The Bambuta Phosphate 

Deposit, LiberiaA Reconnaissance Report 

GEOLOGICAL SURVEY BULLETIN 1480 

Prepared in cooperation with the Li8erian 

Geological Survey under the sponsorship of 

the Agency for International Development) 

U.S. Department of State 

The Bambuta Phosphate 

Deposit, LiberiaA Reconnaissance Report 

By SAM ROSENBLUM and S. P. SRIVASTAVA 

GEOLOGICAL SURVEY BULLETIN 480 

Prepared in cooperation with the Liberian 

Geological Survey under the sponsorship of 

the Agenry for International Development) 

U.S. Department of State 

An unusual deposit of iron and aluminum 

phosphates of at least 1. 5 million tons 

containing 2 8 percent POs 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1979 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Rosenblum, Sam 

The Bambuta Phosphate Deposit, Liberia-A Reconnaissance Report 

(Geological Survey Bulletin 1480) 

Bibliography: P. 25 

1. Phosphates-Liberia. I. Srivastava, S. P., joint author. II. Liberia. Geological Survey. 

III. Title. IV. Series: United States Geological Survey Bulletin 1480. 

QE75.B9 no. 1480 [TN914.L7] 557.3'08s [549'.72] 79-14259 

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

Washington, D.C. 20402 

Stock No. 024-001-03246-5 
CONTENTS 

Page 

Abstract -------------------------------------------------------------------------------------------------------------------- 1 

Introduction -------------------------------------------------------------------------------------------------------------

Location and accessibility---------------------------------------------------------------------------------

Previous work ---------------------------------------------------------------------------------------------------- 2 

Present investigation ---------------------------------------------------------------------------------------- 3 

Acknowledgments --------------------------------------------------------------------------------------------- 4 

Geology --------------------------------------------------------------------------------------------------------------------- 4 

Mineralogy ---------------------------------------------------------------------------------------------------------------- 10 

Physical properties -------------------------------------------------------------------------------------------- 12 

Optical properties ---------------------------------------------------------------------------------------------- 15 

X-ray results ------------------------------------------------------------------------------------------------------ 16 

Chemical composition--------------------------------------------------------------------------------------- 18 

Origin of deposit------------------------------------------------------------------------------------------------------- 18 

Marine precipitation------------------------------------------------------------------------------------------ 20 

Biogenic origin --------------------------------------------------------------------------------------------------- 22 

Reserves -------------------------------------------------------------------------------------------------------------------- 23 

Potential use ------------------------------------------------------------------------------------------------------------- 24 

Recommendations---------------------------------------------------------------------------------------------------- 25 

References cited -------------------------------------------------------------------------------------------------------- 25 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map showing location of phosphate deposit--------------------------- 2 

2. Photograph of ridge capped by iron ore and underlain by phosphate 

rock --------------------------------------------------------------------------------------------------- 3 

3. Geologic map and cross sections of the Bambuta phosphate deposit -------------------------------------------------------------------------------------------------- 6 

4-8. Photographs showing: 

4. Bold magnetite-martite outcrop showing distinct foliation 

parallel to bedding at crest of ridge·----------------------------------- 8 

5. Sheared granitic gneiss west of the Bambuta phosphate 

area --------------------------------------------------------------------------------------- 9 

6. Iron-ore breccia (dark color) in phosphate rock--------------------- 10 

7. Phosphate in iron ore-------------------------------------------------------------- 11 

8. Types of phosphate rock from drill hole BB-38 and several 

outcrops -------------------------------------------------------------------------------- 13 

9. Photomicrographs showing martite replaced by phosphate minerals -------------------------------------------------------------------------------------------------- 14 

10. Photomicrograph of a grain mount of phosphate minerals from 

drill-core BB-38 at 25.3 m depth----------------------------------------------------- 16 

III 
IV CONTENTS 

TABLES 

Page 

TABLE 1. X-ray powder diffraction data on strengite from the phosphate deposit ------------------------------------------------------------------------------------------------------ 17 

2. Phosphate minerals identified in drill cores------------------------------------------- 18 

3. Chemical composition of phosphate samples---------------------------------------- 19 
THE BAMBUTA PHOSPHATE DEPOSIT, 

LIBERIA-A RECONNAISSANCE REPORT 

By SAM ROSENBLUM and S. P. SRIVASTAVA1 

ABSTRACT 

A layer of iron and aluminum phosphates as much as 38 meters (m) near Bambuta, 

Liberia. lies on granitic gneiss and is overlain by metasedimentary iron-formation 

that is an extension of the iron-formation at Bomi Hills. The deposit is 70 km north of 

Monrovia and 25 km east of the Bomi Hills iron mine. 

The phosophate minerals, mainly members of the orthorhombic variscite-strengite 

series, are generally iron bearing in the upper part of the deposit and tend to be more 

aluminous in the lower part. The Bambuta phosphate deposit may be metasedimentary in origin or it may have been formed by replacement of iron-formation and 

granitic gneiss by phosphatic solutions moving laterally from an organic source. 

Diamond drilling has shown minimum reserves of about 1 million metric tons of 

phosphate rock containing about 32 percent P20 5 , or 1.5 million tons containing 28 

percent P 20 5 • Further exploration may increase these estimates of the reserves. 

INTRODUCTION 

LOCATION AND ACCESSIBILITY 

The phosphate deposit near Bambuta is in Liberia Mining Co. 

(LMC) concession Lot 3C, 70 km north-northeast of Monrovia and 25 

km east of Bomi Hills iron mine, the principal mine operated by LMC 

(fig. 1). The nearest town is Bambuta, 1. 7 km northeast of the 

phosphate deposit. That name was used to designate the deposit (Axelrod and others, 1952), but no official name has yet been assigned to 

the small area in which the deposit is located. The Bambuta 

phosphate deposit is easily reached by a good laterite-surfaced road 

from the Bomi Hills mine (fig. 2) or by an inferior road connecting 

Monrovia and Bopolu that passes about 1 km east of the phosphate 

deposit (fig. 1 ). 

The phosphate deposit is associated with iron-formation and iron 

ore that support a prominent ridge crossed by the road to Bomi Hills 

1

With the Liberia Geological Survey, deceased July 1979. 

