addition, due to a great variation in the size of the inherent crystallites of the sedimentary apatites (Jacob and Hill, 1953), it is difficult 

to maintain the standardization necessary to establish

. .

comparable equivalent-uranium results among a variety

to materials, regardless of the fact that they may all be 

ground to the same apparent size prior to radiometric 

assay. It is perhaps for these reasons that the equivalent-uranium figures presented in the excellent summary of Hebert (1947) are somewhat high. The 

discrepancies in Hebert's and in Guntz's data (1952) 

have been pointed out by Davidson and Atkin (1953) 

in their very thorough review and discussion.

URANIUM CONTENT OP REPRESENTATIVE APATITES

Table 5 presents typical and commonly encountered 

values obtained in a variety of apatite' materials studied

TABLE 5. Uranium content of representative apatites

[Asterisk Indicates values considered by the authors to be typical for such material. 

Analyses from U. S. Geological Survey Laboratory]

A pluorapatite, Ca10 (POJ 6Ft, from crystalline rocks

J- Alkali syenite Renfrew Ontario, Canada. ... 0.0022 

2. Gabbro, Henderson, N. Carolina- -__ ___ _. .0006 

3 Shonkinite, Mountain Pass, California. ______ .0049

4. Magnetite deposit, Durango, Mexico. _______ . 0009

5. Magnetite deposit, Mineville, New York__.__ .079 

6- Syenite, Kola Peninsula, U.S.S.R __.._- <. 001 

7. Tonalite, southern California batholith __ ___ .012

g Granodiorite gneiss, Boulder Creek batholith, Colorado _ _______________________

9. Quartz monzonite, Boulder Creek batholith, 

Colorado ___ _ _ -______._._.- _ _ _ _

10 - Hornblende biotite granite, Boulder Creek 

batholith, Colorado. _ _ ___.___._..__._._

n ]yftca granite, Boulder Creek batholith,

Colorado... ___ _._....._._._..._ ___

J2 Quartz diorite ^^ Eouldej. Creek batho.

lith, Colorado ___ _______ _____ _ __

B Carbonate-fiuorapatite (F=> 1 percent), Calo (POt, 

/-./-> > /n/^rrx / j- . i CO»),(F,0-_)M from marine sedimentary phosphorite 

la. Pellets, Hawthorn formation, Florida. _ __._ *. 0061

_ On?^ " JJO - --------------------------------- UU/ D

c. Do--_-_-_-_-_---_--_------_--_------ .0045

2»* Pelle^ Bone Valley formation, Florida. ..... * 009

* ---------------------------------- 

0069

0053

0078

0047

0049

c ^

3a. Pebbles, Bone Valley formation, Florida. 

b Do_________ _________________

c ' Do _

, p 4a. Pellets and oolites, Phosphona formation,

Idaho __ _____________________________

b. Do_. ________________________________

5a" Pellets and replaced fossils, Khouribga, French 

Morocco. _..__-_.____-__ _ _

b Do _

c. Do____ __ __________________________

6- Nodules dredged from the sea 

floor off southern California

(Samples, those analyzed for 

Diets, Emery, and Shepard, 

1942).

Sample No. 

69

106 __ 

121_.__. 

l 

.Oil

*. 016

. 024

* 0061 

.016

.007

*. 013 

.023

0089

0068

0041

01

183 . 0051
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 51

TABLE 5. Continued

C. Fossil Bone, carbonate-fiuor-hydroxylapatite (F=<C 

1 percent) Caw (POt,CO3) 6 (OH,F)i-3

1. RW-4468, mammalian (sp. unknown), Mayport, Florida___.._________._____ 0. 074

2. W-3841, mammalian (sp. unknown), Miles

City, Montana_________________________ . 83

3. Titanothere bone, Hell Creek formation, Camp

Cook, South Dakota__ _ _________ _ . 015

4. Shark tooth, Bone Valley formation, Florida_ . 009

5. Manatee rib, Bone Valley formation, Florida. _ . 006

6. Conodont fragments, Khouribga, French

Morocco....________.___.__.-_.____--_ . 079

D. Guano-derived phosphorites, carbonate-hydroxylapatite Caw (PO^CO^ t(OH)3-i

1. Angaur Island, Pacific Ocean.______--_-_-_-_ .001

2. Mona Island, Puerto Rico__-___-__--__-__--_ . 001

3. Cuthbert Lake Rookery, Tampa Bay, Florida _ <. 001

by the U. S. Geological Survey. In all cases the analyses 

are chemical 1 and represent the contents of individual 

mineral specimens or pure mineral concentrates rather 

than total rock and hence are of additional value in the 

geologic literature and do not duplicate the excellent 

summaries of uranium in certain phosphorite deposits 

(Kusakov, 1953; Hebert, 1947; Davidson and Atkin, 

1953; McKelvey and Nelson, 1950; McKelvey and 

others, 1955; Thompson, 1953; Cathcart, 1956).

It is apparent from the above data that uranium 

content in apatite materials as a group varies from 

barely detectable traces to the concentration of a minor 

constituent, approaching 1 percent. Nevertheless, within 

major occurrences of the same nature, as exemplified by 

the igneous rocks of the Boulder Creek batholith, or all 

of the immense marine phosphates, the level of concentration is of the same order of magnitude, O.OOX to 

0.03 percent. This range in uranium content may be 

taken to embrace the great bulk of apatite in the 

lithosphere and departures from it may be safely considered unusual and examples of special history.

ISLAND PHOSPORITES SECONDARY UPTAKE OF 

URANIUM AND FLUORINE

The unusually low uranium contents of island phosphorite, as illustrated by the Angaur and Mona Island 

analyses (table 5) are attributable to the lack of 

uranium in the parent limestones from which they 

originate by subaerial replacement. Only rarely do

1 The uranium analyses reported in this paper that have been performed in the U. S. Geological Survey laboratories have been done by 

fluorimetric methods. Grimaldi and others (1954) discuss the accuracy 

of these methods pointing out that they may be as good as ± 4 percent 

of the uranium content. They state that under routine conditions, 

"The error generally is greater and may range from d: 8 to 15 percent 

of the uranium content. When errors occur, the results are generally 

low." Most of the analyses in this paper have been done by a 

fluorimetrlc-extraction procedure. For this reason, as well as because 

of the nonroutine nature of the determinations, it is safe to assume 

them to be accurate In the range of dr 5 percent of the uranium present.

island phosphorites contain more than 0.002 percent 

uranium and where they exceed this value the uranium 

in all likelihood has been acquired postdepositionally 

from percolating ground water or from sea water 

with which such deposits are sprayed or washed by 

wind and wave. It is of interest in this regard to compare the fluorine and uranium contents of such materials, as fluorine, not being a primary constituent of 

subaerially formed phosphorites, is a certain indicator 

of postdepositional replacement.

Table 6 and figure 4 show that the percentage of 

uranium bears least relation to the P^O5 content or, 

inferentially, to the amount of apatite present in the 

rock. Thus specimens 3, 7a, and 8 are all virtually pure 

apatite rock yet their uranium contents differ by several 

hundred percent. In contrast, uranium generally increases as fluorine, or as the ratio of fluorine to phosphorus does. The lack of a constant ratio between the 

increases is easily explained by the fact that two independent structural emplacements are involved, F for

TABLE 6. Partial analyses of cave phosphorite from Mona 

Island, Puerto Rico

Sample

7b__-._ 

3__ . 

4 _ _---.

8._. ------

7a___-._-_

PsOji 

(percent)

26. 3

15. 8

41.4

32.4

38. 6

37. 1

F* 

(percent)

0.07

.06

.05

. 24

1.08

.65

U' 

(percent)

0. 0003

.0003

.0005

. 0011

.0028

. 0014

P^X10°

0.27

.38

. 12

.74

2.80

1. 75

F^XIO.000

0. 11

. 19

. 12

.34

.73

. 38

i Analyst, A. B. Caemmerer. 

* Analyst, G. Edglngton. 

»Analyst, C. L. Johnson.

10

10 20 

URANIUM. IN PARTS PER MILLION

30

FIGTJBB 4. Belation of uranium to P«O« In Mona Island cave 

phosphorites.
52 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

OH, and (presumably) U for Ca. That there are so 

few analyses of the required precision available for 

such comparisons is in itself a reflection of the generally recognized lack of uranium in phosphate deposits 

of continental origin. Nevertheless, the facts that secondary uptake of fluorine and uranium are codependent 

and related to the same geological process and time of 

emplacement seem indicated by figure 5, in which the

10 20 30 

URANIUM, IN PARTS PER MILLION

5. Relation of uranium to fluoridation in Mona Island cave 

phosphorites.

admittedly few points exhibit a trend that originates 

at the point of zero content for both percent of 

fluoridation and uranium content.

Similar accord between uranium content and the 

percent of fluorine of Cenozoic bones, in which the 

fluorine is unquestionably of secondary origin, has been 

noted in studies of Oakley (1955a, b).

If we note also that marine apatite nodules of recent 

origin contain appreciable uranium (table 5), it seems 

evident that the large marine phosphorites derive much, 

if not all, of their uranium from the ocean water at 

the time of their formation.

PROBLEMS OF OCCURRENCE OF URANIUM 

IN PHOSPHORITE

POSSIBLE MODES OF OCCURRENCE

Two large problems are posed by the analyses given 

in table 5. In what form and manner does uranium 

occur in phosphorites ? How may its variations within 

the same deposits or among different deposits of essentially similar material be explained ? Thus, the apatite 

nodules from Florida and Morocco are compositionally 

and petrographically quite similar, yet those from 

Morocco contain 2 to 3 times as much uranium. Isolated 

pebbles from the Land Pebble field contain as much 

as 0.2 percent of uranium in contrast to the normal 

concentration.

To explain the nature of uranium in phosphates 

several mechanisms may be postulated. The uranium 

may be present in a separate phase such as UO2 , UF4, 

or Ca(UO2 ) 2 (PO4 ) 2.8H2O (autunite). It may be 

present as isolated U+4 ions or (UO2 )+2 radicals, 

chemically adsorbed on surfaces or internal discontinuities (Hendricks and Hill, 1950). It may be present 

by structural substitution of U+4 in apatite as proposed 

by several investigators (Michael Fleischer, oral communication, 1949; McKelvey and Nelson, 1950; Davidson and Atkin, 1953; Goldschmidt, 1954) on the 

grounds of theoretical plausibility in view of the fact 

that the ionic radii of U+4 (0.97A) and Ca+2 (0.99A) 

(Green, 1953) are virtually identical. Lastly, it may 

be substituted structurally in apatite as (UO2 ) +2 since 

it has been demonstrated that the uranyl radical will 

be abstracted from solution by glycol-ashed bone. Such 

uranium uptake occurs by displacement of two moles 

of calcium per mole of uranium and also renders surface phosphate "* * * nonexchangable to the extent of 

two moles per mole of uranium." (Neuman and others, 

1949a; 1949b). These are puzzling facts for divalent 

uranyl. Moore (1954) has shown that phosphate can 

also remove appreciable (UO2 )+2 from cold water 

solution and that the extracted uranium cannot be 

washed out.

ABSENCE OF URANIUM MINERALS

It is a fact of signal importance that uranium minerals are virtually unknown in phosphorites. Uranyl 

phosphates and vanadates are common in other deposits 

and they are readily found by virtue of their habit as 

highly colored "paints" and the fact that many of the 

species fluoresce brilliantly, thus allowing detection of 

pinpoint disseminations. Yet these minerals have been 

noted in only three or four local concentrations in 

phosphorites (Altschuler and others, 1956; Arambourg 

and Orcel, 1951; McKelvey and others, 1955).

In the Bone Valley formation, the mineral autunite 

was found in a single layer of leached and altered rock, 

2 feet long and 6 inches wide. It occurred in thin square 

tablets with perfect (001) and prominent (010) 

cleavages. Its indices are Nw = 1.603± 0.002 and 

Ne = 1.585± 0.002. Flakes give a uniaxial figure and 

show blue to violet interference colors when tilted. The 

mineral shows a bright emerald-green fluorescence on 

exposure to light of 3650 A wave length and fainter 

fluorescence of the same color at 2537 A. X-ray data 

showed the mineral to be autunite and the optical 

properties, although not characteristic of ideal autunite, 

are nevertheless attributable to autunite as described 

by Fairchild (1929) for synthetic materials.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 53

Within the leached rock in which it occurs, the 

autunite coats cavities or is scattered throughout the 

porous cement. The mineral is absent from the interior 

of solid pebbles. In addition, phosphorite completely 

free of autunite (established by lack of fluorescence 

after powdering), but from the same rock, contains 

appreciable uranium and the autunite was found to 

account for only 5 percent of the total uranium in the 

rock. Thus, even where their presence is established, 

uranium minerals are of secondary origin and too 

insignificant to account for all of the uranium present.

RELATION OF URANIUM CONTENT TO COMPOSITION 

OF PHOSPHORITE

In efforts to understand the occurrence and variation 

of uranium in phosphorites, as well as to concentrate it 

for commercial use, many attempts have been made to 

correlate it with physical and chemical properties of 

apatite and phosphorite. Materials from the Bone 

Valley and Phosphoria formations and from the 

Moroccan deposits have been fractionated according 

to their variation in specific gravity, magnetism, color, 

luster, and particle size. None of these separations led 

to any positive or consistent correlation with uranium. 

This is not surprising, however, as it is not to be 

expected that uranium present in the order of O.OX or 

O.OOX percent, or 1 atom per several thousand unit cells 

of apatite, could sensibly alter the properties of the 

host rock.

FLUORESCENCE AND URANIUM CONTENT

The property of fluorescence is a notable exception, 

however, and it was found that a consistent relation 

could be observed between intensity of fluoresence and 

uranium content within certain samples. In table 7,

TABLE 7. Comparison of fluorescence and uranium content in 

composite samples of apatite nodules from the Bone Valley formation, Boyette mine, Florida

[Analyst, Harry Levlne]

Fluorescence color

Yellow. _ --_ _____

Yellow to brown _ _

Pink or peach ________

Purple or lavender_ _ _ _ _

Percent U

0. 017-0. 021

.009

. 006-. 010

. 003-. 006

Number of 

samples

2

1

2

4

uranium analyses are given for composite samples of 

apatite nodules hand-picked on the basis of their fluorescence under ultraviolet light of 3650 A wavelength. 

Each of the composite samples in table 7 consisted 

of many pellets or pebble fragments, and the fluorescence was characteristic of the entire pellet, or pebble,

rather than pinpoint or spotty. The correlation of 

increased uranium content with enhanced fluorescence 

is, therefore, further indication of the absence of separate uranium phases in phosphorites, even below the 

limits of microscopic detection, as it illustrates a change 

in an inherent property of the host mineral, apatite.

LACK OF PREFERENTIAL SOLUTION OF URANIUM

In solution studies of rock from the Bone Valley and 

Phosphoria formations with nonoxidizing solvents such 

as sulfuric, phosphoric, and citric acids, it was found 

that the percent of uranium extracted was always proportional to the percent of PaOs extracted. This result 

was attained whether the samples were slimes or 

crushed pebbles, whether calcined or treated raw, and 

with highly acid solutions or with small volumes of 

dilute weak acid (Igelsrud and others, 1948, 1949).

It seems evident that uranium in phosphate deposits 

cannot be accounted for by discrete uranium phases, 

by absorption, or by loosely adsorbed ions, radicals, or 

phases. This is indicated by three lines of evidence 

the lack of uranium minerals, the fluorescence of 

uraniferous apatite, and the inability to concentrate or 

separate uranium by physical or chemical means in 

the laboratory.

RELATION OF URANIUM TO P203

Most investigators have found that uranium varies 

approximately as PaOs content in phosphate deposits 

but that a close correlation does not exist between the 

two. The Moroccan deposits illustrate both the general 

accord and the lack of detailed agreement. Figure 6 is 

a scatter diagram in which PgOs is plotted against U 

for 18 samples representing the average daily production from various beds and locations in the Oulad25

20

LLJ 

Q5 10

10 15 20 25 

P2 05, IN PERCENT

30 35

FIGURE 6. Relation of uranium to P2O5 in phosphates from OuladAbdoun Basin, French Morocco.
54 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Abdoun Basin. They thus represent large tonnages. 

The tendency for the two factors to be related is apparent from the plot, in which most of the points fall 

between narrow limits, bounding a region of welldefined slope. However, when only high-grade samples 

are studied (those grouping closer together and containing more than 0.011 percent uranium in figure 6) 

more scattering is noted.

The approximate dependence of uranium on phosphate content is to be expected on two different grounds:

1. Phosphorites are notably devoid of separate uranium 

mineral phases.

2. Uranium is intimately associated within apatite as revealed by lack of preferential solution and by fluorescence.

Furthermore, the other mineral constituents of the 

major phosphorites, generally clays, carbonates, and 

quartz, almost universally contain less than 0.001 percent of uranium. A few samples of clay contain more, 

but such clays have been phosphatized or contain primary, admixed apatite, and their phosphatic material 

can be dissolved with dilute nitric acid leaving a pure 

clay residue that is relatively uranium-free. The ratio 

of uranium to phosphate dissolved is the same as in the 

apatite associated with such clays.

In view of the essentially exclusive association of 

uranium with the apatite within unweathered phosphorites, almost perfect positive correlation between 

uranium and any parameter of apatite content would 

be expected within a group of related rocks. However, 

detailed studies of the relations among uranium and the 

other chemical constituents of apatite show frequent and 

striking departures from pronounced positive correlation. The prevailing relation in the Bone Valley forma-' 

tion is for coarse pebble to be richer in uranium and 

poorer in P2Os content than the finer pebble. Cathcart 

(1956) demonstrated this relation and found that a 

moderate negative correlation ( 0.64) exists between 

P2Os and uranium for samples of -f-150 mesh pellets 

and pebbles throughout the Land Pebble Field. Such

TABLE 8. Size distribution and uranium contents of pelletal 

phosphates from Khouribga, French Morocco

[Analyst, Frank Cuttitta]

Sieve size (mesh)

32

44 __ . . _ ____ - ________

84__. ______________ ________

120

200_. __________________________

325.- -~ 

<325 _ _ _

Diameter 

mm

0.630

.437

.210

. 149

.074

.044

<. 044

Weight 

percent of 

fraction

2.0

2.3

35.2

28.4

25.9

2.9

3.4

Uranium 

percent

0.015

.015

.016

.015

.016

.015

1.012

i The finest fraction contains clay diluent.

negative correlation has not been found elsewhere and 

indeed, even within the land pebble field deposits containing 10 percent P2O5 will contain less uranium than 

deposits with 20 percent P2O5. In the Moroccan deposits 

uranium is uniformly distributed through all size 

grades, as table 8 demonstrates. In the Phosphoria formation McKelvey and Carswell (1956) have demonstrated that maximum uranium content is regionally 

associated with areas of thickset and highest grade 

phosphate accumulation.

An equilibrium condition may exist between apatite 

and uranium in sea water during the precipitation of 

apatite; however, such equilibrium may vary greatly 

with time and marine environment. Thus, assemblies 

of data from different parts of the same formation may 

represent a variety of different equilibria and an average of such varied groups of data may have the effect 

of masking, rather than demonstrating, a universal 

relation. The work of Thompson (1953,1954), who has 

explored the relation of uranium to apatite composition 

in a series of closely spaced, contiguous samples from 

the Phosphoria formation, possibly overcomes this 

difficulty. Her results are reproduced in table 9.

The results developed, and noted by Thompson, are 

that samples with a higher average uranium content 

show the best positive correlation with P2O5 content.

TABLE 9. Correlation coefficients among eU, PzOs, F, COj, and organic matter in samples of the Phosphoria formation (Thompson, 195S, 1954)

Locality and sample

Brazer Canyon WT 605, 604... _ _ ....-- _ . _ ..... _ .

Brazer Canyon WT 603 ________ . _ . _____ . _ .

Coal Canyon WT 700 _._. -...._ ...

Trail Canyon WT 365 ______ . _ . _ . _____ . ....

Beservoir Mountain WT 910 . .... ................

Number 

of 

samples

51 

24 

12 

26 

31

eUXlO* percent

Range

8-29 

7-36 

6-16 

4-28 

7-30

Average

13 

20 

31 

8 

16

PjOj percent

Range

12-34 

3-34 

1-34 

20-35 

31-36

Average

26 

26 

20 

30 

34

Correlation coefficients

eU/PjOj

+0.2 

+.8 

+.9 

+.1 

-.2

eU/organic

-0.2 

-.6 

-.7 

+.5 

-.6

COj/PjOi

+0.62 

+.93 

+.98 

+.29 

n. d.

F/PaOj

+0.98 

+.99 

+.99 

+.95 

n.d.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 55

The same samples also show the best positive correlation between CC>2 and P2Os and the best negative correlation with organic matter. All samples showed 

excellent positive correlation between F and PzOs. 

This last is expectable as the phosphate mineral is a 

carbonate-fluorapatite and although free fluorite is 

common in the Phosphoria, it was not detected in any 

of these samples (Thompson, 1954). One is struck by 

the spread in the correlations between ell and PgOs, 

and by the fact that the samples from Eeservoir Mountain, which show negative and very poor correlation, 

do not differ greatly in average ell content from samples 

WT 603 which show strong positive correlation.

