djurleite digenite transformation

7/25/2019 Djurleite Digenite Transformation

1/8

American

Mineralogist , Volume 79,

pages

308-315,

1994

Djurleite,

digenite,

and

chalcocite:

Intergrowths

and transformations

MrnAr.y

Posrr, Prren R. Busncr

Departments

of Geology and

Chemistry, Arizona

State University,

Tempe, Arizona 85287-1404,U.S.A,

Ansrnq.cr

Intergrowths

between

djurleite

(-Cu,

noS)

nd digenite

(-Cu,.S)

and betweendjurleite

and chalcocite

(CurS)

and the transformation

between djurleite

(dj)

and chalcocite

(cc)

were

studied using high-resolution

transmission electron microscopy.

Pseudohexagonal

wins are

common

in

djurleite; crystal blocks are rotated relative to

each other around

[100],

the normal

of the close-packedayers, by multiples of 60 . Djur-

leite

and digenite

(dg)

bandsare ntergrown, with

(l

I l)u,

parallel

o

(100)o,,

hereby creating

a cubic-hexagonal

alternation in

the sequence

of close-packed

ayers. The

typical orien-

tational

relationship

between

coexistingdjurleite and chalcocite s

where

[001]*

is

parallel

to

[00]0,

and

[010]*

is

parallel

o one of the

(010)

or

(012)

directionsof djurleite.

Ifboth djurleite and chalcocite occur in a sample,chalcociteeasily converts o djurleite

under the electron

beam hrough the rearrangement

f Cu atoms.

A

similar electrochemical

transformation

probably

takes

place

in CurS-CdS solar

cells and

is the reason for

the

instability

ofchalcocite n

such

devices.

IxrnooucrroN

Copper sulfides

are

widespread

and are major

sources

of Cu. Digenite,

djurleite,

and chalcocite

are the Cu-rich

members

of

a seriesof minerals with

compositions rang-

ing

from

CuS

(covellite)

to Cu,S

(chalcocite)

(Table

1).

Djurleite was

discovered as

a

mineral

by Roseboom

(1962),

following

its

synthesisby Djurle

(1958).

Since

chalcociteanddjurleite are not readily distinguished rom

each

other by

optical

methods

(Ramdohr,

1980), rela-

tively little

is known

about their

orientational relation-

ships and intergrowths.

However,

knowledge

of such

re-

lationships

is

useful for

understanding

phase

relations,

transformations,

and reactions

of copper

sulfides.

Besides

being

an important

ore mineral,

chalcocite

has

an important

materials

science

application in the CurS-

CdS couple n

solar

cells

Te

Velde

and Dieleman, 1973).

Copper sulfide

solar

cells

were

considered

n the 1970s

and 1980s

s nexpensive

eplacements

or

costlySi

cells.

However,

a distinct

problem

with

chalcocite

cells s that

they proved to be unstableover time (Moitra and Deb,

I

983).

Low-temperature

chalcocite and djurleite

have

com-

plex

hexagonal

close-packed

structures

with large

unit cells

(chalcocite:

pace

roup

P2,/c,

a: 1.525,

: 1.188,

:

1.349 m,

0:

116.35:- jurleite: pace

roup

P2,/n, a:

2 . 6 9 0 , :

1 . 5 7 5 ,

: 1 . 3 5 7

n m , 0 : 9 0 . 1 3 )

( E v a n s ,

1979).The structureofdigenite

is

basedon

an antifluo-

rite-type subcell n

which

the close-packedCu

* S

layers

follow a cubic stacking scheme

Donnay

et al.,

1958;

Morimoto and Kullerud,

1963).The clustering f

vacan-

cies and Cu atoms

produces

several ypes of digenite su-

perstructures Pierce

and

Buseck, 1978; Conde et al.,

I 978).

The

phase

relations of the copper sulfides

have been

studied

extensively.

Monoclinic chalcocite converts to a

high-temperature hexagonal

polymorph

at 103

oC,

and

the upper

limit

of stability

of djurleite

is

93

'C

(Rose-

boom,

1966;Mathieu and

Rickert, 1972;PoIter, 1917).

According o

Morimoto

and

Koto

(1970)

and

Morimoto

and Gyobu

(197

),

digenite

s

stable

at room temperature

only

if it contains a small amount

of Fe.