1 
2 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

(figs. 1 and 2). The iron ore on the northwest flank of this ridge had 

been mined out before this investigation (1969-70), and mining operations had been started on the south side. 

PREVIOUS WORK 

Phosphate minerals at Bomi Hills and Bambuta were briefly 

described by Axelrod, Carron, Milton, and Thayer (1952) who iden60 

57' 

N. 

0 

_.TO BOMI HILLS 

1 MILE 

SIERRAVJ'"~ go LEONE ( \GUINEA 

/ I /'j- / ~/ ~ ;Bomi aBopolu ( 

'--HillsX ~Bambuta ) IVORY 

....__ . ?_ <"' COAST ~Monrov1a ./" -<9 'l 

60 <1~<1 ·v~ '" " ~h .. , ~ ~ "'r ./ C> "-,, "<-<'/ 

0 100 KILOMETERS ) i---------rJ ' 5o- 0 50 MILES "" ( Oc.e~ 

<l.t,"'~-

FIGURE 1.-Index map showing location of phosphate deposit near Bambuta, Liberia. 

Unlabeled circles are groups of houses. 
INTRODUCTION 3 

FIGURE 2.-Ridge capped by iron ore and underlain by phosphate rock. View east 

along road from Bomi Hills. 

tified phosphosiderite and strengite at both localities; in addition, 

leucophosphite and other phosphate minerals were found at Bomi 

Hills. Moreover, this is one of the earliest reported occurrences of 

leucophosphite. 

The presence of high-grade phosphate rock in the Bambuta area 

became evident in 1969 after two outcrop samples analyzed by the 

Mineralogy Laboratory of the Liberian Geological Survey (LGS) were 

found to consist of aluminum phosphates containing 32 to 36 percent 

phosphate, reported as P20 5 (Srivastava, 1971). About the same time, 

a resource study of the iron-formation and other rocks in the area was 

made by W. E. Stewart and D. A. Dinkins, geologists of LGS, and W. 

A. Freeman, mining engineer of the Mining Service (Stewart, and 

others, 1969). 

PRESENT INVESTIGATION 

The possibility that the phosphate rock near Bambuta might be a 

potential mineral resource led to a preliminary examination of outcrop material by the writers in 1969. This work was done as part of a 

geologic- and minerals-appraisal program undertaken jointly by the 

LGS and the U.S. Geological Survey (USGS) under the sponsorship 

of the government of Liberia and the Agency for International 

Development (USAID), U.S. Department of State. The examination 
4 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

was followed by geologic mapping and core drilling conducted by 

LMC, which operated the property. The drilling results disclosed a 

considerable tonnage of high-grade phosphate rock composed of 

minerals of the variscite-strengite series. These minerals are iron and 

aluminum phosphates not ordinarily used in commerce. However, 

they might serve as fertilizer if used locally under appropriate conditions. The present investigation was made to identify the minerals 

and chemical characteristics of the Bambuta phosphate deposit and 

to evaluate it as a Liberian mineral resource. The estimates of 

reserves are based on the results of exploration by LMC, including 25 

drill holes and reconnaissance mapping (Peter Van Rooijen, written 

communication, 1971). 

ACKNOWLEDGMENTS 

We wish to acknowledge the generous assistance of the following 

individuals and institutions who cooperated with us in this investigation: Charles Milton, George Washington University, Washington, 

D.C., for-X-ray confirmation of the aluminum phosphates, variscite 

and augelite, in the first- two outcrop samples examined; the U.S. 

Tennessee Valley Authority (TV A), Division of Chemical Development, and Rod MacDonald, Engineer, USAID, Liberia, for chemical 

analysis of a phosphate sample; R. E. Guth, General Manager, and 

Peter Van Rooijen, Geologist, LMC, for permission to examine the 

phosphate area and for making available unpublished reports, 

geologic maps, cross sections, and phosphate-rock samples from the 

drill cores; Ronald Ames, Soil Chemist, Liberian Agricultural Company, for testing samples of pulverized phosphate to determine the 

efficiency of the material as a fertilizer; David Pegum, Geophysicist, 

United Nations Mineral Survey, for testing radioactivity of samples; 

J. B. Cathcart, USGS, and E. L. Phillips, LGS, for X-ray identification of the phosphate minerals in several core samples. 

GEOLOGY 

The Bambuta phosphate deposit is exposed just below the crest of 

a ridge that rises 30 to 120m above the general terrain (figs. 2 and 3). 

The ridge is capped by iron-formation that normally lies conformably 

on polymetamorphic, coarsely layered to finely laminated granitic 

(biotitic quartzo-feldspathic) gneiss (figs. 4 and 5). The foliation of the 

gneiss strikes about N. 70° E. and dips from 60° NW. to vertical. 

Iron-formation, consisting chiefly of itabirite, iron-silicate formation, 

lenses of quartzite at least 70 m thick, and masses of amphibolite as 

much as 50 m thick, is commonly folded into the gneiss in this part of 

Liberia (White and Leo, 1969, p. 7 -8). The iron-formation capping the 
GEOLOGY 5 

ridge contains local pods of ore-grade magnetite and/or martite (R. D. 

Jones and Bernard LeFevre, Jr., written communication, 1964). 

Aeromagnetic data indicate the iron-formation near Bambuta is an 

easterly extension of the iron ore at Bomi Hills (Behrendt and W otorson, 1974). Northwest-trending diabase dikes of the Gibi dike system 

crop out a few kilometers to the south of the area, and a few diabase 

dikes are exposed near Bopolu (White and Leo, 1969). 

The metasedimentary rocks of the ridge have sinuous horizontal 

fold axes subparallel to the ridge line. The overall structure appears 

to be similar to the infolded iron-rich metasedimentary synclinoria in 

older granitic gneiss described by Thienhaus and Stobernack (1967) 

in the Bong Range, by Berge (1968) in the Nimba Range, by White 

(1973) and White and Leo (1969) elsewhere in Liberia. However, the 

extent of infolded metasedimentary rocks near Bambuta is much 

smaller than that described at the aforementioned places. Rubidiumstrontium ages (Hedge and others, 1975) ·of rocks from the Bong 

Mine indicate that the iron-formation is at least 2,750 million years 

old. 

