Pyrite contact twins

Two examples of contact twins in pyrite from Peru are described. The first one, from Pasto Bueno ore deposit, shows the pyritohedron {120} as principal form, accompanied by the {111} octahedron and {100} cube as secondary forms, giving a lenticular aspect. (111) is the composition plane, and the twin operation is any one of the three binary axes < 110i > within this plane. The second one (unknown ore deposit) presents two forms, the octahedron {111} and the pyritohedron {120}; the two crystals in the twin are elongated along [101] and [011], respectively, producing a V profile. It is a reflection twin where the twin plane (110) coincides with the composition plane. These twins are by merohedry. Another contact twin is known in the literature, reported by Gaubert [Bull. Soc. Fr. Mine ' ral. Cristallogr. (1928), 51, 211-212] who described it as a spinel twin, i.e. a reflection twin with twin and composition plane (111); here it is shown that it is actually a rotation twin in which the twin operation is a 180 degrees rotation about any of the three equivalent directions < 211 >, contained in the (111) composition plane. The occurrence of these twins as well as the doubtfulness of the spinel twin in pyrite shows a direct relationship with the structural interpretation based on the pseudo-symmetry of the crystallographic orbits.

Automated summary

The article describes two examples of contact twins in pyrite from Peru. The first example is a lenticular twin with the pyritohedron {120} as the principal form and the {111} octahedron and {100} cube as secondary forms. The second example is a V-shaped twin with the octahedron {111} and the pyritohedron {120}. These twins are related to merohedry. The article also discusses a spinel twin described in the literature, which is actually a rotation twin with the twin operation being a 180-degree rotation about any of the three equivalent directions < 211 > contained in the (111) composition plane. The occurrence of these twins and the doubtfulness of the spinel twin in pyrite are related to the structural interpretation based on the pseudo-symmetry of the crystallographic orbits.