How Minerals Form

under UV light sources. It isn't as hard as quartz, even though it looks a lot like it.

HOW MINERALS FORM

You need certain criteria to make a crystal big enough to see. The elements must first be present in sufficient quantities and the right proportions. The physical conditions must also be right for these elements to combine in the right way. They also need to be left undisturbed long enough to grow into their crystalline shape.

The existing temperature, level of oxygenation in the environment, pressure, water availability, and pH all determine how a crystalline mineral grows. Time matters because it makes for bigger crystals without something to disrupt the process.

Most minerals were floating around as part of the liquidy magma deep inside the earth. As magma rose and cooled, the minerals within it also cooled and solidified. Rapid cooling means you'll get smaller crystals, while slow cooling has the opposite effect.

Besides cooling in magma, minerals can form in these other ways:

• The chemical compound can be dissolved in hot water and will crystallize during evaporation or cooling of the water.

• The mineral can precipitate out of a hot gas, such as you'll see emitted from volcanic vents.

• Pressure alone can form a mineral during metamorphosis.

• Exposure to weathering and air can create an oxide mineral.

• Organisms can contribute substrate for a mineral, such as seashell material or calcium carbonate. Teeth and bones also form apatite crystals that can contribute to mineral formation.

Rocks are essentially collections of minerals. Magma is basically melted rock that can exceed 1000 degrees Celsius. It cools as it makes contact with air, water, or the earth's surface. Minerals can become very large, depending on the circumstances.

Granite forms from hot magma that has slowly cooled. The main minerals in granite are:

• quartz

• potassium feldspar —which is pink

• biotite— which is black

• plagioclase feldspar —which is whiter than quartz.

Figure 23 shows granite up close so you can see these features:

Figure 23.

Lava is just magma that has reached the surface of the earth. It cools very rapidly due to our surface temperatures, meaning the crystals inside them just don't grow big. Its chemical composition is the same but the crystals would be extremely small – below the level of microscopic.

Minerals can begin in aqueous solutions. Oceans are great places for solutions of any kind. As the water reaches inland or evaporates, the minerals will crystallize. Some will precipitate early because they aren't very soluble in water in the first place. Halite and

calcite both precipitate out of water easily. Halite is just sodium chloride —table salt. Look for lots of this in the Great Salt Lake in Utah. Tufa towers are calcium-rich stone formations made when spring water rich in this element bubbles into an alkaline pH lake environment. As the lake level drops, towers form. Figure 24 shows these towers:

Figure 24.

Hot underground water picks up dissolved particles in exiting rocks. The water can flow and cool, depositing some of these minerals into the cracks in these rocks. When you see interesting veins in a darker rock, this is from water that has seeped in and left behind crystalline minerals.

You can also get minerals deposited in the open. Inside geodes, amethyst and other silicates can get quite large within the open space of another rock. Usually, geodes are made from gas bubbles trapped in areas where the mineral can grow to large and beautiful sizes.