Barite is a mineral made up barium sulfate (BaSO4).  It's name comes from Greek 'barys' meaning 'heavy', due to its specific gravity of 4.5 which is unusual for a nonmetallic mineral.

Marcasite crystals are commonly metallic pale brass-yellow colored, tabular or pyramidal, often with curved faces often found replacing organic matter, forming fossils, in sedimentary beds, particularly coal beds. It's commonly associated minerals include Barite, Calcite, Dolomite, Fluorite, Galena, Pyrite and Sphalerite and may be replaced by Pyrite.

Pyrite is a brass-yellow mineral with a bright metallic luster. It has a chemical composition of iron sulfide (FeS2) and is the most common sulfide mineral. It forms at high and low temperatures and occurs in igneous, metamorphic, and sedimentary rocks worldwide.
 
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