Nantes University, CNRS, Institut des Matériaux Jean Rouxel, IMN, Nantes, F-44000, France.
Université de Lorraine, CRNS, CRM2, Nancy, France.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2023 Feb 1;79(Pt 1):32-45. doi: 10.1107/S2052520622011714. Epub 2023 Jan 12.
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 ⟨110⟩ 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. Minéral. 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° 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.
描述了来自秘鲁黄铁矿中的两个接触双晶实例。第一个来自帕斯托布埃诺矿床，显示出以五角十二面体{120}为主的形态，其次是八面体{111}和立方体{100}，呈现出透镜状。(111)是组成面，双晶操作为该面内的三个二轴之一 ⟨110⟩。第二个（未知矿床）呈现出两种形态，即八面体{111}和五角十二面体{120}；双晶中的两个晶体分别沿[101]和[011]拉长，形成 V 形。这是一个反映双晶，其中双晶面（110）与组成面重合。这些双晶是由单形对称构成的。文献中还报道了另一个接触双晶，由 Gaubert 描述[Bull. Soc. Fr. Minéral. Cristallogr. (1928), 51, 211-212]，他将其描述为尖晶石双晶，即具有双晶面和组成面（111）的反映双晶；这里表明，它实际上是一个旋转双晶，其中双晶操作为在包含在（111）组成面内的三个等效方向之一 ⟨211⟩上进行 180°旋转。这些双晶的出现以及黄铁矿中尖晶石双晶的不确定性与基于晶体轨道拟对称的结构解释直接相关。
