Large and Rich Phosphorus Resources , 

in the Phosphoria Fortnation 

in the Soda Springs Ar,ea, 

Southeastern Idaho 

GEOLOGICAL SURVEY BULLETIN 1496 

Large and Rich Phosphorus Resources 

in the Phosphoria Formation 

in the Soda Springs Area, 

Southeastern Idaho 

By R. A. GULBRANDSEN and DONATHON J. KRIER 

GEOLOGICAL SURVEY BULLETIN 1496 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: I!IHO 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress catalog-card Number 80-600145 

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

Washington, D. C. 20402 
CONTENTS 

Page 

Abstract------------------------------------------------------------------ .1 

Introduction -------------------------------------------------------------- 1 

Meade Peak Phosphatic Shale Member-------------------------------------- 5 

Distribution of phosphorus in the Meade Peak Phosphatic Shale Member______ 5 

Derivation of resource parameters ------------------------------------------ 10 

Effects of weathering on rock composition________________________________ 10 

Data from the Conda underground section ------------------------------ 12 

Resource parameters for the average underground section ________________ 14 

Resource parameters for the average underground phosphate rock (containing more than 20 weight percent P20 3 ) -------------------------------- 16 

Resource parameters for average surface phosphate rock (containing more 

than 20 weight percent P20 3 ) ---------------------------------------- 16 

Phosphorus resource ------------------------------------------------------ 17 

Method of calculating the average rock composition of a section in terms of three 

components______________________________________________________________ 21 

Composition, porosity, and density of average surface phosphate rock (containing 

more than 20 weight percent 0~) ---------------------------------------- 22 

References cited____________________________________________________________ 24 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

ILLUSTRATIONS 

Page 

Map showing distribution of the Phosphoria Formation in Soda 

Springs area, southeastern Idaho _________________________________ 3 

Columnar section of the Meade Peak Phosphatic Shale Member at 

Mabie Canyon ___________________________________________________ 6 

Frequency diagrams showing distributions of phosphorus in Conda 

underground section and in a composite of 17 surface sections ______ 7 

Graph showing relation of average phosphorus content to proportion 

of rock that contains more than 20 weight percent P20 3 in sections 

of the Meade Peak Phosphatic Shale Member _____________________ 9 

Triangular plot of composition of 17 surface sections, of average surface section, of average underground section, and of Conda underground section -------------------------------------------------12 

Diagrammatic cross section showing resource categories _____________ 18 

TABLES 

Page 

TABLE 1. Distribution of phosphorus in average surface section and in Conda 

underground section _____________________________________________ 8 

2. Average weight percent P20 3 and proportion of rock that contains 

more than 20 weight percent P 20:; in sections of Meade Peak _______ 9 

III 
IV CONTENTS 

Page 

TABLE 3. Average rock composition of each section and average for 17 surface 

sections -------------------------------------------------------11 

4. Composition, density, and porosity of Conda underground section-----13 

5. Composition, density, and porosity of average underground section-----15 

6. Composition, density, and porosity of average underground phosphate rock that contains more than 20 weight percent P20.; _______ 17 

7. Resource of phosphorus in the Meade Peak Phosphatic Shale 

Member in Soda Springs area -----------------------------------19 
LARGE AND RICH PHOSPHORUS RESOURCES 

IN THE PHOSPHORIA FORMATION 

IN THE SODA SPRINGS AREA, 

SOUTHEASTERN IDAHO 

By R. A. GcLBRANDSEN and DoNATHON J. KRIER 

ABSTRACT 

Concern about the capability of world food production to keep abreast of an everincreasing population has led to concern about the adequacy of phosphorus resources 

for fertilizers. Studies show that sufficient phosphorus is available for most population 

projections, but that most resources are of lower grade than those now utilized and that 

the nature of the deposits is poorly known. U.S. production of phosphorus is also of 

concern and is projected to fall short of demand in the early years of the next century, 

on the basis of present-day reserves. 

In this preliminary study we assess the nature and magnitude of deeply buried 

phosphorus resources in the Phosphoria Formation in the Soda Springs area of southeastern Idaho and show that very large and rich phosphate deposits are present. An 

area of about 760 km2 is estimated to be underlain by about 22 billion tons ofphosphate 

rock that contains 2.7 billion tons of phosphorus-enough to supply U.S. needs far into 

the future. Beds that contain more than 20 weight percent P20 5 (avg 27.7 weight 

percent P20:;) make up a total thickness that averages 10.6 m. About 44 percent of 

these resources is shallower than 600 m, and the remainder is no deeper than about 

2,000 m. The rock is estimated to yield 78,000 tons ofP20 5 per hectare of bedding plane. 

How soon these resources can be economically developed has not been well established; 

some indications are that utilization may be near at hand, but much of the required 

technology has yet to be developed. 

INTRODUCTION 

Concern about the adequacy of world food production for projected 

increases in global population has led to concern about the supply of 

phosphorus, an essential element in all living matter, for use in fertilizers. Estimates from a number of studies (McKelvey and others, 

1953b; McKelvey, 1967; Sheldon, 1969; Emigh, 1972, 1973, 1975; 

Cathcart and Gulbrandsen, 1973; Wells, 1975; DeVoto and Stevens, 

1979) show clearly that known phosphate deposits contain sufficient 

phosphorus to last far into the future, even without recycling. Only a 

very small fraction of this phosphorus, however, is classed as reserve, 

that is, as a resource that can be mined and processed at a profit 

under present-day (1979) conditions. Recovery of the greater part of 

this resource will presumably be more costly than at present and will 

require both changes in technology and the development of new 

technology. . 

1 
2 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

Most of these phosphate deposits have received little study, and 

their nature and magnitude are poorly known. If their resource potential is to be utilized when they are needed, there is a compelling 

urgency to begin the studies now. For example, Stowasser's (1975) 

projection of the supply and demand for phosphate rock in the United 

States, based on an estimate of current reserves, shows demand exceeding supply by about 2005 and the deficit enlarging rapidly after 

that time. 

The most recent study of the resource of phosphorus in the Phosphoria Formation in Idaho, as well as in Montana, Wyoming, and 

Utah, is the comprehensive assessment by Bauer and Dunning 

(1979). Although the resource was calculated for all depths, the rock 

parameters used are in most part applicable only to surface and 

near-surface rock that is weathered to varying degrees. Because most 

of the underground rock is unweathered, there is a clear need for 

information about fresh rock if the accuracy of the resource calculation is to be improved. 

