Both types of igneous rocks comprise a mixture of different minerals.

As igneous rocks cool, mineral crystals form following a specific sequence.

These haloes were considered to be the result of damage to the crystal structure of the host minerals caused by high energy alpha particles.

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True granites are relative latecomers on the geologic scene as they required a number of recycles of crustal material to differentiate and concentrate potassium.

In an earlier edition of , Lorence Collins (March/April, 1999) provided a thorough overview of the origin and nature of granitic rocks.

Discoloration haloes in younger rocks tend to be smaller and less intense than in older rocks, indicating that the zone of crystal damage increases with time.

From these observations early attempts were made to use the dimensions of haloes as an age dating technique.

The crystals develop an interlocking texture with some of the trace minerals becoming completely surrounded by later forming crystals.

Volcanic rocks, because they are able to cool and crystalize rapidly, have a very fine-grained texture; the individual mineral grains are too small to see easily with the naked eye.

Granites, for example, have more than 10% quartz and abundant potassium feldspar.

Other plutonic rocks have less quartz and potassium, and different ratios of calcium and sodium feldspar minerals.

Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.

Radiation damage haloes around mineral inclusions are well known from the geological literature.

In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.