The

goals

of

this

paper

are to

investigate the micro-

structuresof

natural

samples

of chalcocite,djurleite, and

digenite

n order to obtain a better understanding

oftheir

relationshipsand to obtain insights nto the processeshat

take

place

in

CurS-CdS

solar cells and that

make

them

unstable.

We used high-resolution transmission

electron

microscopy

(HRTEM)

so that

we

could

obtain simulta-

neous

structural

and textural

information.

TABLE .

Compositions,

structures, and stabilities

of Cu-rich copper sulfide

minerals

Composition

S

packing

System

Stability

References

Chalcocite

low)

Chalcocite

high)

Chalcocite

high-4

Diurleite

Digenite low)

Digenite

high)

Anilite

Cur

-r S

Cur

-r S

CurS

Cu,

-, *S

Cu, u-, S

Cur

-r S

Cur

uS

monoclinic

hexagonal

tetragonal

monoclinic

cubic

cubic

orthorhombic

ncp

ncp

ccp

ncp

ccp

ccp

ccp

r< 103 rc

- 1 0 3 ' , C < f < - 4 3 5 r c

l k b a r < P , f < 5 0 0 ' C

r< 93rc

metastable

m r c < r

T < 7 2 r c

Roseboom

1966)

Roseboom

1966)

Skinner

(1

970)

Potter

(1977)

Morimoto and Koto (1970)

Roseboom

1966)

Morimoto

et al.

(1969)

0003-004x/94l0304-0308$02.00

308


2/8

POSFAI

AND BUSECK:

HRTEM OF COPPER

SULFIDES

30 9

ooo

o @ @

oo

oooo

@ @ @ @

ob

oo

@ @ @ @ e @ @

Fig. . Explanation fthe

pseudohexagonal

winning

ofdjur-

leite.The

openand shaded ircles

epresent

wo layers f S at-

oms;a djurleiteunit cell

s

outlined.

The

arrows

epresent

seu-

dohexagonalxes ndexedon the monoclinic

djurleitecell.

n

twinneddjurleite,

ndividual

crystals re

rotated

around

100]

by

multiples

f 60

elative

o oneanother.

ExpnnnrpNul

We studied

djurleite

from the

Dome

Rock

Mountains,

Aizona, and chalcocite

rom Redruth,

Cornwall

(inven-

tory

nos. N-067

and

A-820 at Eritvils Lor6nd University

Mineral

Collection,

Budapest).Specimens

or HRTEM

studies

were

prepared

both by

ion-beam milling

and by

crushing

the minerals

gently

in

an agate mortar under

chloroform and dispersing

he

particles

onto holey car-

bon

films supported

by Cu

grids.

Since

we noticed that

ion milling

induces ransformations

n

djurleite

and chal-

cocite, the

preferred

method of specimen

preparation

was

grinding.

In this

paper

only the

micrograph of coherently

intergrown chalcocite

and djurleite

(discussed

n the next

section and

labeled

Figure 6)

was

obtained

from

an

ion-

milled sample; all other figures present results from

crushed

minerals.

Electron

microscopy

was

performed

with a

JEOL

4000EX electron

microscope

at a

400-kV

operating

volt-

age

C :

1.0 mm), using a top-entry,

double-tilt

(x,y:

+

20 )

goniometer

stage.

OnsnnvarroNs

Djurleite twinning

Twinning in djurleite

is so common

that

it long ham-

pered

a structuredetermination

Evans,

1979).The twin

laws operating

on djurleite

were

identified by

Takeda et

al. (1967), who distinguishedbetweenpseudohexagonal

and

pseudotetragonal

wins.

Pseudohexagonal

wins

occur

in many crystals

n the

djurleite sample

we studied.

Sectorsare

rotated

relative

to one another

by multiples

of 60' around

[00],

which

is

perpendicular

o

the

hexagonal lose-packed

lanes.

Figure I

displays

two S

layers of the djurleite

structure.

The hexagonalsymmetry

of the S

framework

is reduced

to monoclinic by

the arrangement

of the Cu

atoms. Se-

lected-area electron-diffraction

(SAED) patterns

taken

along the

(010)

and

(012)

zone

axes

are easily distin-

guished Fig.

