Lattice preferred orientation of graphite in a graphite ore as investigated by the anisotropy of out-of-phase magnetic susceptibility

Literature data on single crystals of graphite shows that this mineral is diamagnetic and strongly anisotropic. In addition, it possesses high electrical conductivity and extremely strong conductivity anisotropy. The AC magnetic susceptibility of graphite exhibits weak and negative in-phase component and relatively strong out-of-phase component, which is no doubt due to electrical eddy currents. Consequently, if the graphite crystals are oriented preferentially by crystal lattice (LPO) in graphite ore, one would expect strong anisotropy of magnetic susceptibility (AMS) of the ore. Unfortunately, the standard AMS, which is in fact the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) indicates LPO of graphite, free of the effects of non-conductive paramagnetic and ferromagnetic minerals.

The above theoretical expectations were tested on natural South Bohemian graphite ores occurring in the wide vicinity of the town of Český Krumlov in the Moldanubian Unit and being mined for pencil industry in the past. The ores are metamorphic in origin and one can therefore expect strong LPO of graphite in them. The graphite ores were sampled in two localities near the town of Český Krumlov, one being a road cut outcrop and the other one being a graphite mine. In both cases, the in-phase susceptibility is very low, in the order of 10^-6 [in SI units], being positive in some specimens and negative in the others. This indicates simultaneous and more or less balanced control by graphite and paramagnetic and/or ferromagnetic minerals. On the other hand, the out-of-phase susceptibility is much higher, in the order of 10^-4, and no doubt indicates its graphite control. The degree of opAMS is truly high, P = 2 to 3, and the opAMS foliation is closely related to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.