Merumite A Complex Assemblage of 

Chromium Minerals from Guyana

GEOLOGICAL SURVEY PROFESSIONAL PAPER 887

Prepared in cooperation with 

the Geological Survey of Guyana
MERUMITE A COMPLEX ASSEMBLAGE 

OF CHROMIUM MINERALS FROM GUYANA
r

0 1 mm

0.1 mm

0.1 mm

0.1 mm

Photomicrographs of four merumite thin sections illustrating the diversity of mineral 

composition and structure and the fineness of grain size. Plain transmitted light.

UPPER LEFT: Green eskolaite and red-brown guyanaite. UPPER RIGHT: Banded structure of green eskolaite and yellow 

to brown guyanaite. The very bright polygonal area is a 

cross section of a quartz crystal.

LOWER LEFT : Reddish grimaldiite-mcconnellite aggregate filling LOWER RIGHT : Green eskolaite and yellow-brown guyanaite 

opening in merumite specimen. showing banded structure.
Merumite A Complex Assemblage of 

Chromium Minerals from Guyana

By CHARLES MILTON, D. E. APPLEMAN, M. H. APPLEMAN, 

E. C. T. CHAD, FRANK CUTTITTA, J. I. DINNIN, 

E. J. DWORNIK, B. L. INGRAM, and H. J. ROSE, JR.

GEOLOGICAL SURVEY PROFESSIONAL PAPER 887

Prepared in cooperation with 

the Geological Survey of Guyana

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1976
UNITED STATES DEPARTMENT OF THE INTERIOR 

THOMAS S. KLEPPE, Secretary

GEOLOGICAL SURVEY

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Merumite a complex assemblage of chromium minerals from Guyana.

(Geological Survey professional paper 887) 

Bibliography: p. 

Supt. of Docs, no.: I 19.16:887

1. Merumite Guyana. I. Milton, Charles, 1896- II. Series: United States. Geological Survey. Professional 

paper 887.

QE391.M45M47 549'.7 74-16247

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

Washington, D.C. 20402 (Paper cover) 

Stock No. 024-001-02872-7/Catalog No. I 19.16:887
CONTENTS

Abstract _________________________

Introduction and acknowledgments ____________

Merumite _________________________

Nature and occurrence -____-________

Previous studies _________________

Chemistry _______________________

Older analyses _____________________

Recent analyses _________________

Isotopic study _________________

X-ray powder data ____________________________

Petrography ________________________

Eskolaite ___________________________________

Occurrence in Finland and Guyana, and (possibly) 

Siberia ________________________

Chemistry __________________________

Guyanaite __________________________________

Occurrence in Guyana and Finland, and synthesis 

Chemistry _____________

Analyses ________________________________

Thermal behavior _____________________________

X-ray powder data ____________________

Bracewellite, CrOOH ______________________________

Occurrence and identification ______________

Chemistry _______________________

Analysis ___________________

Page Page

1 Bracewellite, CrOOH Continued

1 Electron-probe study __ __ __ - - - 16

1 X-ray powder data ___________ __ -- - - - 16

1 Grimaldiite CrOOH and mcconnellite CrOOCu 17

6 Occurrence _______________ ___ - - 17

7 Physical properties _____ _ __ - - - 18

7 Chemistry _____________ 18

7 Electron-probe analysis _____ -_ _____ _ 19

7 X-ray data __________________ 19

7 Previous syntheses of rhombohedral CrOOH and

8 CrOOCu _________________________ 19

9 Relation of mcconnellite to delafossite 20 

	y-CrOOH (synthetic) ____________ - 21

9 Synthesis _______________ 21

10 X-ray powder data and cell constants _ 22

10 Chromian gahnite ___________________ _ _ 22

10 Properties _____________________-__ 22

11 Electron-probe analysis _______ __ _ 23

11 X-ray data _________________________ 23

12 Other chromian gahnites and zincian chromites _ __ 23

13 Chromian pyrophyllite __________ _ __ _ 24

13 Gold (and silver) in merumite ________ _ _ 24

13 Accessory minerals from Director Creek _ _ 24

15 Crystallographic relationships of CrOOH compounds 28

15 References cited _________________ __ - 28

ILLUSTRATIONS

[Plates follow references]

FRONTISPIECE. Photomicrographs of four merumite thin sections illustrating diversity of the mineral composition and structure and the fineness of grain size. 

PLATE 1. Photographs of two very large merumite specimens.

2. Photomicrographs of thin sections of merumite showing eskolaite and guyanaite.

3. Photomicrographs of polished sections of merumite showing quartz, grimaldiite-mcconnellite polycrystals, and gold.

4. Photomicrograph of bracewellite crystals and electron-probe analyses of two grimaldiite-mcconnellite polycrystals.

5. Scanning-electron photomicrographs of grimaldiite-mcconnellite polycrystal and chromian gahnite crystals.

6. Photomicrographs of fine-grained topaz aggregates.

FIGURE 1. Index maps showing location of merumite-bearing area in Guyana ____________ ____

2. Sketch map of Merume River area showing location of merumite deposit ___________

3. Geologic map of Merume River valley ______________________________ -

4. Graph of unit cell edge versus composition in natural and synthetic chromian gahnite

TABLES

Page

3

4

5

23

TABLE 1. Older analyses of merumite ___________________ __ __ __ __ __ ______________ __ _____

2. Recent analyses (in percent) of two specimens of merumite (eskolaite and guyanaite) from Guyana and 

of eskolaite from Finland _____________ 

Page

7
VI CONTENTS

Page

TABLE 3. Spectrographic analyses (in percent) of four merumite samples _ 9

4. X-ray powder patterns of merumite (eskolaite 37 percent, guyanaite 63 percent), merumite approaching

	eskolaite (Guyana), guyanaite (Guyana, four strongest lines), eskolaite (Finland), and Cr2O8 9

5. Cell constants of guyanaite (from Guyana and Finland), and of CrOOH, ScOOH, and InOOH 11

6. Analyses of guyanaite ______________________ -_ __________ ______________ ____ _____ _ 12

7. X-ray powder data of (Cr, Al, Fe, Ti, Mn)02 from guyanaite, and CrO2 from CrOOH _ 13

8. X-ray powder data of (Cr, Al, Fe, Ti Mn) 2O8 ____ ---_ _ 13

9. X-ray powder data of guyanaite (from Guyana and Finland), "merumite" and synthetic guyanaite CrOOH 14

10. Analysis of merumite containing 70 percent bracewellite and 30 percent eskolaite ______ _____ __ _ 15

11. Spectrographic analysis of merumite containing 70 percent bracewellite and 30 percent eskolaite _ ______ 15

12. Electron-probe analysis (in percent) of seven crystals of bracewellite and two merumite fragments ______ 16

13. X-ray powder data for bracewellite, goethite, and eskolaite ___________ __ __ __ __ _ ___ 17

14. Chemical analysis of grimaldiite-mcconnellite polycrystals _______ _________________ ____ __ ___ 18

15. Microspectrographic analysis of grimaldiite-mcconnsllite polycrystals _____ __ _ _ __ __ ____ _ 19

16. X-ray powder pattern of grimaldiite and mcconnellite ___________ __ __ __ __ _ ____ __ 20

17. Cell constants of grimaldiite, mcconnellite, and related substances ______ ______ __ _____ ___ 21

18. X-ray powder patterns of 7-CrOOH and lepidocrocite ________ _____________ 22

19. Cell constants of 7-CrOOH and related compounds ___ __ __ ______ ______________ _ ____ ___ 22

20. X-ray powder data for chromian gahnite Zn(Al0 .7, CrOo.sHOi from Guyana and synthetic spinels ________ 23

21. Gold and silver in six fragments of merumite ___ ____ ____ ____ _ 24

22. X-ray powder data for topaz _____________________________________ __ 25

23. Analysis of topaz _________________________________________________ ______ 26

24. Crystallographic relationship of the CrOOH compounds and other metallic hydroxides _____ ______ 27
MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

By CHARLES MILTON, D. E. APPLEMAN, M. H. APPLEMAN, E. C. T. CHAD, FRANK CUTTITTA, J. I. DINNIN,

E. J. DWORNIK, B. L. INGRAM, and H. J. ROSE, JR.

ABSTRACT

Merumite, found as black grains or pebbles in a small 

stream bed in Guyana, is a complex assemblage of hexagonal 

eskolaite (predominant), essentially Cr203 ; guyanaite and 

bracewellite, both orthorhombic and essentially CrOOH; grimaldiite, rhombohedral and also essentially CrOOH; and mcconnellite, rhombohedral CrOOCu, isostructural with grimaldiite 

and invariably intergrown with it. A spinel, chromian gahnite 

Zn(Al0 .TCro.3) 204, chromian pyrophyllite, and free gold are 

present in very minor amounts. With the merumite is found a 

cryptocrystalline reddish or gray topaz, apparently found only 

here in Guyana. Quartz crystals present in merumite are 

evidently older than the merumite; the pyrophyllite is younger. 

Merumite is believed to be of hydrothermal origin.

Iron, aluminum, titanium, and vanadium replaced chromium 

in the merumite minerals.

INTRODUCTION AND ACKNOWLEDGMENTS

Merumite was discovered in 1937 in British 

Guiana (now Guyana) and was described by Bracewell (1946) as a new type of chromium ore, essentially a hydrated chromium oxide, with more than 

80 percent Cr2 03 . In 1950, Michael Fleischer, U.S. 

Geological Survey, suggested to Charles Milton that 

merumite should receive further study. A preliminary examination of a few available specimens indicated a complex and highly varied composition; 

only after years of study by many investigators can 

it be said that the composition of merumite is now 

known. Field studies of the geology of the occurrence and laboratory investigations in the U.S. 

Geological Survey and at the George Washington 

University have clarified its geologic relations and 

probable origin and established its complex mineralogy. In the field, Sat Narain, Guyana Geological 

Survey, and Charles Milton, U.S. Geological Survey 

and George Washington University, have most recently studied the geology, following many previous 

expeditions of others. In the laboratory, microanalyses of various merumite minerals have been 

made by Blanche L. Ingram, Joseph I. Dinnin, and 

Frank Cuttitta, microprobe studies by Edward J.

Dwornik and Harry J. Rose, Jr., and crystallographic X-ray studies by Daniel Appleman, Margaret Hall Appleman, and Edward C. T. Chao. It 

should therefore be clear that this study of merumite was the work of a group, and no individual 

could have succeeded in unraveling the merumite 

enigma without the contributions of his coworkers. 

We are further indebted for the unfailing counsel 

of our colleague Michael Fleischer and for the generous assistance of Dr. R. B. McConnell and Dr. 

Sobharam Singh, formerly of the Guyana Geological 

Survey. Murray R. Director provided us with several excellent specimens of merumite described in 

this report. Dr. A. J. Tousimis, of Biodynamics Research Corporation, Rockville, Md., prepared the 

scanning electron photomicrographs in this report. 

Richard C. Erd, U.S. Geological Survey, has assisted generously in the task of reviewing the 

manuscript.

Milton and others (1968) referred to the new 

species gnyanaite, bracewellite, grimaldiite, and 

mcconnellite; these new mineral names had been 

previously approved by the Commission on New 

Minerals and Mineral Names, International Mineralogical Association.

Specimens of the merumite minerals have been 

deposited with the U.S. National Museum. An account of the geology of the merumite occurrence in 

Guyana has been published recently (Milton and 

Narain, 1969).

MERUMITE 

NATURE AND OCCURRENCE

Merumite, discovered in 1937 in alluvial gravels 

in British Guiana, consists of black rounded grains, 

generally sand sized, although masses several centimetres across have been found; it was first described 

by Bracewell (1946) as a hydrated oxide of chro-
MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

mium with some aluminum. Merumite was considered to be a mineral species until Milton and Chao 

(1958) reported that eskolaite, Cr2O3 , discovered in 

a Finnish base-metal ore by Kouvo and Vuorelainen 

(1958), was a major constituent, along with other 

unidentified chromium minerals. From the present 

study, merumite is known to contain at least seven 

chromium minerals, only three of which, eskolaite, 

guyanaite, and chromian pyrophyllite, have been 

found elsewhere in the world (but only in two 

localities). Except for bracewellite, all the chromium 

compounds discovered in merumite have been made 

in the laboratory.

Although the term merumite is no longer applicable as a mineral name, it should be retained to designate the complex assemblage of minerals described 

in this report. Its characteristic appearance and 

properties easily differentiate it from other hard 

black opaque substances found in nature. It may 

be defined as a fine-grained to aphanitic aggregate 

of (mainly) chromium oxide (eskolaite), three 

hydrous chromium oxides (guyanaite, bracewellite, 

and grimaldiite), a copper-chromium oxide (mcconnellite), a chromian zinc aluminum spinel (chromian 

gahnite), with quartz, gold, and chromian pyrophyllite; all of these may not be present in all 

specimens.

Merumite has a very characteristic appearance, 

and once seen, it is not likely to be mistaken. Its 

specific gravity (about 4.5), hardness (about 5 to 

9), shining black color, rounded contours (often 

seen on specimens which appear to be fragments of 

larger masses), imprint of prismatic quartz crystals 

(which rarely may still be embedded in merumite), 

and generally green (rarely brown) streak are all 

diagnostic.

Plate 1 shows the exterior and sawed interior surfaces of two unusually large specimens of merumite. 

The botryoidal exterior and quartz crystal inclusions 

(see also pi. 3, fig. 1) are characteristic. Granular 

aggregates of grimaldiite-mcconnellite polycrystals 

can also be seen in plate 1, figure 4, as dark mottled 

areas, less than 1 cm across, near the center of the 

specimen.

The presence in merumite of free gold, pyrophyllite rosettes, and doubly terminated quartz crystals 

suggests that merumite was formed in a hydrother: 

mal type of deposit. None of the minerals associated 

with merumite in the placer gravels contains chromium, and only the coarse euhedral quartz, pyrophyllite, and possibly jasper and unusual types of 

topaz found with, but not in, merumite, may be 

genetically related. The hydrothermal merumite,

carrying pyrophyllite and gold, and perhaps the 

topaz may have been deposited in the easily decomposed Roraima(?) sandstone-ash series, of which 

Robello Ridge may be an outlier and which may 

have since disappeared by erosion; the resistant 

merumite and topaz became concentrated, eventually, in Director Creek. 1 Large beds of volcanic ash, 

more or less silicified and carrying euhedral quartz 

and jasper, are contiguous to the merumite placer 

gravels, but have no known chromium mineralization.

Merumite is found most abundantly in the upper 

branches of Director Creek, a small tributary of the 

Merume River which in turn flows into the Mazaruni River (fig. 1). The nearest inhabited place is 

Kamakusa, a government rest house and hydroplane 

stop on the Mazaruni River, about long 60° W., lat 

6° N. The merumite area is about 16 km (10 miles) 

southwest of Kamakusa but, because of the windings of the rivers, the trip takes several hours by 

motorboat. The Merume River is generally navigable 

by small boats up to the First Falls, except in periods of very low water or of very high water when 

the current is dangerously swift. The location of the 

merumite deposit in the Merume River valley is 

shown in figure 2.

The largest concentration of merumite is in a 

strip 3.0-4.5 m (10-15 feet) wide and about 3 km 

(2 miles) long at the base of Robello Ridge 2 , which 

consists of Roraima-like sandstones, conglomerates, 

and volcanic ash, with dips of 30° to the east (fig. 

3). Shale, jaspilite, quartzite, and tuff of the Haimaraka Formation are exposed in the river to the west 

of the ridge. To the east of Robello Ridge, a large 

area of agglomerate was reported by Matthews3 . 

Ten to eleven kilometres (6-7 miles) west, dolerite 

(diabase) intrudes the scarp-forming Roraima Formation.

The merumite deposit is in low-lying swampy and 

forested terrain, about 80 m (260 feet) above sea 

level; sand terraces (White Sand Formation) rise 

some 15 m (50 feet) higher. A few miles to the 

east, west, and south are scarps of the Roraima 

Formation with extensive talus slopes; the scarps 

reach altitudes of 915 m (3,000 feet) above sea level. 

The Merume River and its major tributaries rise on 

the Pakaraima Plateau above the escarpment. Below the First Falls, the river flows through a 

swampy forested plain before joining the Mazaruni

1 Named for Murray R. Director, who examined the merumite deposit in 1957.

2 Named for Joseph Robello of Georgetown, Guyana, who prospected 

the area for diamonds and merumite, and who was helpful in locating 

the deposit.

3 P. F. P. Matthews, November 18, 1953, Report of merumite for African 

Manganese Co., Ltd.; 5-page typescript and map on file in office of 

Guyana Geological Survey, Georgetown.
MERUMITE

60° 20' 59°40'

6° 00' 

5°40

c SORORIENG

\-MOUNTAIN 

onff \

v 

0 5 10 15 20 25 KILOMETRES 

III I I I I

I ^ n r

0 5 10

I

15 MILES

100 150 MILES

FIGURE 1. Index maps showing A, location of merumite-bearing area (shaded) in Guyana, and B, enlarged view of merumite-bearing area (shaded) and surrounding region. Merumite-bearing area is shown in greater detail on figure 2. Escarpments are shown in B by hachured lines.

River. Eight kilometres (5 miles) north of the 

escarpment, and immediately above First Falls, the 

Merume River runs northwards along the western 

side of Robello Ridge, which rises about 92 m (300 

feet) above the river.