LMC's 25 drill holes, which disclosed a layer of phosphate rock as 

much as 38 m thick between the Precambrian iron-formation and the 

granitic gneiss, are along six north-south lines in an area about 150m 

north-south and 360 m east-west along the ridge. Core from hole 

BB-53 (fig. 3) showed small lenses of iron-formation within the layer 

of phosphate rock as well as below the layer. From the drill-hole data 

and the general geology, we inferred that the phosphatic layer lies on 

a mature topography of weathered granitic gneiss and is in turn 

overlain by the main beds of iron-formation. 

Iron ore south of the crest of the ridge is fractured and brecciated, 

and tan phosphate rock fills the fractures and the interstices among 

the breccia fragments in a zone about 30 m vertically below the 

highest peak and downslope to the south (fig. 6). Locally, considerable replacement of magnetite by phosphate minerals has taken 

place along the margins of phosphate-filled fractures (fig. 7). 

Analyses of iron ore overlying the phosphate layer showed tenors of 

P20 5 as high as 2.1 percent. 

Rock exposed in the mined area to the northwest of the peak of the 

ridge is mainly well-fractured blocks of iron ore between minor faults. 

The layering in these blocks dips in various directions and amounts, 

and in places blocks of iron ore a few meters across are underlain by 

soft clayey rubble. Phosphate rock was not recognized in the mined 

area except for two masses in loose rubble on the northeast side of the 

excavation. Other rocks seen in this area were thin layers of ironformation and magnetite-rich schist, but attitudes of these were not 

apparent. 
6 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

60 

56' ,--------------,--------~ ~~---------------,--------------, 

30"N 

50 100 

33,500 

150 METERS 

0 100 200 300 400 FEET 

CONTOUR INTERVAL 50 FEET DATUM IS MEAN SEA LEVEL 

/ 33.000 

Base from Liberia Mining Co., 1971. 

and published by permission. Geology by R. D. Jones and Bernard Lefevre. 1964, 

and P. Van Rooijen, 1971. 

28.500 E 

32.500 N 

EXPLANATION 

Overburden (in cross sections only) 

Iron ore 

Iron formation 

Phosphate rock containing an average of 32 percent P20s 

(in cross sections only) 

Phosphate rock containing less than 28 percent P20s 

(in cross sections only) 

Granite gneiss with intercalated schist 

Area of large boulders of phosphate rock 

Contact, dashed where inferred 

Syncline -+-Anticline 

Strike and dip of foliation 

Dip of foliation (in cross sections only) 

Liberia Mining Company grid 

88 40o Drill hole 

FIGURE 3.-Geologic map and cross sections of the Bambuta phosphate deposit. 
METERS 300 

280 

260 

240 

METERS 300 

280 

260 

240 

220 

METERS 

A 

B 

c 

300 l 280-

260 

240-

GEOLOGY 

220 ~F===+zz:z::8Wv;P" 

METERS 260 

240 

220 

200 

E 

METERS F ~ ~ ;;; F' FEET 

200 ~co ~ t 650 180 co ' h 99 600 9 _ p_ 550 

FIGURE 3.-Continued. 

A' 

B' 

C' 

FEET 

~ 950 

900 

850 

800 

750 

FEET 

850 

800 

750 

700 

650 

600 

7 

A moderately radioactive layer of phosphate rock at least 3 m thick 

and 60 m long dips about 20 ° S. into the north side of the ridge where 

the radioactive layer is exposed in a road cut (north side of map area, 

fig. 3). The level of radioactivity of this layer, as measured by handheld scintillation counter, reached 250-300 counts per second (cps) 

compared to the background value of the granitic gneiss of 50 cps. No 

radioactive mineral was recognized in the field, but a test of a sample 

from this layer by gamma-ray spectrometer indicated that the 

radioactivity was due to uranium. 

Two hundred and fifty meters southeast of the peak and about 90 

m lower in altitude the road crosses a zone of bouldery phosphate 
FIGURE 4.-Bold magnetite-martite outcrop showing distinct foliation parallel to bedding at crest of ridge. View northwest. 

00 

ttl 

> 

~ 

ttl 

e 

>-3 

> 

'"0 

::r: 

0 

en 

'"0 

::r: 

> >-3 

M 

tj 

M 

'"0 

0 

en 

...... 

_>-3 

t""' 

...... 

ttl 

M 

~ 

> 
FIGURE 5.-Sheared granitic gneiss west of the Bambuta phosphate area, showing variation from fine laminae to 

coarse banding, a fold on left side, and boudinage. 

Cl 

trl 

0 

t'"' 

0 

Cl 

--<: 

e.o 
10 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

FIGURE 6.-Iron·ore breccia (dark color) in phosphate rock. 

rock about 80 m wide. About 70-260 m east of this outcrop are two 

more exposures of phosphate rock about 70 m and 100m across (fig. 

3). The level of radioactivity at these outcrops was 120-160 cps. 

In summary, the metasedimentary rocks that support the ridge 

near Bambuta are apparently the same as the sequence containing 

iron-formation and iron ore at Bomi Hills 25 km to the west and at 

Bong Mine 28 km to the east-southeast. However, only at Bambuta 

are these metasedimentary rocks known to include considerable 

phosphate rock underlying and interbedded with the iron-formation. 

MINERALOGY 

Phosphate minerals from this deposit were first identified by Axelrod, Carron, Milton, and Thayer (1952) as phosphosiderite and 

strengite, of which phosphosiderite was the more abundant. Their 

samples presumably were collected from outcrops and consisted of 

phosphate minerals that filled fractures in magnetite or that formed 

cement around iron-ore breccia. 

In the present study, it was learned that the phosphate deposit is 

polymineralic and that the variation in the component phosphate 
MINERALOGY 11 

FIGURE 7.-Phosphate in iron ore; A, phosphate minerals fill fractures near lower contact of foliated iron ore; B, iron ore is partly replaced along fracture filling. 
12 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

minerals is nonsystematic. The main phosphate minerals identified 

by optical and X -ray methods are classified structurally and 

chemically as: 

Composition Orthorhombic 

Variscite 

Barrandite 

FeP04 · 2H20 .. .. .. . Strengite 

Monoclinic 

Clinovariscite (metavariscite) 

Clinobarrandite 

Clinostrengite (metastrengite, 

phosphosiderite) 

In addition, the following were identified in different samples and in 

small amounts: augelite, Al2P04(0H) 3; barbosalite, Fe2+Fe 3+P04 

(0H)2; turquoise, CuAls(P04)4(0H)a ·5H20; and wavellite, Ab(P04)2 

(0H)J.5H20.) 