Equally arresting is the fact that the two groups of 

samples showing best correlations do so for all of the 

constituents compared. This suggests that all of the 

factors are covarying with the same set of environmental conditions and therefore that these two sets 

of samples have sustained the least postdepositional 

change. This follows from the fact that the rocks are 

polymineralic, containing organic matter, carbonates, 

and apatite as separate phases. As the relations among 

these in primary marine deposition would differ considerably from those that would prevail under subaerial weathering or metamorphism, the C02, ell, and 

P2O6 correlations in sets of samples WT603 and WT700 

may all reflect the carbonate-fluorapatite content in 

relatively unaltered rock. On the other hand the poor 

correlation between C02 and P2Os in the other sets may 

reflect secondary alteration in terms of the highly 

mobile calcite, and, as during such alteration uranium 

may be added or subtracted, and apatite may be 

affected, eU^Os correlations are also notably poorer.

MARINE VERSUS GROUND WATER ORIGIN OF 

URANIUM IN PHOSPHORITE

GENERAL STATEMENT AND VIEW

Evidently, comparisons based on composition alone 

do not yield consistent explanations of the occurrence 

of uranium in phosphorites. Davidson and Atkin 

(1953) after surveying the bewildering variation in the 

available chemical data, concluded that in most instances the uranium in phosphorites is postdepositionally emplaced from percolating ground water as a 

proxy for calcium. This explanation adequately accounts for the uranium content of isolated bones and 

some guano deposits, and may account for some part of 

the uranium of other materials, but it is not tenable 

when applied to the preponderant occurrences the 

marine phosphorites, plus the phosphatic shales and 

limestones.

The theory of ground-water percolation as the major 

source and mode of uranium emplacement demands 

that uranium content increase significantly with age. 

Lenoble, Salvan, and Ziegler (1952) observe that radioactivity increases with age in the Moroccan deposits. 

However, they stress the minute nature of the variation, 

and Guntz (1952), in discussing the same deposits, 

emphasizes their regularity of grade, which suggests 

to him uranium coprecipitated during sedimentation. 

It is significant also that the Senonian Moroccan apatite 

contains appreciably more uranium than apatite from 

the Permian Phosphoria, which is much older, and has 

had a more varied history.

In a comparison of the radioactivity of nine Polish 

phosphorites, ranging in age from Cambrian to Upper 

Cretaceous, Pienkowski (1953) noted that "* * * the 

most ancient phosphorites were feebly radioactive; 

whereas the activity increases progressively in the 

younger formations," (table 10). Pienkowski did not

TABLE 10. "Inferred uranium contenf ("in arbitrary: units") of 

Polish phosphorites (Pienkowski, 1953)

"Geologic stage"

Middle Cenomanian ___ . _____________ .

Age, in 106 

years

400

370

350

100

100

95

95

76

60

Uranium

3.7

9.4

5.3

63.0

46.0

47.6

47.8

138.7

132.5

explain this relation and, as his data fall into three 

groups rather than continuously over the time span 

examined, we believe that a progressive change has 

not been significantly demonstrated. It is germane to 

the present discussion, however, that large steplike 

increases in uranium content occur in the younger 

phosphorites. This contradicts the theory that ground 

water is the major source of uranium in phosphorites.

EVIDENCE OF YOUNG PHOSPHORITES AND 

SUBMARINE NODULES

Regarding the question of marine versus groundwater origin, it is most noteworthy that apatite nodules 

dredged from the Gulf of California (table 5), contain 

as much uranium as the pellets from the Permian Phosphoria and Miocene Hawthorn formations. The Phosphoria and Hawthorn materials have been exposed 

subaerially, whereas the nodules from southern California waters have remained submerged and, although
56 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

they may have undergone submarine reworking, most 

of them are thought to be of Quaternary age (Dietz 

and others, 1942).

Further information on the magnitude of oceanderived uranium in recent deposits may be gleaned 

from the analyses of slighty phosphatic materials 

dredged from the Gulf of Mexico during recent U. S. 

Geological Survey studies (H. K. Gould, written communication) . Table 11 presents 10 of many hundreds 

of similar analyses. The samples represent contemporary accumulations of very slightly phosphatized 

shell and marl, and it is most significant that the ratio 

of uranium to phosphate in them far exceeds that of 

most high-grade marine phosphorites.

TABLE 11. Phosphate and uranium contents of bottom samples 

from the Gulf of Mexico

[Analyses from U. S. Geological Survey Laboratory (Qould, H. R., written 

communication, 1953)]

TABLE 12. Relative contents of uranium and P%0& in marine 

phosphorites

Sample

105916- .

105917- ^

105918. .

105919- _

105920 _-

105921 --

105922 

105923 - -

105924- -

105925 ..

Depth 

(fathoms)

24

30

35

45

63

100

80

62

7

8H

North 

latitude

26°50.1'

26°52.1'

26°50.7'

26°50.1'

26°51.5'

26°52.2'

26°34.8'

26°31.3'

26°18.7'

26°20.0'

West longitude

83°16.1'

83°33.5'

83°46.1'

83°57.9'

84°08.0'

84°32.7'

84°15.0'

83°57.9'

83°12.5'

83°22.1'

U

(percent)

0.0002

.0002

.0002

.0002

.0003

.0002

.0002

.0002

.0001

<. 0005

P2 05

(percent)

0.13

.11

.11

.09

.28

.24

.20

.18

.09

.08

Description

Shell sand.

Shell sand.

Algal sand.

Algal sand.

Shell sand.

Foraminiferal sand and

shell.

Foraminiferal sand and

shell

Foraminiferal sand and

shell.

Shell sand.

Quartz sand.

Table 12 lists analyses and U/PaOs ratios of representative large deposits and individual samples of 

apatite, all of materials not demonstrably enriched 

subaerially.

The unusually high U/P2O5 ratio of the phosphatized bottom samples may be due to some special 

regional condition such as oceanographic factors affecting COa concentration which in turn can influence 

uranium solubility (Piggot and Urry, 1941; Bachelet 

and others, 1952), or perhaps unusual uranium concentrations in the sea water of the Gulf as a result of the 

Pleistocene weathering and leaching of the land pebble 

phosphate deposits.

Although the mechanism causing the high uranium 

concentration in the phosphatized shells and marls can 

only be conjectured, the fact remains that enrichment 

of such magnitude can occur in a purely marine environment. Some nodules dredged from coastal CaliSample

Gulf of

Arbitrary mean__ _ _ _

U 

(percent)

Mexico bottom sample

0. 0001-0. 0003

0. 0002

PaOs

(percent)

si

0. 09-0. 28

0. 185

U/PaOj

1/925

Pacific Ocean off southern California -

162 Phosphate nodules__ 

69 Do____ ___ _

158 Do_____ __ _

106 Do___-___-__-

183 Do.. __ _____

121 Do__ _ _ _ _

0. 0125 

.0089

.0081

.0068

.0051

.0041

22.43 

29. 56

29.09

29. 19

29. 66

28.96

1/1800 

1/3300

1/3600

1/4300

1/5800

1/7000

Hawthorn formation, Florida :

Primary apatite pellets__ 0. 0075 

.0045

37 

37

1/4950 

1/8200

Khouribga, French Morocco 4

Do_. _____________

Do________.______

0.015 

.014 

.013 

.013

33.0 

33. 6 

33.0 

33.6

1/2200 

1/2400 

1/2540 

1/2580

Land pebble field, Florida *

Clastic apatite composDo_______________

Do_______________

Do-____-______--_

Do___________ __

0.018

.014

.010

.010

.009

30.7

33. 4

34. 5

35.2

33.0

1/1760

1/2380

1/3450

1/3520

1/3660

1 See table 11.

2 PtOs and sample numbers from Dietz, Emery, and Snepard (1942); uranium on 

the same sample splits by E. S. Clarke, Jr.

s Uranium by E. S. Clarke, Jr.; PjOs, assumed average for such pure apatite 

nodules, will yield conservative U concentration.

* Daily production averages at four stations in Oulad-Abdoun Basin, French 

Morocco.

5 Weighted averages of nodules and pebbles from five areas each representing a 

minimum of 100,000 long tons of phosphorite (data converted from Cathcart, fig. 

3A, 1956).

fornia waters contain as much uranium relative to 

phosphate as the relatively rich large deposits of 

Florida and French Morocco.

NECESSITY OF GEOLOGIC INTERPRETATION

Neglecting the trivial occurrences in guano and bone, 

spectacular as the actual uranium concentration in the
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 57

latter may be, it can be stated that most of the uranium 

in marine sedimentary phosphorite is of marine origin. 

The neutral to basic nature of apatite makes it susceptible to alteration under the prevailing acidities of 

most vadose waters. It is thus to be expected that the 

primary marine uranium may be redistributed by various secondary processes, as indicated by conflicting and 

varied relations of uranium to composition in the same 

and among different deposits, and by the evidence of 

laboratory uptake of uranium by bone and phosphorite. Therefore, it appears that the uranium content of apatite can be explained only in terms of the 

total geologic history of the deposits. In the following 

sections of this report a number of specific deposits are 

examined, and the geologic processes that have governed their uranium content are elucidated in hopes of 

developing a set of principles to explain the range and 

variation of uranium in most phosphorites.

PRINCIPLES GOVERNING URANIUM DISTRIBUTION IN 

PHOSPHORITES

URANIUM IN PHOSPHORITE DURING WEATHERING

FLORIDA LAND PEBBLE PHOSPHATES GENERAL 

GEOLOGY

The land pebble phosphates of Florida present an 

opportunity to study apatite in a variety of geologic 

circumstances and thus to evaluate its uranium content 

in terms of both petrography and petrology. These 

deposits are mainly in the Bone Valley formation of 

Pliocene age (Cathcart and others, 1953) and subordinately in the underlying Hawthorn formation of early 

and middle Miocene age (MacNeil, 1947). The surface 

of the Hawthorn is irregular, its rock is solution-pitted, 

and it contains many small and large slumps, within 

which the overlying Bone Valley thickens (Cathcart, 

1950). The Hawthorn was exposed and weathered during late Miocene time, developing an irregular topography and accumulations of phosphatic residue containing primary apatite nodules, inherited as such from 

the Hawthorn, and secondarily phosphatized limestone 

pebbles. During a period of Pliocene marine transgression this residuum of the Hawthorn was reworked 

into the unconformably overlying Bone Valley formation, with additions of quartz, clay, and probably 

phosphate (Altschuler and others, 1956).

The Bone Valley formation is about 30 feet thick 

and consists of two units (fig. 7). A pebbly and clayey 

sand characterized by graded bedding comprises the 

lower two-thirds and is the unit mined. Grading 

upward from this is a massive-bedded, less phosphatic

468547 58 3

quartz sand, which together with a surface mantle of 

quartz sands, is discarded as overburden during mining 

operations.

The upper part of the Bone Valley formation has 

been leached, altered to aluminum phosphates, and secondarily enriched in uranium in a widespread, transgressive zone that, though discontinuous, underlies 

several hundred square miles in the land pebble district. 

This aluminum phosphate zone is the result of lateritic 

weathering and ground water alteration, and sections 

through it exhibit a progressive variation in mineral 

content and texture (fig. 7). In typical sections pebbles 

of carbonate-fluorapatite still occur at the base, incipiently leached and altered. The calcium-aluminum 

phosphates crandallite and millisite are found in the 

middle of the zone. At the top, the aluminum phosphate wavellite predominates (Altschuler and others, 

1956).

In the land pebble deposits we may thus study and 

contrast the uranium contents of sedimentary apatite 

of the following types:

1. Simple structureless nodules from the Hawthorn formation.

2. Nodules from Hawthorn that have been reworked into 

the Bone Valley and are thus in a second cycle of 

sedimentation.

3. Simple pebbles of phosphatized limestone from the Bone 

Valley formation.

4. Complex reworked pebbles from the Bone Valley formation 

illustrative of several subcycles of sedimentation.

5. Leached and altered pebbles.

6. Secondarily enriched pebbles.

ILLUSTRATION OF LEACHING

In a conglomerate rock composed of pebbles resting 

in a matrix of finer grained material, the pebbles 

usually cause zones of increased permeability in adjoining matrix by disturbing the packing arrangement of 

the smaller grains. These areas would be zones of better 

circulation and as such would be the preferred loci of 

any leaching or enrichment that occurs within the rock. 

Thus the uranium, or the secondary phosphate, might 

be distributed according to the texture within the 

phosphate pebble rock. Graton and Fraser (1935) 

described this type of control and noted its influence 

in gold and copper deposition.

To test the influence of texture on the distribution of 

uranium, six pebbles were selected and analyzed for 

percent of uranium in zones from the surface to the 

center. The pebbles were selected from the aluminum 

phosphate zone of the Bone Valley formation and the 

data given in table 13 are described according to the 

sampling scheme shown in figure 8.
§ 

P

Hawthorn 

formation

Lower part of

Bone Valley

formation

n

aP

5000

a i

o0*

«-f

n

\ 

X

n

W

n'

-o

0

/

----

Calcium 

phosphate

zone 

(matrix)

Upper p

Bone Va

format

»^- » i w * * j A. iiw^^

iN.\Y-Ni »vi i§X '''''i v' 1 '-1 1 * v' >: ' % » %

^\:^x*'»*'- i * v i i V- ,'i i i \.- ' 1 1 % :

> > . NV ' ' *\ . VV-'- ' * !' %

\ .^VVi-'r'ils -.1 i tV".!. 1 , ! %.' "i i v

1 % . A v' L ' I I * k ll*.. I I k -I I t/Q »v Y-'I'UV.-I i ; :'i »' ' ]'/ ;

-»V.i|l;.: l , lr:'-Vil| » i, 1 , 1 '".:1 :^0 ^

Aluminum

phosphate

zone

Overburd
THE GEOCHEMISTRY OP URANIUM IN APATITE AND PHOSPHORITE 59

FIGUEK 8. Sampling scheme used for study of uranium distribution in pebbles.

Table 13 reveals a striking increase in uranium content from the surface inward, in all of the pebbles. 

Except for zone B in pebble 1, this increase is constant 

and of steep, although nonunif orm gradient, and values 

at the center are easily 100 percent higher than those 

at the outside.

This graded distribution suggests leaching of the 

uranium from phosphate pebbles of originally more 

uniform richness. It is significant that the sharpest 

increase in uranium content is in every case, between 

zones A and B. Inside the B zone the increase is 

smaller. This is precisely what would be expected as 

the surface zones would be more easily attacked.

It is evident that subaerial leaching can effect large 

changes in the primary uranium contents of marine 

deposits. The leaching illustrated above occurred after 

deposition, in the course of ground-water alteration. 

Leaching can presumably be equally effective during 

subaerial transport and reworking of similar materials. 

In this latter case the leaching would not be confined 

primarily to conglomeratic material and would probably show greater effects in the finer materials, as these 

have a greater ratio of surface to volume. That leaching did affect the rock's uranium content could probably be demonstrated by the same means or by the use 

of autoradiography.

POSTDEPOSITIONAL KNBICHMENT

ALUMINUM PHOSPHATE ZONE LATERITIC WEATHERING AND 

SUPERGENE ENRICHMENT

The Bone Valley formation yields instructive examples of enrichment as well as of leaching. Unaltered 

phosphorite in the lower part of the Bone Valley contains an average of 0.008 percent of uranium and 10 to 

15 percent of P2Os. In contrast, typical rock from the 

aluminum phosphate zone has 0.012 percent of uranium 

and 8 to 12 percent of P2Os roughly a two-fold enrichment in the phosphatic fraction.

There is much variation in the composition and 

uranium content of the aluminum phosphate zone. 

Nevertheless, the distribution of uranium within it 

closely follows the distribution of calcium (fig. 9). The 

uranium content is greatest in the base of the zone, 

declines gradually toward the middle of the zone, and 

is relatively low in the upper half. The zone of basal 

enrichment corresponds to the zone containing apatite 

pebbles, and analyses for uranium in pebbles, cements, 

and mineral concentrates within the entire zone show 

that the apatite pebbles are the richest uranium components of the zone and that the pebbles in the base of 

the zone are richer than those in the middle. Figure 10 

illustrates these relations and shows the contribution of
60 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 13. Chemical analyses showing distribution of uranium 

within phosphate pebbles

Corrections were made for the percentage of quartz in each portion tested. Analyses 

from U. 8. Geological Survey laboratory]

Pebble

1

2

3

4

5

6

Zone

A

B

C

D

A

B

C

D

A

C

A

C

A

B

D

A

B

C

Percent TJ

0.011

.023

.020

.021

.007

.026

.028

.033

.020

.028

.011

.032

.012

.019

.026

.007

.013

.015

Remarks

Selected at random; a small flattened

spheroid 2.5 by 2 by 1 cm. The pebble

is a microconglomerate, made up of

precipitated phosphate, phosphate pellets, quartz grains, and still smaller

pebbles of similar composition. The

other pebbles are similar in character.

Selected at random.

Selected because of high fluorescence, a

small fractured pebble of originally high

roundness; original central portion is

now exposed surface, hence only surface

and median zone were sampled.

Selected because of high fluorescence.

Selected because of high fluorescence; surface encrustation of brown phosphate

which was obviously deposited by circulating solutions since some of it occurs

as a circular ridge outlining the area of

previous contact with another pebble.

Selected because of high fluorescence.

the various textural components to uranium content of 

the total rock in a section through the aluminum 

phosphate zone.

Additional evidence of secondary enrichment may be 

obtained by comparing the uranium contents of composite samples of pebbles from rock of originally identical petrography above and below the base of the 

aluminum phosphate zone. Pebbles from within the 

zone were found to contain 0.056 percent of uranium 

in contrast to their anologues just outside the zone

which had only 0.023 percent of uranium. Furthermore, individual pebbles from the base of the zone 

contain as much as 0.25 percent of uranium. Concentrates of crandallite and millisite from the middle of 

the aluminum phosphate zone generally contain from 

0.03 to 0.05 percent of uranium. In contrast, pure 

wavellite, the dominant mineral in the upper part of 

the zone, contains only 0.002 to 0.004 percent of 

uranium. Despite the low uranium content in wavellite the upper part of the aluminum phosphate zone 

generally contains from 0.005 to 0.01 percent of uranium 

(fig. 9). This fact has led some authors to assume that 

"* * * radioactivity is as characteristic of wavellite 

* * * as it is of apatite * * *" (Davidson and Atkin, 

p. 13). However, most analyses of the wavellitic part 

of the aluminum phosphate zone reveal the presence of 

trace to minor amounts of calcium (fig. 9), reflecting 

the presence of small amounts of crandallite that can 

be seen microscopically, and in which much of the 

uranium resides.

The aluminum phosphate zone is commonly underlain by ferruginous hardpans that are relict of old 

ground-water levels. It is also characterized by secondary leaching and vesicularity and by mineral 

changes that progress vertically through the section. 

At the bottom, the zone is characterized by softened 

and bleached apatite pebbles. In the middle, holes 

represent former pebbles, and crandallite has replaced 

clay cement and has been deposited from solution 

within pebble cavities. Upward, wavellite needles replace the clayey crandallite deposits and the original 

clay cement (pi. 11, figs. J., B). This alteration has 

caused a depletion of CaO, silicate, and some phosphate 

in the upper part, where there is a complementary 

increase in A12C>3 (fig. 9) combined in acid aluminum phosphate, wavellite (A13 (OH) 3 (PO4)2.5H2O). 

Wavellite yields downward to intermediate Ca-Al 

phosphates (crandallite and millisite), and finally to 

the basic apatite at the base of the zone. These features 

imply a history of lateritic alteration by acid ground

EXPLANATION OF PLATE 11

FIGUEES A, B. Phosphatic quartz sandstone from the aluminum phosphate zone, Florida. A, Plane polarized light; B, crossed 

nicols. Original clay cement (a) is now highly porous and almost completely replaced by lathlike wavelite. 

The large and smoothly bounded cavity (b) is the site of a previous apatite pebble that has been removed by 

leaching. The cavity is now rimmed and partially filled by layered crandallite, with admixed kaolinite and 

goethite. The concavely stratified bottom filling of the cavity indicates the downward direction of the original 

weathering solutions.

C, D. Photograph and autoradiograph of fossil bone, both Xl%.

E, F. Autoradiographs of reworked and first cycle pebbles.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 314 PLATE 11
GEOLOGICAL ST-RVEY PROFESSIONAL PAPER 314 PLATE 12

-»fc-

% $ "W.

;/ ?v &?" itrt> " fe i^ i> , j-o* -;-i f ^ ^ v.% ?. ^-^§*;^v^

-^^->^;v^,; . ^\,^SP^ r ij

 ' ;:^ % ..^ ^^-^^ %, 1

Details of contact between aluminum phosphate zone (white) and overlying ferruginous >and mantle, Clear Springs Mine. 

Bartow, Fla. Photo illustrates the rubbly and concretionary nature of the weathered zone and the irregular and deeply 

embayed contact, which, with the residual concretions visible in the overlying sands, reveals the previously higher level 

of contact.

B. Mined out area of Tennessee brown-rock phosphates, Akin mine, Columbia, Tenn., showing control of rectangular joint system 

in the origin and distribution of the residual phosphates. Pinnacles and blocks of unaltered, crossbedded limestone of the 

Bigby formation occur in rows flanking the valleys created by weathering and mining along the master joints. The subsidiary joint set may be seen dividing and separating individual blocks. Phosphate has been mined from all of the depressions.
GEOLOGICAL SVKVEY PROFESSIONAL PAPER 314 PLATE 13

Photomicrographs of arkosic sandstone with carbonate-fluorapatite cement from the Gas Hills area of Wyoming. A, Thin section 

alone, plane polarized light, showing quartz grains (.a) and intergranular cement (b). B, Photomicrograph focused in the 

plane of the superimposed nuclear emulsion to show alpha tracks emanating from the apatite cement.

z

ID

LU 

O.