In this report we estimate the nature and amount of deeply buried 

phosphorus resources in phosphate rock of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation in the Soda Springs 

area (fig. 1). We define a deeply buried resource as one that underlies 

areas where the surface rock is younger than the Phosphoria Formation (stippled areas, fig. 1). The minimum depth of burial (to top of the 

Meade Peak) is thus the thickness, ranging from about 61 to 76 m, of 

other members of the Phosphoria that are stratigraphically above the 

Meade Peak. All deeply buried, or underground, rock is considered 

fresh and unweathered, although weathered rock is known to occur in 

places beyond the boundary thus defined. The remaining phosphorus 

resource in phosphate rock (calculated below) that lies at or near the 

surface underlies outcrop areas of the Phosphoria Formation (horizontal line pattern, fig. 1). We classify all of this rock as weathered 

surface rock, although it also includes fresh rock in some places. 

FIGURE I.-Distribution of the Phosphoria Formation in Soda Springs area, southeast- ..,.. 

ern Idaho. Dots denote locations of stratigraphic sections of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation. Data for localities 1308, 1309, 

and 1314 are from Davidson, Smart, Peirce, and Weiser (1953); for 1206, 1208, 1211, 

1230, and 1233 from McKelvey, Armstrong, Gulbrandsen, and Campbell (1953a); for 

1200, 1210 and 1265 from McKelvey, Davidson, O'Malley, and Smith (1953c); for 

1258 and 1261 from O'Malley, Davidson, Hoppin, and Sheldon (1953); for 1298, 1303, 

and 1310 from Sheldon, Warner, Thompson, and Peirce (1953); for 1372 from Smart, 

Waring, Cheney, and Sheldon (1954); and for 1306A from Montgomery and Cheney 

(1967). Map modified from Mabey and Oriel (1970). 
R. 42 E. R. 43 E. 

430 

42° 

INTRODUCTION 

R. 44 E. R. 45 E. R. 46 E. 

EXPLANATION 

[<:c;:yj Underlain by the Phosphoria 

Formation 

- Outcrop of the Phosphoria 

Formation 

D Phosphoria Formation not 

present 

T 11 S 

4 6 8 KILOMETERS 

I I II 

0 4 MILES 

3 

T. 5 S. 

T. 6 S. 

T. 7 S. 

T. 8 S. 

T. 9 s. 
4 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

Finally, we calculate the total phosphorus in the entire Meade Peak. 

The parameters needed for our resource calculations are grade, 

thickness, density, and area of rock. The area measurement is 

straightforward; the other parameters, where not measured, are indirectly derived. The key factors for determining the total amount of 

deeply buried resource are: (1) the frequency mode of phosphorus in 

the class of rock that contains more than 20 weight percent P 205 , (2) 

the relation of the amount of rock that contains more than 20 percent 

P20 5 to the average P20 5 content (in weight percent) of a section, and 

(3) the relation between rock density, porosity, and mineral composition. Data from one underground section in the area, the Conda mine, 

are essential to all estimates of these parameters for average underground phosphate rock and for total rock of the Meade Peak. The 

Conda section is about 180 m downdip from the surface. With the 

exception of some parts of the section that have been exposed to 

ground water along joints and highly fractured zones, as well as a few 

soft beds rich in organic matter, the rock is brownish gray to black, 

hard, and well cemented. 

The extensive available information about the phosphate deposits 

and geology of the Soda Springs area is one of our principal reasons 

for selecting this area for study. Broad coverage was provided by 

Mansfield (1927) and Hale (1967a). The entire area (fig. 1) was mapped geologically by Mansfield at scales of 1:62,500 and 1:125,000; 

larger scale geologic maps for much of the area were prepared by 

Cressman and Gulbrandsen (1955), Gulbrandsen, McLaughlin, Hankala, and Clabaugh (1956), Cressman (1964), Montgomery and 

Cheney (1967), Armstrong (1969), and Rioux, Hite, Dyni, and Gere 

(1975). Mabey and Oriel (1970) prepared a geologic map at a scale of 

1:125,000 that includes all of the Soda Springs area. 

The regional structure interpreted by Rubey and Hubbert (1959), 

Armstrong and Cressman (1963), and Cressman (1964) is followed in 

this report and affects our interpretation of the subsurface geology in 

some places. If thrust faults underlie the area, additional phosphate 

deposits of the Meade Peak may also be present in the underlying 

plates. We have not investigated this possibility further, however, 

because of insufficient data for evaluation. Stratigraphic sections of 

the Meade Peak (see fig. 1 for locations) provided most of the basic 

data on the rocks used in the resource assessment (see fig. 1 caption 

for references). 

Acknowledgment. -Many of the data for this study were collected 

directly by V. E. McKelvey or under his supervision. His interest in 

phosphorus resources, especially those in the Phosphoria Formation, 

extends over a period of more than 30 years, and he has contributed 

greatly to our present-day know ledge of these resources and their 
DISTRIBUTION OF PHOSPHORUS IN THE MEADE PARK 5 

origin. Stimulation from his work and through personal association 

with him has aided the senior author in the preparation of this report 

and is greatly appreciated. We also appreciate assistance by J. G. 

Moore in clarifying aspects of rock density. P. Jones and D. Krier 

assisted with organic-matter determinations on samples of the Conda 

underground section. 

MEADE PEAK PHOSPHATIC SHALE MEMBER 

The Meade Peak Phosphatic Shale Member of the Phosphoria Formation contains the principal phosphate deposits of the Soda Springs 

area in southeastern Idaho. These deposits are the richest of all those 

in the Phosphoria Formation of the Western Field and yielded about 

4.1 million tons of phosphate rock from four mines in 1978. The 

reserve in the area, totaling about 1 billion tons of phosphate rock 

(modified from Garrand's (1974) estimate) along the outcrops or margins of segments of marine blanket deposits, makes up only a small 

fraction of the deeply buried resources that extend beneath younger 

units over large areas. 

McKelvey and others (1959) described the stratigraphy and 

nomenclature of the Phosphoria Formation. In the Soda Springs area 

the Phosphoria Formation consists, in ascending order, of the Meade 

Peak Phosphatic Shale Member, the Rex Chert Member, a cherty 

shale member, and, in the eastern part of the area, the Retort Phosphatic Shale Member. The Retort, which interfingers with the cherty 

shale member, is not economically significant in the Soda Springs 

area. 

A columnar section (fig. 2) of the Meade Peak at Mabie Canyon Ooc. 

1210, fig. 1) shows the main ore zones currently being mined and a 

generalized distribution of the three main rock types: phosphate rock, 

mudstone (quartz-silicate rock), and carbonate rock. Thickness of the 

Meade Peak in the area ranges from 33 to about 62 m and is estimated to average 54 m. The average phosphorus content of 17 surface 

sections ranges from about 10 to nearly 17 weight percent P205, and 

the average for the area is estimated at 13.4 weight percent P205. One 

underground section, the Conda mine (loc. 1200, fig. 1), is 60.7 m 

thick and contains an average of 9.4 weight percent P205. 