Surficial samples are locally rich in variscite or strengite, but subsurface samples are mainly aluminian strengite. Core samples include 

minerals in the orthorhombic variscite-strengite series and the 

monoclinic analog of strengite, clinostrengite. Subsurface samples 

generally do not show any systematic changes with depth, except 

that in drill-hole BB-38 (fig. 3) a gradual variation was found from 

iron-rich phosphate at 3 m to an aluminum-rich phosphate at 38 m 

depth. More detailed studies are required to better define the variations in mineralogical characters of this deposit. 

PHYSICAL PROPERTIES 

The physical properties of two surficial samples of phosphate rock 

described by Srivastava (1971) and 25 samples selected from cores 

from 10 drill holes in the Bambuta area are rather similar. The 

phosphate rock is generally well-consolidated earthy material. Most 

hand specimens are massive and chalky, but some are distinctly 

layered, including some outcrop samples (fig. 8, right side) that are 

porous and harder than the core samples. Some specimens of 

phosphate rock contain residual fragments of quartz or iron ore in a 

matrix of secondarily precipitated phosphate minerals. Under the 

microscope, core-samples BB-40/13.1 m and BB-38/39.6 m show partial to almost complete replacement of martite by phosphate minerals 

(fig. 9). Where replacement textures are observed, veins of younger 

phosphate minerals can be seen to cut across the replacing 

phosphate. In outcrops some white pisolitic nodules of phosphate as 

much as 5 millimeters (mm) across were noted locally. Fossils were 

absent from all phosphatic material studied from this deposit. 
FIGURE 8.-Types of phosphate rock from drill hole BB-38 and several outcrops; iron ore is in breccia at upper right, 

and the other samples show porous and layered features. 

s;: 

z 

tr:l 

::0 

;l> 

t'"' 

0 

0 

~ 

1--' 

CIJ 
14 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

FIGURE 9.-Photomicrographs showing martite replaced by phosphate minerals. A, 

Core sample BB-40 at 13.1 m depth, iron oxide (black) partially replaced by amorphous phosphate minerals (gray); pore space is white; plane polarized and incident 

Colors of surficial samples range from buff to brown, depending on 

the content of hydrous iron oxide. Colors of phosphate core samples 

are white, yellow, brown, red brown, and shades of gray. In one core 

sample, small spots of green turquoise were noted in white 

phosphate. 

Densities of powders from outcrop samples are between 2.52 and 

2.66, but the densities of powders from core samples vary by color: 

2.45 for white to yellow phosphate rock, 2.90 for yellow-brown 

phosphate rock, and 2.92 for dark-brown phosphate rock. The density 

of samples of powdered phosphate rock from drill-hole BB-38 

decreases from 3.11 at 3-m depth to 2.36 at 38-m depth, but this 

decrease in density with the depth of the sample was not observed for 

phosphate rock from other drill holes. No attempt was made to 

remove minor amounts of quartz, magnetite, and limonite. Thus, the 

above figures are bulk powder densities. 

Differential-thermal-analysis (DT A) data on two outcrop samples 

showed strong endothermal reactions between 240 o and 260 o C 

characteristic of clinostrengite and similar to data reported for this 
MINERALOGY 15 

light, X80; B, Core sample BB-38 at 39.6 m depth, iron oxides (black) almost completely replaced by phosphate minerals (gray) and a late veinlet of phosphate 

minerals (near left side); plane polarized light, X80. 

mineral by Rao and Da Cunha e Silva (1964, p. 298-299). In contrast, 

core samples yielded endothermal peaks that ranged between 255 o 

and 305 o C for most specimens. 

OPTICAL PROPERTIES 

The presence of several phosphate minerals in the Bambuta deposit 

was confirmed by the optical properties of samples from outcrop and 

the subsurface. Outcrop samples were found by Srivastava (1971) to 

contain colorless and light-brown microcrystalline minerals. The colorless mineral, having refractive indices of 1.543 to 1.550, probably is 

augelite, and the light-brown mineral, having indices of 1.585 to 

1.615, is iron-bearing variscite. An outcrop sample examined by a 

TV A laboratory was reported to have a mean refractive index of 

1.615, indicative of ferrian variscite. One outcrop sample of buff 

microcrystalline material was determined in the present investigation to have mass refractive indices that ranged from 1. 725 to 1. 730, 

showing the minerals to be strengite or clinostrengite (Winchell and 
16 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

Winchell, 1951, p. 186-187). Core samples examined by the writers 

contained microcrystalline material and radial-columnar grains (fig. 

10). The radial-columnar grains are from the late veinlets of crustified 

phosphate minerals (fig. 9B, left side). The microcrystalline material 

has indices between 1.65 and 1. 73 and is most likely barrandite and 

strengite or their monoclinic analogs. The radial-columnar grains 

have indices between 1.59 and 1.62 and are variscite. 

X-RAY RESULTS 

Powder-camera and diffractometry methods of X -ray analysis were 

used to identify the phosphate minerals in 27 samples from 2 areas of 

outcrop and 8 drill holes. One outcrop sample was found by X-ray 

analysis to consist mainly of ferrian variscite and minor augelite 

(Charles Milton, written communication, 1969). Another outcrop 

sample from the southwest part of the deposit (R0-203A, fig. 8) was 

found by diffractometry methods to be mainly strengite (table 1). 

Sufficient data was available on this sample to obtain computerderived cell parameters; a=10.163 angstroms (A), b=9.919 A, 

c=8. 783 A, and cell volume, 885.32 cubic angstroms. 

FIGURE 10.-Photomicrograph of a grain mount of phosphate minerals from drill-core 

BB-38 at 25.3 m depth, showing radial-columnar and microcrystalline grains. 

Refractive-index liquid is 1.54; X240. 
MINERALOGY 17 

In an earlier examination, X-ray diffraction data by J. B. Cathcart 

(written communication, 1971) indicated variable amounts of nine 

phosphate minerals plus quartz, goethite, and kaolinite in six core 

samples. The phosphate minerals he recognized are clinostrengite, 

strengite, barrandite, variscite, ferrian variscite, clinovariscite, 

augelite, barbosalite, and wavellite. 