In addition to the major concentration at the base 

of Robello Ridge, a small amount of merumite was 

found at the edge of a White Sand plateau near the 

point where Gordon Creek joins the Merume River. 

Gordon Creek itself was prospected, and a few 

pieces were found on the right bank, approximately 

0.8 km (0.5 mile) and 1.6 km (1.0 mile) upstream 

from its mouth. A few pieces were found in Copper 

Creek, which flows into the Merume River a short 

distance above Third Falls. Merumite was also found 

in many of the small creeks flowing along the east

side of Robello Ridge for a distance of 2.4 km (1.5 

'miles). Merumite has been reported in the Merume 

River, immediately above White Creek, in a diamondiferous gravel overlain by 1.8-2.4 m (6-8 feet) 

of alluvial sand.

Merumite is recovered from the gravels by the 

same methods used in recovering alluvial diamonds 

and gold. Dredging, with or without suction, is feasible in the river gravels. Where the merumite gravels 

underlie forest growth, as at the base of Robello 

Ridge, arduous clearing of the trees and matted 

roots is necessary. The gravel is washed in Brazilian 

sieves (%-, 14-, and i/8 -inch mesh), and the merumite is picked off the two coarser sieves. The fine 

concentrate on the ^-inch-mesh sieve is set aside 

for washing on a steel batelle. Rarely, small dia-
MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

Form lines indicating Robello Ridge

/35

Strike and dip of Robello Ridge quartzite

Base by Geological Survey of Guyana 400 800 

I I

1200 1600 

J_ 1 I I

2000 MET RES

Geology by Sat Narain, 1967

2000 4000 6000 FEET

FIGURE 2. Sketch map of the Merume River area showing the location of the merumite deposit. Area of this

map is shaded on figure 1.
60°00'

5°45'

Base by Geological Survey of Guyana

3 KILOMETRES

Geology modified from M.G.AIIderidge,1959 

and 1962. Data on merumite deposit from 

Sat Narain, 1971. Reports on file with 

the Geological Survey of Guyana

Y

A

2 MILES

FIGURE 3. Geologic map of the Merume River valley showing Robello Ridge and merumite localities.
MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

monds or gold nuggets have been found on the 

batelle. The general recovery of merumite from the 

Robello Ridge gravels is about 110 g/m3 . The depth 

to the merumite-bearing gravel varies from 20-46 

cm (8 to 18 inches) and the thickness of the gravel 

from 15-30 cm (6 to 12 inches). The gravel consists 

essentially of quartz crystals and milky quartz, with 

red jaspilite, a little topaz (of an unusual chertlike 

type) and brown rutile crystals with other minerals, 

described below, and water-worn fragments of white 

or pink sandstone and volcanic ash in a red clayey 

sand. The clayey sand, quartz, and jaspilite are 

derived from the sandstone of Robello Ridge. 

Rounded pebbles of basaltic rock and dark pebbles 

of a widespread quartz-tourmaline hornfels ("carbon") are found with the merumite and are a possible source of confusion to the inexperienced prospector. The small rutile crystals are sometimes 

termed "tin" by the miners, but the presence of 

cassiterite is very doubtful. Gorceixite has been reported, but actually it may have been topaz. However, cassiterite and gorceixite do occur elsewhere in 

Guyana.

The fact that the occurrence of merumite is limited to the small area shown on the maps implies a 

local origin, and the relative concentration along 

the base of Robello Ridge indicates that the rocks 

of the ridge may have a genetic relationship to 

merumite. The ridge rocks show mild hydro thermal 

metamorphism, the sandstones containing euhedral 

quartz crystals in vugs, similar to those found with 

the merumite. The ash beds are locally silicified, 

having bands of red jaspilite also similar to the 

pebbles found with merumite.

The generally rounded, often polished, shapes in 

which merumite is found suggests mechanical attrition and thus a history of travel. No pyrophyllite 

or gold has been seen on the surface of any specimen of merumite; even quartz is usually completely 

removed. Nevertheless, the distance of travel cannot 

have been great because merumite is found only in a 

limited area, and because the largest known specimens weigh many ounces. Even so, the apparent 

attrition seems hard to reconcile with the considerable hardness of merumite, or at least of eskolaite, 

one of the major components. Likewise, the appearance of fracturing is not easy to explain for material 

as hard and tough as most merumite. Perhaps the 

merumite was both softer and more easily broken 

when first formed than it is now, and hardness and 

toughness result from recrystallization or possibly 

weathering.

The statement of Ramdohr (1960, p. 870) that

merumite is certainly a weathering product of 

chromite cannot be verified. He observed that its 

genesis is obscure, but our observations strongly 

imply a hydrothermal origin. No chromite is known 

in the Merume River valley. Ramdohr further (p. 

884) noted with surprise ("iiberraschend") the report by Milton and Chao (1958) of the occurrence 

of eskolaite in merumite, as he (erroneously, in our 

opinion) views merumite as a weathering product 

of chromite (p. 870). Recently (written commun., 

July 3, 1968) he refers to eskolaite as "by far the 

most important component of merumite." While this 

may be true for the particular specimen or specimens which he examined, guyanaite appears to be 

at least equally abundant generally.

Ramdohr (1969) referred to merumite only to 

note that it has been reported to contain eskolaite 

(p. 945). The eskolaite, from the Outokumpu Mine 

in Finland is described in detail; no reference to 

the other merumite minerals is made.

A detailed report on the history and geology of 

merumite (Milton .and Narain, 1969) summarizes 

the work of many geologists in Guyana.

Vuorelainen and others (1968) described guyanaite (but did not name it) from the Outokumpu mine 

in Finland as an alteration product of eskolaite; both 

are found in sulfide-rich apophyses of the main ore 

body which occur in skarn-bearing quartzite dikes 

emplaced after strong tectonic movements. Besides 

sulfides and eskolaite with guyanaite, zincian chromite, rutile, uraninite, nolanite, corundum, titanite, 

zircon, and graphite are present in the ore body. 

Fuchsite (chromian muscovite) coats some eskolaite guyanaite crystals. From the description by 

Vuorelainen and others (1968), it appears that the 

eskolaite-guyanaite alteration is hydrothermal, and 

not a weathering product. Although eskolaite with 

guyanaite is found in the Outokumpu deposit, nothing like merumite is known from there.

PREVIOUS STUDIES

After the initial recognition of merumite by 

Bracewell (1946), scattered and fragmental data 

appeared, most of them erroneous to some extent. 

They are noted and commented upon in appropriate 

places in this report. The reason for the invalidity 

of earlier studies was nonrecognition of the complexity and diversity of all merumite specimens, 

their superficial similarity notwithstanding. In this 

study, many hundreds of specimens have been examined. Two areas in which little work has been 

done are the microscopic petrography of merumite 

in thin and polished sections and the physical chem-
MERUMITE

istry, particularly phase relations, of its constituent 

minerals.

CHEMISTRY

OLDER ANALYSES

Two published analyses (Bracewell, 1946) and 

one unpublished analysis (made in 1956-57) of 

merumite are given in table 1. The first two were 

of specimens with green and brown streaks respectively, which could indicate either variation in 

mineral content or the (recognized later) variability 

in color of guyanaite. Without mineralogical data, 

the value of these analyses is limited; nevertheless, 

they do show the typical and restricted range in 

composition of what may be presumed to be average 

samples.

RECENT ANALYSES

Analyses made by the U.S. Geological Survey include a merumite containing 37 percent eskolaite 

and 63 percent guyanaite (table 2) and one containing 70 percent bracewellite and 30 percent eskolaite (tables 10 and 11) ; these analyses include 

spectrographic data. Table 2 also includes spectrographic data for a merumite containing more guyanaite than eskolaite (analysis D), and, for comparison, the analysis of eskolaite from Finland 

(Kouvo and Vuorelainen, 1958). Spectrographic 

analyses of four merumite samples of undetermined 

mineral composition are given in table 3; these samples were assayed for gold and silver (table 21). It

TABLE 1. Older analyses of merumite

[n.d., looked for but not detected]

Constituents Analyses (in percent) 

A1 B2 C3

Cr2 03 _ _ __ __ 

AU)3 __ _ _ _ __ _

Fe2O3 (total Fe)

TiO2

V2 03 _ ___ _ ____ _

MnO

H2 0- _ _ _ _ ____ _

H2 0 + ____ ___ ___ _

SiO2

MgO __________________

S _

P

Ca, Ba, Zr, Ni, Nb, Ta __

Total_ __.._.. 

Specific gravity. _____

81.30

6.55

1.58

.75

.15

.06

.10

8.08

1.30

n.d.

n.d.

n.d.

99.87 

4.49

76.74

6.82

5.91

1.40

.15

.06

.11

8.25QP;

n.d.

n.d.

n.d.

100.39

4.44

80.28

2.10

1.78

.36

84.52

1 Analyst, W. H. Bennett, Imperial Institute, London (Bracewell, 1946). 

BraceweJl reported a green streak; Stockley (1955) gave the same 

analysis but erroneously reported the streak as brown (Bennett, oral 

commun., 1969).

2 Analyst, W. H. Bennett, Imperial Institute, London (Stockley, 1955). 

Stockley reported a green streak, but this is erroneous; the original report was "brown steak" (Bennett, oral commun., 1969). Stockley omitted 

the specific gravity, which was determined by Bennett.

3 Analyst Andrews S. McCreath and Son, Harrisburg, Pa. (Murray Director, written commun., 1957)

is not likely that any further analyses will be made 

of merumite, now that it is known to be a mixture 

of many minerals.

The role of A12 03 , Fe2 03 , Ti02 , Mn2 03 , and V2 03 , 

which generally appear in the chemical analyses of 

merumite and its constituent minerals, is one of 

general partial substitution for the dominant O2 03 . 

There are indeed, as will be seen, specific minerals 

characterized by major A12 03 : chromian pyrophyllite, chromian gahnite, and topaz. However the first 

and second are of sporadic and quantitatively negligible occurrence in merumite, and the third accompanies but has never been found in merumite. 

Chemical analyses of merumite and of the constituent oxide-hydroxide minerals were all made on material apparently free from these three Al 2 03-containing minerals.

ISOTOPIC STUDY

Svec and others (1962) studied the relative abundance of the chromium isotopes Cr50 , Cr52 , Cr53 , and 

Cr"54 in a large number of terrestrial and meteoritic 

chromium minerals, including a sample of merumite, 

and found no evidence of any significant isotopic 

variation in any of their specimens.

X-RAY POWDER DATA

Because merumite is a mixture (although, rarely, 

almost monomineralic), X-ray powder patterns are 

basically composites to be resolved as patterns of 

the component minerals.

Unless otherwise noted, the X-ray powder diffraction patterns of this report were obtained on a 

standard 114.6 mm. camera, using Ni-filtered Cu 

radiation. Since the objectives of this report are 

primarily the identification of unknown phases 

present, rather than precise measurements of known 

phases, methods of obtaining high precision, (for 

example, internal standards) were not used. For 

similar reasons, letter scales, such as vs, s, w, and 

so forth, rather than numerical values which imply 

high precision, were used in the tables.

Table 4 gives powder patterns for merumite 

(eskolaite 37 percent, guyanaite 63 percent), for 

merumite approaching eskolaite in composition, for 

guyanaite from Guyana (four strongest lines only), 

for eskolaite from Finland, and for Cr2 03 .

Table 9 gives powder patterns of two merumites 

which are almost wholly guyanaite (although one 

has a little eskolaite and quartz), of pure guyanaite 

crystals from Guyana and Finland, and of synthetic 

guyanaite.
MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

TABLE 2.- Recent analyses of two specimens of merumite 

(eskolaite and guyanaite} from Guyana and of eskolaite from 

Finland

Constituent Amount 

(in percent)

MERUMITE 1 (ESKOLAITE, 37 PERCENT; GUYANAITE,

63 PERCENT) 

_______________ANALYSIS A 3_______________

Cr2 03 _____________________ 88.8

A12 03 _____________________ 3.1

CaO __________________________ .1

H2 0 __________________________ 2.3

Ignition loss _____________________ (3.5)____

Total _____________________. 94^3

ANALYSIS B

Cr203 89.2

ANALYSIS C

Si __ __ _ _ _____ . Al _ __ _ ___ _ _

Fe ______ _ _.

Mg

Ca _

Ti _ __ ____ _ _ _.

Mn _ _

Ag

As _ _ __

Au __ ___ _ _

Be _

Cu __ __ _____

Ga ___ _ ______________

Mo _

Nb _ _

Ni _

Pb _- ___ _ __ ___ . Sb __ _ _____ __ ___

Sr

V

0.02

1.5

.1

.01

.1

.015

.03

.001

.2

.07

.0015

.02

.001

.015

_ __ _ __ __ ___ .03

_ _ - .002

_ _ _ .01

.2

.002

.15

1 This specimen was analyzed to test analytical procedures for similar 

quantities.

2 Analyst, J. I. Dinnin, U.S. Geological Survey. Sample massive, black; 

X-ray film 16827 (table 4). Water and ignition loss were determined by 

ignition of a 100-mg sample in a microcombustion tube swept with argon; 

the increase in weight of an absorption tube packed with Anhydrone 

represented the water content of the sample and the loss in weight of a

cromum y oxaon w ammonum persuaesilver, reduction with ferrous ammonium sulfate, and titration with potassium dichromate solution. A 50-mg sample was dissolved in perchloric 

acid, electrolyzed in a mercury cathode apparatus, and diluted with water. 

An aliquot was used for the determination of calcium by flame photometry.

For discussion of the low summation, see section on thermal behavior of 

synthetic guyanaite. Computation of mineral composition of this merumite 

varies with unknown factors, such as the true percentage of HaO and 

the distribution of AbOa, and so forth, between eskolaite and guyanaite. 

With 6.8 percent H»O (the maximum derived from 89.2 percent CraOs 

and 4.0 percent AkOa, and so forth) and all AbOa, and so forth, in 

guyanaite, a composition of 38 percent eskolaite and 62 percent guyanaite 

follows; and with all AhOa, and so forth, in eskolaite, 36 and 64 percent 

respectively; the mean is then eskolaite 37 and guyanaite 63 percent.

Table 13 gives patterns of two merumite samples, 

both almost wholly bracewellite but having a little 

guyanaite.

PETROGRAPHY

Efforts over many years to prepare standard thin 

sections of merumite were invariably unsuccessful, 

because the intense absorption of most of the merumite minerals necessitated unusually thin sections 

to transmit light, but the almost invariable presence 

of the very hard eskolaite (H = 9.5) associated with 

the much softer minerals made such thin sections

TABLE 2. Recent analyses of two specimens of merumite 

(eskolaite and guyanaite) from Guyana and of eskolaite from 

Finland Continued

Constituent Amount 

(in percent)

ANALYSIS C 4 (SPECTROGRAPHIC)

MERUMITE 1 (ESKOLAITE, 37 PERCENT; GUYANAITE, 

63 PERCENT Continued 

ANALYSIS C1 Continued

K, P. B, Bi, Cd, Ce, Co, Ge, 

Hf, Hg, In, La, Li, Pd, Pt, 

Re, Sc, Sn, Ta, Te, Th, Ti, U, 

W, Y, Yb, Zn, Zr.

Looked for 

but not 

detected.

MERUMITE (GUYANAITE PLUS ESKOLAITE) 

ANALYSIS D (SPECTROGRAPHIC)

Al _ _

Si, Fe, Ti, V _ _ _ __ _ _

Mg, Mn, Ca, Cu, Ge, Sb

Pb, Mo, Be

Ag, Sn, As, Bi, TI, Co, Ni,

Zn, Cd, Zr, B, P, Ba, Sr, Y,

La, U, Th, Cb, W, Pt, Au.

ESKOLAITE

ANALYSIS E°

Cr.0a __ - - 

AU) 3

Fe2 0a _ _ ______ __ __ _

V2 03 _ _ -_ ______ _ _ 

MgO

MnO

Si02 _ __ _ __ _

H3 0 _ _ _

Total

Density

1-9

O.X

O.OX

O.OOX

Looked for

but not

detected.

94.13±0.20

7 0.19

0.55

4.58

7 0.03

7 0.03

7 0.20

0.10

8 99.81

5.18 measured

5.218 calculated.

 " Analysts, H. J. Rose and Frank Cuttitta, U.S. Geological Survey. 

Average of three determinations made by X-ray fluorescence analysis. A 

solution-pellet technique was used to prepare the samples and the K^CraO? 

standards.

CrsOa (percent) 

____ 89.1

Wc-ight sample (in mg)

1.98 __ ____________ __

2.47 __ ___ _ __________________ __ ___----__89.4

2.47 ___-__-___-- _ __-____-------_---- __ __89.2

4 Spectrographic analysis by Helen Worthing, U.S. Geological Survey. 

As to the reliability of the determinations, results are reported in percent 

to the nearest number in the series 1, 0.7. 0.5, 0.3, 0.2, 0.15, 0.1, and so 

forth, which represent approximate midpoints of interval data on a 

geometric scale. The assigned interval for semiquantitative results will 

include the quantitative value about 30 percent of the time.

5 Spectrographic analysis by K. J. Murata, U.S. Geological Survey. X-ray 

film 4864. This was a different sample from the one later analyzed by 

Dinnin, Rose and Cuttitta, and Worthing (A, B, and C, respectively) and 

was -analyzed to identify a (then) new and strange X-ray powder pattern.

"Analyst, Jorma Kinnunen, in Kouvo and Vuorelainen (1958) who gave 

the formula, Cri.nnVo.niFei.mO.i.

