Page 19 - Curriculum Visions Dynamic Book

 A cubic crystal of pyrite or iron sulphide.
ion: an atom, or group of atoms, that has gained or lost one or more electrons and so developed an electrical charge.
pyrite: “mineral of fire”. This name comes from the fact that pyrite (iron sulphide) will give off sparks if struck with a stone.
sulphide: a sulphur compound that contains no oxygen.
Cubic structure
In a pyrite cube, an iron ion occurs at each corner of the cube and the middle of each of the faces. A pair of sulphur ions are found half way along each edge.
The bonds between the iron and sulphur ions are very strong, and they cannot be deformed. This gives the pyrite the property of being hard and brittle.
Sulphur ions
Iron ion
 A diagrammatic representation of the cubic structure of pyrite.
 This bottle used to contain about 10 pyrite cubes. During 20 years of exposure to humid air the pyrite has weathered to leave a grey residue of iron sulphate.
EQUATION: Weathering of pyrite (iron sulphide) in damp air
Iron sulphide + water + oxygen from the air ➪ iron sulphate + sulphuric acid
2FeS2(s) +
2H2O(l) + 7O2(s) ➪ 2FeSO4(s) + 2H2SO4(aq)
 Pyrite nodules are commonly found in rocks such as chalk.
They consist of radiating crystals of iron sulphide, the ends of which can clearly be seen in the picture on the left. The surface is brown because
the iron has oxidised. However, in the picture on the right the change from sulphide to sulphate inside the nodule is causing it to break up.
1
9
19