DISTRIBUTION OF PHOSPHORUS IN THE MEADE PEAK 

PHOSPHATIC SHALE MEMBER 

The distribution of phosphorus in a composite of 17 surface sections 

(fig. 3) shows a marked secondary frequency mode, possibly even two 

modes, in the concentration classes between 20 and 39 weight percent 

P20,,. This mode, which includes most of the rock being mined, shows 

the unusual phosphorus richness of the Meade Peak. Most 

significantly, this same mode also appears in the frequency distribu-
6 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

PERCENT P20s 

40 30 20 10 0 

EXPLANATION 

• Phosphate rock 

[ill - Mudstone 

• Carbonate rock 

~ Chert 

c:> Limestone lens 

Phosphate nodules 

METERS 

UPPER 

MUDSTONE 

ZONE 

UPPER 

PHOSPHATE 

ZONE 

MIDDLE 

MUDSTONE 

ZONE 

LOWER 

PHOSPHATE 

ZONE 

LOWER MUDSTONE ZONE 

astic pho;;phate 

FIGURE 2.-Columnar section of the Meade Peak Phosphatic Shale Member 

at Mabie Canyon <loc. 1210, fig. H Data from McKelvey, Davidson, 

O'Malley, and Smith (1953c). 
DISTRIBUTION OF PHOSPHORUS IN THE MEADE PARK 7 

11 

10 

9 

8 

7 

6 

5 

(/) 4 (/) 

w 

z 

~ 

u 3 

i: 

~ 

...J 2 

~ 0 

~ 

u.. 

0 

w 

(!) 0 

~ z 

w 

u 8 a: 

w 

a.. 

7 

6 

5 

4 

3 

2 

5 10 15 20 25 30 35 40 

PHOSPHORUS, AS WEIGHT PERCENT P205 

FIGURE 3.-Distribution of phosphorus (in 1-

weight-percent-P203 classes) in the Conda underground section (A) and in a composite of 17 surface 

sections (B). 
8 . PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

tion for the Conda underground section (fig. 3). The mode is therefore 

not a product of weathering but a primary feature in the formation of 

the phosphate deposits, important to the future development of the 

deeply buried resource. For this reason we have selected the class of 

phosphate rock that contains more than 20 weight percent P2 0;; for 

the resource calculation. Furthermore, the frequency mode in the 

upper range of P20,, values shows that the abundance of rock 

throughout the entire distribution does not increase regularly with 

decreasing grade-an important factor to consider in the mining of 

lower grade rock. 

The principal difference between these two distributions is the 

much larger proportion in the Conda underground section of rock that 

contains less than 7 weight percent P20 3 • A comparison of the lowgrade (0-10 weight percent P20 5) rock classes for the two distributions (table 1) shows that the Conda section has 18 percent more 

rock of this grade than does the average surface section. This feature 

appears to account for the main part of the difference between the 

average P2 0 5 content of the whole Conda section (9.4 weight percent 

P:!O.;) and the average for the surface sections (13.4 weight percent 

P:!O.;). The Conda section is below average in P2 0 3 content even when 

weathering effects are taken into account, as shown below. 

TABLE !.-Distribution of phosphorus in average surface section and in Conda underground section (in 5-weight-percent-P 0~ classes) 

Class (weight percent P20,,) 

0-5 

5-10 

10-15 

15-20 

20-25 

25-30 

30-35 

35 

Percentage of total thickness 

Average surface section 

29.92 

17.82 

14.53 

9.83 

6.98 

9.94 

9.10 

1.88 

Conda underground section 

41.59 

24.11 

14.01 

6.68 

1.81 

7.48 

4.32 

0 

One important feature of the distribution of phosphorus in the different sections is the relation of average phosphorus content (in 

weight percent P20 5 ) to the proportion (in percent) of rock in the 

section that contains more than 20 weight percent P 20 5 • These data 

(table 2) show that the correlation between these two parameters is 

very good: the correlation coefficient is 0.97. The line fitted to the data 

points (fig. 4) is the calculated line of linear regression. The Conda 

underground section has the lowest average weight percent P20s and 

the lowest proportion of rock that contains more than 20 weight percent P20 5 ; the North Rasmussen Valley section (loc. 1233, fig. 1) has 

the highest of both. The correlation of averages with proportions is an 
DISTRIBUTION OF PHOSPHORUS IN THE MEADE PARK 9 

TABLE 2.-Average weight percent P~O., and proportion of rock that contains more than 

20-weight-percent P~O~ in sections of the Meade Peak 

Locality Name Average weight 

percent P205 Proportion of rock in section that contains more than 20 weight percent 

1200 

1206 

1208 

1210 

1211 

1230 

1233 

1258 

1261 

1265 

1298 

1303 

1306A 

1308 

1309 

1310 

1314 

1372 

Lll 

Conda mine _________________________ _ Trail Canyon _________________________ _ West Dairy ___________________________ _ 

Mabie Canyon ------------------------ South Dry Valley _____________________ _ 

North Wooley Range _________________ _ 

North Rasmussen Valley _____________ _ Kendall Canyon _____________________ _ Blackfoot Narrows ___________________ _ Swan Lake Gulch _____________________ _ 

Lone Pine Springs ___________________ _ 

Pole Canyon --------------------------

Dry Ridge ---------------------------- Gravel Creek Divide _________________ _ 

Henry -------------------------------- Timber Creek _________________________ _ 

Enoch Valley _________________________ _ 

Snowdrift Mountain ___________________ _ 

9.4 

10.5 

12.8 

11.2 

13.3 

15.3 

16.7 

13.8 

13.0 

10.1 

14.2 

15.2 

13.1 

14.8 

13.0 

13.6 

15.3 

14.2 

(percent) 

13.6 

17.6 

26.7 

22.9 

28.2 

32.9 

37.4 

29.0 

25.8 

17.4 

31.7 

29.5 

27.2 

31.2 

28.7 

27.7 

33.6 

32.6 

0 

~ 40.------.-------.------,------,-------.------.------,r------. 

w 

a: 

0 

~ 

a: 35 

0 

1-

z 

w 

u 

ffi 30 a.. 

0 

N 

(!) 

z z 25 

~ z 

0 

u 

~ 20 u 

0 

a: 

LL 

0 

w 15 

~ z 

w 

• u 

a: 

w 

a.. 

