Thermally enhanced mimetic fabric of magnetite in a biotite granite

The thermally induced growth of mimetic magnetite after biotite is here demonstrated by the equivalence of anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) after heating of biotite single crystals and whole rock specimens from a magnetite-free granite (Elba Island, Italy). It is concluded that between 500 degrees and 725 degreesC magnetite nucleates and grows within and parallel to the biotite cleavages yielding a magnetic fabric mimetic of the biotite fabric. At higher temperatures magnetite undergoes partial haematization, accounting for the decrease in remanence, susceptibility and anisotropy degree. This experiment may be used as a proxy for fabric study of granites having composite (tourmaline + biotite) paramagnetic fabrics in which the fabric of biotite can be selectively thermally enhanced.