Question

A mineral specimen has very large crystals. What is the best explanation for how this happened?

A.) The mineral cooled fairly rapidly underground.
B.) The mineral cooled very slowly underground.
C.) Crystal size has nothing to do with cooling.
D.) The mineral cooled very rapidly on the Earth's surface.

Answer 1

B. Crystal size is a function of cooling time. The longer a melt cools the larger the crystals.

Answer 2

B.) The mineral cooled very slowly underground.

Step-by-step explanation:

Minerals are made up of crystals, which can be microscopic in size (and even smaller, as in the case of chalcedony) or up to a few feet long, like crystals found in rock called pegmatite.

Melting substances in a volcanic lava form crystals when the lava cools. Salts dissolved in water form crystals when the water evaporates.

In both cases, crystal formation is a slow process, and without pressure or temperature variation or any other disturbance, crystals will continue to form as long as material is available in the melt or saline.

At this stage of formation, the crystals are said to be growing. So in this case you can say yes, a stone grows.

Once the stone is formed, after the cooling of the lava or evaporation of the saline has ceased, it is normal for the crystals to remain the same size even over millions of years.