10L------L------~----~------~----~------~------~----~ 

9 10 11 12 13 14 15 16 17 

AVERAGE PHOSPHORUS CONTENT, AS WEIGHT PERCENT P20s 

FIGURE 4.-Relation of average phosphorus content (in weight percent P20 5 ) to proportion of rock that contains more than 20 weight percent P20., in sections of the Meade 

Peak Phosphatic Shale Member. Line fitted to data points is calculated line of linear 

regression. 
10 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

inherent relation, but the degree of correlation makes this relation 

particularly useful and is utilized below to estimate the proportion of 

rock in the average underground section that contains more than 20 

weight percent P205 • 

DERIVATION OF RESOURCE PARAMETERS 

EFFECTS OF WEATHERING ON ROCK COMPOSITION 

The principal effect of weathering on fresh rock of the Meade Peak 

Phosphatic Shale Member in the temperate semiarid climate of the 

Soda Springs area is the loss of calcite, dolomite, and organic matter 

by solution and oxidation (Carswell and Gulbrandsen, 1954; Hale 

1967b; Altschuler, 1973; Emigh, 1973). Leaching of these primary 

rock components from the rock creates a porosity equivalent to the 

volume of components lost. Essential retention of the primary rock 

volume is indicated by the coherence of the rock and the preservation 

of bedding features. Where the loss of components is large, however, 

such as from limestone, the primary rock volume may be reduced to 

one supported by the relatively insoluble residual components. Pyrite 

is also destroyed by weathering, but the iron is converted to ferric 

oxide, which remains in the rock, and so any change in rock volume is 

small. The sulfur is oxidized, rarely to native sulfur and more commonly to soluble sulfate that is either carried away or deposited as 

gypsum along fracture surfaces. Gypsum of this origin is common in 

much of the rock of the Conda underground mine. 

To illustrate the effects of weathering on rock composition, we here 

express the average composition of each section in the area in terms 

of three components: 

(1) apatite-used in place of carbonate fluorapatite, the full 

mineral name; 

(2) OM-carb-a combination of organic matter (OM) and the 

carbonate minerals (carb) calcite and dolomite; 

(3). Q-S-the remainder of the rock, chiefly quartz, potassium 

feldspar, buddingtonite, albite, muscovite-illite, kaolinite, 

montmorillonite, pyrite, and possibly goethite. 

The apatite component is the product of the average weight percent 

P205 and the factor 2.6. The OM-carb component is estimated by a 

calculation involving the proportions of OM and the C02 attributed to 

dolomite and calcite in the chemical composition of average phosphorite of the Meade Peak (Gulbrandsen, 1966), average quartz-silicate 

rock, and average carbonate rock of the Meade Peak (Gulbrandsen, 

1975), and involving the percentages of these rock types in surface 

sections of the area (seep. 21). The OM-carb component for the Conda 

section is calculated independently, from organic-matter determina-
DERIVATION OF RESOURCE PARAMETERS 11 

tions on samples of the section and from loss-on-ignition (LOI) values 

reported by McKelvey, Davidson, o~Malley, and Smith (1953c). The 

last rock component, Q-S, constitutes the remainder of the rock. The 

values of the three compositional components for all sections and the 

average for 17 surface sections are given in a comprehensive list 

(table 3) and plotted together on a triangular diagram (fig. 5). 

TABLE 3.-Average rock composition of each section and average for 17 surface sections 

expressed in three components (in weight percent) 

Locality 

1200 

1206 

1208 

1210 

1211 

1230 

1233 

1258 

1261 

1265 

1298 

1303 

1306A 

1308 

1309 

1310 

1314 

1372 

Section Apatite 

Conda mine-------------------------------- 24.4 Trail Canyon ______________________________ 27.3 

West Dairy ________________________________ 33.3 

Mabie Canyon ______________________________ 29.1 

South Dry Valley __________________________ 34.6 

North Wooley Range ________________________ 39.8 

North Rasmussen Valley ____________________ 43.4 

Kendall Canyon ____________________________ 35.9 

Blackfoot Narrows __________________________ 33.8 

Swan Lake Gulch __________________________ 26.3 

Lone Pine Springs-------------------------- 36.9 

Pole Canyon ------------------------------ 39.5 

Dry Ridge __________________________________ 34.1 

Gravel Creek Divide ________________________ 38.4 

llenry ------------------------------------ 33.8 Timber Creek ______________________________ 35.4 

Enoch Valley ____________________ -__ _________ 39.8 

Snowdrift Mountain ________________________ 36.9 

Average of 17 surface sections 

(weighted by section thickness ______________ 34.8 

OM-carb Q-S 

27.8 47.8 

16.3 56.4 

6.8 59.9 

15.9 55.0 

7.3 58.1 

10.3 49.9 

6.3 50.3 

9.3 54.8 

8.9 57.3 

11.2 62.5 

7.7 55.4 

13.7 46.8 

7.0 58.9 

17.4 44.2 

13.8 52.4 

8.0 56.6 

11.2 49.0 

10.0 53.1 

10.6 54.6 

Assuming that only organic matter and carbonate minerals are 

removed from the rock during weathering, the path of weathering for 

each section (fig. 5) is along a line that extends from the OM-carb 

apex, through the section point, to the boundary of the diagram, 

where the component is zero. Such a line is shown for the Conda 

underground section (1200). This potential weathering path passes 

close to the point for Trail Canyon (1206), a nearby surface section. 

The weathering path, which illustrates the removal of OM-carb from 

the rock, is also a line of constant ratio of the other two components; 

the percentage of each component increases as weathering proceeds, 

but their ratio remains the same. The Conda section, for example, has 

24.4 percent apatite, 4 7.8 percent Q-S, and a ratio of apatite to Q-S of 

0.51. If weathering had removed all the OM-carb, the compositional 

point for this section would be at the intersection of the weathering 

path with the boundary of the diagram, and the section would have a 

composition of 33.7 percent apatite and 66.3 percent Q-S in the same 

ratio (0.51) as above. 

The compositions of the surface sections show a range in degree of 

weathering; none shows OM-carb completely removed, but all show a 

marked degree of weathering relative to the Conda underground sec-
12 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

Apatite 

FIGURE 5.-Triangular plot of composition (in weight percent) of 17 surface sections, of 

average surface section, of average underground section, and of Conda underground 

section. OM-carb is combined organic matter and carbonate minerals; Q-S, combined 

quartz, siiicate minerals, and mis~ellaneous minerals. Numbers refer to section localities (fig. 1, tables 2, 3). 

tion. The composition of the average surface section is also plotted 

(fig. 5), and a weathering-path line is drawn through this point. 

Ideally, we would like to reverse the weathering of the average surface section, replace the lost constituents, and thereby obtain a complete composition for the average underground section. All that we 

know directly about the average underground section is that its ratio 

of apatite to Q-S is the same as in the average surface section and that 

its compositional point (fig. 5) lies somewhere on the line between the 

compositional point for the average surface section and the OM-carb 

apex. We may estimate the variables needed-OM-carb and 

porosity-from the data for the Conda section; this estimate allows us 

to calculate the actual proportions of apatite and Q-8 as well as the 

rock density. 