7 Based on spectrographic analysis.

8 Spectrographic analysis also showed traces of Cu and Ni.

impossible to prepare. However, the edges of a few 

sections were sufficiently translucent to reveal some 

textural and mineralogical detail, but, even then, 

very intense illumination, painful to the eye, was 

required. Since those earlier attempts, however, 

modern methods using diamond abrasives and suitable polishing agents have produced good, sufficiently thin, sections as well as polished surfaces for 

study in reflected light.

The frontispiece shows the appearance of some 

specimens of merumite under transmitted light. Although merumite consists of a small number of min-
MERUMITE

TABLE 3. Spectrographic analyses (in percent) of four 

merumite samples * 2

[G. W. Sears, Jr., U.S. Geological Survey, analyst]

Percent

(>10)

7 ______________

5 __________ .

3 __________ ..

2 __ ______ ..

1.5 _________ ..

.7 ____________

.3 ____________

.15 __ _______

.1 ____________

<.05 ___________

.03 ___________

.02 _ _ _______

<.02 _ __ __ ..

.015 __ ___ _

.01 _ .... ____

.007 __ _______

.005 ____ __

<.005 __ ____ ..

.003 _ _ _

.002 _____ _..

.0015 _________

.0007 _ __ _

.0005 _ __

Looked for 

but not 

found.

Analyses

A

Cr

Al

Fe _ __ _.

V 

Si, Ca, Cu 

Ti 

Ce 

Zn . __ _.

Sb, Mg, Au 

Mn

Ni 

Pb, Ga, Mo 

Ba, Co, Ge 

Sr

Ag, Be __ .

Zr

Sc

K, P, As, 

B, Bi, Cd, 

Hf, Hg, 

In, La, 

Li, Nb, 

Pb, Pt, 

Re, Sn, 

Ta, Te, 

Th, Tl, U,

W, Y.

B

Cr

Al. Fe 

Ti _____ .

Si, V 

Ca, Sb, Mn 

Cu 

Ce

Nb 

Pb _ ___ ..

Zn 

Ni, Ga, Ge 

Mg 

Mo, Zr

Co

"Be"""" ' 

Sn _ _ _.

Sc

K, P, Ag, 

As, Au, 

B, Bi, Cd, 

In, La, 

Li, Pd, 

Pt, Re, 

Ta, Te, 

Th, Tl, 

U, W, Y.

C

Cr

Al

Fe, Ti

Si, V 

Mn, Cu 

Ca, Sb 

Ce

Nb

Zn

Ni, Pb, Mg 

Ga, Ge 

Ba, Mo, Zr

Co 

Sr 

Be, Sn

Sc __ _..

K, P, Ag, 

As, Au, 

B, Bi, Cd, 

Hf, Hg, 

In, La, 

Li, Pd, 

Pt, Re, 

Ta, Te, 

Th, Tl, 

U, W, Y.

D

Cr

Al, Fe

Ti

Si, V 

Ca. Zn, Mn 

Sb, Cu 

Ce

Pb

Mg

Ni, Ge 

Ga, Nb, Zr 

Mo 

Ba, Sr 

Co

Be

Sn

Sc

K, P, Ag, 

As, Au, 

B, Bi, 

Cd, Hf, 

Hg, In, 

La, Li, 

Pd, Pt, 

Re, Ta, 

Te, Th, 

Tl, U, 

W, Y.

Not looked for Na, Yb Na, Yb Na. Yb Na, Yb

1 "Results are reported in percent to the nearest number in the series, 

1, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, etc., which represent approximate midpoints of interval data on a geometric scale. The assigned interval for 

semiquantitative results will include the quantitative value about 30 

percent of the time."

2 Assayed for gold and silver, see table 21.

erals, their relative amounts, crystal habits, and relationships to one another vary greatly from specimen to specimen, and almost every thin section

shows some features of special interest. None of the 

minerals of merumite shows alteration or weathering, although the merumite itself is characteristically rounded.

ESKOLAITE

OCCURRENCE IN FINLAND AND GUYANA, AND 

(POSSIBLY) SIBERIA

Eskolaite, essentially Cr2 03 , was discovered by 

Kouvo and Vuorelainen (1958) in a base-metal 

skarn in the Outokumpu mine, Finland, and by Milton and Chao (1958) in merumite in Guyana. The 

parageneses of the two occurrences of eskolaite are 

completely different. In Outokumpu, well-crystallized eskolaite is a very minor constituent in the 

varied rocks and ore, but in Guyana, it is a major 

constituent of merumite. No good crystals of eskolaite have been seen in merumite, only massive 

aggregates which rarely show obscure crystal faces 

on botryoidal surfaces in vugs (pi. 2, fig. 1). In thin 

sections, zoning of the eskolaite is seen locally in 

bands varying from green to nearly black (pi. 2, 

fig. 2).

Such aggregate of eskolaite may (not commonly, 

however) be fairly free from admixed minerals, so 

that the X-ray powder pattern shows only eskolaite 

lines. One such pattern is given in table 4 (second 

analysis) with, for comparison, patterns of eskolaite from Finland (fourth analysis), and of Cr2 03 

(fifth analysis). The same table also gives (first analTABLE 4. X-ray powder patterns of merumite (eskolaite 37 percent, guyanaite 63 percent), eskolaite (Guyana) guyanaite

(Guyana), eskolaite (Finland), and Cr2O3

Merumite,

/»

S

vwb 

w

vs 

vs 

vw 

ww 

s

s 

vs 

vw

ww 

vvw

s 

s 

wwb 

wwb

Merumite at>proaching n^nTee 3'^^.,^ Eskolaite, Finland 

film 168271 eskolaite, film 00188 Guyana (Kouvo and 

16765 Guyana lines? Vuorelainen, 1958)

d 1 3

3.633 vs 

3.433

G 3.220

2.663 vs 

2.478 w 

G 2.4395 _ _ _

2.206 _ __

2.176 w

1.814 w 

1.672 s 

G 1.641

G 1.608 _ _

1.581 _ _ __

1.464 vw 

1.431 w 

1.296 ww 

1.240

d I"

3,601

2.652 _____ __

2.468

2.168 _ __ .

1.807

1.666 _ _ __

1.460 __ __ .

1.426 _ -

1.30 _ _ _ -

d I

96

3.224 _ _

.__ _ __ 97

94

2.432 _ _

_______ 5

.___ ___ ___ 50

_ 60

_ 100

1.636 _ _

.___ _____ 5

1.609 _ _

_ _ _ 5

._ ______ 30

._ _ _ __ _ _ 45

12

5

d

3.63

2.665 

2.479

2.269

2.174

1.8146 

1.6746

1.6139

1.5782 

1.4656 

1.4331 

1.2971 

1.2440

hkl I

 __ 74

100

96

12

38

9

39

_ _ 90

018 _- -

13

25

40

__ _ 20

17

CraOs 

XPDF 6-05042

d

3.633

2.666 

2.480

2.264 

2.176 

2.048 

1.8156 

1.672

1.579 

1.465 

1.4314 

1.296 

1.2398

hkl

012

104 

110

006 

113

202 

024 

116

122 

214 

300 

10.10 

220

1 For analysis, see table 2.

2 X.P.D.F. 6-0504 refers to Index (Inorganic) to the Powder Diffraction File 1972, Joint Committee on 

Penn.

3 ws, very very strong; vs, very strong; s, strong; ms, medium strong; w, weak; vw, very weak; ww, 

line.

Powder Diffraction Standards, Swarthmore, 

very very weak; b, broad line; G, guyanaite
10 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

ysis) the pattern of a merumite specimen which is 

mainly (37 percent) eskolaite.

The occurrence of eskolaite in Guyana is described 

in the section on "Nature and occurrence," of merumite. The Outokumpu mine in Finland is worked 

for its copper content, estimated to be about a million tons, but seven times as much chromium is 

present in serpentine and skarn rocks. Eskolaite, in 

large black (very dark green in transmitted light) 

prismatic to platy euhedral crystals, is found in the 

mine in several associations summarized by Kouvo 

and Vuorelainen (1958, p. 1100) as follows:

1. Chromium-bearing tremolite, skarn, rich in calcite, from 

contact with ore. Mineral habit: long hexagonal prisms 

(up to 5 mm). Associated minerals: pyrrhotite, pentlandite, chalcopyrite, chromium-bearing tremolite, uvarovite, tawmawite, calcite, and talc.

2. Quartzite from the foot wall of the ore. The specimen is 

from a cavity in a vein of milky quartz. Mineral habit: 

long prismatic (up to 1.5 mm). Associated minerals: 

uvarovite, chromium-bearing tremolite, calcite, and pyrite 

(pentagonal dodecahedron).

3. Veins of pyrrhotite. Mineral habit: platy (up to 6-7 mm). 

Associated minerals: pyrrhotite, chromium-bearing tremolite, and chromium tourmaline. In some cases the 

mineral occurs as thin sheets on the surface of quartz.

4. Skarn ore rich in pyrrhotite. Mineral habit: short prismatic. Associated minerals: chromium-bearing tremolite, 

uvarovite, chromium tourmaline, chromium spinel, 

pyrrhotite, pyrite (octahedron), and chalcopyrite.

5. Chlorite seams. Mineral habit: thin platy. Associated 

minerals: chlorite, pyrite, chalcopyrite, pyrrhotite, and 

chromium spinel.

The mineral has not been found to occur with chromium

diopside.

The "chromium spinel" is chromite of very unusual composition, containing 5.8 percent zinc 

(Thayer and others, 1964).

The association in Finland of eskolaite with copper and zinc sulfides and zincian chromite may be 

compared with the presence in merumite of copper 

(in mcconnellite) and zinc (in chromian gahnite).

Ramdohr (1960, p. 332) suggested that an unidentified extremely hard mineral found in native platinum from Nizhne Tagilsk, Ural Mountains, Siberia, 

may be eskolaite. Ramdohr (1969, p. 944-945) gives 

a detailed account of (Finnish) eskolaite, and suggests that fine unmixing products in chremite or 

chromium spinel may be eskolaite rather than, as 

generally held, hematite.

CHEMISTRY

Unlike the eskolaite from Finland, pure eskolaite 

from Guyana has never been found in crystals large 

enough even for microanalysis. Single crystals of 

guyanaite and other merumite minerals, except 

eskolaite, are found only in rare microscopic vugs

or fissures in merumite. Before it was realized that 

merumite is a complex assemblage of minerals, numerous analyses were made (prior to 1950) under 

the mistaken idea that a single definite mineral was 

being analyzed. More recently, however, aware from 

optical and X-ray study that we were dealing with 

relatively simple mixtures, we have made further 

analyses of merumite, because microanalytical procedures for such small quantities of pure minerals 

were not known and experimentation on the few 

milligrams of isolated pure minerals obtained laboriously through months or years of toil would have 

been hazardous, if not reckless. However, on samples 

of several hundred milligrams of known complex 

composition (as determined by X-ray and optical 

studies) experimentation was permissible, and valid 

analytical methods were worked out which were 

later applied to microquantities of the handpicked 

pure crystals.

The Finnish eskolaite (table 2, analysis E) is 94 

percent Cr2 O:! ; almost all of the remaining 6 percent 

is V2 On of very similar molecular weight. The Guyana eskolaite, however, may vary from almost wholly 

Cr2 O3 with very minor A12O3 (table 2, analysis A) 

in a specimen with guyanaite to a theoretical composition with more A12 O3 + Fe2O3 ((Cr0 . 2 5 Fe0 . 4 4 

Alo. 2 9 Tio.oo Vo.o2 )2O3 ; see table 10) than Cr2O3 in a 

specimen with major (70 percent) bracewellite.

Spectrographic analysis of a merumite specimen 

with more (according to the X-ray powder pattern) 

guyanaite than eskolaite and, for comparison, the 

analysis (not spectrographic) of the Finnish eskolaite are given in table 2. The Finnish eskolaite has 

a reported density of 5.218 (powder data) or 5.20 

(single-crystal data) ; pure Cr2 O3 has a density of 

5.25 (Kouvo and Vuorelainen, 1958, table 3, p. 

1,104). Computation of the density of eskolaite in 

an analyzed specimen (table 10) of merumite containing approximately 70 percent bracewellite and 

30 percent eskolaite gives values ranging from 4.86 

to 5.21 with a mean of 5.035 ±0.1755.

GUYANAITE 

OCCURRENCE IN GUYANA AND FINLAND, AND SYNTHESIS

Tombs and others (1964) synthesized orthorhombic CrOOH with unit cell dimensions a = 4.861 

A, 6 = 4.292 A, and c = 2.960 A (table 5) by heating 

CrO2 in water at 450°C under a total pressure of 

40,000 lb/in2 Christensen (1966), using their material, determined the crystal structure. Table 5 

gives the cell constants for guyanaite from Guyana
GUYANAITE 11

and Finland and, for comparison, the constants for 

the synthetic compounds CrOOH (Tombs and 

others, 1964; Christensen, 1966), ScOOH (Christensen, 1971), and InOOH (Christensen and others, 

1964). Until Chenavas and others (1973), reported 

synthesis of isostructural Fe, Ni, V, and Rh oxyhydroxides, these three were the only substances 

known to be isostructural with guyanaite (table 24).

Guyanaite is named for Guyana, the country in 

which the mineral was first found.

Guyanaite, whose identification rests primarily 

on X-ray powder and chemical data, eskolaite, and 

perhaps bracewellite are the major components of

merumite. In thin sections of merumite, the vivid 

green eskolaite contrasts with the brown and red 

(but sometimes green) guyanaite and the darker 

brown bracewellite, but it is hardly possible to isolate such intergrown minerals. However, in tiny 

vugs, prismatic brown pleochroic crystals of guyanaite as much as 0.1 mm long can be isolated (pi. 2, 

fig. 4). By inspection of a large number of merumite 

grains, some specimens can be found which have a 

definitely yellow-brown streak and a sensibly uniform texture and which, by their powder patterns, 

are almost wholly a massive single-phase guyanaite 

(pi. 2, fig. 3).

TABLE 5. Cell constants of guyanaite (from Guyana and Finland), and of CrOOH, ScOOH, and InOOH

Crystal

Guyanaite (Guyana) _ 

Do _ __ ___ _

Guyanaite (Finland) __ 

CrOOH _ ___ __

ScOOH _____

InOOH

a(A)

4.857 

4.862

(2X) 4.864 

4.861

5.150

4.26+0.01

MA)

4.295 

4.314

(2X) 4.305 

4.292

4.600

4.56-1-0.01

Spage 

c(A) group

2.958 Pnnm 

2.951

(2X)2.959

S QfiO P'ww'W

3.197 P2lnm

3.27+0.01 Pnnm

Density 

(calculated 

in 

K cm-3 )

4.57

4.81 

4.57

Reference

This report.

others (1968) computed from Berry 

and Thompson's 

(1962) X-ray powder 

data for "Merumite" 

= guyanaite. 

Vuorelainen and others, 

1968.

Christensen, 1966. 

Christensen, 1971.

Christensen and others,

1964.

Besides the dark-reddish-brown guyanaite, a 

light-green to greenish-black variety is occasionally 

found. This variety forms prismatic microcrystalline 

aggregates, and such a specimen was described 

under the name of merumite by Berry and Thompson (1962). The compound synthesized by Tombs 

and others (1964) is described as olive green and 

light-yellowish-green in transmitted light, with 

n~1.9. Green powdery rounded aggregates as much 

as 1.0 or 2.0 mm in size and the black merumite in 

which the aggregates are found both have a green 

powder and give good X-ray powder patterns of 

guyanaite.

Concerning the color variation in guyanaite, 

Alario Franco and Sing (1972) note that the "color 

of such materials [guyanaite, grimaldiite, and 

y-CrOOH] may be deceptive * * * because it is influenced by the change of particle size and (or) the 

presence at the surface of chromium in different 

oxidation states."

H. F. W. Taylor and R. Edge, University of Aberdeen (written commun., 1967), studied a specimen 

consisting "essentially of guyanaite and esokolaite, 

with a little quartz, ferromagnetic spinel (magnetite?) and an ill-crystalline flaky mineral * * * 

pyrophyllite." A "pinkish mineral" in cavities associated with quartz was probably grimaldiite-mcconnellite. They also studied the specimen's thermal 

behavior, as noted below.

Vuorelainen and others (1968) reported a hydrated oxide of chromium as a new but unnamed 

mineral from the Outokumpu mine in Finland; 

their description leaves no doubt that it is guyanaite. 

The mineral occurs as aggregates of golden-brown 

to greenish-brown fibers which syntactically replace 

smaller crystals (less than 1.0 mm in size) of 

eskolaite.

CHEMISTRY

ANALYSES

Table 6 gives a microchemical analysis (37 mg) 

of guyanaite crystals from Guyana and the analysis 

of guyanaite from Finland by Vuorelainen and 

others (1968). An analysis of merumite consisting 

of about two-thirds guyanaite and one-third eskolaite is given in table 2 (analyses A^C).

Guyanaites from both Guyana and Finland would 

appear from the respective analyses to have com-
12 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

positions closer to 2Cr2 03 -3H2 0, as proposed by 

Vuorelainen and others (1968), than to Cr2 03 -H2 0, 

which is known to be correct on the basis of the 

single-crystal X-ray data. The reason for this difference may be explained by the behavior of CrOOH 

(guyanaite) when heated.