In this investigation, major and minor phases are identified in core 

samples as shown in table 2. 

It is apparent from the table that most of the Bambuta phosphate 

deposit near the crest of the ridge consists of iron-rich barrandite, 

strengite, clinobarrandite, and clinostrengite. Variscite is the major 

phase on the southeast side of the ridge in depth as well as near the 

surface. The heterogeneous mixture is undoubtedly the result of 

nonequilibrium conditions during the formation of the deposit. 

Epigenetic modification in the zone of weathering may have caused 

some of the heterogeneity. 

TABLE 1.-X-ray powder diffraction data on strengite sample R0-203A from the 

phosphate deposit at Bambuta, Liberia 

[Ni-filtered CuKa radiation=l.5418; analyst: Sam Rosenblum; strengite standard from Joint Committee on Powder 

Diffraction Standards card 15-513. Leaders, (---), indicate no data or not indexed] 

R0-203A Strengite Standard R0-203A Strengite Standard 

----ci(~) d(~) 

(obser- (ca1cud(Al 

I ( obser- ( ca 1 cuved) 1 a ted) I hk1 d(A) I/I1 hk1 ved) 1ated) I hk1 I/I 1 hk1 

5.517 5. 521 36 111 5.50 80 111 --- --- --- --- 2.180 40 004,402 

4.959 4.959 45 020 4. 95 60 020 --- --- --- --- 2.134 10 133,104 

4.514 4.522 32 210 --- --- --- --- --- --- --- 2.101 40 142 

4.403 4.392 100 002 4.38 100 002,201 2.086 2.083 11 332 2. 035 10 --- 4.012 4.021 25 211 3.98 60 211 '121 --- --- --- --- 2.001 10 22 3,204 

3.978 3.974 9 121 --- --- --- --- --- --- --- 1. 956 10 --- 3. 729 3.735 8 112 3. 72 40 112 --- --- --- --- 1. 910 10 --- 3.548 3.549 17 220 --- --- --- 1. 900 1. 901 5 151 1. 830 10 --- --- --- --- --- 3.29 20 022,202 --- --- --- --- 1 .803 20 ---

3.162 3.161 8 301 --- --- --- 1. 776 1. 775 8 440 1. 767 10 ---

3.125 3.123 17 122 3 .ll 60 122,212 1. 738 1 . 739 6 441 --- --- --- 3.017 3.011 10 311 3.00 60 311 1. 722 1. 721 6 234 1.720 10 --- 2.963 2.960 17 131 2.95 60 131 1. 702 1. 703 5 252 1 .690 10 --- 2. 926 2. 923 7 003 --- --- --- 1. 664 1.663 12 601 1.650 20 ---

2.875 2.813 19 103 --- --- --- 1. 635 1 .634 7 125 --- --- ---

2.773 2. 771 35 230 --- --- --- 1 .631 1 .632 8 160 1 .625 20 --- --- --- --- --- 2.69 10 113 1. 608 1 .610 7 334 1. 602 10 --- --- --- --- --- 2.63 10 032.231 1 . 596 1. 595 7 352 1. 568 10 --- 2.552 2.556 25 132 2.54 80 132 1 . 559 1. 560 9 305 1. 554 10 --- 2.524 2.521 12 023 --- --- --- --- --- --- --- 1. 540 10 ---

--- --- --- --- 2.445 40 123,21 3 1. 505 1. 506 4 622 --- --- --- --- --- --- --- 2.400 10 140 --- --- --- --- 1 .478 10 --- --- --- --- --- 2.360 10 411 1. 436 1. 437 4 710 --- --- --- 2. 343 2.344 9 232 --- --- --- l. 387 l- 387 l 0 425 --- --- ---

--- --- --- --- 2.273 l 0 331 1. 369 1. 369 7 461 --- --- ---

2.229 2.228 6 240 --- --- --- --- --- --- --- --- --- ---
18 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

TABLE 2.-Phosphate minerals identified in drill cores 

[V, variscite; B, barrandite; S, strengite; CB, clinobarrandite; CS, clinostrengite. Leaders, (---l. none) 

Orthorhombic Monoclinic 

Drill hole Depth (m) t1ajor Minor Major Minor 

88-38--------- 3 8,S C8 cs 

88-38--------- 9 s C8 

88-38--------- 1 7 s cs 

88-40--------- 5 s cs 

88-40--------- 8 s cs CB 

88-41--------- 6 8 s CB cs 

88-42--------- 18 s cs 

88-42--------- 27 8 s cs 

88-52--------- 4 8 s 

88-53--------- 2-7 B v CB cs 

88-53--------- 7-12 8,S 

88-56--------- 0-7 v 

88-57--------- 7-23 B cs 

88-57--------- 38 v 

CHEMICAL COMPOSITION 

Previously reported partial analyses of two outcrop samples of 

phosphate rock from the Bambuta deposit showed 35.71 percent P20 5 

and 7.65 percent iron, and 31.88 percent P20 5 and 9. 73 percent respectively (Srivastava, 1971). Partial analyses of core samples by the 

LMC laboratory indicate as much as 42.85 percent P 20 5 where 

variscite is present. This is an unusually high value. In most of the 

other phosphate-bearing core samples the tenors averaged about 30 

percent P 20 5 , and these tenors are the bases for the reserves discussed below. 

More recent analyses of two outcrop samples are given in table 3 

(Nos. 2 and 3). The method of analysis by the TVA laboratory (written commun., 1970) is not known. The analysis by E. L. Mosier was 

by emission spectrography, but H20 was determined by the senior 

author by weight loss on ignition after drying at 105 oCto constant 

weight. 

ORIGIN OF DEPOSIT 

Any hypothesis for the origin of the Bambuta phosphate deposit 

must account for certain critical relations: 

1. The lenticular layer of phosphate rock underlies a cap of enriched 

iron-formation as much as 35 m thick and replaces the iron ore along 

the contact. 
ORIGIN OF DEPOSIT 19 

2. Minor lenses of iron-formation occur within and below the 

phosphate layer. 

