1625
The Colour of Sphalerite, Pyrite and Chalcopyrite and
What It Says About Their Leaching Behaviour
F.K. Crundwella
Crundwell Metallurgy, London, United Kingdom
ABSTRACT: The rate of dissolution of sulphide minerals is frequently interpreted in terms of the passivating
behaviour of a layer of material on the surface. This interpretation has driven research to identify and possibly
remove this layer from the surface, and thus yield higher rates of dissolution. The identification of such layers
has proved difficult, with different researchers claiming different compositions for possible layers. An alternative
framework has been pursued by Crundwell and co-workers, who have argued that the rate of dissolution
is governed by the electronic structure of the minerals which give rise to their semiconducting properties.
Researchers skeptical of this approach have argued that semiconducting properties are only important in highly
pure substances. In this paper, we review the electronic and semiconducting properties and discuss how they
impact dissolution behaviour. These properties affect their colour and dissolution properties even though the
minerals are not high purity substances.
Keywords: Dissolution, leaching, passivation, semiconductor, chalcopyrite
INTRODUCTION
The dissolution of chalcopyrite (and pyrite and sphal-
erite) is frequently interpreted in terms of a passivating
layer. However, the composition of such a layer has not
been positively identified, and the evidence is inconclusive
(O’Connor, 2018). Crundwell (1988a 2015) proposed
an alternative explanation, which can be referred to as the
semiconductor model. However, the semiconductor model
has been dismissed with the argument that semiconduct-
ing properties are only relevant at high purities, and since
chalcopyrite is a natural mineral, semiconducting proper-
ties are not expected to affect the dissolution and leaching
behaviour. At the same time, the dissolution of pyrite and
sphalerite has been successfully interpreted in terms of the
semiconductor model (Crundwell, 1988 a, 1988b, Bryson
et al., 2015).
The purpose of this paper is to demonstrate that the
semiconducting properties of even impure chalcopyrite,
pyrite and sphalerite are manifest in an obvious property,
their colour. Indeed, this argument will confirm that high
purity is not required for the semiconductor model pro-
posed by Crundwell (1988a). Colour is particularly rele-
vant property because proponents of the passivation model
have claimed that the strong absorption of light in the blue
region is evidence of a semiconducting passive layer (Nicol,
2016), an interpretation that is not correct.
The paper is structured as follows: in the next section,
the passivation model is discussed, followed by an exposi-
tion of the semiconductor model. An integrated interpreta-
tion of various results is then presented.
PASSIVATING-LAYER MODEL
The rate of dissolution of chalcopyrite is slow, with tem-
peratures of above 200°C required for conversion times
of the order of hours. The first suggestion that this slow
rate is caused by passivation, meaning a layer of reaction
The Colour of Sphalerite, Pyrite and Chalcopyrite and
What It Says About Their Leaching Behaviour
F.K. Crundwella
Crundwell Metallurgy, London, United Kingdom
ABSTRACT: The rate of dissolution of sulphide minerals is frequently interpreted in terms of the passivating
behaviour of a layer of material on the surface. This interpretation has driven research to identify and possibly
remove this layer from the surface, and thus yield higher rates of dissolution. The identification of such layers
has proved difficult, with different researchers claiming different compositions for possible layers. An alternative
framework has been pursued by Crundwell and co-workers, who have argued that the rate of dissolution
is governed by the electronic structure of the minerals which give rise to their semiconducting properties.
Researchers skeptical of this approach have argued that semiconducting properties are only important in highly
pure substances. In this paper, we review the electronic and semiconducting properties and discuss how they
impact dissolution behaviour. These properties affect their colour and dissolution properties even though the
minerals are not high purity substances.
Keywords: Dissolution, leaching, passivation, semiconductor, chalcopyrite
INTRODUCTION
The dissolution of chalcopyrite (and pyrite and sphal-
erite) is frequently interpreted in terms of a passivating
layer. However, the composition of such a layer has not
been positively identified, and the evidence is inconclusive
(O’Connor, 2018). Crundwell (1988a 2015) proposed
an alternative explanation, which can be referred to as the
semiconductor model. However, the semiconductor model
has been dismissed with the argument that semiconduct-
ing properties are only relevant at high purities, and since
chalcopyrite is a natural mineral, semiconducting proper-
ties are not expected to affect the dissolution and leaching
behaviour. At the same time, the dissolution of pyrite and
sphalerite has been successfully interpreted in terms of the
semiconductor model (Crundwell, 1988 a, 1988b, Bryson
et al., 2015).
The purpose of this paper is to demonstrate that the
semiconducting properties of even impure chalcopyrite,
pyrite and sphalerite are manifest in an obvious property,
their colour. Indeed, this argument will confirm that high
purity is not required for the semiconductor model pro-
posed by Crundwell (1988a). Colour is particularly rele-
vant property because proponents of the passivation model
have claimed that the strong absorption of light in the blue
region is evidence of a semiconducting passive layer (Nicol,
2016), an interpretation that is not correct.
The paper is structured as follows: in the next section,
the passivation model is discussed, followed by an exposi-
tion of the semiconductor model. An integrated interpreta-
tion of various results is then presented.
PASSIVATING-LAYER MODEL
The rate of dissolution of chalcopyrite is slow, with tem-
peratures of above 200°C required for conversion times
of the order of hours. The first suggestion that this slow
rate is caused by passivation, meaning a layer of reaction