2a,2b). If the crystal

s twinned and

con-

tainsboth

(010)-

and

(012)-type

omains,

composite

diffraction

pattern

like that

in Figure 2c

is

obtained.

Twinned djurleite crystals

may contain

as

many

as

six

distinct

individuals;

however, since the

B

angle deviates

from 90'by only 0.

3',

it is

difficult

o

identify more than

o

@

o

@

o

@

o

@

o

o

o

@

o

@

o

o

o

e

o

@

o o

@

o

t0T

@

o

@

o

101

@

o

Fig. 2.

SAED

patterns

of djurleite taken

from

directions

perpendicular

o

I

I 00].

(a)

The

[0

0]

projection, (b)

t0

2l

projection,

(c)

twinned djurleite. Pattern

c

is

a composite of a and b.


3/8

3 1 0 POSFAI AND

BUSECK: HRTEM OF COPPERSULFIDES

Fig.

3. Domains n

djurleite.A, B, C,

and

D

are

pseudohex-

agonalwins.Domain

A is viewed

alonga

[012]-type

irection,

whereas ,

C, andD

areall

viewed

along

010]-type

irections.

Thearrowsmark

contrast

hangeshat suggesthat B andD are

in

the sameorientation,

ut C is rotated

by

180

around

00]

relativeo B

andD

(e.g.,

fB

and

D

are

010],

henC is

[0T0]).

two individuals

from

SAED

pattern

ike

the one

n

Figure

2c. In

addition

to 60

twins, other types ofrotation

do-

mains

also

occur

(Fig.

3) .

Djurleite-digenite

intergrowths

Narrow

strips having

disordered stacking sequences

commonly

occur

between djurleite twin individuals. Al-

Domainson the wo sides f the

horizontal

oundary

@

vs. A,

B,

C, and

D)

are

elatedo each therby an

-

54 otation round

[010],

which s

perpendicular

o the

plane

of the

micrograph.

The

orientations

were

determined

rom diffraction

Datterns

computedor eachof the domains.

though

from

the

image

alone

it is

difficult to assign a

particular

mineral name to the area marked dg

I

l0] in

Figure 4, the structural characterand orientation of

these

units wereconfirmed from diffraction patternscomputed

from

the digitized image.

We

identified the disordered

bandsbetween

djurleite

units in Figure 4

as digenite,

with

(11

1)dsl l (100)dr.

Fig. 4. Digenite

(dg)

bands

n

twinned djurleite

(dj);

the

zone-axis

ndices mark the direction of

projection

for each structural

unit.


4/8

POSFAI

AND BUSECK: HRTEM OF COPPER

SULFIDES

3 l l

dg1

Fig. 5. Intergrowth

oftwinned digenite

with

twinned djurleite.

The

arrows

mark boundaries

betweenstructural

units.

The I and

2 refer

to the crystal blocks in a twin

relation

to each other. The difraction

patterns

were computed

from the digitized

micrograph;

the

particular

structural

units

to

which

they belong are

marked

on the

microglaph.

X:

djurleite

in

[010]

projection,

Y: djurleite

in

[012] projection, Z: 6a-typedigenite n [1 0] projection.

v

;1

dg 1

Larger

blocks of

digenite also occur in

djurleite.

The

twinned slabs

of digenite in Figure

5 basically have

the

6a-type superstructure

see

he computed

diffraction

pat-

tern marked Z in Fig.

5). The

digenite bands are

a

few

unit

cells thick and

are separatedeither

by twin bound-

aries

or by slabs of djurleite

that

is itself

twinned. The

crystal

in

Figure

5 exhibits a

wide

variety

of structural

features:

l)

orderingofvacancies

nd Cu atoms

hat

pro-

duces the digenite

6a-type superstructure

Conde

et al.,

1978;

Pierceand Buseck,1978),

s seen

n the diffraction

pattern

markedZ, (2) 180 otation twinning around I I l]

in

digenite that introduces

stacking aults into

the cubic

sequence

of close-packed

ayers

[see

he change n

ori-

entation

of the

line

denoting

the

(

I I I

)

plane

on the

right

side of the

figurel,

(3)

60

rotation

twinning

in djurleite,

as

ndicated

by

the diffraction

patterns

marked X and

Y,

and

(4)

alternation

of cubic close-packed

digenite)

and

hexagonalclose-packed

dj

urleite) stacking sequences.