3. Bedding and foliation in phosphate rock and iron-formation are 

roughly conformable with the surface of the subjacent granitic 

gneiss. 

4. The mineralogy of samples from 25 drill holes shows the composition of the phosphatic layer to range from iron to aluminum 

phosphates, the predominant mineral being aluminian strengite, the 

orthorhombic iron phosphate containing some aluminum. Aluminum 

phosphates are generally a minor component of this phosphate 

deposit. 

5. Aluminum phosphate tends to increase and iron phosphate to 

decrease with increasing depth below the ground surface. 

TABLE 3.-Chemical composition of phosphate samples from the Bambuta deposits, 

Liberia 

Sample 

Si02-----

Al 0 ---- 32.28 2 3 

Fe 2o3----

P205----- 44.93 

H20------ 22.79 

Ti02-----

To ta l - - - - l 0 0 . 0 

[Leaders, (···), not reported] 

2 

ll .4 

30.6 

8.9 

32.9 

16.2 

100.0 

3 

15.2 

1.2 

34.4 

32.2 

16.6 

.4 

l 00.0 

SAMPLE DESCRIPTIONS 

l. ALP04 ·2H2

0, variscite, ideal analysis. 

4 5 

l .4 

40.8 42.74 

38 .l 37.99 

19.7 19.27 

100.0 l 00.0 

2. Outcrop sample, analysis by TVA Division of Chemical 

Development. 

3. Outcrop sample, spectrographic analysis by E. L. Mosier, 

U.S. Geological Survey. 

4. Analysis 3 recalculated to 100 percent after removal of 

Si02 and Ti02 values. 

5. FeP04 ·2H2

0, strengite, ideal analysis. 
20 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

Two equivocal modes of origin are possible for the Bambuta 

phosphate deposit: (1) metamorphism of an inorganic marine 

precipitate of Precambrian age; and (2) epigenetic replacement of 

underlying rock by phosphatic solutions from surficial guano 

deposits of late Tertiary or Holocene age. 

Not enough field and laboratory data are available at present to 

permit a clear decision between these two modes of origin. Therefore, 

we present each one with discussions of pertinent processes involved. 

Additional field evidence may help to determine which mode is correct. 

MARINE PRECIPITATION 

The lack of clear biogenic evidence led us to seek information on the 

inorganic precipitation of phosphates and on the association of 

phosphates with iron-formation that would support an inorganic 

origin. A number of reports on inorganic formation of phosphate 

minerals describe geologic conditions that easily account for all the 

critical relations listed above. 

Berge (1970, p. 1) noted that variscite and strengite are frequently 

described from environments that would tend to preclude the fecally 

instigated origin. He showed that variscite will form in acidic environments from interaction of phosphate ions, derived from apatite 

in iron-formation, with aluminum from clay minerals concentrated at 

or near the surface of iron-ore deposits such as those in the Nimba 

Range, Liberia. From experimental evidence and solubility calculations, he concluded that alumina and orthophosphoric acid ions combine at low pH to form aluminum phosphates which are relatively insoluble. Below pH 4, provided sufficient alumina and phosphorus are 

present in solution, variscite or similar minerals should form. At pH 

higher than 4, there is insufficient ionized aluminum to form these 

minerals (Berge, 1970, p. 5-6). 

Iron-formation in western Liberia consists mainly of oxide and 

silicate facies (James, 1966, p. W12; White and Leo, 1969, p. 7-9; 

White, 1973, p. 8-10; Theinhaus and Stobernack, 1967, p. 53). The oxide facies is generally referred to as itabirite and comprises finely 

laminated rocks made up of quartz, iron oxide, and accessory 

minerals. The silicate facies consists of various proportions of quartz 

and iron-silicate minerals such as amphibole, pyroxene, garnet, and 

biotite, with or without iron oxide or pyrite, but where the ironformation has been affected by hydrothermal enrichment, as at Bomi 

Hills, other assemblages of silicate minerals have formed (White and 

Leo, 1969, p. 7). We believe that the oxide facies of the iron-ore 

deposit (magnetite and/or martite) at Bambuta, like that at Bomi 

Hills, was enriched. The enrichment may have been caused by 
ORIGIN OF DEPOSIT 21 

postmetamorphic hydrothermal processes, but it also could have 

been caused by metasomatic replacement during greenschist- or 

epidote-amphibolite-facies metamorphism, but only at temperatures 

below 200 o C. The underlying phosphate had to be affected by the 

same processes, and probable changes within the phosphate layer are 

discussed below. 

At the Bambuta phosphate deposit, the close association with interbeds of iron-formation, as defined by James (1966, p. W 46), and its 

position below the main iron-formation indicate that the phosphate 

was deposited before the main layer of iron-formation. Although 

phosphorus, like iron, is present in small quantities in seawater, it 

may be precipitated in considerable quantities locally if the conditions are favorable. The association of iron silicates such as 

glauconite with some phosphatic deposits suggests that the environments of phosphorite and ironstone deposition have many factors in common (Pettijohn, 1957, p. 475-476). The mechanism of 

phosphate deposition involving marine organisms in a restricted part 

of post-Cambrian seas has been described by McConnell (1965), 

Youssef (1965), and McKelvey (1967), but these organic processes are 

unlikely to apply to a Precambrian deposit. 

We believe that an origin of the Bambuta phosphate deposit by inorganic marine precipitation followed by low-grade metamorphism 

can explain the observed geologic relations. The following sequence 

of events is proposed: 

1. Inorganic marine precipitation of calcium phosphate interbedded 

with thin layers of iron-formation occurred on a granitic gneiss terrane, followed by the deposition of a thick sequence of iron-rich 

sediments, at least 2, 7 50 million years (m.y.) ago. This age is based on 

ages of schist and granitic gneiss from the Bong Mine deposit that 

were dated by Hedge, Marvin, and Naeser (1975, p. 426). 

2. Low-grade regional metamorphism of the sediments enriched the 

iron-bearing sediments to iron ore and transformed the calcium 

phosphate to hydrous iron and iron-aluminum phosphates by 

metasomatic reactions. Along the contact between iron-formation 

and the phosphate layer each bed was affected by the chemistry of 

the other. The upper part of the phosphate layer was converted to 

iron phosphates by diffusion of iron from the overlying ironformation, which itself became somewhat phosphatic. The lower part 

of the phosphate layer became more aluminous than the upper part 

owing to the supply of aluminum from the underlying granitic gneiss. 

