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Through the 'Scope: Unusual Triangular Arrays of Marcasite from Quebec

The annual May symposium of the Canadian Micro Mineral Association (CMMA), held at Brock University in St. Catherines, Ontario, always features a large and generous “freebies table,” where donations from members and friends are eagerly examined by participants looking for something special to add to their collections cost-free. In 2019 a small number of specimens came from the Elder mine, Rouyn-Noranda, Quebec, brought by Claude Gagnier. The minerals were mostly calcite, chalcopyrite, and marcasite. A friend and I were looking at a marcasite specimen, and she pointed out an interesting spot with three marcasite crystals forming a triangular array. She let me have the specimen, with the instruction to figure out what was going on. Examination with the microscope soon revealed other similar triangles. These were clearly not coincidental—they were far too common to have happened by chance. They were nestled here and there among other marcasite crystals that completely covered the matrix, and there was little to see of the rest of such groups—only the triangular surfaces composed of three bladed crystals. My first thought was to compare these with cyclic twins I was familiar with from elsewhere, such as those of marcasite and arsenopyrite (fig. 2) or rutile (fig. 3). These all share the property that they form a quite regular polygonal array, often with a depression or apparent hole in the center. Cyclic twins involve repeated twinning on faces that are also commonly twin planes for simple twins involving just two individuals of the same mineral. For example, cyclic twins of arsenopyrite involve twinning on (101) and (101 –), which meet at 68°, close to the internal angle of a pentagon: 72°. Comparable twins of marcasite meet at 74°389. The only other known (and rare) twin law for marcasite would form a cyclic twin with six members, not three, so neither twin law can explain these triangular arrays. Furthermore, with typical cyclic twins, the interfaces between the individuals are mostly well-defined and bisect the angle between them, whereas these triangular arrays are joined in rather haphazard fashion, as if just shoved up against one another. There is no real sign of a twin plan. The other possible mechanism for forming such an array would be epitaxy, with marcasite systematically oriented on some other “host” mineral. Among the minerals common from the Elder mine, pyrite is the obvious candidate. When marcasite is epitactic on pyrite, it usually grows on the cube faces aligned parallel to their edges. In pyrite, the axes of symmetry that emerge from the middle of the cube faces are not fourfold axes of rotation, as the cube might suggest, but twofold axes. There are also of Marcasite from Quebec