DATA FROM THE CONDA UNDERGROUND SECTION 

From the known composition and the measured density, we may 
DERIVATION OF RESOURCE PARAMETERS 13 

calculate the porosity of the Conda underground section as follows. 

The average bulk density of the section is 2.49 g/cm3 on the basis of 80 

determinations, a value that represents the total weight in grams of 

the mineral components in 1 cm3 of rock (table 4). The grain density is 

2.603 g/cm3 , the total weight of mineral components divided by the 

sum of mineral volumes. The equations for these relations are: 

TABLE 4.-Composition, density, and porosity of Conda underground section. 

Component Weight D' percent (g/cm"l 

Apatite __________________________ 24.4 x3.2 

Carb ---------------------------- 19.8 92.8 

OM ---------------------------- 8.0 101.5 

Q-S ---------------------------- 47.8 112.6 

Subtotal ____ Void ________________________________________________ 

Known variables: weight percent, D, 1 and D1,. 2 · 

Sequence of calculations: 

Total 

(1) to obtain porosity: rV,. .. m1,,.nents, rVsuhtutah D •. " and P;' 

(2) to complete data of the table: rV,,.,.~. rV, ..... ~o V, and W. 

1 D =density of mineral components (g/cm"l. 

1 2 D1, = bulk density of rock (g/cm"l = rV,,.,., = W,,.,.1• 

1 

"D. =grain density of rock (g/cm"l = rV,""'"'"' = W,,.,.,!V,"'""'"'' 

'P,, =porosity of rock= 100 (1- D,.!D.I =100(V,,.,,11. 

"rV =relative volume= weight percent/100D. 

"V =volume (em")= rV calculated to total of 1 em". 

'W =weight (gl =DV. 

------

rV" V" 

(cm"l 

0.0763 0.1900 

.0707 .1760 

.0533 .1327 

.1838 .4577 

12.3841 .9564 

.0175 .0436 

1:1

.4016 1.0000 

"Calculated for carbonate fluroapatite from chemical analyses and unit-cell dimensions 

W' 

(gl 

0.6080 

.4928 

.1991 

1.1900 

22.4899 

"Dolomite-calcite density is intermediate in value between densities of dolomite and calcite and favors dolomite. 

"'Grain density of organic-matter separate 

" Grain density of quartz and silicate mineral separate 

12 rVsuhtotal = sum of rV com,)fmt·nt~ = 1/Dy. 

"'rV,,.,., = rVsuhuotal + rV,,.,d =liD,,. 

and D _ Wtotal 

'' - :-cv=-""':.::..:.....v=-=-- ' · total- \'oid 

where D1

, =bulk density (g/cm=1

), 

Du =grain density (g/cm3 ), 

W =weight (g), 
14 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

and V =volume (em:~). 

The density is related to the porosity P (in volume percent void) of 

rock by 

The porosity of the Conda underground section, therefore, is 4.4 percent, about the same as the lowest porosity that Athy (1930) found for 

shale at a burial depth of about 1,800 m. The Meade Peak in this area 

was buried much deeper, at least 6,000 m, and subjected later in its 

history to additional forces exerted during orogeny. This porosity is 

probably near the minimum attainable by pressure alone without 

recrystallization. 

The volumes of OM-carb (table 4) show that the complete removal 

of those components would create a rock porosity of 35 percent, including the primary porosity. This porosity, particularly as an average, appears likely to be near the maximum attainable without the 

collapse of many beds. Hedberg (1936, p. 277) found in compaction 

studies of clay that significant grain contacts are not attained until 

the porosity is lowered to about 30 to 35 percent. The density for 

Conda rock of 35-percent porosity would be 1.80 g/cma, from the sum 

of the weights of the remaining components. This value is low for rock 

density, particularly because many rocks of the section would therefore have to have densities significantly below this figure. In the 

constant-volume system that we are considering, therefore, the high 

porosity and low density calculated for a section from which all the 

OM-carb has been removed by weathering indicate that the proportion of combined organic matter and dolomite-calcite in the Conda 

underground section (27.8 percent) must be near the maximum for 

the Meade Peak in the area. 

RESOURCE PARAMETERS FOR THE AVERAGE UNDERGROUND SECTION 

On the basis of the above discussion, the proportion of OM-carb in 

the average underground section is taken to be 27.8 percent (8.0 

percent OM and 19.8 percent carb), the same proportion as in the 

Conda underground section and about the maximum that could be 

present. Although the proportion of OM-carb could be less than this 

value, the porous nature of much of the surface rock indicates that it 

would not differ greatly. 

The proportions of apatite and Q-S in the average underground 

section can now be calculated. Apatite and Q-S are present in a ratio 

of 0.637, the same ratio as for the average surface section, and make 

up 72.2 percent (100 minus 27.8 percent for OM-carb) of the total 
DERIVATION OF RESOURCE PARAMETERS 15 

mineral content. Thus the rock contains 28.1 percent apatite and 44.1 

percent Q-S (see data in table 5). This proportion of apatite is equivalent to 10.8 weight percent P20 5 , the grade of the average underground section. The porosity of the average underground rock (4.4 

percent) is also the same as that for the Conda underground section 

and is considered unlikely to be in great error. 

From these data, we calculate the density of the average underground section to be 2.51 g/cm:3 (table 5). The greatest density of the 

underground counterparts of the sections (shown in fig. 5) determined 

in this way is 2.53 g/cm3 for the Gravel Creek Divide (1308) and 

North Rasmussen Valley (1233) sections, which have the highest 

ratios of apatite to Q-S. The smallest calculated density is 2.48 g/cm3 , 

a projection for the Swan Lake Gulch ( 1265) section, which has the 

lowest ratio of apatite to Q-S. The only other known estimate for 

underground rock of the. Meade Peak in southeastern Idaho is one we 

made on a section in the underground Hot Springs mine of the Stauffer Chemical Co., south of this area, near the northeast end of Bear 

Lake. The average density of 21 samples taken systematically across 

this section was 2.57 g/cm3 , a value somewhat higher than the 2.51 

g/cm3 estimated for the average underground section in this area but 

close enough to make us confident in our calculations and to be useful, 

without great error, in estimating the total amount of phosphorus in 

the Meade Peak in the Soda Springs area-that is, the total resource 

present. 

The average thickness of the average underground section, the last 

parameter needed, is taken to be equal to that of the average surface 

TABLE 5.-Composition, density, and porosity of auerage underground section 

[See table 4 for explanation of terms] 

Component Weight 

percent 

Apatite__________________________ 28.1 

Carb ---------------------------- 19.8 

OM ---------------------------- 8.0 

Q-S ---------------------------- 44.1 

D (g/cm") 

3.2 

2.8 

1.5 

2.6 

Subtotal ___ _ Void _______________________________________________ _ 

Total _____ _ 

Known variables: weight percent, D; and P = 4.4 percent = 100V,.,,.1• 

Sequence of calculations: 

<11 To obtain density: rV, ... m""""""' rV,""'"'"~o 

V,ubtotal = Vlotal - V,,,., = 1- 0.0440 = 0.9560, 

rV'"'"' = rV, .. .,,,.,,,IV, .. .,,,.,,, = 0.3814/0.9560 = 0.3990, 

and D1, = 1/rV,.,,a1 = 1/0.3990 = 2.51. 