Djurleite

and chalcocite

The

Cornwall

sample hat

we

studied consists

of chal-

cocite and djurleite.

We found that the

method

used

for

specimen

preparation

affects

the outcome of the

TEM

study. Although we could obtain high-resolution images

from chalcocite

when looking at ion-beam

milled

speci-

mens, we were not able to obtain similar

micrographs


5/8

312

POSFAI AND

BUSECK: HRTEM OF COPPER

SULFIDES

Fig. 6. Coherently ntergrown chalcocite and djurleite in an ion-milled sample(cc: chalcociteviewed down [010]; dj: djurleite

viewed

down

[012]).

using specimens

hat

were

ground

in

an

agate

mortar.

Crushed

grains

of

chalcocite ypically

converted

to djur-

leite when

exposed

o an electron beam

strong enough

o

readily

produce

high-resolution images

(at

a beam cur-

rent

of

-14

pNcm2,

as measured

n the viewing

screen

of the microscope).

On the other hand,

specimens

hinned by ion-beam

milling

may not

reflect

the original

relationship

between

chalcocite

and

djurleite crystals in

the sample. Heating

the specimen o 190 C during embeddingand then bom-

barding

t

with Ar ions

converted

djurleite into

chalcocite

and high

digenite. After

being cooled to room

tempera-

ture and

stored for

several

months,

part

of the material

reverted

to

djurleite. In such

specimens, ntergrowths

of

djurleite and chalcocitewere

stable n

the electron

beam,

and

[010]..

was

commonly

parallel

o

one of the

pseu-

dohexagonal

axes

((010)

or

(012))

of djurleite,

with

[00

]..11

100]dj

Fie.

6).

Although

high-resolution

mages

are not

available

rom

crushed

grains

ofchalcocite, SAED

patterns

confirm

that

in unaltered natural

sampleschalcocite s

typically inter-

grown with djurleite in the same fashion as is seen n

Figure

6.

This

orientational relationship

allows the close-

packed

S

ayers

o

be continuous across

he

interface;

only

the Cu atoms are in diferent

positions

on the two sides

of the boundary.

Figure 7

demonstrates his

relationship

by displaying the structuresof chalcociteand djurleite as

projected

along the

pseudohexagonal

xesofthe S ayers.

When

chalcocite converts to djurleite under the elec-

tron beam, he

framework

of S atoms

remains ntact;

only

the Cu atoms rearrange.Such transformations

were

re-

ported

by

Putnis

(1977).

In

addition to the

reversible

chalcocite

-

djurleite transformation,

we

observed that

the movement of Cu atoms also producesconversions

directly between

different djurleite

orientations. Figure

8

provides

an example of

how four

different SAED

patterns

could be obtained

from

the same crystal while it was

exposed

for

several

minutes

to the electron beam, but

retained n

one

position

throughout the experiment.

First

we recorded

the

pattern

in Figure 8a

(chalcocite

[00]);

then the three SAED

patterns

corresponding o djurleite

(Fig.

8b-8d)

were

observed

n

sequence

ithin

a

few min-

utes.

The four

patterns

appearedand disappeared n cy-

cles and in an apparently random fashion,

except hat the

chalcocite

pattern

only occurred

when

a

low

(


6/8

POSFAI

AND BUSECK:

HRTEM OF COPPER SULFIDES

3 1 3

a

[ 0

o ] d i

+

c/2

c

[o l o ] cc

+

ta l

z

tstnt t

Fig. 7. The

structure f djurleiteas viewed

along

a)

[010]

and

(b)

[012].

c)

The

structure f chalcocite s viewed rom

[010].

Largecircles:

S atoms;small circles:Cu atoms.Parts

b

and c display he orientations

resent

n Fig.

6,

where

he two

domains ontain

S atoms

n identical

positions,

ut Cu atoms

are

n

different rrangements.

relationship

between chalcocite and djurleite

in Figure

8

is the sameas hat found in ion-beam

milled

specimens,

and the three orientationsofdjurleite are in (pseudohex-

agonal) win

relations

to one another.

High-resolution

images

provide

insight

into

the trans-

formation mechanisms.