IO

LU 20

o

X

o

10

THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE

POSITION OF SAMPLE

61

POSITION IN STRATIGRAPHIC SECTION, IN FEET 

FIOCBB 9. Distribution of oxides through aluminum phosphate zone at Homeland mine, Homeland, Fla.

POSITION OF SAMPLE

i I 

Ho20 Ho20A Ho21

0.07

Ho22 Ho23 Ho24 Ho25

LU 

U

g'05 

LU

Z .04

WHITE PEBBLE (APATITE)

.03

.02

.01

\

 SECONDARY CEMENT (CRANDALLITE)

\

\

10i

12

POSITION IN STRATIGRAPHIC SECTION, IN FEET 

FIGURE 10. Uranium distribution in various textural components at Homeland mine, Homeland, Fla.
62 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

water. The pattern of uranium distribution parallels 

that of downward leaching and alteration. Uranium 

is liberated during the leaching and solution of apatite 

pebbles in the upper part of the zone. It is kept soluble 

in the acid ground water and emplaced in the highly 

porous, partly leached apatite pebbles at the base of the 

zone where the acids would be neutralized by the yet 

unaltered calcium phosphate. The correspondence between the curves for CaO and uranium (fig. 9) and the 

basal enrichment of uranium in apatite (fig. 10) suggest that the uranium substitutes for calcium in the 

apatite structure. This presupposes that the uranium 

in apatite is tetravalent, a supposition which is discussed on page 69.

The theory of supergene enrichment presented above 

demands that originally there was 2 or 3 times more 

apatite than now occurs in the aluminum phosphate 

zone. This prerequisite is not supported by the obvious 

field relations as, in unaltered exposures, it is possible 

to see a primary stratigraphic change to a less phosphatic clayey sand, from lower to upper Bone Valley 

(fig. 7). There is evidence, however, that an originally 

thicker section was available to provide the present 

enrichment (pi. 12, fig. A). Briefly, the contact between 

the upper part of the Bone Valley and the surface 

mantle of loose quartz sand is irregular in detail and 

the sands of both zones are virtually identical in size, 

sorting, and heavy-mineral content. Therefore, a large 

part of the loose sand mantle is a residue of weathered 

Bone Valley and the present contact is a lowered one.

THE COOPER MARL RESIDUAL ENRICHMENT

Unusual and widespread subaerial enrichment of 

uranium in apatite also occurs in the phosphate deposits 

of the Cooper marl in the vicinity of Charleston, S. C. 

Being composed principally of limestone, the Cooper 

marl is only slightly phosphatic. However, it has been 

postdepositionally weathered and phosphatized, and in 

the exposures in the vicinity and along the banks of 

the Ashley Eiver, between Drayton and Middleton 

Gardens, the deposits are characterized by a thin uppermost zone of nodular induration. The induration results in irregular, discontinuous encrustation and the 

rock is replaced by secondary phosphate. It is obvious 

that the process was accompanied by moderate leaching the indurated fragments that are loose or easily 

broken out of this zone are marked by cavernous irregularity and conspicuous molds of pebble and fossils, 

although its fine texture is similar to that of the 

unweathered underlying rock.

The unaltered Cooper marl "* * * commonly contains about 75 percent lime carbonate and about 2 percent lime phosphate," (Eogers 1913, p. 187). The

Cooper marl was exposed and eroded during Oligocene 

to Miocene time and later exposed and weathered dur- 

.ing Pliocene to Pleistocene time (Rogers, 1913, and 

Malde, in press). During the last period of weathering 

the calcite was both leached out of the section and 

replaced by apatite. The fact that apatite could reprecipitate as a stable phase during weathering suggests 

that the ground water alteration was only mildly acid. 

This is further indicated by the absence of aluminum 

phosphates or even the intermediate calcium-aluminum 

phosphates, despite the fact that the region contains 

appreciable clay.

An immediate consequence of the re-forming of 

apatite at the site of alteration is the refixation of any 

uranium mobilized by prior solution. This establishes 

a pattern of residual enrichment. The pattern may be 

construed as follows:

1. Mildly acid ground water dissolves calcite and apatite.

2. Some, if not much, of the phosphoric acid in solution is 

immediately neutralized by reaction with the remaining 

calcite of the marl, producing secondary apatite.

3. Most of the uranium in solution is taken up by the newly 

formed apatite.

This scheme indicates a disproportionate enrichment 

of uranium as a result of its preferential uptake in the 

apatite structure. It has been possible to develop conclusive data in support of this in the phosphates of the 

Cooper marl. Malde in his study of the Cooper marl 

noted that "* * * the uranium content is rather uniform, ranging between 0.001 and 0.005 percent, rarely 

as much as 0.007 percent and averaging about 0.003."

In contrast to these observations of the prevailing 

tenor in the unaltered marl, analyses in table 14 illustrate the overall magnitude of enrichment in the hardpan collected in the area between Rantowles Creek and

TABLE 14. Examples of phosphate- and uranium-enriched 

hardpan in Cooper marl near Charleston, S. C.

Locality

Magnolia Gardens, exposure on west bank of

Andrews, borrow pit on west side State Highway 161, one-half mile north of Andrews _ _ 

Ashley River banks, west bank at road from 

Olive Branch Church _ _____

Ashley River banks, east bank 1.25 miles south 

of Lambs ___ ______ ____ __

Bulow Mines, 2 miles southwest of Fort Bull 

on Fort Bull-Red Top Highway ______

Micah Jenkins Nursery, U. S. Highway 61, 

one-half a mile west of St. Peters Church.

Percent U '

0. 048

. 061 

.048

.038

.043

.087

Percent 

PaOs"

22. 1

28. 3 

24. 8

25. 8

27. 1

24. 7

»Analyst, G. J. Daniels. 

3 Analyst, A. R. Schrenk.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 63

Ashley Eiver. The analyses show enrichment of both 

apatite and uranium, especially of uranium. The selective enrichment of uranium is shown by comparing 

analyses of unaltered marl and the overlying fragments 

of cavernous hardpan from the same exposure (table 

15). It is significant that the enrichment in P2O5 is 

only 6-fold whereas that in uranium is 15-fold.

TABLE 15. Comparison of phosphatized and primary Cooper marl 

from Lambs, S. C. (BOO yards above old railroad terminus on west 

bank of Ashley River)

Material

(a) Unaltered marL _____ __ _ _ ________

fb) Phosphatized hardpan. __ ___ _ ______

Percent U i

0.005

.074

15

Percent 

PsOt'

4.0

25.9

6

i Analyst, G. J. Daniels. 

' Analyst, A. R. Schrenk.

Another illustration of preferential uptake of 

uranium during weathering of apatite is obtained by 

comparing different nodules from the same hardpan 

samples of phosphatized Cooper marl. At several 

localities contiguous fragments differed markedly in 

color, from dark brown to light tan or gray. The 

lighter color was ascribed to the bleaching effects of 

oxidative subaerial weathering. On the assumption 

that the lighter fragments had thus undergone more 

of such weathering, light and dark fragments were 

analyzed separately with the thought that differences 

in uranium content would be shown and that differences in other constituents would reveal the relation of 

the uranium content to weathering history. Results 

of chemical and of semiquantitative spectrographic 

analysis of the same samples are given in tables 16 

and 17.

The two samples, obtained from localities about 4 

miles apart, are representative of hardpan and weathering conditions over a wide area; nevertheless, the chemical changes illustrated by the two comparisons are 

remarkably uniform for secondary subaerial processes. 

The spectrographic analyses show that the differences 

between the contiguous dark- and light-colored members of each pair are due to weathering. In each pair 

the relative quantities of silica and aluminum increase 

and the iron content drops in the light member. The 

changes are not large and are of comparable magnitude 

in each pair. The fact that the carbon content is the 

same for both members of one pair and is even greater 

in the light-colored member of the second pair in table 

16, proves that the color change was not brought about 

by oxidation of organic matter as originally supposed. 

Instead, it is obviously the result of leaching of ferric 

iron. The persistence of ferrous iron in each case indicates that the leaching was accomplished by reducing 

solutions, a fact further indicated by the persistence or 

increase in organic carbon.

It is suggested that mildly acid humate solutions, 

derived from the organic-rich soils that are common 

in the Ashley River flood plain, removed the iron by 

the formation of organo-ferric complexes and thus 

effected a partition between ferrous and ferric iron. 

The solutions dissolved out apatite, slight amounts of 

the other soluble constituents, and calcite. This is demonstrated by the uniform decrease in P2O5 content in 

both pairs. This caused a slight residual enrichment 

in the relatively insoluble constituents, quartz and clay; 

hence silica and aluminum both increase in the light 

members (table 17) .2 Thus, the light members of each 

pair reflect substantially more contact with ground 

water solutions, and during such contact appreciable 

quantities of uranium were taken out of solution by the

2 It is possible that In the Ninenjile sample only the relative proportions of Si and F have been reversed. However, in view of the systematic changes in Fe and Al being demonstrated for both samples, it is 

believed that the Si in the light member of the Ninemile sample 

actually has Increased as in the unambiguous case of Lambs.

TABLE 16. Variation of enrichment with weathering in phosphatized Cooper marl, South Carolina

Location

Ninemile Station, Southern Railroad, half a mile northwest of Goodrich, stream bank, east of railroad trestle.

Sample

Dark fragment. __ _ __

Dark fragment _ _ _ _

Light fragment. _____

Percent

U»

0. 035 

. 11 

. 064 

. 12

PiOs 2

27.0 

26. 9 

27. 2 

25. 5

Total 

Fe

1. 04 

. 57 

. 73 

.35

FeO

0. 40

. 38 

. 34 

. 38

Fe2Os 3

1.04 

.40 

.66 

.07

Organic

carbon 4

0.5 

.5 

. 6 

1. 1

1 The uranium analyses were made on the filtrate of a nitric acid leach. Additional 

analyses on the total sample indicate that all the uranium occurs in the soluble, 

phosphatic fraction of the sample. Analyst, O. J. Daniels.

2 Analyst, A. R. Schrenk.

'Analyst, Alexander Sherwood. (Total iron and FeO determined chemically; 

FejOs computed by difference.) 

* Analyst, A. B. Caemmerer.
64 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 17. Semiquantifative spectrographic analyses of phosphatic 

hardpan from the Cooper marl

[Order of precedence gives relative percent content; thus in the dark member at Nine 

mile, F Is greater than Si, and Mg is greater than Sr. Analyst, Katherine E. 

Valentine]

Range, hi percent

>10 ...

5-10- 

1-5. _ ....

0.5-1 

0.1-.6-. _ .......

0.05-.1-.. 

0.01-.05.. ........

0.005-.01... ......

0.001-.005... .....

0.0005-.001.. ..

0.0001-0005.. 

0.00005-.0001..... 

0.00001-.00005 ....

Ninemile, South Carolina

Dark (less 

weathered)

Ca, P

F, Si, Na 

Al, Fe 

Mg, Sr, K

Ti 

B, Ba, V, Y

Li, Mn, Ni 

Zr, Or, Yb, 

La, Sc.

Be

Light (more 

weathered)

Ca, P

Si, P, Al, Na

Fe, Mg, K, 

Sr,U 

Ti 

B, Li, Ba, V, 

Mn, Y. 

Ni 

Or, La, Zr, 

Sc, Yb.

Be

Lambs, South Carolina

Dark (less 

weathered)

Ca, P

Si, F, Na 

Al, Fe 

Mg, K, Sr

Ti 

Li, B, Ba, V, 

Y. 

Ni 

Zr, Cr, Mn, 

La, Yb, Sc.

Be

Light (more 

weathered)

Ca, P 

Si.

Al, F, Na

Mg, Fe, K, 

Sr, U. 

Ti 

B, V, Ba, Y, 

Li.

Ni, Sc, Zr, Cr, 

Mn, La, Yb.

Be

apatite. The fact that the bulk chemical changes are 

small (for example P2O5 data and the date table 17) 

proves that residual enrichment cannot explain the 2- 

to 3-fold uranium enhancement of the light over the 

dark nodules. Patently, the residually enriched uranium 

of the nodules throughout the region is frequently augmented by significant additions of uranium from 

ground water.

CONTRASTING MODES OF ENRICHMENT IN MILD VERSUS 

INTENSE WEATHERING

In comparing uranium distributions within the Bone 

Valley formation and the Cooper marl, it becomes evident that two largely different patterns of regional 

enrichment prevail during weathering, despite the fact 

that the primary mechanism of enrichment is the same, 

namely, the takeup of uranium from ground water 

during weathering. The patterns of enrichment are 

governed by the mineral assemblages which in turn 

reflect the diverse conditions and environments of 

weathering. Figure 11 summarizes and contrasts the 

results.

Strong acid weathering 

(Florida, Senegal, Nigeria, and soils)

Unaltered

Lowered contacts

Mild acid weathering 

(South Carolina)

Unaltered

a o a o & o o o o 

\r ° o o O o ° o

O O O O <

Calcite Quartz Clay Apatite

Florida, Senegal, Nigeria 

Nature of new phases. ....... Acid; A! and Ca, Al phosphates

Process. ..... . . . Phosphatization of clay

Type of enrichment____...... Supergene

Locus of uranium enrich........ At base of weathered section,

ment and hardpans controlled by groundwater

Secondary Crandallite 

apatite 

hardpan

South Carolina

Basic to neutral; secondary apatite. 

Phosphatization of limestone. 

Residual. 

Near surface of active weathering.

Wavellite

FIGURE 11. Contrasting modes of araniam enrichment and weathering in phosphorites.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 65

In South Carolina the process is a simple one of 

simultaneous enrichment of uranium and production of 

apatite at the zone of weathering. The weathering does 

not seem to have been very intense the zone it has 

affected is quite thin and the mineral transformations 

are not profound, involving merely the leaching of 

calcite and apatite and the reprecipitation of apatite. 

As a result the zone of enrichment, coinciding with the 

zone of appreciable weathering, is restricted to the 

uppermost part of the section and migrates only as 

weathering reduces the rock to inert constituents 

(fig. 11).

In Florida, and in Senegal (Capdecomme, 1953) and 

Nigeria (Russ, 1924), the prominence of aluminous 

phases implies a regimen of highly acid, lateritic 

weathering such as would be expected in these tropical 

areas. The development of purely aluminous phosphate 

(wavellite) in the upper parts of the weathering zone 

in turn excludes the concentration of uranium, which 

migrates to where calcic phosphates occur (fig. 11), 

and, accordingly, uranium content increases progressively down the section, reaching moderate concentration in the crandallite zone and causing a supergene 

enrichment in the base of the entire zone where 

weathering is incipient and apatite still abounds.

The deduction of two different intensities of weathering, strongly and weakly acid, from rocks of two different gross petrographies, might be questioned on the 

grounds that the South Carolina deposit, containing 

primary calcite, would have so much greater neutralizing capacity than the Florida deposit that acid phases 

could not be generated. However, it is noteworthy that 

the Sengal deposits contained limestone and dolomite 

but were nevertheless altered to crandallite and other 

aluminum phosphates. In addition it has been shown 

that the excessive enrichment of some of the Cooper 

marl nodules must represent ground-water enhancement beyond the pure residual enrichment of adjoining 

nodules. This ground-water enhancement implies considerable additional ground water seepage; yet the 

apatite, a basic to neutral compound, has remained as a 

stable phase despite the fact that clays abound in the 

overlying marls and metasomatic alteration of apatite 

to aluminum phosphates is rather common under 

similar conditions in soils. Presumably the acidity was 

quite low, as even traces of aluminum phosphates or 

intermediate phases are absent in the hardpan of the 

Cooper marl.

BROWN-ROCK PHOSPHATES OF TENNESSEE RESIDUAL 

CONCENTRATION

The brown-rock phosphates of central Tennessee are 

a prominent example of residual phosphate accumulation. They were therefore studied for residual concentration of uranium. The brown-rock deposits occur 

mainly as a residual blanket surmounting the Bigby 

formation of middle Ordovician age (Hayes and 

Ulrich, 1903), a crossbedded limestone in which apatite 

occurs sparsely concentrated within topset and f oreset 

intercalations, and also weakly disseminated throughout the limestone. The deposits ". . . occur only in areas 

where Bigby-Cannon limestone directly underlies the 

soil . . ." (Alexander, 1953) and are absent when mining 

extends into hillsides where fresh limestone is encountered. It is evident that the deposits were formed during Cenozoic weathering by preferential solution of 

the limestone (pi. 12, B). In many areas the residuum 

occurs as filling of solution-enlarged joints locally 

known as "cutters." Within such "cutters" it is possible 

to sample both the weathering concentrate and the 

adjoining, closely related parent material.3 Analyses 

for one such set of samples from the Akin mine of the 

Tennessee Valley Authority at Columbia, Tennessee 

are presented in table 18. The relation of these samples 

to the stratigraphy is shown in figure 12.

TABLE 18. PzOi and uranium content of wedthered and fresh 

Tennessee phosphates, Akin mine, Columbia, Tennessee

[Analyst, R. S. Clarke, Jr.]

(a) 

(b) 

(c)

Limestone of the Bigby formation....

Enrichment factor (b/a) _______

Percent

PiOt

1.60 

1.60 

27.0 

27.4 

16

U

0. 00004

. 00074 

18

Soil

Residual 

phosphorite

Bigby 

formation r

Crossbedded

phosphatic

limestone

Sample location

FIGURE 12. Sample locations ^ for comparison of brown-rock 

phosphate with parent limestone in Tennessee.

8 Exact Btratigraphic equivalence of these samples cannot be assumed 

due to draping and compaction within the "cutters."
66 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

The figures above demonstrate purely residual enrichment. The differences between the concentration factors 

of 16 and 18 are negligible in view of the difficulties in 

analyzing such small quantities of uranium. The striking contrast of the above to results from the South 

Carolina samples (table 15) where the enrichment 

factor for uranium is far greater than for ?2O5, serves 

to emphasize the partly external origin of the ground 

water enhancement of uranium in the South Carolina 

deposits.

SECONDABY EMPLACEMENT OF UBANIUM IN BONES

Determinations of the radioactivity and uranium 

content of bones have been made by several authors, 

and it can be seen from their publications, and from 

the analyses in table 5, that bones exhibit the gamut 

of uranium content in apatite. The highest radioactivity for any apatite material previously reported is 

by Davidson and Atkin (1953), who listed a specimen 

of fossil fish from the Old Eed sandstone containing 

0.55 percent eU. Bowie and Atkin (1956) have shown 

that the unusual radioactivity of other fossils from the 

same region and formation is due in appreciable 

measure to thorium. They show, however, that the 

thorium does not occur in the bone apatite but rather 

in hydrocarbon concentrations within the bone cavities. 

Although the ell values of fossil bones from the Old 

Eed sandstones may thus not accurately reflect uranium 

in the bone apatite, fossil bones are nevertheless, among 

the most uraniferous apatites. The value of 0.83 percent 

uranium (table 5) is the highest uranium content of 

which we are aware for any apatite substance (bones 

or mineral specimens).

Lacking the economic and geologic interest of the 

phosphorites, bones have not been extensively studied 

for uranium. However, certain well-established generalizations may be drawn from available information. 

Analyses presented by Hebert (1947), Oakley (1955a, 

b), and Jaffe and Sherwood (1951), all show fresh bone 

of marine or terrestrial organisms to be essentially nonuraniferous. In addition, comparisons of radioactivity 

of Cenozoic human and animal bones (Oakley, 1955a, 

b), reveal that uranium content increases with time, an 

indication of postburial uptake. As in the guanoderived phosphorites, uranium uptake in bones parallels 

fluorine uptake; apatite of all types is a favorable 

receptor for both of these elements.

The above facts, and the experimental extraction of 

uranium from solution by apatite (Moore, 1954) and 

by glycol-ashed 4 modern bone (Neuman and others,

* A procedure for removing tissue and organic matter.

1949), clearly demonstrate that the uranium of fossil 

bone is due entirely to secondary enrichment from 

ground water or marine sources. As Davidson and 

Atkin (1953) have pointed out, the most uraniferous 

fossil bones come from continental gravels of high 

permeability. In the highly quartzose and felspathic 

terrestrial deposits, a fossil bone may be the most, and 

possibly the only, favorable host for uranium (and 

fluorine). In such circumstances, ground water charged 

by uranium from regional sources may effect unusual 

concentrations of uranium in isolated bones or phosphorites nodules and cause an enrichment far greater 

than that of the enclosing rocks. Thus, Smith and 

Bradley (1952) report dinosaur bones from the 

Cloverly formation which contain from 0.085 to 0.135 

percent of uranium, whereas the enclosing rocks contain 

only 0.001 to 0.005 percent of uranium.

Denson and Gill (1956) have shown that the highly 

uraniferous lignites of the Fort Union and Hells Creek 

formations in northwestern South Dakota and eastern 

Montana have derived their uranium from percolating 

ground waters which were (and still are) enriched by 

leaching of the overlying tuffs. Undoubtedly, the fossil 

bones from these formations have become uraniferous 

through the same agency. The titanothere bone from 

the Hells Creek formation (table 5) exemplifies this 

type of enrichment. Probably Miles City fossil 

W-3841 (table 5), from an unknown surface exposure 

in the same region, derived its unusual uranium content 

in the same manner.