(21 To complete data of table: rV"'"" V, and W. 

rV 

0.0878 

.0707 

.0533 

.1696 

.3814 

.0176 

.3990 

v 

(cm"l 

0.2201 

.1772 

.1336 

.4251 

.9560 

.0440 

1.0000 

w 

(g) 

0.7043 

.4962 

.2004 

1.1053 

2.5062 
16 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

section (54 m) but is probably somewhat greater owing to the partial 

collapse of some surface beds due to weathering. 

RESOURCE PARAMETERS FOR AVERAGE 

UNDERGROUND PHOSPHATE ROCK 

(CONTAINING MORE THAN 20 WEIGHT PERCENT P~03) 

The relations described above allow us to make reasonable estimates of the parameters for underground phosphate rock (containing 

more than 20 weight percent v~o5) that are needed in resource calculations: grade, thickness, and density. 

An examination of the distribution of rock that contains more than 

20 weight percent P20 2 (fig. 3) shows that the average composition of 

both underground and surface rocks is less than 30 and greater than 

25 weight percent P20 5 . (The actual averages are 27.7 weight percent 

P20 5 for underground rock of the Conda section and 28.0 weight percent P20 5 for the surface rock.) Although the range in P20 5 content 

above 20 weight percent is expectably lower in the Conda underground rock, the modal tendencies of both underground and surface 

rocks suggest that the average P20 5 content of all underground phosphate rock is not likely to be much lower than the average for the 

surface rock and is probably appreciably greater than 25 weight percent P20 5 • The average for the Conda rock is therefore taken as an 

approximation to the true underground average P20 5 content and 

appears likely to fall within 1 weight percent P20 5 of this average. 

The thickness of underground phosphate rock is obtained from the 

relation of thickness to the average P20 5 content for the entire Meade 

Peak Phosphatic Shale Member (fig. 4). The average of 10.8 weight 

percent P205 , therefore, gives a thickness of 10.6 m (19. 7 percent of 

section times total thickness of 54 m) for the underground phosphate 

rock. The thickness of the Conda phosphate rock, by way of comparison, is significantly less: 8.3 m. 

The third parameter, density, is taken to be 2.64 g/cm3 , the average 

measured density of the Conda phosphate rock. Like the grade of rock 

in the average underground section, this density approximation is 

expected to fall close to the true average density of average underground phosphate rock. 

We further use the composition and density of the underground 

phosphate rock and all data on the Conda phosphate rock to calculate 

the rock's porosity (table 6). 

RESOURCE PARAMETERS FOR AVERAGE SURF ACE PHOSPHATE ROCK 

(CONTAINING MORE THAN 20 WEIGHT PERCENT P~03) 

Although the resource of phosphorus in surface phosphate rock is 

not the principal focus of this paper, we calculate it here to estimate 
PHOSPHATE RESOURCE 17 

TABLE 6.-Composition, density, and porosity of average underground phosphate rock 

that contains more than 20 weight percent P20:; 

[See table 4 for explanation of terms. All data on Conda 

rock appear closely to approximate those for average 

underground phosphate rock] 

Component Weight 

percent 

Apatite-------------------------- 72.0 Carb __ _ __ _ _ _ _ __ __ __ __ _ _ _ _______ _ 6.8 

OM ---------------------------- 6.8 

Q-S ---------------------------- 14.4 

D !g/cm") 

3.2 

2.8 

1.5 

2.6 

Subtotal ___ _ Void _______________________________________________ _ 

Total _____ _ 

Known variables: weight percent, D, and D1, = 2.64. 

Sequence of calculations: 

(1) To obtain porosity <Pl: rV, ... m"""''"''' rV,""'"'"J, 

D" = 1/rV,""'""'' = 110.35 = 2.857, 

D, 2.64 

and P = 100 (1 - D,,) = 100 (1- 2.857 ) = 7.6 percent. 

(2) To complete data of table: rV"""' = 1/D1., rV,.,1,1, V, and W. 

rV 

0.2250 

.0243 

.0453 

.0554 

.3500 

.0288 

.4788 

v 

(cm"l 

0.5940 

.0642 

.1196 

.1462 

.9240 

.0760 

1.0000 

w 

(g) 

1.9008 

.1798 

.1794 

.3801 

2.6401 

the total resource of this class in the Soda Springs area. The rock 

parameters we need are the same as those for the underground rock: 

grade, thickness, and density. The grade and thickness have been 

measured for 17 sections and average 28 weight percent P20 5 and 15 

m, respectively. Although samples for density determinations are not 

presently available, we have estimated the average density. (For 

method of calculation, seep. 22.) The calculated density is 2.39 g/cma, 

comparable in magnitude to the value 2.37 g/cma used by Garrand 

(1974) for rock of slightly lower (25.5 weight percent) P20 5 content 

than the 28.0 weight percent P20:1 for surface phosphate rock. 

PHOSPHORUS RESOURCE 

The resource of phosphorus in the Meade Peak Phosphatic Shale 

Member is in the map areas (fig. 1) designated ((Outcrop of the Phosphoria Formation" and ((Underlain by the Phosphoria Formation." 

The outcrop area includes deposits at or near the surface, and the 

area underlain by the Phosphoria comprises the deeply buried, or 

underground, resource. The area of deeply buried phosphorus resources is divided into subareas deeper and shallower than 600 m (fig. 

6). The resource areas, which were measured from geologic maps, 
18 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

A B 

Meade Peak Phosphatic Shale 

Member, Phosphoria Formation 

c 

Rock older than Phosphoria Formation 

FIGURE 6.-Diagrammatic cross section showing resource categories: 

A, Resource underlying outcrop of the Phosphoria Formation. 

B A 

B, Deeply buried resource underlying rock younger than the Phosphoria Formation and shallower than 600 m. 

C, Deeply buried resource underlying rock younger than the Phosphoria Formation and deeper than 600 m. 

using cross sections and utilizing all available structural and stratigraphic information, are bedding-plane areas-that is, map areas 

corrected for the dip of the beds. These areas make up a total area 

about 22 percent greater than the map area. 