Spectacular hanges

ould be ob-

served n real

time on the

TV

screen

hat

was

connected

to the electron microscope.When

the Cu atoms began

o

move,

the

sharp

mage

gradually

becameblurred; after

a

few

secondsno details

could be seen n the image. After

10-20

s, sharp, ordered spots

abruptly appeared

on the

screen, ut

their arrangement ndicated

an orientation

dif-

ferent rom

the

previous

one. Between

ertain

stages fthe

transformation

cycle he

process

did not

go

to completion

in one step; first, only a part (the left side)of the crystal

converted o the

other orientation

(Fig.

9). Figure l0

dis-

plays

wo

stages f the transformation

rom

[32]o,

to

[104]o,

orientation:

(l) part

of the crystal

converted o

the

[104]

orientation

(Fig.

l0a),

and then

(2)

the entire

crystal

switched o

u041,

but the

previous

orientation

boundary

was

preserved

as an antiphase

boundary

fig.

l0b).

In

or-

der to obtain images

of different

parts

of a large

grain,

the

crystal was

translated

under the electron

beam,

causing

uneven exposure

o the electron rradiation.

This

proce-

dure may have

been responsible

or

the separatenucle-

ation events

observed n

the transformation

process.

DrscussroN

The Arizona

djurleite

sample contains both fault-free

and heavily

twinned crystals. Intergrowths with

disor-

dered 6a-type

digenite are associatedwith

the defective

djurleite

crystals.

As

discussed

y

Veblen

(1992),

HRTEM

studies

end to emphasize

pathological

disorder in min-

erals,although t may

also be important

to

know whether

ordered

structures occur in

a

particular

sample. In

the

caseof the

djurleite sample, the large number

of defect-

free

grains

suggests

hat structural disorder s

a

local

phe-

nomenon.

According to Potter (1977), djurleite forms with dige-

nite if

the

value

of Cu/S s

between

1.79

and I .93. Diur-

[ 0 1 2 ] d i

[ 0 2 1 ] -

Fig.

8.

Transformationsetween

ne chalcocite rientation

and hreedjurleiteorientations,s observed nder he electron

beam.

a)

Chalcocite

100],

b)

djurleite

l32],

(c)

djurleite

104],

(d)

djurleite

T32].

See

ext

for

discussion.)

leite

coexists

with digenite in a sample

rom

the

Magma

mine,

Arizona

(Morimoto

and Gyobu,

l97l),

and Mori-

moto

and

Koto

(1970)

synthesized6a-type digenite

with

the composition

of Cu,roS.

However,

several tudies

n-

dicate

that digenite

is not

stable at

room temperature

(Potter,

1977;Morimoto

and Gyobu,

197

;

Putnis,1977);

instead anilite

(Cu,,rS)

is

the stable

mineral

occurring

with djurleite (Table l). Furthermore, Morimoto et al.

(1969)

ound that

grinding

samples hat

containedboth

anilite and djurleite

produced

digenite.

However, we

ground

our samples

gently

and djurleite

was

preserved,

Fig. 9.

HRTEM image

of a

ftrzzy

grain

boundary

(marked

by arrows) between djurleite

u04l

and djurleite

u32l

orienta-

tions.

The left

part

of the

image

corresponds

o the SAED

pattern

in Fig. 8c and the right

part

to

Fig.

8d.


7/8

3t4

POSFAI

AND BUSECK: HRTEM OF COPPER

SULFIDES

Fig. 10. Two

stages

n

the transformation of a crystal

from

djurleite

[32]

into

djurleite

[04]

orientations.

(a)

The left

part

of

the image converted nto the

[04]

orientation, but the right

part

is

still

in

[32]

orientation.

(b)

After a

few

seconds

he

right

part

has

also converted

nto

the

[04]

orientation.

The

previous grain

boundary

is

preserved

as an

antiphase

boundary.

and so

we

think that the electron

micrographs showing

intergrowths of djurleite and digenite

reflect

the original

relationshipof

minerals n

the sample.

The

presence

f untwinned

djurleite crystalssuggests

primary

origin because jurleite crystals

ormed

by solid-

state

transformation of high chalcocite

would

be

heavily

twinned

(Evans,

1979).Apparently,changesn the Cu/S

ratio of the ore-forming fluid controlled

whether

pure

djurleite or assemblages f

digenite and djurleite crystal-

lized. It is likely that the sample that

we

studied

formed

between

72

and 93

'C

(the

upper limits of stability

for

anilite

and djurleite, respectively;

Potter, 1977; Moi-

moto

and

Koto, 1970);

on

cooling o room temperature

the

metastable

6a-type digenite could

persist.