Petrographic evidence of the mode of such postdepositional enrichment is shown by an autoradiograph 

of specimen W-3841 (pi. 11, fig. Z>). The most uraniferous (the lightest) parts of the bone are the peripheral 

zones and the cavernous and highly permeable internal 

structures. In contrast to these areas, the intermediate 

dense areas are of relatively low grade. Obviously, 

uranium concentration bears no relation to the great 

difference in apatite concentration. In addition, it was 

determined by X-ray powder diffraction analysis of 

different portions of the specimen that the apatite is 

of the same type throughout, and further analyses on 

separated material showed that the carbon-rich fraction 

has less uranium than the mineral fraction. Evidently 

accessibility to solutions was the main control on the 

localization of the uranium.

Unlike the contrast between bones and enclosing rock 

in terrestrial deposits bones and other phosphatic fossils in unaltered marine phosphorites are generally 

similar in uranium content to the apatite pebbles and 

pellets from the same deposits. This supports the argument that the unusual postdepositional enrichment of 

continental fossils is attributable to special provenance
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 67

rather than special mechanism. Analyses of 12 fossil 

manatee rib fragments and shark teeth from mines in 

the Bone Valley formation showed a range in uranium 

content of 0.004 to 0.016 percent (Jaffe and Sherwood, 

1951). The P2O5 contents of the same specimens ranged 

from 36.1 to 37.5 percent, indicating that all were relatively pure carbonate-fluorapatite. The manatee and 

shark specimens represented species that ranged from 

Miocene through Pliocene and many of the Bone Valley 

fossils have been reworked from the Hawthorn formation. Nonbiogenic apatite of virtually identical composition and geologic history has essentially the same 

range of uranium content. The simple sand- and 

granule-size apatite pellets from the Hawthorn formation (primary) and the Bone Valley formation (reworked and possibly primary) are pure carbonatefluorapatite containing 36 to 38 percent P2O5. Their 

uranium content (table 5) ranges from 0.004 to 0.012 

percent.

MARINE ENRICHMENT AND REWORKING

It has been shown that postdepositional alteration can 

cause large variation in the uranium contents of apatite 

deposits and that many of these variations can be 

ascribed directly to ground water leaching or enrichment. This does not, however, explain the apparently 

primary variation in unaltered materials. This problem 

has two aspects, illustrated by tables 12 and 19. Table 

12 shows that similar materials from different deposits 

can differ significantly in uranium content. Thus, the 

Moroccan deposits, composed of primary apatite 

nodules of simple texture, have a much higher uranium 

to phosphate ratio than the comparable primary, simple 

nodules of the Hawthorn formation.

Table 19 demonstrates, in terms of the Bone Valley 

and Hawthorn formations, the extent to which the 

components of the same deposits may differ in uranium 

content.

The following facts are evident from table 19:

1. Pebbles, which are complex in texture, are richer as a 

class than pellets, which are simple in texture.

2. Pellets from the Bone Valley formation are richer as a 

class than those from the Hawthorn formation.

The texturally complex pebbles represent materials 

that have gone through two or more cycles of erosion 

and deposition. In addition, the nodules of the Bone 

Valley formation differ from those of the Hawthorn 

formation only by the fact that the former have been 

reworked from the latter. The differences in uranium 

content cannot be explained in terms of relative concentration, as it has been shown (Cathcart, 1956) that the 

coarse pebbles as a class contain less PaOs than the finer

TABLE 19. Uranium content of representative clastic apatite from 

the land pebble phosphate field, Florida

[Analyst, R. S. Clarke, Jr.]

Sample Formation Locality U (percent)

Individual apatite pebbles

B.L.-1_ 

B.L.-2_

B.L.-3_

Ho-14 .

Ho-15 _

Ho-16.

Ho-17 _

Bone Valley __ 

_____do_______

_____do__.____

_____do_______

_____do.______

____.do.______

_____do_______

Bonny Lake mine, Ridgewood, Fla. 

__--.do____. ______________

_----do______--_______- _

Fla. 

_ do._ 

_____do___________________

____-do_____-_--_-____-___

0.011 

.018

.016

.032

.022

.015

.021

Apatite pellets (composite samples)

Va-1 . _ 

Va-7 

Va-10 _

Ho-12.

Ho-13 .

Ho-14 _

_____do_______

__._.do_______

_____do_______

_____do_______

Bone Valley. _ 

__.__do_______

do

__.__do_______

_____do_______

do

 _do_ _ ____ 

_____do_________-__.___-__

Varn mine, Fort Meade, Fla_ 

_____do____----_-____-_-__

do _ __

Homeland mine, Homeland,

Fla. 

_____do___________________

____-do_____--_-__-__----_

0. 0059

.0067

.0045

.0071

.0075

.008 

.011

.0115

.0127

.0123

.0089

materials. This fact is explained by the complex nature 

of the pebbles which generally contain quartz and 

carbonate diluents.

The coarse pebbles of the Bone Valley formation are 

more abundant in the base of the deposits than higher 

in the section. They commonly have internal unphosphatized remnants of dolomite and calcite. In addition, 

they contain clastic quartz and primary phosphate pellets and other pebble fragments. Both the primary and 

the second-cycle laminations or textures within the 

pebbles are discordant to the boundaries.

All these features, coupled with field evidence of 

stratigraphic unconformity between the Bone Valley 

formation and the underlying Hawthorn, indicate a 

record of marine transgression with many pulses of 

reworking. It seems reasonable to suppose that the 

re-exposure to sea water sustained by the reworked 

Bone Valley material allowed additional uptake of 

uranium. Thus, the uranium content of the complex 

pebble components in the Bone Valley is enhanced by 

increments of uranium during successive cycles of 

reworking. The pellets in the Bone Valley, also reworked from the Hawthorn, correspondingly contain
68 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

more uranium than the pellets in the Hawthorn. Within 

the Bone Valley, however, the pellets, as expected, contain less uranium than the pebbles as they evidently 

have not been "worked" as frequently. Thus they are 

more commonly found above the basal conglomeratic 

zone, and they lack the textural complexity of the 

pebbles.

The hypothesis of uranium enrichment during reworking can be tested by comparing the autoradiographs of a complex pebble and a pebble of first generation (pi. 11, figs. E. F). The complex pebble is not 

uniformly radioactive, in contrast to the prevailing 

uniformity of the simple pebble. Moreover, within the 

reworked pebble the most radioactive parts are the 

oldest or reincorporated materials. The strong peripheral enrichment shown by the fossil bone autoradiograph (pi. 11, fig. D) shows the distribution of uranium 

that would be expected if the pebbles owed much of 

their uranium to ground-water enrichment.

The absence of such peripheral enrichment or of any 

gradational enrichment from the surface inward, 

regardless of texture, supports the conclusion that 

uranium was not introduced by ground water. The 

intensity of radioactivity varies with the textures 

created by reworking, providing proof of uranium 

emplacement during the marine reworking. Clearly 

in itself reworking served merely to expose the material to the marine source of uranium. Fortunately, 

however, reworked materials provide a demonstration 

that increased exposure to such a source results in 

increased uranium content.

The effect of total time of exposure to sea water may 

explain differences in uranium content of similar apaties from different marine deposits. Such discussion is 

perforce speculative as it is impossible to assess the 

chemical oceanographic factors controlling the uranium 

solubility or concentration. Nevertheless, it is possible 

to develop a reasonable explanation based on available 

geologic facts. For example, the pellets of the Florida 

and the Morocco deposits are chemically alike (table 3), 

yet the nodules from Morocco contain 2 to 3 times as 

much uranium as those from Florida. The analyes of 

Morocco deposits in table 12 represent phosphatic marls 

containing clastic apatite, clay, and minor chert. They 

are practically devoid of detrital quartz and the bulk 

of their particles are of fine sand size (table 8). In contrast, the Florida phosphates are poorly sorted conglomeratic sands and clays, and their pellets are commonly coarse sand to granules in size. The Florida 

phosphates were undoubtedly much more quickly deposited than the Moroccan phosphates and in view of 

the compositional similarity of the two, the difference

in their uranium content may be largely a reflection of 

the difference in the length of exposure to sea water.

CRYSTALLO-CHEMICAL NATURE OF URANIUM IN 

APATITE

GENERAL STATEMENT

It seems clear that uranium in phosphate deposits 

cannot be accounted for by discrete uranium phases, 

by absorption, or by loosely adsorbed ions, radicals, or 

phases. This follows from several lines of evidence in 

the preceding sections:

1. Lack of uranium minerals.

2. Fluorescence of uraniferous apatite.

3. Lack of correlation of uranium content with any physical 

property such as size, magnetism, color; and inability to 

concentrate uranium by physical means.

4. Inability to selectivity leach uranium.

5. Lack of abnormal concentrations in specific petrographic 

types of apatite.

Moreover, nuclear emulsion studies of many phosphorite samples from the Bone Valley and Phosphoria 

formations have shown that the uranium atoms are 

intimately dispersed within apatite rather than locally 

segregated. Plate 13 is a photomicrograph showing the 

alpha track distribution obtained from an apatitecemented arkose from the Gas Hills area of Fremont 

County, Wyo. Concentrates of cement from this sandstone are found to contain tenths of one percent of 

uranium. The tracks shown in the photomicrograph 

are the recorded paths of alpha particles emitted during the disintegration of individual radioactive atoms. 

The intimate and homogeneous distribution of uranium 

that can be inferred from the lack of clustering or of a 

preferred pattern in the track distribution is typical of 

unaltered phosphorite. The uranium distribution has 

been illustrated with the Fremont County apatite because its high uranium content yields clearer results 

than are normally obtainable with lower grade materials, which need exposures of several months and frequently result in fogged emulsions. The Fremont 

County apatite serves the additional advantage of 

demonstrating that even in highly uraniferous apatite 

random dispersion prevails rather than local concentration.

The evidence summarized above and the nuclear 

emulsion results indicate that uranium occurs as an 

integral part of the apatite structure. The evidence 

from the aluminum phosphate zone of the Bone Valley 

formation in which uranium and calcium contents are 

parallel (fig. 9), despite the fact that one is a mere 

trace and the other a major constituent, indicates that 

uranium substitutes for calcium in apatite.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 69

SUBSTITUTION OF URANIUM FOB CALCIUM 

IN APATITE

THEORETICAL CONSIDERATIONS

The substitution of uranium for calcium is crystallographically plauisible in terms of U+4 which has an 

ionic radius (IE = 0.97 A) virtually identical to that 

of calcium (IE = 0.99 A). The problem of maintaining electrostatic neutrality in substituting a tetravalent 

for a divalent ion may be reasonably discounted in this 

case for two reasons. First, the content of uranium in 

apatite is far too slight to create any appreciable structural dislocation. Thus, a uranium content of 0.01 percent in apatite is equivalent to one atom of uranium for 

every 2362 unit cells [Cai0 (PO4 )6F2 ] or to 1 atom of 

uranium for every 23,620 atoms of calcium! Second, 

the deficiency of positive charge created by other replacements common in apatite is orders of magnitude 

greater than would be required to compensate for the 

excess of positive charge caused by the replacement of 

calcium by uranium. For example, from 0.1 to 1.0 

percent of sodium is common in apatite as a replacement for calcium, and fluorine plus hydroxyl are in 

excess in carbonate-fluorapatite to an extent of 0.5 to 

1.0 percent.

RELATION OF URANIUM TO PHOSPHORUS

The suggestion has been made that uranium ions may 

substitute for phosphate tetrahedra or the phosphorous 

ion in apatite. Several arguments militate against this 

hypothesis:

1. The absorption of uranyl by apatite in the laboratory has been found to render phosphate nonexchangeable rather than to proceed by displacement of phosphate as it does of calcium 

(Neranan and others, 1949a, b).

2. The uranous ion (IB, = 0.97 A) is too large to 

substitute for phosphorus (IB, = 0.35 A) in 

PO* tetrahedra, and it normally requires a 

much larger coordination with oxygen, as displayed by the eight-fold coordination of U(IV) 

inUO2 (Wells, 1950).

3. In the uranium-enriched aluminum phosphate 

zone of the Bone Valley formation, the uranium 

is concentrated preferentially in the calcium 

and calcium-aluminum phosphates and relatively impoverished at the top of the zone (fig. 

9) where only wavellite, the pure aluminum 

phosphate occurs. Although the wavellite 

occurs as an alteration product of apatite and 

crandallite (fig. 9), chemical analyses of pure 

wavellite reveal that very little uranium is retained by the wavellite in the course of its 

replacement of the pre-existing uraniferous 

minerals (Altschuler and others, 1956).

OXIDATION STATE OF URANIUM IN APATITE 

CHEMICAL PROBLEMS

Is the uranium in apatite tetravalent? To answer 

this question and questions concerning the industrial 

recovery of uranium, a method was developed for the 

analysis of U(IV) in apatite. The details of the method 

are treated in Clarke and Altschuler (1958); however, 

as the results are treated below, it is of interest to discuss some of the chemical problems involved in developing the method as they bear cogently on the geochemistry of uranium.

As the total quantity of uranium in most apatite is 

very small, the analyses of that portion of it which is 

tetravalent presents two problems. First, the sample 

must be dissolved under conditions that allow neither 

oxidation nor reduction of its uranium. Second, the 

minute traces of U(IV) must be separated as a uranous 

cupferrate. The separation is easily accomplished by 

the use of titanium, which also forms an insoluble 

cupferrate, and hence can be used as a collector or 

coprecipitant of the traces of uranous cupferrate 

present. The initial problem of maintaining the original valence state of uranium in solution is more vexing, 

however, as it is aggravated by the presence in apatite 

of many elements which directly or catalytically influence the oxidation state of uranium. Iron (present in 

virtually all apatite) and cerium (present in most 

igneous apatites) can, depending on their states, oxidize 

or reduce uranium. In addition, air oxidation of 

uranous solution is catalytically augmented by ions of 

silicate, molybdenum, copper, and cobalt (Pannell, 

1950), all of which may be present in apatite in trace 

quantities. Of all these, iron is the element of greatest 

concern owing to its ubiquitous occurrence and its quantity in phosphorite relative to uranium, as illustrated 

in table 20.

TABLE 20. Iron and uranium content, in weight percent of 

representative apatites from the Bone Valley formation, Florida

[Analyst, R. 8. Clarke, Jr.]

Sample

B.L. 3......

VA 5......

WA 10.....

P.V. 5......

Location Material

..... do - . -

Fe

0.26

33

.60

1.00

Fe<II)

0.14

.06

.04

.06

U

0.016

.021

.0076

.006
70 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

It is known from theoretical and experimental considerations that the iron and uranium couples can react 

in the following manner:

2Fe(II) + U(VI) ^± 2Fe(III) + U(IV)

The extent to which this interaction takes place in 

nature would be governed greatly by the pH of the 

medium, the relative concentrations of the individual 

members of the couples, and the presence of other oxidents, reductants, and stabilizing compounds, capable 

of influencing the direction of the reaction. Of considerable interest is the determination by Baes (1954) that 

the reduction of U(VI) to U(IV) by Fe(II) is greatly 

enhanced in phosphoric acid solutions at room temperature. This production of U(IV) was augmented 

by increasing the phosphoric acid concentration and 

further promoted by the presence of fluoride ion (Baes, 

1954), which forms stable complexes with both Fe (III) 

and U(IV). Schreyer (1954) has shown that uranium 

(IV) forms stable orthophosphate complexes in phosphoric acid solutions to an extent that limits air 

oxidation of U(IV).

Additional verification of the efficacy of the above 

reactions in acid solutions is obtained from experience 

in the industrial recovery of uranium as a byproduct of 

phosphoric acid in triple-superphosphate production. 

In this process, apatite is treated with sulfuric acid 

to produce phosphoric acid and gypsum. The final 

product, monocalcium phosphate is manufactured by 

treatment of additional apatite by the phosphoric acid. 

Uranium is extracted from the phosphoric acid phase 

of this process by use of alkyl pyro-phosphoric acids 

(Ellis, 1952). It has been found in the course of recovery studies that the uranium must be sequestered 

from the phosphoric acid as the tetravalent ion. To 

effect complete recovery, the acids are passed over steel 

filings or punchings prior to extraction.

Thus, in view of the ability of ferrous iron to reduce 

U(VI) to TJ(IV) and the readiness with which both 

U(IV) and Fe(III) form stable compounds with the 

fluoride and orthophosphate ions, one could produce 

U(IV) merely in the course of dissolving apatite in 

order to make the analysis!

Most of the above experimental information on the 

independent behavior and mutual interactions of 

uranium and iron in phosphoric acid systems was 

obtained at room temperature and at concentrations of 

phosphate relative to uranium and iron comparable to 

those prevailing in apatite and many natural solutions. 

The acidities in the laboratory studies, however, are so 

much greater than those prevailing in nature as possibly to invalidate the application of the results to the 

interpretation of uraniferous apatite. Moreover, as it

is impossible to define all of the variables that have 

affected the primary state of uranium in apatite, or all 

of the factors which may have altered it after emplacement, it is still necessary to determine chemically 

whether apatite contains tetravalent uranium. Furthermore, a method for this determination is also valuable 

as a means of assessing the relative quantities of 

hexavalent and tetravalent uranium during various 

stages of the industrial recovery process.

In devising a method for the analysis of U(IV) in 

apatite it was necessary to depress the oxidation and 

reduction, respectively, of the tetravalent and hexavalent uranium inherently present in apatite, and to 

establish that the analytical recovery is both high and 

consistent. Lacking proof that U(IV) actually occurs 

in apatite, experiments were performed on solutions of 

natural apatites and uranium-free synthetic apatites to 

which known spikes of uranous and uranyl solutions 

were added. Table 21 is a representative summary of 

a number of these experiments.

TABLE 21. The recovery, in weight percent, of uranium (IV) 

from 1.5 M phosphoric acid solutions of natural and synthetic 

apatite

[Assembled from Clarke and Altsehuler, 1958]

«

3. 

tn

1 

2 

3 

4

5 

6

Material

Synthetic apatite. _ .- 

Apatite pebbles (B. 

L.-3), Bonny Lake, 

Fla.._ .- - 

Apatite pebbles (B. 

L.-3), Bonny Lake, 

Fla........ _....__ ... 

Apatite pebbles (B. 

L.-3), Bonny Lake, 

Fla._._..._....._._._

Initial 

uranium

Total 

U

0.016

.016 

.016

U

(IV)

0.012

.013 

.013

Uranium 

added

U 

(IV)

0.050

.048

.050

.048 

.048

U

(VI)

0.030

.......

Iron added

Fe 

(II)

0.100

.100

Fe 

(III)

0.100

.100

U(IV) determined

0.050 

.050 

.0025

.061 

.060 

.056

It was determined (table 21, samples 1 and 4) that 

uranous uranium can be recovered from apatite. By 

additions of ferrous and ferric iron it was further 

determined that the reduction of uranyl (table 21, 

samples 3 and 6) and the oxidation of uranous uranium 

(table 21, samples 2 and 5) are both negligible. It 

should be noted that the percent of recovery of U(IV) 

in table 21 is virtually complete in all cases where 

U(IV) was added, and that it is less than 10 percent 

where only U(VI) was added (table 2i, sample 3).

A further demonstration that the U(IV) values 

obtained are not merely governed by the experimental 

conditions is the production of more U(VI) and less 

TJ(IV) by treatment of the sample prior to analysis.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 71

Sample B.L.-3 was heated at 600° C for 4 hours in air. 

This treatment would be expected to oxidize at least 

part of the U(IV) present. A comparison of values 

obtained before and after treatment is given in table 22. 

Thus, by changing the inherent U(IV) in a predictable 

manner prior to analysis, and confirming the change by 

recovery, independent proof is had that the inherent 

U(IV) normally obtained is real, and not a function 

of the method of recovery.

TABLE 22. Percentages of uranium and of U(IV) in sample 

B.L. 3 before and after oxidative heating

[Analyst, R. S. Clarke, Jr.]

Sample

B.L.-3__ 

B.L-3^

B.L.-3__

Condition

Unheated 

Heated at

Heated at

450°

600°

C, 4 hours.

C, 4 hoursTotal

0.016 

.016

.020

U(IV)

0.013 

.0085

[ 2 . 002 

{ 3 . 002

Percent 

reduced 

U

81 

'53

I »

1 Total U not determined on heated sample. Percent reduced U thus slightly high 

due to being uncorrected for weight loss on heating. 

' HjPO4 solution of sample. 

»HCl-NHjOH.HCl solution of sample.

Having established the validity of the method, the 

consistency of the results and the applicability of the 

method to a variety of materials was next studied. It 

was found that igneous fluorapatite, due to its much 

greater crystal size and its lack of carbonate, did not 

dissolve as readily as sedimentary apatite. As a result 

two different methods of solution have been used. Sedimentary apatite is dissolved in cold 1.5 M phosphoric 

acid and igneous apatite is dissolved in cold 1.2 M 

hydrochloric acid containing 1.5 percent hydroxylamine 

hydrochloride. The U(IV) recovery from phosphoric 

acid is virtually complete, as demonstrated in table 21. 

The U(IV) recovery from hydrochloric acid was established from spike experiments to average 79 percent. 

The range in percent recovery was from 67 to 87 percent ; however, the standard deviation from the average 

of 79 percent was found to be only 5.4 percent (Clarke 

and Altschuler, 1958). Therefore, in analyses based on 

hydrochloric acid solutions a 20 percent correction will 

be applied to the value obtained, for a close approximation of the real value. It was found (Clarke and 

Altschuler, 1958) that materials with more than trace 

amounts of oxidizing or reducing elements, like cerium 

or iron, may not yield good results.

TT(IV) CONTENT OF SEDIMENTARY APATITES

The TJ(IV) contents obtained on a variety of sedimentary and igneous apatites are given in tables 23 

and 27.