Using rock parameters measured or derived above, we can calculate: ( 1) the phosphorus resource of the Soda Springs area in deeply 

buried phosphate rock (containing more than 20 weight percent P20 5 ) 

and in surface or near-surface phosphate rock (containing more than 

20 weight percent P2 0,,) underlying outcrop of the Phosphoria Formation; and (2) the total phosphorus resource in phosphate rock (containing more than 20 weight percent P20 5 ) and in the entire Meade 

Peak (table 7). 

We consider our calculation of the phosphorus resources to be reasonably accurate owing to the high quality and large quantity of data 

available for this assessment. Although direct information about the 

deeply buried rock is scanty, the derived parameters appear to us to 

be soundly based and unlikely to be greatly in error. Although the 

areas measured are partly speculative, they are based also on much 

geologic information, and we consider that they are not likely to be in 

error by more than 15 to 20 percent. We believe that the estimated 

resource reported above is probably a minimum for this area because 

all the rock currently being mined for fertilizer products is first calcined to remove organic matter, a process that raises the grade of the 

phosphate rock. Although we did not take this factor into account in 

our calculations, it increases the thickness of deeply buried rock that 

could be included in the phosphate rock class and consequently in-
Rock class 

TABLE 7.-Resource of phosphorus in the Meade Peak Phosphatic Shale Member in 

Soda Springs area 

P20, !average Thickness Area Volume Density Rock P,O, 

weight percent) (ml (km'l (km"l lg/cm"l 110" tons* l (10" tons* l 

Phosphate rock (>20 wt pet P~O,,) 

Deeply buried, total ---------------------- 27.7 10.6 780 8.3 2.64 22.0 6.1 

<600 m deep ---------------------------- 27.7 10.6 350 3.7 2.64 9.8 2.7 

>600 m deep ---------------------------- 27.7 10.6 430 4.6 2.64 12.1 3.4 

Underlying outcrop of Phosphoria Formation ____________________ - __ ------- 28.0 15.0 140 2.1 2.39 5.0 1.4 ---

Total -------------------------------- 920 27.0 7.5 

Entire Meade Peak Phosphatic Shale Member ___________________________ 10.8 54.0 920 49.7 2.51 125.0 13.5 

*Metric tons. 

p 

(10" tons* l 

2.7 

1.2 

1.5 

.6 

3.3 

5.9 

P,O,/hectare 

(10" tons* l 

78 

78 

78 

100 

150 

'1::l 

::r: 

0 

Ul 

'1::l 

::r: 

~ 

~ 

tiJ 

:::0 

tiJ 

Ul 

0 

e 

:::0 

Q 

tiJ 

""""" t.O 
20 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

creases the total phosphorus resource in the area. 

The total resource in the Soda Springs area in the class of rock that 

contains more than 20 weight percent P 20 5 is estimated to be 3.3 

billion tons of phosphorus, about 2. 7 billion tons (82 percent) of which 

is deeply buried. About 44 percent of this resource is shallower than 

600 m, and all is shallower than about 2,000 m. These deeply buried 

deposits are rich in phosphorus and capable of yielding 78,000 tons of 

P20 5 per hectare of bedding plane, a value that promises high efficiency in the ratio of ground handled or disturbed per unit of recovered product. Another measure of richness is that this deeply buried 

phosphate rock contains about 46 percent of the total phosphorus 

content of the entire Meade Peak Phosphatic Shale Member. 

The reserve of phosphate rock in the Soda Springs area, estimated 

currently at about 1 billion ton'§, makes up about 20 percent of the 

resource underlying outcrop of the Phosphoria Formation. Much of 

this resource remaining after the reserve is mined may be exploited 

in the future if surface mining is not restricted by environmental 

considerations. 

The deeply buried phosphorus resource in the Soda Springs area is 

very large yet constitutes probably only about 15 percent of similar 

resources that were calculated by Bauer and Dunning (1979) for all of 

southeastern Idaho below lat 43° N. The very large quantity and 

richness of this resource makes it especially important to the United 

States for maintaining self-sufficiency in the supply of phosphorus far 

into the future. How close this resource is to being profitably mined 

and processed has not presently been established, but our study of the 

nature of the rock, as well as recent underground mining tests, provides information on the status of the problem. 

Methods of processing the deeply buried phosphate rock may not 

differ greatly from those currently used for surface rock. The deeply 

buried rock differs principally in its higher content of organic matter 

and carbonate minerals. The organic matter could be removed by 

ordinary calcining, as is presently done. The increase in the proportion (by about double) of carbonate minerals would be unfavorable, 

especially in the amount of magnesium from dolomite, but may still 

be acceptable. If not, the dolomite could be removed, though at an 

increased processing cost. Other known methods of fertilizer manufacture may be even more efficient and less expensive in processing 

the deeply buried rock. Further studies need to be undertaken in this 

regard. 

Until 1975, large-scale underground mining was not considered 

feasible. However, the plan of Earth Sciences, Inc., for underground 

mining in the Paris-Bloomington area of southeastern Idaho has 

created new interest in this possibility. Even though Earth Sciences, 
METHOD OF CALCULATING AVERAGE ROCK COMPOSITION 21 

Inc., has successfully demonstrated machine mining of the partially 

weathered rock from their shallow mine, the capability of machine 

mining fresh deeply buried rock has yet to be established. The mining 

of all the underground phosphate rock may soon be possible, but 

further experimental work needs to be undertaken promptly to ensure that the technology is available when needed. 

Whatever economic advantage that surface mining had over underground mining has in recent years been declining owing to the 

steadily increasing costs that surface mining must pay for protection 

and reclamation of the surface environment. By way of comparison, 

the surface environmental effects of underground mining are minimal. If environmental concern continues to increase, underground 

mining may be needed even sooner than current production trends 

would indicate. 

METHOD OF CALCULATING THE AVERAGE 

ROCK COMPOSITION OF A SECTION 

IN TERMS OF THREE COMPONENTS 

The calculation of rock composition in terms of three components 

requires the determination of only two of the components, the third 

being the remainder. Apatite, one component, is the product of the 

average weight percent P2 0 5 in a section and the factor 2.6. OM-carb, 

the second component, is derived from chemical analyses of phosphate rock, carbonate rock, and quartz-silicate rock, the three major 

rock types of the Meade Peak Phosphatic Shale Member (Gulbrandsen, 1966, 1975). 

The proportion of carb is calculated from the total C02 determined 

for each rock type minus the amount of C02 in the apatite, which 

averages 1. 7 weight percent C02: 

co2(carbl = co2(totall - co2tapatitcl· 

Multiplying this result by the factor 2.15 yields an estimate of combined dolomite and calcite (carb) that slightly favors dolomite. 