Our

results

confirm

that ifchalcocite and djurleite oc-

cur in the same sample, transformations between hem

are

possible

under the

electron beam. The composition

of djurleiteextends

rom Cu,

ejs

o Cu,

ruS

Potter,

1977).

Djurleite

and chalcocite

coexist if the Cu/S

ratio is

be-

tween

1.96and 2

(Potter,1977).

Based

on

his TEM ob-

servations,

Putnis

(1977)

suggested

hat the composition

ranges

of chalcocite

and djurleite overlap.

If his

sugges-

tion is correct, then our

results

are compatible

with an

isochemical transformation.

On the other hand,

if

the

compositional

values n Table

1

are corect, then the

slight

chemical differencesare compensatedby the diffusion

of

Cu atoms to and

from

other crystals hat

were n contact

with the crystal exposed o the electron beam. The re-

versibility

of the transformations

ndicates hat the

loss

ofS

in the electron

beam

is

not

significant

n our exper-

iments.

Putnis

(1977)

attributed

the

chalcocite

-

djurleite

transformation

to the heating effectof the electronbeam.

However,

Leon

(1990)

showed

that djurleite

directly

transforms

nto high chalcocite

and

high digenite

on

heat-

ing,

without converting

to

monoclinic

chalcocite.

We

did

not observe

he appearance

fhigh

chalcocite

during

our

experiments,

and

djurleite

crystals

n the

Arizona sample

were stable

n the beam

under operating

conditions sim-

ilar to

those used

in the study

of the

Cornwall sample,

suggesting

hat the

temperature

of the

grains

wasnot

raised

above 93

'C.

Instead

we assume

hat the

transformations

result from electrochemical

eactions

causedby

the

flow

of electrons

hrough

the crystal.

Changes

n the electric

currentmake he Cu atomsmove and reorder o a scheme

different

from the

previous

arrangement.

The Cu atoms

switch

their

positions, not only

alternatingly

producing

the djurleite

and chalcocite

structure,

but

also

creating

several

orientational

variants

ofdjurleite.

As

Evans

1979)

put

it,

even

nature

has difficulty

in

finding a stable

ar-

rangement

or

them.

The chalcocite

-

djurleite

transformations

hat

we ob-

served

n a

natural sample

could also

occur

n the copper

sulfide

layer of CurS-CdS

solar

cells.

When such

solar

cells

are

fabricated,

conditions

are optimized

to obtain

monoclinic chalcocite

as

he copper

sulfide

phase

because

chalcociteyields high efficiencies Caswellet al., 1977).

However, djurleite

(Te

Velde and

Dieleman,

1973;Na-


8/8

kayama

et al., l97l)

and the high-pressure,

etragonal

polymorph

of chalcocite

Sands

et al., 1984) were

also

detected n

the copper sulfide layer. It was

suggested

y

Putnis

(1976)

hat the efficiency

fthe cell deterioratesf

the chalcocite

converts to

djurleite.

We

propose

hat this

transformation happens

through

an electrochemical re-

action similar

to

what we

observed n

the electron mi-

croscope.Since solar cells are made with

the

purpose

of

producing

electric current,

electrons nevitably flow

through

the slightly Cu-deficient

chalcocite and

presum-

ably convert t into

djurleite.

AcxNowr,oocMENTS

We thank Istviin D6dony for his

helpful comments

and suggestions.

Reviews

by Carl O. Mosesand EugeneS. lton improved

the manuscript.

This

study

was

supported

by

National

Science

Foundation (NSD

grant

EAR-92-19376. This work is

basedupon research

onducted

with

TEMs

located n

the Center for High Resolution Electron

Microscopy, which is

supportedby the National

Science

Foundation

under

grant

no. DMR-9

l-

l 5680

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Mnxuscnrrr REcETvED

av 17, 1993

Mnmlscrrm ACCEFTEDovelrsen 23, 1993

POSFAI

AND

BUSECK: HRTEM

OF COPPER SULFIDES

djurleite digenite transformation

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