Tombs and others (1964) found that on heating 

their synthetic orthorhombic CrOOH (Cr2 03 -H2 0) 

converted to Cr2 03 in two stages as follows :

at about 335°C

Cr2 03 -H2 0-»Cr2 04 

at about 1,000° C

2Cr2 04 -»2Cr2 03

and, more recently, Alario Franco and Sing (1972) 

report the reversible reaction

CrOOH Air (480°), 02 (400°) 

(guyanaite) H2 (330°) " 

and the irreversible reaction :

Cr02 at 1 am

2Cr02 + H2 (605°) Cr2 03 + H2 0 (atlatm).

Thus, in a Penfield determination of H20+ in 

guyanaite, there is a loss of hydrogen initially with 

formation of Cr02 instead of simple dehydration 

with formation of Cr2 03 . If this hydrogen is lost, 

then Cr02 may remain unreduced to Cr2 03 and the 

H20 weighed may represent less than the whole 

H2 0 in Cr2 03 -H2 0 (CrOOH) ; thus the (erroneous) 

apparent ratio 2Cr2 03 -3H2 0 may arise. In the analysis (table 6) only 8.7 percent H20 was found,

whereas 14.3 percent is required for a formula 

ROOH.

The determination of loss on ignition should give 

a correct figure for water, provided that all the 

Cr2 04 is completely converted to Cr2 03 on ignition; 

if not, then the loss on ignition would also be less 

than the true water content.

If the .analysis of merumite from Guyana (table 

2, analysis A) is recomputed on such a basis, that 

is, if the ignition figure 3.5 percent represents the 

true H2 0 content rather than the Penfield determination of 2.3 percent, the composition of the merumite sample (table 2, analysis A) would be:

mineral constituent percent

eskolaite ___________ Cr2Oa 63.5

CrOOH 28.3 guyanaite ________________ A1OOH 3.6

Total _____________ 95.4

If the X-ray fluorescence determination of Cr2 03 , 

89.2 percent (table 2, analysis B), is used, and the 

1.2 percent of spectrographic constituents is added, 

this, along with 0.1 percent CaO, 3.5 percent H2 0, 

and 3.1 percent A12 03 , raises the summation of the 

analysis to 97.1 percent.

THERMAL BEHAVIOR

The findings of Tombs and others (1964) and of 

Alario Franco and Sing (1972) as to the behavior 

of orthorhombic CrOOH (guyanaite) on heating 

were verified experimentally using a merumite

TABLE 6. Analyses1 of guyanaite from Guyana and Finland

Guyana (microchemical analysis) 2

Constituent

Cr2O3 ___ _ _ __ _ _ 

A12O3 __ ____ _ ___ _ _ .

V2 03 _ _ ____ __ __ .

MruOs H»O+

H2 0- __ _ ___ _ __ _ _____ .

H2O (calculated) _ __ .

Total

Computed composition.

Pernfnt Equivalent H2O R .. 

sample inR2O,H2O Rat>°

* 71 7

5 4 0

«44

. __ 7 .86

. _ 8 .98

._ _ 9 (8.7)

._ __ 9 0 .

. __ 14.3.

96.2

(Cro. 

(Cr,

12.9

.7 

.5

.1 

.1

14.3

89 Alo.04 r 60. 05 1 ifl.Ol

or

.7sAlo.o« Feo.io Tio.oa

0.472 

.039 

.028

.006 

.006

.551

Mno.oi) OOH

2O3 -H2O

Finland (mean of seven electron -probe analyses) 3

Percent 

of 

sample

75.76 

1.03 

1.47 

.____ __ 6.78

14.96 _

100.00

(Cro.886

Mn0. 02 ) (Cri.77.

Equivalent HsO T. 4.. 

inR2Os-H20 Ratl°

13.64

.18 

.16

.82

14.80

iAlo.2o2 Feo.oi5 V 

or

0.492 

.010 

.009 

.045

.556

o.oso) OOH

8 ) 203 -H2O

1 Spectrographic analysis of another (massive) sample containing 

eskolaite gave as minor constituents As2Os, 0.26 percent; V2O3, 0.22 percent; Sb2Os, 0.24 percent; CaO, 0.14 percent; and Au, Cu, Mo, Nb, Pb, and 

Mg, all O.Ox percent.

2 Analyst, Blanche Ingram, U.S. Geological Survey. Approximately 25 

mg of sample was fused with a mixture of K2COs and KCICU. The melt 

was leached with H2O which separated Fe, Ti, and Mn which remained 

in the residue, from Cr and Al which went into solution. Tests showed 

this separation reliable for the amount of Al present. Cr was determined 

on an aliquot of the water solution by titration with standard ferrous 

ammonium sulfate.

3 Vuorelainen and others (1968)

4 Attempt to determine the oxidation state of chromium chemically 

failed. The sample was very difficult to get into solution. It was not possible to determine the amount of oxidant that reacted with the sample.

5 Aluminum was determined colorimetrically by use of alizarin red S 

after chromium was volatilized as chromyl chloride.

6 The residue from the water leach of the fusion melt was dissolved in 

HC1O4, and the iron was determined colorimetrically on an aliquot using 

1, 10-phenanthroline.

T Titanium was determined colorimetrically on an aliquot of the residue 

solution using the Tiron procedure.

8 The manganese in an aliquot of the residue solution was oxidized to 

permanganate and the color measured.

9 A 12 mg sample was dried at 110°C. There was no measurable loss at 

110°C. The sample was then used to determine water by the Penfleld 

method.
GUYANAITE 13

sample consisting largely of guyanaite (X-ray film 

00188). When heated to 335°C, the sample gave the 

powder pattern of Cr02 formed by heating CrOOH 

(Thamer and others, 1957) (table 7). On heating 

to 1,020 °C, a pattern closely resembling that of the 

Finnish eskolaite was obtained (table 8). H. F. W. 

Taylor and R. Edge (written commun., 1967) confirmed that guyanaite (in merumite) ground to 300 

mesh and heated in air to 335°C changes at least 

partially to Cr02 . They suggest that the change involves migration of protons and electrons to the 

surface, as in the formation of oxyamphiboles, with 

no major structural change when guyanaite changes 

to Cr02 .

Alario Franco and Sing (1972) discuss these reactions in further detail. They note the close structural similarity of the orthorhombic unit cell of 

CrOOH (guyanaite structure) and the tetragonal 

unit cell of Cr02 (rutile type) and observe that, 

"The introduction of H atoms into the rutile Cr02 

structure allows the formation of hydrogen bonds 

between the Cr06 octahedra in one plane, i.e. between central octahedra and corner octahedra, but 

the overall crystalline structure is only slightly deformed as indicated by the values of the unit-cell 

parameters. On the other hand, the rhombohedral 

corundum structure of a-Cr2 03 is much more compact (G = 5.21 g cm- 3 vs. 4.0 g cm- 3 for Cr02 ) so 

that hydrogen atoms are not able to force an entry 

into the lattice and therefore Cr2 03 cannot be converted into CrOOH."

X-RAY POWDER DATA

Table 9 gives X-ray powder data of the natural 

brown guyanaite crystals, of the natural greenishblack massive merumite material, of Berry and 

Thompson's (1962) "merumite", all three substances from Guyana, of guyanaite from Finland 

(Vuorelainen and others, 1968), and of synthetic 

CrOOH (Tombs and others, 1964; Christensen, 

1966).

Indexed X-ray powder diffraction patterns show 

guyanaite to be orthorhombic, space group Pnnm, 

a = 4.857 A, 6 = 4.295 A, c = 2.958 A, Z = 2, in close 

agreement with the data of Tombs and others 

(1964) and of Christensen (1966).

BRACEWELLITE, CrOOH 

OCCURRENCE AND IDENTIFICATION

Minute deep-red to black prismatic crystals of 

bracewellite having terminal faces line microscopic

TABLE 7. X-ray powder data of (Cr, Al, Fe. Ti, Mn)O2 

from guyanaite, and CrO2 from CrOOH

(Cr, Al.Fe, Ti, Mn)O-2 

(from guyanaite of film 

00188 heated to 335°C) 

(film 00189)

7*

VW 

WW 

WW

vvs

W

s 

mw 

m 

w 

vvw 

vw 

vs 

m 

mw 

w

m

WW 

WW

mw 

w 

w

d s

B 4.074 _

B 3.617

3.469 _

3.130 

B 2.645

2.430 

2.211 

2.129 

1.981 

B 1.805 _

E 1.6615

1.634 

1.564 

1.454 

1.398

1.316

1.173 

1.131

1.066 

1.045 

1.008

CrO2 (ignited CrOOH) 1 

(Thamer and others, 1957)

I

100

60 

10 

20 

10

75 

25 

15 

15 

_ 10

25 

.__<5

~10 

___ 5

d(calc)

3.126

2.434 

2.211 

2.132 

1.977

1.636 

1.563 

1.458 

1.398 

1.321 

1.315 

1.217 

1.173 

1.130 

1.105 

1.072

d(obs)

3.114

2.424 

2.207 

2.128

1.976

1.634 

1.562 

1.457 

1.398 

1.322 

1.316 

1.219

l".l§6 

1.104

10 1.066 1.067 

5 1.042 1.042 

20 1.009 1.008 

Plus more weak lines.

hkl

110

101 

200 

111 

210

211 

220 

002 

310 

112 

301 

202 

212 

321 

400 

410 

222 

330 

312

., c=2.916A.

2 vvs, very very strong; vs. very strong; s, strong; m, medium; w, weak; 

mw, medium weak; vw, very weak; vvw, very very weak; B, bracewellite? 

line; E, eskolaite? line.

TABLE 8. X-ray powder data of (Cr, Al, Fe. TUMn) 2O3

( Cr, Al, Fe, Ti, Mn ) sO3 synthetic 

(from guyanaite of film 00188 

heated to 1,020°C) (film 00194)

I 1

S

VS

vs

vw

w

w

vvs

vvw

vw

w

vw

vvw

vvw

d

3.618

2.656

2.473

2.397

2.2545

2.169

1.809

1.667

1.5765

1.461

1.428

1.292

1.236

1.209

CraOs (eskolaite), Finland 

Kouvo and Vuorelainen, 1958)

7

96

97

94

5

50

60

100

5

30

45

12

5

d

3.63

2.665

2.479

2.269

2.174

1.8146

1.6746

1.6139

1.4656

1.4331

1.2971

1.244

1 vvs, very very strong; vs, very strong; s, strong; w, weak; vw, very 

weak; vvw, very very weak.

fissures and vugs in merumite. The same substance 

occurs in crystalline masses, admixed with other 

merumite minerals; some merumite grains may consist largely of bracewellite. Chemical analysis of 

such a specimen of massive merumite, mainly bracewellite with eskolaite, is given in table 10. Single 

crystals of bracewellite, from vugs in merumite, 

were analyzed by electron probe (table 12). Table 

13 gives powder data for two merumite specimens 

one (X-ray film 16775), the specimen from which
14 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

TABLE 9. X-ray powder data of guyanaite (from Guyana and Finland), "merumite," and synthetic guyanaite (CrOOH)

Guyana 

Pure brown crystals

/*

VVS

m

vs 

m

m

ms 

s 

ms 

ms

mw

mw

vw

vw

ww

mw

m

mw

m

d(dbs)

3.224

2.524

2.432 

2.178

2.115

1.719 

1.636 

1.609 

1.516 

1.475 

1.420

1.375

1.342

1.289 

1.262

1.212 

1.185

1.138

1.085

1.072

1.056

d ( cole )

3.217

2.52 

. 2.436

2.428 

2.177 

-2.147

2.114 

. 1.964

1.720 

1.636 

1.609 

1.515 

1.479 

1.420 

. 1.413

1.373 

_ 1.348

1.344 

_ 1.293

1.289 

1.263 

 1.246

_1.233

1.218

_ 1.214

1.212 

1.185

1.138 

1.089

1.087 

f 1.074 

jl.072 

J 1.058 

11.057

Guyanaite

Guyana 

1 Greenish-black massive 2

hkl I*

WW

110 vs 

mw

101 .

Oil _ -

200 s 

111 vw 

020 _ _

210 m 

120 _ _ 

vw

211 w 

121 mw 

220 m 

310 mw 

002 .

301 vw 

221 _

130 w 

311 - - -

112 . _ _.

320 _ _

031 _ _ -

202 __ _

131

230 _

022

400 .

212

321 _

410

231 .

222 ._

411

040

330

312 .

420

d

5 Q 3.338 -.

3.221

5 E 2.656 _

2.426

2.174

2.113

5 Q 1.811 _

1.719 

1.636 

1.609 

1.514

1.421

1.377

Synthetic guyanaite

Guyana "Merumite" 

(no. 238, 

Berry and 

Thompson, 

1962)

I

10

2

8 

2

1

6 

7 

2 

2 

1

3

1

2

1

1

d

3.23

2.53

2.44 

2.18

2.12

1.722 

1.639 

1.610 

1.519

1.475 

1.421

1.382

1.342

1.141 

1.090

1.058

Finland 

Vuorelainen and others, 

1968

I

10

2

8 

3

1

4 

6 

2 

1 

1 

3

1

2

1

2

1

d(obs)

3.225

2.525

2.435 

2.180

2.115

1.724 

1.640 

1.610 

1.520 

1.480 

1.422

1.376

1.344

1.141 

1.090

1.058

d(calc)

3.224

2.528

2.432 

2.180

2.117

1.722 

1.639 

1.612 

1.517 

1.480 

1.422

1.376

1.345

1.140 

1.090

1.059

Tombs and 

others, 1964 ' Christensen, 1966

/ * d(obs) I (cole)

S

m 

m+ 

m 

. w 

m 

m 

m+ 

m 

w 

w 

m 

w

m 

w 

w 

w 

w

w

w

3.214

2.522

2.429 

2.174 

2.145 

2.112

1.720 

1.636 

1.608 

1.515 

1.478 

1.421

1.373

1.343

1.288 

1.263 

1.245 

1.233

1.212 

1.185

306

85 

172)

65} 

20 

20

14

101 

117

44 

34 

22 

35

48

40

15 

15

-1!

20

21 

18 j 

17 { 

5

16)

- 9)

d(calc)

3.217

2.528 

2.437 1 

2.430 j 

2.178 

2.146 

2.115

1.721 

1.636 

1.609 

1.516 

1.480 

1.421

1.372

1.345

1.288 

1.264 

1.245 

1.233 

1.218) 

1.215 

1.213 ) 

1.185

1.138 

1.089 1 

1.088 j 

1.073

1.059 ) 

1.057 |

hkl

110

101,200 

Oil 

200 

111 

020 

210

211 

121 

220 

310 

002 

301

130

112

031 

202 

131 

230 

022 

400 

212 

321

231 

222 

411 

040

312 

420

1 X-ray film 00188; a 4.857A, 6 4.295A, c 2.958A, Z=2, dcaJcr=4.53.

2 X-ray film 16821.

3 a 4.861A, 6 4.292A, c 2.960A, Z = 2, d calc=4.57.

4 vvs, very very strong; vs, very strong; s, strong; ms, medium strong; m, medium; w, weak; mw, medium weak; vw, very weak; ww, very very weak.

5 Q, quartz and E, eskolaite.

the cell constants of bracewellite were computed, is 

a massive bracewellite having minor guyanaite, and 

the other (X-ray film 16761), which was chemically 

analyzed (table 10; not enough of the first sample 

was available for analysis) is a merumite having 

bracewellite, eskolaite, and minor guyanaite. Table 

13 also gives powder data for crystals of bracewellite, for goethite (with which bracewellite is isostructural), and for (Guyana) eskolaite. X-ray 

single-crystal study, as well as a least squares refinement of the powder pattern of massive bracewellite 

(given in table 13, film 16775) showed orthorhombic 

symmetry: space group Pbnm, with cell constants 

a = 4.492±0.003 A, b = 9.860±0.005 A, c = 2.974± 

0.002 A, Z = ±.

Bracewellite is isostructural with the common 

minerals diaspore, A100H, and goethite, FeOOH

(table 13), as well as with groutite, MnOOH and 

montroseite, (V,Fe)OOH, and synthetic a-ScOOH, 

synthetic GaOOH, synthetic CoOOH, and synthetic 

LaOOH (perhaps with EuOOH and YbOOH also) 

(table 24). However, no synthesis of bracewellite has 

been reported.

Bracewellite and guyanaite are superficially much 

alike, and their powder patterns have a slight resemblance. Bracewellite, however, is a darker reddish brown (almost black) than guyanaite, and 

the powders of the two minerals are respectively 

dark brown (almost black) and yellowish brown. 

Typical crystals of bracewellite are illustrated in 

plate 4, figure 1.

Bracewellite has been named after Smith Bracewell, formerly director of the British Guiana Geological Survey, who first described merumite.
GRIMALDIITE CrOOH AND MCCONNELLITE CrOOCu 15

CHEMISTRY

ANALYSIS

Chemical and spectrographic analyses of a merumite specimen with major bracewellite and minor 

eskolaite are given in tables 10 and 11.

TABLE 10. Chemical analysis of merumite containing 70 perpercent bracewellite and 30 percent eskolaite *

[Analyst, J. I. Dinnin, U.S. Geological Survey]

Constituent Percent

Equivalent HaO 

( assuming

R2O3-H2O)

Cr2(

A1203 

Ti203 

V2O3

70.6

15.1

6.2

.17

.64

6.9

1.70

1.29 

.02 

.08

8.36

Total ___________ 99.6 3.09 

Ignition loss in argon (at 900°C) 4 __ 6.8 

Additional loss in oxygen (at 1.000°C)__ .6

Total ________________ 7.4 

Specific gravity 40-mg sample _ _ ___ 4.48 ±0.045 

Specific gravity 20-mg sample ____ 4.45 ±0.045

Mean ________________ 4.465±0.045

1 X-ray powder pattern is given in table 13, (second analysis) film 

16761).