The combined OM-carb for each rock type is, then, the sum of the 

percentages of OM and carb, as shown below: 

OM carb OM-carb 

Phosphate rock ________________ 2.31 2.52 4.83 

Carbonate rock ________________ 1.92 75.27 77.19 

Quartz-silicate rock ____________ 2.38 5.29 7.67 

The percentages of OM-carb in the three rock types are multiplied 

next by the respective proportions of each rock type in each of the 17 

surface sections. The sum of these three products yields, finally, the 

OM-carb content of a given section. 
22 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

COMPOSITION, POROSITY, AND DENSITY OF AVERAGE 

SURFACE PHOSPHATE ROCK (CONTAINING MORE THAN 20 

WEIGHT PERCENT P20 5) 

To calculate the density of the surface phosphate rock, we need to 

know its composition and porosity. We know the apatite content (72.8 

percent) from the data on the 17 surface sections, but not the proportions ofOM-carb and Q-8. The OM and carb are obtained from the 

data on average phosphate rock of the Meade Peak Phosphatic Shale 

Member (Gulbrandsen, 1966), adjusted to the slightly lower apatite 

content of the surface phosphate rock. The Q-S component, then, 

makes up the remainder of the rock. The composition of the surface 

phosphate rock determined in this way is: 

apatite = 72.8 percent 

OM = 2.8 percent 

carb = 3.0 percent 

Q-S = 21.4 percent 

An estimate of the porosity of the surface rock is obtained by 

((weathering" the underground phosphate rock and thereby reducing 

the contents of OM and carb to the proportions present in the surface 

rock, as shown above, while holding the remaining rock volume constant. The volumes and weights of apatite and Q-8, therefore, remain 

the same as those shown in table 6. The other known parameters are 

the final percentages of OM (2.8 percent) and carb (3.0 percent). The 

data on this ((weathered" rock are summarized below in tabular form 

(see table 4 for explanation of terms): 

Component Weight 

percent 

Apatite ------------------------------ 78.5 

Carb -------------------------------- 3.0 

OM__________________________________ 2.8 

D (g/cm") 

3.2 

2.8 

1.5 

v 

(em") 

0.5940 

.0259 

.0452 

Q-S -------------------------------- 15.7 2.6 .1462 

Subtotal ________ .8113 

\Toid -------------------------------------------------------- .1887 

Total ____________ 1.0000 

We know that 

w 

(g) 

1.9008 

.0726 

.0678 

.3801 

Wa 11 + W<~-s = 100 percent - 2.8 percent (carb) - 3.0 percent (OM) = 

94.2 percent, yielding a total weight for 1 cm=J (that is, a density) 

of Wtotat = Wa 11+ W<~-s = 2.2809 g= 2.4213 g; 

0.942 0.942 

therefore, 

Wo:vi = percent OM X Wtotat = 2.8 X 2.4213 = 0.0678 g, 

100 100 
COMPOSITION, POROSITY, AND DENSITY OF AVERAGE SURFACE ROCK 23 

Wcarb = percent carb X Wtotal = 3.0 = X 2.4213 = 0.0726 g, 

100 100 

VoM = WoM = 0.0678 g= 0.0452 cm3 , 

DoM 1.5 

Vcarb = Wcarb = 0.0726 g = 0.0259 cm3 , 

Dcarb 2.8 

Vap + OM + carb + li-S = 0.8113 cm3

, 

Vvoid = Vtotal - Vap + OM + carb + li-S = 1 - 0.8113 = 0.1887 em\ 

and P = Vvoid X 100 = 0.1887 X 100 = 18.87 percent. 

To complete the data calculations, the percentages of apatite and 

Q-8 a:re given by their respective weights divided by the total weight 

of the rock. 

Using the above estimate of porosity and the composition given 

previously, we can now calculate the density of the surface phosphate 

rock. We note that the ((weathered" underground phosphate rock from 

which the value for porosity was derived has different amounts of 

apatite and Q-8 from the surface phosphate rock because the surface 

rock includes the rock that in its unweathered state contains less 

than 20 weight percent P 20;;. 

The data and a description of the calculation are presented below 

(see table 4 for explanation of terms): 

Component Weight 

percent 

Apatite __________________ 72.8 

Carb ____________________ 3.0 

OM______________________ 2.8 

Q-S -------------------- 21.4 

D rV (g/cm') 

3.2 0.2275 

2.8 . 0107 

1.5 .0187 

2.6 .0823 

Subtotal ______ .3392 

Void -------------------------------------------- .0789 

Total______ .4181 

v w 

(em") (g) 

0.54413 1.7412 

.02560 . .0716 

.04473 .0671 

.19684 .5118 

M13 

.1887 

1.0000 2.391 

Known variables: weight percent, D, and porosity (P) = 18.87 percent 

= 100V\'oid· 

Sequence of calculations: 

and 

(1) to obtain density: rVcomponents, rVsubtotab Vsuhtotal = Vtotal - v\'Oid 

1-0.1887 = 0.8113 cm3

, rVtotal = rVsuhtotalVsuhtotal = ~:~~i~ = 

0.4181, 

Db = 1/rVtotal = 2.39; 

(2) to complete data of table: rVvoi(" V, and W. 
24 PHOSPHORUS RESOURCES IN PHOSPHORIA FORMATION, IDAHO 

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Garrand, L. J., 1974, Phosphate reserves of southeastern Idaho: Pocatello, Idaho, U.S. 

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Formation: Geochimica et Cosmochimica Acta, v. 30, no. 8, p. 769-778. 

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U.S. Geological Survey Open-File Report 75-554, 45 p. 

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Survey Bulletin 1042-A, 23 p. 
REFERENCES CITED 25 

Hale, L. A., ed., 1967a, Anatomy of the western phosphate field-a guide to the 

geologic occurrence, exploration methods, mining engineering, recovery technology: Salt Lake City, Utah, Intermountain Association of Geologists, 287 p. 

--1967b, Phosphate exploration using gamma-radiation logs, Dry Valley, Idaho, 

in Hale, L.A., ed., Anatomy of the western phosphate field-a guide to the geologic 

occurrence, exploration methods mining engineering, recovery technology: Salt 

Lake City, Utah, Intermountain Association of Geologists, p. 147-160. 

Hedberg, H. D., 1936, Gravitational compaction of clays and shales: American Journal 

of Science, ser. 5, v. 31, no. 184, p. 241-287. 

Mabey, D. R., and Oriel, S. S., 1970, Gravity and magnetic anomalies in the Soda 

Springs region, southeastern Idaho: U.S. Geological Survey Professional Paper 

646-E, p. E1-E15. 

McKelvey, V. E., 1967, Phosphate deposits: U.S. Geological Survey Bulletin 1252-D, p. 

D1-D21. 

McKelvey, V. E., Armstrong, F. C., Gulbrandsen, R. A., and Campbell, R. M., 1953a, 

Stratigraphic sections of the Phosphoria Formation in Idaho, 1947-48, part 2: U.S. 

Geological Survey Circular 301, 58 p. 

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