The analyses in table 23 reveal a wide spread in the 

percentage of tetravalent uranium in apatite. The 

remainder, in all cases, is safely presumed to be hexavalent in view of the instability, and the absence in 

natural substances, of tervalent and pentavalent 

uranium (Garrels, 1955) and of the fact that monovalent and divalent uranium are competely unknown in 

natural solutions (McKelvey and others, 1955).

Several prevalent notions are open to serious question as a result of these analyses. It was heretofore 

generally assumed that the traces of uranium are of 

the same chemical nature in all apatite deposits; that, 

once fixed in apatite, uranium is not changed except 

insofar as the host mineral is altered; and that the 

uranium within any single specimen of apatite is 

entirely hexavalent or tetravalent. These assumptions 

must all be discarded.

Despite the variety displayed in table 23, the distribution of U(IV) proportions is not haphazard and 

several important generalizations emerge if the data 

are regarded in the light of geologic occurrences.

U(IV)/U PERCENTS REFLECT GEOLOGIC HISTORY

Initially, to establish the validity of subsequent inference, it should be noted that materials with identical 

histories yield identical results. Thus, samples Ho-14 

through Ho-17 in table 23 are composites of the same 

petrographic fraction from each of four successive beds 

in the Bone Valley formation. The particular strata 

are thin and therefore have the same general relation 

to the zone of aluminum phosphate alteration higher 

in the section. Although the total uranium content 

varies by as much as 100 percent among the samples, 

the percentage of U(IV) is virtually identical throughout, within the limits of the fluorimetric analyses of 

uranium.

The reflection of geologic history in the U(IV)/U 

percentages is further illustrated by several other 

groups of samples. Nodules from several localities in 

the Pacific Ocean off the coast of southern California, 

have sustained different degrees of submarine reworking and are of somewhat composite age, having also 

sustained accretion to differing extents since Miocene 

time. They are similar but not identical. Likewise 

their U(IV)/U percentages are similar but not identical. The apatite pellets from the Hawthorn formation 

were collected from two beaches along the central westFlorida coast. Both samples represent present-day 

marine reworking of Miocene apatite. Their U(IV)/U 

percentages are virtually identical.
72 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TABLE 23. Total and tetravalent uranium content of sedimentary

apatites

[Analyst, E. 8. Clarke, Jr.]

Sample Material Total U 

(percent)

U(IV) 

(percent)

U(IV) 

U 

percentage

Bone Valley formation (Pliocene)

B.L.-l.....

B.L.-2 

B.L.-3..... 

Ho-14 . 

Ho-15. ...

Ho-16 

Ho-17.. 

Va-7 

Va-7a ...

WA-10 .... 

PV-5......

.....do. ......... ........,-.- ...

 do 

..... do..... 

 do... .

. do 

__ .do... ___ . _____ . .... ... .

0.0011 

.0089 

.016 

.032 

.022 

.015 

.021 

.011 

.009 

.0075 

.007

0.010 

.0056 

.013 

.016 

.011 

.007 

.010 

.005 

.006 

.003 

.003

91 

63 

81 

50 

50 

42 

48 

45 

67 

40 

43

Hawthorn formation (Miocene, recently reworked on Gulf Coast beaches)

QP-1......

GP-2. ..... do . 

0.0067 

.0059

0.0039 

.0037

58 

63

Tennessee phosphates, Bigby formation (Ordovician)

AV_o

Ak-1 ... . Brown-rock phosphate (Cenozoic 

weathering).

0.00004 

.00074

0.00001 

.00002

25 

3

Moroccan phosphates (Senonian)

Mor-3.....

Mor-11 ....

 do 

.....do .... ..... ......... ...

0.012 

.012 

.008

0.0018 

.0015 

.0006

15 

13 

8

Phosphoria formation (Permian)

Conda-300. 

KAH-185-. .... .do... ..... . ..

0.0063 

.017

0.0015 

.003

24 

18

Pacific Ocean, southern California (post-Miocene)

fiQ

106 .

127 

158 ...

162.........

183.. ....

..... do ... .... ...

..... do... .... ......... 

.. do.... . _ 

 do.. 

.... . do...... ...... . - 

0.0089 

.0063 

.0041 

.0081 

.0125 

.0051

0.0061 

.0040 

.0028 

.0050 

.0093 

.0028

69 

59 

68 

62 

74 

56

Fossil bone

1 .......

RW-4468--

W-3841 .... .... .do... ... ... ... ....... ..... ... ....

0.015

\ .078

J .071 

.85

.82

0.0004

.054

.050 

i.40

.50

3

J 70-76

61

Phosphatic arkose

Gas Hills, Wyo... .................. 0.74 «O.J4

.17 23

i Determination from HCI solution; uncorrected for 20 percent deficiency in recovery.

PRIMARY MARINE URANIUM IN YOUNG PHOSPHORITES 

IS MAINLY U(IV)

In marine materials that have not been postdepositionally oxidized and that are of relatively young age, 

more than half of the uranium is tetravalent. The 

nodules from the Pacific serve as an example as they 

have been interpreted by Dietz, Emery, and Shepard 

(1942) to have been submerged since their formation 

in post-Miocene time. Samples B.L.-1, B.L.-2, and 

B.L.-3 (table 23), in which 63 to 91 percent of the 

uranium is tetravalent, were collected from clayey, 

impermeable strata in the basal conglomerate zone of 

the Bone Valley formation. Sample B.L.-l has minute 

pyrite crystals and petroliferous organic matter dispersed throughout; however, the clay in which it rested 

was devoid of organic matter and pyrite. The pebble 

thus reflects the reducing conditions of its time and 

environment of formation and has not been postdepositionally weathered. Its content of U(IV), 91 percent 

of the total, is the highest we have found. Samples 

B.L.-2 and B.L.-3 are from higher strata, contain less 

organic matter, are lighter colored than B.L.-1, and 

have no pyrite. Less of their uranium is tetravalent.

SUBAERIAL WEATHERING ALTERS TJ(IV) TO U(VI) IN 

MARINE PHOSPHORITE

Material that is obviously weathered contains proportionally less U(IV) than comparable or equivalent 

unweathered material. This may be illustrated by comparing the U(IV)/U percentage obtained from the 

slightly phosphatic limestone of the Bigby formation 

with the much lower value obtained from the equivalent 

"brown-rock" phosphates derived from the Bigby by 

subaerial residual concentration. See table 23 and fig. 

12 for relations of the samples.) A second illustration 

is obtained by comparing samples Ho-14 through 

Ho-17 with samples B.L.-1, B.L.-2, and B.L.-3. The 

two groups were collected from different mines (Homeland and Bonny Lake) and from different parts of the 

stratigraphic column and, hence, are not precisely 

equivalent. They are comparable, however, in their 

common origin, in their textural complexity, and in 

their overall composition. The Homeland samples were 

collected from an oxidized zone in the middle of the 

Bone Valley formation. Their associated clays are 

mottled and limonite stained, and the pebbles constituting the samples are light colored, and somewhat 

friable in contrast to the dark pebbles from the green, 

clayey, conglomerate at the base of the Bonny Lake 

mine. The appreciably lower U (IV) percentages in the 

Homeland materials demonstrate that some uranium 

may be oxidized in place in the host apatite. The 

enormous internal porosity of most phosphorite
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 73

(Hendricks and Hill, 1950; Jacob and Hill, 1953) 

would greatly facilitate such oxidation, which, in fact, 

is visible in the form of limonite impregnations and 

micro-concretions dispersed throughout the apatite 

pebbles.

POSSIBLE RELATION OF TT(VI) CONTENT TO RADIOACTIVITY 

AND AGE

It may be that the U(IV)/U percentages in marine 

phosphorites are a function of radioactive disintegration and thus, indirectly, of age. This hypothesis presupposes that all, or most, of their uranium is initially 

TJ(IV), a suggestion favored strongly by the analyses 

of samples from the Bonny Lake mine and of the 

Pacific Ocean nodules. The thesis further demands 

that the U(VI) present is produced by auto-oxidation, 

or by some secondary effect whose onset is governed, 

both as to rate and intensity, by the progress of radioactive decay. If the U(VT) content of apatite is related 

to age, rather than an independent result of oxidative 

weathering, it should be possible to demonstrate a progressive increase in U(VI) content in a series of progressively older apatites. Such tests may soon be 

practicable for igneous apatites but their application 

to phosphorites is unfortunately prevented at present 

by the difficulty of obtaining materials assuredly free 

of oxidation from normal weathering. In addition to 

the difficulty of obtaining such materials, the few criteria at our command for establishing such freedom 

from oxidative weathering may be relatively insensitive. Such criteria are the presence of syngenetic 

glauconite, pyrite, and organic matter, or the presence 

of reduced ions in the apatite. Earely, it may be possible to develop preclusive geologic evidence, such as a 

history of continual protective burial.

In exploring the possible relations of U(VI) content 

to age in phosphorites, only three groups of samples 

may be compared with a modicum of assurance that 

they have not been weathered. They are (table 23) the 

Pliocene Bonny Lake pebbles, which contain pyrite and 

organic matter and were found in green clay beds; the 

Quaternary Pacific Ocean nodules, which have remained submerged since their formation; and possibly 

the pellets from the Permian Phosphoria formation, as 

these contain minor amounts of organic matter. It may 

be significant that the two Phosphoria samples show 

considerably lower U(IV)/U percentages than the 

other two groups, and that the two latter groups are 

moderately close in value. However, the extreme view 

that all of the uranium in apatite is initially U(IV) 

is not supported by the data. Although, in some samples (as in the Bonny Lake pebbles), favorable reducing conditions may cause substantially all the

uranium in apatite to be taken up as U(IV), the existence of significant amounts of U(VI) in the young and 

recently dredged Pacific Ocean nodules suggests that 

the initial U(IV)/U(VI) ratio in apatite is influenced 

by the oxidation conditions in the marine environment. 

It is also possible however that submarine, postdepositional oxidation can lower the initial U(IV)/U(VI) 

ratio.

SECONDARY SOURCES OF U(VI) IN PHOSPHORITE

Only three analyses for U(IV) have been made on 

fossil bone; however, the spread in relative proportions 

of U(IV) and U(VI) (table 23) is indicative of the 

diversity that might be expected in view of the conceivable variations in the composition of the transporting ground water from which the uranium is emplaced, 

in the environment of deposition, and in the state of 

degradation of the organic matter that may be associated with the bones.

Although it is true that apatite readily abstracts 

U(IV) from solution, in a strongly oxidizing environment the mere presence of apatite is not necessarily 

effective in producing or even retaining U(IV). To 

the contrary, it was found that samples of Morocco 

apatite pellets and fossil bone that are richly encrusted 

by hematite have the capacity to oxidize uranous ions 

in phosphoric acid solutions, as the experiment in table 

24 demonstrates. Iron free synthetic and natural apatite used as "controls" do not cause such oxidation.

TABLE 24. Experiment showing reduction of U(IV) by phosphoric 

acid solutions of hematite-rich apatite samples

[Analyses in weight percent. Analyst, R. S. Olarke, Jr.]

Material

Initial uranium

Total U

0.016 

.008 

.015

U(IV)

0.013 

.0009? 

.0004?

U(IV) 

added

0.050 

.050 

.050 

.050

U(IV) determined

0.050 

.061 

.046 

.020

The data given in table 24 show that the complexing 

capacities of phosphate and fluoride ions for Fe(III) 

and U (IV) respectively are not adequate to prevent the 

oxidation of U(IV) in the presence of excess ferric 

iron. As this is true in solutions of freely mobile ions, 

it indicates that slowly dissolving apatite in the environment of such reactions would not prevent the 

oxidation of U(IV) by excess ferric iron. They also 

show that the very low values for U(IV) in samples 

Mor-3, 5, and 11 and Hell Creek 1 (table 23) may be 

due to oxidation effects during analysis, although it is 

to be expected that they would be low inherently. In
74 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

any case the method is not trustworthy for materials 

with more than trace quantities of oxidizing impurities. 

It is clear from the above experiment, however, that 

high U(VI)/U percentages may be anticipated where 

uranium is emplaced primarily from ground water.

The phosphatic arkose sandstone from the Gas Hills 

region of Fremont County, Wyo., is a striking illustration of secondary uranium fixation by apatite. X-ray 

powder diffraction analyses show the material to be a 

carbonate-fluorapatite. Deposits of this material occur 

as cements scattered through the arkosic sandstones and 

conglomerates of the upper part of the Wind Eiver 

formation.5 Associated with them are "paints" of 

autunite and other secondary uranyl minerals; however, the sample used was shown by nuclear emulsion 

study and inspection under ultra-violet light to be free 

of uranium minerals. The apatite cement is attributed 

to secondary phosphatization, and the uranium is 

attributed mainly to secondary emplacement in the 

porous cement (R. G. Coleman, oral communication, 

1956). This interpretation is supported by the occurrence of the uranyl phosphate and vanadate minerals 

throughout the region.

The alpha-track film of the specimen (pi. 13) reveals 

intimate and homogeneous dispersion of uranium 

throughout the cement, and a virtual absence of any 

uranium atom concentrations. It cannot be determined 

whether much or most of the uranium was originally 

fixed as U(IV) and subsequently oxidized or whether 

the U(VI) now present was emplaced substantially as 

uranyl. The local abundance of uranyl minerals and of 

limonite films in rocks other than phosphorite indicates 

that oxidizing solutions have permeated the area and 

have introduced uranium as uranyl.

The material emphasizes that U(VI) like U(IV) 

can be thoroughly dispersed in apatite and may be 

emplaced in a manner analogous to that demonstrated 

by Neuman and his coworkers (1948, 1949a, b). They 

have shown that glycol-ashed bone will fix appreciable 

quantities of uranyl ion by exchange with surface 

calcium. This reaction was found to be surface limited 

(Neuman and others, 1949b) and involved the displacing of 2 moles of calcium for every mole of 

(UO2 )+2 taken from solution and the rendering of 2 

moles of phosphate nonexchangeable. They suggest the 

formation of a compound analogous to uranyl pyrophosphate to explain these results.

6 All information on occurrence has been obtained from R. G. Coleman, who kindly supplied the specimen, the second most uraniferous 

known to us.

Regarding the form in which uranyl complexes with 

apatite, several types of compounds should be considered. Uranyl pyrophosphates, or uranyl orthophosphates of the torbernite type could both be stable within 

the range of conditions presumable (oxidative, weakly 

alkaline to weakly acid). In addition, complexes of the 

type UO2HPO4 could presumably form by surface reaction with apatite; F. S. Grimaldi (oral communication, 

1955) verifies that UO2HPO4 could be stable under 

such conditions. We believe that uranyl orthophosphate 

compounds are more probably the form in which primary U( VI) would occur in phosphorites. These would 

not require the polymerization entailed in pyrophosphate formation and would avoid the anomalous exchange of 2 divalent calciums for 1 divalent uranyl 

group. It is quite possible that an appreciable part of 

the U(VI) present in unweathered phosphorites is 

chemisorbed on the surfaces of growing apatite crystallites and, upon the complete development of the apatite 

nodules, the U(VI) appears to be intimately and randomly dispersed throughout the apatite. Sheldon 

(written communication, 1957) also favors the chemisorption of some hexavalent uranium in the rocks of 

the Phosphoria formation.

RELATIVE STRUCTURAL FAVORABILITY OF U(IV) AND 

tJ(VI) IN APATITE

The above mechanisms have one characteristic in 

common. Their surface-limited nature is virtually predictable in view of the asymmetry and large size of the 

uranyl anion (3.4 X 1.4A, Connick and Hugus, 1952) 

relative to the calcium ion (IR 0.99 A). Thus, 

although the reactions so clearly documented by Neuman and his colleagues involve ion exchange, the results 

obtained indicate extensive structural rearrangement. 

It is therefore difficult to visualize any intra-crystalline 

solid solution by such a mechanism, first, because of 

the structural dislocation involved in formation of 

pyrophosphate and displacement of two calcium for 

one uranyl ion, and second, because the large uranyl 

anion could not migrate to any extent through the 

apatite structure. The channels within the hexagonal 

network of the fluorapatite structure, being barely wide 

enough to accept the fluoride ion (IR = 1.36 A, Green, 

1953), could not accommodate the uranyl anion. Sixty 

percent of the calcium ions in apatite line the vertical 

channels and would thus be inaccessible to postdepositional substitution by uranyl. On the other hand, the 

U(IV) ion could fit easily, and migrate a short distance 

within the channels, which are not as fully occupied
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 75

as the walls and hence permit greater mobility to 

migrating ions. That such migration occurs is fully 

substantiated in the replacement of (OH) by F that 

takes place in f ossilization.

Structural considerations thus dictate that much 

more calcium is accessible for replacement by U(IV) 

than by U( VI). Furthermore, the U(IV) substitution 

does not involve any structural dislocation and the 

positive charge excess that would result is easily compensated by other known replacements of greater magnitude. It is not unusual, therefore, that samples as 

uraniferous as RW-4468 and W-3841 (table 23) should 

have as much as 70 and 61 percent respectively of their 

uranium in tetravalent form, despite the fact that 

ground water may be presumed to carry mainly uranyl 

ions.

TEST OP URANIUM AND FLUORINE INTERDEPENDENCE IN APATITE

There are several compelling reasons for detailed 

examination of the relations between excess fluorine 

and uranium in marine apatite, as follows:

1. Excess fluorine is characteristic of marine sedimentary 

apatite, in contrast to normal igneous fluorapatite.

2. Marine apatite is characteristically more uraniferous than 

igneous apatite.

3. UF4 could be stable tinder the pH and Eh conditions of 

formation, and of subaerial preservation, of marine 

phosphorite. Although the formation of independent 

fluoride minerals is conceivable only under conditions 

of much greater salinity than those characterizing 

marine apatite formation (Kazakov, 1937), the possibility of a complex similar to UF4, as an integral part 

of apatite, is strongly suggested by the following: 

A. The calcium ions lining the channels in the apatite 

structure are located in the same horizontal plane 

as the adjacent fluorine ions.

B. These channel calciums are coordinated to fluorine 

and oxygen (fig. 3), and as the excess fluorine could 

reasonably replace oxygen, or lodge within the 

channels in the apatite structure, the existence of 

UF4 in apatite may be reasonably postulated, particularly as U*4 may be considered to replace 

calcium.

4. The postdepositional uptake of fluorine and uranium by 

various forms of sedimentary apatite and bone involve 

separate replacements; however, their concomitant 

operation may mask, or be augmented by, their mutual 

complexing ability.

A general correlation between total fluorine and total 

uranium was demonstrated in studies of seven Pennsylvanian phosphatic shales of Kansas by Runnels and 

others (1953). In all likelihood this reflects only the 

fact that uranium content varies directly as apatite 

content in unaltered rocks of similar composition in

which apatite is the only, or major, uranium host. 

Similar relationship could be expected in a group of 

phosphatic limestones and has been demonstrated for 

Mona Island rocks (fig. 3) and in some suites of rock 

from the Phosphoria formation (table 9).

The possible genetic relation between excess fluorine 

and U(IV) is marine apatite should be tested in two 

ways. On the assumption that all of the uranium was 

originally fixed as U(IV), excess fluorine should be 

compared with total uranium. Figure 13 is a scatter 

diagram on which total uranium is plotted against 

excess fluorine,6 as computed from a random selection 

of 50 of the analyses given by Thompson (1953, 1954) 

for Phosphoria samples from Trail Canyon, Utah; 

Coal Canyon, Wyo.; and Reservoir Mountain, Idaho. 

It can be seen that total uranium is independent of 

excess fluorine in rocks of the Phosphoria.

Assuming that only the U(IV) demonstrably present 

in apatite was emplaced as such, a comparison was 

made (fig. 14 of the few data available (table 25)).

The wide scatter of the points in figure 14 indicates 

that U(IV) and excess fluorine are independent of one 

another in marine phosphorite. The data are admittedly 

few; however, the well-defined trends that can be demonstrated to exist among total fluorine, P2O5 , and 

uranium, for groups of samples no larger than this 

(fig. 5, and Runnels and others, 1953) exemplify the 

extent of mutual dependence that could be expected 

between F and U(IV) if uranium occurred in apatite 

as a result of UF4 formation.

EFFECTS OF REWORKING ON U(IV) CONTENT 

OF APATITE

The demonstrated secondary enrichment of uranium 

in apatite during the course of marine reworking allows 

the establishment of controls for studying the nature 

of the uranium extracted from the ocean source. If the 

uranium additions incident to reworking are mainly 

U(IV), the U(IV)/U percentages should be higher 

in the reworked materials of a deposit. Samples were 

taken in the phosphatic terrace gravels in the vicinity 

of Charleston, South Carolina, for tests of this thesis. 

The phosphatic fraction of the samples consisted of 

sand-size pellets and detrital bone apatite, mainly 

water-worn shark tooth fragments, plus rounded 

granules and pebbles of bone and phosporite. In addition, the samples contained quartz and shell fragments. 

The coarser material was found to be texturally more 

complex, indicating more reworking. Accordingly, each

6 Based on 3.8 percent of fluorine in pure fluorapatite and 38 percent 

P2OS in carbonate-fluorapatite. Therefore excess fluorine is amount in 

excess of theoretical F/P2O5 weight ratio of 1/10.
76 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

0.06

.05

UJ 

Q_

X

g

UJ

Z>

o

.03

.02

.01

0.1 0.2 0.3 0.4 0.5

EXCESS FLUORINE, IN WEIGHT PERCENT

FIOURB 13. Uranium vs excess fluorine in Phosphoria formation (data recomputed from Thompson, 1953).

