others
tarnish rapidly. This is especially true of marcasite, the
ortho-rhombic form of iron sulphide, which is sometimes confounded with
pyrite. This even decomposes and crumbles away in time.
Pyrite
was used to quite an extent in earlier times for mirrors, large pieces
of it being ground and polished until they gave a good polish. Among
the remains of the Incas of Peru have been found large numbers of these
pyrite mirrors.