Pyrrhotite varieties from the 9.1 km deep borehole of the KTB project

We used transmission electron microscopy (TEM) to study pyrrhotite from the German Continental Deep Drilling ("Kontinentale Tiefbohrung," KTB) project . Our goals were to determine the distribution of structure types with dept h and to establish relationships between the bulk thermomagnetic behavior a nd the microstructures of pyrrhotite. In samples from the deep section of t he borehole (9080 m below surface, which is equivalent to an in-situ temper ature of similar to 260 degrees C), the dominant variety of pyrrhotite is 1 C that has a mostly disordered vacancy distribution. Faint, diffuse superst ructure reflections in the selected-area electron diffraction (SAED) patter ns indicate some nC-like ordering occurs, probably in small domains. In add ition to the disordered pyrrhotite, a 5C type is also present. According to bulk thermomagnetic measurements, pyrrhotite grains from 9080 m are antife rromagnetic at room temperature; we attribute this behavior to the dominanc e of the disordered 1C type. In the upper section of the hole (at 564 and 2 325 m) several pyrrhotite varieties occur, but the 4C type is most common, in agreement with the ferrimagnetic character of most pyrrhotite grains. Optical microscopy of pyrrhotite grains that are covered with a magnetic co lloid reveal intergrown ferrimagnetic and antiferromagnetic lamellae. TEM i mages show that these grains are intergrowths of 4C, 5C, and nC types. We i nterpret these microstructural variations to be responsible for the variati on in magnetic properties within single pyrrhotite grains.