0.6
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 77

0.014

.012

2 .010

LU

U

or

LU

°- .008

.006

> .004

.002

TABLE 26. Relative contents of U, U(IV), and PgOg in phosphatic 

terrace gravel in the vicinity of Charleston, S. C.

[Analyst, R. S. Clarko, Jr.]

0.1 0.2 0.3 0.4 0.5 0.6 0.7

EXCESS FLUORINE, IN WEIGHT PERCENT 

FIGURE 14. U(IV) vs excess fluorine in various phosphates.

TABLE 25. Data on excess fluorine and U(IV) in miscellaneous 

apatite and phosphorite

[All analyses from U. S. Geological Survey Laboratory; F and P2 0son Pacific nodules 

are given in Dietz, Emery, and Shepard (1942)]

Sample

P V-5- - 

Wa-10- 

BL-3-

Mor-11 

69 

106-. 

127 

158 

162 

183 

Material

Apatite 

pellets, 

 do 

 do 

 do _

Phosphate 

nodule.

 do ...

 do ...

 do 

 do 

 do _

Locality

Peace Valley 

mine, Fla. 

Watson mine, 

Fla. 

Bonny Lake 

mine, Fla. 

Khouribga, 

Morocco. 

Pacific 

Ocean.

 __do -

 .do... 

 do 

.. do .

 do 

Percent

F

3.8 

3.8 

3.8 

4.05 

3.31

3.12 

3.07 

3.15 

2.47 

3.36

P2 06

34.5 

36.6 

34.9 

33.4 

29.56

29.19 

28.96 

29.09 

22.43 

29.66

Excess 

F

0.3 

.1 

.3

.7 

.35

.20 

.17 

.24 

.23 

.39

U(IV)

0.0027 

.0029 

.013 

.005 

.006

.004 

.003 

.005 

.009 

.003

Remarks

Bone Valley formation. 

Do.

Do.

Average sample, 

daily production. 

Sample members, 

these of Dietz and 

others (1942). 

Do. 

Do. 

Do. 

Do. 

Do.

sample was split into two fractions on the basis of size 

and analyzed for PzOs, U, and U(IT). The results are 

given in table 26.

The samples from each locality were fractionated by 

screening and the obvious shell fragments were handpicked from the coarse fraction in each case. It was not 

practical to remove shell fragments from the fine fraction. Table 26 shows that the coarse fraction contains 

less PzOz at all 4 localities and more uranium in 3 of 

the 4. The lower PgOs reflects the greater textural

Sample

U 

Material (percent)

P2 0B (percent)

TJX1000

P2 05

TJ(IV) (percent)

U(IV)

U 

percentage

Accabee Flats, Ashley River, Charleston, S. C.

Acc-1 _ 

Acc-2 _

0.020 

.020 

.018

28.4 

31.7 

30.2

0.70 

.63 

.60

0.016 

.013 

.013

80

65 

72

Micah Jenkins Nursery, Johns Island, S. C.

Micah-1-

Micah-1 Medium to fine sand ___ ._

.027 

.015

27.6 

31.2

.98

.48

0.021 

.011

78 

73

Ninemile Station, S. C.

Nme-1 - 

Nine-2.. .

.037 

.021

27.5 

31.4

1.35

.67

0.025 

.010

68 

48

Lambs, S. C.

Lamb-1 __ 

Lamb-l 

.015 

.025

28.6 

31.5

.52 

.79

0. 0097 

.012

65

48

complexity and greater percentage of inclusions resulting from reworking. The generally higher uranium 

content reflects the enhancement of uranium during 

reworking. The fact that in all samples the U(IV)/U 

percentages are significantly higher in the reworked 

materials proves that the additional increments of 

uranium are largely tetravalent.

SUMMARY OF EVIDENCE BEARING ON THE PRIMARY 

STATE OF URANIUM IN MARINE PHOSPHORITES

The theoretically plausible occurrence of U(IV) in 

apatite has been confirmed in these investigations by 

demonstrating that TJ(IV) actually does occur and 

indeed is the predominant ion species of uranium in 

some phosphorites. In addition, considerations of the 

crystal chemistry of apatite lead to the conclusion that 

structural emplacement of (U02) +2 is most probably 

surface-limited and, therefore, not as favorable as structural emplacement of the U+4 ion. Nevertheless, the 

fine-grained nature of most phosphorite results in an 

exceptionally large inherent surface area (Hendricks 

and Hill, 1950; Jacob and Hill, 1953) which can accommodate all of the uranium in typical phosphorite as 

chemically adsorbed uranyl, as has been pointed out 

by Hendricks and Hill (1950) and Sheldon (written 

communication, 1957). It is thus important to examine 

the significance of the wide range in the U(IV)/U
78 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

percentages of marine phosphorites (table 23). Do the 

data accurately represent the primary state of the 

uranium deposited in apatite?

In answering the above question we must consider 

laboratory and field evidence bearing on the relative 

stabilities of U+4 and UO2+2 under subaerial exposure. 

Garrels has shown (oral communication, 1955, and 

McKelvey and others, 1955) that the reduction of 

uranyl to uranous uranium can be effective at 25° C 

and a pH of 4 or higher, for small amounts of uranyl 

and a wide range of ferrous iron concentration. The 

previously cited stabilizing effect of phosphate and 

fluoride on this reaction can appreciably increase the 

production of U(IV). It is also probable that U(IV) 

may be produced from U(VI) by the oxidation of 

organic matter or H2S. Both Gruner (1952) and Miller 

and Kerr (1954) have successfully precipitated UO2 

by such reactions at low temperature and pressure. 

The presence of apatite in the milieu of such reactions 

would fix the U(IV) so produced, by ion exchange 

with calcium. Beyond this, by its extracting action, 

apatite would actually cause further production of 

U(IV) for its own fixation, as it would disturb the 

achievement of equilibrium. This action would be even 

more pronounced than the stabilizing effect of complexing with phosphate or fluoride in solution as 

equilibrium may be reached in the latter case.

All of the above reactions favoring production of 

U+4 presuppose the interaction of uranium with 

another reducing couple. On the other hand the relative instabiilty of U+4, in the form of UO2, to air 

oxidation is revealed by the many secondary minerals 

associated with vein deposits of uranium and the fact 

that uraninite or pitchblende is virtually never pure 

UO2, but more commonly UsOg, or some intermediate 

value. We have shown experimentally (table 24) that 

apatite pellets encrusted with iron oxide have the 

capacity to oxidize U(IV) in solution to uranyl. From 

the above evidence, as well as the fact that (UC^)"1" 2 is 

the dominant ion under most natural conditions (Garrels, 1955), it is reasonable to expect that weathering 

causes a lowering of the initial U(IV)/U(VI) ratio 

in marine phosphorites. The very features of excessive 

porosity and surface area that permit contemplation of 

adsorbed (UO2) +2 in phosphorites also permit thorough permeation by air and oxygen-bearing water. 

Such extensive contact with oxygen could effect appreciable alteration of U+4 to (UO2 ) +Z, despite the fact 

that other, more obvious, weathering changes might not 

occur. External oxidation would be augmented by the 

disruptive effects of radioactive disintegration on the 

crystal structure and by the liberation of oxygen from

water as a result of apha particle bombardment 

(Ellsworth, 1925).

Conclusive evidence that weathering has lowered the 

initial U(IV)/U(VI) ratio is obtained in the Tennessee phosphorites. There, the residually accumulated 

"brown-rock" ores maintain essentially the same 

PaOs/U ratio as the underlying parent limestone (table 

18), thus proving that uranium has not been added or 

subtracted. In contrast the U(IV) content has been 

lowered from 25 percent to 3 percent of the total 

uranium (table 23).

The fact that the highest U(IV)/U percentages (and 

therefore the highest U(IV)/U(VI) ratios) are obtained from youngest and least weathered rocks becomes doubly significant in view of the above evidence. 

The high U(IV) contents of pebbles from the basal 

Bone Valley formation of Pliocene age and of nodules 

dredged off the coast of California (p. 72 and table 23) 

suggest that the uranium fixed by apatite from marine 

waters is largely tetravalent and that long exposure has 

allowed oxidation to lower the U(IV)/U perventages 

in the Phosphoria and Bigby formations.

Few samples of the Phosphoria and Bigby formations have been studied and the hypothesis of initially 

higher U(IV)/U(VI) ratios must be tested with more 

data and specific comparisons of weathered and obviously unweathered materials. It can be urged in support of the idea, however, that McKelvey and Carswell 

(1956) infer a history of weathering from changes in 

PsOs and uranium contents in various exposures of the 

Phosphoria formation, and they note unquestionable 

instances of weathering in other exposures. In addition, it is difficult to assume that the U(IV)/U percent 

of 25, for limestone of the Bigby formation in the Tennessee phosphate field, represents the initial U(IV) 

content, and not a lowered value, superimposed by 

oxidative weathering. In view of the striking field 

evidences of weathering in the origin of the Tennessee 

"brown-rock" ores (pi. 12) and the substantial decrease 

of their U(IV)/U percentage from 25 to 3, it is highly 

probably that the immediately underlying parent limestone, the Bigby formation, was also altered and that 

its primary U(IV)/U percentage was appreciably 

higher than 25.

It is not possible to suggest how much of the primary 

uranium taken up by marine apatite was U(IV). It is 

clear however that significant amounts of U(VI) are 

present initially, even though U(IV) may be the dominant species, and that the primary U(IV) /U( VI) ratio 

in apatite is undoubtedly influenced by the Eh of the 

marine waters.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 79

URANIUM IN IGNEOUS APATITE

GENERAL STATEMENT AND ANALYSES

In accordance with Goldschmidt's empirical rules, 

U(IV) should be preferentially emplaced or "captured" in apatite due to its size identity with the Ca+2 

ion and the greater binding capacity of its higher 

charge. Considering the existence of such preferred 

emplacement in sedimentary apatite (see p. 59), and 

also the progressive increase in uranium content in the 

younger products of differentiation (Larsen and Phair, 

1954; Adams, 1954), apatite might be expected to contain more uranium in the more felsic members of an 

intrusive series. Unfortunately, too few groups of 

related intrusive bodies have been investigated extensively enough to test such generalizations. To demonstrate the type of progressive concentration that might 

be a consequence of capture, it is necessary to compare 

the uranium contents and the relative amounts of apatite and other uranium-bearing minerals in each rock 

type, and also the uranium content and the relative 

volumes of the different rock types. It is, in effect, the 

problem of illuminating the total budget of uranium 

before one can understand individual expenditures of it. 

The determinations of U and U(IV) in apatite require as much as 100 milligrams of pure mineral. To 

obtain this quantity of a small accessory mineral large 

samples of representative rock specimens must be carefully processed through several time-consuming operations. The paucity of uranium analyses for apatite in 

the literature reflects the prohibitive nature of the 

task. The analyses 7 given in table 27 are hardly representative of all igneous apatite. They, nevertheless, 

give some idea of the range and typical values for 

uranium content. The apatites, all cases, were optically 

clear, subhedral to euhedral, and contained few inclusions. An exception is the apatite from Mineville, N. Y., 

which is not properly igneous in origin, is exceedingly 

high in rare earths, and has many inclusions (McKeown 

and Klemic, 1956). Excluding the unusual Mineville 

material, all the apatites fall within the range of 0.001 

to 0.012 percent of uranium. Most of them fall within 

the narrower limits of 0.004 to 0.008 percent of uranium. 

The rocks sampled come mainly from a few batholiths 

and the selection is heavily weighted in favor of intermediate "acid" igneous types. As exemplified by the 

Boulder Creek batholith, they reveal the fact that the 

range in uranium content of apatites from a group of 

related rocks is not large in contrast to the dispersion 

that may obtain in sedimentary deposits. Significantly,

TABLE 27. Uranium and U(IV) content of igneous apatites

[TJ(IV) was determined after solution of sample In HCl-NHjOH-HCl solution. 

Values shown are corrected to Compensate for the 80 percent recovery by this 

method (Clarke and Altschuler (1958). Analyst, R. S. Clarke, Jr.]

Sample Material Area

Total TJ (percent)

U(IV) (percent)

U(IV)

U

percentage

Boulder Creek batholith

GP-17--

OP-WOP-^ 

np-i9Q

GP-49 

GP-50 .. 

OP-61 .. 

OP-63- 

OP-61 

Quartz monzonite.

Quartz monzonite. 

Quartz monzonite.

GoldHffl, Colo.... ...

West of Boulder, Colo.

South Beaver Creek,

Colo. 

West of Tremont 

Mountain, Colo. 

West of Tremont 

Mountain, Colo.

Springs, Colo.

O ftflfiQ

.0053

.0078

(YUQ

.0011

.0048 

.0044 

.0023

.0005

n rwiQ

.0012

.0019 

.0012

.00030

.0011 

.0012

(WKtl

n.d.

28

23

24 

24

27

23

27 

10

'Silver Plume" dikes intrusive into Boulder Creek batholith

OP-28. 

OP-41 ~

OP-100 

Granite dike.. . 

.....do . .......

 ..do ... - 

Gordon Gulch, Colo.-

Flagstafl Mountain,

Colo.

0.00044 

.0022

.0017

0.00014 

.00027

.00039

32

12

23

Idaho batholith

CPR-117-

CPR-118-

CPR-119-.

CPR-122..

Diorite 

 do 

 ..do ... ...

Gneissic granite 

Horseshoe Bend,

Idaho.

Idaho.

Hamilton, Mont ......

0.0059

.0032

.0015

.0013

0.0025

.0017

.00086

.00086

42

53

57

66

Miscellaneous

Dur-l -

P-4. 

P-22 

P-Jt

53-BE-3a.

53-BE-3b.

KO-I .

FK-3- 

G-25 

Apatite-magnetite, deposit.

deposit.

Durango, Mexico. . _

batholith, Lakeview Mountain, 

Calif.

Canada. 

Shelby, N. 0. ....

Shelby, N. 0 ....

Kola Peninsula.USSB.

Mineville, N. Y.__._

0.0010 

.012

.00066

.0023

.0061

.0065

.001

.079

rvuo

0.0006 

.0057

.00036

ftftnp»9

.0032

IVU1

n.d.

n.d.

n.d.

60 

47

54

23

52

63

T We are greatly Indebted to H. W. Jaffe, David Gottfrled, E. S. 

Larsen, Jr., and George Phair, for supplying many of the apatite 

samples.

these analyses also show that the changes are not necessarily progressive, as apatites from the more felsic 

rocks are not more uraniferous than those from intermediate rocks. The explanation may lie in the progressive increase in the quantity of the other uraniferous 

accessory minerals. Studies of E. S. Larsen, Jr. and 

George Phair, and their colleagues confirm this suggestion (oral communication, 1957).
80 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

VARIATION OF URANIUM IN APATITE WITH 

URANIUM IN HOST ROCK

It is noteworthy that the uranium content of the 

apatites from the Boulder Creek batholith varies almost 

directly as the uranium content of the host rocks.8 

This is shown in figure 15. The rocks are slightly to 

moderately different in composition and are from 

localities as much as several miles apart. The fact that 

most of the analyses conform so closely to a uniform 

trend of concentration of uranium in apatite relative 

to host rock indicates that the partition of uranium 

between apatite and the crystallizing magma can maintain a constant ratio over a significant interval of the 

differentiation span of the magma. That such equilibrium does not necessarily prevail throughout differentiation is indicated by the aberrant point on the 

plot, GP-63.

. GP-34 QUARTZ MONZONITE

GP-17 GRANODIORITE

'GP-19 QUARTZ MONZONITE

/GP-129 QUARTZ DIORITE 

GP-50 QUARTZ MONZONITE

GP-51 QUARTZ MONZONITE

GP-63 GRANITE

GP-49 GRANDDIORITE

GP-61 QUARTZ DIORITE

012345

TOTAL URANIUM IN ROCK, IN PARTS PER MILLION

FIGUIIE 15. Total uranium in rock vs. total uranium in apatite for 

rocks from Boulder Creek batholith.

8 We are indebted to George Phair for uranium determinations 

of total rock for this comparison.

OXIDATION STATE OF URANIUM IN IGNEOUS 

APATITE

In the absence of analytical data the uranium in 

igneous apatite has been reasonably presumed to be 

entirely tetravalent (Tomkeieff, 1946; Goldschmidt, 

1954; Larsen and Phair, 1954; McKelvey and others, 

1955). The conclusion is virtually dictated by the 

following:

1. The common occurrence of uraninite in the same or related rocks;

2. The implications of uranium occurring largely in other 

calcium and cerium-bearing minerals such as sphene, 

monazite, xenotime, and allanite, a fact presumably 

governed by the ability of U+4 to substitute for the 

Ce+s and Ca+2 ions;

3. The occurrence of cerium and thorium in igneous apatite;

4. Independent evidence of the effectiveness of reducing conditions during the crystallization of the host rocks (Larsen and Phair, 1954; Adams, 1954).

It is reassuring to demonstrate that U(IV) actually 

occurs in igneous apatite in view of the geologic arguments favoring'this occurrence. Fewer data, gathered 

earlier (Altschuler and others, 1954), showed that more 

than half the uranium in some igneous apatite was 

reduced. Appreciable U(IV) has been found in all 

samples analyzed; however, with more comprehensive 

sampling it is now seen that many apatites may have 

more TJ(VI) than U(IV). It is not yet possible to 

define the controls of the relative U(IV) and U(VI) 

concentrations in igneous apatite. Many more analyses 

are required, not only for determination of the valence 

state of uranium in apatite, but also for other constituents of the parent rock and particularly for isotopes of 

lead (see discussion p. 82). It is possible, however, 

to discuss several alternative hypothesis on the basis 

of the data presented in table 27.

Of paramount importance is the fact (noted earlier, 

under sedimentary apatite) that consanguinous rocks 

yield similiar results when their apatites are analyzed 

for U(IV). The virtual identity in the U(IV)/U percentages of all but one of the Boulder Creek samples 

could not necessarily be anticipated unless apatites generally showed predominantly one or the other valence 

state. Eocks from the Idaho batholith show slightly 

more spread; the lithologies represented also differ 

more. In attempting to explain the obtained distributions of U(IV) it is important to note that U(IV) percentages have been found to range from 10 to 66, and 

it is possible that more analyses may substantially 

widen the range.

It is conceivable that igneous apatite initially fixes 

more U(IV) during its crystallization than it now 

contains and that the increase in U(VI) is superimposed by normal oxidative weathering. However, the
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 81

lack of greater diversity in groups of related specimens 

argues strongly against so simple a hypothesis. Thus 

the total uranium content of the Boulder Creek apatites 

range from 0.0011 to 0.0078 percent. In view of this 

range and of the demonstrated possible range in U(IV) 

(10 to 66 percent of the total), the regularity that 

prevails in the U(IV)/U percentages of the Boulder 

Creek apatites would be remarkable if it were caused 

by so variable a process as weathering, in which local, 

external factors (such as permeability and groundwater composition) play so large a role. It is pertinent 

to observe that the original rock samples and the apatite 

separates were carefully screened for evidence of alteration, and only fresh materials were used.

U(IV)/U(VI) BATIO BEFLECTS MAGMATIC 

EQUILIBBIA

Two other hypothesis may be considered to account 

for the consistent U(IV)/U(VI) ratios 9 in groups of 

apatites with much more diverse total uranium 

contents:

1. The U(IV)/U(VI) ratios may be entirely a function of the 

conditions prevailing during the crystallation of the 

magma and be inherited unchanged.

2. The U(IV)/U(VI) ratios may be related to radioactivity 

and to age. This second explanation does not preclude 

the application of the first, as the percent of U(IV) may 

have been initially determined by the prevalent equilibria 

of the magma during crystallization and it may have 

since been altered regularly by radioactivity.

Much evidence can be marshalled for the first of the 

above viewpoints. Figure 15 demonstrates that the 

uranium in the apatite is in equilibrium with the total 

uranium in the rock and therefore, assuming the confined crystallization of phanerocrystalline rocks, with 

the magma from which these rocks crystallized. It 

follows that the uniform U(IV)/U(VI) ratio in these 

apatites varies directly with the total uranium in the 

crystallizing magma. The progressive change in 

uranium content with differentiation in a series of 

related igneous rocks has been documented in the radioactivity studies of Billings and Keevil (1946) and the 

chemical studies of Larsen and others (1956). In addition, Adams has shown that total uranium has a linear 

relation to total K2O in the volcanic rocks of the Lassen 

Peak region. In these rocks "potassium increases systematically during differentiation" (Adams, 1954).

Obviously uranium responds to the major chemical 

equilibria that control differentiation. The above evidence that trace percentages of U(IV) in apatite are

9 Previous sections were concerned with the amounts of the total 

uranium that is tetravalent; hence the percentage U(IV)/U was used. 

Here, relative quantities of TJ(IV) and TJ(VI) are discussed and their 

ratio is used. Obviously the ratio and the percentage are based on the 

same figures and demonstrate the same trends.

directly related to uranium in the total rock (fig. 15) 

reveals that the U(IV)/U(VI) ratio in apatite might 

vary regularly with differentiation. Goldschmidt 

(1954) pointed out the progressive development of 

"magmatic oxidation" displayed by the increasing ratio 

of ferric to ferrous iron in mafic to felsic rocks, and he 

also noted that the iron oxide and sulfide mineral assemblages are compatible with the ferric-ferrous ratio. 

It is reasonable to suppose that uranium in plutonic 

apatite would similarly reflect the overall oxidation 

capacity of the magma and that the U(IV)/U(VI) 

ratio would change accordingly in a large series of 

related rocks. Precise proof of this, however, must 

await a much larger assemblage of data.

