acoid, a
{001}, and two square pyramids, b {101} and c {201}. Crystals are
usually twinned, and are often complex and difficult to decipher. There
are three twin-laws, the twin-planes being (111), (101) and (110)
respectively. Twinning according to the first law is effected by
rotation about an axis normal to the sphenoidal face (111), the
resulting form resembling the twins of blende and spinel. Twinning
according to the second law can only be explained by reflection across
the plane (101), not by rotation about an axis; chalcopyrite affords an
excellent example of this comparatively rare type of symmetric twinning.
Interpenetration twins (fig. 2) with (110) as twin-plane are of very
rare occurrence.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

Crystals have imperfect cleavages parallel to the eight faces of the
pyramid c {201}. The fracture is conchoidal, and the material is
brittle. Hardness 4; specific gravity 4.2. The colour is brass-yellow,
and the lustre metallic; the streak, or colour of the powder, is
greenish-black. The mineral is especially liable to surface alteration,
tarnishing with beautiful iridescent colours; a blue colour usually
predominates, owing probably to the alteration of the chalcopyrite to
covellite (CuS). The massive and compact mineral frequently exhibits
this iridescent tarnish, and is consequently known to miners as "peacock
ore" or "peacock copper." The massive mineral sometimes occurs in
mammillary and botryoidal forms with a smooth brassy surface, and is
then known to Cornish miners as "blister-copper-ore."

Chalcopyrite or copper-pyrites may be readily distinguished from
iron-pyrites (or pyrites), which it somewhat resembles in appearance,
by its deeper colour and lower degree of hardness: the former is easily
scratched by a knife, whilst the latter can only be scratched with
difficulty or not at all. Chalcopyrite is decomposed by nitric acid with
separation of sulphur and formation of a green solution; ammonia added
in excess to this solution changes the green colour to deep blue and
precipitates red ferric hydroxide.

The chemical formula CuFeS2 corresponds with the percentage
composition Cu=34.5, Fe=30.5, S=35.0. Analyses usually, however, show
the presence of more iron, owing to the intimate admixture of
iron-pyrites. Traces of gold, silver, selenium or thallium are sometimes
present, and the mineral is sometimes worked as an ore of gold or
silver.

Chalcopyrite is o