2 Chromium: A sample was decomposed by digestion with fuming per 

ch'oric acid, diluted with water, oxidized with permanganate the excess of 

which was reduced with hydrochloric acid. Dichromate was reduced with 

ferrous ammonium sulfate, and titrated with a standard solution of 

dichromate.

3 Iron: (A) Sample was decomposed by digestion with fuming mixture 

of sulfuric and phosphoric acids, diluted with water, passed through a 

silver reductor, and titrated with standardized dichromate. (B) Sample 

was decomposed by digestion with fuming perchloric acid and treated as 

above.

4 H2O and ignition loss: A sample in a quartz boat was ignited at 900°C 

in an argon atmosphere, water being swept through an ignition tube and 

caught in an Anhydrone absorption tube. The weight of the quartz beat 

before and after ignition represents the ignition loss of the sample. The 

quartz boat was then ignited at 1,000°C in an oxygen atmosphere. The 

residue from the ignition was used for the determination of iron. A 

weighed portion of the ignited sample was digested with fuming perchloric acid, diluted with water and electrolyzed on a mercury cathode to 

remove chromium and iron. The solution was then diluted to volume, and 

aliquots used for the following determinations: 

Al: Alizarin Red S colorimetric procedure. 

Ti: Tiron colorimetric procedure. 

V: Phosphotungstomolybdate colormetric procedure.

TABLE 11. Spectrographic analysis of menimite containing 70 

percent bracewellite and 30 percent eskolaite

[Analyst, H. W. Worthing, U.S. Geological Surveyl

Constituent

Cr, Fe _ _ _ _ _ .

Al _ _ __ _____

V

Ti __ _____ _

Si _ _ _ __ _

Co _ __ _ _ __ ____

Mg, Pb, Mn __ _ _ _

Cu _ _ _ _ _ _

Ba, Zr _ _ __ _ _

Ni _______ _ _ _ Mo _ _ _ _

Be, Sr _

Sc, Y, Yb __ ___

Ag ________________________

Looked for but 

not detected.

Percent l

. _ -Major (>10)

3

.5

.3

.1

.03

.02

.01

_ _ _ _ .005

_ _ .002

.001

.0007

._ __ ___ .0005

.00002

K, P, As, Au, B, Bi, Cd, 

Ce, Ga, Ge, Hf , Hg, 

In, La, Li, Nb, Pd, 

Pt, Sb, Sn, Ta, Th, 

TI, U, W, Zn.

There is an indication, as seen in table 13 (where 

the strongest line of guyanaite, G, appears as weak 

in tiie X-ray pattern) that a small amount of guyanaite may be present with major bracewellite and 

eskolaite. It has been disregarded in the following 

computations.

The analyses in table 10 can be used in two ways, 

both of which are unlikely extremes, to compute the 

composition (and density) of the bracewellite (and 

eskolaite); the values representing the true compositions are somewhere in between the computer 

values. The first method would be to assume that all 

of the H2O is combined with Cr2O3 , giving 

Cr2O3 -H2O and (Cr,Fe,Al,Ti,V) 2O3 . The second 

method would be to assume that all of the H2O is 

combined with all of the minor oxides, plus some 

Cr2O3 ; this gives (Cr,Fe,Al,Ti,V) 203 -H2O + Cr2O3 . 

The two computations follow:

Case 1

Cr2O3 

Fe203 

A1203 

Ti2 03 

V2 03 

H20 _

Cr2O3

A12 033 

Ti203 

V2 03 

H20 .

Percent

_ _ _ _ 70.6

_ 15.1

fi 9

_ .17

_ .64

_ 7.4

Total __ 100.1 

69.9 Cr2O3 - 

30.2 (Cr0 .«

100.1

Percent

70.6

1 c i

__ 6.2

.17

_ _ .64

_ 7.4

Mole 

ratio

464 

95 .

61 .

1 .

5 .

411

Mole 

ratio

411

411

'Old ~

Case 2 

Mole Mole 

ratio ratio

464 249 

95 95 

61 61 

1 1 

5 5 

411 411

Percent

(62.5)

(7.4) .

oo V o.na)^0.:

Percent

(37.9)

(15.1) 

(6.2) 

(.17) 

(-64)

(7.4)

Mole 

ratio

53 

__95

._ 1

. 5

Mole 

ratio

215

Percent

(8.1) 

(15.1) 

(6.2) 

(.17) 

(.64)

(30.2)

Percent

(32.7)

Total __100.1 __________(67.4)___ (32.7)

67.4 (Cro.6iFeo. 23Alo.i5Tio.ooVo.oi) 2O3 -H2O 

32.7 Cr2O3

100.1

In the computation of analysis to give density for 

case 1, where the analyzed sample is computed as 

69.9 percent Cr2O3 -H2O plus 30.2 percent (Cr0. 25 

Feo.^Alo.^Ti o.ooVo.o 2 ) 2 03 , we have

69.9 30.2 100.1

 4- ~ = >

*-*"C r 0.25 Pe 0.44A1 0.29 TI 0.00 V0.02 ) 2°3 4.465

1 See footnote on reliability of determinations, table 2, footnote 4.

and the computed density of the eskolaite in this 

sample is 4.86 gem- 3 . The Gcroon computes to 4.31 

gem- 3 . These values give a computed density of 

4.480 gem" 3 for the analyzed sample as compared 

with a measured 4.465 ±0.045.

For case 2, where the analyzed sample is computed as 67.4 percent (Cro.eiFeo^Alo.wTio.ooVo.oOaOsH2 0 plus 32.7 Cr_03 , we have
16 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

67.4 32.7 100.0

k~<Cr0.61F <i 0.23 AI 0.15T1 O.OO v0.01>2 <VH2 0 5.21 4.465

and the computed density of the bracewellite in this 

sample is 4.18 gem- 3 . These values give a computed 

density of 4.52 gem- 3 for the analyzed sample as 

compared with a measured 4.465 ±0.045.

These two sets of values may be averaged thus:

Eskolaite

Compositon

(Cro.25Feo.4iAlo.2o Tio. 

Cr2 03

Density 

gem-3 )

4.86

5.21_____

5.035 ±0.175

Bracewellite

Compositon Density 

gcm~3 )

CrOOH _________________________ 4.31 (case 1)

(Cro. 61Feo. 23Alo. 15 Tio.ooV0 . 01 ) 2O3 -H2O_______ 4.18 (case 2)

(Cro.soBFeo.uaAlo.msTio.ooVo.cBjaOs'HsO __________ 4.24 ±0.065

Finally, these averaged values give computed 

densities of 4.48 gem- 3 (case 1) and 4.50 gcm~ 3 

(case 2) for the analyzed sample as compared with 

the measured 4.465 ±0.045, an agreement which indicates the acceptability of the computed values.

An alternative method of computing the density 

of the bracewellite in this sample is to use the unit 

cell constants.

mass of unit cell 

Thus G = ,

volume of unit cell 

and as for case 1

4X85.01

= 4.29 gem- 3 ,

6.02 x 1023 x 131.72 x 10~24

and as G(cro 2gPeo 44A , 0 2gTlo ooVo oM has been computed 

to 4.86 gcm~ 3 the computed density of the analyzed 

sample is 4.46 gcm~3 as compared with the measured 

4.465 + 0.045. 

r or case i "r <cr 0 _ 61Fe 0-23 Ai 0 _ 15Ti 0 _ 00 v0 _ 01) 2 o3 .H 2 o 

4 x 82.1

=4.14,

6.02 x 1023 x 131.72 X 10~ 24

and as

the computed density of the analysed sample is 4.49 

gcm~3 as compared to measured 4.465 ±0.045 

Then,

From published data From analysis From X-ray

4.86

(eskolaite). 

Cr3O3 _ 5.21

CrOOH (bracewellite structure) ___

(Cro.sosFeo.nsAlu.OTsTlo.ooo Vo.oos)

OOH.

4.18 

4.31

4.14 

(case 2) 

4.29 

(case 1)

Since both mean values are equally acceptable, 

their average, 4.23 may be taken as the specific gravity of the bracewellite in the analysed sample, and 

4.16 as the specific gravity of bracewellite-structure 

CrOOH.

ELECTRON PROBE STUDY

Results of an electron probe study of seven crystals of bracewellite and two fragments of merumite 

are given in table 12. Seven elongate crystals (60- 

200 fj. long) were mounted in epoxy resin and polished; the crystals were prepared in this manner 

to minimize effects of geometry on X-ray intensities. 

Two larger fragments of merumite (0.5 mm), from 

which the individual bracewellite crystals were 

picked, were 'similarly prepared for analysis.

Twelve different crystals of bracewellite were 

dispersed on the surface of a polished beryllium 

block and were scanned qualitatively for Pe, Al, and 

Cr. Seven of the 12 crystals showed Fe present as 

a major constituent, Al significantly above background, and Cr, of course, a major constituent. In 

6 of the 7 crystals, raw intensities for Cr decreased 

in the presence of significant Fe.

Although the results are obviously imprecise, they 

do indicate that bracewellite varies rather widely in 

iron content (1-12 percent Fe2 03 ) and, to a lesser 

degree, in alumina (2-5 percent A12 03 ).

TABLE 12. Electron-probe analysis (in percent) of seven 

crystals of bracewellite and two merumite fragments

["Spec" pure CrzOs, analyzed FeS2, and AbOs were used as reference 

standards. CrKa and FeKa were measured using the LiF analyzing crystal; 

AlKa was measured using potassium acid phthalate. All analyses were 

performed at 20 kv with a specimen current of 0.01 /tamp. Analyst, E. J. 

Dwornik, U.S. Geological Survey]

Crystal AbOs Fe2O3 CrcOs Equivalent 

H2O

Sum

Bracewellite

A _

B __ _

C _ __

D _

E

F _

G _ _ __

5 

2 

3 

5

4 

5 

3

12 

<1 

<1 

<1 

<1 

12 

1

64

94 

95 

100 

91 

64 

95

8.7 

10.3 

10.5 

11.35 

10.25 

8.73 

10.61

89.7 

106.3 

108.5 

116.3 

105.2 

89.7 

109.6

Merumite

A _ _ _

B _

3 

2

<1 

<1

98 

94

10.84 

10.26

111.8 

106.3

(Probable bracewellite composi- Mean 4.245 Mean 4.215 

tion in analysed sample).

X-RAY POWDER DATA

In table 13, X-ray powder data for three samples 

of bracewellite are compared with those for isostructural goethite and eskolaite. It is evident from 

the X-ray data in the second column that the specimen analyzed in tables 10 and 11 contained, along 

with bracewellite, considerable eskolaite and very 

minor guyanaite.
GUIMALDIITE CrOOH AND MCCONNELLITE CrOOCu 17

TABLE 13. X-ray powder data for bracewellite, goethite, and eskolaite

rmw 

vs

vw

w 

s 

ms

mw 

ms 

mw 

ms

w

vw 

w 

mw 

w 

vw 

w 

vwb

w

wb

vw 

m

Bracewellite (massive) 

(X-ray film 16775) 

Indexed 

d(obs) d(calc)

4.92 

4.084

3.319

2 G 3.236

2.655 

2.547

2.467 

2.402 

2.216 

2.159

1.763 

1.722 

1.685 

1.666 

1.641 

1.574 

1.542 

1.484

1.461

1.428

1.341 

1.293

4.930 

4.088

3.320

2.653 

2.546

2.465 

2.405 

2.215 

2.161 

._ 2.044

- 1.979

_ 1.806

1.763 

1.748 

1.684 

1.666 

1.643 

1.574 

1.543 

1.482

1.450

1.424

1.344 

1.297

Merumite ( massive 

bracewellite + eskolaite) 1 

(X-ray film 16761) 

hkl I- d(obs)

020 _ ______

110 ws

0

120 _ __

130 ws 

021 vw

040 s 

111 w 

121 _ __ _

140 wb 

220 _ _

131 __ __

150 w

141 _ ___ .

221 ._ __

240 sb 

060 ._

231_ ._ __

160 . _

250_

241__ ___ _

022 w

170_ _ _ .

132 vw

4.077 

2 E 3.604

2 G 3.232 _

2.656 

2.546

2.471 

2.398

2.168

1.809

1.672__

1.430 __

1.298_ _

Bracewellite (red 

pure crystals ) 

(X-ray film 16763) 

1- d(obs)

._ WW

VVS

VVW

vs 

vw

w

vs

w

__wb

vw

4.901 

4.065 

2 E 3.604 _.

3.314 _ .

2.648 

2.546

2.458 

2.404 

2.209 

2.149

1.678

1.5395 

1.485

1.417

G°ethite SyLnal 

(Brown, 1961) (X-rlyfiTm 16765) 

/ d(obs) hkl P d(obs)

10 

100

_ _10

30

8 

4

16 

25 

10 

20

_2

6

8

__ 2

20 

_10

__ 4

_ _6

16 

10

4

. 10

2

. _8

_8

8 

2

2

2

4.98 

4.18

3.38

2.69 

2.58 

2.520 

2.490 

2.452 

2.252 

2.192

2.009 

1.920

1.799 

1.770 

1.721 

1.694 

1.661 

1.606 

1.546 

1.509 

1.467

1.453

1.418 

1.392 

1.357

1.317 

1.264 

1.241 

1.198

020 _ ________

110

_ _ _ __vs

120 __ ___ -

130 vs 

021 _____

101

040 __ ___ _

111 s 

121- _ ____ _

140 w

131 _

041 _ .

__ w

211 _______

141 _ __ _ .

221 _ _ .

240 s 

060 _ __ _ .

231_ _ __ _ -

151, 160_ _ .

250,002 _

320 _ __ .

_vw

061 _ __

_ _ _w

112 ___ _.

330 _ _ .

331 _ _ __ .

321 vvw 

331 __ __ .

142

341 _ __

3.601

2.652

2.468

2.168

1.807

1.666

1.460

1.426

1.30

1 1 See table 10 for analysis. analysis.

- ws, very very strong; vs, very strong; s, strong; ms, medium strong; m, medium; w, wea

eak; b, broad; E, strongest line of eskolaite; G, strongest line of guyanaite.

k; mw. medium weak; vw, very weak, vvw, very very

GRIMALDIITE CrOOH and 

MCCONNELLITE CrOOCu

OCCURRENCE

Well developed tabular deep-red rhombohedral 

crystals as much as 1 mm in size and having perfect 

cleavage, found in vugs in the merumite, proved to 

be inseparable intergrowths (polycrystals) of two 

isostructural phases, CrOOH and CrOOCu, both 

rhombohedral. Their identification has been possible 

because of the close correspondence of their chemical 

and X-ray data with those of the synthetic substances. These phases have been named grimaldiite 

and mcconnellite, respectively, for Dr. F. S. Grimaldi 

of the U. S. Geological Survey and for Dr. R. B. 

McConnell, former Director of the British Guiana 

Geological Survey.

A typical mode of occurrence is illustrated in 

plate 1, figure 4, a sawed face of merumite, and in 

plate 3, figure 1. Small granular aggregates of pink

grimaldiite-mcconnellite fill vugs (frontispiece, 

lower left). The aggregates themselves consist of 

more or less euhedral single crystals such as those 

shown in plate 5, figure 1.

Only very rarely has pure natural rhombohedral 

CrOOH been observed as clear deep-red fragments 

of microscopic crystals broken in the crushing of 

specimens of merumite for study; these fragments 

have been sufficient for single-crystal X-ray study 

but not for chemical analysis. Almost all known 

grimaldiite occurs as well-developed tabular rhombohedral crystals which are intergrowths of grimaldiite and the isostructural mcconnellite. These 

composite crystals are characteristically turbid, 

translucent rather than clear, and vary as to X-ray 

powder pattern and optical character according to 

the relative amounts of the two substances forming 

the aggregate. Chemical analyses have also, necessarily, been made on such composite crystals. Nevertheless, the two minerals can be clearly resolved
18 MERUMITE A COMPLEX ASSEMBLAGE OP CHROMIUM MINERALS PROM GUYANA

with the electron probe, and their distribution in the 

crystals has been individually photographed by 

X-ray fluorescence (pi. 4, fig. 2).

PHYSICAL PROPERTIES

The physical properties refractive indices and 

density vary from one polycrystal to another, as 

is shown by the natural crystals. As the electronprobe photographs indicate, consistent optical measurements cannot be obtained.

The available optical data for (synthetic) hexagonal CrOOH are e = 1.975, « = 2.155 (measured in 

sodium light), giving a geometric mean of 2.093; 

the mean refractive index from specific refractive 

energies is 2.1 (Douglass, 1957). Earlier, a measured refractive index of approximately 2.0 ±0.1 was 

given by Shafer and Roy (1954). For the density 

Douglass (1957) reported 4.11 ±0.03 gcm~3 as a 

measured value and calculated 4.10 from X-ray 

data; even earlier, Laubengayer and McCune (1952) 

reported 4.12 gem- 3 (measured value). No optical 

data were reported for CrOOH (mcconnellite), but 

Ys (2 w + e ) can be calculated as 2.29 by the Gladstone-Dale formula, using the measured density 5.49 

gem- 3 (calculated, 5.609 gem- 3 ) of Dannhauser and 

Vaughan (1955). Stroupe (1949) gave a "rough" 

density of 7.0 gem- 3 .