EELATIOtf OF U(IV)/U(VI) BATIO TO AGE 

AUTO-OXIDATION

The hypothesis that the U(IV)/U(VI) ratio in 

igneous apatite is a function of radioactivity and varies 

regularly with age is more difficult to apply. Essentially, it proposes that the U(IV)/U(VI) ratio is 

governed by auto-oxidation. The concept of autooxidation was first proposed by Ellsworth (1924,1925) 

who stated that the disintegration of UO2 or ThO2 to 

Pb must automatically oxidize TJO2 , or other reduced 

species present, in the following manner

UO2 -> Pb + 20 

20 + UO2 + Pb -> UO3 + PbO

Subsequent decay of the UO3 thus produced will, in 

turn, oxidize two more UO2 (Holmes, 1948) as follows:

UO3 -» Pb + 30 

30 + 2UO2 + Pb -» 2UO3 + PbO

Thus, in net, one mole of UO3 is produced for each 

mole of radiogenic lead and, ideally, one might use 

U(VI)/U ratios to compute the ages of rocks and 

minerals in substantially the same manner that leaduranium ratios are now used, as was proposed by 

Lane (1934).

Three assumptions underlie the use of auto-oxidation 

as an age method. First, all or most of the uranium 

present in a mineral was originally tetravalent; second, 

none of the U(VI) now present was produced by 

weathering; third, uranium has not been differentially 

leached or added to rock.

Tomkeieff (1946) found that UO3 in uraninite cannot 

be explained solely by auto-oxidation. In studying the 

lead and UO3 contents of 52 collected analyses, he 

demonstrated that natural uraninite varies greatly and 

always contains more UO3 than is demanded by the 

auto-oxidation hypothesis.
82 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

These findings are not surprising in view of the ease 

with which UO2 may be weathered. Indeed, Brooker 

and Nuffield (1952) have found that surface oxidation 

affects the peripheral zones of individual pitchblende 

specimens enough to change their cell edge constants 

relative to the material from the interior of the same 

specimens.

Despite the fact that pure UO2 is not known in 

nature, it is possible that uraninite may crystallize 

substantially free of UOs. If such material is unweathered and relatively free of water and Fe+2, Ce+3 , 

or other reduced species, it can be used to test the theory 

that UO3 is formd by auto-oxidation and that the 

UOs/U ratio is an index of geologic age. Tomkeieff 

(1956), by averaging analyses, found that auto-oxidation might account for a significant part of the UOs 

in uraninite. The demonstration by Holmes (1948) 

that the Khito Island uraninites yield UOs ages that 

are in very good agreement with the lead-uranium ages 

derived from the same specimens shows that autooxidation may be the principal mechanism in producing 

UO3 in many minerals.

The uniformity of the UO3/U percentages of the 

apatites from the Boulder Creek batholith and, to a 

lesser extent, the Idaho batholith, plus the fact that 

the apatite from the much younger Idaho batholith 

rocks have much lower UOs/U percentages, suggest 

that the UOs was produced by an internal process that 

is a function of time, rather than superimposed by an 

external process such as oxidative weathering.

It is difficult, however, to visualize the direct operation of auto-oxidation in producing UOs in igneous 

apatite in view of the widespread dispersion of uranium 

in apatite (0.01 percent of uranium equals 1 atom of 

uranium per 2400 unit cells of apatite) and the fact 

that Fe and Ce in igneous apatite equal or exceed 

uranium.

We can evaluate auto-oxidation in apatite quite 

simply by premising all initial uranium in apatite as 

U(IV). On this basis the age of the apatite can be 

determined by using the charts or the formula published by Wickman (1939). As discussed above, we 

may substitute the U(VI)/U ratio for the Pb206/Pb238 

ratio.

The U(VI)/U ratios of the Boulder Creek apatites 

(excluding sample GP-63) yield ages between 4.0 and 

4.5 billion years. Ages obtained by the ratio of lead to 

alpha activity (Larsen and others, 1952) in zircon from 

the same rock are close to 1 billion years (David Gottfried, U. S. Geological Survey, oral communication, 

1956). This latter age demands a TJ(VI) content of 

approximately 15 percent of the total U rather than 

75 percent of it.

Ages based on U(VI)/U ratios of the Idaho batholith apatites range from 2.4 to 3.6 billion years. Ages 

from the lead-alpha activity ratios of zircon from the 

same rock range from 60 to 110 million years (H. Jaffe, 

U. S. Geological Survey, oral communication, 1957). 

These would demand TJ(VI) contents of only 0.8 to 1.5 

percent of the total U rather than 34 to 58 percent of it.

EXTERNAL WEATHERING CONTROLLED BY RADIOACTIVE DECAY

Despite the fact that the contribution of thorium has 

not been taken into account in the above evaluations, 

the results show so great an excess of UOs as to invalidate the hypothesis that auto-oxidation produced the 

UOs, or even contributed significantly to it. It is possible, however, that radioactive disintegration can indirectly facilitate and control the ratio of external oxidation by solutions and gases present in the rocks and, 

hence, produce an effect like auto-oxidation. Such control on crystal permeation by oxidizing agents could be 

established by progressive increase in crystal damage 

because of radioactive decay. Ellsworth (1925) pointed 

out the following disruptive effects of radiation on 

crystal structure:

1. Intracrystalline fracturing due to volume changes caused 

by auto-oxidation of Ce, Fe, and U(IV),

2. Fracturing due to volume changes caused by radioactive 

heat,

3. Structural dislocation caused by recoil forces in atoms 

emitting alpha particles,

4. Dislocation caused by alpha particle bombardment.

The effectiveness of radiation damage in increasing 

the cell dimensions of quartz and zircon has been demonstrated by Hurley and Fairbairn (1953). In addition, oxygen liberated by the decomposition of structural or absorbed water as a result of alpha radiation 

would effect further oxidation.

In summary, the relative U(IV) and U(VI) contents of igneous apatites are most readily attributed to 

oxidation conditions during the crystallization of the 

host rocks. The consistency evident among some groups 

of related rocks, however, indicates that initial UOs 

contents may be progressively augmented by autooxidation or external oxidation, at a systematic rate, 

and it is of interest to investigate the relations of radioactive decay to such contributions to UOs content. To 

do so requires much more uranium data and a comparison of U(VI)/U ratios with radiogenic lead.

CONCLUSIONS

SUMMARY

Uranium in apatite ranges from a few parts per 

million (O.OOOX percent) in the insular phosphorites 

to almost 1 percent in subaerially enriched deposits.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 83

The most typical deposits, those comprising the greatest 

concentrations of P2Os in the lithosphere, are far more 

restricted in range. Primary apatite, both igneous and 

sedimentary, generally, has between 0.005 and 0.015 

percent of uranium. Thus, igneous apatite seldom 

exceeds 0.01 percent and primary apatite pellets from 

the Hawthorn and Phosphoria formations typically 

contain 0.005-0.008 percent of uranium; the rich 

Moroccan phosphorites contain about 0.012 to 0.014 

percent (with remarkable consistency).

The uranium in apatite may be leached or enriched, 

and its enrichment may take place subaerially from 

ground water or during submarine reworking. Marine 

enrichment commonly enhances uranium content in 

apatite to 0.02 to 0.03 percent, as shown in the Bone 

Valley formation and the South Carolina terrace 

gravels. The fact that unweathered marine phosphorite 

seldom exceeds 0.03 percent of uranium, however, can 

be attributed only to the infinitesimally small concentrations in the marine source, rather than to any limitations of the apatite structure or to the competition of 

other ions for replacement sites. This is evident from 

the much greater content of uranium in subaerially 

enriched apatite, in which much additional uranium is 

emplaced long after the normal suite of other trace 

elements substituting for calcium. Thus pebbles in the 

Bone Valley formation are supergenely enriched to 

0.2 percent of uranium, and fossil bones contain as 

much as 0.83 percent of uranium. Significantly, the 

U(IV) content in each of the above is more than half 

of the total uranium.

The characteristics of geologically young marine 

phosphorites enlighten us both as to the capacity and 

the nature of marine sources of uranium. Phosphatic 

marls from the Gulf of Mexico and phosphorite nodules 

from the Pacific Ocean have a U/P2O5 ratio as high or 

higher than those of any of the world's major phosphate deposits that have been investigated, thus proving 

that ocean water is an adequate source of uranium in 

typical marine phosphorite.

NATURE OF URANIUM FIXATION IN PHOSPHORITES

The suggestion that uranium in phosphorites is dominantly U(VI) fixed as a surface-coordinated uranyl 

pyrophosphate has been advanced (Bowie and Atkin, 

1956) to account for the lack of uranium in calcareous 

sediments. However, the well-established ability of 

carbonate to complex uranyl ion and thus increase the 

solubility of uranium (Bachelet and others, 1952) more 

probably accounts for the relative lack of uranium in 

carbonate rocks. Furthermore, the surface-limited reactions of uranyl ion with bone apatite demonstrated by

Neuman and others (1949a, b) are structurally much 

more limited that the isomorphous replacement of Ca+2 

by U+4 , and it is not surprising that an apatite material with as much as 0.83 percent total uranium should 

contain more than half the uranium as U(IV). Some 

of the U(VI) in unweathered phosphorites is probably 

chemisorbed by surface reaction of uranyl on the growing crystallites of sedimentary apatite. Such reactions 

are, in nature, more likely to involve orthophosphate 

than pyrophosphate formation. In addition, the high 

U(IV) content of young phosphorites suggests that 

postdepositional oxidation has also produced appreciable U( VI) in the older rocks.

Were mere adventitious extraction the sole factor in 

uranium fixation by precipitating apatite, the ratio of 

uranium to phosphate in apatite might approximate 

their relative concentrations in the sea. Uranium in sea 

water is 1-2 parts per billion. P2O5 in sea water of the 

Continental Shelf is 50 mg/m3 (Kazakov 1937). The 

IT/P2C>5 ratio in sea water is thus 0.02-0.04 and thereby 

exceeds by far the ratio of U/P2O5 found in primary 

marine phosphorites (table 12). In addition, comparison of U/P2O5 ratios both within and among the primary pellets and nodules from the Hawthorn formation, Florida, the Gulf of Mexico, the Pacific Ocean, 

and French Morocco (table 12), reveals considerable 

variety for comparable materials. Thus, direct and 

indiscriminate fixation of uranium that is available 

fails to explain the known uranium contents in apatite. 

Nor can these known contents be explained merely by 

citing an "affinity" of uranium for phosphate that is 

demonstrated by the many uranium phosphate minerals, 

as these are entirely uranyl minerals, and we have found 

that in relatively young, primary phosphorite most of 

the uranium is present in the tetravalent or, uranous, 

form.

It seems, indicated from the above, that the uranium 

actually available to apatite must be limited and largely 

contingent on special circumstances, presumably, the 

production of U(IV). Garrels (1955) and Sheldon 

(written communication, 1957) have shown from thermodynamic considerations of the isolated couple 

(UO2 )+2 and U+4 that the uranyl concentration of 

normal sea water would be many orders of magnitude 

greater than that of the uranous ion. It is only in an 

acid reducing environment that the U(IV) concentration becomes significant (Garrels, 1955). Even under 

these conditions (pH 6, Eh = 0.3), the 

(UO2 )+2/U+4 ratio is about 100 (Sheldon, written 

communication, 1950); however, apatite cannot be considered to form from such water of relatively high 

acidity.
84 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

It follows from the insolubility of U(IV) in ground 

water solutions that uranium is delivered to the ocean 

as (UO2)+2 . However, uranyl ion may be readily 

reduced at low temperature and pressure to U(IV) and 

as^the concentrations of ferrous iron and organic matter 

in marine phosphorites is generally far greater than 

the concentration of uranium, the necessary marine 

reduction of U(VI) prior to apatite fixation may be 

readily presumed.

The substantial ground-water and marine enhancements of uranium in apatite reveal the potency of 

apatite to remove uranium selectively from terrestrial 

or marine solutions. U(IV) produced by reduction 

reaction between (UO2 ) +2 and Fe+2 , or organic matter, 

would be taken up as a proxy for calcium during formation of apatite, or by ion exchange of calcium during 

marine reworking of previously formed apatite.

Unweathered pebbles from the Bone Valley formation and nodules of Quaternary age from the Pacific 

Ocean contain dominantly tetravalent uranium. This 

indicates that the hexavalent uranium in sea water is, 

in fact, reduced prior to emplacement in apatite. 

Further proof of this is obtained from the demonstration that new increments of marine uranium in the 

transgressively reworked Pleistocene terrace gravels 

near Charleston, South Carolina are mainly U(IV). 

In contrast to these findings, the marine phosphorites 

containing prominent U(VI) have been obviously 

weathered or are geologically much older (and probably weathered). It is therefore believed that the 

U(VI) present in marine phosphorites is derived 

largely from postdepositional oxidation of U(IV).

REGENERATIVE CAPTURE

Iii contrast to igneous apatite the occurrence of 

uranium in marine sedimentary apatite is not governed 

solely by the equilibrium conditions prevailing during 

the precipitation of the apatite. This follows from the 

evidence of secondary enrichment during marine reworking ; namely, the host mineral acquiring new increments of the trace element by re-exposure to the same 

or similar solutions. It is also indicated by the wide 

variation in the U/P2O5 ratios in unaltered primary, 

or first-cycle, phosphorites, a fact which would otherwise demand considerable variation in the uranium 

content of the marine sources, for which there is no 

evidence. It is proposed that apatite actively affects 

the course of the reduction reactions which might normally produce U(IV) in the following manner:

(UO2 )+2 + Keductant ^± U+4 -f Oxidized reductant

The presence of apatite as a precipitating or solid 

phase which can sequester the U+4 by solid solution or

ion exchange for calcium, interferes with the attainment of equilibrium in the above reaction and drives it 

instead in the direction:

(UOo)+2 + Keductant -> U+4 + Oxidized reductant

I

Thus by removal of U(IV) in an insoluble phase, 

apatite promotes an additional reduction of uranyl and 

thereby insures a continuing supply of U(IV) for its 

own uptake. The removal of U(IV) from sea water is 

thus not dependent on continuous precipitation of 

apatite, as uptake may proceed equally by ion-exchange 

reaction with previously formed apatite. This is indicated by the higher total U and U(IV) contents of 

reworked phosphate nodules.

The name, regenerative capture, is proposed to 

describe this type of trace element concentration in 

which a stable complex or insoluble phase, by effectively removing an ion species that is normally present 

in insignificant quantities, prevents the attainment of 

equilibrium in the oxidation-reduction reaction producing this ion, and causes continuous production of 

the ion. Thus, as long as removal continues, the supply 

is maintained and a mineral is able to fix unusual concentrations of an ion species which, from mere stability 

or concentration considerations, would not be expected 

in large quantities. Regenerative capture can be maintained only as long as the host mineral remains unsaturated with respect to the trace constituent. The 

fact that apatite can concentrate as much as 0.5 percent of U(IV) proves that saturation conditions are 

seldom realized in typical marine apatite (0.01-0.02 

percent of U).

The idea of regenerative capture clearly invokes no 

new chemical principle. Its merit lies in explaining 

unexpectedly large mineral concentrations of ions present in insignificant concentrations in solution. Regenerative capture applies to uranium in apatite in two 

important ways. It helps to explain the high U(IV)/ 

U(VI) ratios of some marine phosphorites despite the 

fact that uranium in the ocean source most probably 

occurs predominantly as U(VI). It implies that a 

strongly reducing environment is not a requirement for 

the fixation of U(IV) by apatite but rather that appreciable U(IV) may be fixed by apatite so long as any 

reduction of U(VI) is possible.

EQUILIBRIUM FIXATION OF URANIUM IN 

IGNEOUS APATITE

A completely different situation prevails in igneous 

apatite. The presence of many other good hosts makes 

the entire distribution of uranium much more complex 

and the amount of uranium available to apatite is regu-
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 85

lated not only by its inherent capacity but by the 

capacities of other hosts coextensive with it and thus 

reflects the total equilibrium in the magma. This is 

clearly evident from the systematic variation of both 

the total uranium and the U(IV)/U(VI) ratio in 

apatite with the total uranium in the rock, for suites 

of related rocks.

SELECTED REFERENCES

Adams, J. A. S., 1954, Uranium and thorium contents of volcanic rocks, in Faul, Henry, ed., Nuclear geology, p. 89-98: 

New York, John Wiley and Sons.

Alexander, Frank, 1953, Guidebook, flield trip to central Tennessee phosphate district Southeastern Mineral Symposium, Geol. Soc. America: Nashville, Vanderbilt Univ., 

Tennessee Div. Geology, 8 p.

Altschuler, Z. S., Cisney, F. A., and Barlow, I. H., 1953, X-ray 

evidence of the nature of carbonate-apatite (abs.) : 19th 

Internat. Geol. Gong., Algiers, Comptes rendus, sec. 11, 

fasc. 11, p. 9.

Altschuler, Z. S., Clarke, R. S., and Young, E. J., 1954, Uranium 

in apatite (abs.) : Geol. Soc. America Bull., v. 65, no. 12, 

pt. 2, p. 1225-1266.

Altschuler, Z. S., Jaffe, E. B., and Outtitta, Frank, 1956, The 

aluminum phosphate zone of the Bone Valley formation, 

Florida, and its uranium deposits: Proc. Internat. Conf. 

on Peaceful Uses of Atomic Energy, Geneva, 1955, v. 6, 

p. 507-513; U. S. Geol. Survey Prof. Paper 300, p. 495-504.

Arambourg, Camille, and Orcel, Jean, 1951, Observations preliminaires sur a presence d'un vanadate d'urane dans les 

gisements de phosphate du Maroc: Acad. Sci. Paris Comptes 

rendus, v. 233, no. 25, p. 1635-1636.

Bachelet, M., Cheylan, E., Douis, M., and Goulette, J. C., 1952, 

Preparation et proprietes des uranylcarbonates; 2" note: 

Uranylcarbonates alcino-terreux: Soc. chim. France Bull., 

p. 565-569.

Baes, C. F., Jr., 1954, The production of U4+ by ferrous ion in 

phosphoric acid solution; the formal electrode potential of 

the U4+ U6+ couple: U. S. Atomic Energy Comm. ORNL1581 or AECD-3594. (Microcard copy on file in U. S. 

Atomic Energy Comm. depository libraries.)

Barr, J. R., Jr., 1955, Uranium production from phosphate rock: 

Remarks at 39th Ann. Mtg. Florida Eng. Soc., Daytona 

Beach, Florida, U. S. Atomic Energy Comm.

Beevers, C. A., and Mclntyre, D. B., 1946, The atomic structure 

of fluorapatite and its relation to tooth and bone material: 

Mineralog. Mag., v. 27, no. 194, p. 254-257.

Billings, M. P., and Keevil, N. B., 1946, Petrography and radioactivity of four Paleozoic magma series in New Hampshire: 

Geol. Soc. America Bull., v. 57, p. 797-828.

Borneman-Starinkevitch, I. D., 1938, On some isomorphic substitutions in apatite: Acad. Sci. U.R.S.S., Comptes rendus 

(Doklady), v. 19, no. 4, p. 253-255.

Bowie, S. H. U., and Atkin, D., 1956, An unusually radioactive 

fossil fish from Thurso, Scotland: Nature, v. 177, no. 4506, 

p. 487-488.

Brooker, E. J., and Nuffield, E. W., 1952, Studies of radioactive 

compounds, IV Pitchlende from Lake Athabaska, Canada : 

Am. Mineralogist, v. 37, p. 363-385.

Capdecomme, Laurent, 1953 Etude mineralogique des gites dephosphates alumineux de la region de Thies (Senegal) : Internat. Geol. Gong., 19th, Algiers, Comptes rendus, sec. 11, 

fasc. 11, p. 103-117.

Capdecomme, Laurent, and Pulou, Rene, 1954, Sur la radioactivite" des phosphate de la region de Thies (Se'ne'gal) : 

Acad. Sci. Paris Comptes Rendus, v. 239, p. 288-290.

Carnot, Adolphe, 1893, Recherches sur la composition generate et 

la teneur en fluor des os modernes et des os fossiles des 

differents ages: Annales des mines, ser. 9, v. 3, p. 155-195.

Cathcart, J. B., 1950, Notes on the land-pebble phosphate deposits of Florida, in Snyder, F. G., ed., Symposium on mineral resources of the southeastern United States, 1949 proceedings : Knoxville, Tenn. Univ. Press, p. 131-151.

 1956, Distribution and occurrence of uranium in the 

calcium phosphate zone of the land-pebble phosphate district of Florida : Proc. Internat. Conf. on the Peaceful Uses 

of Atomic Energy, Geneva, 1955, v. 6, p. 514-519; U. S. Geol. 

Survey Prof. Paper 30, p. 489-494.

Cathcart, J. B., Blade, L. V., Davidson, D. F., and Ketner, K. B., 

1953, The geology of the Florida land-pebble phosphate deposits : 19th Internat. Geol. Cong., Algiers, Comptes rendus, 

sec. 11, fasc. 11, p. 77-91.

Clarke, R. S., Jr., and Altschuler, Z. S., 1958, Determination of 

the oxidation state of uranium in apatite and phosphorite 

deposits: Geochimica et Cosmochimica Acta, v. 13, nos. 2/3, 

p. 127-142

Connick, R. E., and Hugus, Z. Z., The participation of / orbitals 

in bonding in uranium and transuranium elements: Am. 

Chem. Soc. Jour., v. 74, p. 6012-6015.

