Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Marcasite (FeS2) has the same formula as pyrite but a different structure. The term "marcasite" is often used to refer to pyrite as a gem stone, and this usage actually predates the mineralogical usage of the name "marcasite." Marcasite is notorious for crumbling in collections because the sulfur reacts with atmospheric misture. The structure consists of iron atoms surrounded by octahedra of sulfur, each joined to a neighbor by sharing vertices.
Above is a single unit cell of marcasite and below is a view of the lattice. Note the nearly planar arrangement of sulfur atoms shown in blue.
Below is a view perpendicular to the views above. At top is a view of three octahedra and below that is a side view showing the vertical chains of edge-joined octahedra.
A view of the kinked plane of sulfur atoms shown in blue above. The arrangement is approximately close packed with rows of iron atoms (green) alternating on each side of the sheet. On the left side, triangles with iron atoms above the layer are yellow and those with iron atoms below are in green. Iron atoms on top are brown.
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Created 21 March 2011, last update