Accordingly, for the natural complex grimaldiitemcconnellite crystals, observed refractive indices 

should range from e of CrOOH (1.975) to the « of 

CrOOCu (somewhat higher than 2.29), taking into 

account slight changes which reflect the Al and Fe

in the two compounds. However, because of the very 

irregular distribution of the two compounds in the 

crystal, it is unlikely that any precise determinations 

are feasible.

CHEMISTRY

A chemical analysis of a large number of crystals 

of the grimaldiite-mcconnellite intergrowth and supplementary microspeetrographic data are given in 

tables 14 and 15. An X-ray pattern (table 16) can 

be interpreted from our knowledge of the X-ray 

crystallography of the synthetic phases; the only 

ambiguities are the roles of A12 03 and Fe203 as 

substitutions for Cr2 03 in the two compounds. Depending on this, the crystal 'aggregates in the analyzed sample consist of about 80.5-86.5 percent 

grimaldiite, the remainder being mostly mcconnellite.

The loss on ignition is presumably low by 0.55 

percent because of oxidation of Cu2 0, and the analysis is adjusted .'accordingly. Tombs and others 

(1964) found that rhombohedral CrOOH, unlike the 

orthorhombic form (which is guyanaite), decomposed only partially at 350°C to a mixture of Cr02 

and Cr2 03 . The ignition loss, however, should represent all of the water in the grimaldiite (mcconnellite 

being assumed to have no hydrogen substituting for 

copper).

The chemical data are consistent with the average 

relative proportions of CrOOH and CrOOCu shown 

by electron probe (pi. 4, fig. 2).

TABLE 14. Chemical analysis 1 of grimaldiite-mcconnellite polycrystals

[Analysts, Frank Cuttitta and H. J. Rose, Jr., U.S. Geological Survey]

Cr203 _- ______

CuO

A1203 __ _ _ __

Fe203 _ _ __

SiO __ _________

H20 __ __ _

Ignition loss

wt.

percent

__ 77.3

_ __ 5.5

_ 4.3

.85

.42__

.00

_ __ 11.5

wt.

percent 

recomputed

77.3 

(Cu20)4.95 

4.3 

.85

.00_ .

(H2 0 + ) 12.05

(Al, Fe)2Oa-H2O

Mole 

ratio

509 __.

35-

42 

5

669

Mole 

ratio

42 

5

47

Percent

4.3__

.85_

.85__

CraOs-CueO OraOa-HaO

Mole 

ratio

__35

__35

Percent Mole Percent 

ratio

5.3 474 72.0 _.

4.95__ __ _ _____ .

_ _ _ __ 474 8.5

HaO

Mole Percent 

ratio

148 2.7

Total percent,_ 99.87 99.45

Total mole ratio _________________1,260 =

3.00__ +__._10.25__ + __80.5___ + __2.7 

94__ +___70__.__ +___948__ +___148____.

1 Mieroanalysis (total sample 9.35 mg).

The analytical methods used for the microdeterminations were: Cr and Cu by X-ray fluorescence; Si. Al and Fe by spectrophotometry, using1 heteropolymolybdenum blue, alizarin S, and 1, 10-phenanthroline, respectively, as chromogenic agent; and HsQ- and loss on ignition by graviinetry.

Copper is recomputed as CusO, rather than the CuO reported by the analyst. On ignition, the CusO would presumably oxidize to CuO, affecting1 

the ignition loss negatively by gain of 0.6 percent of oxygen. The thermal decomposition of rhombohedral CrOOH is discussed in some detail by 

Tombs and others (1964); its behavior is quite different from that of orthorhombic CrOOH. Whereas the latter readily and completely converts to 

stable CrOa at 350°C, rhombohedral CrOOH at this temperature changes slowly and partially to a mixture of CrO, and CraOs; when the rhombohedral 

CrOOH is completely changed, there is no reduction of CrOs to CraOs.

The recomputed ignition loss 12.05 percent, is obtained by adding to 11.5 percent the oxygen gain (0.55 percent) of Cu2O + O ^2CuO.

Both analysis and X-ray pattern (table 16) indicate the presence of quartz (less than 0.5 percent). Because the analyzed material was obtained 

from a merumite-quartz aggregate, this is not surprising.

Interpretation of analysis

Constituent

Grimaldiite (Cr, Al, Fe)OOH .

Mcconnellite (Cr, Al, Fe) OOCu

HaO (excess) __.._____.___

Total __-_____ __-

Percent 

80.5 -86.5 

16.25-10.25

2.7 - 2.7

99.45-99.45
GRIMALDIITE CrOOH AND MCCONNELLITE CROOCu 19

TABLE 15. Microspectrographic analysis of grimaldiitemcconneUite poll/crystals 1

[Analyst, C. L. Waring, U.S. Geological Survey]

Constituent

Cr _ _ _ _ _ _ _____ __.

Cu _________ _____ _.

Al __ __ _ ___ __ .

Fe, Zn 3

Ti __ _ __ ___ _ ___ _.

Mg, Ca, Si _ _ _ _ _ _.

Percent "

____________ >10

10

._ ___ _ 3

_ _ _ _. .3

__ _ .03

. __ _ _ _ .01

1 Sample (1 mg) is different from that in table 12.

2 See footnote on reliability of determinations, table 2, footnote 4.

3 The Zn may represent chromian gahnite.

ELECTRON-PROBE ANALYSIS

Electron-probe analysis of the polycrystals (pi. 

4, fig. 2) showed a high chromium content throughout the intergrowth. Parallel to the cleavage, layers 

having a relatively high copper content alternate 

with copper-free layers (within the limits of detection). X-ray powder diffraction (table 16) and 

single-crystal (table 17) studies showed that these 

polycrystals are composed of both CrOOH (grimaldiite) and CrOOCu (mcconnellite).

Electron-beam scanning images of the two new 

interlayered mineral phases were prepared to show 

distribution of copper and chromium within several 

grains. The individual "books" were prepared by 

imbedding the grains in epoxy, orienting them so 

that the electron beam would be scanning at a slight 

angle to the basal cleavage plane, or, essentially, 

so that the beam would be "looking at the edge." 

Several grains show alternating Cu-Cr and Cr layers. On other grains, copper was detectable only 

on the outer edges of the layers on which it appeared.

H. T. Evans, Jr. (oral commun., 1966) noted that 

in the isostructural grimaldiite and mcconnellite, 

the CrO2 complex forms layers which are then connected by the cations in this case H+ and Cu+. 

Interlayer substitution in this respect should take 

place easily, but the large difference in atomic diameters of H+ and Cu+ interferes. Alternation of 

layers at the atomic level is possible, however. It 

may also be noted that the a parameters are almost 

identical and that the c parameters vary according 

to the diameter of the cation.

Plate 4, fig. 2, shows electron-beam scans of the 

composite grimaldiite-mcconnellite crystals; the 

distribution of the mcconnellite in a well-defined 

second phase, not in random distribution, is clearly 

shown by the fluorescence of the copper.

X-RAY DATA

Single-crystal study of isolated crystals of pure 

grimaldiite (these are very rare; almost all grimaldiite is intergrown with mcconnellite) showed 

that it is rhombohedral, space group R3m, with unit 

cell (in hexagonal setting) a = 2.986±0.009 A, c = 

13.40 ±0.04 A, in rhombohedral setting a = 4.787 ± 

0.005 A, arh =36.3 ±0.1°, Z = l. Douglass (1957) reported a = 2.984 ±0.003 A, c = 13.40 ±0.01 A for 

synthetic rhombohedral CrOOH prepared by thermal decomposition of aqueous chromic acid. The 

strongest X-ray lines of grimaldiite (table 16) are 

in good agreement with the data of Thamer and 

others (1957) for the synthetic compound. The 

strongest X-ray diffraction lines of mcconnellite 

(table 16), from a pattern of a mixed crystals of 

mcconnellite and grimaldiite, agree closely with the 

data of Stroupe (1949) for synthetic CrOOCu prepared by thermal decomposition of precipitated copper ammonium chromate.

Unit-cell dimensions from least squares refinement of indexed X-ray powder-diffraction data for 

mcconnellite are in good agreement with those obtained by Dannhauser and Vaughan (1955), who 

showed that CrOOCu is rhombohedral and determined the crystal structure. The crystal structures 

determined by Douglass (1957) and by Dannhauser 

and Vaughan (1955) show that grimaldiite and 

mcconnellite are isostructural, so their oriented intergrowth is understandable.

Strunz (1970, p. 175) groups mcconnellite and 

delafossite with AlOOCu and GaOOCu as oxides 

and (p. 218) groups grimaldiite and heterogenite 

as hydroxides.

From the d-spacings of film 16794 (table 16), 

the cell constants of both grimaldiite and mcconnellite were obtained and are given in table 17. 

With these are shown the similar cell constants for 

other isotypic minerals and synthetic compounds.

PREVIOUS SYNTHESES OF RHOMBOHEDRAL 

CrOOH AND CrOOCu

Laubengayer and McCune (1952) prepared rhombohedral CrOOH hydrothermally at 145°C, and 

Shafer and Roy (1954) cite Simon and others 

(1930) as having first prepared crystalline chromium oxyhydroxide hydrothermally at 200°C and 

15 atm. Shafer and Roy (1954) prepared crystalline 

CrOOH from a gel at 230°C and 4 atm for 322 hr. 

Thamer and others (1957) synthesized rhombohedral CrOOH by heating aqueous chromic acid in 

sulfuric acid solutions to 300°-325°C; they gave 

comprehensive data on its morphology, physical 

properties, and X-ray powder pattern (table 16). 

Douglass (1957) determined the structure of rhombohedral CrOOH, and L. H. Jones (in Douglass,
20 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

TABLE 16. X-ray powder pattern of grimaldiite and mcconnellite

Grimaldiitemcconnellite 1 

( + quartz ) 

X-ray film 16794

IVW

vw 

ws 

w 

vw 

ms

w 

ms

vs 

vw 

vw 

vw 

vvw

vs

mw

ms

vw 

w

w

d(obs)

10.05 .

4.97__. ...

4.47 

Q 3.33____.

3.22__._.

M 2.85__._.

2.528- 

M 2.462__.

2.401. 

2.232_ 

M 2.21- . .

2.040

1.99____.

1.857-

1.537-

1.486

M 1.420 ...

1.409-

1.267.

HCrOa 

XPDF 9-331 

(Thamer and others 1957)

/

100

5

30 

5

5

25

10

15

10 

5

<5 

5

. _ Plus

d(obs)

4.44

2.529

2.407 

2.227

2.042

1.858

1.536

1.490

1.414 

1.404

1.284 

1.266 

additional

d(ealc) a

4.47

2.537

2.411 

2.233

2.046

1.860

1.538 

1.492 

1.489

1.415 

1.406

1.286 

1.269 

weak lin

hkl*

003

101

019

006 .

104 .

015

107 .

110 .

009

113 

018 .

021 .

202

"ff-CrOOH"

(Torokin and 

others, 1968)

/ d(obs) /

<5

3 4.46 .

 _ _ - 40

__ - _ 20

9 2.51 . _ .

.___ ___ __ ___100

- _ 40

_ __ __. 5

2 1.861- .

. ____ ____ ._ _45

10 1.489 35 

._ __ __ _ . _5

. __ _____ _30

2 1.414 . _ _ _

.____ ___ ___ __ __ 20

.___ _ __ ___ __ _15

2 1.263. - -

_ _ _ Plus ad

CusO CrsOs 

XPDF 5-668 

(Stroupe, 1949)

d(obs)

5.69

2.85 

2.570

2.470

2.210

1.911

1.646

1.488 

1.439 

1.426

1.319 

1.275

[ditional

d(calc) 1

5.70

2.850 

2.547

2.467

2.207

1.900

1.645

1.487 

1.493 

1.425

1.319 

1.274

Mcconnellite 

(film 16794)

hid 4- d(obs)

003 _ . _.

006 2.85 

101 __ 

012 2.462

104 2.21

009 _ _

018 1.64

110 _ _ 

113_ __ _

0012 1.420

116 _ __ _

202 _ _ _

weak lines Plus additional

d(calc) 5

__ -5.720

2.860 

__ _2.555

2.474

2.213

1.907

1.650 

 1.4915

 1.4915

1.443

1.430 

1.429

 1.322

_ 1.277

weak lines

1 Unit-cell refinements (table 17) of the two phases have been successfully carried out using Evans and others (1963) program.

~ ws, very very strong; vs, very strong; ms, medium strong; w, weak; mw, medium weak; vw, very weak; vvw, very very weak; Q strongest 

quartz line; M, mcconnellite lines.

3 Computed on hexagonal cell having a=2.984A, c=13.40A by Thamer and others (1957) who give slightly different d(calc).

* Computed from a=2.9747±0.00014A, e=rl7.1015±0.0005A.

5 Computed from X-ray powder film 16794 (above, first column) using a and c of Dannhauser and Vaughan (1955) (see table 17) as initial parameters for the least squares analysis.

1957) studied the infrared absorption spectrum, 

finding that it had no resemblance to that of diaspore, A100H.

Torokin and others (1968) have also formed 

grimaldiite by heating (synthetic) y-CrOOH to 

340°-360°C at 300-350 atm pressure for 2 hr. With 

little change in water content (from 1.54 to 1.51 

moles per mole Cr2 03 ) the color changes from emerald green to dirty violet, and the X-ray powder pattern changes from that given in table 18 to that 

(a-CrOOH) given in table 16. Grimaldiite was also 

formed directly by heating an ammoniacal chromium nitrate solution at 320 °C and 200 atm for 

2hr.

Groger (1912) appears to have been the first to 

synthesize rhombohedral CrOOCu by heating basic 

cupric chromite (CuCr2 04 ) for 6 hr at 1,000°C. 

Stroupe (1949) synthesized the compound in two 

other ways: first, by igniting cupric chromite

>900° 

2 CuCr2 04 > Cu2Cr2 04 (that is CrOOCu)

+ Cr2 03 +[0];

second, by heating equimolar cupric chromite and 

cupric oxide:

>900° 

CuCr2 04 + CuO > Cu2Cr2 04 + [0].

Stroupe repeated Groger's synthesis but did not 

obtain usable single crystals; those he did obtain 

by his own methods afforded good single-crystal 

X-ray data as well as characteristic powder patterns (table 16).

Dannhauser and Vaughan (1955) determined the 

structure of CrOOCu and indicated its relation to 

delafossite FeOOCu. They reported that CrOOCu 

("cuprous chromite") is rhombohedral, with apparent hexagonal symmetry arising from extensive 

twinning, the twins being related by 60° rotation 

about the hexagonal c axis. Their cell constants are 

comparable with data for the isostructural CrOOH 

(table 17). These data are for the mcconnellite hexagonal lattice; for the primitive rhombohedral cell, 

the parameters are a = 5.9536±0.0002 A, « = 28°56.0' 

±0.1'.

RELATION OF MCCONNELLITE TO DELAFOSSITE

The structural relationship of mcconnellite, 

CrOOCu, to delafossite, FeOOCu, indicated by 

Dannhauser and Vaughan (1955) appears to be
GRIMALDIITE CrOOH AND MCCONNELLITE CrOOCu 21

TABLE 17. Cell constants of grimaldiite, mcconnellite, and 

related substances

[Ditrigonal^scalenohedral R3m]

(angstroms) (angstroms) Reference

Grimaldiite _ 

CrOOH _ __

Heterogenite 

(stainierite) 

CoOOH.

Mcconnellite _ 

CrOOCu ___

FeOOAg __

Delafossite 

FeOOCu. 

AlOOCu _

GaOOCu _

CoOOPt

CoOOPd __

CrOOPd _____

RhOOPd ____ 

CoOOCu _

RhOOCu ____ 

CoOOAg __ 

CrOOAg _

RhOOAg __ 

GaOOAg ____ 

InOOAg _ _

ScOOAg __

TlOOAg _ _

AlOOAg ____

Feitkneohtite

MnOOH.

2.973 +0.002A 

2.984 +0.003

2.855

2.983 ±0.004 

2.9747+0.0014

3.041

3.038 

2.85

3.03

2.8300+0.0004

2.8300+0.0003

2.9230+0.0003

3.0209+0.0002 

2.8488+0.0001

3.074 +0.001 

2.8729+0.0002 

2.9843+0.0004

3.0684+0.0004 

2.9889+0.0002 

3.2772+0.0003

3.2112+0.0002

3.568 +0.002

2.890 +0.002

3.1? _ _

13.392 +0.01A 

13.40 +0.01

13.156

17.160 +0.031 

17.1015+0.0005

18.55

17.00 

16.9

17.1 __ __

17.837 +0.002

17.743 +0.002

18.087 +0.003

18.083 +0.002 

16.920 +0.002

17.094 +0.009 

18.336 +0.002 

18.511 +0.004

18.579 ±0.004 

18.534 +0.002 

18.881 +0.003

18.538 +0.002

18.818 +0.006

18.27 ±0.02 

. 13.18

This 

report.

1957. 

Kondrashev 

and 

Federova, 

1954. 

This 

report. 

Dannhauser

and 

Vaughan, 

1955. 

Croft, 

Tombs, 

and 

England, 

1964. 

Pabst, 

1938. 

Hahn and

de Lorent, 

1955. 

Do.

Rogers, 

and 

Prewitt, 

1969. 

Do.

Do.

Do. 

Do.

Do. 

Do. 

Do.

Do. 

