instruction: How many iron atoms are in a single molecule of the main component substance of ironsand?
context: Ironsand, also known as iron-sand or iron sand, is a type of sand with heavy concentrations of iron. It is typically dark grey or blackish in colour.
  
  It is composed mainly of magnetite, Fe3O4, and also contains small amounts of titanium, silica, manganese, calcium and vanadium.
  
  Ironsand has a tendency to heat up in direct sunlight, causing temperatures high enough to cause minor burns. As such it forms a hazard in New Zealand at popular west-coast surf beaches such as Piha.
response: The main component of ironsand is magnetite, which has chemical formula Fe3O4. Because Fe is the chemical symbol for iron and it is followed by a 3 in this formula, there are 3 iron atoms per magnetite molecule.