Little is known about temperature-pressure conditions of stability 

of these phosphates. Considering DT A breakdown conditions of 

215 °-300 o C at one atmosphere pressure and extrapolating to 

moderate burial pressures at depths of 1-5 km, then these hydrous 
22 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

phosphates had to form at temperatures less than 200 o C, perhaps in 

the lowest grade of amphibolite facies conditions. If they are unstable 

under these conditions, then lower temperature postmetamorphic 

hydrothermal processes need to be invoked to explain the replacement of the original calcium phosphate by hydrous iron and 

aluminum phosphates. Either by metamorphic or hydrothermal processes, metasomatic fluids had to remove large quantities of silica 

from the iron-formation at the same time that similar large quantities 

of iron and aluminum replaced calcium in the phosphate layer. 

Lacking definite criteria for metamorphic or hydrothermal effects 

in the phosphate rock as well as in the overlying iron ore, we are 

unable to decide which of these two processes was responsible for the 

enrichment of the iron ore and for the transformation of the 

phosphate minerals in the Bambuta deposit. However, during or 

after metamorphism, plastically squeezed phosphate filled open fractures in and partly replaced the more dense and brittle iron ore along 

the contact, completely enclosing some iron-ore breccia (fig. 6). The 

metamorphism probably took place during the 1,600-m.y. metamorphic event (Stobernack, 1968, p. 46-49); similarly, a postmetamorphic 

injection and replacement of iron-formation by the underlying 

phosphate could have been as young as the 500± m.y. Pan-American 

event (Hedge and others, 1975, p. 426). 

3. Erosion stripped most of the overlying metasedimentary rocks, 

leaving a synclinal relic that is more resistant than the surrounding 

crystalline rocks. Near-surface chemical reactions of iron and ironaluminum phosphates with aluminous materials favors replacement 

of relict iron phosphates by aluminum phosphates. Thus, we infer 

that aluminum phosphates in this deposit may have had two modes 

of origin: (1), metamorphic replacement of calcium phosphate at 

depth during Precambrian time, and (2), surficial replacement of iron 

phosphate in Holocene time. The aluminum phosphates produced by 

these two processes should be different minerals, but we are unable to 

confirm this at present. 

BIOGENIC ORIGIN 

A biogenic origin for the iron phosphate minerals at Bomi Hills was 

proposed by Axelrod, Carron, Milton, and Thayer (1952, p. 883) who 

believed that they "***formed by the interaction of bat dung, or 

substances derived from it, with exposed iron oxide***the 

phosphates from Bambuta differ from those at Bomi Hills; the difference, however, may reflect incomplete knowledge of the two 

deposits rather than any significant difference of origin. In both, the 

source of the phosphorus is believed to be the dung of bats.'' 
RESERVES 23 

In support of a biogenic origin, a test of the radioactivity of 12 

powdered drill-core samples and one outcrop sample from the Bambuta deposit showed a maximum of 0.002 percent uranium. The data 

falls in the range of uranium (0.001 to 0.003 percent) indicated by 

Cathcart (1978) for phosphates derived from guano deposits. The 

range for uranium in marine phosphorites (0.005 to 0.030 percent 

uranium) is at least 5 to 10 times greater. 

Precipitation of phosphate minerals from guano-derived solutions 

is a simpler process than the marine-precipitation-andmetamorphism mode discussed above, but there is no evidence, at 

present, of a guano source for the phosphatic solutions. Presuming 

that such a source did exist long enough to supply all the phosphate 

in the Bambuta deposit, we propose the following history: 

1. At least 2,750 m.y. ago (Hedge and others, 1975, p. 426), ironrich sediments were deposited on a granitic terrane, and these were 

metamorphosed to iron-formation and granitic gneiss. 

2. In late Tertiary or Holocene time, while this terrane was still exposed and probably at shallow depth, guano-derived phosphatic solutions traveled laterally along the contact between the iron-formation 

and the granitic gneiss, and both rock types were replaced by 

phosphate minerals. Iron phosphates were formed from ironformation, and aluminum-rich phosphates were formed from granitic 

gneiss. The foliation in each rock type was preserved in places by the 

new phosphate materials. 

3. During or after replacement by the phosphatic solutions, fracturing in the iron-formation took place, and phosphate minerals worked 

upward into open fractures by plastic deformation (fig. 6). 

4. Erosion in Holocene time stripped the guano source of the 

phosphatic solutions and part of the iron-formation through which 

the solutions penetrated. The remaining iron-formation supports the 

crest of the ridge over the phosphate deposit. 

RESERVES 

Stewart, Dinkins, and Freeman (1969) indicated 450,000 metric 

tons of phosphate rock in the Bambuta phosphate deposit, but the 

basis for these reserves was not given. Data from 25 drill holes that 

intersected the phosphate rock were used in conjunction with 

geologic maps and cross sections by Peter Van Rooijen (written commun. 1971) to calculate a minimum reserve of 1.5 million metric tons 

of phosphate rock containing 28 percent P20 5 or 1 million tons containing 32 percent P20 5 • These estimates are tentative because the 

structure is not well enough known to permit a valid interpretation of 

the number of phosphate layers and their true thicknesses. 
24 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

POTENTIAL USE 

Little is known about the release of phosphate from iron-rich 

members of the variscite-strengite series of phosphate minerals. Inasmuch as aluminian strengite is the apparent major mineral in the 

Bambuta deposit, the treatment required to make the phosphate 

available will affect the potential use of the phosphate rock as fertilizer. 

Untreated phosphate rock from the Bambuta deposit was compared by LAC to imported calcium phosphate fertilizer and found to 

be deficient in its release of phosphate. Experiments were then undertaken by TV A to determine the effects of calcining the phosphate 

rock at 300 ° C. The results showed that calcining caused 20 to 25 percent of the total phosphate to become citrate soluble and thus 

available to plants. 