Do. 

Da

Do.

Do.

Gessner, 

W., 1967. 

R. C. Erd,

written 

commun., 

1972. 

. XPDF-18

804.

questioned by the work of Buist and others (1966), 

who found in an experimental study of the system 

Cu-Fe-0 no phase FeOOCu (delafossite), only one 

of composition given as 3Cu2 0-Fe3Oi. Moreover, 

with the X-ray diffraction patterns of natural delafossite from three localities, they give that of their 

3Cu2 0-Fe3 04 ; the patterns are closely similar.

However, the existence of 3Cu2 0-Fe3 04 was discredited by others. Yund and Kullerud (1964) determined equilibrium assemblages in the system 

Cu-Fe-0 at 800°C and below, finding delafossite 

CuFe02 and copper ferrite CuFe2 04 (a spinel) were 

the only ternary phases. Similarly, Wiedersich and 

others (1968) showed that from initial mixtures of

Cu2 0 or CuO and Fe2 03 , or of Cu2 0 and FeO or 

Fe3 04, only CuFe02 (delafossite) and sometimes a 

spinel phase formed; no mixture yielded a compound 

Cu6Fe3 07 (3Cu2 0-Fe3 04 ). Finally, the structural 

study of Douglass (1957) establishes the composition of mcconnellite (CrOOCu) as valid.

Mcconnellite, CrOOCu, is therefore, as suggested 

by Dannhauser and Vaughan (1955), isostructural 

with delafossite, FeOOCu. Hey (1968) has analyzed 

natural delafossite from Nizhnii Tagil, U.S.S.R. 

(the type locality) and Kimberly, Nevada, confirming the accepted formula FeOOCu. He also observes 

that natural delafossite is clearly not a high-temperature phase, as it is often closely associated with 

kaolinite; this may be of interest with reference 

to the merumite mineralogy, including mcconnellite, 

with which another hydrous silicate, pyrophyllite, 

is commonly associated.

Both mcconnellite and grimaldiite are isostructural with CoOOH (heterogenite), MnOOH (feitknechtite), and NiOOH (synthetic), and with 

NaHF2 and CsICl2 (table 24).

rCrOOH (SYNTHETIC)

Besides the three new natural polymorphs of 

CrOOH found in merumite, a fourth is known only 

as a synthetic product. It is termed y-CrOOH and is 

very probably isostructural with the common orthorhombic dipyramidal minerals boehmite (y-A100H), 

lepidocrocite (y-FeOOH), and synthetic y-ScOOH, 

whose cell constants are all given in table 19. Alario 

Franco and Sing (1972) refer to "green y-CrOOH" 

as "similar to boehmite." However, no single-crystal 

work has been done on y-CrOOH; Christensen (written commun., 1971) notes that "y-CrOOH 

is very difficult to prepare in the laboratory, so it is 

not so surprising that this form is not found in 

nature."

SYNTHESIS

Hund (1959) describes y-CrOOH as green, prepared by heating in an autoclave a solution of sodium 

chromate with sodium formate or sulfur (as reducing agents) for 10 hr at 250°-270°C. Hund also 

gives infrared absorption curves for the y forms of 

A100H, CrOOH, and FeOOH, further showing the 

isotypy.

By reduction of sodium chromate with hydrogen, 

at 290°C, 140 atm pressure, for 2 hr, Torokin and 

others (1968) obtained an emerald-green hydrous 

oxide having 2.29 moles of H2 0 per one mole of 

Cr2 03 . On further heating to 320°C, the water content decreased to 1.54 moles and at 340°-360°C, with
22 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

slight loss of water content (to 1.51 moles), there 

was a color change to dirty violet, interpreted as an 

inversion from y-CrOOH to a-CrOOH (grimaldiite). 

The latter compound was also formed (from chromium nitrate solution and ammonia) at 320°, 200 atm 

for 2 hr.

X-RAY POWDER DATA AND CELL CONSTANTS

Torokin and others (1968) discuss y-CrOOH, giving powder data. Their data, with that of lepidocrocite (Rooksby, 1951), are given in table 18. Their 

d spacings have been computed to give the cell constants of table 19; these have been used to index 

the pattern.

CHROMIAN GAHNITE 

PROPERTDZS

Violet octahedral crystals (plate 5, fig. 2) of 

chromian gahnite, Zn(Alo. 7Cro. 3 ) 2 04, are frequently

associated with grimaldiite-mcconnellite polycrystals 

but are inconspicuous because of their extremely 

small size (usually less than 0.02 mm) and similarity 

in color to the grimaldiite-mcconnellite. Optically, 

they are anisotropic and show marked complex twinning, opposite octants appearing to extinguish together. The index of refraction is about 1.915. The 

mineral gave excellent X-j~ay powder patterns. Ball 

mounts were used because too little material was 

available for standard spindles. The X-ray data are 

given in table 20.

Table 15 gives a microspectrographic analysis of 

grimaldiite-mcconnellite containing 0.3 percent Zn; 

if this is assumed to be in the chromian gahnite, there 

would be nearly 1 percent of this mineral present in 

the sample, somewhat more than is usually observed.

Figure 4 is a graph of unit cell edge plotted 

against compositions from 100 percent ZnAl2 04 to 

100 percent ZnCr2 04 using the data of table 20. 

According to this graph the composition of the 

Guyana chromian gahnite is Zn(Alo.7Cr0.3) 2 04.

TABLE 18. X-ray powder patterns of 7-CrOOH and lepidocrocite

7-CrOOH (Torokin and others, 1968)

/

40 

40

90

100 

10

10

60

30

d

6.38 

3.30

2.40

1.962 

1.885

1.512

1.406

1.183

d(calc) 1

6.35

3.31

2 84

2.40

2

OQ

911

1.943 

1.916 

1.858 

__ 1.718

_ _ 1.588

1.511

1.470

_ _ 1.432

1.422

1.364

_ _1.270

1.229

___ 1.208

1.207

1.199 

1.179 

__ __ 1.105

1.095 

__ 1.074

hM * 

020 

120 

Oil 

031 

111 

060 

200 

051 

160 

151 

080

231

180 

171 

022

251 

091 

280 

191 1-10-0 

340 

311 

360 

242 

082

Lepidocrocite (Rooksby, 1951)

/

100 

90 

10 R

80 

20 

20 

70

20 

40 

20 

_ __ 20

40 

___ _10R

10 

20 

10 

__ __ 10

30 

10 

10 

20 

20

20 B

40

d

6.26 

3.29 

2.79 

2.47 

2.36 

2.09 

1.937

1.848 

1.732 

1.566 

1.535 

1.524 

1.496 

1.449 

1.433 

1.418 

1.389 

1.367 

1.261 

1.213 

1.196 

1.189

1.10 __ .

1.075 ___.

hkl

020 

120 

Oil 

031 

111 

131, 060 

051, 200

220 

151 

080 

002 

201 

022 

180 

171 

260 

122 

251 

091, 320 

280 

022. 191 1-10-0

1 Calculated from Torokin and others (1968), X-ray powder data giving approximate cell parameters in table 19.

TABLE 19. Cell constants (in A) of 7-CrOOH and related compounds

7-CrOOH 1 ____ __ __ _

Bohmite (-yAlOOH) __________

Lepidocrocite (7-FeO OH) _ _ _ _

Do ____ __

7-ScOOH ___ ___ __ __

a

__ _ 3.89 +0.02

___ 3.69

_ _ _ 3.88

4.01

4.015

b

12.70 +0.13

12.2

12.54

13.01

13.04

c

2.92 +0.02

2.86

3.07

3.24

3.244

Reference

This report

Reichert and Yost, 1946

Rooksby, 1951

Milligan and McAtee, 1956

Auer-Welsbach and

Seifert, 1962

1 Computed from powder data of Torokin and others (1968); the values are approximate.
CHROMIAN GAHNITE 23

8.32p

 /Guyana Crgahnite(AL 70 Cr030 ) (a = 8.1625)_

Synthetic Zn(AI 0 7 5 Cr025 ),O4 (a=8.1491)

8.08

0 10 20 30 40 50 60 70 80 90 100

Zn AI 2 O 4 (ASTM 5-0669) Zn Cr2 O 4 (ASTM 1-1123)

COMPOSITION, IN PERCENT

FIGURE 4. Graph of unit cell edge versus composition in 

natural and synthetic chromian gahnite. A, Composition of 

chromian gahnite from Guyana was computed from cubic 

cell constant. B, Cubic cell constant of synthetic chromian 

gahnite was computed from X-ray powder pattern. C, Composition and cubic cell constant of ZnALOi. D, Composition 

and cubic cell constant of

ELECTRON-PROBE ANALYSIS

A procedure for probe analysis was developed by 

E. J. Dwornik and H. J. Rose, Jr. using willemite 

and sphalerite as standards for zinc; pure alumina, 

synthetic MgAl2O4 , and analyzed chromites as standards for aluminum; and analyzed chromite as standard for chromium. Synthesized spinels ranging in 

composition from ZnAl2 04 to ZnCr2 04 could not conveniently be used as reference standard because of 

extremely fine grain size and inhomogeneity, as exhibited in preliminary electron-probe intensity 

measurements and variations in luminescence under 

electron excitation. The probe was operated at 20 kv 

with a specimen current of 0.05 joamp.

An average composition (in percent) for 10 crystals, varying in size from 20-40 /*m, is ZnO, 46; 

A12 03 ,30; and Cr2 03 ,25; all ±10 percent.

The composition computed (in percent) from unitcell data is ZnO, 41; A12 03, 36; and Cr2 03, 23.

X-RAY DATA

Table 20 gives X-ray powder data for the chromian gahnite from Guyana and for related spinels.

OTHER CHROMIAN GAHNITES AND 

ZINCIAN CHROMITES

Chromian spinels are usually black chromites, or 

colored spinels, having a small chromium content. 

The Guyana gahnite contains 23.4 percent Cr2 03 , by 

far the highest percentage yet known. John L. Baum 

(written commun., 1969) reports minute reddishbrown octahedral chromium zinc spinel from the 

Sterling mine at Ogdensburg, N. J. Clifford Frondel

TABLE 20. X-ray powder data for chromian gahnite Zn(Al0 . 

from Guyana and synthetic spinels

ZnCrsCU1

/ d

48 2.92 

100 2.50 

3 2.40 __.

10 2.07

25 1.69 

50 1.60 

60 1.47

5 1.31 

8 1.27 

1 1.20 __.

7 1.11 

12 1.08 _ .

4 1 04

Zn,AJo.7,Cro.3)2O42

7 s

VS

vvs

w

mw

s 

s

vw 

w

vwb

d(obs)

2.888 

2.463

2.040

1.663 

1.570 

1.442

1.291 

1.246

1.095

.965 

.944

d(calc)

2.886 

2.461

2.040

1.666 

1.571 

1.443

1.290 

1.245

1.091 _ __

.962__ _

.942 __

Zn ( AIo. 75, Cro.26 ) aO*3

J 5

VS 

VVS

w

s

VS

VS

w 

mw

*

d

2.884 

2.456

2.038 

1.871 

1.664 

1.568 

1.437

1.289 

1.243

/

3 

84 

100 

1

8 

10 

24 

40 

43 

__ 1

6 

9 

_ _ 1

1

_9

12

_4

_5

9

ZnAI2CM

d

4.67

2.861 

2.438 

2.335 

2.021 

1.855 

1.650 

1.556 

1.429 

1.367 

1.278 

1.233 

1.219 

1.1670 

1.322 

1.0803 

1.0525 

1.0104 

.9875 

.9527 

.9334 

Plus additional

hkl

111 

220 

311 

222 

400 

331 

422 

511 

440 

531 

620 

533 

622 

444 

711 

642 

731 

800 

733 

822 

751 

weak lines.

1 Synthetic zinc chromite, ZnCrsOi, prepared by New Jersey Zinc Co.. XPDF card 1-1122. a=8.32A. 

a Chromian gahnite, Zn(AJo.7,Cro.3)2O4, from Guyana. <j=8.1625A.

3 Synthetic compound, Zn(Alo.75,Cro.25)2O(t, prepared by Jun Ito, Harvard University (written commun. 

* Synthetic zine aluminate (gahnite), ZnAhO*. prepared by EGA Laboratory, XPDP 5-0669. a=8.0848A. 

5 s, strong; vs, very strong; vvs, very very strong; w, weak; mw, moderately weak;; vw, very weak.

1968). a=8.H91A.
24 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

(written commun., 1968) reports that microprobe 

analyses of these crystals showed more aluminum 

than chromium; they were dark green to brownish 

black in color.

In the Outokumpu mine, Finland, where eskolaite 

was found, there is a chromite with 5.8 percent ZnO 

(Thayer and others, 1964). A chromite from Norway has 2.62 percent ZnO (Donath, 1931) and a 

chromite from Maryland contains 0.6 percent ZnO 

(Pearre and Heyl, 1960) . Seeliger and Miicke (1969) 

describe donathite as a tetragonal analog of ferrian 

chromite that contains up to 2.62 percent ZnO, is 

opaque, dark brown streak, strongly magnetic, having composition (Feo. 787Mgo. 13 7Zno.o76) (Cr0 .642Fe0 .35o

S/

CHROMIAN PYROPHYLLITE

Pyrophyllite is almost as abundant an accessory 

mineral in merumite as is quartz, although, because 

of pyrophyllite's softness, it has never been observed 

on the naturally worn surface of merumite specimens, whereas quartz occasionally is. However, on 

breaking open a merumite specimen, radial aggregates of white, bluish-green, or sometimes yellowish-brown pyrophyllite, as much as 1-2 mm across, 

are often seen.

Two X-ray films, 178 and 305, the latter of the 

blue-green pyrophyllite, identify the mineral.

The chromium content of two selected greenish 

flakes was determined by X-ray fluorescence (E. J. 

Dwornik, analyst) as 2.2 percent and 2.6 percent 

Cr2 03 respectively; therefore, about a tenth of 

A12 03 is replaced by Cr2 03 .

Chromian pyrophyllite has also been reported 

from Salzburg, Austria (Meixner, 1961), with about 

1 percent Cr (=2.93 percent Cr2 03 ). Here it is 

found, associated with quartz and ankeritic carbonates, in a bleached zone around gersdorffite-bearing 

veins and is derived from hydrothermal alteration 

of sericite.

GOLD (AND SILVER) IN MERUMITE

Gold is present in merumite as irregularly subrounded grains, a few showing vague octahedral outlines, some as large as 0.05 cm but generally much 

smaller. The large merumite specimen having abundant grimaldiite-mcconnellite and quartz (plate 1, 

fig. 4) is especially rich; the gold content is estimated as over 10 oz/ton. The gold is bright yellow, 

indicating little silver, and shows no evidence of 

wear or attrition. It is believed to be a primary mineral, formed with the chromium mineralization following that of quartz.

Plate 3, figure 2, shows gold in a polished section 

of merumite.

Several attempts were made to determine the gold 

content of selected merumite specimens. The results 

are given in table 3 and in table 21. In summary, 

six samples of merumite were analyzed for gold and 

silver; gold was found in all, although some methods 

of analysis (spectrographic) did not reveal small 

amounts; silver accompanies the gold, although in a 

very small ratio, and did not exceed 1 ppm in samples with less than 4 ppm gold; in two samples gold 

was as high as 2 oz/ton and 23 oz/ton.

ACCESSORY MINERALS FROM DIRECTOR CREEK

A sample of White Sand Formation, about 0.8 km 

(0.5 mile) northeast of the Director Creek merumite 

deposit, contained a rather scanty assemblage of 

heavy minerals about 80 percent rutile, 10 percent 

zircon, also apatite, tourmaline, andalusite, and possibly, hypersthene and chromite. Chromite is a common though rather sparing detrital mineral elsewhere in the region. Hypersthene and andalusite occur characteristically downstream from dolerite intrusions.

The rutile is similar to that found with merumite;

TABLE 21. Gold and silver in six fragments of merumite

Sample 1 Au

A __ 60 ppm (2 oz/ton) 2

0.03 percent (10 oz/ton) 4

B __ _4 ppm 2

Not detected*

C __ 2 ppm 2

Not detected*

D __ 0.5 ppm 2

Not detected*

E_ __ Looked for but

Not detected 5.

0.7 ppm 6

0.9 ppm 7

P __ 0.07 percent (23 oz/ton) 8

Ag

3 ppm 3

0.003 percent 4

<1 ppm 3

Not detected.4

<1 ppm 3

Not detected.4

<1 ppm 3

Not detected.4

0.00002 percent 5 (0.

0.0017 percent 8 (10

,2 ppm)

ppm)

1 Sample:

A Fragments of merumite not themselves showing visible gold but 

from a specimen (figs. 3 and 4) known to be rich in visible 

gold. Because sample A was selected by deleting all fragments 

in which gold was visible, the value of 2 oz/ton is probably much 

lower than the true value for this specimen.

B, C, D Fragments of merumite in which no visible gold was known.

E Fragments of merumite, consisting of about 30 percent eskolaite 

and 70 percent bracewellite, in which gold was not known to be 

present. See tables 10 and 11.

F Fragments of merumite, consisting of about 62 percent eskolaite 

and 38 percent guyanaite, in which gold was not known to be 

present. See table 2, analysis C. 

2Cyanide-atomic absorption method; 10-gram samples; analyst, J. D.

Mensik.

3 HNOs boil-atomic absorption method; 1-gram samples; analysts, J. D. 

Mensik and J. A. Thomas.

