Grain boundary energy as a clue to mineral segregations — A preliminary study

Dihedral angles between two grains of quartz and one of pyrrhotite, sphalerite resp. hematite are measured at triple junctions. An approach to equilibrium is noted. The sulphides have dihedral angles greater than 120 ° and the oxide less than 120 °. No significant influence of sphalerite composition on dihedral angle can be proved. It is concluded that the average grain boundary energy between two grains of quartz is smaller than that between quartz and a sulphide but greater than that between quartz and hematite. Sulphide — biotite interfaces are found to be parallel to the biotite cleavage. Parallelism is concluded to meet a requirement of minimum grain boundary energy. Assuming random distribution of grains, the interfacial area between sulphides and transparent minerals is found to be too small in comparison with that between sulphides. The sulphide clustering, observed and inferred, is suggested to depend on a combination of grain boundary energy and epitaxial growth, but may to some extent be inherited from the ore-generation stage.