A study of the uptake of phosphorus by corn from amorphous 

aluminum phosphate indicated that crop yields in acid soil (pH 5.5) 

were between 60 and 70 percent of those from soils to which a comparable application of monocalcium phosphate had been made, but 

that in calcareous soil (pH 8.5 ), both amorphous aluminum phosphate 

and the crystalline phase, variscite, were better sources of phosphate 

than monocalcium phosphate (Taylor and others, 1963, p. 150). Thus, 

lime is required with aluminum phosphates to ensure adequate 

availability of phosphate to plants. 

From general chemical considerations under surface conditions, 

iron phosphates are somewhat more water soluble than aluminum 

phosphates and presumably they would yield more phosphate ions. 

However, in acidic soils such as those overlying the laterite of 

Liberia, the phosphate ions are likely to precipitate rapidly as a 

relatively insoluble aluminum phosphate before they can be taken up 

by plants. Clearly, iron-bearing phosphates from the Bambuta area, 

though they may not need heat treatment, will require the addition of 

lime to keep the phosphate ions available. 

Local sources of limestone and marble for producing lime are 

unknown in Liberia. However, lime may be made from deposits of 

marine shells on the beaches of eastern Liberia or in offshore seabottom deposits. These shells could be recovered easily and roasted 

for lime. Tests by the LGS Mineralogy Laboratory showed that up to 

60 percent by weight of the beach sand consists of shells and shell 

fragments coarser than one-eighth of a millimeter (120-mesh, U.S. 

Standard screen). Similarly, shell fragments coarser than 120-mesh 

make up as much as 85 percent by weight of the sea-bottom 

sediments in the vicinity of Cape Mount. Presumably the same 

percentage of shells may be found in other offshore areas of Liberia. 
REFERENCES CITED 25 

RECOMMENDATIONS 

The estimated reserve of 1.5 million metric tons of high-grade 

phosphate rock justifies the formulation of plans for mining and 

marketing. A feasibility study is recommended for the possible mining and marketing of the material in this deposit for its obvious fertilizer potential. Other studies are recommended to determine the use 

of these phosphates in detergents, baking powder, ceramics, sugar 

refining, water softeners, pharmaceuticals, dental cements, fireproofing materials, and other industrial products. Detailed geologic mapping and additional drilling should be done to permit accurate 

calculation of reserves of phosphate rock. Further analyses of 

phosphate samples are required from surface and subsurface locations to ascertain the range in grade of P20 5 in this material. Additional mineralogical work is recommended to detect changes in 

mineralogy from place to place in the deposit that could have critical 

effects on utilization. 

REFERENCES CITED 

Axelrod, J. M., Carron, M. K., Milton, C., and Thayer, T. P., 1952, Phosphate 

mineralization at Bomi Hill and Bambuta, Liberia, West Africa: American 

Mineralogist, v. 37, nos. 11-12, p. 883-909. 

Behrendt, J. C., and Wotorson, C. S., 1974, Aeromagnetic map of the Monrovia 

Quadrangle, Liberia: U.S. Geological Survey Map I-775 B., scale 1:250,000, 1 

sheet. 

Berge, J. W., 1968, A proposed structural and stratigraphic interpretation of the 

Nimba-Gbahm Ridge area, Liberia: Geological, Mining, and Metallurgical Society of Liberia Bulletin, v. 3, p. 28-4 7. 

__ 1970, Implications of phosphorus fixation during chemical weathering in the 

genesis of supergene iron ores: Geological, Mining, and Metallurgical Society of 

Liberia Bulletin, v. 4, p. 33-43 [1971]. 

Cathcart, J. B., 1978, Uranium in phosphate rock: U.S. Geological Survey Professional Paper 988-A, 6 p. 

Hedge, C. E., Marvin, R. F., and Naeser, C. W., 1975, Age provinces in the basement 

rocks of Liberia. U.S. Geological Survey Journal of Research, v. 3, no. 4, p. 

425-429. 

James, H. L., 1966, Chemistry of the iron-rich sedimentary rocks, in Data of 

geochemistry (6th ed.): U.S. Geological Survey Professional Paper 440-W, p. 

W1-61. 

McConnell, Duncan, 1965, Precipitation of phosphates in seawater: Economic Geology,v.60,no.5,p. 1059-1062. 

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

p. D1-D21. 

Pettijohn, F. J., 1957, Sedimentary rocks (2d ed.): New York, Harper and Bros., 718 p. 

Rao, A. B., and da Cunha e Silva, Judson, 1964, Differential thermal analysis of some 

Brazilian phosphates: Anais da Academia Brasiliera Ciencias, v. 36, p. 295-303. 
26 BAMBUTA PHOSPHATE DEPOSIT, LIBERIA 

Srivastava, S. P., 1970, Mineralogy of phosphate rock from Bambuta: Geological, 

Mining, and Metallurgical Society Liberia Bulletin, v. 4, p. 26-32 [1971]. 

Stewart, W. E., Dinkins, D. A., and Freeman, W. A., 1969, Field investigation of Lot 

No. 3 near Bambuta, Lofa County: Liberia Geological Survey Memorandum 

Report 53, 9 p. 

Stobernack, Just, 1968, Stratigraphie und Metamorphose des prekambrischen Grundgebirges der Bong Range in Liberia: Ph. D. thesis, Technische Universitat Clausthai, Germany, 185 p. 

Taylor, A. W., Lindsay, L., Huffman, E. 0., and Gurney, E. L. 1963, Potassium and 

ammonium taranakites, amorphous aluminum phosphate, and variscite as 

sources of phosphate for plants: Soil Science Society of America Proceedings, v. 

27,no.2,p.148-151. 

Thienhaus, R., and Stobernack, J., 1967, Investigations on facies, metamorphism, 

and tectonics of the Precambrian basement of the Bong Range, Liberia: Geological, Mining, and Metallurgical Society of Liberia Bulletin, v. 2, p. 48-58. 

White, R. W., 1973, Progressive metamorphism of iron-formation and associated rocks 

in the Wologizi Range, Liberia: U.S. Geological Survey Bulletin 1302, 50 p. 

[1974]. 

White, R. W., and Leo, G. W., 1969, Geological reconnaissance in western Liberia: 

Liberia Geological Survey Special Paper 1, 18 p. 

Winchell, A. N., and Winchell, Horace, 1951, Elements of optical mineralogy, Part II, 

Descriptions of minerals (4th ed.): New York, John Wiley and Sons, 551 p. 

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p. 590-600. 

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