4 Spectrographic method; analyst G. W. Sears, Jr. "Results are rereported in percent to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 

0.15, and 0.1, etc., which represent approximate mid-points of interval 

data on a geometric scale. The assigned interval for semiquantitative results will include the quantative value about 30 percent of the time."

5 Spectrographic method, as above; analyst, Helen W. Worthing.

6 Cyanide-lead decomposition; 1-gram sample; analyst, John Marinenko. 

Gold determined fluorimetrically as the Rhodamine B Chloraurate.

7 Aqua regia leach decomposition and Te precipitation; 1-gram sample; 

analyst, John Marinenko.

Gold determined fluorimetrically as the Rhodamine B Chloraurate.

8 Spectrographic, as above, analyst, Helen W. Worthing.
ACCESSORY MINERALS FROM DIRECTOR CREEK 25

the zircon occurs in both well-rounded and prismatic 

pinkish-brown grains, the tourmaline in brown 

rounded grains, and the andalusite (pleochroic) in 

pink grains. The chromite(?) is in rounded, also 

octahedral, grains.

Overlying the merumite in Director Creek is a 

pinkish to red clayey soil with numerous quartz 

pebbles. Very little merumite was found in it, but 

what was found occurred with reddish mottled 

quartzite, some showing secondary quartz overgrowths, and friable quartz-hydromuscovite grains.

Below the merumite is about 0-3 m of reddish 

clayey and pebbly sand having much hydromuscovite and some pumiceous fragments; rutile, zircon, 

and tourmaline were present.

Besides quartz, rutile, zircon, and topaz, the pinkish sand from which the merumite is washed in the 

batelle contains, in much less quantity, tourmaline, 

staurolite, and spinel. The rutile is quite variable in 

color, from lead gray to reddish brown and mottled;

some grains contain anatase. Many of the brown 

grains show a bipyramidal habit. The zircon, usually altered, is in anhedral grains, and shows anomalously low indices of refraction and birefringence. It 

varies from cream to pinkish in color. The topaz 

occurs in two varieties, both rather unusual; one is 

as rounded cream-colored grains, composed of a 

submicroscopic aggregate resembling chert (pi. 6), 

the other as reddish-white mottled grains in which 

a columnar-aggregate structure is rarely seen. The 

spinel is a yellowish to greenish gahnite, sometimes 

showing octahedral faces. Tourmaline-quartz fels 

occurs sparingly and staurolite very rarely.

Table 22 gives the powder patterns of reddishbrown cryptocrystalline topaz from the batelle concentrate and a standard (XPDF 12-765) pattern of 

topaz (average of Brazilian, Mexican, and Utah 

topazes). Table 23 gives the chemical analyses of 

this topaz and of similar chert-like topaz from South 

Carolina (referred to below).

TABLE 22. X-ray powder data for topaz

X-ray film 678 

Guvana 1

J«

VW

w

VW

vvs 

s

vvs 

s

vs

vs

vs 

ww

vvw 

ms 

ms

s 

vvs 

w

w 

w 

ms

ms 

ms

ww 

vs 

ww 

w

VW

ww

d

4.429 

4.151

3.914 

3.712 

3.538

3.216 

3.039

2.943

2.495

2.379 

2.357__ __

2.256 

2.237 

2.176

2.108 

2.064 

1.996

1.945 

1.900 __ _

1.862

1.828 

1.793

1.702 

1.677 

1.659 

1.628 

1.605

1.576

X-ray film 687 

Guyana 2

J*

VVW

w

ww

ws 

ww

vvs

VW

vvs

s

s

vvw

VW

VW

vs 

s

VW

ww

vs

ww 

vvw

s 

ww

VW

ww

ww

d

4.418 

4.150

3.921 

3.702 

3.562 

3.349_

3.205 

3.035

2.935 

9 fiQ7

2.489

2.368

2.312 

2.255 

2.207 

2.176

2.104 

2.060 

1.991

1.946

1.867

1.829 

1.805 

1.784

1.676 

1.659 

1.626 

1.603

1.577

Average of topaz from Brazil, 

Mexico, and Utah 3 

XPDF 12-765

/

5

4 

11 

6 

60

66 

37 

_ ___ 23

100

22 

_ _ _ 10

26

_ 45

_ _ 8

9 

6 

10 

12

__ _ 8

44 

24 

7 

_ _ 9

4 

__ _ 5

24 

____ 26

12 

8 

5

27 

8 

11 

2 

__ _ 3

1 

3 

<1

d

4.395 

4.194 

4.111 

3.896 

3.693

3.195 

3.037 

2.986 

2.937

2.4804 

2.3966 

2.3783 

2.3609 

2.3247 

2.3130 

2.2470 

2.1989 

2.1711 

2.1269 

2.1049 

2.0555 

1.9872 

1.9816 

1.9470 

1.9340 

1.8691 

1.8553 

1.8212 

1.7969 

1.7796

1.6706 

1.6561 

1.6203 

1.6014 

1.5974 

1.5791 

1.5684 

1.5573

hkl

020 

200 

Oil 

120 

111

021 

220 

121 

211

031 

301 

131 

320 

002 

311 

012 

040 

112 

140 

321 

022 

041 

212 

240 

141 

411 

331 

032 

241 

132

232 

322 

341 

431 

042 

501 

142, 520 

402
26 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

TABLE 22. X-ray powder data for topaz Continued

X-ray film 678 

Guyana x

/* d

s 1.533

s 1.467

w 1.451

vs 1.425 

vs 1.410 

ww 1.391

vs 1.361

vw 1.336 

vw 1.306 

vvw 1.286 

mw 1.260 

ww 1.250

mw 1.221 

vvw 1.209 

mw 1.170 

mw 1.149

X-ray film 687 

Guyana 2

/ 4

S

WW

ww

s 

mw 

mw

s

ww 

ww 

ww 

vvw

ww 

ww

d

1.534

1.506

1.488 

1.470

1.450

1.425 

1.412 

1.398 

1.381 __ .

1.362

1.349_ _ _ .

1.335 __

1.307 _ __ .

1.285 _ _ .

1.260_ _ .

1.239

1 999

1 9fiQ

Average of topaz from Brazil, 

Mexico, and Utah 3 

XPDF 12-765

/

_1

. - 4

12 

.__ _ 16

. _ _6

<1

2 

3 

5

. ___ 9

8

4 

26 

25

d

1.5544 

1.5491 

1.5335 

1.5264 

1.5180 

1.5006 

1.4296 

1.4860 

1.4647 

1.4614 

1.4433 

_ _ 1.4398

1.4344 

1.4180 

1.4023

hkl

511 

003 

412 

031, 332 

440 

113 

242 

521 

060 

023 

160, 441 

123 

213 

351 

052

1 Light flesh-colored rounded grains of topaz, Director Creek, Guyana.

2 Reddish-brown topaz, Director Greek, Guyana.

3 Swanson and others (1962). . v. 

* ws, very very strong; vs, very strong s, strong; ms, moderately strong; mw; moderately weak; w, weak; vw, very weak: vvw, very very weaK.

TABLE 23. Analysis of topaz

Constituent

A12 03 _ __ ___ ___ _

TiO2 - __ _ ___

SiO2 _ _ ______

-H2 0 ___ _ _ ___ _

+H2O _ _ __

F _______

Fes>O3 _ _ _ _

Total C as CO2 _ _ __

Sub total _ _

Less O equivalent of F __

Total _

Specific gravity ' _

Guyana 1 

(percent)

3 55.2

4 0.47

3 32.7

5 0.12 52.8

6 15.1

5 0.35

106.74

6.36

100.38

3.55

South Carolina 2 

(percent)

56.76

33.00

0.04

2.67

13.23

105.70

5.57

100.13

3.509

1 Blanche Ingram, U.S. Geological Survey, analyst; Claude Waring, 

U.S. Geological Survey, determined (spectrographically) Fe 0.03 percent, Mg 0.01 percent.

2 Pardee, Glass, and Stevens, 1937.

3 The sample (2 mg) was fused with NaOH. and the solution from 

the fusion was used to determine AkOo and SiOa spectrophotometrically. 

AlaOa was determined with alizarin red S and SiOs as the molybdenum 

blue complex. The amount of F in the aliquot used for measurement of 

AhOs was below the interference level.

* The sample (approximately 25 mg) was fused with Na2COs and dissolved in perchloric acid; Ti was determined spectrophotometrically with 

Tiron.

5 The sample (100 mg) was heated at 110°C to constant weight to determine H2O. The sample was then mixed with twice its weight of 

freshly fused sodium tungstate, and +H:>O and total C were determined 

by igniting the sample at 900° C in a tube combustion furnace. The tube 

contained MgO in the packing to retain F. HaO and COa were absorbed 

and weighed.

6 F was determined by a specific ion-electrode procedure. Samples of 

10 mg each were decomposed by fusinj? with NaaCOs-ZnO, and the F was 

leached with HsO. Measurement of F was made in a 0.5 M sodium citrate 

solution to prevent Al interference.

7 Specific gravity was determined using a specific gravity bottle and 

toluene as liquid.

Cryptocrystalline detrital topaz, either creamcolored or reddish, does not appear to have been 

previously noted elsewhere in Guyana and according to Dr. Leandert Krook of the Surinam Geological and Mining Service (written commun., 1970), is 

unknown after many years of detrital-sediment 

study in Surinam. In both countries, however, the 

normal variety of topaz is a fairly common detrital 

mineral. Dr. D. Bleackley, of the Institute of Geological Sciences, London, notes that the cryptocrystalline topaz could well have been misidentified as 

gorceixite (written commun., 1969).

In all observable respects, the cream-colored or 

reddish topaz resembles a cryptocrystalline topaz 

described by Pardee and others (1937) from quartz 

veins in the Brewer gold mine in South Carolina. 

This topaz contained the highest H2 0 (2.71 percent) and the lowest fluorine (13.23 percent) ever 

reported.

This cream-colored or reddish cryptocrystalline 

topaz appears to occur only with the merumite; it 

has not been found in the overlying local White Sand 

nor in the underlying altered volcanic ash. Clear 

transparent well-crystallized topaz is found else-
TABLE 24. Crystallographic relationship of the CrOOH compounds and other metallic hydroxides (values in angstroms)

Orthorhombic

Pnnm

OO(OH) Guyanaite 1 ^' 10 

AJO(OH) -

FeO(OH) FeO(OH) 10 

ScO(OH) ScOOH " 10 

InO(OH) InOOH*. 

CoO(OH) __ ___ 

NiO(OH) NiOOH 10 

MnO(OH) ________ -

( V.Fe) O (OH) VOOH <a 

GaO(OH) - - 

LaO(OH) ____ _____

RhO(OH) RhOOH 10 

NaHFs - _- __ __

CsIGla - - ______

a

4.86

4.937 

5.150 

5.26

4.791

4.862

4.924

2=2

b

4.295

4.432 

4.600 

4.56

4.226

4.446

4.291

c

2.95

2.994 

3.197 

3.27

2.905

2.962

3.082

Pbnm

Bracewellite 1 > 2

Goethite 2 

a-ScOOH 3

.CoOOH 10

a-NiOOH 12

Montroseite s 

.GaOOH*

.LaOOHs

a

4.492 

4.41 

4.65 

4.755

4.353 

4.31 

4.56 

4.54 

4.51 

4.92

2=4

b

9.860 

9.40 

10.02 

10.301

9.402 

13.90 

10.70 

9.97 

9.75 

11.3

Rhombohedr

Amam 2=4 RSm

o a b c

2.97 a-CiOOH x 3.89 12.7 2.92 Grimaldiite * 2 

2.84 Boehmite 2 3.69 12.2 2.86 . ___________ .

3.04 Lepidocrocite 3 3.88 12.54 3.07 . ____________ .

3.209 a-ScOOH 2 ' 3 4.01 13.01 3.24 . ____________ .

2.840 _ ___ . ____ _. ________ .._. _ Heterogenite 2- 10

2.925 a-NiOOH u 4.40 13.8 2.82 0-NiOOH 10

3.03 _.__ ________________________ ... __________ .

2.965 _____________________________ . ____________ .

3.44 __________________________ ... ____________ .

. __ --_-. - . ...... ...NaHFa2- 6

. -- -- ______ _ __ CsTCl2'

Z=3

a 

2.99 13

. 

. ________

. __ ___

. ____ -_

2.85 13

2.99 14

3.1?2 13

.__ _ 

..... . 

3.468 13

6.328 12

1. This report. 

2. Strunz, 1970. 

3. Christensen and Jensen, 1967. 

4. Lehmann and others, 1970. 

5. Yamaguchi and Katsurai, 1967. 

6. Swanson and others, 1955. 

7. Swanson and others, 1954. 

8. Christensen, 1971. 

9. Erd, R. C. U.S. Geological Survey, written commun., 1972. 

10. Chenavas and others, 1973. 

11. Bode, 1961. 

12. Evans, Howard, U.S. Geological Survey, written commun.. 1974.
28 MERUMITE A COMPLEX ASSEMBLAGE OF CHROMIUM MINERALS FROM GUYANA

where in many Guyana localities, but the reddish 

and cream-colored cryptocrystalline varieties are 

not.

This topaz may have been mistaken for gorceixite; 

Dixon (1957) states that a report on merumite by 

Smith Bracewell to the Hon. Colonial Secretary, 

September 17, 1945, referred to the "abundance of 

gorceixite." Gorceixite, however, is known to occur 

elsewhere in Guyana.

CRYSTALLOGRAPHIC RELATIONSHIPS OF 

CrOOH COMPOUNDS

As already mentioned, the three mineral polymorphs of CrOOH, guyanaite, bracewellite, grimaldiite, and the fourth, an artificial compound not 

known as a mineral, have interesting crystallographic relationships to other metallic oxyhydroxides with composition ROOH. These relationships 

are indicated in table 24, which is an amplification 

of data in Strunz (1970).

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Groger, M., 1912, Chromite aus basischen Chromaten: Zeitschr. Anorg. u. Allg. Chemie, v. 76, p. 30-38.

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ternarer Oxyde des Aluminums, Galliums, und Indiums 

mit einwertigen Kupfer und Silber: Zeitschr. Anorg. u. 

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p. 229-231.

Kouvo, Olavi, and Vuorelainen, Yrjo, 1958, Eskolaite, a new 

chromium mineral: Am. Mineralogist, v. 43, p. 1098-1106.

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Am. Chem. Soc. Jour., v. 74, p. 2362-2364.

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Milton, Charles, and Chao, E. C. T., 1958, Eskolaite, Cr2 03, in 

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 fr U.S. GOVERNMENT PRINTING OFFICE : 1976 O 217-900
PLATES 1-6
PLATE 1

FIGURE 1. Very large (8 cm) specimen of merumite. Exterior view showing botryoidal surface and inclusions of 

quartz.

2. Sawed face of merumite specimen in fig. 1 showing quartz inclusions.

3. Exterior view of another very large (8 cm) merumite specimen showing botryoidal surface and quartz 

inclusions. Note that some of the quartz inclusions are well crystallized.

4. Sawed face of merumite specimen in fig. 3 showing quartz inclusions and several areas of grimaldiite^ 

mcconnellite (mottled areas indicated by arrows).
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 1
PLATE 2

FIGURE 1. Photomicrograph of a thin section of merumite with green subhedral eskolaite 

enclosing yellow-brown guyanaite. Plain transmitted light.

2. Thin section of merumite showing banded green eskolaite and yellow-brown 

guyanaite. Plain transmitted light.

3. Thin section of merumite consisting almost entirely of guyanaite with very 

little eskolaite. Most of the guyanaite occurs in extremely fine-grained 

spheroidal aggregates, but coarse prismatic guyanaite is also present 

(see fig. 4). Plane-polarized light.

4. Detail of fig. 3 showing both the coarse prismatic and fine-grained habits 

of guyanaite. Plane-polarized light.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 2
PLATE 3

FIGURE 1. Polished surface of merumite showing quartz inclusions (note broken crystal) 

and mottled areas (indicated by arrows) which are granular aggregates 

of grimaldiite-mcconnellite polycrystals. Reflected light. 

2. Polished section of merumite showing gold (at arrow). Reflected light.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 3
PLATE 4

FIGURE 1. Bracewellite crystals (deep brown, almost black), showing prismatic habit,

from vugs within merumite. Plain transmitted light.

2. Electron-probe analyses of two grimaldiite-mcconnellite polycrystals. A and 

D, Electron-probe photomicrographs of the edges of two specimens. Width 

of each edge is approximately 70 vm. B and E, Scanning X-rays images 

in CrKa radiation showing distribution of chromium in specimens A and 

D, respectively. C and F, Scanning X-ray images in CuKa radiation 

showing distribution of copper in specimens A and D, respectively.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 4
PLATE 5

FIGURE 1. Scanning-electron photomicrograph of a grimaldiite-mcconnellite polycrystal.

Taken by A. J. Tousimis.

2. Scanning-electron photomicrograph of octahedral crystals of chromian gahnite 

and a flake of grimaldiite-mcconnellite (upper left). Edge of octahedron 

(upper right) is 35 ^m. Taken by A. J. Tousimis.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 5
PLATE 6

FIGURES 1, 2. Photomicrographs of topaz showing structure of fine-grained aggregate. 

Fig. 1 taken in plane-polarized light and fig. 2 with crossed polarizers. 

3, 4. Another thin section of a topaz aggregate. Fig. 3 taken in plane-polarized 

light and fig. 4 with crossed polarizers.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 887 PLATE 6