Answer:  

Dr. Martin Schoonen, who is actually studying iron pyrite, kindly answered the question above:
  
   

Iron pyrite, also known as fool's gold, will react very slowly with oxygen and water from the air to form new sulfur salts on its surface. Iron pyrite is widely distributed over the Earth. Most of it is present as cubic crystals, some are present as concentric concretions. The concentric concretions are actually a different mineral by the name marcasite. The mineral marcasite has the same chemical composition, two sulfur atoms for each iron as pyrite, but these are arranged in a different pattern. Marcasite reacts faster with water and oxygen than pyrite. It is possible that the crystal you refer to is actually marcasite.

Iron pyrite    Courtesy of Freespirit Emporium

    
My research group has been studying the chemical properties of iron sulfides and related minerals for more than a decade. It is thought that iron sulfides played a role in the origin of life. In addition, we are exploring the possible use of iron pyrite and related minerals in environmental cleanup. 
   
If you like to know more about what we do, you can point your browser to http://pyrite.geo.stonybrook.edu/
  
  
Acknowledgement
We would like to thank Dr. Martin Schoonen for his kind answer.
 
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