Davidson, C. F., and Atkin, D., 1953. On the occurrence of 

uranium in phosphate rock: 19th Internat. Geol. Cong., 

Algiers, Comptes rendus, sec. 11, fasc. 11, p. 13-31.

Denson, N. M., and Gill, J. R., 1956, Uranium-bearing lignite 

and its relation to volcanic tuffs in eastern Montana and 

North and South Dakota: Proc. Internat. Conf. on the 

Peaceful Uses of Atomic Energy, Geneva, 1955, v. 6, p. 464- 

467; U. S. Geol. Survey Prof. Paper 300, p. 413-418.

Dietz, R. S., Emery, K. O., and Shepard, F. P., 1942, Phosphorite 

deposits on the sea floor off southern California: Geol. Soc. 

America Bull., v. 53, p. 815-848.

Du Toit, A. L., 1917, The phosphates of Suldanha Bay: South 

African Geol. Survey Mem. 10, 34 p.

Ellis, D. A., 1952, The recovery of uranium from industrial 

phosphoric acids by solvent extraction: U. S. Atomic Energy 

Comm., DOW-81, 79 p.

Ellsworth, H. V., 1924, Atomic disintegration as a measure of 

geologic time: Pan,, Am. Geologist, v. 42, p. 273-280.

 1925, Radioactive minerals as geologic age indicators: 

Am. Jour. Sci., 5th ser., v. 9, p. 127-144.,

Fairchild, J. G., 1929, Base exchange in artificial autunites: 

Am. Mineralogist, v. 14, p. 265-275.

Garrels, R. M., 1955, Some thermodynamic relations among the 

uranium oxides and their relation to the oxidation states of 

the uranium ores of the Colorado Plateaus: Am. Mineralogist, v. 40, nos. 11 and 12, p. 1004-1021.

Goldschmidt, V. M. (by Alex Muir, ed.), 1954, Geochemistry: 

Oxford, Clarendon Press, 730 p.

Graton, L. C., and Fraser, H. J., 1935, Systematic packing of 

spheres with particular relation to porosity and permeability: Am. Assoc. Petroeum Geologists Bull., v. 43, no. 8, p. 

785-909.
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Green, Jack, 1953, Geochemical table of the elements for 1953: 

Geol. Soc. America Bull., v. 64, p. 1001-1012.

Grimaldi, F. S., May, Irving, and Fletcher, M. H., 1952, U. S. 

Geological Survey fluorimetric methods of uranium analysis : U. S. Geol. Survey Circ. 199, 20 p.

Grimaldi, F. S., May, Irving, Fletcher, M. H., and Titcomb, 

Jane (Compilers), 1954, Collected papers on methods of 

analysis for uranium and thorium: U. S. Geol. Survey 

Bull. 1006, 184 p.

Gruner, J. W., 1952, New data on syntheses of uranium minerals. 

 Part I. Annual Report for July 1, 1951 to June 30, 1952: 

U. S. Atomic Energy Comm. RMO-983, 26 p.

Gruner, J. W., and McConnell, Duncan, 1937, The problem 

of the carbonate-apatites. The structure of francolite: 

Zeitschr. Kristallographie, v. 97, p. 208-215.

Guntz, A. A., 1952, Sur la presence d'uranium dans les 

phosphates Nord-Africains: Acad. Sci. Paris Comptes 

Rendus, v. 234, p. 868-870.

Hayes, C. W., and Ulrich, E. O., 1903, Description of the Columbia quadrangle, Tennessee U. S. Geol. Survey. Geol. Atlas, 

Columbia folio, 95.

Hebert, Claude, 1947, Contribution a 1'etude de la chimie des 

phosphates de calcium: Annales des mines, Mem., v. 136, 

no. 4, 93 p.

Hendricks, S. B., and Hill, W. L., 1950, The nature of bone and 

phosphate rock: Natl. Acad. Sci. Proc., v. 36, p. 731-737.

Holmes, Arthur, 1948, The oldest known minerals and rocks: 

Edinburgh Geol. Soc. Trans., v. 14, pt. 2, p. 176-194.

Hurley, P. M., and Fairbairn, H. W., 1953, Radiation damage in 

zircon a possible age method: Geol. Soc. America Bull., v. 

64, p. 659-673.

Igelsrud, Iver, Chocholak, John, Stephan, E. F., and others, 1949, 

Recovery of uranium from phosphate rock by the Battelle 

monocalcium phosphate process: U. S. Atomic Energy 

Comm. BMI-JDS-201, 99 p.

Igelsrud, Iver, Stephan, E. F., Choeholak, John, and others, 

1948: Chemical process to recover uranium from phosphate 

rock: U. S. Atomic Energy Comm., BMI-JDS-126, 29 p.

Jacob, K. D., Hill, W. L., Marshall, H. L., and Reynolds, D. S., 

1933, The composition and distribution of phosphate rock 

with special reference to the United States: U. S. Dept. 

Agriculture Tech. Bull. 364, 90 p.

Jacob, K. D., and Hill, W. L., 1953, Laboratory evaluation of 

phosphate fertilizers in Pierre, W. H., and Norman, A. G., 

ed., Soil and fertilizer phosphorus in crop nutrition, v. 4, 

chap. 11, p. 299-345, of Agronomy, [a series of monographs 

prepared under the auspices of the American Society of 

Agronomy] : New York, Academic Press.

Jaffe, E. B., and Sherwood, A. M., 1951, Physical and chemical 

comparison of modern and fossil tooth and bone material: 

U. S. Geol. Survey TEM-149, 19 p., issued by U. S. Atomic 

Energy Comm. Tech. Inf. Service Extension, Oak Ridge, 

Tenn.

Kazakov, A. V., 1937, The phosphorite facies and the genesis of 

phosphorites in Geological investigations of agricultural 

ores USSR: Sci. Inst. Fertilizers and Insectofungicides 

Trans. (USSR), no. 142, p. 95-113. (Special issue in 

English published for 17th Internat. Geol. Gong.)

Lane, A. C., 1934, Fivefold check of uraninite age?: Am. 

Mineralogist, v. 19, no. 1, p. 1-13.

Larsen, E. S., Jr., Keevil, N. B., and Harrison, H. C., 1952, 

Method for determining the age of igneous rocks using the 

accessory minerals: Geol. Soc. America Bull., v. 63, p. 1045- 

1052.

Larsen, E. S., Jr., and Phair, George, 1954, The distribution of 

uranium and thorium in igneous rocks, in Faul, Henry, ed., 

Nuclear geology, p. 75-89: New York, John Wiley and Sons.

Larsen, E. S., Jr., Phair, George, Gottfried, David, and Smith, 

W. L., 1956, Uranium in magmatic differentiation: Proc. 

Internat. Conf. on Peaceful Uses of Atomic Energy, Geneva, 

1955, v. 6, p. 240-247; U. S. Geol. Survey Prof. Paper 300, 

p. 65-74.

LeCornec, J., 1951, Estimate of world supplies of the principal 

plant nutrients by cost range: U. N. Sci. Conf. Conserv. and 

Utiliz. Res. Proc., v. 2, Mineral resources, p. 270-274.

Lenoble, Andr6, Salcan, Henri, and Ziegler, Valery, 1952, Decouverte de 1'uranium dans les niveaux phosphates du 

Maroc: Acad. Sci. Paris Comptes rendus, v. 234; no. 9, p. 

976-977.

McConnell, Duncan, 1938, A structural investigation of isomorphism of the apatite group: Am. Mineralogist ,v. 23, no. 

1, p. 1-19.

McKelvey, V. E., and Carswell, L. D., 1956, Uranium in Phosphoria formation: Proc. Internat. Conf. in the Peaceful 

Uses of Atomic Energy, Geneva, 1955, v. 6, p. 503-506; U. S. 

Geol. Survey Prof. Paper 300, p. 483-487.

McKelvey, V. E., Cathcart, J. B., Altschuler, Z. S., and others, 

1953, Domestic phosphate deposits, in Pierre, W. H., and 

Norman, A. G., eds., Soil and fertilizer phosphorus in crop 

nutrition, v. 4, chap. 11, p. 347-376, of Agronomy, a series 

of monographs, prepared under the auspices American 

Society of Agronomy: New York, Academic Press.

McKelvey, V. E., Everhart, D. L., and Garrels, R. M., 1955, 

Origin of uranium deposits: Econ. Geology, 50th Anniversary volume, 1905-1955, pt. 1, p. 464-533.

McKelvey, V. E., and Nelson, J. M., 1950, Characteristics of 

marine uranium-bearing sedimentary rocks: Econ. Geology, 

v. 45, p. 35-53.

McKeown, F. A., and Klemic, Harry, 1956, Rare-earth-bearing 

apatite at Mineville, Essex County, New York: U. S. Geol. 

Survey Bull. 1046-B, p. 9-23.

MacNeil, F. S., 1947, Correlation chart for the outcropping Tertiary formations of the eastern Gulf region: U. S. Geol. 

Survey Oil and Gas Inv. Prelim. Chart 29, with text.

Malde, H. E., Geology of the Charleston phosphate area, South 

Carolina: U. S. Geol. Survey Bull. (in press).

Miller, L. J., and Kerr, P. F., 1954, Progress report on the 

chemical environment of pitchblende in Annual Report for 

June 30, 1953 to April 1, 1954, pt. 2: U. S. Atomic Energy 

Comm. RME-3096, 98 p.; see p. 72-92.

Moore, G. W., 1954, Extraction of uranium from aqueous solution by coal and other materials: Econ. Geology, v. 49, p. 

652-658.

Neuman, M. W., and Neuman, W. F., 1948, The deposition of 

uranium in bone, II Radioautograph studies: Jour. Biol. 

Chemistry, v. 175, no. 2, p. 711-714.

Neuman, W. F., Neuman, M. W., Main, E. R., and Mulryan, 

B. J., 1949a, The deposition of uranium in bone, V Ion 

exchange studies: Jour. Biol. Chemistry, v. 179, p. 335-340.

 1949b, The deposition of uranium in bone, VI Ion 

competition studies: Jour. Biol. Chemistry, v. 179, p. 

341-348.
THE GEOCHEMISTRY OF URANIUM IN APATITE AND PHOSPHORITE 87

Oakley, K. P., 1955a, Analytical methods of dating bones, in 

Techniques in Archaeology: Advancement of Science, v. 11, 

no. 45; p. 3-8.

 1955b, The composition of the Piltdown hominoid remains : British Mus. Nat. History Bull., Geology, v. 2, no. 6, 

p. 254-265.

Palache, Charles, Berman, Harry, and Frondel, Clifford, 1951, 

Dana's system of mineralogy, v. 2, 7th ed.: New York, John 

Wiley and Sons, 1224 p.

Pannell, J. H., 1950, Aid oxidation of uranous solutions: U. S. 

Atomic Energy Comm. MITG-261, 18 p.

Pienkowski, S., 1953, RadioactivS d'un groupe de phosphorites 

polonais I: Acad. Polonaise Sci. Bull., Cl. Ill, v. 1, p. 39-41.

Piggot, C. S., and Urry, W. D., 1941, Radioactive relations in 

ocean water and bottom sediments: Am. Jour. Sci., v. 239, 

p. 81-91.

Rogers, G. S., 1913, The phosphate deposits of South Carolina: 

U. S. Geol. Survey Bull. 580-J, p. 183-220. [1915].

Runnels, R. T., Schleicher, J. A., and Van Nortwick, H. S., 

1953, Composition of some uranium-bearing phosphate 

nodules from Kansas shales: Kansas Geol. Survey Bull. 102, 

pt. 3, p. 93-104.

Rusakov, V. P., 1933, O soderzhanii radiya i toriya v fosforitakh 

[The radium and thorium content of phosphorites] : Akad. 

Nauk SSSR, Leningrad, Doklady (Compte rendus) ser. A, 

no. 3, p. 25-33.

Russ, W., 1924, The phosphate deposits of Abeokuta Province: 

Nigeria Geol. Survey Bull. 7, p. 9-38.

Schreyer, J. M., 1954, The chemistry of uranium (IV) orthophosphate solutions. Part I. The solubility of uranium 

(IV) orthaphosphates in phosphoric acid solutions: U. S. 

Atomic Energy Comm. ORNL-1747, 34 p.

Silverman, S. R., Fuyat, R. K., and Weiser, Jeanne D., 1952, 

Quantitative determination of calcite associated with carbonate-bearing apatites: Am. Mineralogists, v. 37, p. 211-222.

Smith, K. G., and Bradley, D. A., 1952, Radioactive dinosaur 

bones from the Camp Da vis region, western Wyoming: 

Michigan Acad. Sci. Papers 1951, v. 37, p. 257-263.

Strutt, R. J., 1906, On the distribution of radium in the earth's 

crust: Royal Soc. London Proc. Ser. A, v. 78, p. 150-153.

 1908, On the accumulation of helium in geologic time: 

Royal Soc. London Proc. Ser. A, v. 81, p. 272-277.

Thompson, M. E., 1953, Distribution of uranium in rich phosphate beds of the Phosphoria formation: U. S. Geol. Survey 

Bull. 988-D, p. 45-67.

 1954, Further studies of the distribution of uranium In 

rich phosphate beds of the Phosphoria formation: U. S. 

Geol. Survey Bull. 1009-D, p. 107-123.

Tomkeieff, S. I., 1946, The geochemistry of uranium: Sci. Progress, v. 34, no. 136, p. 696-712.

Volodchenkova, A. I., and Melentiev, B. N., 1943, Apatites of two 

textural types from apatite-nepheline rocks of Chibiny: 

Acad. Sci. U.R.S.S., Comptes rendus (Doklady) v. 39, no. 1, 

p. 34-35.

Weiner, J. S., Oakley, K. P., and Le Gros Clark, W. E., 1953, 

The solution of the Piltdown problem: British Museum 

(Natural History) Bull., Geology, v. 2, no. 3, p. 141-146.

Wells, A. F., 1950, Structural inorganic chemistry, 2d ed., 727 

p.; Oxford, Clarendon Press.

Wickman, Frans E., 1939, Some graphs on the calculation of 

geological age: Stockholm, Sveriges geol. Undersokning, 

ser. C, no. 427, Arsbok 33, no. 7, 8 p.

INDEX

Page

Abstract- .- _____________._____________________ 45 

Acknowledgments__________________________________ 47 

Akin mine, Tennessee________________________________ 65 

Aluminum phosphate zone, distribution of chemical elements ------ _____ 60-61

field relations__________.________________________ 57-58 

origin., _____________________________________ 57,62 

petrography ......... ._..________ _._ 60-61

Aluminum phosphates....____________________________ 57-61,65

Analyses, chemical... __.__._____________.. 49,51,56,62,63,65,69,77 

methods of...._______________.________________ 51,69-71

Angaur Island, Pacific Ocean__________________________ 51

Apatite, complete chemical analyses______________________ 49

mineralogy..___________________________________ 47-50

spectrographic analyses___________._______________ 49

structure of._________________________________ 47-48,74

uranium content_ __ __________________________ 50-57 

Ashley River______________________________________ 62 

Ashley River flood plain.______________________________ 63 

Autoradiograph..___________________________________ 66 

Auto-oxidation.____________________________________ 81-82 

Autunite._____________________________________ 52,74

Bigby-Cannon limestone.._____________________________ 65 

Bigby formation____________________________________ 65,78 

Bone Valley____________________________________ 62 

uranium content in fossils from_________________________ 67 

Bone Valley formation........_....................._............ 52,53,54,57,67

stratigraphy____________________________________ 57-58 

weathering..___________________________________ 57-60 

Bones, uranium content___________________ __ _________ 66 

Bonny Lake mine, Florida.--___________________________ 69,72 

Bonny Lake pebbles. _______________________________ 73 

Boulder Creek batholith _.. ._-_____________-___ 51,80,82 

Boyette mine, Florida________________________________ 53 

Brown-rock phosphate_____________________________ 65,72,78

Calcite .___._.- ....- .. . ...... .. . .__...... 62,64

Calcium, leaching and replacement of .____________________ 60

substitution for________________________________ 47,52,62,69 

Calcium aluminum phosphates..________________________ 57-60 

Carbonate-apatite________________________________ 48-49 

Cave phosphates, partial analyses...________________________ 51-52 

Cerium, in apatite___________________________________ 69,80

oxidation or reduction of uranium by._____________________ 69,82 

Cerium phosphates________________________________ 47 

Cleverly formation, uranium content of bones. __________________ 66 

Conclusions.._____________________________________ 82-85 

Cooper marl.____________________________________ 62, 63, 64,65 

Coupled diodochy, in phosphate minerals..-._________________ 47-48 

Crandallite .. __...._................................................. 57, 60

Enrichment, factors for PsOi and U.________________________ 63, 65 

marine-....____________________________________ 67-68 

postdepositional______________________________ 59 

residual. ____._.....__________________________ 62 

supergene...______...___________________________ 62 

uranium____________________________________ 68

Equivalent uranium_____________.___________________ 50

Fluorapatite....__________________________________ 48, 49, 74

Fluorescence, in apatite nodules. _____.__________________ 53

Fluorine, excess in apatite__________________________ 48-49, 75, 77

in apatite structure.--.. _________________.___ 47, 75, 77

replacement of OH____________________________ 47, 49, 51

Fort Union formation, uranium content._____________________ 66

Fremont County, Wyo., apatite deposit ____________ ._____. 68

Granule, term defined__ . ____-___._______--... 

Guano-derived phosphorites_________._.________.... 

Gulf of California-____________. ______________ 

Gulf of Mexico, phosphate and uranium contents of bottom samples.

49

51

55

56

Page

Hawthorn formation .___._____.... __._ - 55, 56, 67,83 

Hells Creek formation, uranium content.. . __ 66

Idaho batholith _.... .. .... . 79

age of apatites of-__ ____ 82 

Introduction_____ . 45-47 

Island phosphorite 51

Khito Island, age of uraninites of.-... 82 

Khouribga, French Morocco. 54, 56

Land Pebble field, Florida. . - 52, 54, 56, 57, 58

Lateritic weathering- .. 59 

Leaching...__............ - . . . 59, 62

Lignites, uraniferous ... 66

Magmatic oxidation.. 81 

Marine phosphate deposits... 49, 52 

Marl.. 62, 64, 83

Mllllsite . 57, 60

Mona Island, Puerto Rico, analyses of cave phosphorite . 51-52

Nodule, term defined . 49 

Nodules, phosphorite, from Pacific Ocean 73, 83 

Nuclear emulsion studies.- ,. 68

Old Red sandstone... 66 

Oolite, term defined.___._____ 49 

Organic content, correlation with uranium 54-55 

relationship to reducing conditions_ .._._ . 63, 72, 84 

Oulad-Abdoun, French Morocco, phosphate beds_ 53-54

Peace Valley mine, Florida 69 

Pebble apatite. ..._. ... 67, 68

Phosphate deposits, Florida 51, 53, 56, 57, 65, 68

French Morocco.. ____ 53, 54, 56, 67, 68

Tennessee-__ 78 

terrace gravel near Charleston, S. C. 77 

Phosphate fertilizer, production in United States.. 46 

Phosphate rock, term defined 50 

Phosphoria formation.. 53, 54,78 

Phosphorite, spectrographic analyses.. 64 

term defined_ 50 

Phosphorite pebble, term defined 50 

Photomicrograph 68 

Polish phosphorite-_ 55 

Purpose of report. 46

Radioactive decay. - 82 

Rantowles Creek.__ . 62 

Recovery methods 69-71 

References, selected. . 85-87 

Replacement, subaerial. 51 

Reserves, PsOj, apatite, and U.... 46

Reservoir Mountain.. 55

Reworking, marine - 57,67-68,71,83

subaerial---_ - 59

Sedimentary apatite.. - 57 

Sedimentary carbonate-apatites, composition . 48-49 

Senonian Moroccan apatite. 55 

Spectrographic analyses..- . 49,64

Terminology, petrographic... 49-50

Thorium, content in Florida pebble phosphorite.. 50

in igneous rocks and minerals 47,80

source of radioactivity in fossils. 66

Titanium________________ - 69

89
90 INDEX

Page

Uraninite, oxides and auto-oxidation in_____________________ 81-82 

Uranium, as byproduct of phosphate production__________..____ 46 

correlation with other elements. .. _________... .... 54-55 

crystallo-chemical nature in apatite.____________________ 68 

distribution in phosphate pebbles______________________ 60 

experimental oxidation and reduction of.-_________________ 70-71 

form and manner of occurrence in phosphorites_______ 52-55 

in igneous apatite_._____________________....____ 79-82 

in sea water. _._______________________________ 83,84 

marine sources of- ___________________________ 55-56,83-84 

marine versus ground-water origin in phosphorite_____________ 55-57 

oxidation or reduction by other elements_ _____ 69-71,73,82,84 

oxidation state in apatite..___ ____________.__ ..... 69-74 

reserves in phosphorite____________________________ 46 

secondary emplacement in bones... _____________ _.__._.__ 66-67 

substitution for calcium in apatite_____________________ 69-75

Page

Uranium contents, igneous apatite- 50-51,79

sedimentary apatite______ - . 50-51,72

Uranyl minerals. ..._ 52,74

Uranyl pyrophosphates_.. . 74

Vanadates 52 

Yarn mine, Florida. 69

Watson mine, Florida________ .. .._ .... 69

Wavellite . ..-... . . - . 57,60,69

Weathering._____...... 64,65

lateritie ________....__.._.... __ ____ -__.... 65

redistribution of elements during. 60-61,63-64

uranium oxidation during. ___ - . . - 72-73,82,84

Weathering patterns... 60,64-66

X-ray powder diffraction analysis 68

o