The only attempts to investigate the crystal structure of marcasite have been made by Huggins and by de Jong. Huggins’ contribution has been of the nature of guesswork. The structure proposed by him on the basis of Lewis’ atomic theory ultimately proves correct, although his crystallographic substantiation of it is fortuitous and incorrect. De Jong, who attempted the only rc-ray work on marcasite, arrived at dimensions for the unit cell similar to those given beyond, except that his a-axis is doubled. He gave Vh11 as the space group.

In this paper, the details of a complete rt-ray determination of the structure of marcasite are given. No essential assumptions have been made other than that marcasite is holohedral. The rotation and oscillation method of analysis was alone used, with the crystal completely bathed in molybdenum Kα radiation. The determination leads to the following structure for marcasite:

(Conventional marcasite orientation, origin at a symmetry center).

There is no complete agreement among the authors giving analytical expressions or equivalent positions of Vh12. The equivalent positions are therefore rederived. It proves that Schiebold gives incorrect positions for symmetry centers and screw axes, and that Mark gives an incorrect origin of coordinates for the equivalent positions of this space group.

Marcasite possesses a 6-3 coordination structure. It is made up of units essentially equivalent to units discernable in the pyrite structure. Pyrite has a different linking of the groups which is also consistent with 6-3 coordination. The rutile structure is a special case of the marcasite structure, and the rutile and marcasite groups display a very similar crystal habit.

Marcasite appears to be composed of iron and sulfur in the atomic state. Both kinds of atoms have almost identical radii, 1.12 Å.

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