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[Roger Warin]

Pyrite and marcasite intergrowths.

Pyrite microcrystals are attached to marcasite crystal in systematic orientations.
Pete Richards described intergrowths of pyrite and marcasite very similar to those observed on Moroccan specimens (The Mineralogical Record Vol. 26 March-April 1995).
On my specimen, marcasite is hidden by subsequent overgrowths of pyrite. The group resembles a fir tree.
Pete Richards says that the crystallization of pyrite is subsequent to that of marcasite, including epitactic growth of pyrite on marcasite. The pyrite forms bars. Richards's hypothesis which explains the geometric observations is that the pyrite is epitactic on an acicular marcasite crystal which is completely overgrown by the pyrite. For more information, see the Richards's article.
https://www.phy.mtu.edu/~jaszczak/SB734_05.pdf