Iron pyrite

Iron pyrite 

Frequency	common
Hardness	6-6.5
Density	5
Chemical Formula	{{#Chem:FeS2}}
Core Elements	Fe
Thermal Decomposition	{{#Chem:FeS2}} 540-850°C {{#Chem:Fe2O3}}

pyrite, along with Hematite and Magnetite is a primary ore of iron.

Weathering

In contact with a wet natural environment (water, air, perhaps sunlight), pyrite weathers producing ferrous (and perhaps ferric) sulfates:

{{#Chem: 4FeS2(s) + 14O2(g) + 4H2O(l)  =  4FeII + 8SO42- + 8H+}}
{{#Chem: 4FeII + O2 + 4H+  =   4FeIII + 2H2O}}
{{#Chem: 4FeIII + 12 H2O  =    4Fe(OH)3 + 12H+}}