One way to think about the closed system of the crystal is to compare it to an hourglass.
The grains of sand in the top half of the hourglass are the radioactive parents, and those falling to the bottom are the stable daughters.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
After one half-life, 50 percent of the original parents remains; after two, only 25 percent remains, and so on.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.Therefore, their ages indicate when they were formed.Because all parts of the solar system are thought to have formed at the same time (based on the solar nebula theory), the Earth must be the same age as the moon and meteorites--that is, about 4.6 billion years old.Ideally, the mineral crystals in igneous rocks form a closed system--nothing leaves or enters the crystal once it is formed.This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal.